Analysis of a fatal accident in a highly enriched uranium facility

•Coupled neutron kinetics with elastic equations.•Coupled neutron kinetics with thermal hydraulics transients.•Solve the system of coupled nonlinear first order differential equations.•Compute energy yield.•Compute thermodynamics conditions. A fatal critical accident in a compact, water moderated and highly enriched (90%) MTR facility is analyzed. A very detailed Monte Carlo model was used to define the initial conditions and the reactivity coefficients. The MCNP code was used to model all the heterogeneities of the facility. Once the reactivity parameters were calculated the kinetics equations were solved coupling them with the thermodynamics conditions of the core and the steady water. The power pulse was then calculated and compared with radiological data.