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Motivated by the interest in steam explosion in chemical solutions and seawater, a series of tests were carried out in the MISTEE facility at KTH to investigate steam explosion characteristics as multiple molten droplets of tin were falling through a coolant pool containing deionized water, boric acid solution, neutral solution of boric acid and sodium phosphate, and seawater, separately. The experimental results revealed distinct and complex characteristics of steam explosion of multiple droplets, which were not observed in previous single-droplet steam explosion experiments. The tin melt samples of 5 g and 20 g were employed to formulate different numbers of multiple droplets. In the test with 5 g melt, steam explosion was more energetic at a deeper explosion location − a similar trend found in the single-droplet steam explosion test with 1 g melt. However, the test of 20 g melt did not show a clear trend in a wide range of explosion depth. The peak pressure and impulse increased with increasing mass of melt sample. The steam explosion occurred more closely to the coolant pool surfaces in the seawater and chemical solutions than in deionized water. Steam explosion intensity was significantly reduced in a neutral solution containing 1.2 wt.% boric acid and sodium phosphate. The influence of the chemical solutions on steam explosion was diminishing in the tests with multiple droplets., QC 20240527

The main results of the major international activities related to fuel coolant interactions (FCI) of the last 4-year period are presented and a summary of the knowledge gained regarding understanding and the improvements of modelling is provided. At first, the major outcomes of the OECD SERENA-2 program are presented and discussed. Important clarifications were obtained on the so-called material effect and on FCI code capabilities. We then summarise complementary analytical analyses and experimental programs performed in the frame of the SARNET community. The focus was put on the role of melt fragmentation and solidification, the impact of void on the intensity of an explosion and the triggering mechanisms. As a conclusion, tables summarising the improvements are proposed as well as research priorities., QC 20150109

Ex-vessel severe accident mitigation strategy in Nordic type Boiling Water Reactors (BWRs) imply that melt released into a deep pool of water below reactor vessel will form a coolable by natural circulation porous debris bed. However, if liquid melt is not completely fragmented and quenched when it reaches the bottom of the pool it can cause agglomeration of debris, increasing hydraulic resistance and thus worsening coolability of the bed. In the previous work we have developed and validated an approach to prediction of mass fractions of agglomerated debris using Fuel-Coolant Interaction (FCI) code VAPEX-P. This paper discusses development of a surrogate model (SM) which can predict fraction of agglomerated debris with high computational efficiency. Such model is a must for affordable sensitivity and uncertainty analysis in different accident scenarios. Details of the SM development and verification against full model are provided in the paper., QC 20141230

Ex-vessel termination of accident progression in Swedish type Boiling Water Reactors (BWRs) is contingent upon efficacy of melt fragmentation, quenching, solidification and formation of a coolable by natural circulation porous debris bed in a deep pool of water below reactor vessel. When liquid melt reaches the bottom of the pool it can cause formation of agglomerated debris and "cake" regions, which affect hydraulic resistance and thus coolability of the bed. This paper discusses development and validation of conservative-mechanistic and best estimate approaches to quantifying mass fractions of agglomerated debris at given conditions of melt release from the vessel. Fuel coolant interaction (FCI) code VAPEX-P is used as a computational vehicle for modeling. Experimental data from the DEFOR-A (Debris Bed Formation and Agglomeration) tests with binary oxidic simulant material melt is used for validation of developed methods. The paper discusses the influence of different inherent uncertainties in the prediction of the fraction of agglomerated debris., QC 20130930

After four and a half years of operation in the frame of the 6th Framework Programme (FP6) of the European Commission, SARNET (Severe Accidents Research NETwork of excellence) continues in the FP7 (project named SARNET2) from April 2009 for 4 years. Forty-one organisations from 21 countries network their capacities of research in order to resolve the most important remaining uncertainties and safety issues on severe accidents (SA) in existing and future water-cooled nuclear power plants (NPPs). It includes a large majority of the Europeanactors involved in SA research plus a few non-European important ones. The objective is to perform the common research programmes that have been defined in the network first phase and to continue to improve the common computer tools and methodologies for NPP safety assessment. It will consolidate the sustainable integration of the European SA research capacities. These research programmes concern essentially the six highest priority safety issues that were identified after ranking in the first phase of the network: in-vessel core coolability, molten-corium-concrete-interaction, fuel-coolant interaction, hydrogen mixing and combustion in containment, impact of oxidising conditions on source term, and iodine chemistry. The Joint Programme of Activities includes the following main tasks: Performing new experiments on the above mentioned issues andjointly analysing their results in order to elaborate a common understanding of the concerned physical phenomena; Continuing the development and assessment of the ASTEC integral computer code (jointly developed by IRSN and GRS to predict the NPP behaviour during a postulated SA), which capitalizes in terms of models the knowledge produced in the network. In particular efforts are being extended to its applicability to BWR and CANDU NPP types; Continuing the storage of the SA experimental results in a scientific database, based on the STRESA JRC tool; Promoting educational and training courses, QC 20140825

Ex-vessel debris bed coolability is cornerstone for severe core melt accident mitigation scheme adopted in Swedish type BWRs. Debris agglomeration can significantly affect coolability of the debris bed. The paper discusses an approach for conservative-mechanistic assessment of the debris agglomeration. In order to tackle with considerable aleatory and epistemic uncertainties which present in accident scenario and in the debris agglomeration phenomena we apply mechanistic simulation tool VAPEX code and conservative assumptions about agglomeration related phenomena and accident scenario. Results of systematic analysis arc presented in a form of agglomeration mode map. The map shows conditions for transition between different modes of debris agglomeration. Assessments of the influence of epistemic uncertainties and of numerical discretization errors on simulation result are provided. Safety implication aspects of the proposed agglomeration mode map are discussed in the present paper., QC 20110201

Cooling and solidification of metal oxide droplets in water are considered, using a single-particle model which takes into account heat conduction and thermal radiation transfer within the particle. It is shown that, for millimeter-size particles, near-infrared absorption of the particle's substance determines the solidification pattern and dynamics. For semi-transparent aluminum oxide particles, the rate of surface solidification is controlled by convective heat transfer. For opaque corium particles, thermal radiation from the particle surface leads to fast surface solidification. The impact of so-formed crust layer on subsequent particle fragmentation is discussed with respect to its influence on steam explosion., QC 20100525