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Projects / Programmes source: ARIS

Understanding stratified steam explosions in reactor conditions

Research activity

Code Science Field Subfield
2.03.02  Engineering sciences and technologies  Energy engineering  Fuels and energy conversion technology 

Code Science Field
T160  Technological sciences  Nuclear engineering and technology 

Code Science Field
2.03  Engineering and Technology  Mechanical engineering 
Keywords
nuclear safety, severe accident, fuel-coolant interaction, steam explosion, stratified configuration, multi-phase flow, numerical simulation
Evaluation (rules)
source: COBISS
Researchers (9)
no. Code Name and surname Research area Role Period No. of publicationsNo. of publications
1.  33647  Sandi Cimerman    Technical associate  2020 - 2021 
2.  33540  PhD Martin Draksler  Energy engineering  Researcher  2019 - 2023  107 
3.  39141  PhD Janez Kokalj  Energy engineering  Junior researcher  2019 - 2020  84 
4.  14572  PhD Matjaž Leskovar  Energy engineering  Head  2019 - 2023  434 
5.  35548  PhD Jure Oder  Energy engineering  Researcher  2019 - 2021  58 
6.  08661  PhD Andrej Prošek  Energy engineering  Researcher  2019 - 2023  599 
7.  35549  PhD Matej Tekavčič  Process engineering  Researcher  2019 - 2023  93 
8.  12057  PhD Iztok Tiselj  Energy engineering  Researcher  2019 - 2023  467 
9.  29182  PhD Mitja Uršič  Process engineering  Researcher  2019 - 2023  265 
Organisations (1)
no. Code Research organisation City Registration number No. of publicationsNo. of publications
1.  0106  Jožef Stefan Institute  Ljubljana  5051606000  90,600 
Abstract
Steam explosions are one of the most feared potential hazards that may occur in the course of a severe accident in a nuclear power plant with core meltdown. It can threaten the integrity of the reactor containment and lead to early release of radioactivity. A steam explosion is an energetic fuel coolant interaction process, which may occur when the hot reactor core melt comes into contact with the coolant water. Recently an important safety related uncertainty was revealed, which is related to the experimental observation of unexpected strong spontaneous steam explosions in stratified melt-coolant configurations. One of the important conditions for the possible occurrence of an energetic steam explosion is the formation of an appropriate premixture of pre-fragmented melt and coolant. Stratified melt-coolant configurations, i.e. a molten corium layer below a coolant layer, were believed as being unable to generate strong explosive interactions because of the limited amount of existing premixture in favourable conditions for steam explosion. However, the recently performed experiments in the PULiMS and SES (KTH, Sweden) facilities with high melting temperature oxidic simulants of corium revealed that strong steam explosions may develop spontaneously in stratified melt-coolant configurations. In the tests the formation of a considerable melt-coolant premixed layer was observed prior to the explosions. The observed strong spontaneous stratified steam explosions have an important impact on the safety related issue of fuel coolant interactions in nuclear power plants. The purpose of the project is to improve the understanding and modelling of steam explosions in stratified configurations and the application of the developed model to reactor conditions. The main objectives are the following: Analysis of stratified steam explosion experiments in the PULiMS and SES facilities to improve the understanding of stratified steam explosions and development of premixed layer formation model.   Simulation of stratified steam explosion in reactor conditions to demonstrate the applicability of the developed model and to get an estimation of the expected pressure loads. The important issue of stratified steam explosions and its impact on nuclear safety was discovered only recently. Various hypotheses about the possible physical mechanisms responsible for the interface instabilities and the formation of the premixed layer were already proposed, but were not elaborated further and were not checked against experimental results. Also no models for the simulation of such stratified steam explosions have been developed yet. Thus, the results obtained in the frame of the proposed project will be original in respect to both objectives of the project, i.e. the improved understanding and modelling of stratified steam explosions and the estimation of pressure loads during a stratified steam explosion in reactor conditions.
Significance for science
One of the most important remaining issues in core melt progression during a severe nuclear reactor accident is the likelihood and the consequences of energetic fuel-coolant interactions. The emerging issue of stratified steam explosions and related phenomena was recognized internationally. It is addressed in the OECD Status Report on Ex-Vessel Steam Explosion, published in 2017, and in the 7.FP EC SAFEST project. The unexpected strong spontaneous steam explosions in stratified melt-coolant configurations were observed recently. The complex phenomenon is not well understood and no models for the simulation of such stratified steam explosions have been developed yet. Thus the results of the proposed project will be an important and original contribution to the improved understanding and modelling of stratified steam explosions and the estimation of pressure loads during a stratified steam explosion in reactor conditions. Based on the research results it will be possible to more reliably predict the pressure loads during a steam explosion in a nuclear power plant, which is important for safety analyses. Safety analyses are necessary for the risk management to be able to implement the optimal severe accident management approaches. The important issue of stratified steam explosions and its impact on nuclear safety was discovered recently, thus the proposed research is original and may lead to the development of new research directions. The expected project results will present a scientific contribution in the field of fuel-coolant interaction research related to nuclear safety.
Significance for the country
Despite the growing share of wind and solar electricity in the past two decades, nuclear power still represents the second largest (after hydro) low-carbon source of electricity on the planet. The last few years of the German “Energiewende” demonstrate that it will be very difficult to decarbonize our electricity systems without the contribution from nuclear power. These developments justify investments in nuclear engineering R&D, which is an important pillar for the safe operation of nuclear power plants. The results of the proposed research project are the theoretical basis for safety analyses, related to the safe operation of the Krško nuclear power plant (and for other nuclear power plants in Europe and in the World), and to the technical projects for the Krško NPP and the Slovenian Nuclear Safety Administration. The results contribute to the higher level of safety culture in Slovenia. The research directly contributes to the sustainable stable supply of electricity, to environment protection and public health. The results of the project will enable the assessment of the likelihood and the consequences of a steam explosion in the Krško NPP. The research contributes to creating, keeping and improving of own technical knowledge and makes independence on foreign expertise in support of safe Krško NPP operation and helps the regulatory body in the inspection of Krško NPP operation and maintenance. All this contributes to the increase of the economic growth and the increase of the competitive position of Slovenia.
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