Analysis of Complete Loss of Coolant Accident Scenario in Open Pool Research Reactor

The present work deals with a hypothetical case of a complete loss of coolant accident (LOCA) in 5MW pool-type research reactor using MTR-type fuel with Graphite reflectors. In this scenario The residual decay heat generated in the fuel is mainly removed by conduction to the grid plate, natural convention of air, and radiation heat transfer (if the fuel reaches high enough temperatures). As part of reactors' safety requirements, it is usually important to properly analyze this extreme scenario and examine both the conditions and requirements to ensure no fuel melt will occur; this analysis is important for operational decisions by emergency response teams in case of such a severe accident (e.g., evacuation due to potential fuel melt).
We present a thermal analysis of a complete LOCA, where the core is totally exposed to ambient air. This scenario is initiated due to a break of the primary water pipe causing the pool water level to drop rapidly, until the core is fully uncovered. In order to analyze this problem, we developed a one-dimensional thermal model that takes into account the important heat loss mechanisms, including heat transfer by natural convection of air, heat transfer by radiation to the ambient environment, and radiation heat transfer between the fuel assemblies/graphite, and to the grid plate. Importantly, this new model uses conservative, but not overly stringent bounding assumptions, in order to serve as a basis for safety analysis.
In the model we used the decay heat correlation of ANS 5.1 for U-Al fuels, and assume local heating of the fuel assemblies by their own decay heat, with the decay heat distributed un-evenly in the reactor with high power peaking factor. Furthermore, our model takes into account an axial temperature distribution which is important to identify the initiation of melting.
We found that while the core is exposed to air the fuel assemblies cooled mainly by conduction to the grid plate and by natural convection of air. Later on, when the fuel temperature exceeds 350oC, radiation heat transfer becomes significant as well. Furthermore, we found that the heat generation distribution has signification affect, due to the fact that the heat generation in the central assemblies are much higher than the peripheral and they are more thermally isolated from the environment, which means they will be the first to heat up and possibly melt. Overall, we found that the time duration, from the reactor shutdown until the fuel exposure, that needed to ensures that no failure occurs to the fuel cladding ranged between 10-20 hours, depending on the power of the reactor.