Advanced LWR concepts with high conversion ratio and negative reactivity feedback
The development and construction of liquid metal cooled breeder reactors is highly desirable for their superior waste managing capability as compared to that of light water reactors (LWR). However, their construction is constrained by, among other factors, (a) Higher construction and operational costs as compared to that of LWR; (b) No presently available industrial infrastructure; (c) Lack of operational experience among the nuclear industry, which instead has accumulated vast experience with light water as coolant for the current fleet of nuclear power plants. Hitachi has developed an innovative Resource-Renewable BWR (RBWR) nuclear reactor core design, which is expected to fission almost all the transuranic (TRU) elements generated in the nuclear energy system. In 2008/2009 UCB was asked to evaluate the ability of the RBWR technology to offer designs that could compete with the corresponding SFR designs. Based on the conclusions of our analysis performed during this work, it appears that the RBWR-AC holds considerable promise to improve the sustainability of nuclear energy. However, it was also found that few concerns still remain on the safe operability of this system, mainly 1) the sign of the void reactivity coefficients and 2) adequacy of the control rod shutdown margin in fully voided conditions. Moreover, the feasibility of increasing the conversion ratio above 1 appears of significant interest. It is our aim to evaluate the feasibility of designing a similar core with negative reactivity coefficients and sufficient control rod shutdown margin in all conceivable conditions. We will first study the performance of a core similar to the RBWR-AC, but using thorium instead of depleted uranium as fertile material. If successful, such a system would address the void coefficient of reactivity problems, while possibly increasing the destruction rate of accumulated TRU and retaining an overall fissile conversion ratio of 1 (or possibly more). It is also possible that the use of thorium would increase the control rod shutdown margin without further system modifications. Although in the past little attention has been devoted by the scientific community to the possibility of light water breeder reactors, it is our believe that the successful accomplishments of the tasks set forth here, will attract the deserved attention to the potential for this technology to improve the sustainability of nuclear power ? by dramatically enhancing the uranium resources while virtually eliminating the actinides from the high level waste stream; all this while using the industrial infrastructure of BWR already in place, thus substantially reducing development cost and time relative to those required for pursuing the SFR route.