Core debris interactions with concrete

Most boiling water reactors have automatic depressurisation systems. Severe accidents predicted to occur at boiling water reactors often involve failure of these automatic systems to ftmction. Improvements of the systems are being investigated. 

The second effect of an overlying water pool (or due to cool-down) is to form a solidified crust at the surface of core debris interacting with concrete. This crust will certainly eliminate aerosol generation by melt entrainment in gases sparging through the core debris. It may also act to trap by interception and by impaction particles produced by vaporisation and condensation of volatile debris constituents. Quantitative assessments of the effects of the crustal material on radionuclide release during core debris interactions with concrete have not been reported. 

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Powers, D. A, et al., I985, VANESA, A Mechanistic Model of Radionuclide Release and Aerosol Generation during Core Debris Interaction with Concrete, NUREG/CR-4308. 

The proceedings of these meetings provide a good indication of the range of experimental and analytical activities devoted to the study of core debris interactions with concrete. Most of the work has dealt with concrete ablation and combustible gas generation. Relatively less work has gone into the study of radionuclide release during core debris interactions with concrete. 

S-19. D.A. Powers, J.E. Brockmann, A.W. Shiver, VANESA A Mechanistic Model of Radionuclide Release and Aerosol Generation During Core Debris Interactions with Concrete,... 

Currently, there is great interest in the possibility that accidents can be arrested by cooling the external surfaces of a reactor vessel. Any cooling of the core debris that can be produced this way will, of course, slow the rate of radionuclide release from core debris. The simultaneous additions of water to debris beds that accumulate within the reactor vessel will also trap radionuclides that are released. The physical phenomena involved are analogous to those involved with water overlying core debris interacting with concrete discussed in Chapter IV. 

Ex-5. D.A. Powers and J.L. Sprung, A Simplified Model of Aerosol Scrubbing by a Water Pool Overlying Core Debris Interacting With Concrete, NUREG/CR-5901, SAND92-1422, Sandia National Laboratories, Albuquerque, NM, November 1993. 

This report presents a discussion of the physical phenomena that lead to attenuation of aerosol production during core debris interactions with concrete by an overl5ang water pool. 

Finally, mention must be made of the effects of other materials released into water in the containment over the course of an accident. Some of these materials are released to the containment during core degradation. The effects of these materials on water pH are not well understood. Hot water will leach calcium hydroxide from concrete and this can cause an increase in pH. Core debris interactions with concrete release copious quantities of aerosol that are rich in species like CaO, Na20 and K2O that will dissolve in water to form hydroxide ions and raise the pH. On the other hand, CaO can precipitate buffers intended to control the water pH. 

Powers, D. A., Sprung, J. I. A simplified model of aerosol scrubbing by a water pool overlying core debris interacting with concrete. Report NUREG/CR-5901 (1992)... 

This report, though dated, provides a thorough discussion of the physical phenomena that affect fission product release and aerosol formation during the interactions of core debris with concrete. 

E. R. Copus, "Core-Concrete Interactions with Overlying Water Pools," 2nd CSNI Specialist Meeting on Core Debris-Concrete Interactions, Karlsruhe, FRG, April 1-3, 1992. 

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