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Ex-Vessel Steam Explosion Analysis for Pressurized Water Reactor and Boiling Water Reactor

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Nuclear Engineering and Technology 48 (2016) 72e86 Available online at ScienceDirect Nuclear Engineering and Technology journal homepage: www.elsevier.com/locate/net Original Article Ex-vessel Steam Explosion Analysis for Pressurized Water Reactor and Boiling Water Reactor * Matjaz Leskovar and Mitja Ursic Jozef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia article info abstract Article history: A steam explosion may occur during a severe accident, when the molten core comes into Received 8 June 2015 contact with water. The pressurized water reactor and boiling water reactor ex-vessel Received in revised form steam explosion study, which was carried out with the multicomponent three- 20 August 2015 dimensional Eulerian fuelecoolant interaction code under the conditions of the Organi- Accepted 23 August 2015 sation for Economic Co-operation and Development (OECD) Steam Explosion Resolution for Available online 28 October 2015 Nuclear Applications project reactor exercise, is presented and discussed. In reactor cal- culations, the largest uncertainties in the prediction of the steam explosion strength are Keywords: expected to be caused by the large uncertainties related to the jet breakup. To obtain some Boiling Water Reactor insight into these uncertainties, premixing simulations were performed with both avail- Fuel-Coolant Interaction able jet breakup models, i.e., the global and the local models. The simulations revealed that Pressure Load weaker explosions are predicted by the local model, compared to the global model, due to Pressurized Water Reactor the predicted smaller melt droplet size, resulting in increased melt solidification and Reactor Cavity increased void buildup, both reducing the explosion strength. Despite the lower active melt Severe Accident mass predicted for the pressurized water reactor case, pressure loads at the cavity walls Steam Explosion are typically higher than that for the boiling water reactor case. This is because of the significantly larger boiling water reactor cavity, where the explosion pressure wave origi- nating from the premixture in the center of the cavity has already been significantly weakened on reaching the distant cavity wall. Copyright © 2015, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society. 1. Introduction within a very short time. High pressure and fast expansion of vapor could potentially induce high loading on the sur- Steam explosion, in the frame of nuclear reactor safety, is a rounding structures. A steam explosion is also called an en- process resulting from the interaction between the core melt ergetic fuelecoolant interaction (FCI). In the case of an ex- (corium) and water [1,2]. Energy transfer from the corium to vessel steam explosion, cavity walls might not be able to the coolant is so fast that a large amount of vapor is produced bear such dynamic loads. Then, the cavity or even the * Corresponding author. E-mail address: [email protected] (M. Leskovar). This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http:// creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any me- dium, provided the original work is properly cited. http://dx.doi.org/10.1016/j.net.2015.08.012 1738-5733/Copyright © 2015, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society. Nuclear Engineering and Technology 48 (2016) 72e86 73 containment might be at risk of damage or even failure [3e5]. liquid, and vapor, without considering the local velocities. The Direct or bypassed loss of the containment integrity can lead local jet breakup model is based on the KelvineHelmholtz to the release of radioactive materials into the environment, instability model, which also considers the local velocities. To threatening the safety of the general public. get some insight into these uncertainties related to jet Details of processes taking place prior to and during a breakup, the premixing simulations were performed with steam explosion have been experimentally studied for a both available jet breakup models. number of years, with adjunct efforts in modeling these pro- The global jet breakup model is, strictly speaking, appli- cesses to address the scaling of experimental results to reactor cable only for single large, very hot jets in a water pool, so that conditions [1,6,7]. Despite great efforts in steam explosion fragmentation occurs due to the friction of the vapor film, research, the confidence in predicting reactor situations is not whose characteristics are governed mainly by buoyant forces. such that an unambiguous decision can be taken whether The model was validated on the FARO facility steam explosion there would be an early failure of the containment due to a tests [12], so extrapolations to situations far from those of steam explosion or not. To resolve the remaining open issues FARO are questionable. In this model, the rate of volumetric on the FCI processes and their effect on ex-vessel steam ex- jet fragmentation into droplets is deduced from the compar- plosion energetics, the OECD project Steam Explosion Reso- ison with a standard case: lution for Nuclear Applications (SERENA) was launched in vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u ! : u 0:5 2007, consisting of an experimental and an analytical part 0 75u m s r G ¼ G T0 t g 0 0 ; [8,9]. To verify the progress made in the understanding and f 0 (1) T m ; s r j g 0 ¼ j j modeling of key FCI processes for reactor applications, a p 1bar reactor exercise was performed at the end of the project. The where typical FARO conditions are chosen for the standard exercise comprises three cases: a pressurized water reactor G ¼ 3 2 case: reference fragmentation rate 0 0.1 m /m /s, jet tem- (PWR) central melt release, a PWR side release, and a boiling ¼ m ¼ À3 perature T0 3,000 K, vapor viscosity g,0 10 kg/m/s, jet water reactor (BWR) central release. r ¼ 3 s ¼ density 0 8,000 kg/m , and jet surface tension 0 0.5 N m. In our ex-vessel steam explosion study, conditions of the The diameter of the created droplets is a user input parameter SERENA project reactor exercise for the PWR and BWR central with a default value of 3 mm, which is the typical average melt release cases were considered. Simulations were carried Sauter diameter in the FARO experiments. out by applying two different jet breakup modeling ap- The local jet breakup model is based on the Kel- proaches. In the following sections, the modeling approach vineHelmholtz instability model, which was modified to take and the considered ex-vessel FCI cases are described first. into account the multiphase aspect. In this model, the volu- Next, the PWR and BWR simulations that were performed are metric jet fragmentation rate is calculated with the following presented. Various premixing and explosion-phase simula- equation: tion results are provided and discussed. Finally, the PWR and rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi À Á BWR simulation results are discussed in comparison. r r À 2 À s r þ r j amb vj vamb jkmax j amb G ¼ ; f Nf r þ r À j Áamb (2) r r À 2 ¼ 2 j amb vj vamb ; 2. Modeling and calculation conditions kmax r þ r s 3 j amb j Simulations were performed with the computer code MC3D where the subscript j stands for the jet and the subscript amb (multicomponent three-dimensional Eulerian fuelecoolant stands for the ambient fluid, the properties of which are interaction code), version 3.6.8 [10,11]. MC3D is a multidi- calculated by averaging considering the phases volume frac- mensional Eulerian code devoted to the study of multiphase tions. Nf is the jet fragmentation parameter with an expected and multiconstituent flows in the field of nuclear safety. The value between 1 and 6. Direct comparisons with the FARO ¼ steam explosion simulation is usually carried out in two steps. experiments lead to the use of Nf 2 [13]. In this model, the First, the premixing phase is simulated followed by the diameter of the created droplets dd is related to the wave- l simulation of the succeeding explosion phase, using the pre- length of instability, which is established from the wave mixing simulation results as initial conditions and applying number kmax [Eq. (2)]: an explosion trigger. p ¼ l ; l ¼ 2 : In reactor calculations, the largest uncertainties in the dd Nd (3) kmax prediction of the steam explosion strength are expected to be caused by the large uncertainties related to jet breakup. These Nd is the droplet diameter parameter with an expected value uncertainties propagate through different premixing pro- between 0.1 and 0.5; the recommend value, based on com- ¼ cesses and result in uncertainties in the generation rate and parisons with the FARO experiments, is Nd 0.2 [13]. size of the melt droplets, distribution of the melt droplets in In this study, conditions of the SERENA project reactor the premixture, droplet solidification, and void fraction, all of exercise for the PWR and BWR central melt release cases were which influence the steam explosion strength [9]. In MC3D, considered. A purpose of the reactor exercise was to verify two jet breakup models are provided: a global model and a whether the pressure loads calculated by various FCI codes local one. The global jet breakup model is based on the hy- are consistent with each other. This objective can be reached pothesis that jet breakup can be achieved through a correla- by applying the codes to a limited number of geometries and tion considering only the local physical properties of the melt, conditions that are generic enough to hold all the 74 Nuclear Engineering and Technology 48 (2016) 72e86 Fig. 1 e PWR scheme with initial conditions [14] and calculation model (vertical/horizontal scale ¼ 0.3).
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