Core—concrete interaction

Its unique design suggests several accident scenarios that could not occur at other reactors. For example, failure to supply ECC to 1/16 of the core due to the failure of an ECC inlet valve. On the other hand, some phenomena of concern to other types of reactors seem impossible (e.g., core-concrete interactions). The list of phenomena for consideration came from previous studies, comments of an external review group and from literature review. From this, came the issues selected for the accident progression event tree (APET) according to uncertainty and point estimates. 

Cole, R. K., Kelley, D. P. and Ellis, M. A. (1984) CORCON-Mod 2 A computer programme for analysis of molten core-concrete interactions , NUREG/CR-3920, August. 

An analysis should also be carried out to determine how the containment basemat might fail as a result of the molten core-concrete interaction which would occur after pressure vessel failure. Estimates should be made of the conditional probability of basemat failure as a function of the residual heat level and the coohng available to the molten material. Special care should be taken when the basemat of the containment has additional compartments above so that penetration of the basemat could lead to a radioactive release via unfiltered pathways. 

Ex-6b. R.E. Blose et al., Core-Concrete Interactions with Overlying Water Pools, NUREG/CR-5907, SAND92-1563, Sandia National Laboratories, Albuquerque, NM, November 1993. 

In the following description of the reactions occurring during this stage of a severe core damage accident, three different topics will be discussed the release of fission products from the fuel, the release of constituents of the core structural and control rod materials (although these two sources develop almost simultaneously in the reactor pressure vessel so that the volatilized substances can be assumed to enter the gas flow as a mixture) and, finally, volatilization of substances during the molten core - concrete interaction phase. The current state of the art will be discussed with special emphasis on the important chemical phenomena no attempts will be made to establish numerical values of source terms from the results of these experimental and theoretical efforts. 

Upon contact, the molten core material (the so-called corium ) starts to react with the material of the basemat concrete, with the details of this reaction depending on the particular conditions. During the first hours of the molten core — concrete interaction, the temperature of the melt decreases from 2400 to about 1500 C. Due to the radial progress of the melt in the reactor cavity, in the KWU plant design siunp water is expected to pour down onto the melt about 6 to 7 hours after the beginning of the core - concrete interaction. When the reaction zone is flooded by sump water, an exothermal reaction between the metallic constituents of the melt which have not yet been oxidized inside the reactor pressure vessel (in particular Ziicaloy) and the water may start. The consequence of this reaction is a considerable increase in temperature, so that it is assumed that in this phase of the accident the highest temperatures might be reached. 

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... 

Table 7.10. Calculated release fractions of different elements during molten core - concrete interaction...

According to the qualitative results obtained from informal Sascha experiments (Albrecht, 1987 b), silver is largely vaporized under the conditions of the molten core — concrete interaction. This means that any silver not volatilized during the in-vessel phase would become airborne in this late stage of the accident and be transported to the containment sump water phase, i. e. at a moment when volatilized iodine would have been largely plated out in the containment sump water. 

A major task of the ACE Phase C experiments was validation of data obtained from code calculations. Measured concrete ablation rates agreed to within about 35% with the predictions obtained with various codes such as CORCON, Wechsl etc. Agreement of the fission product release data between the experiments and model predictions, however, was considerably worse, demonstrating that chemical models in which the formation of silicates and zirconates during the core - concrete interaction phase is not taken into account predict release fractions that are one to two orders of magnitude higher than the measured values. 

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. 

The reactor vessel lower head has no vessel penetrations, thus eliminating penetration failure as a potential vessel failure mode. Preventing the relocation of molten core debris to the containment eliminates the occurrence of several severe accident phenomena, such as exvessel fuel-coolant interactions and core-concrete interaction, which may threaten the containment integrity. Therefore, AP 1000, through the prevention of core debris relocation to the containment, significantly reduces the likelihood of contaimnent feilure. 

Melt Release, RPV Failure, No Core-Concrete Interaction... 

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. 

Based on melt inventory at start of core-concrete interaction. 

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