Feasibility Study on Passive Residual Heat Removal System Coupled with Dry Air Cooling Tower for Small Modular Reactor ATOM Using MARS-KS Code

Abstract

The small modular reactors (SMRs) of the integrated pressurized water reactor (IPWR) type have been developed due to their design characteristics for inherent safety. In designing an integrated type reactor, the major components are installed inside the reactor pressure vessel. This distinctive arrangement of primary system provides suitable conditions to adopt passive safety systems by securing sufficient height difference and reducing flow resistance.

Recently, an advanced concept of SMR of IPWR type, called Autonomous Transportable On-demand reactor Module (ATOM) has been developed by the Korean research group. As the ATOM aims at inherent reactor safety, the passive residual heat removal system (PRHRS) is adopted as one of main passive safety systems (PSS). As the PRHRS removes residual heat using latent heat of the water stored in the emergency cooldown tank (ECT), it bears a technical limitation that the PRHRS may lose its cooling capability after the depletion of the stored water. It indicates that the power supply system should be restored before the depletion of the stored water under the station blackout (SBO). However, through Fukushima accident in 2011, it was learned that the prompt restoration of power system was practically infeasible. Therefore, the enhanced PRHRS is needed to extend grace period for responding to accident management.

In this thesis, an advanced design concept of PRHRS has been proposed. The PRHRS is indirectly coupled with the dry air cooling tower (DACT) through the intermediate loop to ensure an indefinite grace period, called indefinite PRHRS. Through the design calculation by using MATLAB code, several key design parameters of the indefinite PRHRS were determined. For the transient simulation, the MARS-KS, which is the best estimate thermal-hydraulics system code, was used. The MARS-KS model of the ATOM with indefinite PRHRS was also developed. Through the long-term transient simulations for 72 hours, the feasibility of the indefinite PRHRS was assessed from two perspectives: cooling capability and cooling sustainability of the indefinite PRHRS. The indefinite PRHRS successfully cooled down the reactor core of ATOM by extracting residual heat for 72 hours. The cooling sustainability of the indefinite PRHRS can be indefinitely extended without depleting the stored water assisted by the DACT. The parametric analysis of environmental temperature was conducted to confirm the effect of the environmental temperature on cooling capability and sustainability of the indefinite PRHRS. Additionally, the performance of the indefinite PRHRS under the extreme environmental conditions such as siting in a desert was evaluated.