Unit 2 reactor (TMI-2), heavily damaged and melted fuel assemblies formed a large
amount of fuel debris whose composition was unknown except the possible highest
235 U/U enrichment, 3 wt%, whose geometry is uncertain, and whose mass is larger
than the minimum critical mass derived from the enrichment. Moreover, the fuel
debris had to be cooled and shielded by water. To overcome this uncertain situation,
the coolant water was borated with a concentration, >4,350 ppm, sufficient to
secure the subcritical condition [2].
The situation of the damaged reactors in Fukushima Daiichi Nuclear Power
Station (1FNPS) is more severe than that of TMI-2 because of the water issue. The
most major difference is that the coolant water flow is practically “once through.”
Boron should be ceaselessly added in the water to maintain its lowest concentration
necessary to secure the subcritical condition, which is not feasible. The water is not
borated relying on the circumstantial evidence that the xenon gas monitoring in the
containment vessels (CVs) does not show a sign of criticality. Although the fuel
debris will not be touched for a while, its condition may change because of a
gradual drop of its temperature or change of its geometry by aftershocks. The
condition will be intentionally changed when the fuel debris is retrieved. Every
such change may lead to the criticality of fuel debris [3].
To avoid criticality and its severe consequences, a certain principle of criticality
control must be established. There may be options, such as prevention of criticality
by coolant water boration or by neutronic monitoring, prevention of the severe
consequences of criticality, etc. Each has merits and demerits.
It is necessary to understand the actual condition of the fuel debris regarding the
selection of an appropriate principle from those options and the realization of
certain criticality control following the selected principle. Adequate observation,
sample taking, and analysis of the fuel debris must be conducted.
21.2 Present Condition of 1FNPS Fuel Debris
Fuel assemblies with the design called “BWR STEP 3” had been loaded in the
reactors. Each new fuel assembly contains six kinds of uranium dioxide (UO 2 ) fuel
(Fig. 21.1, Table 21.1). The most popular initial
235 U/U enrichment in the fuels is
4.4 wt%, whose inventory per assembly is 76.8 kgU. The fuel of 9.6 kgU per
assembly has the highest initial enrichment of 4.9 wt%. The initial uranium
inventory in total is 170.9 kgU per assembly, including fuels of other enrichments
and of the UO 2 -gadolinium oxide (Gd 2 O 3 ) composite [4].
The Unit 1 reactor in 1FNPS had 400 assemblies, which consisted of six batches
of burn-up. Each of the Unit 2 and 3 reactors had 548 assemblies of five batches.
Among these assemblies, 64 in the Unit 1 reactor, and 116 in the Unit 2 reactor, had
a low burn-up of only 3–5 GWD/t (Table 21.2). Other assemblies of the same
number are older but still have a burn-up as low as 15–16 GWD/t. The oldest
assemblies have a burn-up of about 40 GWD/t [5].
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K. Tonoike et al.
amount of fuel debris whose composition was unknown except the possible highest
235 U/U enrichment, 3 wt%, whose geometry is uncertain, and whose mass is larger
than the minimum critical mass derived from the enrichment. Moreover, the fuel
debris had to be cooled and shielded by water. To overcome this uncertain situation,
the coolant water was borated with a concentration, >4,350 ppm, sufficient to
secure the subcritical condition [2].
The situation of the damaged reactors in Fukushima Daiichi Nuclear Power
Station (1FNPS) is more severe than that of TMI-2 because of the water issue. The
most major difference is that the coolant water flow is practically “once through.”
Boron should be ceaselessly added in the water to maintain its lowest concentration
necessary to secure the subcritical condition, which is not feasible. The water is not
borated relying on the circumstantial evidence that the xenon gas monitoring in the
containment vessels (CVs) does not show a sign of criticality. Although the fuel
debris will not be touched for a while, its condition may change because of a
gradual drop of its temperature or change of its geometry by aftershocks. The
condition will be intentionally changed when the fuel debris is retrieved. Every
such change may lead to the criticality of fuel debris [3].
To avoid criticality and its severe consequences, a certain principle of criticality
control must be established. There may be options, such as prevention of criticality
by coolant water boration or by neutronic monitoring, prevention of the severe
consequences of criticality, etc. Each has merits and demerits.
It is necessary to understand the actual condition of the fuel debris regarding the
selection of an appropriate principle from those options and the realization of
certain criticality control following the selected principle. Adequate observation,
sample taking, and analysis of the fuel debris must be conducted.
21.2 Present Condition of 1FNPS Fuel Debris
Fuel assemblies with the design called “BWR STEP 3” had been loaded in the
reactors. Each new fuel assembly contains six kinds of uranium dioxide (UO 2 ) fuel
(Fig. 21.1, Table 21.1). The most popular initial
235 U/U enrichment in the fuels is
4.4 wt%, whose inventory per assembly is 76.8 kgU. The fuel of 9.6 kgU per
assembly has the highest initial enrichment of 4.9 wt%. The initial uranium
inventory in total is 170.9 kgU per assembly, including fuels of other enrichments
and of the UO 2 -gadolinium oxide (Gd 2 O 3 ) composite [4].
The Unit 1 reactor in 1FNPS had 400 assemblies, which consisted of six batches
of burn-up. Each of the Unit 2 and 3 reactors had 548 assemblies of five batches.
Among these assemblies, 64 in the Unit 1 reactor, and 116 in the Unit 2 reactor, had
a low burn-up of only 3–5 GWD/t (Table 21.2). Other assemblies of the same
number are older but still have a burn-up as low as 15–16 GWD/t. The oldest
assemblies have a burn-up of about 40 GWD/t [5].
252
K. Tonoike et al.