Japan’s development of high burnup BWR fuel has been advanced by stages,

Step I, Step II, and Step III, as shown in Fig. 3.24, with each step confirming the

usage results.

Step I fuel has been used practically since 1987. It employs the zirconium liner fuel, while having the same structure as the previous 8 x 8 type lattice (8 x 8 RJ). By applying the uranium-saving technology using power peaking such as the installation of natural uranium blankets on the core upper and lower end parts, the discharge burnup for the same average enrichment was increased by about 10 % from about 29.5 GWd/t to 33.0 GWd/t and the fuel cycle cost was also reduced by about 10 %.

The high burnup 8 x 8 fuel of Step II has been used as a reload fuel since 1991. It has a higher enrichment for high burnup and economy. The average discharge burnup is 39.5 GWd/t and the maximum burnup of fuel assembly was increased to 50 GWd/t from 40 GWd/t of the Step I fuel. To suppress fuel temperature and rod internal pressure rise during reactor operation, the initial pressure of helium gas was increased from about 0.3 MPa to 0.5 MPa and the theoretical density of pellets was raised from about 95 % to 97 %. Four fuel rods in the center of the fuel assemblies were replaced with a large diameter water rod and the H/U ratio was increased to enhance neutron moderation efficiency. Ring-type spacers were used to improve the thermal margin (power limit) of fuel rods and upper tie plates were designed as a low pressure drop type with a small resistance against water flow.

The 9 x 9 fuel of Step III, which has been used on a full scale since 1999, gives a higher discharge burnup of 45 GWd/t by increasing fuel enrichment and a higher maximum burnup of fuel assemblies of 55 GWd/t. The change to the 9 x 9 arrangement of fuel rods, which increases the number of fuel rods per fuel assembly, reduces the average linear heat generation rate and increases the nuclear design flexibility. There are two design types for Step III fuel assem­blies; types A and B. The type A assembly consists of 74 fuel rods and two large

diameter water rods. A high pressure drop at the non-boiling fuel assembly inlet and a low pressure drop in the boiling region can stabilize the coolant flow in the fuel assemblies. The assembly adopts a high pressure drop type lower tie plate and partial length fuel rods (about 2/3 the usual length). The type B assembly is composed of 72 fuel rods and a square water channel to increase the H/U ratio and to optimize neutron moderation. It improves the nuclear and thermal-hydraulic characteristics such as core safety. Both fuel assembly types have a higher initial pressure of helium gas, 1.0 MPa, to improve the heat transfer between pellet and cladding and to mitigate the internal pressure rise of fuel rods with neutron irradiation.

The BWR fuel assembly has been improved with respect to intra-assembly design points such as fuel rod size and arrangement without considerably changing the fuel assembly size and therefore it can be applied to existing reactors. Application of high burnup fuel through those three phases has extended the average discharge burnup to about 1.5 times than that of the early 8 x 8 fuel and reduced the fuel cycle cost by about 30 %. The spent fuel amount is also reduced in inverse proportion to the discharge burnup increase.

As a restriction in the fuel cycle for high burnup, there are limited accep­tances to reprocessing facilities (maximum burnup of fuel assembly < 55 GWd/ t at Rokkasho, Japan) and transport and processing facilities (maximum ura­nium enrichment <5 wt%). Since Step III, the development of high burnup fuel has been continued within the restriction to increase the burnup and reduce the fuel cycle cost and spent fuel amount.