Study on transmutation of minor actinides discharged from high burn-up PWR-MOX spent fuel in the force-free helical reactor

Abstract

The force-free helical reactor (FFHR) is a demo relevant helical-type D-T fusion reactor. In this study, the burning and/or transmutation (B/T) of minor actinides (MAs) has been investigated for an operation period (OP) of up to 10 years in the FFHR by 75% plant factor ( η) under a neutron wall load (P) of 1.5 MW/m 2. In order to incinerate and transmute the MAs effectively, transmutation zone (TZ), containing the mixture of MA nuclides discharged from high burn-up pressured water reactor (PWR)-MOX spent fuel, has been located in the blanket of the FFHR. The MA mixture has been spherically prepared, and cladded with SiC to prevent fission products from contaminating coolant and the MAs from contacting coolant. Helium has been selected for the nuclear heat transfer in the TZ. Effect of the MA volume fraction in the zone on the B/T has been also investigated. The energy multiplication ratio ( M), which is one of the main parameters in a fusion-fission hybrid reactor and relates to fission rate ( R F), is quite high and increases from 5.8 to 9.4 in the case of a MA fraction of 10% and from 15.3 to 31.8 in that of 20% depending on the OP. The neutron multiplication coefficient ( k eff), also relating to the R F, is less than 0.9 in all investaged cases during the OP. Tritium breeding ratio (TBR) value is greater than 1.1 for all investigated cases so that tritium self-sufficiency is maintained for (D,T) fusion driver. Its value reaches to 3.2 in the case of a MA fraction of 20% at the end of operation period (EOP). The spatial non-uniformity of the fission energy density can be expressed with the help of the peak-to-average fission power density ratio (Γ). The Γ value varies in the range of 1.05–1.14 depending on the MA fraction and the OP. These values show that the TZ has a good flat fission power density profile in all investigated cases during the OP. At the beginning of operation period (BOP), the total transmutation rate (TR) values of 237Np, 241Am and 243Am are 1489 and 1206 kg/GW thyr in the cases of 10 and 20% MA, respectively. The TRs of 237Np, 241Am and 243Am decrease exponentially in all investigated cases during the OP. At the EOP, in the case of a MA fraction of 20%, the effective half-lives of 237Np, 241Am and 243Am decrease to 5.4, 5.1 and 6.8 years, repectively, and the net transmutation fractions (TFs) for the whole TRUs and MAs are obtained as 40 and 60%, respectively. These consequences bring out that the blanket carries out the B/T of MA effectively.