section, but the iron in steel may increase the critical mass of fuel debris because it
has strong neutron absorption.
The MCCI product would be a composite of UO 2 and concrete. The major
content of concrete is silicon dioxide, which has also a small neutron absorption
cross section and neutron moderation capability. The critical mass of the UO 2 –
concrete composite has been evaluated as 400 kg for the fresh UO 2 of 5 wt%
235 U/U
enrichment. For the fuel burned up to 12 GWD/t, the critical mass can be as small as
800 or 2,000 kg, depending on how the effect of fission products is considered. Only
the water bonded in concrete is considered in the evaluation; therefore, the critical
masses can be smaller when the MCCI product is submerged in the coolant water
[9]. The mass of 2,000 kg is equivalent to 12 fuel assemblies. It is also known that a
certain cluster of 16 assemblies in the Unit 2 reactor has an average burn-up of
about 14 GWD/t. Thus, this evaluation is not far from reality.
Before knowing the actual condition of fuel debris, it is possible to compute
critical conditions. Such work has been already conducted for many years to
produce a handbook or a database for criticality safety. It is easy to extend these
standards to wider conditions such as UO 2 –steel composite or UO 2 –concrete
composite. The computation will supply a new set of “criticality maps of fuel
debris.” These maps will indicate (Fig. 21.2) subcritical and critical conditions,
and supercritical conditions that would likely bring severe consequences. In
Fig. 21.2, the horizontal line represents variation of composition, and the vertical
line represents variation of geometry. Composition on the right has higher reactivity
and smaller critical volume. On the left, the composition is certainly subcritical,
which can be excluded from the criticality control.
The actual criticality situation will be assessed by placing onto the map the fuel
debris condition revealed by observations or sample analyses. It is also necessary to
study how the condition can move on this map from expected changes such as
temperature drop in the fuel debris or geometry changes caused by retrieval work of
fuel debris, etc.
21.4 Options of Criticality Control Principles
21.4.1 Prevention of Criticality by Poison or Dry Process
The boration of coolant water was practiced in TMI-2 and is most preferable.
Borated water bounds the criticality characteristics of all debris into a small region,
indicated as “Boration” in Fig. 21.3, and keeps the region far from critical condition
no matter how much temperature or geometry changes. By securing the lowest
boron concentration in water, the subcritical condition can be guaranteed as well.
The water issue, however, must be fixed to implement this option. Moreover, a
structure made of carbon steel or aluminum will act as the water boundary when a
21 Options of Principles of Fuel Debris Criticality Control in Fukushima. . .
255
has strong neutron absorption.
The MCCI product would be a composite of UO 2 and concrete. The major
content of concrete is silicon dioxide, which has also a small neutron absorption
cross section and neutron moderation capability. The critical mass of the UO 2 –
concrete composite has been evaluated as 400 kg for the fresh UO 2 of 5 wt%
235 U/U
enrichment. For the fuel burned up to 12 GWD/t, the critical mass can be as small as
800 or 2,000 kg, depending on how the effect of fission products is considered. Only
the water bonded in concrete is considered in the evaluation; therefore, the critical
masses can be smaller when the MCCI product is submerged in the coolant water
[9]. The mass of 2,000 kg is equivalent to 12 fuel assemblies. It is also known that a
certain cluster of 16 assemblies in the Unit 2 reactor has an average burn-up of
about 14 GWD/t. Thus, this evaluation is not far from reality.
Before knowing the actual condition of fuel debris, it is possible to compute
critical conditions. Such work has been already conducted for many years to
produce a handbook or a database for criticality safety. It is easy to extend these
standards to wider conditions such as UO 2 –steel composite or UO 2 –concrete
composite. The computation will supply a new set of “criticality maps of fuel
debris.” These maps will indicate (Fig. 21.2) subcritical and critical conditions,
and supercritical conditions that would likely bring severe consequences. In
Fig. 21.2, the horizontal line represents variation of composition, and the vertical
line represents variation of geometry. Composition on the right has higher reactivity
and smaller critical volume. On the left, the composition is certainly subcritical,
which can be excluded from the criticality control.
The actual criticality situation will be assessed by placing onto the map the fuel
debris condition revealed by observations or sample analyses. It is also necessary to
study how the condition can move on this map from expected changes such as
temperature drop in the fuel debris or geometry changes caused by retrieval work of
fuel debris, etc.
21.4 Options of Criticality Control Principles
21.4.1 Prevention of Criticality by Poison or Dry Process
The boration of coolant water was practiced in TMI-2 and is most preferable.
Borated water bounds the criticality characteristics of all debris into a small region,
indicated as “Boration” in Fig. 21.3, and keeps the region far from critical condition
no matter how much temperature or geometry changes. By securing the lowest
boron concentration in water, the subcritical condition can be guaranteed as well.
The water issue, however, must be fixed to implement this option. Moreover, a
structure made of carbon steel or aluminum will act as the water boundary when a
21 Options of Principles of Fuel Debris Criticality Control in Fukushima. . .
255