In the one-batch design in the previous section, keff drop of Pu ADSs is 14 %dk, which is too large to be compensated by burnable poison or control rods. As the first step of design improvement, a multi-batch design is introduced. Theoretically, an

Operation day

Time evolution of criticality for one-batch design

N-batch core can reduce keff drop by 1/N, that is, a drop of 14 %dk can be reduced to

2.3 %dk by six-batch design. Figure 19.4 illustrates the criticality change with an expansion for operation date of 0-1,200 days. The criticality drop for the early operation date is larger than the limit of 3 %dk, that is, 0.94 of keff at end of burn-up; the drop decreases in the equilibrium cycle. The maximum drop is 5 %dk for Pu-ADS and 6.5 %dk for Pu+U ADS, which can be compensated by control rods or burnable poison or mitigated by shorter operation in the early cycle in future improvements. The drop in the equilibrium cycle is approximately 2 %dk, which is comparable to that of the reference MA-ADS.

Volume fractions and inventories are listed in Table 19.6. Six-batch cores generally require more inventory than a one-batch core because an averaged keff during operation of multi-batch cores is higher than that of the one-batch core. Transmutation amounts of Pu — and Pu+U-ADSs are much smaller than that of the MA-ADS because the operation period is, respectively, only 50 and 100 days.

To evaluate transmutation half-life, operation and cycle efficiencies must be determined. The short operation period of 50 or 100 days implies frequent fuel exchange and low operation efficiency. There are two kinds of interval: fuel exchange and plant maintenance. We assumed that fuel exchange of a 1/6 core requires 15, 30, or 60 days for the Pu — and Pu+U-ADS and that plant maintenance including accelerator needs 60 days. Because fuel exchange for 15 days is very short, considering shutdown and startup of the ADS plant is included, tentative storage inside a core vessel should be applied for such a short interval. In the case of Pu-ADS, the 50-day operation and 15-, 30-, or 60-day interval are repeated five times, then 50-day operation and 60-day maintenance are done. In the case of Pu +U-ADS, 100-day operation and 15-, 30-, or 60-day interval are repeated two times, then 100-day operation and 60-day maintenance are done. The total operation period before a long plant maintenance of 60 days in both ADSs is 300 days.

Operation day

Table 19.6 ADS inventories for six-batch design (equilibrium core) ADS case MA (Ref., 1-batch) Pu Pu+U Volume fraction of inert matrix (%) 69.8 85.4 62.5 Core inventory at BOC (t) U 0.19 0.01 3.05 Pu 1.83 1.84 2.04 MA 2.37 0.19 0.20 Core inventory at EOC (t) U 0.18 0.01 3.01 Pu 1.79 1.80 2.00 MA 1.92 0.19 0.20 Transmutation, BOC-EOC (t) U 0.00 0.000 0.037 Pu 0.04 0.041 0.043 MA 0.45 0.000 0.000 Specific heat, h (MW/tHM) 182 393 151

Based on the foregoing assumptions, operation efficiency and cycle efficiency are determined as listed in Table 19.7, with specific heat and resulting transmuta­tion half-life. Operation efficiency multiplied by cycle efficiency of the Pu-ADS is the poorest, but the transmutation half-life is the shortest because of the high specific heat. In the present study, a 30-day interval for fuel exchange is adopted as a nominal case. The transmutation half-life of the Pu-ADS is 24.8 years in the nominal case, which is applied to scenario analysis.

Another observation is that the impact of the out-core period on cycle efficiency is significant. The out-core period is presumed considering the half-life of 242Cm of

126.8 days. If a shorter out-core period is accomplished by corresponding design of the reprocessing and fabrication plant, cycle efficiency and resulted transmutation half-life can be improved. Table 19.8 shows comparison of a 3-year and 1-year out-core period. An impact on the transmutation half-life of the Pu-ADS is a factor of around 2, and the transmutation half-life becomes as short as 13.5 years. Although 3 years of out-core period is applied as the nominal case, a shorter out-core period should be pursued in future study.