Chennai, July 2 (IANS) India's atomic energy regulator has said unusual corrosion spots were found in the coolant channels in the power reactors at Kakrapar Atomic Power Station (KAPS) owned by Nuclear Power Corporation of India Ltd (NPCIL).
"During the ongoing inspections in KAPS units, unusual indications of corrosion spots were noticed in the coolant channels," Atomic Energy Regulatory Board (AERB) said in a statement on Friday.
"How these have been formed and whether these are linked to the failure is being checked and it can fully be established after the failed channel is removed and examined," the statement added.
On the morning of March 11, the first reactor at KAPS shut down automatically following leakage of heavy water from its coolant channel. The leak was plugged ten days later.
According to AERB, as its regulatory focus is on operating units to assure their safety on priority, it had asked for inspection of coolant channels of other units to rule out the possibility of similar corrosion.
"The inspections done so far in different reactors indicate that presence of local corrosion spots on coolant channels is specific to KAPS units alone and no evidence of this phenomenon is seen in other reactors inspected so far," AERB said.
"We studied the pressure tubes and leak detection systems in all other PHWRs in the country and found them to be satisfactory. We didn't find anything amiss in them that might result in an incident similar to that happened at Kakrapar," Shiv Abilash Bhardwaj, Chairman, AERB told IANS on June 30.
According to AERB officials the focus is to see whether the corrosion is specific to KAPS units and whether it is linked to operational factors.
Incidentally the pressure tubes in the coolant channels of KAPS unit 1 were changed in 2011.
"However the incident of leak from a coolant channel at KAPS unit 1 at an early stage of its life has raised some concerns," AERB said.
Once the actual cause of corrosion is identified then other aspects would be looked into-quality of metal, fabrication process and others, officials told IANS on Saturday.
For this the ruptured coolant channel has to be removed first.
"Special tools have to be developed so that the evidence (the cause of rupture) is not disturbed while removing the affected channel," Bhardwaj had told IANS earlier.
As the affected channel is in the reactor's core area, it has to be removed carefully due to radiation and transported to a lab.
The affected channel needs radiation shielding. The equipments have to be brought in and set up for removing the affected channel.
A 220 MW pressurised heavy water reactor (PHWR) like the one at KAPS has 306 coolant channels and each channel will have 12 fuel bundles.
Officials had earlier said 12 fuel bundles were removed safely from the coolant channel before plugging the leak and there was no need to take out all the fuel bundles from the reactor.
According to AERB officials, the checking for corrosion spots in the coolant channels was done on the globally accepted statistical sampling basis.
One has to remove fuel bundles -- a complicated exercise -- from the coolant channel for inspection.
India's atomic power plant operator NPCIL has two 220 MW units at KAPS. Following the heavy water leak, unit 1 is under cold shut down. The second unit is under maintenance shut-down since July 2015.
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