GENES4/ANP2003, Sep. 15-19, 2003, Kyoto, JAPAN Paper 1018 Full MOX Core Design in ABWR Toshiteru Ihara1*, Takaaki Mochida2, Sadayuki Izutsu3 and Shingo Fujimaki3 1Nuclear Power Department, Electric Power Development Co., Ltd., Tokyo, 104-8165, Japan 2Nuclear Plant Engineering Department, Hitachi, Ltd., Hitachi, Ibaraki, 319-1188, Japan 3Core Design Group, Global Nuclear Fuel Japan Co., Ltd., Yokosuka, Kanagawa, 239-0836, Japan Electric Power Development Co., Ltd. (EPDC) has been investigating an ABWR plant for construction at Oma-machi in Aomori Prefecture. The reactor, termed FULL MOX-ABWR will have its reactor core eventually loaded entirely with mixed-oxide (MOX) fuel. Extended use of MOX fuel in the plant is expected to play important roles in the country’s nuclear fuel recycling policy. MOX fuel bundles will initially be loaded only to less than one-third of the reactor, but will be increased to cover its entire core eventually. The number of MOX fuel bundles in the core thus varies anywhere from 0 to 264 for the initial cycle and, 0 to 872 for equilibrium cycles. The safety design of the FULL MOX-ABWR briefly stated next considers any probable MOX loading combinations out of such MOX bundle usage scheme, starting from full UO2 to full MOX cores. KEYWORDS: Full MOX, ABWR, Core Design I. Introduction Table 2 respectively. The core design from full UO2 to full MOX loaded of (1) The MOX bundle uses the well-proven design of STEP-2 1) 2) 3) ABWR has been performed. The MOX fuel is 8x8 UO2 bundle (50GWd/t maximum exposure) having much bundle configuration with a large central water rod, with operational experience. The bundle features a large water 40GWd/t maximum burnup, and it is compatible with 9x9 rod in the center of 8x8 fuel rod configuration, as shown high burnup UO2 fuel. The shutdown and thermal margins of in Fig.1. the MOX core, from partially- to fully- loaded, are (2) The MOX bundle maximum exposures conservatively comparable to that of full UO2 core. Safety analyses based are selected to be 40 GWd/t based on the MOX on MOX loaded core characteristics and MOX fuel property irradiation experience. This maximum exposure is equal have shown its conformity to the design criteria in Japan. to that of STEP-I UO2 bundle. This paper shows the safety design for full MOX cores of (3) The MOX fuel bundle consists of MOX fuel rods using ABWR. UO2-PuO2 and UO2 fuel rods using UO2-Gd2O3 as a fuel material. The bundle average fissile material content is II. Full MOX Core selected to be about 2.9wt% of fissile plutonium (Puf) 1. Design Concept content and about 1.2wt% of U-235 enrichment for the One of the ABWR core features is a wider fuel bundle conditions of 13-month cycle length and the reference pitch. Specifications of fuel bundle are the same as those of plutonium composition (67 wt% Puf ratio). the current BWR lattice. As a result, the non-boiling water area outside the channel box (bypass flow area) increases in the ABWR core. The bypass flow area can thermalize Table 1 Basic specification of core design neutrons more effectively before absorption by the fuel Items Specification material. The wider fuel pitch of the ABWR core decreases Core the absolute value of void reactivity coefficient and Type Advanced Boiling Water increases the shutdown margin, which collectively makes Reactor ABWR well-adopted for loading its core fully with MOX (ABWR) fuel. Thermal power (MW) 3,926 The main specifications are not changed from those of the Core flow (t/h) 52.2x103 (100%rated) standard ABWR, such as 3926MWt thermal power, 872 fuel Core pressure (MPa[abs]) 7.17 bundles and 205 control rods. The main design concepts of Number of fuel bundles 872 MOX fuel are the following, and the basic specifications of Number of control rods 205 core are shown in Table 1 and those of fuel are shown in Fuel bundle pitch (cm) 15.5 * Corresponding author, Tel. +81-3-3546-2211, Fax. +81-3-3546- 2805, E-mail: [email protected] Table 2 Basic specification of fuel design lifetime. Also, though MOX fuel tends to become higher rod internal pressure owing to increased FP gas and He gas Items Specification release, the rod internal pressure remains still equivalent to Fuel assembly 9x9 UO fuel rod at the end of lifetime due to increased gas Lattice configuration 2 plenum volume for MOX rod. The rod internal pressure of MOX fuel 8x8 the MOX fuel is shown in comparison with 9x9 UO2 fuel in UO2 fuel (STEP-III 9x9) 9x9 Average 235U content *1(wt%) Fig.2. MOX fuel 1.2 The neutron multiplication factor (k-infinity) of MOX fuel is shown in Fig.3 in comparison with 9x9 UO2 fuel. UO2 fuel (STEP-III 9x9) 3.8 Average Puf content *1(wt%) The change in k-infinity with exposure is smaller for MOX MOX fuel 2.9 fuel than that for UO2 fuel. This, in turn, leads to a smaller UO2 fuel (STEP-III 9x9) - bundle peaking of full MOX core than full UO2 core and 1/3 Maximum exposure (MWd/t) MOX core (i.e. consist of 512 9x9 UO2 fuels and 360 MOX MOX fuel 40,000 fuels). This brings about the decrease of the maximum UO2 fuel (STEP-III 9x9) 55,000 control rod worth and the mitigation of the shutdown margin Batch averaged exposure (MWd/t) decrease for full MOX core. MOX fuel 33,000 Figure 4 shows the fuel loading pattern of full MOX UO2 fuel (STEP-III 9x9) 45,000 equilibrium core. Shutdown margin, Maximum Linear Heat Number of fuel rods Generation Rate (MLHGR), and Minimum Critical Power MOX fuel 60 Ratio (MCPR) for the equilibrium cycle are shown in Fig.5 UO fuel (STEP-III 9x9) 74 *3 2 the comparison among full UO2 core, 1/3 MOX core and full Pellet material MOX core. MOX fuel UO2-PuO2(MOX rods) The shutdown and thermal margins of the MOX core, UO2-Gd2O3(UO2 rods) from partially- to fully- loaded, are comparable to that of full UO2 fuel (STEP-III 9x9) UO2,UO2-Gd2O3 UO2 core. These results satisfy the design targets and the Cladding outside diameter (mm) operational limits. 12.3 MOX fuel 11.2 UO2 fuel (STEP-III 9x9) Control rod Cladding wall thickness *2(mm) MOX fuel 0.86 43222234 UO2 fuel (STEP-III 9x9) 0.71 Cladding Material 31113 MOX fuel Zirc-2(with Zr liner) 211 1112 UO2 fuel (STEP-III 9x9) Zirc-2(with Zr liner) Number of water rods 2 112 MOX fuel 1 w 2 12 UO2 fuel (STEP-III 9x9) 2 1 Water rod outside diameter (mm) 2111 112 34.0 MOX fuel 311 13 UO2 fuel (STEP-III 9x9) 24.9 43222234 *1 For reload fuel W ： Water rod *2 Including Zirconium liner thickness of about 0.1mm 1 ：4 MOX fuel rod *3 Including 8 partial length rods (Pu content 4<3<2<1) ： UO2 fuel rod containing Gd2O3 2. Fuel and Core Design Rod arrangement of the MOX fuel bundle is shown in Fig.1 Rod arrangement of MOX fuel bundle Fig.1. Of the 60 fuel rods, 48 contain MOX and the rest UO2 bearing Gd2O3. Four kinds of MOX rods with different plutonium contents in the bundle are employed in order to reduce the local peaking. Although MOX fuel tends to become higher pellet temperature owing to lower thermal conductivity and increased FP gas release compared with UO2 fuel, the pellet center temperature remains sufficiently lower against the fuel melting temperature through the 6 8.0 ) 80 k Full MOX fuel core Coolant pressure △ 5 9X9 UO2 fuel core % ( 4 1/3 MOX fuel core in 6.0 g 60 3 2 a) 2 Design target 4.0 40 1 (kg/cm 0 Shutdown mar Shutdown 01234567891011 Cycle exposure (GWd/t) 2.0 MOX fuel 20 9X9 UO2 fuel (a) Shutdown margin Fuel rodFuel internal pressure (MPa[abs]) 0.0 0 0 20406080 50 Full MOX fuel core Peak pellet exposure (GWd/t) Operating limit 45 9X9 UO2 fuel core Fig.2 Fuel rod internal pressure 1/3 MOX fuel core 40 1.3 35 1.2 Hot operation 30 MLHGR (kW/m) 25 1.1 01234567891011 1.0 Cycle exposure (GWd/t) (b) MLHGR 0.9 MOX fuel bundle 0.8 9X9 UO2 fuel bundle 1.9 Full MOX fuel core 1.8 Neutron multiplication factor multiplication Neutron 0.7 9X9 UO2 fuel core 1.7 1/3 MOX fuel core 0 1020304050 1.6 Exposure (GWd/t) 1.5 Fig.3 Neutron multiplication factor MCPR 1.4 Operating limit Full MOX 1.3 1.2 1/3 MOX 1.1 01234567891011 Cycle exposure (GWd/t) (c) MCPR Fig.5 Comparison of core performance in equilibrium 3. Characteristics of MOX core 3.1 Reactivity coefficient and dynamic parameter The MOX core is characterized by an increase in the absolute void coefficient due to large neutron absorption cross section of plutonium relative to uranium. Void MOX fuel coefficient, Doppler coefficient, delayed neutron fraction Fresh reload bundle and prompt neutron lifetime change depending on the MOX Bundle in 2nd cycle of operation fuel loading fraction (Fig.6). While the void coefficient of Bundle in 3rd cycle of operation the full MOX core is about 20% larger in absolute value Bundle in 4th cycle of operation than the full UO2 core, the delayed neutron fraction of the Fig.4 Fuel loading pattern of full MOX equilibrium cycle full MOX core is about 20% smaller than the full UO2 core.