Thus, this option does not differ, essentially, from the first option, which is
prevention of criticality by poison. Monitoring still makes sense if we integrate it
with the first option and use it as an implementation of the “double contingency
principle.”
21.4.3 Prevention of Severe Consequence
The last option is, in fact, being currently applied. The defense line consists of
xenon gas monitoring and the injection of borated water. The monitoring sensitivity
is not sufficient to measure subcriticality but can detect the event beyond the
occurrence of critical condition before severe consequences result. The borated
water on standby will be injected when the monitoring detects the criticality.
A study is under way to improve the monitoring sensitivity to make the detection
and intervention quicker and to reduce the risk of this option.
A much bolder idea is also being brought up, which is to consider such quick
detection and intervention as a regular reactivity control. A small-scale, controlled
chain reaction is permissible in the concept, and the resumption of fuel debris
retrieval is allowed after suppressing the criticality. To realize this kind of criticality control, its risk must be fully understood.
21.4.4 Risk Assessment
The risk study is necessary regardless of which option is chosen because the
subcritical condition is not secured at present. Even though the fuel debris will
not be touched for a while, the temperature of the fuel debris may drop gradually in
time, which slowly increases reactivity. The risk of “low probability and high
consequence events” must be also evaluated. An aftershock of large magnitude
may change the fuel debris geometry greatly. The extreme event would be the fall
of fuel debris in the PV onto the other in CV.
The fuel debris retrieval must be assessed carefully, of course, if it is conducted
under nonborated water. The first step of the risk analysis is to understand the actual
conditions of fuel debris. Exhaustive observation of the fuel debris should be
conducted as early as possible, which enables us to complete the maps described
in the previous sections.
According to each option, engineering work should be performed in parallel to
establish design requirements. For the prevention of criticality by borated water, its
required lowest concentration must be established. For the prevention of criticality
by monitoring, requirements of sensitivity and time response of the monitoring and
time response of an intervention measure must be clarified. For the prevention of
severe consequences, an allowable limit of fission number must first be set. Then,
the time response of detection and intervention must be defined to regulate fission
numbers of supposed criticality events within the limit.
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K. Tonoike et al.
prevention of criticality by poison. Monitoring still makes sense if we integrate it
with the first option and use it as an implementation of the “double contingency
principle.”
21.4.3 Prevention of Severe Consequence
The last option is, in fact, being currently applied. The defense line consists of
xenon gas monitoring and the injection of borated water. The monitoring sensitivity
is not sufficient to measure subcriticality but can detect the event beyond the
occurrence of critical condition before severe consequences result. The borated
water on standby will be injected when the monitoring detects the criticality.
A study is under way to improve the monitoring sensitivity to make the detection
and intervention quicker and to reduce the risk of this option.
A much bolder idea is also being brought up, which is to consider such quick
detection and intervention as a regular reactivity control. A small-scale, controlled
chain reaction is permissible in the concept, and the resumption of fuel debris
retrieval is allowed after suppressing the criticality. To realize this kind of criticality control, its risk must be fully understood.
21.4.4 Risk Assessment
The risk study is necessary regardless of which option is chosen because the
subcritical condition is not secured at present. Even though the fuel debris will
not be touched for a while, the temperature of the fuel debris may drop gradually in
time, which slowly increases reactivity. The risk of “low probability and high
consequence events” must be also evaluated. An aftershock of large magnitude
may change the fuel debris geometry greatly. The extreme event would be the fall
of fuel debris in the PV onto the other in CV.
The fuel debris retrieval must be assessed carefully, of course, if it is conducted
under nonborated water. The first step of the risk analysis is to understand the actual
conditions of fuel debris. Exhaustive observation of the fuel debris should be
conducted as early as possible, which enables us to complete the maps described
in the previous sections.
According to each option, engineering work should be performed in parallel to
establish design requirements. For the prevention of criticality by borated water, its
required lowest concentration must be established. For the prevention of criticality
by monitoring, requirements of sensitivity and time response of the monitoring and
time response of an intervention measure must be clarified. For the prevention of
severe consequences, an allowable limit of fission number must first be set. Then,
the time response of detection and intervention must be defined to regulate fission
numbers of supposed criticality events within the limit.
258
K. Tonoike et al.