Graphite behavior and its effects on MSBR performance

Graphite behavior under Molten-Salt Breeder Reactor (MSBR) conditions is reviewed and its influence on MSBR performance estimated. Based on the irradiation behavior of small-sized graphite specimens, a permissible reactor exposure for MSBR graphite is about 3 × 1022 neutrons/cm2 (E > 50 keV). The stresses generated in the graphite due to differential growth and thermal gradients are relieved by radiation-induced creep, such that the maximum stress during reactor exposure is less than 1000 psi for reactor designs having a peak core power density of about 100 kW/liter and reactor exposures less than about 212 years. The corresponding power costs for single-fluid MSBR's would be about 3.1 mills/kWhr(e) based on a capital charge rate of 12% per year and an 80% load factor. Experimental data on graphite behavior also indicate that graphites with improved dimensional stability under irradiation can be developed, which would lead to improved reactor performance. The deposition of fission products on graphite does not appear to be large (10 to 35% of the “noble-metal” fission products based on MSRE experience); taking into account graphite replacement every two years, fission product deposition reduces the MSBR breeding ratio by about 0.002. Also, it appears that xenon poisoning can be kept at a 0.5% fraction poisoning level by using pyrolytic carbon as a pore impregnant which seals the surface of MSBR graphite and/or by efficient gas stripping of the fuel salt fluid by injection and removal of helium gas bubbles. It is concluded that good MSBR performance can be obtained by using graphite having combined properties presently demonstrated by small-size samples, and that development of MSBR graphite having such properties is feasible.