Based on the lessons learned from the TEPCO’s Fukushima Daiichi NPPs’ accidents, evaluation of fission products (FPs) transfer during the severe accidents is quite important in the regulation activities.
In core expansion phase caused by core disruptive accident (CDA) of sodium-cooled fast reactors (SFRs), a large bubble consisting of mixture of core materials and sodium vapor inside, so-called CDA bubble, expands rapidly and accelerates the sodium slug of the upper plenum to the shielding plug to threaten the reactor vessel integrity. If the reactor vessel integrity fails, FPs contained in the coolant and in the cover gas may release into the containment vessel.
To evaluate the FPs transfer in the reactor vessel during core expansion phase, it is important to simulate the FP transfer by coupling with CDA bubble behavior such as vaporization and condensation. Based on this background, S/NRA/R has started developing the method by utilizing the calculation codes: ACTOR and ASTERIA-SFR.
ACTOR  has been developed for evaluation of FPs transfer in the primary cooling system during Protected Loss-of-Heat-Sink accident. ACTOR has models to simulate major phenomena: FP release from the fuel pin, bubble transport including break-up and coalescence in sodium, FPs transfer from bubble to coolant, and FP diffusion into the sodium. These analytical models, however, are insufficient to simulate FP transfer of CDA bubble behavior.
ASTERIA-SFR  is a CDA analysis code which has capability to calculate interfacial area of CDA bubble considering vaporization and condensation of core materials and sodium, and to calculate transfer of the materials based on mechanistic fluid dynamics calculation. ASTERIA-SFR has potential to simulate not only thermal-hydraulics behavior of CDA bubble but also the FP transfer adequately.
In order to enhance the capability, S/NRA/R has started development of methodology on in-vessel source term evaluation during the ULOF event. The methodology utilizes two computer codes: ACTOR and ASTERIA-SFR. In this framework, ACTOR is incorporated to a plant dynamics analysis code in order to simulate FPs transfer behavior based on more precise thermo-hydraulics calculation. To evaluate FPs transfer behavior with CDA bubble, fluid dynamics calculation module of ASTERIA-SFR is improved to model cesium component.
This paper describes the plan and progress of the development of the in-vessel source term evaluation method. It is also discussed that calculation results of CDA analysis focusing on FPs transfer behavior during the ULOF event.
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|Affiliation/Organization||Nuclear Regulation Authority, Japan|