Analysis of Postulated Pool Draining Accidents in the MNR

Abstract

A safety analysis for the McMaster Nuclear Reactor has been carried out for postulated scenarios of loss or termination of forced flow in the reactor core in a state of shutdown, with loss of pool inventory of different magnitudes including core uncovery. Models were developed to evaluate the natural convection flow through the core assemblies for the different conditions within the aforementioned envelope. The flow rate was used to get the temperature or enthalpy rise along the heated channel in order to estimate the corresponding clad temperatures in the given scenarios. The models were constructed from first principles using the one-dimensional momentum conservation law, incorporating the Boussinesq approximation for the single-phase case and the Homogeneous Equilibrium Model assumptions when a two-phase mixture was present. In order to obtain the flow rate and enthalpy rise along the channel, knowledge of the assembly power and inlet temperature is required. The power was calculated using a well known decay power correlation. The pool temperature which was used as the assembly inlet temperature was calculated via a lumped parameter model using a simple energy balance between the core output (again by using the decay-heat profile) and the pool heatup. Heat losses from the pool were neglected and the model allowed for reaching saturation temperature in the pool. In this case, water vaporization was calculated using the latent heat to assess pool inventory loss rate. For all scenarios before core uncovery, the models predict that clad and fuel temperatures remained well below limits associated with clad blistering or melting. Consequently, it is asserted natural convection and acceptable temperatures will be sustained in the McMaster Nuclear Reactor while the core remains covered. In the most severe draining before uncovery, in which the pool drains to just before exposing the core, it takes approximately a week (180 hours) after shutdown for boiling to start in the core’s hottest channel. For core uncovery, the models predict that the clad remains below the blistering temperature for pool height at 9.4% of the heated channel’s height (corresponding to exposing about 61.7 cm of the assembly), and below melting temperature for pool height at 8.1% of the heated channel’s height (corresponding to exposing about 62.5 cm of the assembly). Both heights are below the height of the bottom of the lowest beam tube, at which the worst draining case will end.