Response of actinides to flux changes in high-flux systems

When discussing the transmutation of actinides in accelerator-based transmutation of waste (ATW) systems, there has been some concern about the dynamics of the actinides under high transient fluxes. For the condition of a pure neptunium feed, it has been estimated that the 238Np/237Np ratio increase due to an increasing flux may lead to an unstable, positive reactivity growth. In the analysis here a perturbation method is used to calculate the response of the entire set of actinides in a general way, that allows for more species than just Np. The time response of the system can be calculated, i.e., a plot of fuel composition and reactivity versus time after a change in flux can be made. The effects of fission products can also be included. The procedure extremely accurate on short time scales (∼1000 sec) for the flux levels we contemplate. Calculational results indicate that the reactivity insertions are always smaller than previously estimated