The main objective of the coordinated research project (CRP) on is to simulate the fission product transportation behavior of the reference pool-type sodium-cooled fast reactor (SFR) of 1250 MWth capacity with mixed oxide fuel under severe accident conditions. The accident considered is an unprotected loss of flow accident resulting in a core damage event with release of radionuclides. The work package 3 (WP-3) essentially models and simulates the ‘in-containment phenomena’ after the postulated severe accident, which includes sodium chemical reactions and aerosol mass evolution in the containment.
Seven organizations from six countries participated in WP-3. CIAE, CEA, and TerraPower used CONTAIN-LMR code and its derivatives. XJTU used REBAC-SFR code. IGCAR used SOFIRE based fire simulation code, PFIRE, and PANDICA for in-containment aerosol evolution. IBRAE RAN used multiphysics EUCLID/V2 code and CIEMAT used ASTEC-Na code.
To decouple this part of analysis from previous stages of calculation, the stand-alone calculation was defined for WP-3, which uses a set of pre-defined release fractions. The stand-alone case is appropriate for inter-comparison with respect to assessing the tools of WP-3 calculations. In the coupled case, the release fractions of radionuclides computed at the previous work packages were used as initial conditions. Both IBRAE RAN and IGCAR participated in this coupled simulation. The purpose of this effort is to demonstrate the conservatism built in the WP-3 inputs such as release fractions, chemical forms, and sodium ejection amounts. The obtained discrepancies in activities of airborne fission products (FP) in containment take place mainly due to differences in modeling of FP release from molten fuel and of aerosol growth and deposition.
In both integral and stand-alone calculations, the sodium fire and subsequent radionuclide release are modeled in two separate cases: (1) sodium spray fire with instantaneous reaction that results in the highest containment pressure while the containment does not leak, and (2) sodium pool fire that results in a prolonged burning of sodium in a compartment while the containment is leaking at the design leak rate.
The sodium spray fire exercise sets the baseline for the participants to compare the results based on the relatively straightforward boundary conditions. The pool fire case shows observable differences among the organizations due to the complexity of the sodium reaction phenomena, which also drive the aerosol release into the containment via the small compartment. In conclusion, there is broad consensus among the predicted results in WP-3 with respect to the stand-alone case.
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