Effect of Reactivity Addition Rate and of Weak Neutron Source on the Fission Yield of Uranium Solutions

The fission yield of an accidental assembly of a supercritical volume of uranyl-nitrate solution will depend on several variables, two of which are discussed here: the rate of reactivity addition, and the effect of the weak-neutron flux in the solution. Fission yields for a 53 g/liter uranyl-nitrate solution subjected to various rates of reactivity addition were calculated. The small number of neutrons from the solution will cause the excursion to occur after the assembly has reached a critical mass and has become slightly supercritical. The effect of the delay in initiation on the fission yield of the assembly is calculated with curves of the probability of initiation vs time. The effect on the fission yield of different neutron source strengths, the addition of sodium carbonate to the solution, and changes in the uranium concentrations of the solutions are discussed. A comparison is made of the predicted values obtained from these calculations and the reported yields from six accidental excursions.