MA/LLFP Transmutation Experiment Options in the Future Monju Core

Transcription

1 MA/LLFP Transmutation Experiment Options in the Future Monju Core Akihiro KITANO 1, Hiroshi NISHI 1*, Junichi ISHIBASHI 1 and Mitsuaki YAMAOKA 2 1 International Cooperation and Technology Development Center, Japan Nuclear Cycle Development Institute, Tsuruga, Fukui, , Japan 2 Power and Industrial Systems Research and Development Center, Toshiba Co., Kawasaki, Kanagawa, , Japan Future experimental irradiation test options for the minor actinide (MA) and long-lived fission product (LLFP) transmutation in Monju core have been studied to search for and to demonstrate the possible contribution of Monju to the future commercialization of FBR technology. It was shown that MA incineration rate for 148 EFPDs by 5 cycles operation was evaluated to be 26% to 32% for the core region loading case and 13% for the radial blanket region loading case. As regards LLFP irradiation test, the transmutation rate was evaluated to be 0.7%/year for without-moderator case and 1.2%/year to 1.4%/year for with-moderator case. These results showed the capability of Monju core for profitable experimental data acquisition, without any significant disturbance on the core characteristics with some slight design modifications. KEYWORDS: fast reactors, minor actinides, long-lived fission products, irradiation experiment, transmutation I. Introduction Deposit of high-level radioactive wastes (HLW) is indispensable to nuclear fuel cycle. Reduction of environmental burden can be achieved by reducing the radioactive hazard of the minor actinides (MAs: Np, Am, Cm) and the long-lived fission products (LLFPs: 99 Tc, 129 I, 135 Cs and so on), contained in the HLW as a result of nuclear power generation. It was known that the radioactive hazard of the HLW would be decreased to that of the original natural uranium ore after several hundreds years assuming that all the MAs and LLFPs are separated from the HLW and transmuted into short-lived radioactive nuclides or stable nuclides in the nuclear reactors. Transmutation of MA/LLFP can be efficiently and effectively achieved by the fast neutron spectrum system because of its high neutron flux level and its neutron surplus available for transmutation when compared with the thermal or other spectrum systems. Much larger fission-to-capture cross-section ratio of MAs in the fast spectrum system substantially makes it as the almost only effective way of MA transmutation 1-4). Negligibly small contribution of thermal neutrons in the active core region in the fast spectrum system also enables effective MA transmutation, especially with large amount of core loading. Fast reactors can offer wider range of neutron spectrum and higher level of neutron flux than that of LWRs. In the MA/LLFP separation and transmutation studies, fast reactor has been and still is considered to be one of the most feasible and promising candidates 5). MA/LLFP transmutation experiment options in the future Monju core are to be proposed based on the above mentioned fundamental considerations in this study. * Corresponding author, Tel , FAX , nishi@t-hq.jnc.go.jp are to be proposed based on the above mentioned fundamental considerations in this study. Investigation of the MA transmutation experiment has been performed based on the preceding studies in this field. Reactor design studies on MA transmutation in FBR cores have revealed that it reduces burn-up reactivity swing while increasing the sodium void reactivity and reducing the Doppler coefficient, which needs some safety considerations. Because of these safety considerations the maximum ratio of MA mixture in the core fuel is said to be limited below 5% and to be allowed up to 10% with some penalty on core characteristics. Taking into account the influence of the MA-mixed test fuel assembly loading on the core characteristics, both the nine test assemblies loaded in the core region case and the eighteen assemblies loaded in the radial blanket region case were studied. And the radial blanket region loading case was evaluated to be preferable to avoid any significant disturbance on the core characteristics. As regards LLFP transmutation, 99 Tc was selected as the typical target nuclide to be irradiated 6) because of its large neutron absorption cross section, chemically inert form in the reactor core as a metal and no need for isotopic separation. LLFP loading affects the core characteristics as well as MA loading. Fifty-four test assemblies, at the maximum, loaded in the radial blanket region case was studied. Zirconium-hydride (ZrH 1.7 ; The hydrogen ratio is an assumed value) was partially loaded in the test assembly as the neutron spectrum moderator. This maximum number of assemblies was assumed in order to evaluate the allowable number of loaded assemblies. The results showed the capability of Monju core for profitable experimental data acquisition, without any significant disturbance on the core characteristics.

2 Monju reference core layout Core layout for MA transmutation experiment (Homogeneous loading type) Core layout for MA transmutation experiment (Heterogeneous loading type) Core layout for LLFP transmutation experiment Inner core fuel assembly Outer core fuel assembly Radial blanket fuel assembly Control rod (Primary) Control rod (Back up) Irradiation test rig (MA) Irradiation test rig (LLFP) Neutron source assembly Fig. 1 Core layout of Monju

3 II. Prototype Fast Breeder Reactor Monju Experimental irradiation of MA/LLFP transmutation was assumed in the future Monju core in this study. Monju is the prototype fast breeder reactor in our country with an electric power of 280MWe (714MWt), a sodium-cooled loop-type reactor. The Monju core consists of 715 assemblies (198 core fuel assemblies, 172 radial blanket fuel assemblies, 19 control rod assemblies, etc.) and is fueled with UO 2 -PuO 2 mixed oxide fuel. The core layout of Monju is shown in Fig. 1 and the fundamental core specifications are shown in Table 1. Figure 2 shows the side view of Monju core. Monju has been shut down since December 8, 1995 when the sodium leak accident occurred in the secondary heat transport system. The Role of Monju was clearly redefined in the "Long-Term Program for Research, Development and Utilization of Nuclear Energy, revised by Atomic Energy Commission in November 2000, after the accident. Monju can be considered as one of the assets of the humankind, which can demonstrate the prominent FBR characteristics in the nearly commercialized scale. The confirmation of the fundamental performances, such as breeding ratio, etc. has been and continues to be its mission. This is essential for the prototype reactor. Monju should be restarted at the earliest stage possible, and the sodium handling technology should be established for that purpose. The demonstration of the reliability as a power station should be pursued simultaneously. The earliest demonstration of the world s most advanced technology will be the next mission. Adjustment of the plutonium stockpile and incineration of trans-uranics, etc., are to be demonstrated as pursued in the Feasibility Study on Commercialization of FBR Cycle Technology, conducted by JNC. The possibility of Monju, to contribute to the future commercialization of FBR technology, should be pursued and demonstrated. Accordingly, the irradiation experiments of Fig Upper Axial Blanket Inner Core Outer Core Lower Axial Blanket Radial Blanket Monju core side view Unit : mm MA/LLFP transmutation are to be performed just after the confirmation of the current core characteristics. III. Analytical Calculations and Results MA and LLFP transmutation irradiation experiments were studied individually because the disturbances, to be estimated, on the core characteristics of the MA and LLFP loading are different depending on the irradiation Table 1 Monju core specifications Reactor type Sodium-Cooled Loop-Type Thermal power 714MW Electrical power 280MW Operational cycle length 148 days/cycle Core dimensions Equivalent diameter 179cm Height 93cm Burn-up reactivity swing 2.5 % k/kk' Core fuel burn-up (GWd/t) Average 80 Assembly maximum 94 Maximum linear heat rate 360 W/cm Fuel No. of driver assemblies 198 Fuel type UO 2 -PuO 2 Plutonium enrichment (Inner core/ Outer core) Initial core 15/20 Pu fissile % Equilibrium core 16/21 Pu fissile % Fuel inventory Core (U+Pu metal) 5.9ton Blanket (U metal) 17.5ton Cladding outer diameter 0.65cm Cladding thickness 0.047cm Cladding material SUS316 No. of pins per assembly 169 Duct flat-to-flat 11.1cm Duct pitch 11.6cm Blanket Material UO 2 Axial blanket Top length 30cm Bottom length 35cm Radial blanket No. of assemblies 172 Pellet diameter 1.04cm Pin outer diameter 1.16cm No. of pins per assembly 61

4 experiment concepts. Core characteristics were evaluated by two-dimensional R-Z 7 energy groups diffusion calculations taking burn-up effect into account. Seven energy group cross sections were derived from the 70 energy group cross section set JFS-3-J3.2 7) generated from the JENDL-3.2 library 8) by collapsing. The energy group structure adopted is shown in Table 2. The cross sections of the rare earth metals, which were considered to be inevitably mixed up with MAs up to 20%, were synthesized from the individual cross sections as a lumped FP to preserve the total neutron absorption cross section. 1. MA Transmutation Experiment MAs can be incinerated more efficiently in fast spectrum than thermal spectrum because MA nuclides have some threshold reaction and much larger fission-to-capture cross-section ratio in the fast neutron spectrum. Following two concepts of irradiation experiment were assumed for the MA transmutation in this study. The one is MOX fuel mixed with MAs, which is called Homogeneous loading type, assuming the future homogeneous whole core loading of MAs. Rare earth metals were assumed to be included in MAs up to 20%, which were considered to be difficult to separate from MAs completely in the reprocessing procedure. The ratio of MA in the MOX fuel was assumed to be 5 or 10 wt% because MA mixture could cause a deterioration of material properties such as thermal conductivity or melting point. The maximum ratio of MA mixture in the core fuel is also limited to avoid any significant disturbance on the core characteristics. Table 2 7 energy groups structure Energy Group Energy Range MeV-1.35MeV MeV-388keV 3 388keV-86.5keV keV-9.12keV keV-961eV 6 961eV-101eV 7 101eV eV MA Nuclide Table 3 Composition (%) Np 42.7 Am 33.5 Cm 3.8 Rare earth* 20 Composition of MAs Isotopes Composition (%) 237 Np Am Am Am Cm Cm Cm Cm Cm 0.5 *Decontamination factor: La/52, Ce/72, Pr/28, Nd/46, Pm/20, Sm/20, Eu/10, Gd/10 Table 4 Effect of MA transmutation test rig loading on the core characteristics Item 5%MA in MOX fuel 10%MA in MOX fuel 30%MA mixed with MgO Number of test assemblies loaded MA transmutation rate (%/740EFPD) Total amount of transmuted MA (kg/740efpd) Reactivity change (% k/kk ) Burn-up reactivity swing* Coolant density coefficient* Doppler coefficient* Power peaking factor* * Normalized to the Monju reference core (1.0)

5 MA content (kg/assembly) MA content (kg/assembly) MA content (kg/assembly) Homogeneous loading type (5%MA) Homogeneous loading type (10%MA) in the material property aspects such as thermal conductivity or melting point, compared with the homogeneous loading type, while some disadvantage in the nuclear characteristics aspects, such as coolant density coefficient or Doppler reactivity feedback, should be mitigated. The MA assemblies of heterogeneous loading type were assumed to be loaded in the radial blanket region to avoid any significant disturbance on the core characteristics. The core layouts of each case are shown in Fig. 1. MA isotopic composition was estimated based on the LWR spent fuel composition of 33GWd/t burn-up, reprocessed after 5 years cooling-off interval from the discharge. In both cases, nine isotopes of MAs, shown in Table 3, were selected as the targeted nuclides to be transmuted. The irradiation interval was assumed to be 148 EFPDs by 5 cycles as same as the current driver fuels. The results of the core characteristics analysis are shown in Table 4. In case of homogeneous loading type, the 10% MA loading affected the core characteristics more than the 5% of MA loading by double. The resulting MA transmutation rate was evaluated to be 28% (5% MA loading case) to 32% (10% MA loading case). The Doppler coefficient was evaluated to be decreased by 7% and the coolant density coefficient be increased by 10% at the maximum. MAs are strong absorbers and fissile materials especially in the fast neutron spectrum, resulted in harder neutron spectrum. The control rod worth was estimated to be reduced by 1% due to the same reason. Special emphasize was put forward on the power peaking increase by 2% (5% MA loading case) to 4% (10% MA loading case). On the other hand, as for the heterogeneous loading type, the MA transmutation rate was evaluated to be 13%, while the influence of the MA irradiation assembly loading resulted in negligible core characteristics change, compared with the homogeneous loading type. So, it seems to be possible to load more than 18 assemblies without any significant disturbance on the core characteristics. Although the MA transmutation rate is not so large in this case, the heterogeneous loading type in the radial blanket region is preferable for large amount of MA loading. The details of MA content change by irradiation for each isotope are shown in Fig. 3. Fig. 3 Heterogeneous loading type (30%MA) MA content change by irradiation The other is MA diluted with MgO by 30%, which is called Heterogeneous loading type, assuming the future heterogeneous core loading of special MA incineration assemblies. Higher MA mixture ratio can be achieved in this case because this type of MA loading has an advantage 2. LLFP Transmutation Experiment As for the LLFP transmutation experiment, 99 Tc was selected as the typical targeted nuclide, which needs no isotopic separation because of its single isotopic composition. 99 Tc has relatively larger absorption cross section of approximately 1 barn under fast neutron spectrum and has stable chemical form in the reactor core as a metal, which is favorable for in-core irradiation. Experimental assemblies with both 99 Tc pins and moderator (ZrH 1.7 ) pins were assumed to be irradiated in the radial blanket region. The number of test rigs loaded was assumed to be 54 at the maximum. The ratio of the moderator pins was varied parametrically from 0% (without

6 100% Tc-99 pins 78.7% Tc-99 pins 60.7% Tc-99 pins ZrH 1.7 pin 99 Tc pin Fig. 4 Pin arrangement in the test rigs for 99 Tc irradiation experiments Absorption cross section (barn) Tc absorption cross section 2.0 Total flux Volume ratio of 99 Tc ; [ 99 Tc / ( 99 Tc + ZrH )] Total flux ( n/cm 2 /sec) Power density (W/cc) Reference 99 Tc 100% 99 Tc 60.7% Power peak Distance from center (cm) Fig. 5 Correlation of total flux and absorption cross section on the ratio of 99 Tc Fig. 6 Power distribution in the core for 99 Tc transmutation experiment Table 5 Effect of LLFP(Tc-99) transmutation test rig loading on the core characteristics Item 100% 99 Tc pins 78.7% 99 Tc pins 60.7% 99 Tc pins Number of test assemblies loaded LLFP transmutation rate (%/year) Amount of transmuted LLFP (kg/ year) Reactivity change (% k/kk ) Burn-up reactivity swing* Sodium void reactivity* * Normalized to the Monju reference core (1.0) moderator) to 40%, as shown in Fig. 4, to investigate the effect of the moderator on the transmutation rate and the possible amount of loaded 99 Tc. Core layout is shown in Fig. 1. The results of the core characteristics analysis are shown in Table 5. The transmutation rate was evaluated to be 0.7%/year for without-moderator case and 1.2%/year to 1.4%/year for with-moderator case. The reactivity was decreased by 3.3% k/kk in case of 99 Tc loading without moderator, while the reactivity decrease with moderator was

7 estimated to be 4.5% k/kk. Sodium void reactivity was reduced approximately by half compared with the reference core in case with moderator. The disturbance on the core characteristics seemed to be not negligible in this maximum loading case. The total amount of transmuted 99 Tc per year remained as the same level in both the with-moderator and the without-moderator cases, in spite the transmutation rate was improved in the with-moderator case. This comes from the fact that the increase of the moderator pins limits the number of 99 Tc pins resulting in decrease of the total amount of loaded 99 Tc. Figure 5 shows the correlation between the moderator ratio and the effective absorption cross section. The higher moderator ratio leads to softer neutron energy spectrum, and results in larger effective absorption cross section. On the other hand, in the with-moderator case, a steep power skew at the adjacent fuel assemblies was observed around the test rigs. Some design modification, such as local adjustment of plutonium enrichment, etc. might be necessary to mitigate this power skew, shown in Fig. 6. As a result, the number of test assemblies was found to be limited below one third or one sixth of the maximum loading case, for example, to avoid any significant disturbance on the core characteristics. IV. Conclusion 1. MA Transmutation Experiment In case of MA incineration experiment in the Monju core, nine test assemblies with MOX fuel mixed with 5% MA loaded in the core region seems to be feasible, taking into account the power peaking increase. In case of radial blanket region loading, over eighteen test assemblies with MA diluted with MgO by 30% seems to be feasible without any significant disturbance on the core characteristics. Certain transmutation rate of over 10% to 30% can be achieved in both cases. So the options can be chosen based on the purpose of the experiment. 2. LLFP Transmutation Experiment In case of LLFP ( 99 Tc) transmutation experiment in the Monju core, the number of test assemblies, fifty four at the maximum, loaded in the radial blanket region seems to be limited below one third or one sixth of the maximum loading case, for example, to avoid significant disturbance on the core characteristics. Some design modifications might be necessary to mitigate the steep power skew around the test rigs, especially in the with-moderator case. The irradiation experiment seems to be feasible under these conditions. Transmutation rate of approximately 0.7%/year to 1.4%/year can be obtained. This implies the possibility of profitable experiment in the Monju core for LLFP transmutation. The capability of the Monju core for MA/LLFP transmutation irradiation experiment has been confirmed, which contributes to the future commercialization of FBR technology. So, Monju should be restarted at the earliest stage possible not only for this purpose but also to establish the sodium handling technology and to demonstrate its reliability as a power station. References 1) T. Wakabayashi, et al., Nucl. Technol., 118, 14 (1997). 2) M. Yamaoka, M. Ishikawa, T. Wakabayashi, et al., Proc. Int. Fast Reactors and Related Fuel Cycles, FR 91, Kyoto, Vo. IV (1991). 3) M. Yamaoka, T. Wakabayashi, Proc. Int. Conf. Design and Safety of Advanced Nuclear Power Plants, ANP 92, Tokyo, Japan, I3.3-1 (1992). 4) K. Fujimura, et al., J. Nucl. Sci. Technol., 38[10], 879 (2001). 5) A. Mizutani., Trans. Am. Nucl. Soc., 81, 294 (1999). 6) M. Salvatores, I. Slessarev, A. Tchistiakov, et al., Nucl. Sci. Eng., 130, 309 (1998). 7) H. Takano, Proc Symp. on Nucl. Data, Tokai, Japan, JAERI-Conf , 47 (1995). 8) T. Nakagawa, et al., J. Nucl. Sci. Technol., 32, 1259 (1995).