Upgradation of Emergency Cooling System (ECS) Logic of Dhruva

Dhruva is a 100MWth Research Reactor utilized for the production of radioisotopes, and offer irradiation facilities for testing various materials such as nuclear fuel & structural materials. During normal operation, the primary cooling is achieved by circulation of Heavy Water through Main Coolant Pumps (MCPs) and the secondary and tertiary cooling is achieved by Process Water Pumps (PWPs) and Sea Water Pumps (SWPs). However, on power failure or outage of pumps, Heavy Water is circulated through core by means of Auxiliary Coolant Pumps (ACPs). Each of the ACPs has two prime movers- one water driven turbine and an electric motor on class-II supply. Various process interlocks for the operations viz. opening/closing of valves, starting/stopping of motors etc. are controlled by a hardwired electronic based system called Emergency Cooling System Logic (ECS Logic). The ECS Logic System is classified as Class-IA system. There are three identical but totally independent channels in ECS Logic System. The outputs of three channels are used in a ladder network to obtain two-out-of-three logic which is then used to actuate appropriate valves and motors. To monitor the health of ECS logic in an online mode, an independent test and monitoring system in the form of Fine Impulse Testing (FIT-ECS) is also deployed. To take care of aging & obsolesces in ECS Logic, upgradation of ECS Logic System was taken up. The new ECS channels have the same inputs/outputs/logic implementation and use circuits & components identical to old ECS channel. Interface to existing FIT-ECS System is kept same as earlier. However, the numbers of cards were reduced and unused components of the functions not utilized in old boards were deleted. New ECS Logic has PCB based backplane, Euro connectors for card to backplane connection and IDC on backplane for Field I/O interface. These modifications in the upgraded system were incorporated to achieve high reliability and availability. For new ECS Logic, Mean Time Between Failure (MTBF) and Probability of Failure per Hour (PFH) have been calculated to be 1,79,985 h and 2.414E-07 h−1 respectively, considering MTTR of 8 h, proof test interval of 1 month and no diagnostic coverage. The paper provides detailed account of the upgradation of the ECS logic, its salient features, advantages gained, qualification tests performed & its results, methodology of testing & commissioning of the system and performance of the system.