EFFECT OF RADIOLYTIC GAS ON NUCLEAR EXCURSIONS IN AQUEOUS SOLUTIONS

Abstract

Knowledge of the consequences of a nuclear criticality accident in aqueous fissile solutions is necessary to design the processing equipment for such solutions. The data at the disposal of designers before 1967 was provided by actual critically accidents. In 1968, the Service d'Etudes de Criticite of the French Commissariat a L'Energie Atomique initiated a program of systematic experimental aqueous solution nuclear excursions which were initiated intentionally to obtain solution criticality accident data. This program was designated "Consequence Radiologiques d'un Accident de Criticite" (CRAC). Although not intended to study the evolution of a solution nuclear criticality accident, the Kinetic Experiment on Water Boiler (KEWB) demonstrated the dependence of the nuclear excursion on parameters such as solution temperature and radiolytic gas. Similarly, the CRAC program results indicated the excursion was governed by parameters such as the solution addition rate, initial neutron population, solute concentration, and thermal and radiolytic gas feedback. The energy deposited in a fissile solution is the sum of the energies contributed by the radiation sources. The majority of the energy is deposited by the fission fragments. One feature of the energy deposition is a commensurate increase in the system temperatures which affects the solution volume and thereby the neutron leakage probability. A second feature is the decomposition of the water molecule which results in release of H(,2) and O(,2) in the solution. Microbubbles are nucleated in the fissile solution by a localized thermal spike generated by a fission fragment. Initially, the microbubble contains a mixture of radiolytic gas and water vapor. Below the boiling point the vapor condenses quickly, leaving a gas microbubble. Unless the solution is supersaturated, the gas bubble will dissolve in a few microseconds. However, in a supersaturated solution the bubble will grow and produce negative feedback by increasing neutron leakage. The analysis for this study employs two mathematical models for the radiolytic gas feedback. One assumes the radiolytic gas concentration is a linear function of the energy release and the nucleation rate is a linear function of the power (Energy model). The other assumes a correlation between the system pressure and the radiolytic gas feedback (Pressure model). Both models have been incorporated into a space-independent kinetic computer code, MACKIN, while the pressure model was also incorporated into a space-dependent code, AZPAD, (Space-dependent model). The model incorporation provides a numerical tool with which to analyze a nuclear excursion in an aqueous fissile solution. The models have been successful in predicting the peak power, burst energy, and maximum system pressure for the first burst in both KEWB and CRAC experiments.