Numerical study of uranium nitride dissociation time at various temperatures using the EVKLID/V2 code

Background

Dense nitride fuel is promising for reactors on fast neutrons (fast reactors) with liquid-metal coolant to solve the problems of closing the nuclear fuel cycle. An important aspect of research involves the study of the nitride fuel dissociation rate under various conditions, including that characteristic of severe accidents.

Aim

To numerically study the time of uranium nitride fuel dissociation under various external conditions characteristic of severe accidents with core destruction.

Materials and methods

The object of the study is uranium nitride. The research method is numerical simulation using the validated SAFR severe accident module of the EVKLID/V2 integral code developed by the IBRAE RAS, Afrikantov OKBM JSC, NIKIET JSC, and NRC Kurchatov Institute (Russian Federation). The SAFR module uses a model for calculating the behavior of nitride fuel elements in severe accidents. Computational methods of the SAFR module include enthalpy formulation to solve the thermal conductivity equation; the melt flow is simulated by solving one-dimensional equations of mass, energy, and momentum conservation.

Results

Depending on the properties of the contact environment, time of fuel dissociation may vary by several orders of magnitude. In a leaking fuel element with a helium atmosphere without nitrogen at a relatively high temperature of 1800 ℃, the dissociation time for 50% of uranium nitride ranges from 40 days to ~3 years, depending on the conditions of wetting the fuel column by liquid uranium. At high values, close to the nitride melting point of 2600 ℃, the dissociation time is from hundreds of seconds to 1 h. The presence of nitrogen with a partial pressure of ~2.5 atm completely suppresses dissociation up to melting.

Conclusion

The longest time of uranium nitride dissociation, and therefore its greatest thermochemical stability, is observed during dissociation into a gas atmosphere in the presence of a residual melt film on the fuel surface.