| 摘要 |
PROBLEM TO BE SOLVED: To secure a subcritical margin by removing fissionable substances whose concentration is already known after confirming with the change in neutron counting rates the subcriticality in the process of loading the fissionable substances into a container. SOLUTION: A subcriticality monitor 1 which includes fissionable substances whose concentration, composition and distribution are already known and a neutron detector is installed in a container 2. While spent fuel assemblies 3 are loaded into the container 2, subsequently, the change in neutron counting rates associated with the loading is measured by the neutron detector of the monitor 1 to confirm the subcriticality in the container 2. According to the increase in the number of loaded assemblies, a neutron multiplication factor (Keff) heightens and the increase of the Keff caused by the loading of the monitor 1 lowers. Then, the monitor 1 is taken out from the container 2 after the change in the neutron counting rates is measured on completion of the loading and the subcriticality in the container 2 is confirmed. In this case, a subcritical margin can be secured without fail by determining the concentration, composition and distribution of the fissionable substances stored in the monitor 1 so that the increase of the Keff by the monitor 1 may beΔK0 , a subcritical margin prescribed beforehand, or more.
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