Fukushima Daiichi accident study : status as of April 2012.

Abstract

In response to the accident at the Fukushima Daiichi nuclear power station in Japan, the U.S. Nuclear Regulatory Commission (NRC) and Department of Energy agreed to jointly sponsor an accident reconstruction study as a means of assessing severe accident modeling capability of the MELCOR code. MELCOR is the state-of-the-art system-level severe accident analysis code used by the NRC to provide information for its decision-making process in this area. The objectives of the project were: (1) collect, verify, and document data on the accidents by developing an information portal system; (2) reconstruct the accident progressions using computer models and accident data; and (3) validate the MELCOR code and the Fukushima models, and suggest potential future data needs. Idaho National Laboratory (INL) developed an information portal for the Fukushima Daiichi accident information. Sandia National Laboratories (SNL) developed MELCOR 2.1 models of the Fukushima Daiichi Units 1, 2, and 3 reactors and the Unit 4 spent fuel pool. Oak Ridge National Laboratory (ORNL) developed a MELCOR 1.8.5 model of the Unit 3 reactor and a TRACE model of the Unit 4 spent fuel pool. The good correlation of the results from the SNL models with the data from the plants and with the ORNL model results provides additional confidence in the MELCOR code. The modeling effort has also provided insights into future data needs for both model development and validation.

Technical Approach and Results from the Fuels Pathway on an Alternative Selection Case Study

“…One such case study is a margins-based comparison of the behavior of Zircaloy cladding with new cladding designs based on silicon carbide (SiC). Zircaloy cladding works well under normal conditions, but it is well known that loss of core cooling can lead to core melt, with release of radioactive material into containment, as occurred at Three Mile Island in 1979 [2], and release of radioactive material from containment, as occurred at Fukushima in 2011 [3]. In addition to loss of the fuel cladding barrier in these accidents, a significant amount of combustible hydrogen was also produced, leading to additional damage and affecting the release of radioactive material at Fukushima.…”

“…In addition, the research work has identified new phenomena, such as breakaway oxidation and oxygen diffusion from the cladding inside surfaces, which are believed to further adversely affect the fuel cladding embrittlement process. Thus, post quench ductility (which is necessary to ensure coolable core geometry) 3 is not guaranteed following a postulated LOCA. The proposed rule would establish new requirements for zirconium-based alloys to prevent breakaway oxidation and account for oxygen diffusion from the oxide fuel pellet during the operating life of the fuel.…”

Material Selection for Accident Tolerant Fuel Cladding

“…SINCE the nuclear accident in Japan in 2011, [1,2] considerable research has been devoted to identify light water reactor (LWR) fuel systems that tolerate severe accident scenarios (i.e., beyond design basis accidents). [3][4][5][6][7][8][9] The objective is to provide larger safety margins, i.e., delay the onset of severe LWR core degradation by reducing the rate of H 2 and heat generated by the rapid oxidation of Zr-base alloy cladding and increase the coping time after an accident.…”

Analysis of Pb-Bi Vessel Wall Ablation Experiment with High Temperature Liquid by MPS Method

“…However, there seems to be large uncertainties among different codes (e.g., MELCOR, MAAP and SAMPSON) in predictions of RPV lower head failures [5][6][7]. In these system analysis codes, where phenomena associated with lower head failure are poorly understood, such as penetration failure, the models are very simple and parametric.…”