Two cases of Pu feed were assumed in the present design for Pu transmutation: Pu-ADS, to which only Pu is provided from reprocessing process of LWR SF, and Pu+U-ADS, to which Pu accompanied by U with 50 % weight ratio is provided from the process. Treatment of pure Pu raises proliferation concern in a country without nuclear weapons, and the reprocessing plant is designed to add depleted

Fig. 19.2 R-Z model of accelerator-driven system (ADS)

238 239

U to Pu just after separation. Addition of U results in 9Pu production and

generally is undesirable for Pu transmutation. Table 19.3 lists two compositions. Other conditions are similar to the MA-ADS.

19.3.2 Result of One-Batch Core

The reference ADS for MA transmutation is designed with a one-batch core, which means that all the fuel is loaded and unloaded simultaneously. At first, this one-batch design was adopted to Pu transmuters. Table 19.4 lists the in-core and out-core time with operation efficiency. Out-core time of the ADSs is 3 years, which allows decay of 244Cm. In-core time of the reference MA-ADS is 2 years, although that of Pu-ADS is reduced to 1 year because the decrease of criticality is

too rapid for this ADS. The operation efficiency, eo, is 82.1 % assuming 300 days operation annually.

Design and transmutation performance are summarized in Table 19.5. Volume fraction of the inert matrix, ZrN, of the MA-ADS core is 69.8 %, adjusted so that k-effective at the beginning of the cycle (BOC) of the equilibrium core becomes 0.97. The equilibrium core is obtained after calculating ten cycles of burning, cooling, and recycling. Volume fraction of the Pu-ADS is more and that of the Pu+U-ADS is almost the same. The inventory at BOC of the heavy metal in Table 19.5 is proportional to a one-volume fraction of ZrN. An interesting observa­tion is that the amounts of Pu at BOC are equal among three ADSs, which means U and MA contribute very little to the criticality before depletion. However, impacts on the criticality drop after depletion is significant (Fig. 19.3). kgff drop of the MA-ADS is as small as 1.5 %dk, although others lose 14 %dk even at the equilibrium cycle around 6,000 days, which means MA is a better fertile than 238U. The Pu-ADS has a steeper decrease than Pu+U-ADS because of the absence of 238U. The huge drop of the Pu — and Pu+U-ADS is not acceptable in the current design of accelerator and target for the MA-ADS; the acceptable drop is about 3 %dk in the MA-ADS.

The effective transmutation rate and transmutation half-life are listed at the bottom of Table 19.5. The half-life of the Pu-ADS is shortest because its specific heat is twofold larger than others although its cycle efficiency, ec, is much smaller than others.