Britta Bienen

R T Klinkvort

S Fan

J Black

S Bayton

L Thorel

M Blanc

G S P Madabhushi

S Haigh

T Broad

V Zania

A Askarinejad

Q Li

D S Kim

S Park

M Almeida

M F W Barra

L Prendergast

D Kong

B Zhu

Centrifuge benchmark testing of laterally loaded monopiles in sand

Keywords: Centrifuge, Monopiles, Offshore Wind turbines, Sand

The large diameter monopile is a commonly used foundation concept for offshore wind turbines. The advantages of geometrical simplicity and reliable performance make it often the most attractive solution. Despite the concept's high popularity, optimisation of the current design models can still be made. To address fundamental understanding of modelling effects in centrifuge testing of laterally loaded monopiles in sand, a large coordinated centrifuge-testing program across 11 different centrifuge centres worldwide is ongoing. This extended abstract presents the initial results of global benchmark testing.

Monopile foundations are still the preferred foundation solution for offshore wind turbines, due to the simple geometry (a hollow steel pile) and the experience gathered over the years (Wind Europe, 2018). However, despite their wide use and continuous optimisation, there are still areas of uncertainty in monopile design. The continued lack of fundamental understanding of monopile behaviour has been highlighted by a review of the effects influencing the response of a monopile in the centrifuge [START_REF] Klinkvort | A review of modelling effects in centrifuge monopile testing in sand[END_REF].

Based on this review and inspired by the multi-facility research on CPT's presented in [START_REF] Bolton | Centrifuge cone penetration test in sand[END_REF], this extended abstract presents initial results from a multi-facility test program on lateral loaded monopiles in dry sand. The test program is ongoing at different centrifuges with different setups, sizes, model monopiles and sands. Through careful design of the testing program, several of the effects that control the lateral response of a monopile were investigated. The study focuses on the monotonic lateral response of the monopile, but many of the findings are also valid for more complex load situations. All testing was performed in beam centrifuges. The centrifuges have different sizes, actuators and data acquisition techniques, but the testing principles are the same for all of them.

The sands used at the different centrifuge facilities were first tested in the same laboratory (at NGI). All are poorly graded fine silica sands. The grain size distributions are shown together with the average grain size diameter in Figure 1 and is seen to have similar granin size distributions.

The maximum and minimum dry unit weight of sand is known to depend on the testing methodology and it is therefore difficult to define a unique value [START_REF] Lunne | Methods used to determine maximum and minimum dry unit weights of sand: Is there a need fro a new standard? Canadien Geotechnical Journal[END_REF]. However, relative density is commonly used to describe the state of sand and this is the reason for using it here also. Because all sands were tested in the same laboratory the methodology is the same between the tests which enhances the comparison of the achieved relative density.

Benchmark testing in 6 centrifuges is currently finalised, the dimension of the test piles are given in Table 1 and the results pile head response at sand surface are presented in Figure 2. All test was design so penetration depth was L=5D and the lateral load was applied with an eccentricity of e=5D. Figure 2a shows that pile with same diameters gives similar responses in model scale, with the exception of the tests from DTU. Figure 2b shows a good match between the normalised results in the tests from CEIGR, IFFSTAR, COFS, TU Delft and KAIST confirming the modelling and normalisation methodology. After displacements above 0.1D the responses starts to deviate, but here displacements are getting large and is well above the serviceability limits. Explanation of the differences may be related to slightly different soil stress conditions, relative density etc. The response from the tests at DTU shows a stiffer responses. This is most likely related to the test setup and the vertical displacement constrained at pile top. Very large downward axial loads (Vmax>6000N) were measured during the tests, leading to a different load conditions for these tests compared to the other. The results in Figure 2 confirms that the scaling technique used in centrifuge modelling is appropriate if care is taken to scaling and test setup.

  Before the testing in the centrifuge all sands were first tested tested at the Norwegian Geotechnical Institute (NGI). Here the sand were tested to determine Grains size distributions, solid/maximum/minimum density, shear stiffness and traixial stress-strain behaviour. The centrifuge facilities in question are located at the Centre for Engineering Infrastructure Ground Research (CEIGR) at the University of Sheffield, the Centre for Offshore Foundation Systems at the University of Western Australia (COFS, UWA), University of Cambridge Engineering Department (CUED), Technical University of Denmark (DTU), The French Institute of Science and Technology for Transport, Development and Networks (IFSTTAR), Korea Advanced Institute of Science & Technology (KAIST), Technical University of Delft (TU Delft), Federal University of Juiz de Fora (COPPE), University of Nottinham (UN) and Zhejiang University (ZU).

Fig. 1 .

 1 Fig. 1. Grains size distribution of all tested sands

Fig. 2 .

 2 Fig. 2. Total pile head response of the test piles scaled to a references stress similar to a pile diameter of Dproto=2m

Table 1 .

 1 Test pile geometry

	Centrifuge	D (mm)	T (mm)
	CEIGR	50	3.2
	COFS	52.2	2.1
	DTU	40	2
	IFSTTAR	40	2
	TU DELFT	18.2	1.2
	KAIST	80	1.5


Marcos Dracos
email: marcos.dracos@in2p3.fr

Short and Medium baseline Reactor Neutrino Experiments

Keywords: neutrino oscillations, mass hierarchy, reactor neutrinos, short baseline, medium baseline

published or not. The documents may come

Introduction

Nuclear reactors are copious neutrino sources used to study the neutrino properties. The neutrino detection method used since the first observation of neutrinos by F. Reines and C. Cowan in 1956 remains the same up to today. This method uses the inverse beta decay (IBD) of an antineutrino interacting with a proton of the detector target producing a positron annihilating with an electron (prompt signal) and a neutron captured after (delayed signal).

Up to 2012, reactor neutrino experiments were using only one "far" detector to extract their results. The unoscillated neutrino spectrum was extracted using the reactor predictions taking into account the reactor operation conditions. In order to reduce the systematic errors, new generation experiments as Double Chooz, Daya Bay and RENO, used near detectors to monitor the reactor neutrino flux. This allowed to measure for the first time the last neutrino oscillation mixing angle θ 13 [START_REF] Abe | Indication for the disappearance of reactor electron antineutrinos in the Double Chooz experiment[END_REF][START_REF] An | Observation of electron-antineutrino disappearance at Daya Bay[END_REF][START_REF] Ahn | Observation of Reactor Electron Antineutrino Disappearance in the RENO Experiment[END_REF] . The measurement accuracy achieved up to now by these experiments allow to perform high precision measurements.

Short baseline neutrino experiments

By 2011, three short baseline reactor neutrino experiments started taking data with the main objective to measure the mixing angle θ 13 or at least set a severe limit on this parameter. These experiments were Double Chooz in France 1 , Daya Bay 2 in China and RENO 3 in South Korea. The configuration of detectors and reactors for the three projects is depicted by Fig. 1. Double Chooz disposes of two detectors and two reactors, Daya Bay of three reactors and three detector sides, and RENO of six reactors and two detectors clearly identified as near and far detectors. Table 1 gives the present reactor power, the detector volume, overburden and starting date of the experiments. The detectors of all three experiments are similar. They mainly include the liquid scintillator target doped with Gadolinium to reduce the neutron lifetime to about 30 µs, a gamma catcher filled with liquid scintillator without Gadolinium to detect escaping gammas from the neutrino target, a buffer protecting from the PMT radioactivity and a veto for cosmic ray rejection. For each experiment, the detectors on all locations are identical in order to reduce the systematic errors. Fig. 2 presents the far Double Chooz detector.

Double Chooz

θ 13 measurement

In 2011 all three experiments, Double Chooz, Daya Bay and RENO, have started taking data. Double Chooz has started with only the far detector (the near detector was only ready beginning of 2015), while Daya Bay was disposing of six detectors over 8. RENO had started with a complete configuration. End of 2011, Double Chooz has presented the first results 1 with a significance of excluding a zero θ 13 value of 1.7 σ. It has been followed by Daya Bay (March 2012) [START_REF] An | Observation of electron-antineutrino disappearance at Daya Bay[END_REF] and RENO (April 2012) 3 reporting a non-zero significance of 5.2 σ and 4.9 σ, respectively. Fig. 3 shows the neutrino energy spectrum of the three experiments at that time. The latest results for the three experiments for sin 2 2θ 13 give:

• Double Chooz: 0.119 ± 0.016 (stat. + syst.) (using data collected up to 2016) 4 • Daya Bay: 0.0841 ± 0.0027 (stat.) ± 0.0019 (syst.) (using data collected up to July 2015) 5 • RENO: 0.086 ± 0.006 (stat.) ± 0.005 (syst.) (using data collected up to September 2015) [START_REF] Seo | New Results from RENO using 1500 Days of Data[END_REF] As it can be seen, Daya Bay already reached a 3.9% accuracy on θ 13 while RENO and Double Chooz remain at 9.1% and 13.4%, respectively. Fig. 4 shows the dependence of the ν disappearance probability on L/E for Daya Bay and RENO. The observed shape is as expected by the neutrino oscillation formalism. By fitting this distribution the parameter ∆m 2 ee = cos 2 θ 12 |∆m 2 21 |+sin 2 θ 12 |∆m 2 32 | can directly be extracted from the probability distribution:

P (ν e → νe ) ≈ 1 -cos 4 θ 13 sin 2 θ 12 sin 2 ∆m 2 21 L 4E ν small for L=few km -sin 2 2θ 13 sin 2 ∆m 2 ee L 4E ν
The extracted values for ∆m 2 ee are 5,6 :

• Daya Bay: [2.50 ± 0.06 (stat.) ± 0.06 (syst.)] × 10 -3 eV 2

• RENO: [2.61 ± +0.06 -0.16 (stat.) ± +0.09 -0.09 (syst.)] × 10 -3 eV 2

Using the Daya Bay value one can get: Daya Bay, the remaining uncertainty on energy reconstruction is kept at the level of 1%. For this experiment, the energy resolution is given by:

σ E E rec = 0.016 2 + 0.081 2 E rec + 0.026 2 E 2 rec
giving an energy resolution of 8.7% at 1 MeV.

Reactor Neutrino Anomalies

With the achieved energy resolution Double Chooz, Daya Bay and RENO are able to better measure the reactor neutrino energy spectrum. All three experiments reported an excess around 5 MeV compared to predictions [START_REF] Seo | New Results from RENO using 1500 Days of Data[END_REF][START_REF] An | Improved Measurement of the Reactor Antineutrino Flux and Spectrum at Daya Bay[END_REF][START_REF] Crespo-Anadn | Double Chooz: Latest results[END_REF] . Fig. 6 presents the neutrino energy distribution compared to predictions for the three experiments. The excess around 5 MeV depends on the reactor thermal power excluding an external origine. Some possible explanations already have been presented in several publications (see e.g. 9 ) based on β-decay branch contribution (thousands of individual beta-decay branches) of the main isotopes contributing to the neutrino production. This bump could also be explained by considering residual non-linearities of the experiments [START_REF] Mention | Re-actor antineutrino shoulder explained by energy scale nonlinearities?[END_REF] . Further investigations are ongoing but this shows the fragility of the predicted spectrum. On reactor antineutrino anomaly, the latest results from Daya Bay 7 and RENO [START_REF] Seo | Short-baseline reactor neutrino oscillations[END_REF] give for the antineutrino flux compared to the Huber 13 /Mueller 12 model (Fig. 7):

• Daya Bay: R = 0.946 ± 0.020 • RENO: R = 0.946 ± 0.021 while the world average including Daya Bay 7 results becomes R = 0.9430 ± 0.008(exp.) ± 0.023(model). The deficit of observed reactor neutrino flux relative to Daya Bay has studied the reactor neutrino flux and spectrum evolution with the reactor fuel isotope composition 14 using 2.2 million IBDs detected by four of its detectors. Fig. 8 shows the relative IDB yield per fission as a function of 239 Pu and 235 U fractions, for four neutrino energy ranges. A clear energy-dependent evolution is observed that induce a change in spectral shape as fuel composition evolves.

Fig. 9 presents the neutrino cross-section/fission as a function of the effective fission fraction of 239 Pu and 235 U, compared to the Huber-Mueller model prediction (rescaled by 5.1% just for better slope comparison). The hypothesis of a constant antineutrino flux as a function of the 239 Pu fission fraction is rejected with a signif-icance corresponding to 10 σ. The measured evolution of the reactor neutrino flux is by 3.1 σ incompatible with the predictions. This indicates that the uncertainties of model calculations are underestimated. For this study, only the four primary fission isotopes, 235 U, 238 U 239 Pu and 241 Pu, are taken into account representing more than 99.7% of the total fissions. Fig. 10 presents the neutrino IBD yield per fission of 239 Pu versus the one of 235 U. It is clearly seen that the disagreement with the model from the 235 U evolution. A possible explanation, with 2.8 σ significance, is that there is an overestimation of the antineutrino flux from 235 U in reactor models, instead of an anomaly produced by sterile neutrinos. Tentatives by independent analyses to combine these new Daya Bay data with all previous data coming from many other experiments have the tendency to decrease the importance of 235 U isotope effect [START_REF] Giunti | Reactor Fuel Fraction Information on the Antineutrino Anomaly[END_REF] . However, this new Daya Bay observation constitutes an unaccounted systematic error by the reactor models.

Daya Bay doesn't report indications that the 5 MeV "bump" could come from a particular isotope. Improvements in systematic uncertainties and increasing statistics could help to better understand this particular problem.

Using the spectrum of L/E of Fig. 4 and the probability of having a 4th sterile neutrino family: P (ν e → νe ) ≈ 1 -cos 4 θ 14 sin 2 θ 13 sin 2 ∆m 2 ee L 4E ν -sin 2 2θ 14 sin 2 ∆m 2 41 L 4E ν exclusion limits can be established. Fig. 11 shows the exclusion limits obtained by RENO experiment [START_REF] Seo | Short-baseline reactor neutrino oscillations[END_REF] and the one obtained combining Daya Bay 16 results with those of MINOS and Bugey-3.

Plans and Prospects

Double Chooz will continue data taking up to beginning of 2018 after which the detector decommissioning will start. The experiment will profite of this phase to well measure the number of protons in each part of its detectors in order to decrease the systematic errors. RENO will take data up to end of 2018 with a possible extension up to 2021. The goal will be to reach a precision of 6% on θ 13 . Daya Bay plans to take data up to 2020. The goal is to reach a 3% systematic error on θ 13 by better understanding of the systematic errors (non-linearities, calibration,...) and better background reduction. Fig. 12 17 summarises the uncertainty evolution on θ 13 and |∆ 2 ee |.

Fig. 12. Daya Bay uncertainty evolution.

Medium baseline experiments

Within the context of relatively large θ 13 , medium baseline reactor experiments could determine the neutrino mass hierarchy, provided that they will have a sufficient (and challenging) energy resolution [START_REF] Petcov | The LMA MSW solution of the solar neutrino problem, inverted neutrino mass hierarchy and reactor neutrino experiments[END_REF][START_REF] Choubey | Precision neutrino oscillation physics with an intermediate baseline reactor neutrino experiment[END_REF][START_REF] Learned | Determination of neutrino mass hierarchy and theta(13) with a remote detector of reactor antineutrinos[END_REF][START_REF] Zhan | Determination of the Neutrino Mass Hierarchy at an Intermediate Baseline[END_REF][START_REF] Zhan | Experimental Requirements to Determine the Neutrino Mass Hierarchy Using Reactor Neutrinos[END_REF] . Indeed, the survival probability of reactor neutrinos can be written as: The ± at the fourth term is for Normal and Inverted hierarchy. It has to be noted that this probability doesn't depend on the CP violation parameter δ CP , also the matter effect for the considered baselines is negligible. The experiments must be placed at a distance to maximise the contribution of this last term in order to be significantly sensitive to the neutrino mass hierarchy. It comes out that this distance must be around 50 km. Fig. 13 presents the oscillated neutrino energy spectrum for the two hierarchies compared to the unoscillated one. It can be clearly seen that a very good energy resolution is needed to discriminate the two cases.

P (ν e → νe ) ≈ 1 -
Two projects propose to exploit this effect, JUNO [START_REF] An | Neutrino Physics with JUNO[END_REF] in China and RENO50 24 in South Korea. Both experiments proposes to use a large (∼20 kt) liquid scintillator detectors. Thanks to the size of these detectors, other physics subjects can be treated together with the reactor neutrino program, as the detection of supernova explosions and relic neutrinos, solar and atmospheric neutrinos, geoneutrinos, proton lifetime and exotic searches.

JUNO

The JUNO detector will be located in the South of China near Jiangmen at a distance of 53 km from the nuclear power plants of Yangjiang and Taishan. The thermal power of these two nuclear power plants is expected to be between 26 GWth and 35 GWth by the beginning of next decade. JUNO plans to collect 100 kIBDs over 6 years to determine the neutrino mass hierarchy. Fig. 14 illustrates the observed neutrino spectrum which could be observed by JUNO together with the main background contributions. Thanks to the ability to observe the "solar" and "atmospheric" oscillation contributions in the same de-tector, JUNO will be able to precisely measure the oscillation parameters ∆m [START_REF] An | Observation of electron-antineutrino disappearance at Daya Bay[END_REF] 21 , ∆m [START_REF] An | Observation of electron-antineutrino disappearance at Daya Bay[END_REF] ee and sin 2 θ 12 . Table 2 presents the JUNO performance for these parameters compared to the current accuracy. A sub-percent precision on all three parameters is expected. For the mass hierarchy determination, JUNO has to reach an energy resolution of at leat 3% at 1 MeV. For this, JUNO detector must be able to detect more than 1200 photoelectrons par deposited MeV. To achieve this, JUNO, compared to previous liquid scintillator detectors, plans to optimise the liquid scintillator quality and composition [START_REF] Djurcic | JUNO Conceptual Design Report[END_REF] . The photocathode coverage will also considerably increase to reach a value of nearly 80%. The two last points to be improved is the photomultiplier quantum efficiency and the calibration of the detector.

The JUNO detector

The JUNO central detector will be spherical compared to the cylindrical shape of SuperKamiokande. A schematic view of the JUNO detector is depicted by Fig. 15.

The neutrino interactions will occur inside an acrylic sphere (12 cm thick) immersed in water (35 kt) and containing the liquid scintillator and having a diameter of 35.4 m. Outside this sphere a mechanical structure will support all 20" PMTs (about 17000). In between the 20" PMTs small PMTs (3") will be placed to mainly increase the energy dynamic range of the whole system. All the surrounding water will be used as Water Cherenkov veto detector with about 2000 20" PMTs. In order to compensate the earth magnetic field significantly degrading the PMT detection performance, two magnetic coils, a vertical and horizontal ones, will be placed inside the veto detector. On top of the central detector, a Top Tracker will be placed in order to well study the cosmogenic background. This Top Tracker based on plastic scintillator strips consists of a recycling of the OPERA Target Tracker 26 . All parts of the detector are in an R&D period or in a construction phase. The 20" PMTs will be provided by NNVT (15000) and Hamamatsu (5000) companies. The HZC company will provide all the 25000 3" PMTs. The delivery and tests of the 20" PMTs have already started. A distillation plant has been installed in the Daya Bay site for tests in order to find the best way to produce the JUNO liquid scintillation. Very promising results have already been obtained up to now with an attenuation length higher than 22 m (20 m required).

Taking as example Daya Bay, JUNO has elaborated a very extensive calibration program to well calibrate all the detector positions at various energies. This includes displacement of radioactive sources with ultrasonic positioning system for energy calibration, and fast 1 ns laser system for timing calibration.

The excavations of the underground laboratory to host the JUNO detector have started beginning of 2015. A 1300 m length slope tunnel to reach the detector (overburden of 700 m of rock) and a vertical shaft are already excavated. The civil engineering will finish by beginning of 2019 when the installation of the detector will start. The data taking period is supposed to start beginning of next decade.

Mass hierarchy performance

The JUNO performance to solve the neutrino mass hierarchy problem using the above described detector is shown by Fig. 16. This Fig. shows the χ 2 difference between the two hierarchies versus the parameter |∆ 2 ee | in case of normal hierarchy (similar performance is obtained in case of inverted hierarchy). This ∆χ 2 is of the order of 11 (σ ∼ 3.3) in real conditions and could go up to 16 (σ ∼ 4) in case the dispersion of the reactors in both nuclear plants is ignored.

RENO50

RENO50 has a similar physics program and proposal than JUNO with a liquid scintillator cylindrical detector (18 kt) instead of a spherical one. This project proposes to use the same nuclear plants than those used by the RENO experiment with the detector placed at a distance of 50 km away. An extensive R&D period took place up to summer 2017 in many directions as in liquid scintillator purification and reduction of radioactivity. Unfortunately, efforts to obtain a full construction fund failed. For this reason, the Collaboration decided to stop the project.

Conclusions

Short baseline reactor experiments are greatly contributing in the understanding of neutrino oscillations and in measuring the oscillation parameters. They allowed in 2012 to measure the last unmeasured oscillation angle θ 13 and thus opened the way to observe a possible CP violation in the neutrino sector and determine the neutrino mass hierarchy as well. These experiments also contribute to better understand the so called neutrino anomalies and are setting limits on sterile neutrino existence.

Thanks to the relatively large θ 13 value, medium baseline reactor experiments, as JUNO, could determine the neutrino mass hierarchy provided that they will have a very good energy resolution better than 3% at 1 MeV. These experiments can also mesure precisely three of the oscillation parameters allowing to start unitarity tests of the PMNS mixing matrix. The very large volume (∼20 kt) of these experiments will allow them to also have a very rich astroparticle physics program.

Fig. 1 .

 1 Fig. 1. Reactor and detector configuration for Double Chooz, Daya Bay and RENO experiments.

  .2 m) • acrylic vessel (8 mm) • 8.3 tons Gd-scintillator Gamma Catcher (e=0.55 m) • scintillator Buffer (e=1.05 m) • steel (3 mm) • 80 tons "oil" • 390 PMTs (10")

Fig. 2 .

 2 Fig. 2. The Far Double Chooz detector.

Fig. 3 .

 3 Fig. 3. Observed energy spectrum of the prompt signal for Double Chooz, Daya Bay and RENO experiments as published in 2012.

Fig. 4 .

 4 Fig. 4. Reactor antineutrino survival probability as a function of L ef f /Eν for Daya Bay and RENO experiments.

Fig. 5

 5 Fig. 5 presents sin 2 2θ 13 versus ∆m 2 ee for Daya Bay 5 and RENO 6 experiments. All these precision results have been obtained using an extensive calibration program mainly based on radioactive sources deployed inside the detectors. A better background reduction and understanding of the systematic errors also helped. For

Fig. 6 .Fig. 7 .

 67 Fig. 6. Measured neutrino energy spectrum compared to predictions for RENO, Daya Bay and Double Chooz.

Fig. 8 .

 8 Fig. 8. Relative neutrino flux measurements versus the 235 U and 239 Pu fission fractions for four energy ranges.

Fig. 9 .

 9 Fig. 9. Measured IBD yield versus the 235 U and 239 Pu fission fractions compared to predictions.

Fig. 10 .

 10 Fig. 10. IBD yield/fission for 235 U and 239 Pu compared to predictions.

3 Fig. 11 .

 311 Fig. 11. Exclusion plots for sterile neutrino searches.

Fig. 13 .

 13 Fig.13. Neutrino energy spectrum for both heirarchies compared to the unoscillated spectrum.

Fig. 14 .

 14 Fig. 14. Neutrino energy spectrum with main background contributions.

Fig. 15 .

 15 Fig. 15. Schematic view of the JUNO detector.

Fig. 16 .

 16 Fig. 16. Mass hierarchy JUNO performance.

Table 1 .

 1 Reactor power, detector volume, overburden and starting date of the experiment.

	Experiment	Reactor power Volume	overburden	start of data taking
		(GWth)	(tons)	(near/far-mwe)
	Double Chooz	8.5	2x6	80/300	April 2011
	Daya Bay	17.4	8x20	270/950	August 2011
	RENO	16.8	2xyy	90/440	August 2011

  cos 4 θ 13 sin 2 2θ 12 sin 2 ∆m 2 21 L 4E ν -sin 2 2θ 13 sin 2 ∆m 2 31 L 4E ν -sin 2 θ 12 sin 2 2θ 13 sin 2 ∆m 2

				21 L 4E ν	cos	2 ∆m 2 31 L 4E ν
	±	sin 2 θ 12 2	sin 2 2θ 13 sin	2∆m 2 21 L 4E ν	sin	2 ∆m 2 31 L 4E ν

Table 2 .

 2 Precision measurements with the JUNO detector.

		current precision JUNO precision
	∆m 2 21 ∆m 2 ee sin 2 θ 12	2.3% 4% 6%	0.6% 0.5% 0.7%