still continues to emit radiation and heat. For decommissioning of the Fukushima
Daiichi NPS, all the fuel debris should be retrieved from the pressure and containment vessels of each reactor unit.
In preparation for retrieval of the fuel debris from the Fukushima Daiichi NPS,
however, there remain the following serious problems: (1) leakage of cooling water
from containment vessels, (2) inflow of groundwater into reactor buildings,
(3) maintenance of subcritical state of the fuel debris, and (4) shielding of radiation
from the fuel debris [2]. The cooling water of the fuel debris concerns these four
problems. In a similar accident that occurred at Three Mile Island NPS Unit
2 (TMI-2), where its pressure vessel was not seriously damaged, all these problems
were settled or did not arise because the pressure vessel could be filled with cooling
water containing highly concentrated boron as a neutron absorber and radiation
shield [3]. In contrast, all four problems make it extremely difficult to retrieve the
fuel debris from each reactor unit of the Fukushima Daiichi NPS.
The fuel debris, which has uncertain chemical composition and physical state,
needs to be treated with great care from the aspect of criticality safety. In particular,
large blocks of fuel debris can cause a change in physical state, such as size and
water content, when they are broken into fragments to be retrieved in cooling water.
Furthermore, a recent study on fuel debris resulting from the molten core–concrete
interaction has revealed its potential for criticality [4]. There will probably be no
risk of a criticality accident if it is possible to keep a high concentration of boron in
the cooling water and take the criticality control measures that were used in the
TMI-2 accident. However, these measures will be difficult unless both (1) the
leakage of cooling water and (2) the inflow of groundwater are completely stopped.
If not, retrieval of the fuel debris will require alternative approaches to criticality
control in cooling water or dry retrieval with radiation shielding.
The authors focus on the former approach: new criticality control measures for
fuel debris, such as criticality safety standards and criticality monitoring methodology [5]. This report summarizes a facility development project for an experimental study on criticality control for fuel debris.
22.2 Experimental Study on Criticality Control
for Fuel Debris
22.2.1 Modification of STACY
To implement the new criticality control measures for fuel debris, the Japan Atomic
Energy Agency (JAEA) has been carrying a project to modify the Static Experiment
Critical Facility (STACY) to pursue critical experiments on fuel debris
[6]. STACY, a facility using solution fuel (low-enriched uranyl nitrate), is to be
converted into a thermal critical assembly using fuel rods and a light water
moderator.
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H. Sono et al.
Daiichi NPS, all the fuel debris should be retrieved from the pressure and containment vessels of each reactor unit.
In preparation for retrieval of the fuel debris from the Fukushima Daiichi NPS,
however, there remain the following serious problems: (1) leakage of cooling water
from containment vessels, (2) inflow of groundwater into reactor buildings,
(3) maintenance of subcritical state of the fuel debris, and (4) shielding of radiation
from the fuel debris [2]. The cooling water of the fuel debris concerns these four
problems. In a similar accident that occurred at Three Mile Island NPS Unit
2 (TMI-2), where its pressure vessel was not seriously damaged, all these problems
were settled or did not arise because the pressure vessel could be filled with cooling
water containing highly concentrated boron as a neutron absorber and radiation
shield [3]. In contrast, all four problems make it extremely difficult to retrieve the
fuel debris from each reactor unit of the Fukushima Daiichi NPS.
The fuel debris, which has uncertain chemical composition and physical state,
needs to be treated with great care from the aspect of criticality safety. In particular,
large blocks of fuel debris can cause a change in physical state, such as size and
water content, when they are broken into fragments to be retrieved in cooling water.
Furthermore, a recent study on fuel debris resulting from the molten core–concrete
interaction has revealed its potential for criticality [4]. There will probably be no
risk of a criticality accident if it is possible to keep a high concentration of boron in
the cooling water and take the criticality control measures that were used in the
TMI-2 accident. However, these measures will be difficult unless both (1) the
leakage of cooling water and (2) the inflow of groundwater are completely stopped.
If not, retrieval of the fuel debris will require alternative approaches to criticality
control in cooling water or dry retrieval with radiation shielding.
The authors focus on the former approach: new criticality control measures for
fuel debris, such as criticality safety standards and criticality monitoring methodology [5]. This report summarizes a facility development project for an experimental study on criticality control for fuel debris.
22.2 Experimental Study on Criticality Control
for Fuel Debris
22.2.1 Modification of STACY
To implement the new criticality control measures for fuel debris, the Japan Atomic
Energy Agency (JAEA) has been carrying a project to modify the Static Experiment
Critical Facility (STACY) to pursue critical experiments on fuel debris
[6]. STACY, a facility using solution fuel (low-enriched uranyl nitrate), is to be
converted into a thermal critical assembly using fuel rods and a light water
moderator.
262
H. Sono et al.