Experimental research on the influence of rolling motions on the convection behaviors inside two-layer corium pools

Highlights

• Transient behaviors of two-layer corium pool under rolling conditions were studied.

• Thermal stratification can be weakened by rolling motions.

• Heat transfer capacity can be generally enhanced by rolling motions.

Abstract

In realms of nuclear safety analysis, In-vessel retention (IVR) has been a research focus as it is a promising mitigation strategy for the postulated severe accidents in pressurized water reactors (PWRs). However, the recent development of ocean floating reactors for civil use has greatly motivated the pertinent studies under motion conditions, which can be much different from the known investigations conducted under static condition without the additional acceleration forces induced by motions. Thus, through experimental research, this paper extensively studied the influence of rolling motions on the convection behaviors inside two-layer corium pools, which are the major concerns for IVR research. Transient behaviors of the layered corium pools were obtained, and sensitivity analyses were conducted on three critical parameters, i.e. maximum rolling angle, rolling period and the thickness of metal layer. Experimental results show that the two-layer fluids can stay basically stratified under rolling conditions but mixing phenomena can be observed in the interface nearby region when rolling motions become intense enough. Generally, rolling motions can weaken the thermal stratification inside the layered corium pool, and enhance heat removal capacity from the corium pool to the outside cooling boundary. These phenomena indicate a good chance for the efficient decay heat removal from the corium pool under rolling motions. This study is supposed to provide valuable references to assess the success probability of IVR design applied in reactors operated under ocean conditions.

Introduction

With the development of world economy, a reliable and steady energy supply has become an important issue. Compared with the convectional fossil fuel energy, the nuclear energy is low-carbon and more environmental-friendly, and is increasingly welcomed around the world because of the global warming problem. However, safe regulation of heat removal is one of the most challenging aspects in employing nuclear fission energy, especially in accident situations. Therefore the safety analysis is one critical area in nuclear energy application. Existing research suggested that during a postulated severe accident in light water reactors, which is the most widely-used reactor type, the reactor core may melt down. If no sufficient cooling is provided, the molten core will further relocate into the lower plenum of the reactor vessel, forming a corium pool (Bonnet, 1999). RASPLAV and MASCA corium experiments (Asmolov and Tsurikov, 2004, Asmolov et al., 2001) suggested that the initial homogenous corium pool can stratify into two layers with a molten ceramic pool of UO₂–ZrO₂ lying beneath a metallic layer of Fe–Zr due to the density difference between the different compositions of the corium. This non-homogeneous structure can directly change the distribution of thermal loading along curved walls and pose a greater threat to the management of nuclear safety.

As the reactor pressure vessel is a critical barrier to keep the radioactive material inside the reactor, it's of vital necessity to guarantee its integrity. To achieve this goal, in-vessel retention (IVR) of the molten corium has been proposed as one of the key severe accident management strategies. And External Reactor Vessel Cooling (ERVC) is an effective method for successful application of IVR by flooding and submerging the reactor lower plenum (Rempe et al., 1997; Theofanous, 1989; Zhang et al., 2011). Based on the IVR-EVRC concept, some typical experiments have been designed to research the convection behaviors and heat transfer characteristics of the corium pool, e.g. COPO (Helle et al., 1999; Kymäläinen et al., 1994), ACOPO (Theofanous et al., 1997), BALI (Bonnet, 1999), RASPLAV (Asmolov et al., 2000) SIMECO (Stepanyan et al., 2004), LIVE (Gaus-Liu and Miassoedov, 2013; Gaus-Liu et al., 2010; Miassoedov et al., 2013) and COPRA (Luo et al., 2018; Zhang et al., 2016a, b, c, d; Zhou et al., 2018). These experiments can be used to model the thermal behavior in steady state of corium pools under static condition. More details of the relevant experimental research on corium pool behaviors can be referred to the review work by Zhang et al. (2015). As for the numerical analyses, Li et al. (2014) applied moving particle semi-implicit (MPS) method to investigate the stratification and solidification/melting phenomena of the corium pool and moreover the melt stratification and migration in debris bed (Li et al., 2020a). Also, a comprehensive review was performed on MPS method developments and applications in fields of nuclear engineering, such as the analysis of corium behavior (Li et al., 2020b). In addition, Zhang (Zhang et al., 2010, 2011) developed IVRASA code to analyze the heat transfer characteristics of the multi-layer corium pool. Zhang et al. (2018) and Luo et al. (2019) employed large eddy simulation method to investigate the thermal behavior of corium pool configurations in HPR1000 reactor and the simulants effects on the corium pool behaviors. Moreover, effect of stratified interface instability on thermal focusing effect was numerically studied by Ge et al. (2019).

From the above literature review, we can see that the relevant studies conducted so far on the corium pool are all confined to static condition. However, for Ocean Floating Reactor (OFR), the corium pool behaviors are inevitably influenced by ocean motion conditions, such as inclining, heaving and rolling, which may change the flow and heat transfer phenomena due to the induced acceleration forces. And hitherto, few studies of convection in layered corium pools under motion conditions are found. Thus, in this paper, we aim to study the influence of rolling motions on the flow and heat transfer phenomena of two-layer corium pools. The results of this paper are supposed to provide valuable references to assess the success probability of IVR design applied in ocean floating reactors.