Abstract:

Steam explosions may be encountered in severe accidents of light water reactors (LWRs), which are thermal detonations caused by rapid and intense vaporization of the coolant upon its direct contact with the core melt (corium). Motivated by the interest in understanding and mitigation of severe accident progression, many studies have been conducted to investigate the steam explosion phenomena during severe accidents. However, most of the previous studies did not consider the effect of chemical additives in the coolant of nuclear power plants, such as additions of H3BO3, NaOH and Na3PO4 for water chemistry control, and direct utilization of seawater(NaCl additive) under an extreme condition like the Fukushima accident. The present thesis work is motivated to fill the knowledge gap concerning the impacts of chemical additives (H3BO3, NaOH, Na3PO4, and NaCl) on steam explosions.The primary objective of the present research is to obtain characteristics of steam explosions in seawater and chemical solutions of H3BO3/NaOH/Na3PO4 with prototypical concentrations. To achieve this goal, a series of experiments have been carried out in the MISTEE experimental platform at KTH, involving single droplets and multiple droplets falling into a variety of coolant pools filled with seawater or chemical solutions of H3BO3/NaOH/Na3PO4 additives. The thesis work consists of four parts as follows.The first part is a description of the experimental methodology developed in the present study.Two experimental facilities, dubbed MISTEE-CE and MISTEE-SEA of respective mechanicalplug and aerodynamic levitation for melt delivery, were designed on the MISTEE platform. Both setups were equipped with high-speed cameras for visualization, a pressure sensor for dynamic pressure measurement, and a fragment catcher for debris collection. A double-crucible design was employed to enable induction heating while avoiding melt contamination. The aerodynamic levitation system was implemented in MISTEE-SEA to reduce the disturbance of the mechanical plug. All chemical solutions were prepared in the laboratory with degassed deionized water. Tin(Sn) was chosen as the melt material due to benign properties suitable for safe handling in the laboratory.The second part is the presentation of visual observations and parameters selected to characterise explosions. The visualization includes the phenomena of droplet-coolant interactions and steam explosion occurrences. A molten single droplet falling into the coolant pool with deionised water or chemical solution might experience one of the three typical phenomena: deformation without fragmentation, minor fragmentation, or spontaneous steam explosion. In contrast, multi-droplet tests might involve merging and multiple explosions of droplets, resulting in a more complex set of phenomena. The quench depth and the lateral deformation ratio were defined and used to analyze the dynamic process of a single droplet in the coolant, while the peak pressure was employed to compare steam explosion energetics. In addition, the size distribution of debris particles was scrutinized.The third part is a summary and highlights of the experimental study on single-droplet steam explosion in different chemical solutions, using 1g of melt sample. The results revealed that the H3BO3 additive had little impact on steam explosion when the H3BO3 concentration was lower than 1.2 wt.%, but the risk of steam explosion in 3.2 wt.% H3BO3 solution was higher. The addition of NaOH and Na3PO4 to an H3BO3 solution significantly offset the influence on steam explosion. This suggests that the presence of PO43- and H+ ions play a significant role in spontaneous steam explosions. Additionally, seawater enhanced the occurrence of spontaneous steam explosions, with a clear correlation between increasing salinity and a higher likelihood of steam explosion. Compared to deionized water, chemical solutions (including seawater) caused more pronounced deformation in molten droplets at equivalent depths prior to direct contact of melt with coolant. Furthermore, the peak pressures of steam explosions in chemical solutions had the potential to reach notably higher values than those in deionized water. The chemical solutions except for the one of 1.2 wt.% H3BO3 tended to produce higher fractions of finer debris particles.The fourth part is about the experimental results of an investigation on steam explosion involving multiple droplets falling into deionized water and chemical solutions, using 5 g and 20 g of melt samples, respectively. It was found that under identical test conditions, the peak pressure of steam explosion increased with melt sample mass, resulting in a noticeably higher fraction of fine debris particles in the case of a 20 g melt sample. The steam explosion location was concentrated within a shallower range when using chemical solutions instead of deionized water. In contrast to single-droplet experiments, the influence of the chemical solutions on the steam explosion was diminished in the tests with multiple droplets.