Journal of Power and Energy Systems

Transcription

1 Energy Systems Advanced Recycling Core Accommodating Oxide Fuel and Metal Fuel for Closed Fuel Cycle* Kazumi IKEDA**, James W. MADDOX***, Wataru NAKAZATO**** and Shigeru KUNISHIMA**** **Mitsubishi FBR systems, Inc , Jingumae 2-chome, Shibuya-ku, Tokyo, , Japan, ***AREVA Federal Services LLC, 3315 Old Forest Road Lynchburg, VA 24501, USA ****Mitsubishi Heavy Industries, Ltd., 16-5, Konan 2-chome, Minato-ku, Tokyo, , Japan Abstract This report presents a unique TRU burning core capable of accommodating oxide fuel and metal fuel and easy to change oxide core to metal core conforming to the design requirements. For the homogeneous oxide fueled core containing transuranics (TRU) fuel with 12% of the moderator pins, the results of calculation show the TRU conversion ratio (ratio of loss of TRU to loss of heavy metal) of 0.33 and the TRU burning capability (ratio of loss of TRU per electric generation) of 67 kg/tweh. On the other hand, the calculations replacing from oxide fuel assemblies to metal fuel assemblies have indicated the TRU transmutation capability of 69 kg/tweh with the TRU conversion ratio of As the result of simulation calculations, three ordinary fuel exchanges transform the oxide equilibrium core to the full metal core by way of transitional cores, where the maximum linear heat rates are still equal to the metal equilibrium core or less. With this, the presented core concept is concluded that a full oxide core, a full metal core, mixed fueled cores can be materialized in the presented first unit of Advanced Recycling Reactor (ARR1). Key words: Advanced Recycling Reactor, Dual fueled core, TRU burning core, Sodium Fast Reactor, Oxide Fuel, Metal Fuel, Transitional Core 1. Introduction *Received 22 Oct., 2009 (No ) [DOI: /jpes.4.234] Copyright 2010 by JSME In the international program supported by the Department of Energy (DOE) of the USA (1), AREVA and Mitsubishi Heavy Industries, Ltd. (MHI) seek to develop an Advanced Recycling Reactor (ARR) in concern with a Consolidated Recycling Facility (CRF) for closed fuel cycle. This paper discusses about a dual fueled core capable of utilizing two kinds of fuels, oxide fuels and metal fuels with regard to the core for which the TRU burning capability is maximized. It is thought useful to be an amphibious system in promoting the fuel development of fast reactors or ARRs, as in the case of the dual fueled cars with utilization of gasoline and ethanol. Which would be more promising fuel for fast reactor, oxide or metal? Although the points at issue including the order of priority, e.g., individual experience of those concerned, achievement, or performance, and differences of breeding properties are well-known, but the conclusion is not yet reached. In the fast reactor development in Japan also, oxides are positioned as a major concept, while metals are regarded as another candidate. This selection prevails widely and we have chosen the oxide fuels based on our own experience. Then, some experts expressed their opinions 234

2 that metal fuels should preferably be available. So we have studied a concept not only accommodating oxide fuel and metal fuel, but also with more smooth and better transformation from an oxide fuel core to a metal fuel core only by ordinary fuel exchange than the scheme that all oxide fuels are exchanged to metal fuels once. According to the work done by ANL, the comparisons of the properties have been done between the oxide fuel core and the metal fuel core over a 250 MWth Advanced Burner Test Reactor (ABTR), and it has been confirmed that either of the cores ensures similar level of TRU burning properties (2). In this study, it is aimed to verify that both oxide fuel core and metal fuel core satisfy the design requirements, and to indicate that it is possible to convert the oxide core, initially arranged by oxide fuel and then loaded with the metal fuel assemblies as fresh fuels at fuel exchange, into metal fuel core configuration conforming to the design requirements. Before this study, some works for the ARR1 for closed fuel cycle have performed. That is to say, it was designed stressing the harmonization of the safety design requirements and the technical feasibility together with the TRU burning capability. It has been presented on ARWIF 2008 in Fukui of Japan (3) that the parametric study was conducted to optimize dimensions and specifications of oxide fueled core and to confirm the recycling-ability of TRU in the ARR1. The accommodation of several kinds of TRU has been discussed at ICAPP 2008 in Anaheim, CA of the USA; LWR-UO 2 used fuel, TRU after long storage used fuel, mixed (Pu, U) oxide used fuel in LWR, recycled TRU in ARR (4). For the enhancement of TRU burning capability and the increase of minor actinide (MA) transmutation capability, the subsequent work has relaxed the design requirements and minimized the limitations, expecting future achievements in the related field of technologies. As a result, the TRU burning capability has been boosted by 50% and the Am transmutation capability of blanket has also been raised up to 48 kg/tweh (5). At first, Section 2 shows issues of fuel for fast reactor. Then methodology is explained in Section 3, design condition in Section 4, definition in Section 5, specification and properties of both cores and transitional cores in Section 6. Basic thought of duel fueled core, and so on are discussed in Section 7. Finally conclusions are remarked. Nomenclature ABTR Advanced Burner Test Reactor ARR Advanced Recycling Reactor ARR1 ARR s first unit CRF Consolidated Recycling Facility C.R. conversion ratio DOE Department of Energy HM Heavy Metal I.C. Inner Core JSFR Japan Sodium cooled Fast Reactor MA Minor Actinide MHI Mitsubishi Heavy Industries, Ltd. O.C. Outer Core UNF Used Nuclear Fuel TRU transuranics 2. Feasibility Issue of Core Accommodating Oxide Fuel and Metal Fuel In this study, it is assumed that a dual fueled core is feasible under the following conditions. Both oxide and metal fuel subassemblies have the same configurations so as to be 235

3 accommodable in ARR1. Three type cores; oxide, metal and transitional core, satisfy the criticality at normal operation and the safety requirements, which are assumed to be identical to Japan Sodium cooled Fast Reactor (JSFR). The sodium void reactivity (defined in active core) is set to $6 or less (oxide), $8 or less (metal). In the event of one-rod stuck at any accident, the control rods of one system (primary shutdown system) can be designed to attain a cold shutdown status and another system (back-up shutdown system) can attain a hot stand-by status. The reactor shutdown margin is provided $1 (0.3 to 0.4 % k). The maximum linear heat rate shall be set considering the Am contents (430 W/cm for several percentages of Am, 350 W/cm for Am 20%). A special concern in this study is an extraordinary power peak in the transitional core mixed with oxide and metal fuels in the case that the Pu enrichments of both fuels will be determined so that the power distribution becomes flat in each equilibrium core and the maximum linear heat rate is not adjusted in the transitional core. The TRU conversion ratio and the TRU burning capability should be enhanced, comparing with the previous design, where the TRU conversion ratio was 0.50 to 0.55 and the TRU burning capability was 45 to 51 kg/tweh, depending on TRU isotope ratios. Actually, a targeted concept in this study is enhanced the TRU burning and the Am transmutation. 3. Calculation Methodology The used nuclear methodology is based on the core design of JSFR. The calculations have been conducted; using 70-group core constants JFS3-J3.3 made from the nuclear data file JENDL-3.3 (6), 3-D triangular mesh code TRISTAN (7), and perturbation code TRI-PERT. It has been checked that this methodology is valid because moderator shifts the neutron flux energy spectrum to lower energy side a little. TRISTAN is of a corner type having the calculation points at the vertices of the triangular prisms, different from the type having the mesh point at the centre. It has been verified that the calculated eigenvalue coincides within the difference of or less with CITATION (8) in terms of the value extrapolated to the infinite mesh number. Its mesh effect of calculated eigenvalue is about half as large as CITATION. 4. Design Conditions This study assumes the following conditions: The TRU constituents will be present in used nuclear fuel (UNF) discharged from the light water reactors with a burnup of 50 GWd/t; Np %/ Pu %/ Pu %/ Pu %/ Pu %/ Pu %/ Am %/ Am %/ Cm % (wt.). TRU nuclides to be included in the transmutation fuel will be Pu, Americium (Am) and Neptunium (Np), where Curium (Cm) is not considered in the calculations. The ratio of Pu to Np is the same as the UNF, while the ratio of Am to (Pu, Np) is variable. The amount of impurities is not taken into account in the calculations. The maximum fraction of TRU to heavy metal (HM) is limited to 50%. 5. TRU Burning Related Definition 5.1 TRU Conversion Ratio TRU conversion ratio (TRU C.R.) was defined by INL as follows (9). 236

4 TRU C.R. = (RHM RTRU)/RHM (1) where RHM: mass consumption of HM between beginning and end in life, RTRU: mass consumption of TRU between beginning and end in life. 5.2 TRU Burning Capability TRU burning capability is defined as follows. TRU burning capability = (TRU-D TRU-P)/EL-P (2) where TRU-D: destructed TRU (kg), TRU-P: produced TRU (kg), EL-P: produced electricity (TWeh). 5.3 Am Transmutation Capability Am transmutation capability including transmutation to other elements is defined as follows. Am transmutation capability = (AM-D AM-P)/ EL-P (3) where AM-D: destructed or transmuted Am (kg), including decay and capture reaction to the other elements, AM-P: produced Am (kg). 6. Specification of Duel Fueled Core and Properties In this study, in the first place, both of oxide fuel and metal fuel are designed to have common outer dimensions to enable core where either fuel is usable. Item ARR-1 Electric power 500 MWe Thermal power 1180 MWt Fuel type Oxide Av. burn-up 150 GWd/t Cycle length 720 days Max LHR <430 W/cm Void Reactivity <$ 6 Fig. 1 Core concept of configuration of ARR1 6.1 Specifications of Core and Fuel The designed ARR1 is a 500 MWe, 1180 MWth oxide fueled and sodium cooled reactor, with the 700 mm core height, and the enhanced volume fraction of structures (cladding tubes thickness: 1.0 mm), while the moderator pins share 12% in pins of assembly (Fig. 1 and Table 1). The same core height, the same burn-up, the fuel assemblies with the same heavy metal inventory, and the same external dimensions of subassembly and fuel element are set. The inner diameter of pins has been arranged to attain smear density of 80 %TD for oxide fuel, and 75 %TD for metal fuel as shown in Fig. 2. The position of plenum is located in the upper part taking sodium-bonded metal fuels into consideration. 237

5 By these arrangements, the TRU enrichments are enabling the assurance of almost the same criticality at the end of the cycle. This design is considered advantageous in ensuring the interchange-ability. Item Coolant Fuel and Type Effective Height Table 1 Calculated specifications of ARR1 Specification Sodium Oxide and annular 700 mm No. of Assembly. 272/342 ( /66) *1 Moderator B 4C(Enriched B-11) No. of Pins (moderator/fuel) 40/291 /Subassembly No. of PCR 31 No. of BCR 6 Fuel exchange 3 batches Avg. burn-up 150 GWd/t *1: (inner core + outer core + Am blanket) Oxide fuel Metal fuel Pellet Cladding Cladding Smear Density 80 %TD 75 %TD Fig. 2 Dimensions of oxide fuel and metal fuel element [mm] Number indicates batches for fuel exchange. Fig. 3 Three batches for fuel exchange 238

6 The fuel batches are set as shown in Fig. 3. In the transitional core, the metal fuel subassemblies are loaded as reload fuels on the oxide fuel equilibrium core. After the first fuel exchange, the metal fuel comprises 1/3, and the oxide fuel comprises 2/3 of the core. After the next fuel exchange, the metal fuel comprises 2/3, and the oxide fuel comprises 1/3, respectively. 6.2 Properties in Equilibrium Cores and Transitional Cores Both of the oxide fuel core and the metal core are designed as shown in Table 2; these cores satisfy the design requirements with the sodium void reactivity of active core of $6 or less (oxide), and $8 or less (metal). The core shutdown margin of 1.9 % k/kk for the primary shutdown system and 0.7 % k/kk for the back-up shutdown system has been secured at the oxide fuel core, and superior values are secured for the metal fuel core. The maximum linear heat rate has been 360 W/cm or lower. That is to say, the maximum linear heat rate of the metal core is lower by about 1% to 354 W/cm and does not exceed the value of the oxide fuel equilibrium core. Table 2 Core characteristics of oxide core and metal core with moderator CORE Oxide fuel Metal fuel Difference Vol. Fraction (Vf/ Vs/ Vc) *1 (%) 37.0/33.9/ /42.5/36.5 *4 - Pu Enrichments I.C. wt% 32.7% 32.6% 0.3% O.C. wt% 38.9% 39.2% 0.8% MA fraction I.C. wt% 17.3% 17.4% 0.6% O.C. wt% 11.2% 10.8% 3.6% Reactivity Loss Over Cycle k/kk' 5.8% 6.2% 6.9% TRU transmutation capability (kg/tweh) % TRU C.R % Average burnup (GWd/t) % Doppler Coefficient *2 Tdk/dT % Sodium Void Reactivity *3 ($) % Delayed Neutron Fraction * % *1 fuel/structure/void/coolant, Vf is defined by inner diameter of cladding, *2 at EOEC, *3 Active Core at EOEC, *4 including bond material Maximum Linear Heat Rate (W/cm) MOX MOX/Metal 2/1 1/2 Inner core Outer core No. of Cycle Fig. 4 Maximum linear heat rate among oxide core, transitional core and metal core Metal The simulation calculation has been conducted in 12 cycles to confirm successful fuel exchange. In the 4 th cycle that is the first transitional core, which the metal fuel comprises 1/3, and the oxide fuel comprises 2/3 and the maximum linear heat rate at the inner core significantly decreases by 10 W/cm and by 3 W/cm at the outer core as shown in Fig. 4. In the 6 th cycle, all fuels have been changed to metal, and in the 7 th cycle, the maximum linear heat rates are equalized to the equilibrium cores between inner and outer cores at

7 W/cm. The both distributions of the maximum linear heat rates are as shown in Fig. 5. Oxide MLHR:357W/cm Metal MLHR:352W/cm W/cm Fig. 5 Distribution of maximum linear heat rate over fuel life of oxide core and metal core The TRU conversion ratios are 0.33 for the equilibrium oxide core and 0.30 for the metal as shown in Table 2, while the comparison of TRU burning capability shows that the metal fuel increases to 69.1 kg/tweh and the oxide fuel remains 66.6 kg/tweh as shown in Fig. 6. The Am transmutation capability becomes 5% lower than the oxide core. That is, the oxide core shows 29.3 kg/tweh, while the metal core decreases to 27.8 kg/tweh. The net Pu consumption is calculated to be 10% greater, while the Cm production amount becomes 6% smaller, compared with the oxide core. With this, it seems natural to conclude that the presented dual type core is feasible, satisfying the conditions in Section 2. TRU burning capability (kg/tweh) Am Np Pu Cm Oxide Metal 5.1 Fig. 6 TRU burning capability of oxide core and metal core 7. Discussion 7.1 Basic Thought of Design in Dual Fueled Core Figure 7 indicates a basic thought in this design study of dual fueled core. The concept of this study is a dual fueled fast reactor core with small TRU C.R. and a smooth transition from oxide to metal core. Oxide and metal fuel should be able to be accommodated in a single core, satisfying the safety requirements. The deployment of moderator pins is found to be effective for the reduction of TRU C.R. and the void reactivity (5) by the spectrum shift in neutron energy. Another issue in the transition core by fuel exchange will be a power peak owing to 240

8 mixed arrangement of different kinds of fuel. The core characteristics will be determined from the core configuration consisted of dimension, composition and arrangement. That is, two cores; oxide and metal cores, will have similar core characteristics when they have similar core configurations. In the same manner, in the transitional core that has different kinds of fuel, two fuels have similar characteristics and the accommodation of oxide and metal fuels is feasible, when there are totally similar configurations in both fuel subassemblies. Concept Dual Fueled Core with Oxide and Metal Fast Reactor Core with Small TRU C.R. Smooth Transition from Oxide to Metal Core Issue Compatibility of Oxide and Metal Fuel Safety Requirements: Void Reactivity etc. Transition by Fuel Exchange and Power Peak Issue Design Same Configuration of Oxide and Metal Fuel Moderator Pins and Enhanced Cladding Thickness More Enhanced Cladding Thickness in Metal Reactor Physics Evenness in Geometry of Dual Fuels Spectrum Shift in Neutron Energy Evenness in Neutron Reactions in Dual Fuels Characteristics Same Enrichment and Similar Characteristics in Oxide and Metal Cores Smaller TRU C.R. and Satisfying Requirements, Transitional Core without No Extraordinary Power Peak by Ordinary Fuel Exchange Fig. 7 Basic thought in the dual fueled core design In this study, the cladding thickness is designed to increase in metal fuel pin because metal fuel pellet has smaller number of nuclides to moderate neutron than oxide fuel and enhanced irons will compensate this difference partially. That is to say, increasing the cladding thickness will reduce differences in the configuration and in the moderator powers between oxide and metal pins. From the calculations, it is found that they have similar Pu enrichments and there is no extraordinary power peak in the transitional core. 7.2 Flat Power Distribution of Transitional Cores Why does the maximum linear heat rate in the inner core decrease in the transitional cores? The Pu enrichment of the oxide core at the inner core is 0.1% greater in the metal core, while at the outer core 0.3% greater in the metal core. The maximum linear heat rate of the inner core has showed 3% lowering in the 4 th cycle. The first reason can be that the metal fuel is hard to decelerate neutrons so that the neutrons do not tend to be absorbed in each assembly, thereby allowing the neutrons to leak into the neighboring assemblies. For this reason, the metal fuel of fresh fuel has showed relative decrease compared with the oxide fuel by 2%. The other reason is that the metal fuel equalizes globally the power distribution because its neutron spectra are harder and the mean free path of fast neutron is longer. This effect lowers the maximum linear heat rate by 1%, compared with the previous oxide core. 241

9 7.3 Different properties of Oxide Fuel and Metal Fuel Why is the TRU conversion ratio of the metal fuel core smaller? Although the TRU enrichments of fresh fuel are the same 50%, the Pu inventory of core as a whole has the small difference of 0.05% between oxide fuel core and metal fuel core, the TRU conversion ratio shows the difference of 0.33 (oxide) and 0.30 (metal). The trend has been also confirmed without the moderator pins that TRU C.R. are (oxide) and (metal) as shown in Table 3. Table 3 Core characteristics of oxide core and metal core without moderator CORE Oxide fuel Metal fuel Difference Vol. Fraction (Vf/Vs/Vc) (%) 35.9/31.6/ /40.1/ Pu Enrichments I.C. wt% 25.8% 24.4% 5.6% O.C. wt% 31.0% 32.9% 6.2% MA fraction I.C. wt% 3.6% 3.4% 5.6% O.C. wt% 4.3% 4. 8% 6.2% Reactivity Loss Over Cycle k/kk' 6.34% 6.26% 1.2% TRU C.R % Average Burnup (GWd/t) % This can be attributable to the difference of relative reaction rate caused by the difference of neutron spectra as Fig. 8. Table 4 indicates that the U-238 capture reaction rate will decrease in the metal core. By the same reason, the fission and capture reaction rates of Am-241, Am-242m, and Am-243 become 11 to 16% smaller in the metal fuel core, even if the inventory are the same. On the other hand, the Am transmutation capability will be greater in the oxide fuel. 1.0E E+02 Flux (relative) 1.0E E E-03 Oxide core Metal core 1.00E E E-01 Cross Section (barn) U-238 capture Pu-239 fission 1.0E E E E E E E E+07 Energy [ev] Fig. 8 Fission of Pu-239, capture of U-238, and flux spectra between oxide core and metal core. Table 4 One group cross sections (barn) Oxide Metal Difference Pu-239 f % Pu-239 c 4.82E E % U-238 f 4.95E E % U-238 c 2.75E E % f: fission reaction, c: capture reaction. 242

10 8. Conclusions As a core concept of ARR1, the presented dual type core is confirmed as follows, (1) The duel fueled core is designed that the moderator pins are deployed and the cladding thickness of oxide fuel pins is set to 1.0mm and it is increased to 1.5 mm in metal pins. The objective of this concept is a dual fueled fast reactor core with small TRU C.R. and smooth transition from oxide to metal core by the evenness in geometry and neutron reactions of dual fuels and the spectrum shift in neutron energy. (2) By this neutron spectrum shift by the enhanced cladding thickness and the deployment of moderator pins, the smaller TRU C.R. and the acceptable void reactivity becomes feasible. The oxide fuels and metal fuels can be made easily changeable by providing the fuel pins having the same outer diameter and the same geometry of assembly, and beside, it has been confirmed that the design requirements have been met and the difference in properties have been not significant except the Doppler coefficient. It has been also concluded through the simulation analysis along the replacement of fuels that it is easy to transform the oxide fuel core of ARR1 to the metal fuel core observing the design requirements. (3) It has been found that the U-238 capture reaction tends harder to occur in the metal core under the same enrichment, with resultant lowering of the TRU conversion ratio, compared with the oxide fuel core. This is attributed to the fact that oxygen softens the neutron flux spectrum in energy at the oxide core compared with the metal core. Acknowledgments We will express our thanks for cooperation to this study by members of the technical team in AREVA Federal Services LLC, AREVA NP Inc., AREVA SAS, Mitsubishi FBR Systems, Inc. and Mitsubishi Heavy Industries, Ltd. Also, we thank Mr. Hiroyuki Moriwaki for his contribution in the calculation works, Mr. Denis Verrier for some comments, Dr. Dorothy R. Davidson for encouragement and leadership. This material is based upon work supported by the Department of Energy under Award Number DE-FCO1-07NE References (1) DOE, Office of Nuclear Energy & Office of Fuel Cycle Management, GNEP-16732, Global Nuclear Energy Partnership Strategic Plan, (2006). (2) W. S. Yang, T. K. Kim, and R. N. Hill, Core Design Studies for Advanced Burner Test Core, Proc. of ICAPP2007, paper7263, Nice, France (2007) (3) K. Ikeda, K. Stein, W. Nakazato, and M. Mito, Core Design of Advanced Fast Reactor toward Reduction of Environmental Burden, submitting for ARWIF2008, Tsuruga, Japan (2008) (4) K. Ikeda, K. Stein, W. Nakazato and M. Mito, Global Cooperation and Conceptual Design ARR and Recycling Strategy, Proc. of ICAPP 2008, paper8379, Anaheim, CA, USA (2008). (5) K. Ikeda, J. W. Maddox, W. Nakazato and S. Kunishima, Global Cooperation and Conceptual Design toward GNEP/ Enhanced TRU Burning Fast Reactor, Proc. of PBNC16, paper1296,aomori, Japan (2008).- (6) K. Shibata, et al, Japanese Evaluated Nuclear Data Library Version 3 Revision-3: JENDL-3.3, Journal Nucl. Sci. Technol. Vol. 39, 1125, (2002). (7) K. Ikeda, H. Moriwaki, W. Nakazato, Nuclear, Calculation Methodology and Development of 3-D Transport Nuclear Design Code, submitting for FR 09, Kyoto, Japan (2009). (8) ORNL-TM-2496, Rev.2: The CITATION Code Report Distribution, (1971) (9) R. Ferrer, M. Asgari, S. Bays, B. Forget, Computational Neutronics Methods and Transmutation Performance Analyses for Fast Reactor, INL/EXT , (2007) 243