Performance evaluation of start-up counters and ion chambers used in protective and regulatory channels of power reactors

The responsibility for safe installation of nuclear power plant is vested in Safety Review Committee for Operating Plants (SARCOP). It is essential that procedures leading to criticality be conducted according to documented technical specifications that are approved by SARCOP. SARCOP, based on the reviews of the Unit Safety Committee and on its own assessment makes recommendations for implementation. According to documented commissioning procedure, the neutron counters designated for the central thimble and the vault as well as the ion chambers used in the 'Regulatory, Primary Shut-down System and Secondary Shut-down System' have to be source checked prior to heavy water flushing of moderator and auxiliary systems. The paper describes calibration of the said detectors at laboratory and at site using systems developed and procedures adopted by Radiation Safety Systems Division. The systems developed include the portable water moderated jig for calibration of the aforesaid detectors and a digital pico-ammeter for measurement of low current output from the ion chambers. The contribution of extra cameral current at low neutron fluence rates in ion chambers is eliminated by averaging the output from both polarities. In case of start-up counters and delayed neutron monitors, operating parameters are fixed after obtaining elaborate voltage and discriminator plateaus using the respective electronic channels designated for the purpose. (author)