Method of measuring the degree of subcriticality

Purpose: To obtain the degree of subcriticality, that is, negative reactivity, of a composite subcriticality system (such as a reactor core while shut-down, spent fuel transfer vessel and fuel rack) or a unit subcriticality system (such as a fuel assembly having an integral neutron source) and make use of the obtained results for economical design of the reactor, evaluation of safety and so forth. Constitution: The degree of subcriticality of a unit subcriticality system including a fuel assembly having a neutron source is assumed while calculating the degree of subcriticality of a composite subcriticality of unknown subcriticality from the degree of subcriticality of the unit subcriticality system, to thereby correct the afore-mentioned assumed degree of subcriticality of the unit subcriticality system from the ratio between the afore-said known degree of subcriticality and the calculated degree of subcriticality. Examples of the measurement of the degree of subcriticality of the composite subcriticality system are measurements of the degree of subcriticality of spent fuel transfer vessels, fuel racks and new fuel storage vehicles for storing such fuel as plutonium, while measurement of the degree of subcriticality of the spent fuel is an example of the measurement of the degree of subcriticality of the unit subcriticality system. (Horiuchi, T.)