Severe accident source term problem is the calculation of transport and release of radionuclides (RNs) from the reactor core to the environment for a hypothetical accident sequence of low probability. Generally, the source term problem is decoupled into four source terms: 1) In-vessel source term, 2) Interface source term 3) In-containment source term and 4) Environmental source term.
In present work, a medium-size oxide fuelled sodium-cooled fast reactor (1250 MWt) is chosen as the reference design for the calculations. For the analysis, a whole core melt accident initiated by the loss of flow to coolant pumps with the failure of both the shutdown systems (ULOFA) is considered. The temperature and pressure transients due to the burning of ejected sodium are studied. Additionally, the evolution of the RN and sodium aerosols generated from the sodium fire is analysed. For realistic estimates of the in-containment source term, in-vessel release fractions calculated from MINICHEM are used as an input.
The present calculations are performed for the two sets of in-vessel release fractions viz., 1) No mixture 2) Real mixture to be used as input. We found that following an accident, Cs, Rb, I, Xe, and Kr are the dominant suspended species in the containment. Additionally, it is observed that after 5 hours, less than ~15% of the total aerosol mass is suspended. After 24 hours less than ~2% of the total aerosol mass are suspended. The present calculations indicate that an effective containment, holding RN for the first 24 hours would be helpful to reduce the radioactivity available for release. The calculations performed here is valid for the short-term/instantaneous source term. Work is in progress to include aerosol generation models and multi-component aerosol modelling.
|Affiliation/Organization||Indira Gandhi Centre for Atomic Research, India|
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