Determination of the subcriticality level using the Cf source-detector method

Measurement and monitoring of reactivity in a subcritical state, e.g. during the loading of a power reactor, has a clear safety relevance. The methods currently available for the measurement of k_eff in stationary subcritical conditions should be improved as they refer to the critical state. This is also very important in the framework of ADS (accelerator driven systems) where the measurement of a subcritical level without knowledge of the critical state is looked for. An alternative way to achieve this is by mean of the ²⁵²Cf source-detector method. The method makes use of three detectors inserted in the reactor: two “ordinary” neutron detectors and one ²⁵²Cf source-detector which contains a small amount of ²⁵²Cf that introduces neutrons in the system through spontaneous fission. By observing fissions through the detection system and correlating the signals of the three detectors, the reactivity ρ (and hence the multiplication factor k) can be determined.