Research and Development Relating to the Accident at the TEPCO’s Fukushima Daiichi NPS

1-15 Prevention of Re-Criticality in Fuel Debris in the Damaged Coresｒ

Fuel debris: Composite of damaged
fuel and core components

Fig.1-32 Fuel debris in damaged coreTens of thousands of kilograms of fueldebris, namely, a composite of fuel andcore components, has been generated
as a result of severe core meltdown atthe 1F.

Concretemoderator

Fuel particle

Fig.1-33 Example of calculation
model of fuel debris
This model represents an exampleof supposed fuel debris consistingof small fuel particles dispersed inconcrete.

The Great East Japan Earthquake on March 11, 2011 caused
a severe accident at the TEPCO’s Fukushima Daiichi NPS (1F).
As shown in Fig.1-32, the cores of the reactors in 1F1–1F3
were seriously damaged owing to a failure of the core coolingsystem. The amount of fuel debris, which is a composite ofdamaged fuel and other core components, is estimated to betens of thousands of kilograms in each unit.
There has been no indication of re-criticality of the fueldebris since the accident. The risk of re-criticality during
removal of the fuel debris from each core should still beconsidered carefully because the minimum critical mass ofthe fresh fuel is only tens of kilograms. Although criticality
control of normal fuel handling is designed on the basis ofsome determined fuel conditions, there are many unknowns
regarding the fuel debris, such as its composition or shape.
Therefore, an exhaustive criticality database including any
conjectured conditions of the fuel debris, called a “criticalitymap,” needs to be prepared to judge the criticality risk quicklyon the disclosure of its actual conditions.
We are analyzing the criticality of fuel debris considering
many parameters, such as water content or fuel burn-up, by

1.6
1.4
1.2

←

The core had melted
and dropped down

Fuel : Concrete = 1 : 7
Mirror reflection(simulating infinitematerial)

Can be critical

Damaged Reactor

Infinite multiplication factor－ Criticality Characteristics of Fuel Debris Containing Concrete －1.0
0.8
235U 5wt% fresh fuelSpent fuel without FPs
Spent fuel with FPs

0.6
0.4
0.2

0.1

110Heterogeneity (radius r in Fig.1-33) (cm)Fig.1-34 Infinite multiplication factor of supposed
fuel debris
It is indicated that the fuel debris modeled in Fig.1-33
can be critical under various conditions. Such fueldebris must be controlled carefully to prevent recriticality.

referring to previous studies in the literature.
An example of the calculation model is shown in Fig.1-33.
The model represents fuel dispersed in concrete, which is amajor material of the pedestal floor of the containment vessel.
The infinite multiplication factors of the fuel debris areshown in Fig.1-34. The lines in the red area indicate theconditions under which the fuel debris may be critical.
Moreover, another calculation shows that the minimum criticalmass of the fuel debris can be less than several thousand
kilograms, which is significantly smaller than the amountof fuel in each core. The fuel debris containing concrete,
therefore, must be handled cautiously to prevent re-criticality.
We continue the effort to complete the criticality map bycomputation and critical experiments considering fuel debris
under diverse conditions, such as a submerged fuel–concrete
mixture and a composite of fuel and the steel of the pressure
vessel. The results are expected to provide useful informationfor judging the criticality risk for any actual fuel debris, andto contribute to the development of reliable re-criticality
prevention and detection measures.

Reference
Izawa, K. et al., Infinite Multiplication Factor of Low-Enriched UO2-Concrete System, Journal of Nuclear Science and Technology, vol.49, no.11, 2012,
p.1043-1047.

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JAEA R&D Review 2013