MOX fuel assemblies have a zoning configuration with three different contents of plutonium as shown in Fig. 3.64. The reactivity of MOX fuel assembly is determined by the assembly-averaged Puf content. Plutonium used in MOX fuel has been reprocessed and recovered from spent fuel of various types of reactors and therefore it has various isotope compositions. The assembly-averaged Puf content is set to give a proper reactivity corresponding to plutonium isotope composition. As a currently implemented method to set the assembly-averaged Puf content, the MOX fuel assembly to give the same reactivity as 235U 4.1 wt% uranium fuel at 28,100 MWd/t is used as a criterion for the appropriate reactivity [41]. The enrichment of 4.1 wt% and burnup of 28,100 MWd/t are the enrich­ment and the core average burnup at EOC for the typical uranium fuel in the standard 3-loop PWR (See Problem 6 at the end of this chapter). To match reactivity at the core average burnup of EOC means that MOX-fueled core has the same operating cycle length as the uranium-fueled core.

Figure 3.65 compares variation in the infinite multiplication factor between typical MOX fuel and 235U 4.1 wt% uranium fuel. The MOX fuel shows a milder variation in infinite multiplication factor with burnup than the uranium fuel and the line intersects that of the uranium fuel at a point. The infinite multiplication factor line of MOX fuel moves up and down with almost the same slope as Puf content increases and decreases. Therefore, a proper reactivity of MOX fuel can be obtained by adjusting Puf content responding to plutonium isotope compositions in MOX fuel

— Uranium Fuel with 4.1wt% Enrichment — MOX Fuel

28 100 MWd/t

Burnup [MWd/t]

fabrication. Practically, the reactivity worth of each plutonium isotope is replaced with an equivalent reactivity worth of 239Pu, referred to as the equivalent fissile method [41]. Figure 3.66 shows the relation between Puf content and Puf fraction set by the equivalent fissile method.

[2] Reactor Power Uprating

Reactor Power Uprating for PWRs are evaluated considering the same effects as for BWRs mentioned in the list [5] of Sect. 3.2.6 and the range of facility modification is determined. It has been reported that advanced safety analysis methods and development and introduction of advanced fuel are important for significant power uprating [42].