Operation and safety in 540 MWe PHWR

Large sized reactor such as 540 MWe Pressurised Heavy Water Reactor (PHWR) are neutronically loosely coupled and prone to xenon induced spatial oscillations at high power levels. Local variations of neutron fluxes and hence power in various zones can independently take place. Neutron flux detection devices are required to monitor these local variations and the control systems must be capable of responding to these variations so as to avoid any undesirable transients or power distribution. To provide continuous in core monitoring of local flux for effective control and protection, about 198 self powered neutron detectors (SPNDs) of the straight individually replaceable type are distributed in the reactor core for reactor control and protection. The 540 MWe PHWR has two fully independent shutdown systems. The first shutdown system (SDS-1) comprises of 28 spring assisted, vertical gravity drop shut-off rods, each consisting of a cadmium absorber sandwiched between stainless steel tubes. The second shutdown system (SDS-2) constitutes six poison tanks connected to respective zircaloy injection tubes. This system is capable of high speed injection of gadolinium nitrate solution (in D2O) into the moderator through these tubes. The present paper details the advances in physics made in the control and protection systems for 540 MWe PHWR. (author)