Experimental Results on Pouring and Underwater Liquid Melt Spreading and Energetic Melt-coolant Interaction

In a hypothetical light water reactor (LWR) core-melt accident with corium release from the reactor  vessel,  the  ultimate  containment  integrity  is  contingent  on  coolability  of  the decay-heated core debris. Pouring of melt into a pool of water located in the reactor cavity is considered in several designs of existing and new LWRs  as a part of severe accident (SA) management strategies. At certain conditions of melt release into the pool (e.g. large ratio of the  vessel  breach  size  to  the  pool  depth),  liquid  melt  can  spread  under  water  and  reach  a coolable configuration. Coolability of the melt depends on decay heat generated per unit area of the spread melt which is directly proportional to the terminal spread thickness of the melt layer. Thus a success of the debris bed coolability depends on the efficacy of the molten core materials spreading which is limited by rapid solidification  of the melt due to melt-coolant heat transfer. Among the factors which can reduce spreading effectiveness are heat and mass losses of the liquid melt due to fragmentation, cooling and solidification in the process of melt jet pouring into the pool. Previous extensive experimental and analytical works on liquid melt spreading and solidification were focused mostly on analysis of melt spreading in case of melt release through an inclined channel. Melt spreading under water as a result of a jet pouring into a pool, has not been addressed systematically. This paper summarizes first experimental results obtained in the frame of Pouring and Underwater Liquid Melt Spreading (PULiMS) research program. The work is an extension of previously reported by Kudinov et al. [1-4] studies  on debris  bed formation and  agglomeration (DEFOR-A)  phenomena.  In contrast  to DEFOR-A experiments, PULiMS exploratory tests (PULiMS-E) discussed in this work have been performed with a shallow (20 cm) water pool. Up to 78 kg of high melting temperature core melt simulant materials (eutectic mixtures of the binary oxides such as Bi 2 O 3 -WO 3  and ZrO 2 -WO 3 )  is  used  in  each  test.  Initial  melt  superheat  varied  from  70  up  to  300ºC.  In  the paper we discuss: (i) experimental observations  of the  jet  pouring into  a  shallow pool  and underwater  liquid  melt  spreading  on  a  flat  surface;  (ii)  characterization  of  solidified  melt debris;  (iii)  key  physical  processes  as  well  as  melt  material  properties  and  experimental conditions  most  influencing  the  melt  spreading  and  solidification  phenomena.  Produced experimental data can be used for validation of the models for prediction of the underwater liquid melt spreading in case of melt jet pouring in a shallow water pool.QC 20131212</p

Experimental and Analytical Study of the Particulate Debris Bed Self-leveling

Melt fragmentation, quenching and long term coolability in a deep pool of water under reactor vessel is employed as a severe accident (SA) mitigation strategy in several designs of light water reactors (LWR). Geometrical configuration of the debris bed is one of the factors which define if the decay heat can be removed from the debris bed by natural circulation. Boiling and two-phase flow inside the bed also serves as a source of mechanical energy which can reduce the height of the debris bed by so called “self-leveling” phenomenon. However, to be effective in providing a coolable geometrical configuration, self-leveling time scale has to be smaller than the time scale for drying out and onset of re-melting of the bed. This paper presents results of experimental and analytical studies concerning the self-leveling phenomenon. The goal of this work is to assess characteristic time scale of particulate debris spreading. In the experiments on the particulate debris spreading air injection at the bottom of the bed is used to simulate steam flow through the porous debris bed. A series of test have been carried out to study the influence of particles size and density, roughness of the spreading plate, gas flow rate etc. on particulate spreading. A semi-empirical model for predicting the spreading of particulate debris has been developed using experimental closures for debris mass flow rate as a function of local (i) angle of the bed and (ii) gas flux. The comparison between the model prediction and the experimental observations shows a good agreement.QC 20131216</p

Experimental Results on Pouring and Underwater Liquid Melt Spreading and Energetic Melt-coolant Interaction

In a hypothetical light water reactor (LWR) core-melt accident with corium release from the reactor  vessel,  the  ultimate  containment  integrity  is  contingent  on  coolability  of  the decay-heated core debris. Pouring of melt into a pool of water located in the reactor cavity is considered in several designs of existing and new LWRs  as a part of severe accident (SA) management strategies. At certain conditions of melt release into the pool (e.g. large ratio of the  vessel  breach  size  to  the  pool  depth),  liquid  melt  can  spread  under  water  and  reach  a coolable configuration. Coolability of the melt depends on decay heat generated per unit area of the spread melt which is directly proportional to the terminal spread thickness of the melt layer. Thus a success of the debris bed coolability depends on the efficacy of the molten core materials spreading which is limited by rapid solidification  of the melt due to melt-coolant heat transfer. Among the factors which can reduce spreading effectiveness are heat and mass losses of the liquid melt due to fragmentation, cooling and solidification in the process of melt jet pouring into the pool. Previous extensive experimental and analytical works on liquid melt spreading and solidification were focused mostly on analysis of melt spreading in case of melt release through an inclined channel. Melt spreading under water as a result of a jet pouring into a pool, has not been addressed systematically. This paper summarizes first experimental results obtained in the frame of Pouring and Underwater Liquid Melt Spreading (PULiMS) research program. The work is an extension of previously reported by Kudinov et al. [1-4] studies  on debris  bed formation and  agglomeration (DEFOR-A)  phenomena.  In contrast  to DEFOR-A experiments, PULiMS exploratory tests (PULiMS-E) discussed in this work have been performed with a shallow (20 cm) water pool. Up to 78 kg of high melting temperature core melt simulant materials (eutectic mixtures of the binary oxides such as Bi 2 O 3 -WO 3  and ZrO 2 -WO 3 )  is  used  in  each  test.  Initial  melt  superheat  varied  from  70  up  to  300ºC.  In  the paper we discuss: (i) experimental observations  of the  jet  pouring into  a  shallow pool  and underwater  liquid  melt  spreading  on  a  flat  surface;  (ii)  characterization  of  solidified  melt debris;  (iii)  key  physical  processes  as  well  as  melt  material  properties  and  experimental conditions  most  influencing  the  melt  spreading  and  solidification  phenomena.  Produced experimental data can be used for validation of the models for prediction of the underwater liquid melt spreading in case of melt jet pouring in a shallow water pool.QC 20131212</p

Insight into steam explosion in stratified melt-coolant configuration

Release of core melt from failed reactor vessel into a pool of water is adopted in several existing designs of light water reactors (LWRs) as an element of severe accident  mitigation  strategy.  When  vessel  breach  is  large  and  water  pool  is shallow,  released  corium  melt  can  reach  containment  floor  in  liquid  form  and spread under water creating a stratified configuration of melt covered by coolant. Steam  explosion  in  such  stratified  configuration  was  long  believed  as  of secondary importance for reactor safety because it was assumed that considerable mass of melt cannot be premixed with the coolant. In this work we revisit these assumptions  using  recent  experimental  observations  from  the  stratified  steam explosion tests  in  PULiMS  facility.  We  demonstrate  that  (i)  considerable  melt-coolant premixing layer can be formed in the stratified configuration with high temperature  melts,  (ii)  mechanism  responsible  for  the  premixing  is  apparently more  efficient  than  previously  assumed  Rayleigh-Taylor  or  Kelvin-Helmholtz instabilities.  We  also  provide  data  on  measured  and  estimated  impulses, energetics  of  steam  explosion,  and  resulting  thermal  to  mechanical  energy conversion ratios. QC 20131212</p

Insight into steam explosion in stratified melt-coolant configuration

Release of core melt from failed reactor vessel into a pool of water is adopted in several existing designs of light water reactors (LWRs) as an element of severe accident  mitigation  strategy.  When  vessel  breach  is  large  and  water  pool  is shallow,  released  corium  melt  can  reach  containment  floor  in  liquid  form  and spread under water creating a stratified configuration of melt covered by coolant. Steam  explosion  in  such  stratified  configuration  was  long  believed  as  of secondary importance for reactor safety because it was assumed that considerable mass of melt cannot be premixed with the coolant. In this work we revisit these assumptions  using  recent  experimental  observations  from  the  stratified  steam explosion tests  in  PULiMS  facility.  We  demonstrate  that  (i)  considerable  melt-coolant premixing layer can be formed in the stratified configuration with high temperature  melts,  (ii)  mechanism  responsible  for  the  premixing  is  apparently more  efficient  than  previously  assumed  Rayleigh-Taylor  or  Kelvin-Helmholtz instabilities.  We  also  provide  data  on  measured  and  estimated  impulses, energetics  of  steam  explosion,  and  resulting  thermal  to  mechanical  energy conversion ratios. QC 20131212</p