Interpretation of the CABRI-RAFT TPA2 Test

Yamano, Hidemasa   ; Onoda, Yuichi ; Tobita, Yoshiharu; Sato, Ikkenn 

During the course of core disruptive accidents in liquid-metal fast reactors, a boiling pool of molten fuel/steel mixture could be formed. The stability of this boiling-pool, for which in-pile experimental data with real reactor materials are very limited, plays an important role in the determination of the accident scenarios. In the TPA2 test of the CABRI-RAFT program (from 1996 to 2002), the fuel-to-steel heat transfer characteristic governing the pool behavior was investigated as a joint study with the French 'Institut de Radioprotection et de Surete Nucleaire' (IRSN). This test was performed in the CABRI reactor in 2001 using a test capsule that contains fresh 12.3% enriched UO pellets with embedded stainless steel balls. Following a pre-heating phase, the capsule was submitted to a transient overpower resulting in fuel melting and steel vaporization. The steel vapor-pressure build-up observed during the transient was quite weak, suggesting the presence of a strong mechanism to limit the fuel-to-steel heat transfer. The detailed experimental data evaluation suggested a phenomenon that the steel vaporization at the surface of steel ball blanketed the steel from molten fuel. This vapor blanketing seems to be a mechanism reducing the fuel-to-steel heat transfer. An analysis with the SIMMER-III code, a multi-component multi-phase thermal-hydraulics code, was performed in this study. This code simulation could well reproduce the pressure buildup and boiling pool behavior which occurred in the test by applying specifically reduced heat transfer coefficients.