An online reactivity meter is developed for use in the Indian Nuclear Power Plants (INPP). The reactivity meter is required to generate reactor trip whenever the calculated reactivity exceeds the predefined limits. The computation of reactivity is based on inverse point kinetic model of the reactor with six groups of delayed neutrons. The recursive equations representing the change in reactivity in response to change in neutron flux have been formulated and solved online for determining the reactivity at any instant using Gate Array. The online diagnostics features have been integrated with the reactivity computation to verify its correctness and healthiness of the meter. The online diagnostic scheme encompasses checking of all the major hardware components, viz. EPROM, ADC, DAC and their interfaces with the Gate Array. It also monitors the robustness of Gate Array and the correctness of the computational cycle algorithms implemented in Gate Array by incorporating suitable checks for stuck bits, improper timings, faulty logic, range crossing, etc., including floating point exceptions. Upon detecting fault(s), limited consecutive attempts towards validation of fault are performed. On confirmation of fault the reactivity computation is halted and output goes to a failsafe state.

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