Reactivity definition in the subcritical system irradiated by shot neutron pulses

The adapted equation derived originally to describe a critical condition was proposed for the case of transient process in subcritical systems with a neutron multiplication factor Keff = 0.96-0.98. The calculations used the experimental data of MUSE (multiplication with an external source). A simple expression due to a very short lifetime of prompt neutrons τpr = 5.8·10-7 s was obtained to determine the reactivity on the intensity of the neutron flux, at times up to 60 microseconds. (authors)