Worth measurement of reactivity devices using inverse kinetics method

There are different techniques to measure reactivity like dynamic and inverse kinetics. In the dynamic technique stable reactor period is measured and reactivity is determined from the in-hour formula but this is more accurate for positive reactor period only. In the inverse kinetics method, the variation in neutron density with time is used to determine tile precursor concentration, c_i(t) with time. The computation of c_i(t) can be undertaken in two different ways. (i) c_i(t) is computed in terms of n(t) by directly solving the differential equation of c_i(t) (ii) direct integration of the differential equation assuming linear variation of c_i(t). The process begins by assuming a functional form of the neutron concentration variation between two successive time internals. This leads to an approximate estimate of the precursor concentration and thus the reactivity at the current time. The inverse kinetics method can be applied to obtain reactivity worth of any reactivity device using critical or subcritical method. In the critical method i.e. rod drop method, reactor is initially maintained in the steady and critical state, while in the subcritical method, the reactor is initially maintained in steady and subcritical state. Care must be taken while using the method in low reactor power regime and subcritical regime where the contribution from source term becomes important. In the paper, different cases have been discussed wherein standard methods have been applied to measure reactivity of shut off rods under different conditions, bringing out the importance of the source term, wherever it arises. (author)