Raoudha Gaha
email: raoudha.gaha@gmail.com

Abdelmajid Benamara
email: abdel.benamara@enim.rnu.tn

Bernard Yannou
email: bernard.yannou@ecp.fr

An Environmental Impact/Cost model for evaluating Multiple Feature-Based Machining Methods

Keywords: Environmental evaluation, cost estimation, features technology, detail design, CAD, LCA

For a designed part, if different sets of features are used for machining, the manufacturing Environmental impacts (EIs)as well as cost may vary. In this research, a new methodology based on feature technology is proposed in order to help designer till detail design phase to choose the most optimal compromise Environmental Impact/Cost in manufacturing process. Hence an Environmental impact/Cost model is presented. This model follow the activities required for machining a set of features which are tool set-ups, fixturing set-ups and machining tool paths. The paper starts by presenting features based models which are the base of the methodology proposed. Then, a state of the art about using Feature technology in reducing on one side manufacturing cost and on the other side manufacturing environmental impacts is presented. Finally an environmental impact/cost methodology for evaluating multiple feature-base machining methods is proposed.

Introduction

Today, energy, resources and environmental problems become more and more serious. That's why they are highlighted in regulations frameworks such as WEEE (European Union,, 2003a), RoHS (European Union,, 2003b), EuP (European Union,, 2005) directives and the REACH regulation (European Union,, 2006), which are pushing industrials to adopt sustainable strategies to develop their products.

The eco-design approach, presents, firstly, as a multi-criteria approach taking into account all environmental impacts and secondly, it is also an approach multistep that takes into account all stages of the life cycle. Hence it is largely studied, this last decade, in order to be used in industries. The most studies realized tried to integrate environment constraint in the early design stages such as conceptual design phase which decides about 70%-80% property of the product [START_REF] Song | Expression of scheme case and scheme generation in product's conceptual design[END_REF]. However, choices done in detailed design phase are responsible on about 80% of both environmental impacts and costs. How to eco-design in CAD phase with a low cost, if it is possible, is the key for a successful green and less expensive design. After a state of the art on works based on FT to reduce manufacturing cost or EI or both and based on the work of [START_REF] Tseng | Evaluating multiple feature-based machining methods using an activity-based cost analysis model[END_REF], firstly, in this paper, we put forward the concept of "eco-feature", and then it is used for environmental and cost evaluation in order to choose the most acceptable compromise Environmental impacts/cost. Secondly, a methodology is proposed based on the eco-feature concept. Finally the application of the proposed methodology on a case study is realized.

Features based models

A feature is a local shape of a product directly related to the manufacturing process. [START_REF] Smith | Manufacturing feature identification for intelligent design -Intelligent Systems in Design and Manufacturing[END_REF] define features as high-level design primitives with their attributes, their qualifiers and restrictions which affect the functionality and/or the ability for the product to be manufactured.

There are different types of features : Shape Features, Constraint Features, Precision Features (or tolerance), Assembly Features, Functional Features, Material Features and Primitive Features.

In combination with surface representation, geometric representation of features shape is oriented to volume representation (3D) such as the representation based on B-REP (Boundary Representation), on CSG (Constructive Solid Geometry), as well as hybrid representation as shown in [START_REF] Wang | A scheme to represent features, dimensions, and tolerances in geometric modeling[END_REF].

Techniques developed to create models based on features are divided into two main approaches according to [START_REF] Allada | Feature-based modelling approaches for integrated manufacturing: state-of-theart survey and future research directions[END_REF] and [START_REF] Shah | Parametric and Feature Based CAD/Cam: Concepts, Techniques, and Applications[END_REF]:

 Features recognition: features are recognized from a geometric model applying rules of recognition. More features models, describing the same product, can therefore be derived.

 Design features: a product model is developed based on predefined features.

In this case, a product's explicit geometry is created from the features model. To evaluate a part environmentally, non-geometric data has to be attached to the geometric model for eco-designing products in the detail design phase.

State of the art

Current product development activities in manufacturing companies are still predominantly driven by quality/cost concerns. This does not prevent the existence of some works coupling cost and sustainability. Feature technology (FT) is used in cost estimation of a manufacturing process plan such as cost estimation models of product based on CAD/CAPP integration based on features-based modeling presented in [START_REF] Tseng | Evaluating multiple feature-based machining methods using an activity-based cost analysis model[END_REF], (Shuangxia et al., 2002) (Jinks et al., 2010). In the side of sustainability FT is used to evaluate environmental impacts of manufacturing processes, it is oriented to reducing or avoiding processes which have an impact on ecosystems, human health, etc. [START_REF] Sheng | Multi-objective process planning in environmentally conscious manufacturing: a feature-based approach[END_REF] developed a hierarchical part planning strategy for environmentally conscious machining in order to choose the least impact set of machining sequences order because a feature can be machined with different scenarios. Features are not only used for EE but also for green process planning selection, such as methods developed by [START_REF] Cao | Study on model set based process planning strategy for green manufacturing[END_REF] and [START_REF] Tan | Improved methods for process routing in enterprise production processes in terms of sustainable development II[END_REF] for green CAPP selection have to be developed in order to be used as numerical methods integrated in CAD systems and also a method for environmentally conscious process planning proposed by [START_REF] Zhao | Toward the development of process plans with reduced environmental impacts[END_REF]. Finally FT is also explored in the side of environmental evaluation and cost. In fact, we can find works integrating Life cycle assessment (LCA) to life cycle costing (LCC) such as the novel redesign approach propose by [START_REF] Bovea | Redesign methodology for developing environmentally conscious Products[END_REF] that integrates Quality Function Deployment (QFD) , LCA, life cycle cost (LCC), and contingent valuation (CV). [START_REF] Bevilacqua | Development of a sustainable product lifecycle in manufacturing firms: a case study[END_REF] also evaluated the environmental impact of circuit board design based on both LCA technique and economic aspects, [START_REF] Kloepffer | State-of-the-Art in Life Cycle Sustainability Assessment (LCSA) Life cycle sustainability assessment of products[END_REF] proposed a new direction which summarizes environmental LCA, LCC, and Social LCA and the eco-design tool prototype "CAST-tool" developed by Morbidoni et al. (2011). However many disadvantages are detected as high dependence on data accuracy, less intuition of evaluation result, lack of special software and databases and long evaluation cycle, etc. [START_REF] Xin | Green feature modeling for mechanical and electrical product conceptual design[END_REF]. Hence, it is necessary to consider environmental and cost constraints of a manufacturing process till feature attribution in detail design phase.

Proposed methodology

Our methodology consists in exploring features technology to evaluate the environmental impact (EI) as well as cost of a part manufacturing process till the detail design phase. In fact the influence of geometrical characteristics of a product on reducing the EI was proved on previous works (Gaha et al., 2011) [START_REF] Gaha | Eco-design of a basin mixer in geometric modeling phase[END_REF]. For the cost reducing it is proved by [START_REF] Tseng | Evaluating multiple feature-based machining methods using an activity-based cost analysis model[END_REF].

According to [START_REF] Tseng | Evaluating multiple feature-based machining methods using an activity-based cost analysis model[END_REF] there are three activities in manufacturing process that are related to manufacturing costs and can be evaluated:

1. Tool set-ups.

2. Fixturing set-ups.

3. Machining tool paths.

We adopt these activities which can be also evaluated in terms of environmental impacts. Evaluating manufacturing costs and environmental impacts for Multiple Sets of features is a way to localize, the set of features that can be machined with the lowest manufacturing compromise cost Cmin /Environmental impact EImin which is considered a "good" set of features. So it gives both indications about the cost and about the environmental impact. If we are more interested in eco-design, the best environmental profiles set features is then selected if they are superimposed and we cannot decide then the least expensive set of features is selected

The feature set that can be machined with the lowest activity-based cost/Environmental impact is considered the most suitable method for producing the part.

Conclusion

This paper is developed base on the work of Tseng and Jiang. It presents and discusses the need for integrating cost and environmental performances into manufacturing process based on feature technology in order to optimize the manufacturing part process in terms of cost as well as environmental impact till detail design phase. In fact, for a given design, different machining scenarios are possible based on different sets of features. Hence manufacturing costs and environmental impacts may vary. A methodology for analyzing different manufacturing costs and environmental impacts for machining using different sets of features were proposed using an activity-based cost/environmental impact model. The activities used in cost and environmental impact analysis were tool set-ups, fixturing set-ups and machining tool paths.

Fig. 1

 1 Fig. 1 Proposed methodology based on features technology for cost estimation and environmental evaluations of possible process plans

  Find the smallest in Ci as Cmin = Min {Ci, i = 1, %, n} Find the smallest in EIi as EImin = Min {EIi, i = 1, %, n} 6. CFmin is the feature set with the lowest manufacturing cost and EIFmin is the feature set with the lowest manufacturing environmental impact.

	Given feature set Fi, i=1, %, n, performs the following steps:
	1. Calculate tool set-up cost CFSi and environmental impact EIFSi.
	2. Calculate fixturing set-up cost CFXi and environmental impact EIFXi .
	3. Calculate cost of machining tool paths CFTi and environmental impact
	EIFTi.
	4. The manufacturing cost and Environmental impacts for machining feature
	set Fi are:
	Ci = CFSi + CFXi + CFTi.
	EI i = EIFSi + EIFXi + EIFTi.
	5.


L Buiron

Low Void Effect (CFV) Core Concept Flexibility: from Self-breeder to Burner Core

In the frame of the French strategy on sustainable nuclear energy, several scenarios consider fuel cycle transition toward a plutonium multirecycling strategy in Sodium cooled Fast Reactor (SFR). Basically, most of these scenarios consider the deployment of a 60 GWe SFR fleet in two steps to renew the French PWR fleet. As scenarios do investigate long term deployment configurations, some of them require tools for nuclear phase-out studies. Instead of designing new reactors, the adopted strategy does focus on adaptation of existing ones into burner configurations. This is what was done in the frame of the EFR project at the end of the 90's using the CAPRA approach (French acronym for Enhance Plutonium Consumption in Fast Reactor). The EFR burner configuration was obtained by inserting neutronic penalties inside the core (absorber material and/or diluent subassembly). Starting from the preliminary industrial image of a SFR 3600MWth core based on Low Sodium Void concept (CFV in French), a "CAPRA-like" approach has been studied. As the CFV self-breeding is ensured by fertile blankets, a first modification consisted in the substitution of the corresponding depleted uranium by "inert" or absorber material leading to a "natural burner" core with only small impact on flux distribution. The next step toward CAPRA configuration was the substitution of 1/3 of the fuel pins by "dummy" pins (MgO pellets). The small spectrum shift due to MgO material insertion leads to an increase Doppler constant which exceeds the value of the reference case. As the core sodium void worth value is conserved, the CFV CAPRA core "safety" potential is quite similar to the one of the reference core. Fuel thermo-mechanical requirements are met by both nominal core power and fuel time residence reduction. However, these reduction factors are lower than those obtained for EFR core. The management of the enhanced reactivity swing is discussed.

I. INTRODUCTION

For the next generation fast reactor design, the Generation IV International Forum (GIF) defined global objectives in terms of safety improvement, sustainability, waste minimization and non-proliferation.. Among the possibilities studied at CEA, Sodium cooled Fast Reactor (SFR) are studied as potential industrial tools for next decade's deployment. Many efforts have been made in the last years to obtain advanced industrial core designs that comply with both sustainability and safety concerns [START_REF] Buiron | Innovative cored design for Generation IV sodium cooled fast reactors[END_REF][START_REF] Sciora | Low Void Effect Core Design Applied ton 2400 MWth SFR[END_REF] .

Large power core designs studies focused on Low Sodium Void Core concept (CFV acronym in French) for which sodium void worth is close to zero, even negative, at end of cycle. This "low sodium void value" is achieved by a heterogeneous geometrical design involving both internal fertile blankets and upper sodium plenum. This configuration not only yields a near zero void effect by also a global improvement on the thermal sodium expansion feedback coefficient. This leads to global improvement toward a "natural safe behavior" of the core for unprotected loss of flow transients.

This specific design has been used for SFR deployment scenarios studies in the current century. As a matter of flexibility scenarios have to investigate several strategies including net nuclear energy growth and nuclear phase out. In both cases, inherent properties of SFR can bring solutions in terms of design adaptation to meet the associated requirements.

In the case of phase out studies, the main goal is to be able to switch from self-breeder core to plutonium burner for inventory minimization concerns.

Starting from the current design of the large power industrial core used in the scenario a preliminary image of a plutonium burner core has been investigated following guidelines of the CAPRA 3 approach issued from EFR [START_REF]Fast Reactor Database 2006 Update[END_REF] project in the 90's.

II. SCENARIO NEEDS

II. Phase out

The reference deployment of next generation nuclear reactors consider fuel cycle based on plutonium multirecycling strategy in Sodium cooled Fast Reactor (SFR). Basically, most of the current scenarios [START_REF] Coquelet-Pascal | Comparison of Different Scenarios for the Deployment of Fast Reactors in France -Results Obtained with COSI[END_REF] consider the deployment of a 60 GWe SFR fleet in two steps to renew the French PWR fleet. Fig. 1 shows a typical transition from PWR (involving today second and third generation of these reactors) to SFR, these latter being deployment around 2040. In order to achieve sustainability, SFR core has to exhibit self-breeding performances to master the plutonium inventory when equilibrium is reached. As scenarios do investigate long term deployment configurations, some of them require tools for nuclear phase-out studies. For these particular scenarios, the need does focus on nuclear systems that enable to reduce the fleet plutonium inventory in an efficient way. To do so, one can choose either to develop dedicated tools such as Accelerator Driven System or to rely on inherent flexibility of SFR.

This "exercise" has already been performed during the 90's in the frame of the European Fast Reactor (EFR) within the French Act on Waste management. The main purpose was to have fast reactor core that could "burn" plutonium coming from multirecycling MOX PWR. The core design, labeled CAPRA (French acronym for Enhance Plutonium Consumption in Fast Reactor), was derived from self-breeder EFR configuration. The CAPRA approach involving many core changes leads to a burner core.

II.A. Reference core

The reference core is a MOX 3600 MWth, axially heterogeneous core based on the Low Void Concept (hereafter labeled CFV core) proposed by CEA. The Fig. 2 displays schematic axial and radial cuts of core. The radial plot shows

• mixture of fissile (red) and fertile (yellow) layers for the inner plutonium content. • lower fertile zone for the outer plutonium content, • a large upper sodium plenum. This combination leads to an enhanced neutrons leakage in the sodium voided configurations and also improvement in the coolant thermal expansion feedback coefficient. Self-breeding is achieved by internal fertile zones (fertile pellets inside the fuel pin).

Inner Diluant S/A

Control Rod S/A Inner Fuel S/A: Shutdown Rod S/A Outer Fuel S/A Reflector S/A: Fig. 2. Radial and axial cut of the reference core.

Core performances were calculated using a plutonium quality corresponding to a representative isotopic content available around 2035 (hereafter labeled "Pu2035", see Table I.) for the start of the deployment scenario. The main core characteristics are presented in Table II. The core configuration leads to a self-breeder core with a low reactivity swing. The reference cladding material is a representative martensitic Oxide Strengthened Steel (ODS) that enable to reach higher maximal damage rate than ferritic steel like AIM1 used by EFR concept. The total sodium void worth is slightly negative at the end of the cycle.

II.A. Scenario Requirements

The phase out study needs specific assumptions for fuel cycle management (nuclear material flow, reprocessing and manufacturing capacities, reactor shutdown schedule, etc..). For reactor, there is a need of neutronic system with efficient plutonium burning capacities. In order to minimize the impact on the reactor design, the burner core configuration has to be obtained while conserving some of the reference core characteristics such as:

• Concerning the nominal core performances, the CAPRA configuration has to comply with some specific requirements:

• "Manageable" cycle reactivity loss. The value has to be less that 20 pcm/efpd which correspond to Phenix case. • « no degradation » of feedback coefficient.

Sodium void worth and Doppler constant (KD) could be taken as global estimators to quantify performances.

III. FROM BREEDER TO BURNER CORE

III.A. Philosophy of CAPRA approach

There are several ways to move from a self-breeder configuration into a burner one but all needs some more or less in depth adaptations of the core design:

• change in the power density (by means of initial fuel S/A in the core ), as for PRISM or ABR concept 6,7 , • change in the fuel nature (plutonium on inert matrix), • insertion of neutronic penalties in the core.

The latter case offers a wide range of possibilities and was the basis of the CAPRA approach used in the frame of the EFR project. "Neutronic penalties" can be obtained by either modifying the geometry of the core to enhance neutron leakage or inserting non-heavy nuclei material ("inert" or absorber material) to decrease the fissile volume fraction. These modifications lead to increase the fissile content in order to reach acceptable cycle length. As the plutonium content reaches high level, the U 238 proportion goes down and the core exhibits less abilities to produce fissile material.

In the present case, the CFV reference core has several fertile zones inside the fuel element itself and thus shows additional degree of freedom for CAPRA approach compared to EFR homogeneous core. • fertile material (yellow) substitutions by inert material, • insertion of "inert" pins in the fuel subassembly (green pins) .

Two solutions investigated in the CAPRA project were not implemented here:

• MOX fuel nature change to nitride or uranium-less fuel (inert matrix). As the phase-out scenario is based on reprocessing strategy, MOX fuel nature is the most suitable form for industrial purpose. • Insertion of diluent (with or without absorber material) subassembly. As local penalties involve perturbation in the flux distribution compared to the reference core, a fine tuning of the core characteristic's is needed especially in order to meet several basic requirements such as maximum linear heat rating, reactivity swing during cycle, safety margin for shutdown issues, etc.. This solution could be investigated in a next phase of the study.

.

III.B. Calculation tools

Calculations

were performed using the ECCO/ERANOS 8 code package together with JEFF3.1.1 cross sections library [START_REF] Koning | The JEFF-3.1 Nuclear Data Library[END_REF] . The transport option has been used for all calculations with a 33-group working library which has been generated from a 1968-group master library. For comparison, fine-group (1968 groups) ECCO cell calculations were also performed, and provide an accurate description of the reaction thresholds and resonances. The fine-group calculation has been performed for the 2dimensional description of a sub-assembly. Because the broad 33-group library has been generated from this master fine group library, it has been that differences on the core parameters are small.

For the core geometry model of the core, a cylindrical (RZ) model is adopted. Validity of the RZ model was assessed against the results of heterogeneous (hexagonal-Z) calculation using the TGV/VARIANT [START_REF] Doriath | Variational Nodal Method (VNM) to Solve 3D Transport Equation: Applications to EFR Design[END_REF] 3-dimensional nodal transport code. Heavy nuclei depletion calculations are performed with an extended chain up to Cf252 isotope. Sodium void reactivity effects have been obtain by set sodium concentration near zero for the fuel zones including fissile and fertile or inert medium zones of the inner plutonium content. The upper sodium plenum has also been considered.

For Doppler constant estimation, only fissile zones were considered in order to have a coherent comparison between configurations.

IV. BURNER CORE CONFIGURATION

IV.A. Impact of incremental changes

In order to evaluate individual impact of any modification from the reference configuration, incremental change strategy was applied. The evaluated impacts on core characteristics as well as plutonium consumption (assuming 1450 MWe electricity gross) are displayed in Table III. Configuration A:

Starting from the reference core, the main geometrical modifications applied consisted in:

• Substitution of fertile pellets by MgO pellets • Reduction of the fuel residence time to 3x500 efpd to meet criterion on maximum burnup The choice of the MgO inert material was driven by safety considerations. Its low atomic number composition lead to a soften neutron spectrum that bring important margins for both sodium void worth and Doppler constant. As further geometrical modifications will lead to higher initial plutonium content, these margins will help keeping the same feedback level as what is observed in the reference core.

Beside improvement in the feedback coefficients, the main impact of these modifications relies on the cycle reactivity loss which increases up to 7 pcm/efpd.

Configuration B and C:

In order to introduce inert material inside the fissile part of the subassembly, fuel bundle has to been redesign. As the new configuration has to meet the maximum linear heat rating criteria the fuel pins number has to remain quite constant. To do so, pin diameter was first decreased to be able to move from 331 pins to 469 (configuration B). This new bundle leads to new fuel, structures and sodium volume fractions:

• Fuel volume fraction goes down from 44% to 36 % • Sodium volume fraction increases from 28 to 32 %. This leads to higher plutonium content that does enhance the plutonium consumption, but also degrades feedback coefficients as well as cycle reactivity loss compared to previous configuration.

Configuration C corresponds to substitution of around one third of the fuel pins by inert pins (MgO pellets instead of MOX ones). The corresponding fuel volume fraction inside the fuel bundle drops to 26%. The associated plutonium content is 35% which leads to a reactivity loss around -18 pcm/efpd, close to the acceptable limit. The evaluated maximum burnup value (233 GWd/t) is beyond the required criterion. This high level plutonium content leads to harder neutron spectrum that favors neutron leakage so that sodium void worth level is still below the one of the reference case. Doppler constant value is a bit lower but is still higher but still showes improvement compared to the self-breeder core.

Configuration D:

In depth analysis of the configuration C shows that the increase of maximum burnup value comes from the combined effect of the radial insertion of the MgO inside the fuel part of the subassembly. Large volumes of MgO (both the fuel part and in the medium and lower inert zone of the inner core) lead to strong spectrum transition at the fuel/inert MgO interfaces. The local effect can be seen in the Fig. 5 which represents a 1D Monte-Carlo model of the inner fuel pin. Here, several materials of the central inert zone fuel have been tested for fission form factor evaluation. Several solutions have been tested in order to decrease the peaking value. The most efficient consists in inserting absorber material in the transition zone. One example in which 1 cm of steel containing boron carbide have been located in both lower and upper part of the central inert zone. The associated result is shown by the green curve in Fig. 5.

This solution has been applied in configuration. The absorption level of the core is slightly increased and the plutonium content need to be updated. If the maximum burnup value obtained by the RZ calculation scheme does not seem to be impacted, more realistic 3D calculations show a 20% reduction of the peaking factor. However the associated maximum linear heat rating is still beyond the requirement.

Configuration E:

The last parameter that can help reaching the targeted value is the core nominal power. Moving from 3600 MWth to 3000 MWth gives acceptable maximum value without degrading plutonium consumption. In addition, the change in power density gives lower cycle reactivity loss. The associated feedback coefficients values meet the requirement in terms of "safety" level:

• Sodium void worth is still negative • Doppler constant is not degraded • Both values show small gain compared to the reference case. This latter configuration was the basis of a full 3D core description to validate the observed performances. Starting from configuration E, cores modifications were included in a 3D core description using "best estimate" calculation scheme:

IV. B. 3D Burner core configuration

• 3D flux transport calculation scheme • Reloading scheme using 1/5 pattern with control inserted at beginning of cycle • Equilibrium search (each subassembly is replaced twice) To do so, some adaptations have been performed:

• MgO pellets have been put into inert pins only • MgO pellets of the lower and central inert zones (former fertile zones) have been replaced by ODS ones to limit fission peaking factor inside the fuel pin. The pins configuration is displayed in Fig. 6. • Natural boron carbide used in the control rod design has been replaced by enriched B 10 boron carbide (90% B 10 ) in order to deal with the marge reactivity swing of the burner configuration.

Fig. 6. Schematic axial description of the fuel and inert pin of the fuel bundle.

The main core performances are listed in Table IV. Most of them are very similar to those obtained for configuration E using a "low" level calculation scheme. The main improvement is the 3D validation of the maximum linear heat rating that fits the initial requirement. If the maximum burnup exceeds the value of the selfbreeder core, it seems acceptable from the thermomechanical point of view. The plutonium consumption evaluated at the end of irradiation is 56 kg/TWeh. The characteristics of the CAPRA core image has to be compared to results obtained for EFR core. One of the main differences relies on the level of the Doppler constant. For EFR CAPRA the Doppler constant was 25% lower than the one of the reference EFR MOX core because of the use of diluent subassemblies inside the core that increased the power density and leads to higher initial plutonium content.

The current CFV CAPRA design based on simple material and geometrical modifications helps improving feedback coefficient while maintaining acceptable plutonium consumption.

IV. CONTROL ROD DESIGN

In order to obtain a realistic design of the core a large amount of work has been done for the fuel element especially in terms of neutronic effects of the different materials involved in the diverse inert zones inside or outside the fuel pin itself.

The same kind of approach needs to be done for the control rod design. The only modification that was applied for rod design was the substitution of the natural boron by enriched one. A lot of work remains to be done to ensure that the current modification is able to satisfy the whole set of criteria relative to reactivity control and reactor shutdown.

If we put aside complex core configurations such as fuel handling errors fro hot and cold core states, control rods efficiency has to comply with:

• reactivity management at beginning of cycle,

• reactor shutdown at beginning and the end of cycle In addition, control rod efficiency has to be compatible with "reasonable" margin on boron carbide melting temperature. With the modified boron content rod design, equilibrium beginning of cycle criticality is achieved with 25 cm rods insertion in the inner fissile zone. If we assume a conservative value for the 90% B 10 4C conductivity and a representative power peeking factor (~1.5), maximum boron carbide centerline temperature can be estimated. The original rod design has 37 absorber natural born carbide rods corresponding to 24% volume fraction. Fig. 6 shows the margin to melting evaluated from the maximum center line boron carbide temperature after a certain number of irradiation cycles. Here, several pin designs have been tested for which we made the assumption that the total absorber volume fraction inside the control rod subassembly remains constant. The current design based on 37 pins rods does not show any realistic margin since melting occurs before the end of the first irradiation cycle. With smaller rod pin diameter, solution can be found. At least 91 pins are required to have design that can remain two cycles or more in the core. The use of the natural boron carbide instead of 90% B 10 boron carbide gives significant margin on fuel melting since 37 pins design is still 1400 K above fuel melting after 5 irradiation cycles.

Fig. 6. Boron carbide margin to melting as a function of rod pin number in the control rod design

For reactor shutdown issues, the total control rod worth required for the CAPRA core has to account for the following items:

• reactivity loss during cycle : ~ 4600 pcm • reactivity Doppler effect from nominal full power state to cold isotherm shutdown : ~1000 pcm • management of fuel handling errors : ~2000 pcm • integration of incertainty level : ~1000 pcm

The sum of all these effects gives a total close to 9000 pcm. Assuming that the control rod worth does not depend on the number of pin for an imposed volume fraction, several material and geometrical rod configurations based on 127 absorber rod pins were investigated. Starting from a 127 absorber pin rods design (see Fig. 7) the effect of B 10 content on control rods worth has been investigated. As brief sensitivity study showed that control rod absorption level can be enhanced with the use of moderator material, some versions of the design with ZrH2 and other similar material has been tested. As shown in Table V, Initial 90% B 10 configuration leads roughly to a 3000 pcm margin. Criteria on total control rod worth can be achieved with either 30% or 48% B 10 content. With these configurations the estimated boron carbide maximum centerline temperatures decrease and the gain can be used to find optimized solutions that can meet the whole set of control rods requirements.

The analysis of configurations with moderator material showed that such pins can bring new degree of freedom in order to:

• boost the global absorption rate level by shifting neutrons to lower energy • decrease the "shadow" effect between absorber pins (spatial self-shielding) and thus decrease the peaking form factor CFV CAPRA core control rod design is currently ongoing using the above results. The optimized solution will have to be validated on the basis of Monte-Carlo calculations.

V. CONCLUSIONS

The reference deployment of next generation nuclear reactors consider fuel cycle based on plutonium multirecycling strategy in Sodium cooled Fast Reactor (SFR). As scenarios do investigate long term deployment configurations, some of them require tools for nuclear phase-out studies. Instead of designing new reactors, the adopted strategy does focus on adaptation of existing ones into burner configurations.

Starting from the low sodium void core configuration (CFV), the self-breeder reference core has been turned into a plutonium burner core using the CAPRA-like approach already used in the 90's in the frame of the EFR project.

As the CFV self-breeding is ensured by fertile blankets, a first modification consisted of the substitution of the corresponding depleted uranium by "inert" or absorber material leading to a "natural burner" core with only small impact on flux distribution. The next step toward CAPRA configuration was the substitution of 1/3 of the fuel pins by "dummy" pins (MgO pellets). The small spectrum shift due to MgO material insertion leads to an increase Doppler constant which exceeds the value of the reference case. As the core sodium void worth value is conserved, the CFV CAPRA core "safety" potential is quite similar to the one of the reference core.

Preliminary studies on control performances show that the initial design can not meet the whole set of reactor shutdown requirements, especially the margin to melting issues. A set of material and geometrical modifications of the design gives clear insight of possible solutions that have to be confirmed by reference calculations.

The resulting core configuration exhibits acceptable plutonium consumption without degrading basic feedback coefficient values compared to the self-bredeer reference core. Reactivity management as well as reactor shutdown capacity seem achievable with some minor design adaptations.

The move from a self-breeder reference core to a burner core presented here has been performed while keeping the main feedback characteristics of the CFV core concept. Plutonium burning efficiency can be tuned according to scenarios demands and this kind of design offers a high degree of flexibility for nuclear material management.

Fig. 1 .

 1 Fig. 1. Typical transition scenario for the 60 GWe French fllet (dark=current PWR, green=EPR, blue=SFR).

Fig 3

 3 displays schematic radial and axial cut of core and subassembly for reference and CAPRA core configurations. The main adaptations consist in:

Fig. 3 .

 3 Fig. 3. Radial and axial cut of the reference (right) and CAPRA (left) cores.

Fig. 4 .

 4 Fig. 4. Radial of the fuel bundle: pins (left); 469 pins (center) and 342+127 CAPRA (rigth).

Fig. 5 .

 5 Fig. 5. Fission form factor as a function of the material composition of the central inert zone. The red curve corresponds to the same steel composition as the cladding material (ODS) while the blue one corresponds to MgO material. Fuel plutonium content was set to 35% according to configuration D characteristics. The light elements of MgO material give a local modification of the flux coming from fissile zones and the fission form factor reaches 1.7 compared to 1.3 for steel composition.Several solutions have been tested in order to decrease the peaking value. The most efficient consists in inserting absorber material in the transition zone. One example in which 1 cm of steel containing boron carbide have been located in both lower and upper part of the central inert zone. The associated result is shown by the green curve in Fig.5.This solution has been applied in configuration. The absorption level of the core is slightly increased and the plutonium content need to be updated. If the maximum burnup value obtained by the RZ calculation scheme does not seem to be impacted, more realistic 3D calculations show a 20% reduction of the peaking factor. However the associated maximum linear heat rating is still beyond the requirement.

  Proceedings of ICAPP 2015May 03-06, 2015 -Nice (France)Paper 15091

Fig. 7 .

 7 Fig. 7. Schematic view of 127 pins control rod designs with or without moderator (gray pins).

TABLE III CAPRA

 III

			configurations characteristics
	Conf.	Ref	A	B	C	D	E
	Power (MWth)	3600	3600	3600	3600	3600	3000
	Pu Content (%)	22.3	21.7	25.4	36.5	37.5	35.8
	Max BU (GWd/t)	163	171	194	233	220	187
	Δρ cycle (pcm)	-800	-2600	-3600	-5500	-5500	-4600
	βeff (pcm)	365	361	350	324	323	325
	Sodium Void ($)	-0.40	-1.6	-0.7	-1.1	-0.92	-1.15
	KD ($)	-1.70	-3.1	-2.9	-2.5	-2.2	-2.4
	Cons. (kg/TWeh)		-23.4	-34.0	-55.1	-57.5	-55.6

TABLE V Control

 V

		rod (CR) worth of various design versions
	B4C	B10	Mod.	Mod.	CR worth
	pins	content	pins	material	(pcm)
		(%)
	127	19.8	0	None	-7 700
	127	30	0	None	-9 000
	127	48	0	None	-10 100
	127	90	0	None	-12 500
	108	90	19	ZrH2	-12 300


P Marc

F Charlier

J Godard

A Magnaldo

E Schaer

Study and modelling of heterogeneous catalytic reactions. application to uranium dioxide dissolution.

Abstract Submission

Dissolution is a key step in several industrial processes. It is especially a milestone of the head-end of many hydrometallurgical processes. For example, in recycling of spent nuclear fuel, the solubilization of the chemical elements is essential before performing the liquid-liquid extraction steps to separate reusable material and final waste. One of the most complex scenarios is that of heterogeneous autocatalytic reactions. Today, there are few satisfying models for these cases due to a lack of comprehension of their mechanisms.

We focus here on the dissolution of uranium dioxide in nitric medium. In order to propose optimized processes for dissolution, this study aims at better understanding the chemical, physico-chemical and hydrodynamic phenomena of such reactions. This study is also part of a modeling approach aiming, on one hand, at expressing the intrinsic reaction rates and describing the physico-chemical phenomena at interfaces and, on the other hand, at developing a general model for dissolution reactors.

Results :

Optical microscopy observation confirmed the highly autocatalytic nature of the reaction and led to measurements, for the very first time, of "true" chemical kinetics of the reaction. The acid attack of sintering-manufactured solids occurs through preferential attack sites. It develops cracks in the solids that can lead to their cleavage. This inhomogeneous attack is made possible by the establishment of bubbling in the cracks which allows periodic renewal of the reagents and thus maintains the reaction within the cracks. This point is a key component of the mechanism: a strong link among the development of cracks, bubbling through the cracks, and overall dissolution kinetics is demonstrated in this work.

A model coupling material balance to the structural evolution of the solid and liquid phase compositions, and taking into account the interfacial transport is proposed. The simulations based on this model are close to the experimental observations, and allow to replicate the effect of various reaction parameters for the very first time, such as the reduction of overall kinetics when turbulence increases.

Type of presentation :: Oral Highlight 1: Optical microscopy is used to determine liquid-solid reactions mechanisms and kinetics.

  Chemical reaction engineering / ECCE10 / Reaction mechanism and kinetics STUDY AND MODELLING OF HETEROGENEOUS CATALYTIC REACTIONS. APPLICATION TO URANIUM DIOXIDE DISSOLUTION. Philippe MARC 1 , Florence CHARLIER * 1 , Jérémy GODARD 1 , Alastair MAGNALDO 1 , Eric SCHAER 2 1 DEN/MAR/DRCP, CEA Marcoule, Bagnols-sur-Cèze, 2 LRGP, ENSIC, Nancy, France Authors of the abstract come from :: Industry Submit your abstract below (400 words):