Fission waves can oscillate

TL;DR

This paper demonstrates that self-sustaining fission waves in uranium can undergo oscillations due to a Hopf bifurcation caused by delayed neutron production, challenging the assumption of their inherent stability.

Contribution

It reveals that coupling of the neutron field with the decay chain can induce oscillations in fission waves, a phenomenon not previously recognized.

Findings

Fission waves can oscillate due to a Hopf bifurcation.

Delay in neutron production leads to wave instability.

Numerical and Monte Carlo methods confirm oscillatory behavior.

Abstract

Under the right conditions, self sustaining fission waves can form in fertile nuclear materials. These waves result from the transport and absorption of neutrons and the resulting production of fissile isotopes. When these fission, additional neutrons are produced and the chain reaction propagates until it is poisoned by the buildup of fission products. It is typically assumed that fission waves are soliton-like and self stabilizing. However, we show that in uranium, coupling of the neutron field to the 239U->239Np->239Pu decay chain can lead to a Hopf bifurcation. The fission reaction then ramps up and down, along with the wave velocity. The critical driver for the instability is a delay, caused by the half-life of 239U, between the time evolution of the neutron field and the production of 239Pu. This allows the 239Pu to accumulate and burn out in a self limiting oscillation that is characteristic of a Hopf bifurcation. Time dependent results are obtained using a numerical implementation of a reduced order reaction-diffusion model for a fast neutron field. Monte Carlo simulations in combination with a linear stability analysis are used to confirm the results for the full system and to establish the parameter space where the Hopf occurs.