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Estimation and understanding of the state of the fuel debris and fission products inside the plant comprise an essential step in the decommissioning of Tokyo Electric Power Company Holdings’ Fukush...

In the TPA2 test of the CABRI-RAFT program, which is part of a fast reactor safety study, fuel-to-steel heat transfer characteristics within a molten fuel/steel mixture system have been investigated. This test was performed in the French CABRI reactor and used a test capsule that contained fresh 12.3%-enriched UO2 pellets with embedded stainless steel balls. Following a preheating phase, the capsule was subjected to a transient overpower that resulted in fuel melting and steel vaporization. The observed steel vapor pressure buildup was quite low, which suggested the presence of a mechanism that significantly reduced the fuel-to-steel heat transfer. A detailed experimental data evaluation by SIMMER-III led to one possible interpretation that the steel vaporization at the surface of the steel ball blanketed the steel from the molten fuel.

In the CABRI-2 program, 12 tests were performed under various transient conditions covering a wide range of accident scenarios using two types of preirradiated fast breeder reactor (FBR) fuel pins with different smear densities and burnups. For each fuel, a nonfailure-transient test was performed, and it provided basic information such as fuel thermal condition, fuel swelling, and gas release. From the failure tests, information on failure mode, failure time, and axial location was obtained. Based on this information, failure conditions such as fuel enthalpy and cladding temperature were evaluated. These failure conditions were compared with the CABRI-1 tests in which different fuels as well as different transient conditions were used. This comparison, together with supporting information available from existing in-pile and out-of-pile experiments, allowed an effective understanding on failure mechanisms depending on fuel and transient conditions. It is concluded that pellet-cladding mechanical interaction (PCMI) due to fuel thermal expansion and fission-gas-induced swelling is playing an important role on mechanical clad loading especially with high smear density and low fuel-heating-rate conditions. At very high heating-rate conditions, there is no sufficient time to allow significant fuel swelling, so that cavity pressurization with fuel melting becomes the likely failure mechanism. Fuel smear densitymore » and fission-gas retention have a strong impact both on PCMI and cavity pressurization. Furthermore, pin failure is strongly dependent on cladding temperature, which plays an important role in the axial failure location. With the low smear-density fuel, considerable PCMI mitigation is possible leading to a high failure threshold as well as in-pin molten-fuel relocation along the central hole. However, even with the low smear density fuel, PCMI failure could take place with an elevated cladding-temperature condition. On the other hand, in case of a sufficiently long transient timescale, such low smear density fuel has a potential to allow gas escape to plenum leading to a very effective mitigation of swelling-induced PCMI.In case of very high cladding temperature near its melting point, plenum-gas blowout at cladding rupture takes place before fuel disintegration. Fuel-disintegration behavior under this condition is dominated by fuel enthalpy, and no special effect of the high burnup can be identified through comparison with the CABRI-1 test results.« less

Slow ramp-type transient-overpower tests were performed within the framework of the international CABRI-2 experimental program. The implemented power transients of ~1% nominal power/s correspond to...

In this paper an improved method to evaluate key phenomena in the initiating-phase energetics of unprotected loss-of-flow (ULOF) whole-core accidents in liquid-metal fast breeder reactors is presented. Three phenomena, namely, axial fuel expansion, fuel failure, and postfailure fuel motion, have been examined through the CABRI-1 in-pile experiments and analyses with special emphasis on the self-limiting mechanisms of the energetics potential. For the experiment analyses, the SAD3D, PAPAS-2S, and SAD4A computer codes are employed selectively to obtain a detailed investigation of the phenomena and to validate physical models. The improved knowledge obtained through the research efforts in CABRI-1 and relevant safety experiments has been implemented in the revised SAS3D code. This evaluation method, which accounts for the self-limiting mechanisms, has been applied to a reactor analysis of an energetic ULOF sequence. The results of the application study confirm the importance and effectiveness of the method.