Experimental study on counter-current flow limitation for passive emergency core cooling system in SMR

Abstract

Because of economy of scale, the single unit capacity of conventional light water reactors (LWR) continues to increase and nuclear power plants become more economical. However, in order to remove decay heat safely, active safety systems are essential in the conventional large LWRs. The Fukushima Daiichi nuclear accident, which occurred on March 11, 2011, showed the limitations of the conventional large LWRs. Therefore, we chose the different strategy from the economy of scale. That is to ensure both public acceptable safety and economic feasibility of nuclear systems through maximizing passive and inherent safety features and simplifying nuclear systems, especially safety systems.

Firstly, we applied several inherent and passive safety features of high temperature gas-cooled reactors (HTGR) (e.g. ceramic coated fuel, lower power density core, and annular core) to a LWR for desalination which operates in the low-pressure low-temperature condition. A low-pressure inherent heat sink nuclear desalination plant (LIND) was designed and a scoping analysis for the dedicated nuclear desalination system was performed. In thermal-hydraulic analysis, we verified that the LIND system has a sufficient decay heat removal capability even if all active safety features fail during an extended SBO accident. In reactor core neutronic analysis using the MCNP code, we confirmed the possibility of the concepts related with the reactor core and estimated that the cycle length of the LIND core would be around 6 years under 200 $MW_{th}$ and 4.5% enrichment.

Based on the LIND system, we extended the operating condition to the high-pressure high-temperature condition for generating electricity and proposed the conceptual designs of a public acceptable simple SMR (PASS) and a new passive emergency core cooling system (PECCS). The comparison of safety systems between typical LWR and PASS showed simplification of the PASS system. From the comparison of between NuScale-PECCS and PASS-PECCS, we showed the PASS-PECCS may solve the limitation of the previous PECCS for indefinite cooling. The PASS system provides passive safety systems (passive decay heat removal system (PDHRS) and PECCS) which can remove decay heat for an unlimited period. From the structural safety assessment of SiC cladding in LOCA, we found that the PASS system has more than 30 days grace time even in the core uncovery accidents with SBO, failures of the key passive safety systems (PDHRS and PECCS).

In order to understand CCFL phenomena appearing in PASS-PECCS and to suggest its optimal design, down-scaled experiments were conducted in an air/water condition. From the experimental study, we investigated the effects of water head in an upper tank on CCFL and the scaling effects of a cavity pipe on CCFL. We found that there is a significant enhancement of water penetration in water head cases compared with a no water head case (L/D = 3). We found that there is an optimal water head condition which gives the highest water penetration rate and the CCFL characteristics in the water head case are mainly governed by bubbling dynamics. The most important scaling parameter is the diameter of the cavity pipe. As the diameter of the cavity pipe increases, the enhancement of water penetration by the water head in the upper tank increases and the CCFL line of the optimal water head tends to converge.

From the system code analysis on PASS-PECCS, the feasible diameters of a cavity valve and a rupture disc were proposed and we investigated the performance of PASS-PECCS. As the diameter of the cavity valve is equal to or larger than 0.155 m, decay heat can be removed safely for an unlimited period. In the case of the rupture disc, as the diameter is smaller than 0.045 m, the PASS system can have more than 30 days grace time.