Commissioning and operation of new liquid poison injection based shut down system in TAPP-3 and 4

Shut Down System - 2 (SDS - 2) of TAPP-3 and 4 works on the principle of rapid injection of gadolinium nitrate poison solution into bulk moderator in calandria using high pressure helium to shut down the reactor. This is a new system, in the context of Indian PHWRs, designed, engineered, commissioned and being operated in TAPP-3 and 4. The system design incorporates passive features such as floating polyethylene ball with ball-ball seat arrangement and locked open isolation ball valves with key interlock arrangement. This arrangement eliminates active valves downstream of poison tanks during SDS - 2 actuation. A series parallel arrangement of fast acting pilot controlled air operated valves, which keep the high pressure helium isolated from poison tanks in poised state, are the only active components. During commissioning and initial period of operation of TAPP-4, problems were encountered and were resolved by suitable modifications and the same were incorporated in TAPP-3. Measurement of poison level in the poison tanks, during the system actuation, using differential pressure level transmitters, was found inaccurate. An innovative method, using pressure transmitters was evolved and implemented successfully to measure poison injection time during commissioning of the system. Problems were encountered during surveillance testing of the fast acting valves. Circuit and logic modifications were carried out to overcome these problems. Elimination of active valves downstream of poison tanks poses challenges in maintaining stable poison-moderator interface during various disturbances either in calandria or in the SDS - 2 circuit. Problems in this regard were resolved by modifications in the balance line connecting moderator cover gas system and helium atmosphere above poison tanks. Subsequent to the modifications, the system has been trouble free during its poised condition in TAPP-3 and 4. This paper brings out commissioning and operating experience of SDS - 2 in TAPP-3 and 4 with details of various problems encountered and modifications incorporated. (author)