Feasibility study of passive cooling capability of phase change material loaded in emergency core tank of small modular reactor ATOM using MELCOR code

Abstract

Small Modular Reactor (SMR) is getting attention as a new concept of nuclear power plant with passively safe design in recent years. As part of its safety feature, Passive Residual Heat Removal System (PRHRS) is adopted to remove decay heat to Emergency Cooldown Tank (ECT). As the accident proceeds, the coolant within ECT may evaporate and PRHRS loses its capability to remove the decay heat. Therefore, the additional heat sink may extend the grace time and guarantee much longer time period with full capability to cool down the reactor core from its decay heat. In this thesis, Phase Change Material (PCM) was applied as a possible method to provide efficient functionality on ECT. To assess performance of PCM, a set of severe accident analyses was performed with various materials and structures of PCMs installed on a SMR design, Autonomous Transportable On-demand reactor Module (ATOM). For the assessment, ATOM input was transferred from MARS-KS to MELCOR. PCMs were modeled with variations on material, structure, and volume. Four cases were performed with two different materials and two different replacement ratios. 30% replacement and 50% replacement with two different materials were simulated to observe the effect of replacement on cooling ability. Initiating accident scenario was Station Blackout (SBO), and simulation was done for 72 hours. When additional PCM was absent, PRHRS itself resulted in complete dryout of ECT at the end of accident scenario. Besides, core inlet and outlet temperature increased again after 40 hours. When PCM replaced part of ECT coolant, it may delay time for the coolant to reach saturation temperature. However, since it has less volumetric latent heat, material with much higher volumetric latent heat must be applied for sufficient amount of heat removal compared to ECT with coolant only. The research concluded that PCM can be plausible option for enhancement of cooling capability of ECT after further development on material itself. Current PCM may extend the grace time only if the PCM is added to the ECT.