Kakrapar investigations continue

Three cracks have been found in the coolant channel which failed at India's Kakrapar 1, although the reasons for its failure will not be known for some time, the country's Atomic Energy Regulatory Board (AERB) said in its latest update on the March incident which led to a leak of heavy water.

Preparatory work prior to the removal of the damaged channel is in progress, with the regulator reviewing each step of the procedure in advance to ensure that vital information on the nature and causes of the failure are not lost in the process.

Kakrapar Atomic Power Station - also referred to as KAPS - comprises two Indian-designed 202 MWe (net) pressurized heavy water reactors. Unit 1 shut down automatically following the coolant leak on 11 March, and remains off line. The plant's safety systems, including its backup cooling system, functioned as intended. Shortly after the incident, the AERB confirmed that fuel bundles removed from the leaking channel were undamaged.

Kakrapar 1's core contains 306 coolant channels made of zirconium-niobium alloy, each holding 12 bundles of uranium fuel. The fuel is cooled by a flow of heavy water under high pressure - the primary coolant - which generates steam in a secondary circuit to drive the electricity-generating turbines.

The AERB said that incidents of leakages from coolant channels are "not unexpected" events, with over 70 cases of leaks or defects in coolant channels reported internationally for similar reactors. Reactors are accordingly equipped with sensitive leak detection systems. It also said that India's standards for the manufacture, construction, operation and life management of coolant channels had been developed taking into account "all the available experience" from the operation of PHWRs around the world.

However, it said, the Kakrapar leak had "raised some concerns" and the regulator had "as a first priority" ordered checks on the sensitivity of the leak detection systems at all of India's operating PHWRs, with improvements made where necessary. This work has now been completed by the nuclear operator the Nuclear Power Corporation of India Ltd, the AERB said.

The AERB said that the cause of unusual corrosion spots noticed on Kakrapar's coolant channels, and whether they are linked to the failure, may not be fully established until the failed channel is removed and examined. Inspections of coolant channels at other units requested by the AERB have so far not found any similar corrosion. "The inspections done so far in different reactors indicate that the presence of local corrosion spots on coolant channels is specific to KAPS units alone," the AERB said.

The AERB said its "regulatory priority" is the safety of India's other operating units. "Exact reasons for the failure at KAPS 1 can be established after the completion of the failure analysis, which is expected to take considerable time," it said.

Researched and written
by World Nuclear News