Surveillance on Nuclear Instrumentation of Dhruva Research Reactor

Dhruva is a 100 MWth research reactor utilized for the production of radioisotopes and offers irradiation facilities for testing various materials such as nuclear fuel and structural materials, cooled by heavy water, moderated by heavy water, and fueled by natural metallic uranium. The Nuclear Instrumentation (NI) monitors the neutron flux in the reactor using neutron detectors which are located out of core and feeds appropriate signals to the Reactor Regulating System (RRS) and Reactor Protection System (RPS). NI plays an important role in the safety of the reactor. The log power (log P), linear power (lin P), log rate (log R), i.e., the inverse of reactor period and linear rate (Lin-R) are generated using neutron detector signals based on which reactor (power) regulation and protection are achieved. NI also generates signals for post-accident analysis. For covering fourteen decades of in-core neutron flux, triplicated nuclear channels consisting of fission chambers (FCs) and pulse processing electronics, triplicated log and linear safety channels consisting of B-10 lined uncompensated ion chambers (UICs) and DC amplifiers, multi-range DC channels (MRDC) based on UICs for main control room (MCR) and supplementary control panel (SCP) are used. Additionally, for post-accident analysis, high range channels based on UICs are provided. The neutron detectors are located in triplicated basket arrangement around radial beam holes at the nearly same elevation and spread at 120 degrees w.r.t. each other, in out of core region. Considering the importance of NI in reactor regulation and protection and with the fact that reactor design lifetime is much more than the NI design lifetime, commensurate surveillance including preventive maintenance (PM) as per plant technical specification is followed. The surveillance mechanisms, which have evolved over a period of time, based on operational data, help in ensuring intended performance of NI and thereby assuring overall safety. The objective of this paper is to present a detailed account of surveillance of nuclear instrumentation of Dhruva which includes various diagnostic features available and maintenance methodology formulated.