The High Gain Light Water Breeder Reactor with a Uranium-Plutonium Cycle

The view has long been held that breeding in light water cores is possible only with the thorium cycle, at a rate slightly above the break-even point. If we utilize the uranium-plutonium cycle (plutonium fuel with ²³⁸U fertile material), we find that in a typical light water spectrum the value of η (number of neutrons emitted per neutron absorbed) for ²³⁹Pu, the principal plutonium isotope in standard light water reactor (LWR) spent fuel, is <2, which is the minimum value necessary for break-even in fissile fuel content. The reason for the low yield of fission neutrons from ²³⁹Pu absorption is that nearly one-third of the time ²³⁹Pu does not fission but instead forms ²⁴⁰Pu, a relatively nonfissile isotope. However, in studying the effect of neutron absorption by ²⁴⁰Pu and then by the subsequent ²⁴¹Pu, which is highly fissionable with a large value of η, it is found that a net neutron gain is obtained. This, combined with the neutrons obtained from ²³⁹Pu fission and the fast effect (²³⁸U fissions), yields sufficient neutrons for a high gain breeding potential. Unfortunately, in a conventional LWR with plutonium fuel, we cannot run long enough to produce neutron absorption in ²⁴⁰Pu and in the resultant ²⁴¹Pu because the ²⁴⁰Pu forms too quickly and its absorption is so high that it rapidly depletes the initial core reactivity. In the present concept, this difficulty has been overcome by a novel method of fuel management, which assumes the availability of rapid fuel reprocessing. In the design concept presented, two seed-blanket cores are utilized, a prebreeder and a breeder. The prebreeder core is fueled with plutonium obtained from standard LWR spent fuel and generates plutonium with a high isotopic content of ²⁴⁰Pu and ²⁴¹Pu, which is used to fuel the breeder core. The initial fissile fuel for a 950-MW(electric) prebreeder is between 3000 and 4000 kg. Assuming the availability of rapid fuel reprocessing and refabrication, overall breeding of well over 10%/6 yr is feasible. The time between core refuelings is >1 yr, rather than once every 3 months as required in the case of liquid-metal fast breeder reactors, so that the fuel inventories are substantially reduced for our system. Thermal-hydraulic analysis indicates that each of the two seed-blanket cores can fit into a standard pressurized water reactor pressure vessel and meet safety requirements. Major advantages of the concept are continued utilization of present LWR plants and strong negative void coefficients for the core. It is anticipated that at equilibrium conditions, when plutonium for the prebreeder and breeder cores is recycled into standard LWRs, there will be a further gain in breeding.