Estimation of current status inside reactors

Since high radiation dose prevents us from going into the reactor building, it is difficult to know where fuel debris is or how the reactor was damaged. This information regarding the status inside the reactor is quite important to determine the removal method of fuel debris.

Analytically estimating inside of the reactors

◎ Objective

Due to the reduction of the water level in the reactor core during the accident, the fuel assemblies are considered to have been exposed for 6 to 14 hours until the alternative coolant was injected. The temperature of the fuel assemblies exceeded the melting temperature of UO2 and part of the molten fuel (= fuel debris) might have relocated to the primary containment vessel through the reactor pressure vessel at Units 1, 2 and 3.

"The council for the decommissioning of TEPCO's Fukushima Daiichi Nuclear Power Plant" plans to start those fuel debris removals by the end of FY 2020. To make that operation efficient and reliable, it is necessary to know the current status of the reactors including the debris distribution.

◎ R&D activities

The R&D regarding various phenomena inside the reactor in severe accidents has been widely conducted since the Three Mile Island accident (US) in 1979. However, the knowledge obtained by these activities is not enough to evaluate the Fukushima case with sufficient accuracy.

Considering the applicability of the previous studies and taking advantage of recent progress in analytical and experimental techniques, various R&D is performed regarding thermal hydraulics, fuel degradation, failure of pressure vessel lower head and accident analysis with computer codes.

◎ Related facilities

Phenomena in the reactor during a severe accident

An example of simulated molten material falling into water
(Left:experiment, right:calculation)

Detailed 3D analytical model of pressure vessel lower head with weld part

This R&D contributes to part of the "Mid-and-Long-Term Roadmap towards the Decommissioning of TEPCO's Fukushima Daiichi Nuclear Power Station Units 1-4".

  • (2-(2)-1) Further Advancement of Technologies for Analysis of Accident Progression to enable Understanding of Status of Reactor Interiors
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