Refueling is carried out in every operation cycle. After a cycle, the neces­sary number of discharged and loaded assemblies and their positions for achieving criticality throughout the next cycle are determined in consider­ation of the operation conditions. The scattered batch refueling method is adopted. In this method, the fraction of discharged fuel assemblies in a particular region (inner core, outer core or blanket) over the total fuel assemblies within that region is kept equal among the three regions. In that way, the power distribution and other neutronic characteristics are kept virtually unchanged between operation cycles.

In design of the refueling core, the refueling plan (i. e. the number and position of discharged fuel assemblies) is made for ensuring designed cycle length and achieving a safe and economical core configuration. By assum­ing the range of the content of available plutonium, the core reactivity and the core characteristics (such as power distribution, reactivity coefficients, and burnup) are evaluated. According to the evaluations, the core charac­teristics and safety in the target cycle are confirmed based on the plutonium content used in the detailed design and the refueling plan. The designed core characteristics are confirmed again by the reactor physics tests at actual reactor startup.

(2) Variation of plutonium content and the equivalent fissile content method [10]

(a) Variation of plutonium content

Before being used as nuclear fuel, plutonium was irradiated in various reactors and reprocessed. The plutonium content of MOX fuel depends on the burnup of the original spent fuel, cooling time etc. The higher the burnup of the original spent fuel, the larger the fraction of higher order isotopes such as 240Pu, 241Pu and 242Pu becomes. 241Pu in the reprocessed plutonium decays into 241Am with a half-life of 14.2 years. The decay of the fissile 241Pu reduces the reactivity of MOX fuel. 241Am has larger capture cross section than 240Pu and 242Pu, and it has a much smaller fission cross section than fissile materials like 239Pu. Accumulation of 241Am decreases the reactivity of MOX fuel. Figure 4.5 shows the cross sections of plutonium isotopes and 241Am. As the decay of 241Pu and production of 241Am continue after fabricating the fuel elements, the designed plutonium enrichment in the fabricating process must be determined by considering the duration to the loading in the core.

(b) The equivalent fissile content method

Since the contribution of plutonium to the core reactivity largely depends on the plutonium content, the necessary plutonium enrichment

Table 4.2 Plutonium Equivalent Worths in FBR [10]

also depends on the plutonium content. However, the core reactivity can be accurately estimated by equivalently treating the fissile enrichment in the fuel. This method is called the The Equivalent Fissile Content Method.

In this method, the contribution of 239Pu, which has the highest fraction among plutonium isotopes, is set as the reference (1.0). The relative contributions by other plutonium isotopes including 241Am (treated as one of the plutonium isotopes) are defined as the plutonium equivalent worths (nPU, nU). Examples of the plutonium equivalent worths are listed in Table 4.2. By using them, the adjusted fissile enrichments corresponding to the plutonium enrichments of the inner core and outer core are calculated as Eq. (4.7).

ЇЇ239=єри’ЕІаігіГи+(1—єри)’Е&гі}] (4.7)

epu : Plutonium enrichment

ai : Fraction of plutonium isotopes i

Pj : Fraction of uranium isotopes j

nPU : Plutonium equivalent worth of plutonium isotope i

nU : Plutonium equivalent worth of uranium isotope i

Although the content of the obtained plutonium at the moment of fuel fabrication is different from the plutonium content at the moment of design, equal core reactivity can be achieved by adjusting the plutonium enrich­ment during the fabrication so that the equivalent fissile enrichment is equal to the design value.