Early-phase core degradation interpretation by interaction between fuel cladding and fuel components in Fukushima Daiichi nuclear power plant Unit 2

ABSTRACT

The early phase of the core degradation at the Fukushima Daiichi Nuclear Power Station Unit 2 was investigated in terms of a fuel-component degradation by materials interaction by carrying out the accident analysis, evaluation of chemical conditions and experimental works. First, the accident scenario was evaluated to specify a target period (from 18:40 to 22:40 on NaN Invalid Date NaN), and then the temperature and H₂/H₂O pressure ratio in the core region during this period were calculated based on plant parameters. Subsequently, semi-integral experiments were performed using the mocked-up fuel components of a boiling water reactor (BWR). Furthermore, separate-effects tests using Ni-alloy/Zircaloy4 were also performed to understand the phenomenon observed in the semi-integral experiments, where the Ni-Zr reaction occurred even with the oxide layer on the Zr-based alloy. Consequently, it was estimated that the Zr-Ni eutectic melting reaction above 1100°C under steam-starved conditions resulted in a fuel failure at 19:08, melting of spacer locations at 19:37, and upper-tie-plate at 21:26. The first and second degradations could have caused the release of radioactive elements or particles leading to the neutron detection at 21:00 in front of the main gate, and the third degradation could have resulted in the separation of the upper-tie-plate.

KEYWORDS:

• Fukushima Daiichi nuclear power station
• unit 2
• core degradation
• upper-tie-plate degradation
• early accident phase

Nomenclature

$\mathit{C}$	=	Mole fraction of element [-]
$\mathit{D}$	=	Diffusion coefficient [cm2 s−1]
$\mathit{k}$	=	Volumetric mass transfer coefficient [1/s]
$\mathit{L}$	=	Thickness of unreacted Ni-alloy [m]
$\mathit{S}$	=	Surface area of interface region [m2]
$\mathit{t}$	=	Time [s]
$\mathit{V}$	=	Volume of fluid [m3]

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The separate-effects test in this work was funded by the Tokyo Electronic Power Company Holdings, and it was performed in the framework of “TEPCO Decommissioning Frontier Technology Creation Collaborative Research Base” at the Tokyo Institute of Technology. The Zircaloy4 plate was supplied by Nippon Nuclear Fuel Development Co., Ltd.