Study on Transient Heat Transfer Characteristics of Molten Stainless Steel and Liquid Sodium

Abstract

The sodium-cooled fast reactor (SFR) not only has the advantages of high coolant temperature, nuclear fuel breeding and nuclear waste transmutation capabilities, but also has the characteristics of good inherent safety. It is one of the fastest-growing and most experienced fourth-generation reactor types. The main circuit system of the SFR is close to normal pressure, the coolant has good thermal conductivity and large boiling margin, the design of three circuits, the combination of active heat removal system and passive residual heat removal system, etc., so that the SFR has good safety characteristics. However, the power density of the SFR is high. In accidents such as unprotected loss of flow (ULOF), if the decay heat cannot be removed by natural circulation in time, the liquid sodium in the core will boil explosively, which may cause the core to melt. Since the void reactivity of sodium is positive, the boiling of sodium in the core may lead to a risk of recriticality of the core, and causing the core to melt. The debris bed formed after the relocation of the molten debris is mainly cooled by the natural circulation of sodium, and the heat of the sodium pool is discharged to the outside of the reactor through an independent heat exchanger placed in the sodium pool, thereby realizing the long-term cooling of the debris bed. During the release and relocation of the core molten material, the molten material will interact with the sodium, affecting the release of the molten material and the formation of the debris bed, and ultimately affecting the long-term cooling of the debris bed under severe accidents. Therefore, the molten fuel-coolant interaction (MFCI) is a complex and significant issue in the safety analysis of severe accidents in SFR. In response to this phenomenon, many experiments on prototype materials and alternative materials were carried out, and a clear understanding of the fragmentation process, fragmentation mechanism, heat transfer mode, fragment size distribution and whether steam explosion occurs in the MFCI phenomenon was obtained, and many mechanism models and theories were proposed. However, since the interaction process between the molten fuel and the coolant is a complex process with multiple components and multiple phases, only a few programs can be used to simulate this phenomenon. There are even fewer analyses of the transient heat transfer characteristics of molten stainless steel and liquid sodium. COSA is a test facility built by Xi’an Jiaotong University to study the interaction phenomenon between molten material and liquid sodium, and experiments on the interaction between molten stainless steel and liquid sodium were carried out on this test facility. The severe accident analysis program ACENA developed by Xi’an Jiaotong University was used to analyze the transient heat transfer characteristics between molten stainless steel and liquid sodium, further understand the phenomenon and mechanisms in the test, and verify the analysis ability of the ACENA program. The verification results show that the program can calculate that the molten jet enters the sodium pool and quickly exchanges heat in a short time, causing the liquid sodium in the center region to boil. After that, the molten material reaches the bottom and causes the temperature of the liquid sodium at the bottom to rise rapidly, and the liquid sodium with a higher temperature moves upward under the effect of buoyancy. Secondly, the program will overestimate the heat exchange when the molten stainless steel jet contacts the sodium pool. The program cannot take into account the process of solidification and formation of a hard shell on the surface of molten stainless steel, which causes the program to overestimate the heat exchange rate between stainless steel and liquid sodium.

Keywords

• : sodium-cooled fast reactor
• molten fuel-coolant interaction
• acena program