Simulations of step reactivity insertion limits in typical research reactors

Reactivity insertions limits imposed by clad melting temperature in a typical Material Test and Research reactor (Mtr) have been investigated. Simulations were conducted using modified PARET code having Low Enriched Uranium (LEU) cores. The transient problem was forced through the specification of extremely inserted reactivity as a function of time. For all transitory studies the reactor was initially kept critical at a very low power of 1 watt. The behavior of power, reactivity, and centerline temperatures of the core were investigated for less than 1 second. Scram disabled transients for ramp reactivity insertions reveal the initialization of clad melting at $2.0 and $2.2 per 0.5 sec in above mentioned cores. Simulations show that the clad failure in LEU core occurs at step reactivity insertions of $2.1. Oscillations in the power, fuel and clad temperatures have been observed. The secondary power peak within 0.1 second of the primary peak causes non-linear rise in temperatures within the fuel which lead to the clad meltdown. The reactivity insertion limits are dependent on the combined effect of heat capacity, thermal conductivity and coefficients of reactivity. (author)