 INES 2 |
Actuation of secondary shutdown system (SSS) on slow insertion of more than one rod of primary shutdown system (PSS) |
Kakrapar Atomic Power Station is provided with two fast acting shutdown systems, Primary Shutdown System (PSS) and Secondary Shutdown System (SSS). PSS consists of 14 shut off rods kept above the core which drop on a trip signal. SSS consists of 4 banks of Lithium Pentaborate solution which will be pushed into the tubes which are located inside the core. In addition to the above, a system called Automated Liquid Poison Addition System (ALPAS) is provided for long time shutdown requirements.
On 18 May, 1997, unit was operating at 205 MWe. At 1731 hrs. SSS solenoid Valve Testing was being done as per schedule. When SSS bank no. 3 testing was being done, liquid poison got injected into the tube. Reactor tripped on low Primary Heat Transport System Pressure. During the trip, 2 PSS rods registered slow insertion time. This triggered the actuation of SSS and ALPAS. The event is rated at level 2 on INES.
Justification:
The initiator "reactor Trip" is an expected initiator. Safety function availability was within operational limits and conditions. Hence a basic rating of 1/2. However, on the previous day, a deficiency in drop time for one of the rods of PSS was noticed. But the Unit was started without rectifying the deficiency, which is a violation of procedural requirement. Hence the event is upgraded to level 2, based on additional factor of deficiency in safety culture.
Event date: Sun, 18-05-1997
