Numerical Assessment of Long-Term Coolability with Core Blockage via OECD-ATLAS

Abstract

The present CFD numerical study is intended to investigate the effect of the core inlet blockage on the long-term coolability of the reactor core. The aim of emergency core cooling system (ECCS) is to cool down the reactor core during postulated accidents such as loss of coolant accident (LOCA). For the blowdown phase, cold coolant from the safety injection tank (SIT) is injected into the primary system. In the long-term cooling (LTC) phase of LOCA, the cooling process continues using the water discharged from the break into the containment building and collected in the sump. For a large break LOCA situation, the debris may be produced and flowed in different paths inside the containment. Sump screens are installed in the in-containment refueling water storage tank (IRWST) to filter the amount of debris that could be flowed into the primary system and to minimize the impact of debris on the core cooling. Some fine debris, however, may pass through the sump screen, especially during the early stage of the long-term cooling phase when the screens are relatively clean and have a large gap, and may flow into the core. The passed debris may be deposited on the inlet of fuel assembly. The coolant flow may be blocked by the debris deposition and accumulation in the fuel assemblies. Due to the partial blockage of the core, the degradation of core cooling may lead to core damage. To assess the core blockage associated phenomena, the OECD-ATLAS facility has been recently equipped with a core blockage system installed in the lower plenum of the core pressurized vessel. The initial experimental tests carried out by the facility operator has revealed that there is a dependence of the pressure drop, within the blockage system, on the blockage ratio. Unfortunately, the experiment could not be repeated due to the identified mechanical problems that might endanger the facility integrity in the long-term operation. The CFD analysis was conducted to numerically examine the core blockage effects on the long-term coolability of the core. Several cases were conducted to study the behavior of the flow and calculate the K-Factor for each case. The obtained results reveal that the K-factor is a function of the Reynolds number and blockage ratio. In particular, it varies slowly up to blockage ratio of about 40 % but reaches much higher values at blockage ratio values above 80%. Using these CFD predictions, a correlation has been produced to mimic the K-factor behavior and should allow for future system codes analysis to estimate the blockage ratio effects on the core's long-term coolability.

Date of Award	Dec 2018
Original language	American English