Velocity characteristic and stability of wave solutions for a candle reactor with thermal feedback

We study the stationary nuclear fission wave (NFW) in the CANDLE fission wave reactor analytically and numerically. The focus of this work is to elucidate in a universally applicable way the variation of wave velocity and power with respect to parameters of reactor composition and design. We also study the stability of such waves solving the time-dependent problem numerically. A one-dimensional model of an infinite cylindrical reactor with U-Pu fuel and non burnable absorber is used, including a qualitative model of thermal feedback. A new analytical approach is proposed to determine the velocity of the stationary wave. We show that there are three main mechanisms which determine the wave velocity, and thus, the power and the amplitude of the neutron flux distribution. The thermal feedback mechanism and the mechanism related to the kinetics of 239Np contribute to wave velocity formation in a similar way. These mechanisms compete together with the effect of slow β-decay of 241Pu. The latter dominates at lower velocities, leading to instability of the stationary wave solutions and to the existence of a minimal possible velocity of real stationary waves. Negative thermal feedback decreases wave velocity and lowers the upper margin of possible absorber densities. Under realistic conditions, both nuclear density mechanisms and thermal feedback may be important for wave velocity formation in CANDLE reactors with uranium fuel.