PROPOSAL OF INTEGRAL CRITICAL EXPERIMENTS FOR LOW-MODERATED MOX FISSILE MEDIA

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1 Integrating Criticality Safety in the Resurgence of Nuclear Power Knoxville, Tennessee, September 19 22, 2005, on CD-ROM, American Nuclear Society, LaGrange Park, IL (2005) PROPOSAL OF INTEGRAL CRITICAL EXPERIMENTS FOR LOW-MODERATED MOX FISSILE MEDIA I. DUHAMEL Institut de Radioprotection et de Sûreté Nucléaire (IRSN) BP Fontenay aux Roses Cedex, France E. GIRAULT Commissariat à l Energie Amique (CEA) CEAValduc, Is-sur-Tille, France emmanuel.girault@cea.fr C. VENARD Commissariat à l Energie Amique (CEA) CEA Cadarache, Saint-Paul-lez-Durance Cedex, France chrisphe.venard@cea.fr ABSTRACT The assessment of the available benchmarks, performed by the OECD/NEA Working Party on Nuclear Criticality Safety Expert Group on Experimental Needs, has highlighted the need of additional experiments dealing with low-moderated MOX powders, which are mainly encountered in MOX fuel manufacturing. For the different configurations assessed in manufacry safety report, the neutron flux varies between the fast and the intermediate energy range. As the fissile media heterogeneity has not a great impact in such spectrum, IRSN has studied the possibility of performing integral critical experiments with low water-moderated MOX fuel rods assemblies. The proposal consists of different configurations dealing with various tight triangular lattices pitches, which would be assembled in two French criticality facilities. Design has been optimized taking in account the feasibility of the experiment, the possibility of using the existing materials and equipment, the fuel fabrication cost and the representativity of the experiments regarding powders encountered in MOX fuel manufacturing. Conclusions are based on the comparisons of spectral, balance and sensitivity data between, on one hand, the proposed experiments and, on the other hand, application configurations involving MOX powders dealing with different PuO 2 and water contents. They point out that the proposed experiments provide a good representation of the application configurations, especially for water reflected configurations with % content of reacr grade plunium and 3 5 % water concentration. Key Words: MOX fuel, low-moderated, integral experiment 1 INTRODUCTION In the framework of calculation codes validation, the use of mixed oxide fuel (MOX) in nuclear power plants, recycle LWR-Pu from reprocessing plants or convert surplus Weapons- Grade (WG) plunium, has created a need for benchmarks that deal with MOX fuel.

2 I. DUHAMEL, E. GIRAULT, C. VENARD For the past 5 years, the OECD/NEA Working Party on Nuclear Criticality Safety (WPNCS) Expert Group on Experimental Needs has highlighted MOX fuel manufacturing process as an area in which there is a specific need for additional experimental data for validation purposes. The results of this lack of available benchmarks are unknown computational biases and relatively large uncertainties, which industries and regulars have accommodate with. In order significantly decrease the uncertainties linked the configuration multiplication facr prediction, new and accurate experiments dealing with low-moderated Mox media are needed. This paper aims presenting the integral experiments proposed by IRSN and CEA and highlighting how this proposal allows, at least in part, filling the need. 2 MOX FUEL MANUFACTURING The MOX fuel fabrication plants must be designed prevent the risk of a nuclear criticality accident under all normal and credible abnormal conditions. During the different steps of the fabrication processes, low quantities of hydrogenated materials (lubricants, poreformers) are involved. As reliance solely on geometric favorable equipments is not possible, controls on both fissile material masses and hydrogen content are necessary in the main units of the plant. The main parameters be considered for MOX powders fabrication units are described in the table I. Table I. Main characteristics of MOX powders encountered in fabrication process. PuO 2 content Reacr-Grade (RG) plunium 30 % (primary blend) 12.5 % (final blend) Weapon-Grade (WG) plunium 22 % (primary blend) 6.5 % (final blend) Powder density 4.6 g/cm 3 or 5.5 g/cm g/cm 3 Water content 3 % (normal conditions) 5 % (abnormal conditions) 1 % 5 % 235 U (wt %) Less than 1.2 % Less than 1.2 % 240 Pu (wt %) More than 17 % About 4 % 3 PROPOSAL TO ADDRESS EXPERIMENTAL NEEDS 3.1 Apparatus B Valduc criticality laborary Major facility of the Valduc critical laborary [1, 2], Apparatus B, is a multi-purpose subcritical assembly that enables reproducing a great variety of criticality configurations, representative in particular of the fabrication, srage, transport and reprocessing of fissile materials in the form of rods or solutions. Moderating materials, absorbers, reflecrs or shields (lead, concrete, polyethylene ) can be added under various forms. Page 2 of 12

3 Proposal of integral critical experiments for low-moderated MOX fissile media Apparatus B is mainly an experimental tank, which contains the fissile core be studied, and the associated instrumentation. The sub-critical approach is carried out by increasing slowly the liquid level in the core and uses the neutron amplification technique. During the experiments, the criticality is approached up the value of k = (1 - β/10), where k is the effective multiplication coefficient and β the difference between the prompt critical state and the delayed critical state. In order perform high quality experiments that can be used as benchmarks, and meet requirements with regulations and the management of nuclear materials, specific equipments are operated in the installation, such as physico-chemical analysis laboraries for nuclear materials. Thus, associated with rods diameters measurements (clad, oxide), it could allow obtaining very small experimental uncertainties. For example, experiments involving low-moderated rods arrays (UO 2 enriched with % of 235 U with tight pitches) have already been performed at Valduc laborary in 1998 and the experimental uncertainties effect on k eff (according ICSBEP method) was evaluated be less than 90 pcm (1 σ). 3.2 The EOLE experimental reacr The EOLE reacr located at Cadarache [3] is an experimental facility operating a maximum power of 100 W. It is mainly dedicated the neutronic studies of Light Water Reacrs and High Conversion Reacrs. EOLE is an easily adaptable facility composed of a concrete structure offering biological shield for flux levels up 109 n.cm -2.s -1, a cylindrical vessel in AG3 with an over structure in stainless steel, able contain various types of core structures, 4 control rods and a pilot rod. The water circuit is coupled with a thermoregulation station in order maintain a constant moderar temperature in MOX lattices and allow reactivity temperature coefficient measurements from 5 C up 80 C. Core criticality is obtained either by radial critical size adjustment or by soluble boron dilution; fuel pins are always fully immersed, which enables accurate 2D-core modeling. Reactivity worth measurements are performed by critical and sub-critical MSM techniques. 3D fluxes and fission rates are measured in the core and the reflecrs by miniature fission chambers (various fast and thermal response functions). Reaction rates and spectral indices are also available through irradiation of metallic foils. Fission radial maps are obtained by direct γ-spectrometry on fuel pins. 3.3 Proposed experiments Obviously, the most representative experiments corresponding the fabrication process would be integral experiments dealing with MOX powders with different density and various moisture contents (from 1 about 5 %). Unfortunately, the use of powders raises many difficulties as, for example, maintain a definite temperature, guarantee a safe containment of the plunium and assure the accuracy of the moisture content and the density. Moreover, depending on the powders density, it would lead high quantities of fissile materials. Therefore, IRSN [4] has studied the possibility of performing experiments with low water-moderated MOX fuel rods arrays. Indeed, damp MOX powders can be simulated with assemblies of tightly packed MOX fuel rods when keeping similar moderation ratio [5]; in fact, Page 3 of 12

4 I. DUHAMEL, E. GIRAULT, C. VENARD for water content lower than 5 %, which leads fast and intermediate neutron spectra, the fissile media heterogeneity has not a great impact, regarding the neutron mean free path Experimental configuration Thus, the experimental regular core involves a hexagonal MOX fuel rods array in water. The proposal consists of five configurations that would be assembled on the Apparatus B facility at Valduc Criticality Laborary (see figure1). Various tight-pitch lattices will be investigated in order cover different moderation ratios. Figure1. Experimental configuration. MOX rods Design has been optimized taking in account the feasibility of the experiment, the possibility of using the existing materials and equipment, the fuel fabrication cost and the representativity of the experiment in terms of neutron spectrum (see table V). The composition of the fuel material is representative of reacr grade MOX with a PuO 2 content of about 27.5 % (wt), which corresponds the primary blend mixture in French fabrication process. Thus about 2200 new fuel rods must be manufactured with the following characteristics: Isope 238 Pu Table II. MOX fuel isopic content. 239 Pu 240 Pu 241 Pu 242 Pu 241 Am Weight % U 238 U Page 4 of 12

5 Proposal of integral critical experiments for low-moderated MOX fissile media Table III. Main MOX rods characteristics. Pellet diameter (cm) Zr4 Clad Outside diameter (cm) 0.94 Inside diameter (cm) 0.82 Column height (cm) 100 Fuel density (g/cm3) 10.4 Pu/(U+Pu+Am) weight ratio 27.5 Different tight pitches are considered cover different moderation ratios close powder water--fuel ratios. Criticality is achieved by rising the water level, so that the immersed fissile column height is about 90 cm. Table IV. Main hexagonal arrays characteristics. Pitch (cm) Hexagonal array Rods number H 2 O content V H2O /V MOX The two main configurations (V H2O /V MOX = 0.3 and 0.6) could also be investigated in the EOLE reacr at Cadarache. The combination of the two facilities allows the determination of various parameters (critical mass, material buckling, spectral indices and reactivity temperature coefficients). Since the fuel fabrication represents an important part of the tal cost of the foreseen program, a possible extension of the program is increase the arrays pitch contribute calculation codes validation for powders (and also MOX assemblies) with higher moderation ratios and also involve different absorber or moderar materials Experimental measurements The critical masses of these 5 configurations will be measured in Apparatus-B through the critical height Hc of the water level in the tank. The main components of the neutron balance (Capt( 238 U)/Fission, fission rates of 239 Pu, 241 Pu, 238 U and 240 Pu) and Material Buckling (B 2 m = (K -1)/M 2 ) could be measured in the EOLE reacr for two of these 5 configurations. Finally, the reactivity temperature coefficient, which is a challenging calculation problem in MOX media [6], could also be accurately measured in EOLE (± 0.2 pcm/ C in 1σ) from 5 C up 80 C. Page 5 of 12

6 I. DUHAMEL, E. GIRAULT, C. VENARD 4 MAIN NEUTRONIC CHARACTERISTICS AND SIMILARITY WITH MOX POWDERS In order make a judgment upon the ability of the planned experiments match the neutronic characteristics of MOX powders of interest, several physic parameters were compared. 4.1 Computer codes and calculation methods Calculations were performed with the multigroup route of CRISTAL, which is the French package used for criticality-safety studies. This route deals with the APOLLO2 assembly code, the CEA group energy library and the MORET4 Monte Carlo code. The CEA93 application library using the 172-group Xmas energy structure was derived from JEF2.2 and was processed using NJOY-THEMIS. The assembly code APOLLO2 is used for self-shielding (using the generalised Livolant-Jeanpierre formalism), for flux calculations (using the Pij method) and for sensitivity studies. Then, selfshielded and homogenised cross-sections are used in the Monte Carlo code MORET4 for 3D calculations. Taking in account the energy groups boundaries in the 172-groups energy mesh, the adopted five-groups structure is as follows: - Group 1: 111 kev < E < 19.6 MeV - Group 2: kev < E < 111 kev - Group 3: ev < E < kev - Group 4: 0.1 ev < E < ev - Group 5: E < 0.1 ev 4.2 Main neutronic characteristics comparison Calculations have been performed for Reacr-Grade (RG) and Weapon-Grade (WG) MOX powders with the different PuO 2 and water contents given previously. Maximal density (5.5 g/cm3) and uranium enrichment (1.2 % of 235 U) and minimal 240 Pu content were studied (4 % for WG and 17 % for RG Plunium). Regarding the Reacr Grade plunium, the isopic composition assessed in the MELOX French MOX fuel manufacry safety report (71 % of 239 Pu; 17 % of 240 Pu; 11 % of 241 Pu; 1 % of 242 Pu) was considered in this paper. Several physic parameters were calculated (see table V): - q -4eV : Slowing down density, which is the number of neutrons per fission neutron that slow down past an energy of 4 ev. A value of 1 indicates a thermal spectrum in fissile media, a value of 0 a very fast media (metallic systems), - GMF: Average group of neutrons causing fission in the 172-groups energy structure and its corresponding energy boundaries, - 5-group flux, fission and capture data (in percent), - 5-group detailed balance for the main isopes, - Sensitivity coefficients the main nuclear reactions for uranium and plunium isopes. Page 6 of 12

7 Proposal of integral critical experiments for low-moderated MOX fissile media Table V. Neutronic characteristics of some integral experiments and MOX powders. Proposed experiments Reacr Grade Weapon Grade Pitch (cm) Vmod/Vox Water content H/Puf q -4eV GMF Average Energy (ev) ev ev For all of these powders and for the proposed experiments, the neutron spectrum varies between the fast and the intermediate energy range (see figure 2). In fact, less than 5 % of neutron flux occur under 10 ev (groups 4 and 5) and more than 50 % above 100 kev (group 1). Figure 2: Neutron spectra in infinite media. Page 7 of 12

8 I. DUHAMEL, E. GIRAULT, C. VENARD Moreover, main parts of absorptions occur in group 3. Sensitivity profiles of k eff nuclear data highlight that absorption is mainly due the 238 U capture in the resonance range between 5 and 1000 ev (see figure 3), the 239 Pu fission (see figure 4), the 239 Pu capture below 10 kev (see figure 5), and the 240 Pu capture in the first large resonances (see figure 6). Figure 3. k eff Sensitivity Profile 238 U capture cross section Figure 4. k eff Sensitivity Profile 239 Pu fission cross section sensivity coefficients RG Primary blend - water content 3 % RG Primary blend - water content 5 % RG Final blend - water content 3 % RG Final blend - water content 5 % proposed experiment pitch 0.96 cm proposed experiment pitch 1.04 cm E-10 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+ Energy (MeV) Page 8 of 12

9 Proposal of integral critical experiments for low-moderated MOX fissile media Figure 5. k eff Sensitivity Profile 239 Pu capture cross section Figure 6. k eff Sensitivity Profile 240 Pu Capture Cross section 0.00 Energy (Mev) 1.E-10 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E sensivity coefficients RG Primary blend - water content 3 % RG Primary blend - water content 5 % RG Final blend - water content 3 % RG Final blend - water content 5 % proposed experiment pitch 1.04 cm proposed experiment pitch 0.96 cm Page 9 of 12

10 I. DUHAMEL, E. GIRAULT, C. VENARD As shown in the figures above, the representativity of the proposed experiments the primary blend and final blend is satisfacry. In fact, major isopes reactions sensitivities are preserved in the continuum range, as well as in resolved and unresolved resonances domains. Table VI: Comparison of fission and capture balance in the 5-groups energy structure Proposed experiment: Pitch 1.04 cm RG MOX Powder 12.5 % PuO 2 3 % H 2 O Discrepancies Proposed RG MOX Powder experiment: 30 % PuO 2 Pitch 0.96 cm 3 % H 2 O Discrepancies Isopes Groups Capture Fission Capture Fission Capture Fission Capture Fission Capture Fission Capture Fission 238 U 239 Pu 240 Pu % % % % % % % % % % % % % % % % % % % % % % % % % 8.765% % % % % % 0.186% % % % 4.799% % % % % % % % % % % % % % % % 9.206% % % % % % % % % Regarding the sensitivity calculation results and the fission and capture balances for major isopes (see table VI), one can conclude that the proposed experiments with pitch of 0.96 cm is close RG primary blend with a water content of 3 % and the one with the pitch of 1.04 cm is close RG final blend with a water content of 3 %. In fact, the large contributions of continuum and unresolved resonances range 238 U capture is preserved, as well as the predominant component of the 10 ev-10 kev domain 239 Pu absorptions. 4.3 Independent assessment These proposed experiments were presented during the Workshop on The need for integral experiments with low-moderated MOX fuels organized by OECD/NEA in April of Based on the outcome of the Workshop, the Nuclear Science Committee requested the WPNCS assemble an international assessment team in order review the different experimental programs that were proposed. The experiments were assessed according five previously established criteria namely, neutronic criteria, the type of experiments, financial aspects, anticipated schedule and other aspects. The most significant criteria, technically, is the ability of the proposed experimental Page 10 of 12

11 Proposal of integral critical experiments for low-moderated MOX fissile media programs match the neutronic characteristics of the application configurations. Core average values were compared. Detailed sensitivity-uncertainty analyses were performed by scientists at Oak Ridge National Laborary (ORNL). Rather than specifying actual configurations encountered in the MOX fabrication plants, simplified geometries were considered and the range of important parameters was defined (moderation ratio, plunium content and origin of the plunium weapon grade or reacr grade). Finally, the assessment team concluded that the experiments proposed by IRSN and CEA provide a good representation of the application configurations especially in the following conditions: water reflected configurations with % content of reacr grade plunium and 3 5 % water concentration. Considering the experiments already performed in this facility, the anticipated accuracy and quality of the measurement are judged achievable. Finally, the experiments proposed by IRSN and CEA were judge able fill the identified need and complementary with the Institute of Physics and Power Engineering (IPPE Russia) proposal. 5 CONCLUSION In the framework of calculation codes validation, the use of mixed oxide fuel in nuclear power plants has created a need for benchmarks that deal with MOX fuel cycle facilities. The assessment of the available benchmarks has highlighted the lack of experiments dealing with low-moderated MOX powders, which are mainly encountered in MOX fuel fabrication plants. For the different configurations assessed in MOX fuel manufacry safety report, the neutron flux varies in the fast and the intermediate energy range. In such spectrum, the neutron mean free path is very high, thus the fissile media heterogeneity has not a great impact. Therefore, integral experiments using UO 2 -PuO 2 rods are proposed be performed in Apparatus-B at Valduc Criticality Laborary and in EOLE reacr at Cadarache. Neutronic parameters comparisons and sensitivity studies have highlighted that the proposed experiments are similar reacr-grade powders with low water content (about 3 %). Thus, this kind of experiments would enable validate calculation codes for the different powders encountered in MOX fuel fabrication plants, especially for reacr grade MOX powders. Besides, as they involve only one kind of fissile media, these experiments would be easy investigate with all calculation codes. The important technical know-how accumulating during the several years of operating, the specific equipments available at CEA, such as physico-chemical analysis laboraries, and the manufacturing of new MOX rods, which could allow performing lot of measurements, would lead high quality experiments with low experimental uncertainties. Finally, this program could be easily extended higher pitches, cover a large range of moderation ratios and various configurations : arrays in interaction, heterogeneities in rods array, poisoning moderar (B, Gd, fission products ), various reflecr and/or absorber solid materials. Page 11 of 12

12 I. DUHAMEL, E. GIRAULT, C. VENARD 6 REFERENCES 1. F. Barbry, A. Laviron, P. Fouillaud, P. Cousinou, G. Poullot, Experimental study programs and test potential of the IPSN/Valduc criticality laborary, Proceeding of International Conference on Nuclear Criticality Safety ICNC 99, Versailles (France), September 1999, Vol. IV, pp (1999). 2. P. Fouillaud, E. Girault, P. Grivot, J. Legendre, The Criticality laborary of Valduc (France) and its ability meet the experimental needs for low moderated «MOX» fissile media, Proceeding of the Workshop on the need of integral critical experiments with low-moderated MOX fuels, Paris (France), April 2004, Vol. I, pp (2004). 3 P. Blaise, P. Fougeras, A. Santamarina, S. Cathalau, Integral needs for MOX powders : State of the art at CEA-Cadarache on MOX fuel experiments, Proceeding of the Workshop on the need of integral critical experiments with low-moderated MOX fuels, Paris (France), April 2004, Vol. I, pp (2004). 4 V. Rouyer, I. Duhamel, G. Poullot, P. Cousinou, F. Barbry, P. Fouillaud, E. Girault, IRSN projects for critical experiments: Low-moderated MOX fuel project and others, Proceeding of International Conference on Nuclear Criticality Safety ICNC2003, Tokai Mura (Japan), Ocber 2003, Vol. I, pp (2003). 5 I. Duhamel, V. Rouyer, A. Santamarina, C. Venard, Criticality calculation codes validation : Experimental needs for low-moderated Mox media, Proceeding of the Workshop on the need of integral critical experiments with low-moderated MOX fuels, Paris (France), April 2004, Vol. I, pp (2004). 6 I. Erradi, A. Santamarina, O. Litaize, The Reactivity Temperature Coefficient Analysis in Light Water Moderated UO2 and UO2-PuO2 Lattices, Nuclear Science and Engineering, 144, pp (2003). Page 12 of 12