Aerosol core-concrete interaction

When source terms for a complete core melt accident (in which the melt progress could not be stopped within the reactor pressure vessel) have to be calculated, the aerosol production during the core - concrete interaction phase also has to be taken into consideration. In the Reactor Safety Study (US NRC, 1975), an empiric approach was used with respect to the fission product release during this phase, in recognition of the fact that during this stage the environment is chemically oxidizing and that a metallic iron phase is present. From this approach, it was concluded that the remainder of the volatile fission products still present in the molten corium... 

In the gas—steam flow which enters the containment in the low-pressure accident sequence, maximum aerosol densities in the range of 20 g/m may occur. When the core — concrete interaction begins, about 1 to 3 Mg of aerosols in total are assumed to be present in the containment atmosphere, according to corresponding calculations. 

As the consequence of the molten core - concrete interaction, the venting gas will contain huge amounts of CO2. However, due to the presence of NaOH in the scrubber solution, its pH is stabilized at a value of about 9 by a COs —HCOs"-CO2 equilibrium as a result, the retention efficiency of the scrubber solution for elemental iodine is not adversely affected by the continuing CO2 flow. Other substances which are potentially transported to the scrubber solution (such as boric acid, aerosols, decomposition products of organic compounds etc.) do not lead to a pH decrease down to values at which significant decomposition of thiosulphate is to be expected. 

Basemat meltthrough is a long term result of core-concrete interactions. These interactions can generate hydrogen and other noncondensible gases, generate copious amounts of radioactive and nonradioactive aerosols, and eventually fail the basemat. Core-concrete interactions will be discussed in more detail in a later section. 

For the containment the natural discretisation is into interconnected subcompartments (unless a fully three-dimensional treatment is envisaged), as a discussion between experts on CONTAIN and JERICHO emphasised. Each volume may then have sub-databases describing the walls, internal structures, sump, and atmosphere. This last will have its own database including thermal-hydraulic variables plus an aerosols database and so on. The details will become clearer as code integration proceeds, but the overall philosophy is clear. The description for all systems (core, circuit, containment) is to be in terms of volumes, with a well-defined and natural tree structure being imposed on the data describing each volume. Database definitions are now available for the core/bundle, circuit and containment (including iodine chemistry), and are underdevelopment for core-concrete interaction. 

There is no experiment (not even the TMI accident) which represents all features of a severe accident (i.e., primary system thermal/hydraulics in-vessd core damage fission product and aerosol release, transport and deposition ex-vessel core-concrete interaction containment thermal/hydraulics and hydrogen transport and combustion), and only the TMI accident is at full plant scale. It is therefore necessary for severe acddent codes to supplement standard assessment against experiment (and against simple problems with analytic or otherwise obvious solutions) with plant calculations that cannot be hilly verified, but that can be judged using expert opinion for reasonableness and internal self-consistency (particularly using sensitivity studies) and also can be compared to other code calculations for consistency. 

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. 

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,... 

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. 

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. 

Experimental demonstrations of the power of an overlying water pool to attenuate aerosol production during melt-concrete interactions even though the core debris cannot be quenched. 

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. 

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