Celia Lepeytre
email: celia.lepeytre@cea.fr

Fabien Frances

Marie-Sophie Charvolin

Amélie Ludwig

Esther Le Toquin

Emmanuel Comoy

Agnes Grandjean

Alban Gossard

Célia Lepeytre

Agnès Grandjean

C Lepeytre

Colloidal gel as an efficient process to treat CBR and prion contaminated solid surfaces

Keywords: Bacillus anthracis, VX, soman, sulfur mustard, Sodium hypochlorite, shear thinning Bacillus anthracis Bt: Bacillus thuringiensis CFU: colony-forming unit CWAs: Chemical Warfare Agents LB: Lysogeny broth PrP: Prion Proteins

Introduction

Chemical, biological, radiological and nuclear (CBRN) threats and the use of CBRN weapons in the Tokyo subway sarin attack in 1995, the 2001 anthrax attacks in the USA in 2001 [START_REF] De Queiroz | Disinfection of Bacillus subtilis spore-contaminated surface materials with a sodium hypochlorite and a hydrogen peroxide-based sanitizer[END_REF] and most recently, the chlorine attacks in Syria in 2013 and 2017 [START_REF] Nations | Seventh report of the Organisation for the Prohibition of Chemical Weapons-United Nations Joint Investigative Mechanism[END_REF][START_REF] Kumar | Highly selective and sensitive chromogenic detection of nerve agents (sarin, tabun and VX): a multianalyte detection approach[END_REF] have led to the investigation of surface decontamination methods in several countries [4,[START_REF] Greunig | Decontamination in front of the hospital. Management concepts for the mass casualty event with CRBN injuries[END_REF]. Furthermore, in post accidental situation, the contamination can be easily disseminated inside and outside the civilian infrastructure [START_REF] Richter | Inactivation of Bacillus anthracis spores to decontaminate subway railcar and related materials via the fogging of peracetic acid and hydrogen peroxide sporicidal liquids[END_REF][START_REF] Kaminski | Wide-area decontamination in an urban environment after radiological dispersion: A review and perspectives[END_REF], making the development of specific, effective decontamination products and processes crucial. Currently available CBRN decontamination technologies include physical, chemical and biological processes [4,[START_REF] Volchek | Decontamination in the event of a chemical or radiological terrorist attack[END_REF][START_REF] Khan | Chemical, biological, radiological, and nuclear threats-Decontamination technologies and recent patents: A review[END_REF]. Chemical decontamination products often contain an oxidant such as peroxides or hypochlorite ions [START_REF] Volchek | Decontamination in the event of a chemical or radiological terrorist attack[END_REF][START_REF] Khan | Chemical, biological, radiological, and nuclear threats-Decontamination technologies and recent patents: A review[END_REF][START_REF] Kumar | Chemical, biological, radiological, and nuclear decontamination: Recent trends and future perspective[END_REF], which are effective against biological and chemical contaminations [START_REF] Munro | The sources, fate, and toxicity of chemical warfare agent degradation products[END_REF][START_REF] Epa | Compilation of available data on building decontamination alternatives[END_REF], with hypochlorite also being particularly effective against a large variety of spores and microorganisms [START_REF] De Queiroz | Disinfection of Bacillus subtilis spore-contaminated surface materials with a sodium hypochlorite and a hydrogen peroxide-based sanitizer[END_REF][START_REF] Mcdonnell | Antiseptics and disinfectants: Activity, action, and resistance[END_REF][START_REF] Young | Mechanisms of killing of Bacillus subtilis spores by hypochlorite and chlorine dioxide[END_REF][START_REF] March | The differential susceptibility of spores from virulent and attenuated Bacillus anthracis strains to aldehyde-and hypochlorite-based disinfectants[END_REF]. Against chemical agents, while a basic medium effectively hydrolyzes (and thus neutralizes) sulfur mustard and nerve agents such as soman [START_REF] Munro | The sources, fate, and toxicity of chemical warfare agent degradation products[END_REF][START_REF] Yang | Decontamination of chemical warfare agents[END_REF][START_REF] Dell'amico | New insights into the biodegradation of thiodiglycol, the hydrolysis product of Yperite (sulfur mustard gas)[END_REF], an oxidant is required to neutralize other nerve agents such as VX. Various commercial CBRN decontamination products containing oxidative compounds have been developed over the years in liquid, gas (fumigant), foam, gel forms [START_REF] Volchek | Decontamination in the event of a chemical or radiological terrorist attack[END_REF][START_REF] Epa | Compilation of available data on building decontamination alternatives[END_REF][START_REF] Wagner | All-Weather Hydrogen Peroxide-Based Decontamination of CBRN Contaminants[END_REF].

The decontamination of solid surfaces is also an important issue in the nuclear industry, notably to make decommissioning waste easier to handle by reducing its radioactivity. In this case, the aim of the decontamination process is to concentrate the radioactivity and maximally reduce the final waste volume. Colloidal gels have been developed over the past 15 years for this purpose [START_REF] Faure | Vaccumable gel for decontaminating surfaces and use thereof[END_REF][START_REF] Castellani | Efficiency enhancement of decontamination gels by a superabsorbent polymer[END_REF][START_REF] Castellani | A topping gel for the treatment of nuclear contaminated small items[END_REF]. These gels, also named "vacuumable gels", consist of a surface-adapted decontaminating solution in which inorganic particles (typically silica or alumina) are used as a thickening agent and dispersed in a decontaminating solution to form a gel-like concentrated colloidal suspension. The gel's viscosity decreases under shearing but returns almost immediately, making them easy to implement at large scale and at a distance from the contamination by spraying [START_REF] Faure | Vaccumable gel for decontaminating surfaces and use thereof[END_REF][START_REF] Lemesre | Hybrid colloidal suspensions tailored as gels to remove radioactive bitumen stains[END_REF][START_REF] Ludwig | Oxidizing alkaline biodecontamination gel and surface biodecontamination method using said gel[END_REF]. The gels remain attached even to vertical surfaces, increasing the contact time between the decontaminating solution contained in the gel and the contaminated surface. The gels crack as they dry to form non-dusting solid flakes in which the contaminants are trapped and which are easily to remove by brushing or vacuum cleaning. Vacuumable gels have recently been applied to treat other surfaces, such as bitumen stains [START_REF] Lemesre | Hybrid colloidal suspensions tailored as gels to remove radioactive bitumen stains[END_REF], and other contaminants, such as heavy metals [START_REF] Gossard | An innovative green process for the depollution of Cr(VI)contaminatedsurfaces using TiO2-based photocatalytic gels[END_REF]. Based on our experience in the formulation of active colloidal gels, we have been investigating the possibility of using vacuumable gels to treat building materials contaminated by CBR agents. We propose then here to add hypochlorite ions and sodium hydroxide in the decontaminating solution to obtain an innovative CBR gel that neutralizes both biological and chemical contaminations [START_REF] Ludwig | Oxidizing alkaline biodecontamination gel and surface biodecontamination method using said gel[END_REF][START_REF] Ludwig | Pigmented decontaminating gel and method for decontaminating surfaces using said gel[END_REF]. As vacuumable gels, they should be effective against radiological contaminations. In this study, we also investigated their decontamination efficiency against prions, which are problematic in healthcare and veterinary settings because of their transmissibility and wide spectrum of resistance [START_REF] Georgsson | Infectious agent of sheep scrapie may persist in the environment for at least 16 years[END_REF].

After characterizing the physicochemical properties of the CBR gel, we tested the decontamination efficiency of fresh and aged gel samples, to evaluate its efficiency in time, against a radiological agent ( 137 Cs), the chemical agents soman, sulfur mustard, and VX, spores of Bacillus thuringiensis (Bt) and of Bacillus anthracis (Ba) as biological agents, and hamster prion protein (PrP) as a model diseaseassociated prion contamination. The decontamination mechanisms involved in each case are discussed.

Experimental section

Materials and sample preparation

Alumina was purchased as Aeroxide Alu C (specific surface area, 100 m².g -1 ) from Evonik Industries. Pluronic PE 6200 was purchased from BASF and iron oxide (Ferroxide 212M) was purchased from Rockwood Pigments. Nitric acid, sodium hydroxide, acetone, heptane, potassium iodide and starch were purchased from Sigma Aldrich; sulfuric acid from VWR; bleach (sodium hypochlorite) from Fisher, and sodium thiosulfate from Fluka. The chemical agents sulfur mustard (HD), VX, and soman (GD) were provided by the French Directorate General of Armaments (Direction Générale de l'Armement). Bt DSM 5815 was provided by the Leibniz-Institute DSMZ (Braunschweig, Germany) and Ba (Sterne strain NCTC-08234 Weybridge Lot 100816) was provided by NCTC (National Collection of Type Cultures) and stored at 4 °C. 137 CsCl was purchased from Orano's Radioactive Standard Laboratory (Laboratoire d'Étalons d'Activité). Prion studies were performed using brain homogenate from Syrian hamsters infected with hamster-adapted scrapie strain 263 K [START_REF] Fichet | Novel methods for disinfection of prion-contaminated medical devices[END_REF].

Three kinds of substrates were used: stainless steel (for chemical, biological and radiological experiments), ceramic tiles (9 × 2 cm, identical to the ones used in the Paris metro, for the biological experiments) and glass (prion experiments). Stainless steel and ceramic tiles are reference materials for CBRN experiments. The chemical and radiological experiments, which are very complicated to conduct safely, were only performed on stainless steel because of financial constraints. The radiological decontamination experiments were performed on 7 cm 2 316L stainless steel discs and the chemical decontamination experiments were performed on 5 x 5 cm coupons of 304L stainless steel according to NATO standards [START_REF]NATO-AEP-7[END_REF]. The prion decontamination experiments were performed on 7.6 × 2.6 cm pieces of glass as proof of concept.

Gel formulation and aging

The different operations and reactions were performed under ambient conditions. The CBR gel is a patented formulation of 14 wt.% Aeroxide Alu C alumina as a thickening agent, 1 wt.% Pluronic PE 6200 as a surfactant, and 0.1 wt. % iron oxide as a pigment and the remainder (84.9 wt. %) of a decontaminating solution of 0.5 M NaOH containing hypochlorite ions, which is equivalent to the active chlorine amount [START_REF] Ludwig | Oxidizing alkaline biodecontamination gel and surface biodecontamination method using said gel[END_REF]. Typically, for 100 g of CBR gel, 1 g Pluronic PE 6200 was first mixed with 84.9 g of a 0.5 M NaOH and active chlorine solution using a three-blade stirrer at 200 rpm for 5 min. 0.1 g of iron oxide and 14 g of alumina particles were then gradually added, while increasing the stirring speed up to 500 rpm as the viscosity increased. Stirring was maintained for 5 min after all the compounds had been added to homogenize the gel. Samples of a second type of gel (referred to as NaOH-gel) were prepared in the same manner but without presence of hypochlorite ions. For the prion decontamination experiments, a third gel was tested, which was prepared with a 1 M (rather than a 0.5 M) solution of NaOH.

The gels were either studied within 3 months of synthesis (fresh), after storing for 15-27 months at room temperature, or after storing for 8 weeks at 40 °C in an environmental chamber (Table 1). The active chlorine content of the gels was determined using a specific in-house titration method (see Supplementary Material). In this study, we express the amount of active chlorine in the gels using the ratio Ra.c. defined as followed:

𝑅 𝑎.𝑐. (%) = ( Active chlorine measured in the gel after aging Initial active chlorine in the fresh gel ) . 100

(1) The Fig. 1 presents the evolution of the Ra.c. ratio as a function of the time and depending on the storing conditions. Note that, as the Ra.c. ratios of the gels stored at 40 °C for 8 weeks and aged for 24 months at room temperature were similar (14.5 and 13.7 respectively), we decided to use the gels stored at 40 °C for 8 weeks as more rapidly prepared surrogates for the gels aged for 24 months to facilitate the evaluation of the chemical decontamination experiments.

CBR decontamination experiments and analytical methods

Preliminary gel characterization

The pH of the gels was measured using a glass combination electrode connected to a pH-meter calibrated at pH = 4.01, 7.00 and 9.18, in a suspension of 1 g of gel in 10 mL of water. NaOH concentrations were then specifically measured in this diluted solution by acid-base titration using a 0.1 M solution of HCl. The in-house titration method used to measure the active chlorine percentage in the gels is described in the Supplementary Material. Rheological measurements were performed with a Discovery HR-1 plate-plate rheometer (TA Instruments; diameter, 40 mm; gap between the plates, 1 mm). The viscosity of the gel was measured using stepped flow tests at shear rates ranging from 0.01 s -1 to 100 s -1 . A 1 mm thick layer of gel was deposited on a vertical stainless steel substrates and left to dry at room temperature to observe the formation of flakes. In the rest of the study, the gel will be considered as dried as soon as the formation of flakes.

Radiological decontamination experiments

The contaminant for these experiments was a certified acidic solution of CsCl. A volume of CsCl corresponding to 5000 Bq of 137 Cs was deposited on the surface of the substrates prior to evaporation under a fume hood. The initial activity (Bq) of each substrate was measured by gamma spectrometry using a Eurisys Mesures coaxial germanium detector. This was connected to an Itech measurement chain controlled using the Interwinner 7.0 software program. 137 Cs activity was measured at an energy of 661.66 keV. A 1 mm thick layer of the studied gel was then applied with a spatula on each substrate. After drying for a few hours, the flakes were removed from the substrate with a paintbrush. Finally, the activity of the substrates was measured again to calculate the decontamination factor (the ratio of the initial and final activities).

Chemical decontamination experiments

Chemical decontamination experiments were performed with both the CBR gel and the NaOHgel to highlight the effect of active chlorine on the decontamination efficiency. For each of the three agents (sulfur mustard, VX and soman, see chemical structures Fig. SI 1), eight stainless steel coupons were contaminated at 10 g•m -2 by dropwise deposition of these CWAs. To confirm the level of contamination, three of the substrates were mixed in a 90/10 vol.% heptane/acetone mixture for 1.5 h to extract the chemical agents and analyze their concentration by gas chromatography with helium as carrier gas. The five remaining samples and a non-contaminated coupon used as a blank were placed inside a closed dispersion chamber before the gels were applied with a spray gun through a slot. The thickness of the gel layers was measured by gravimetry for each sample based on the gel density (calculated as 1.11 g.cm -3 , see detailed in Supplementary Material) and estimated at 1 ± 0.2 mm. After applying the gel, the samples were dried overnight at room temperature under a fume hood. The millimeter-scale flakes that had formed were retrieved using a spatula. Residual levels of contamination were determined by dipping the substrate samples in a 90/10 vol.% heptane/acetone mixture for 1.5 h.

In parallel, the flakes were also stirred (without dissolution) in a 90/10 vol.% heptane/acetone mixture for 1.5 h , to extract and quantify any remaining chemical agents. The chemical agents solubilized into the heptane/acetone mixture were analyzed by gas chromatography. The samples were injected in splitless mode. A flame ionization detector (Perkin Clarus 680) was used for HD and a flame photometry detector (Shimadzu GC2010) was used for VX and GD. The decontamination efficiency was calculated by determining the two following decontamination factors:

Substrate decontamination (%) = (1 - Residual contamination level Initial contamination level ) . 100 (2)
Residual contamination in flakes (%) = ( Contamination extracted from residual flakes Initial contamination level × 𝐴

) . 100

In Eq. ( 2), the initial and residual contamination levels (in µg•cm -2 ) are respectively the amounts of chemical agent on the substrate before and after the decontamination step. In Eq. ( 3), the mass of chemical agent extracted from the flakes (µg) is divided by the initial contamination level multiplied by the contaminated area (𝐴, cm 2 ). The results of the decontamination tests are expressed as the mean and standard deviation of five repeats.

Biological decontamination experiments

Preliminary experiments were performed with Bt, which is typically used as a surrogate for Ba [START_REF] Tufts | Bacillus thuringiensis as a surrogate for Bacillus anthracis in aerosol research[END_REF]. Experiments were repeated with Ba if the log reduction in colony forming units (CFU) was greater than 6. DSM 5815 was prepared as described by Wood et al. [START_REF] Wood | A simple decontamination approach using hydrogen peroxide vapour for Bacillus anthracis spore inactivation[END_REF]. The Bt concentration and the protocol used for deposition on the substrates were as described by Le Toquin et al. [START_REF] Le Toquin | New Biocide Foam Containing Hydrogen Peroxide for the Decontamination of Vertical Surface Contaminated With Bacillus thuringiensis Spores[END_REF]. A bacterial suspension of Bt spores was prepared in distilled water at a concentration of 2 × 10 8 CFU•mL -1 . The experiments with Ba were performed at biosafety level 3. An aliquot of 100 µL (equivalent to an initial amount of 2 × 10 7 CFU onto the coupons) was inoculated on the surface of each substrate using a micropipette. After inoculation, the substrates were air dried for 1 h in a 20 °C biosafety cabinet and the gel was applied with a sterilized spatula (layer around 1 mm thick). The different substrates covered with a layer of gel were then placed inside open Falcon tubes lying on their sides and left to dry for a few hours. The dried flakes were recovered and mixed in 10 mL of lysogeny broth (LB) medium (Becton Dickinson Co., Sparks, MD, United States) in Falcon tubes. In parallel, the treated substrates were also placed in Falcon tubes, in 20 mL of LB medium. All these Falcon tubes were incubated for 1 h at 30 °C. To isolate the spores from the residual flakes in suspension in LB, each tube was centrifuged at 4500 rpm for 3 min at 4 °C and the supernatant was separated. Several serial dilutions in LB medium were plated for each Falcon tube and covered with LB agar. After drying, the substrates were incubated for 24 h at 30 °C for Bt and 24 h at 37 °C for Ba and, finally, the number of viable CFU formed in the LB agar were counted visually in the plates containing 30-300 CFU. The gel decontamination efficiency was finally calculated as the log reduction in the number of microorganisms after decontamination:

LR = -log 10 ( CFU𝑓 CFU𝑖 ) (3)
where CFUi and CFUf are the counts respectively before and after the decontamination process recovered from the substrates.

Prion decontamination experiments

Brain homogenate from Syrian hamsters infected with scrapie strain 263 K were first dispersed in a PBS solution at 10 wt/vol %. Then, 25 µL of this solution is deposited on glass substrates, to make them contaminated with 2.5 mg of brain homogenate, and the liquid solution is let to evaporate [START_REF] Fichet | Novel methods for disinfection of prion-contaminated medical devices[END_REF].

After drying and rinsing in deionized water, the contaminated substrates were covered with a layer (around 1 mm thick) of one of three gels (the CBR gels with 0.5 and 1 M NaOH and the NaOH-gel) and left to dry for a few hours. The solid flakes were retrieved and there were no visible residues on the substrates. The substrates were nonetheless scraped and the presence of residual PrP was analyzed by western blotting as previously described [START_REF] Fichet | Novel methods for disinfection of prion-contaminated medical devices[END_REF]. Briefly, the samples are purified according to the Bio-Rad TeSeE protocol with or without proteinase K treatment (4 g of enzyme per mg of protein) to detect either abnormal protease-resistant PrP (PrP-res), or total PrP, respectively. The purified samples were loaded on acrylamide gel, transferred onto a PVDF membrane and the presence of PrP was evidenced using Sha-31 anti-PrP monoclonal antibody [START_REF] Feraudet | Screening of 145 anti-PrP monoclonal antibodies for their capacity to inhibit PrPSc replication in infected cells[END_REF].

In parallel, four control contaminated substrates were respectively immersed in different solutions (ineffective or effective treatments) to confirm the relevance of the protocol: (i) in water for 24 h at room temperature (ineffective treatment); (ii) in water for 10 min at 55°C (ineffective treatment);

(iii) in 1 M sodium hydroxide, pH = 14 for 10 min at 55°C (the reference effective treatment) and (iv) in 0.1 mM sodium hydroxide, pH = 10 for 10 min at 55°C (a second, partially effective reference treatment). The substrates were then rinsed, dried and the residues were dissolved in 100 µL of 10 wt/vol % homogenate of brains from PrP-deficient mice [START_REF] Bueler | NORMAL DEVELOPMENT AND BEHAVIOR OF MICE LACKING THE NEURONAL CELL-SURFACE PRP PROTEIN[END_REF]. The presence of PrP in the samples was detected by western blotting as described above.

Results and discussion

Physicochemical properties of the CBR gel

In the different experiment presented in this study, the gel was applied manually using a spatula because tests were performed on small surface areas. However, for an industrial use of this process, these gels have to be easily sprayable on large surfaces such as walls or floors. These properties have been confirmed using rheological analyses. The flow curve of the CBR gel, representing the stress as a function of an applied shear, is shown in Fig. 2a. The fact that the experimental data are well fitted by a Herschel-Bulkley model (described in the Supplementary Material) indicates that the gel is a shear thinning yield stress fluid, meaning that its viscosity decreases drastically under high shearing forces. This makes the gel easy to spray, the shearing imparted by the spraying device ensuring that the gel is fluid enough to pass through the nozzle without sticking. The gel's yield stress of 44 Pa (obtained from the Herschel-Bulkley fit, see Supplementary

Material) means that it only begins to flow above this threshold and thus that a layer of gel sprayed on a vertical surface remains attached under its own weight (assuming perfect adhesion of the gel to the surface).

These rheological properties are highlighted by the photographs of a 1 mm thick layer of the CBR gel straight after spraying (Fig. 2b) and after drying (Fig. 2c), the two black marks drawn on the substrate showing that it remains rigidly attached throughout the drying process. This ensures prolonged contact between the surface and the gel, which increases the efficiency of the decontamination process.

Note that the red color of the gel comes from the iron oxide pigment that was added to facilitate visual inspection [START_REF] Ludwig | Pigmented decontaminating gel and method for decontaminating surfaces using said gel[END_REF]. Fig. 2c shows the millimeter-sized solid flakes that form during the drying process, with some having fallen off under the effect of gravity. The remaining flakes were easily removed by manual brushing.

In summary, the CBR gel is well suited to be used as a vacuumable gel. It could be easily sprayed and remains attached to vertical surfaces in layers 1 mm thick, and forms solid flakes after drying, which can be removed effortlessly from the substrate.

Gel efficiency against radiological agents and associated decontamination mechanisms

The 137 Cs γ activities measured on the stainless plates, before and after application of fresh samples of CBR gel and NaOH gel are listed in Table 2 along with the calculated decontamination factors. The decontamination factors are similar to those obtained with a vacuumable gel specifically designed for nuclear applications [START_REF] Faure | Vaccumable gel for decontaminating surfaces and use thereof[END_REF][START_REF] Castellani | Efficiency enhancement of decontamination gels by a superabsorbent polymer[END_REF][START_REF] Lemesre | Hybrid colloidal suspensions tailored as gels to remove radioactive bitumen stains[END_REF][START_REF] Ludwig | Oxidizing alkaline biodecontamination gel and surface biodecontamination method using said gel[END_REF][START_REF] Gossard | An innovative green process for the depollution of Cr(VI)contaminatedsurfaces using TiO2-based photocatalytic gels[END_REF], where decontamination factors of 15-20 are considered satisfactory. The CBR gel is therefore suitable for radiological decontamination. The proposed mechanism is that the aqueous solution in the gel solubilizes the radioelement (here 137 Cs) on the substrate. This is enhanced by the presence of surfactants (Pluronic PE 6200) in the gel, which improve its degree of wetting on the surface [START_REF] Faure | Vaccumable gel for decontaminating surfaces and use thereof[END_REF]. The solubilized radiological contamination is then trapped in the flakes obtained after drying. The similar results obtained with the NaOH gel show that the absence of hypochlorite ions did not significantly affect the decontamination efficiency, which is consistent with the proposed solubilization mechanism. 1). Table 3 lists also the residual contamination in flakes rates. Furthermore, the evolution of the substrate decontamination percentage and residual contamination in flakes associated to these results are represented on Fig. 3.

Fig. 3

Table 3 Initial and residual contamination levels of chemical warfare agents deposited on the stainless steel substrates before and after the decontamination step, substrate decontamination rates, contamination extracted from the flakes and residual contamination in flakes rates.

Chemical agent CBR gel (fresh)

CBR gel (Ra.c. = 14.5 %)

NaOH-gel (fresh)

Sulfur mustard All the gels had substrate decontamination efficiency higher than 99.97 % for HD, 99.98 % for GD and 99.66 % for VX (Fig. 3a). Fig. 3The decontamination efficiency depends little therefore on the active compounds present in the gel. Only the NaOH concentration has a slight influence on VX because the presence of NaOH makes the solubilization of CWA, which are organic compounds easier. Regarding the degradation of the chemical agents, while HD and GD are almost completely neutralized in the flakes (residue contamination in flakes efficiencies respectively lower than 0.2 % and 0.02 %), a significant amount of residual VX is still detected in the flakes for all gels (18.9 to 32.2 % of the initial contamination, depending on the nature of gel) (Fig. 3b). These results suggest a two-step decontamination mechanism: first, solubilization of the chemical contaminant on the substrate (in the similar manner as for radiological contamination), and second, chemical degradation of the contaminant by active compounds in the gel.

Initial
Under basic conditions, HD is hydrolyzed into unchlorinated and thiodiglycol compounds [START_REF] Munro | The sources, fate, and toxicity of chemical warfare agent degradation products[END_REF][START_REF] Dell'amico | New insights into the biodegradation of thiodiglycol, the hydrolysis product of Yperite (sulfur mustard gas)[END_REF]. These two degradation compounds are probably produced in the gels because of the presence of NaOH. In further possibility is that HD is oxidized by active chlorine, producing the corresponding sulfoxide (HDO) [START_REF] Picard | Introduction to chemical warfare agents, relevant simulants and modern neutralisation methods[END_REF]. In this case however, since the NaOH gel and both CBR gel flakes had similarly low residual contamination in the flakes efficiencies, this suggests that only NaOH-related degradation occurred. Soman is hydrolyzed into phosphonic acid in alkaline media (pH > 10) [START_REF] Yang | Decontamination of chemical warfare agents[END_REF]. The high pH of the tested gels (Table 1) thus makes them highly effective at neutralizing GD and the resulting flake residues have insignificant residual contamination efficiencies. This point is of high interest because no further treatment are required and the waste can be directly conditioned. In the case of VX, the higher residue contamination efficiencies indicate that the active compounds in the gels are less effective, meaning that the second, "chemical" step of the decontamination process (degradation of chemical agent) is incomplete. VX is slowly hydrolyzed by NaOH, leading mainly to an acid after breakage of the P-S bond [START_REF] Yang | Decontamination of chemical warfare agents[END_REF], which explains why the least alkaline gel (the aged CBR gel, Table 1) had the lowest degradation efficiency. However, since sulfur is oxidized in the presence of hypochlorite ions, this should facilitate cleavage of the P-S bond [START_REF] Yang | Decontamination of chemical warfare agents[END_REF]. Recently indeed, VX has successfully been degraded in hypochlorite solutions into non-toxic N,N-diisopropylformamide and N,N-diisopropylamine [START_REF] Hopkins | Chemical analysis of bleach and hydroxide-based solutions after decontamination of the chemical warfare agent O-ethyl S-2-diisopropylaminoethyl methylphosphonothiolate (VX)[END_REF]. This mechanism is consistent with the higher degradation efficiency obtained with fresh CBR gel compared to fresh NaOH-gel, which have the same NaOH concentration, because of the presence of hypochlorite ions in the fresh CBR gel. Furthermore, the highest degradation efficiency obtained here (81%) compares favorably with the results of a previous study in which a degradation efficiency of 67% was measured after applying a solution at 0.5 wt.% sodium hypochlorite for 24 h on stainless steel surfaces contaminated with VX [START_REF] Love | Efficacy of liquid and foam decontamination technologies for chemical warfare agents on indoor surfaces[END_REF].

In summary, the CBR gel fully decontaminated the stainless steel surfaces onto which chemical warfare agents (HD, GD and VX) had been deposited and the extracted agents were neutralized by the active compounds in the gel, reducing the toxicity of the waste. NaOH plays an important role in the degradation process, especially for HD and GD. Although VX was not fully neutralized, the presence of hypochlorite ions increases the efficiency of the gel in degrading VX and VX-OH, a highly toxic product of VX hydrolysis. Furthermore, one of the main benefits of the vacuumable gel process is that the final waste is produced in the form of solid flakes, which entrap any residual contaminants and prevent their release into the environment.

Gel efficiency against biological agents and associated decontamination mechanisms 3.4.1. Gel efficiency on Bacillus thuringiensis and Bacillus anthracis spores

Fig. 4 shows the spore neutralization efficiency (log reduction in CFU) for Bt on stainless steel and ceramic tile substrates after application of differently aged CBR gels and fresh NaOH-gel. The removal of Bt spores was complete for all the CBR gels, even the one stored for 27 months, with no CFU counted on any of the substrates or in any of the flakes after decontamination ( Table 4). The CBR gel therefore remains effective against Bt even after important reductions in its active chlorine percentage (Table 1). This is probably because of the long contact time (few hours) between the gel and the spores on the surfaces, as discussed in the following paragraph. The biocidal activity of the gel thus involves two steps spore absorption-spore inactivation. First, the Bt spores are adsorbed into the gel and, as the gel dries, the known biocidal activity of the hypochlorite ions [START_REF] De Queiroz | Disinfection of Bacillus subtilis spore-contaminated surface materials with a sodium hypochlorite and a hydrogen peroxide-based sanitizer[END_REF][START_REF] Mcdonnell | Antiseptics and disinfectants: Activity, action, and resistance[END_REF][START_REF] March | The differential susceptibility of spores from virulent and attenuated Bacillus anthracis strains to aldehyde-and hypochlorite-based disinfectants[END_REF] ensures that the Bt spores are neutralized. This mechanism is consistent with the fact that the NaOH-gel is ineffective against Bt spores, with a log reduction in CFU of less than 1.1 obtained with a fresh sample (Fig. 4).

Furthermore,

Table 4 shows that the number of CFU in the flakes is higher than the number remaining on the surface.

The spores are thus absorbed into the gel but NaOH has no neutralizing effect on them.

In liquid decontamination solutions, results in the literature indicate that the higher the active chlorine percentage, the shorter the decontamination time is. Muhammad et al. have notably studied the decontamination efficiency of chlorine bleach solutions against B. atrophaus spores as a function of the active chlorine percentage and the contact time [START_REF] Muhammad | Inactivation of Bacillus spores in decontamination wash down wastewater using chlorine bleach solution[END_REF]. At room temperature, the log reductions obtained were 6.96 in 30 min for a solution with 2 % liquid bleach and up to 7 in 10 min for solutions with 5 % liquid bleach. Another study focusing on Geobacillus stearothermophilus decontamination found that 5.8 % bleach was necessary to obtain a log reduction greater than 6 in 20 min [START_REF] Sabbah | Use of a Mixture of Surrogates for Infectious Bioagents in a Standard Approach to Assessing Disinfection of Environmental Surfaces[END_REF]. The higher log reduction values we obtained in this work can therefore be explained by the much longer contact time between the CBR gel and the contaminants, highlighting once again the value of its specific rheological properties. The volume of decontaminating solution is also much lower than with liquid decontamination, and the residues can be disposed of without further treatment. The initial number of colony forming units on the substrates obtained from the positive control was 2 × 10 7

NM, not measured.

Gel efficiency on Bacillus anthracis spores

Similar results were obtained with Ba (Fig. 5), namely complete removal with CBR gel aged for 27 months on both stainless steel and ceramic tile and very low decontamination efficiency with fresh NaOH-gel, confirming the tendency observed with Bt. These results indicate that the CBR gel acts against PrP contamination with the same two-step mechanism as against biological agents, namely that PrP is first absorbed into the gel, then destabilized by NaOH and hypochlorite during the drying process.

Gel efficiency against prion proteins and associated decontamination mechanisms

Conclusions

This paper presents an efficient process to decontaminate solid surfaces after CBRN contamination using a inorganic CBR gel. This CBR gel can be sprayed at a distance across large

Fig. 1 :

 1 Fig. 1: Evolution of the active chlorine percentage in the gels as a function of the storage time. Black squares, red circle and blue triangle represent the CBR gel aged at RT and 40°C and the NaOH aged at RT respectively.

Fig. 2 .

 2 Fig. 2. (a) Stress in the CBR gel as a function of the applied shear rate. The black squares are the experimental data and the red line is a fit using the Herschel-Bulkley model. (b, c) Photographs of a 1 mm thick layer of CBR gel deposited on a vertical stainless steel substrate (b) before and (c) after drying.

Fig. 3 :

 3 Fig. 3: (a) Substrate decontamination efficiencies and (b) Residual contamination in flakes calculated after the decontamination step using fresh CBR gel (Ra.c. = 100 %), fresh NaOH gel (Ra.c. = 0 %) and CBR gel aged for 8 weeks at 40 °C (Ra.c. = 14.5 %).

Fig. 4 .

 4 Fig. 4. Log reduction of Bacillus thuringiensis colony forming units on stainless steel and ceramic tile surfaces after treatment with different gels.

Fig. 5 .

 5 Fig. 5. Log reduction of Bacillus anthracis colony forming units on stainless steel and ceramic tile surfaces after treatment with different vacuumable gels.

  Western blot analyses of PrP after treatments with different gels and control inefficient treatments are shown in Fig.6. The CBR gel prepared with 1 M NaOH was as effective as the first reference treatment (Ref #1, effective, 1 M NaOH for 10 min at 55°C), with no PrP detected on the substrate, suggesting a greater than 3-log reduction in PrP concentration given the detection limits of the technique. The CBR gel with 0.5 M NaOH was less effective, with one of the three triplicates showing just a ~2-log reduction in concentration (Fig.6a), similar to the second reference treatment (Ref #2, partially effective, NaOH 0.1 mM for 10 minutes at 55°C). However, no PrPres was detected for the CBR gels after proteolysis (Fig.6b) indicating that a 0.5 M NaOH concentration in the gel is sufficient to inactivate PrP, in keeping with the known effects of millimolar NaOH solutions on PrP[START_REF] Kasermann | Sodium hydroxide renders the prion protein PrPSc sensitive to proteinase K[END_REF].

Fig. 6 .

 6 Fig. 6. Western blot analysis of the efficacy of different vacuumable gels and reference treatments against prion protein (PrP) contamination. (a) Total remaining PrP (no proteolysis with proteinase K) and (b) total remaining protease-resistant PrP (after proteolysis with proteinase K) after treatment with CBR gels prepared with 0.5 and 1 M NaOH and two reference treatments (Ref #1, 1 M NaOH for 10 min at 55°C; Ref #2, 0.1 M NaOH for 10 min at 55°C), and in serial dilutions of non-treated contaminated substrate.

  surfaces and remain attached, even on vertical walls, without flowing. Once in contact with the contaminated surface, the CBR gel acts with different mechanisms depending on the nature of the contamination. First, the gel wets the surface and either solubilizes (radiological and chemical agents) or absorbs (biological agents and prion proteins) the contaminants. The contaminants remain trapped in the gel as it dries. Chemical agents, bacteria and prions are neutralized by NaOH and hypochlorite in the process. The resulting waste in the form of non-dusting solid flake is easily removed from the surface by brushing or vacuuming. In this way, no liquid effluent is produced and no secondary treatment required, although the solid waste should be treated as toxic if any residual contamination remains. The efficacy of the CBR gel was tested here on actual radiological ( 137 Cs), chemical (sulfur mustard, VX and soman) and biological (B. thuringiensis and B. anthracis) contaminations with satisfactory decontamination factors, even after two years' storage. The preliminary results presented here on prion decontamination are also promising. The wide spectrum of action of the gel should make it an attractive option to decontaminate CBRN agents but also in more conventional health and safety applications against various pathogens. In the context of the ongoing COVID-19 pandemic, it would be particularly interesting to investigate the efficiency of this CBR gel against viral particles.

Table 1

 1 Compositions of the gels used in the study

	Name	Aging	Ra.c. (%)	NaOH (M)	pH	Decontamination tests
		Fresh	100	0.5	13	R, B, C, prion
		18 months, RT	23.3	NM	13	B
	CBR gel	24 months, RT 27 months, RT	13.7 7.7	0.16 NM	12 12	B B
		8 weeks, 40 °C	14.5	0.23	12	C
		Fresh for Prion	100	1	13	Prion
	NaOH-gel	Fresh	0	0.5	NM	R, B, C, prion
	R, radiological; B, biological; C, chemical; NM, not measured; RT, room temperature

Table 2

 2 Radioactivity and measurement accuracy of substrates contaminated with 137 Cs before and after gel treatment.

	Gel	Initial 137 Cs activity (Bq)	Final 137 Cs activity (Bq)	Decontamination factor a
	NaOH gel	5000 ± 12%	160 ± 12%	31
	CBR gel	5200 ± 12%	270 ± 12%	19
	a Initial activity / final activity

Table 4 .

 4 Bacterial colony forming units after decontamination by CBR and NaOH-gels of different substrates.

		Bacillus thuringiensis			Bacillus anthracis
		Stainless steel	Ceramic tile	Stainless steel	Ceramic tile
	Gel
		On surface	In flakes	On surface	In flakes	On surface	In flakes	On surface	In flakes
	CBR gel (fresh)	0	0	0	0	NM	NM	NM	NM
	CBR gel (18 months)	0	0	0	0	NM	NM	NM	NM
	CBR gel (27 months)	0	0	0	0	0	0	0	0

Acknowledgments

This work was supported by an ANR GIFT-CBRN grant and by the CEA through the interministerial R&D program against CBRN-E threats. The authors thank the French Directorate General of

Armaments for all the tests carried out with chemical warfare agents and Fabienne Gas (CEA) for access to her laboratory for the tests on biological agents.


Corentin Sanchez

Philippe Xu

Alexandre Armand

Philippe Bonnifait

Lane level context and hidden space

Lane level context and hidden space characterization for autonomous driving

Corentin Sanchez 1 , Philippe Xu 1 , Alexandre Armand 2 , Philippe Bonnifait 1

Abstract-For an autonomous vehicle, situation understanding is a key capability towards safe and comfortable decisionmaking and navigation. Information is in general provided by multiple sources. Prior information about the road topology and traffic laws can be given by a High Definition (HD) map while the perception system provides the description of the space and of road entities evolving in the vehicle surroundings. In complex situations such as those encountered in urban areas, the road user behaviors are governed by strong interactions with the others, and with the road network. In such situations, reliable situation understanding is therefore mandatory to avoid inappropriate decisions. Nevertheless, situation understanding is a complex task that requires access to a consistent and non-misleading representation of the vehicle surroundings. This paper proposes a formalism (an interaction lane grid) which allows to represent, with different levels of abstraction, the navigable and interacting spaces which must be considered for safe navigation. A top-down approach is chosen to assess and characterize the relevant information of the situation. On a high level of abstraction, the identification of the areas of interest where the vehicle should pay attention is depicted. On a lower level, it enables to characterize the spatial information in a unified representation and to infer additional information in occluded areas by reasoning with dynamic objects.

I. INTRODUCTION

An autonomous vehicle senses its surrounding environment through its sensors and continuously takes decisions. Level 3 autonomous driving systems are able to handle very structured and clear situations like highways. However, there is still a lack for operating in urban situations where the decision-making has to be performed in much more complex situations. Several processes are involved before taking the final decision. The data provided by perception sensors such as LiDARs or cameras is processed by perception algorithms in order to supply information about the surroundings. Then, the situation must be understood so that the decision-making can plan the maneuver to adopt. The work presented in this article focuses on the lane level situation modeling for autonomous driving. In figure 1, the gray module defines the working area in which this work takes place.

The notions of scene and situation defined in [START_REF] Ulbrich | Defining and Substantiating the Terms Scene, Situation, and Scenario for Automated Driving[END_REF] are bases for describing the driving situation context. The selection process, or "prioritization" as described in [START_REF] Refaat | Agent Prioritization for Autonomous Navigation[END_REF], enables to infer the most relevant information. There is a need to analyze a situation by aggregating all available information from different sources, i.e., perception, map, localization. Situation understanding implies to have a representation which provides non-misleading information for decisionmaking. Further, it also allows to reason by predicting the evolution of the elements of the environment. Depending on the situation, e.g., a roundabout or a pedestrian crossing, the areas of interest where the vehicle has to pay attention are different. These areas must be identified all along the journey.

Sensors Actuators

Perception Decision Making

Context Modeling

Relevant road users that are in interaction or may interact with the vehicle should be identified while others can be ignored.

A list of agents can be supplied by the perception system as in [START_REF] Hubmann | Automated Driving in Uncertain Environments: Planning With Interaction and Uncertain Maneuver Prediction[END_REF], [START_REF] Noh | Decision-Making Framework for Autonomous Driving at Road Intersections: Safeguarding Against Collision, Overly Conservative Behavior, and Violation Vehicles[END_REF] in the form of object maps. However, in case of missed detections, absence of object does not mean that the space is free as there may be a lack of visibility. This can lead the vehicle to an inconsistent situation interpretation. Another approach is to discretize the environment with grids. Each cell can supply information like occupancy which can be used by the decision-making to plan the best trajectory [START_REF] Laugier | Situation Awareness & Decision-making for Autonomous Driving[END_REF]. Evidential grids have the advantage to distinguish a cell that has not been observed, i.e., missing information, from a cell that is conflicting between occupied and non-occupied states [START_REF] Moras | Moving Objects Detection by Conflict Analysis in Evidential Grids[END_REF]. However, classification and prediction of dynamic occupied cells is more complicated, grids are less suited to manage dynamic road users. The current trend is to use a prior map with several layers like a geometric layer, a topological layer and a semantic layer (see [START_REF] Poggenhans | Lanelet2: A high-definition map framework for the future of automated driving[END_REF]) which can be used jointly with the detected objects as in [START_REF] Ulbrich | Graph-based context representation, environment modeling and information aggregation for automated driving[END_REF] or with the free space characterized by an occupancy grid [START_REF] Mouhagir | Using evidential occupancy grid for vehicle trajectory planning under uncertainty with tentacles[END_REF].

The environment representation is therefore a key point to understand the situation and to infer knowledge. Missing information comes from occlusions [START_REF] Sun | Behavior Planning of Autonomous Cars with Social Perception[END_REF], [START_REF] Schorner | Predictive Trajectory Planning in Situations with Hidden Road Users Using Partially Observable Markov Decision Processes[END_REF], [START_REF] Hubmann | A POMDP Maneuver Planner For Occlusions in Urban Scenarios[END_REF], [START_REF] Hoermann | Entering crossroads with blind corners. A safe strategy for autonomous vehicles[END_REF] but also from limited range of the vehicle field of view [START_REF] Orzechowski | Tackling Occlusions & Limited Sensor Range with Set-based Safety Verification[END_REF]. Depending on the situation encountered and areas of interest, this will not have the same impact.

This paper aims at defining a lane level grid representation based on areas of interest. We propose a spatial representation that handles occlusions and infers additional knowledge in hidden areas to facilitate safe decision-making. Indeed, some hidden parts do not present any danger to the egovehicle. We therefore propose a formalism that allows to characterize and manipulate them in real-time. These are our main contributions. Section III and IV depict the selection of these areas and the classification process. Finally, some case studies are presented.

II. RELATED WORK

Decisions operated by the autonomous vehicle are taken at different levels of abstraction. Early with [START_REF] Michon | A Critical View of Driver Behavior Models: What Do We Know, What Should We Do?[END_REF], the notion of abstraction levels is presented and reused in several architectures, especially for the decision-making [START_REF] Ulbrich | Graph-based context representation, environment modeling and information aggregation for automated driving[END_REF]. The Operational Level concerns dense information updated at a high rate and is reactive to events that occur close in time and space, e.g., the local trajectory planned with its related speed profile. The upper level is the tactical level that corresponds to bigger events which happen in a longer time horizon like maneuvers [START_REF] Hubmann | A generic driving strategy for urban environments[END_REF]. The most abstracted layer is the Strategic level. The mission planer plans a global trajectory over a larger horizon of time and space.

Each level of abstraction contributes to the representation of the situation and enables reasoning. In several architectures, all processes that refer to situation understanding or context modeling notions are not defined in a unified point of view. In [START_REF] Ulbrich | Towards a Functional System Architecture for Automated Vehicles[END_REF], the scene modeling and situation modeling are separated whereas in [START_REF] Tas | Functional system architectures towards fully automated driving[END_REF] they rather stand in the perception module. The notion of a separated world model module is used as well [START_REF] Albus | 4D/RCS: A reference model architecture for intelligent unmanned ground vehicles[END_REF], [START_REF] Gheţa | World modeling for autonomous systems[END_REF], [START_REF] Furda | Enabling Safe Autonomous Driving in Real-World City Traffic Using Multiple Criteria Decision Making[END_REF]. The purpose of such a module is to provide a representation of the situation of the vehicle, with prediction and analysis to the decision-making.

The representation of information can be done either in a continuous or discrete manner. In [START_REF] Papp | World modeling for cooperative intelligent vehicles[END_REF], the Local Dynamic Map structures all the objects and features of the world. A map is also used and as in [START_REF] Poggenhans | Lanelet2: A high-definition map framework for the future of automated driving[END_REF], a metrical layer of information is stored. The notion of a Parametric Free Space [START_REF] Schreier | Compact Representation of Dynamic Driving Environments for ADAS by Parametric Free Space and Dynamic Object Maps[END_REF] gives information on the free space and its contour, i.e., obstacle boundary or unknown boundary. When the representation is discrete, the use of grids is popular [START_REF] Mouhagir | Using evidential occupancy grid for vehicle trajectory planning under uncertainty with tentacles[END_REF], [START_REF] Hoermann | Entering crossroads with blind corners. A safe strategy for autonomous vehicles[END_REF].

As presented before, there is a need to combine these representations, as it is presented in [START_REF] Tas | Functional system architectures towards fully automated driving[END_REF], [START_REF] Ulbrich | Graph-based context representation, environment modeling and information aggregation for automated driving[END_REF] architectures. Occlusions in specific situations need to be addressed and may need objects representation. Several decision-making approaches enable to take into account occluded areas. POMDP are used for handling occlusions for static obstacles [START_REF] Bouton | Scalable Decision Making with Sensor Occlusions for Autonomous Driving[END_REF] as well as dynamic ones [START_REF] Hubmann | A POMDP Maneuver Planner For Occlusions in Urban Scenarios[END_REF], [START_REF] Schorner | Predictive Trajectory Planning in Situations with Hidden Road Users Using Partially Observable Markov Decision Processes[END_REF]. The authors in [START_REF] Hoermann | Entering crossroads with blind corners. A safe strategy for autonomous vehicles[END_REF] uses several layers of information: a dynamic grid, an object list and a map of unobservable regions to assess likely collisions. Some articles handle directly polygons of occluded areas [START_REF] Narksri | Crossing Blind Intersections from a Full Stop Using Estimated Visibility of Approaching Vehicles[END_REF], in some cases the centerline is used [START_REF] Hubmann | A POMDP Maneuver Planner For Occlusions in Urban Scenarios[END_REF] and in other cases the segment border is used [START_REF] Orzechowski | Tackling Occlusions & Limited Sensor Range with Set-based Safety Verification[END_REF].

When hidden areas appear, one needs to assess whether all of them are relevant for a safe decision making. The notion of areas of interest will be presented as there is a need to identify where relevant information is missing. As demonstrated in [START_REF] Sun | Behavior Planning of Autonomous Cars with Social Perception[END_REF], the term "Social perception" is defined in order to infer information in occluded zones from the behavior of road users. In specific situations, additional information is inferred. A method patented [START_REF] Burca | Determining visibility distances based on a dynamic field of view of a vehicle[END_REF] shows the mechanism to determine a visibility distance by intersecting the path of a vehicle from a map with the dynamic field of view. This example shows a characterization of the following path of the vehicle by two states: the free space that is of interest and the hidden one. The Responsibility Sensitive Safety (RSS) model presented in [START_REF] Shalev-Shwartz | On a Formal Model of Safe and Scalable Self-driving Cars[END_REF], also highlights the spatial safety requirement with objects representation. The concept of a safety distance is introduced for several situations.

Several works need a spatial representation handling occlusions and objects for tasks like decision-making [START_REF] Schorner | Predictive Trajectory Planning in Situations with Hidden Road Users Using Partially Observable Markov Decision Processes[END_REF], risk analysis [START_REF] Yu | Occlusion-Aware Risk Assessment for Autonomous Driving in Urban Environments[END_REF] or safety evaluation [START_REF] Hoermann | Entering crossroads with blind corners. A safe strategy for autonomous vehicles[END_REF]. In [START_REF] Orzechowski | Tackling Occlusions & Limited Sensor Range with Set-based Safety Verification[END_REF], reachable sets are used for predicting the path and intention of a potential vehicle that could be hidden in an occluded area. Particle filters are also used in order to determine the plausibility of a hidden vehicle [START_REF] Narksri | Crossing Blind Intersections from a Full Stop Using Estimated Visibility of Approaching Vehicles[END_REF]. Then, as presented in [START_REF] Hubmann | A generic driving strategy for urban environments[END_REF], a global method for motion planning could use non-misleading information provided by such a context modeling module.

In this paper, the different levels of abstraction of the information allows to identify the relevant parts of the situation that the autonomous vehicle encounters. Then an analysis of the areas of interest is presented. Additional information that can be inferred from occluded zones and from objects are integrated in a unified representation.

III. INTERACTING LANE GRID

The three levels of abstraction provide a hierarchy of levels in which several processes take place. Each level has a specific type of task to achieve in a world model module.

A. Strategic level of abstraction (high level)

This level has the duty to compute and infer high-level information. At this level the mission planning process provides the intended path of the autonomous vehicle on a topological representation of the map. From this intended path, one can define all the driving lanes that may interact with the car during its cruise. Two types of interacting lanes are differentiated. The primary order lanes are directly in interaction, i.e., shares a common driving space, with the ego vehicle intended path. These lanes are the ones for which the vehicle needs to have information, typically whether they are occupied by another road user, for decision-making. The second order encompasses lanes that have an indirect interaction with the autonomous vehicle, in the sense that they have a direct interaction (i.e., primary order) with the primary order lanes of the ego vehicle itself. The vehicle does not necessarily need information about the second order lanes to drive, but they are able to provide additional information about the primary ones.

In this paper, we consider fours types of interactions that widely cover common driving situations: lane keeping, changing, merging and crossing interactions. In table I the interacting paths are defined based on the topological layer of the road. In lane keeping, the followed lane belongs to the primary order area of interest. The changing lanes belong to the secondary order, as they are the primary order lanes of the lane changing interaction type.

A similar approach can also be considered towards vulnerable road users. A map with the cycle paths, sidewalks or TABLE I: Lanes of interest (road users specific) represented in gray are based on the longitudinal importance order of the lateral interaction type. The autonomous vehicle planned path is represented in blue. For visibility issues the secondary order shows the lanes of interest corresponding to one lane of the primary order (surrounded in black)

Interaction type vs

Lane keeping Lane Changing Lane Merging Lane Crossing Lanes importance Primary order Secondary order pedestrian crossing would enable to define additional types of interaction. These cases are out of the scope of this work.

B. Tactical level of abstraction (intermediate level)

At the tactical level, the autonomous vehicle has to convert the topological information provided at the strategic level into a metrical representation. Moreover, the vehicle has to focus on areas of interest that are in its vicinity in terms of time and distance.

Using a metrical HD map, the spatial areas of a lane can be extracted as a 2D polyline, i.e., a sequence of line segments, representing the center of the lane along with a width. The advantage of coding the center of a lane compared to coding only the lane borders, e.g., with Lanelets [START_REF] Poggenhans | Lanelet2: A high-definition map framework for the future of automated driving[END_REF], is that it is straightforward to discretize in the longitudinal direction. By defining a threshold in terms of interacting distance, we prune geometrically the interacting lanes and construct a spatial Area Of Interest (AOI). The AOI is therefore composed of the primary AOI (1) and the secondary one AOI (2) , all the space outside of the AOI = AOI (1) ∪ AOI (2) , denoted as AOI, is considered out of interest as there will be no interaction with the ego vehicle.

The next step is to characterize the AOI using perception information from exteroceptive sensors such as cameras or LiDAR.The sensor setup of the vehicle can be characterized beforehand in order to determine its physical perception range. Prior to the perception itself, it is possible to define the spatial Field Of View (F OV ) of the vehicle. Its complementary F OV , corresponds to regions where the vehicle has no means to perceive anything using its own embedded sensors.

For decision-making at the tactical level, we propose to use a lane level grid representation that discretizes the lanes within AOI along their longitudinal direction.

C. Operational level of abstraction (low level)

The lowest level of abstraction corresponds to a representation at the sensor level. The space characterization is classically done using grids [START_REF] Hoermann | Entering crossroads with blind corners. A safe strategy for autonomous vehicles[END_REF] or parametric free space [START_REF] Schreier | Compact Representation of Dynamic Driving Environments for ADAS by Parametric Free Space and Dynamic Object Maps[END_REF].

The frame of discernment Ω, i.e., the space state, is then decomposed into three categories: free (F ), occupied (O) and unknown (U ).

So far, the frame Ω has been decomposed in three different ways:

Ω = AOI ∪ AOI = F OV ∪ F OV = F ∪ O ∪ U (1)
By crossing these three breakdowns, we obtain the space depicted in figure 2a. One can see that the free and occupied sets are necessarily inside the F OV . Conversely, the unknown set U = F ∪ O has non-empty intersection with the F OV . Therefore, we will distinguish the hidden set H = U ∩ F OV , that represents the space within the F OV of the sensors but cannot be characterized because of occlusions, from the rest of unknown space U ∩ F OV that are out of reach from the sensors range. As we are only interested in the space inside the AOI, the remaining sets of interests becomes:

F * = F ∩ AOI (2) O * = O ∩ AOI (3) H * = H ∩ AOI = U ∩ F OV ∩ AOI (4) U * = U ∩ F OV ∩ AOI (5)
These four sets F , O, H, U ∩ F OV are represented in figure 2a by the colors green, brown, red and gray, respectively. The subsets of interest [ ] * are represented in filled color whereas their complement is hatched.

IV. INTERACTING OBJECTS

Static objects have an impact on the decision-making process. They are typically well represented in occupancy grids. At the opposite, dynamic objects (e.g., other road users) interact with the surrounding environment. More complex information can be inferred from their presence. The detected objects that are outside of the AOI are discarded. On figure 2b, an entity defined as a dynamic object is denoted E and gives information about the occupied O and hidden H space states. Its complementary E is all but dynamic objects.

A. Safety area

When driving, the space in front of the vehicle needs to be obstacle-free for obvious safety reason. This space is defined by a safety distance that is an increasing function of the vehicle velocity. The faster a vehicle is driving the more space needs to be kept as free in front of it. This safety distance encompasses the driver's reaction time, the minimum braking distance and safety regulations.

Therefore, when a vehicle is detected and its speed estimated, a safety area at its front denoted S is defined. In the case where this safety area is visible and characterized as free (as it should be) or occupied (it means that the theoretical safety distance is not respected), then the safety property can be ignored as it is less informative than the free or occupied knowledge. The safety property is therefore only useful to further characterize an unknown area which could be either hidden or out of the F OV . Its corresponding set of interest is:

S * = S ∩ AOI ( 6
)
This set is represented in yellow in figure 2b.

B. Protected area

One important aspect of the interaction between road users is that they impose physical constraints among each other. Once a vehicle occupies a given space of the road, no other road users can cross this space without causing an accident.

In our case study, we propose to use objects belonging to the second order AOI (2) to further characterize the first order AOI (1) . Because the lanes in AOI (2) have a direct interaction, e.g., crossing, with the ones in AOI (1) , a vehicle in AOI (2) may obstruct the circulation in AOI (1) . This will happen when a vehicle belongs to AOI (2) and occupies a spatial space that also belongs to a lane in AOI (1) . All the space behind this vehicle along the lane in AOI (1) is protected in the sense that no other road users can go through the obstacle. This protected area is denoted P .

Like the safety case, the protected property is only relevant within the unknown category and can be modeled by the following set of interest:

P * = P ∩ AOI ( 7
)
This set is represented in purple in figure 2b. Table II depicts only the classification of the AOI. As areas of interest are linked to the ego-vehicle main path, their representation can also be seen as a graph of successive states whose edges are represented by a distance to the egovehicle state. A whole cell can be classified as been occupied regardless of the object size. Indeed, the distance to this occupied state and the area of interest which it belongs will be supplied to the decision-making.

This characterization process brings information and enables to structure the environment understanding. It should be noticed that if additional information characterization can be refined, the model is able to extend the process to integrate potentially new sets of information.

For clarity, in the rest of the paper, everything outside of the AOI (dashed set in figure 2) will be ignored and the notation [ ] * will be omitted.

V. CASE STUDIES

To illustrate the concepts introduced in the previous sections, let us consider the two case studies pictured in figure 3 and figure 4. The ego vehicle is represented by the blue car which follows the dotted intend path. The set of all colored lanes constitutes the AOI as defined in table I, these lanes are discretized in cells along their longitudinal direction. The light blue circle depicts the F OV of the ego vehicle. The orange cars are other road users.

A. T-intersection

The first situation is at a T-intersection as shown in figure 3. The ego-vehicle intends to turn right and has to give way to the vehicles coming from the left merging lane. The two red cars on the adjacent left lane do not interact with the ego vehicle but cross the merging lane and cause occlusions. This example represents a situation where the ego-vehicle should be able to take a decision even under occlusion at a give-way intersection.

In this situation, the ego-vehicle has two primary order interacting lanes: a lane keeping one, i.e., the right-turn lane, and a lane merging one, i.e., the lane coming from the left. The adjacent left lane of the ego vehicle is a secondary order interacting lane as it crosses one of the primary ones. The set of all these lanes are then pruned down to a distance horizon of interest to constitute the AOI. One can note that the second order lane is pruned very early as it becomes out of interest. Within the visible area the free cells (F ) are shown in green. The cells occupied by the red cars are in brown (O). The black cells are unknown (U ) as they are out of the F OV . The red cells are also unknown but, as they are inside the F OV and not observed because of occlusions caused by obstacles, they are categorized as hidden (H). Last, the purple cells highlight the protected (P ) area. Like U and H, P is not observed, but its space is physically obstructed by the presence of other road users. It should be noted that the protected area may reach outside of the F OV .

B. Overtaking

Let us consider a second situation where the ego vehicle intends to overtake a vehicle positioned on its right side as pictured in figure 4. As explained in section IV-A, a safety area is defined in front of every detected object by using its estimated speed. It is assumed here that vehicles constantly keep safe inter-distance with other vehicles so they can safely perform emergency braking to avoid collision in case of dangerous situations. It means that it is likely that the immediate space in front of any vehicle is free. In figure 4a the orange vehicle on the right is stopped. In this case, there is no safety space, therefore the unknown area in front of it is characterized as hidden (red). In figure 4b, the same vehicle is moving, therefore a portion of the space in front of it is safe (orange). Note that for the car on the left side of the ego vehicle, the safe property of the cells in front of it (orange rectangle) is ignored as the cells have already been classified as free, which is more informative.

C. Experimental setup and implementation

The methodology presented in the previous sections has been implemented on a Renault ZOE experimental vehicle of the Heudiasyc laboratory. Experiments have been conducted in the city of Compiègne, France, where an HD map has been constructed. The lanes in the map are represented by their center as a polyline with their nodes being geo-referenced with centimeter accuracy. The borders of each lane are also encoded. The graph structure behind the map is used at both the topological and metrical levels. It enables to compute the ego vehicle intended path as well as all the primary and secondary order interacting lanes. For the computation of the AOI, we set a longitudinal distance of interest of 100m and an arbitrary cell discretization step of 1m.

A NovAtel SPAN-CPT solution, combining an IMU and GNSS with RTK corrections, was used to have an accurate localization of the ego-vehicle. A Velodyne VLP32-C LiDAR was used for the environment perception. This sensor has a 360 • field of view with a theoretical range of 100 meters. A simple geometric ground fitting and clustering algorithm was used to measure the free space and object surrounding the ego vehicle [START_REF] Zermas | Fast segmentation of 3D point clouds: A paradigm on LiDAR data for autonomous vehicle applications[END_REF]. For the computation of the safety space, an emergency braking model with a deceleration of a = -6 m •s -2 is used. The safety distance associated to a vehicle driving at a speed V is computed as d safe = -V 2 /(2a).

The ego-vehicle and paths of interest are extracted recursively from the HD map as they can be preprocessed thanks to the topological layer. Areas of interest are represented by polygons using the center line with its width. The ROS middle-ware was used for the implementation and the Shapely Python library was used for the geometric operations between areas of interest and the free space polygons. The difference characterizes the hidden space. Figure 5 shows the results from real data in a round about scenario.

D. Discussions

Figure 5 shows the result from real data in a roundabout scenario at a given time.

The hypothesis made in our experiments can easily be extended to more complex assumptions. The interaction distance of the AOI can be set dynamically depending on the ego vehicle dynamic state. The AOI cell discretization step can also be set dynamically instead of a constant. The further we are, the more the step increases. The free space can be replaced by a more reliable occupancy grid that integrates information over time and more sophisticated perception algorithms, e.g., deep learning-based, can be used of object detection and tracking. Extension to include vulnerable road users can also be done based on the same principles.

It is important to note that the illustrations shown for the two case studies are instantaneous representation of the world at a given time. For the T-intersection case, the decision of the ego vehicle to enter in the intersection relies on the fact that the protected area remains protected during the whole duration of the insertion maneuver. Our representation space should therefore also be used for prediction purposes where the predicted positions of the tracked objects are used instead.

VI. CONCLUSION

In this work, an intermediate lane level information representation approach has been proposed with an interacting lane grid. Interacting lanes are extracted from the map and define the interactions encountered by the autonomous vehicle. These interacting areas enable the autonomous vehicle to focus on relevant space parts and to look for the corresponding potential interactions. This lane level grid is discretized and each cell is classified from most informative state: free, occupied, protected, safety zone, to less informative: hidden, out of field of view. This process for the cell grid classification has been depicted. We build information on space representation from spatial perception (as grids) and from objects. This information representation is provided for decision-making at the maneuver planner level.

In future work, objects detection, tracking and behavior prediction will be implemented in order to improve cells classification and situation predictions will be explored.

Fig. 1 :

 1 Fig. 1: Place of Context Modeling in a simplified architecture.

Fig. 2 :

 2 Fig. 2: Set diagram of an area of interest. Colors show the characterization of the area of interest from section III-C for figure 2a and from section IV for figure 2b.

Fig. 3 :

 3 Fig. 3: T-intersection. Areas of interest are modeled and characterized: Free (green), Occupied (brown), Out of range (black), Hidden (red), Protected (purple)

Fig. 4 :

 4 Fig. 4: Straight road with only lane change situations. Areas of interest are modeled and characterized : Free (green), Occupied (brown), Hidden (red), Safety zone (orange). On figure 4a, the right side vehicle is stopped . On figure 4b the right side vehicle has a constant speed.

Fig. 5 :

 5 Fig. 5: Rviz display of real data using ROS. The ego-vehicle is displayed in blue. The figure on the top left shows the free-space polygon (Green), operational level. The figure on the top right illustrates the AOI (blue : lane keeping, red: lane merging), tactical level. The figure on the bottom left depicts the path (as a graph) of the ego-vehicle (blue) and its primary order interacting paths (red). The figure on the bottom right shows final representation. In this scenario, only the hidden space is shown characterized (black) thanks to the free-space polygon.

TABLE II :

 II Characterization sets table of areas of interest

			F OV	F OV
		Free	Occupied	Hidden Unknown
	AOI	F

* O * H * S * : Safety U * P * : Protected

This work is carried out within SIVALab, a shared laboratory between Renault and Heudiasyc (UTC/CNRS), and financed by the CNRS.