Fresh uranium fuel is usually enriched to about 3-4 wt% of 235U on average for a fuel assembly. 235U decreases with burnup and part of the 238U is converted to plutonium through neutron capture. Typical spent fuel contains about 1 wt% of 235U and about 1 wt% of plutonium converted from 238U. 239Pu and 241Pu account for about 60-70 % of the plutonium. MOX fuel is an oxide mixture of plutonium recovered from spent fuel and natural or depleted uranium, containing about 3-5 wt% of plutonium per fuel assembly. The difference between MOX fuel and uranium fuel is that plutonium is blended into the fresh fuel and its amount is larger than that in uranium fuel. This brings about a change in core characteristics due to the different nuclear characteristics of plutonium. As shown in Fig. 3.25, plutonium has a larger neutron absorption cross section in thermal and resonance regions than uranium, and a smaller delay neutron fraction. The different characteristics have the following effects on core characteristics.

(i) MOX fuel reduces the number of thermal neutrons and the neutron spectrum is hardened, and therefore it causes a decrease in neutron absorption of the control rods and boric acid water. It is necessary to assure the core shutdown capability even with one control rod stuck in the fully withdrawn position or with boric acid water injection under the condition of all control rods stuck.

(ii) In connection with the large resonance absorption cross section of 240Pu, MOX-fueled BWR cores have a larger reactivity change relative to a change in void fraction than uranium-fueled cores. This tends to give a more negative void reactivity coefficient. It is, therefore, necessary to check the impact on core characteristics and stability at transients.

(iii) Since MOX fuel rods tend to generate a relatively higher power when loaded together with uranium fuel rods in the fuel assembly, it is necessary to assume that the maximum linear heat generation rate and MCPR at normal operation meet the operation criteria. Especially, fuel rods facing the water gap region generate a large power and therefore particular attention should be given to the fuel rod arrangement in the fuel assembly.

In use of MOX fuel, the plutonium enrichment and MOX fuel inventory are adjusted based on the effects mentioned above. Actually, MOX-fueled cores are designed to have a margin to meet changes of the characteristics, consid­ering the range of variation in various factors due to nuclear calculation error and future fuel design changes. Existing reactors have been evaluated as able to operate with MOX fuel replacing about 1/3 of the core fuel [23].

The BWR MOX fuel used in the initial step of the Japanese plan for Pu-thermal utilization has the same structure as that of Step II fuel because of the rich operating experiences already available for uranium fuel. The dis­charge burnup of the MOX fuel is about 33 GWd/t, which is just slightly lower than the 39.5 GWd/t discharge burnup of Step II uranium fuel. The MOX fuel is being introduced with repeated usage experience. The Step II fuel assembly developed for high burnup has a large diameter water rod to improve the neutron moderation effect. The water rod has the effect of getting a less negative void reactivity coefficient. In design of the MOX fuel rod, the active height of the fuel rod is shorter considering that the FP gas release rate is

Uranium Fuel Rods

slightly higher than that of uranium fuel. Figure 3.26 shows a fuel rod arrange­ment in an MOX-fueled assembly. Low enrichment plutonium fuel rods are arranged on the periphery as a measure to get local power peaking like done for use of uranium fuel. Experienced gadolinia-added uranium fuel rods are arranged among MOX fuel rods in the assembly as burnable poison rods for excess reactivity control.

In ABWRs, the fuel assembly size is the same as that of BWRs, but the fuel assembly gap was expanded to enlarge the non-boiling region outside the channel box, which increases the water-to-fuel volume ratio. This mitigates an increase in negative void reactivity coefficient by MOX fuel loading and a reduction in reactivity worth by control rods and boric acid water. Sufficient thermal margin and reactor shutdown capability were obtained from a 100 % MOX-fueled ABWR core and the MOX fuel loading has been confirmed to have a high flexibility [24].

Differently from uranium fuel, MOX fuel does not need an enrichment process, and the fraction of plutonium oxide to be mixed with depleted uranium oxide is only increased for high burnup. Such a high burnup with no enrichment cost increase has a large effect on the fuel cycle cost. A high burnup MOX fuel is being developed through irradiation tests in experimental reactors the same approach as taken for developing the high burnup uranium fuel.

Plutonium is an a emitter. Secondary reactions of the released a particles and the self-fission of 240Pu release neutrons. MOX fuel treatment must pay more attention to radiation protection and heat generation than uranium fuel treat­ment. Features and measures of radiation and decay heat in MOX fuel are mentioned in Sect. 3.3.6.