Scientists at India’s Bhabha Atomic Research Centre (BARC) investigating the corrosion of toughened radiation-resistant pipes which is causing leaks at two reactors at the Kakrapar NPP in Gujarat now believe the cause may be contaminated carbon dioxide gas.
In March 2016, the 220MWe reactor Kakrapar 1 developed a serious water leak in its primary coolant channel and a plant emergency was declared at the site. The indigenously built nuclear plant was shut down, but no worker was exposed and there were no radiation leaks, the Department of Atomic Energy confirmed. Operator Nuclear Power Corporation of India Limited (NPCIL) said the reactor had shut down safely, and confirmed that safety systems had functioned normally. To avoid further problems, Atomic Energy Regulatory Board (AERB) shut down the affected plants until the cause had been found. Nuclear experts say the pipes showed a nodular type of corrosion “like smallpox”, which had spread to all the critical tubes at the two Indian Pressurised Heavy Water Reactors (PHWRs). The investigation has been ongoing for a year.
Experts initially sought to discover why the leak detection system had failed. “There is a leak detection system in place in all PHWRs, but in this case it failed,” said AERB Chairman SA Bhardwaj. The AERB suspects the crack developed so rapidly that the electronic leak detection system just did not have the time to react.
Subsequent investigations revealed that the leak detection system was fully functioning and the operator had not shut it down to cut costs. The initial probe found four big cracks in a coolant tube that had led to the leak. Further investigations established that the outside of the tube, the part not exposed to high-temperature heavy water, was corroded. This was difficult to explain, since the outside of the failed tube was exposed only to high-temperature carbon dioxide and there had been no recorded case of a similar corrosion on the outside of any other tube.
The AERB then ordered an investigation of all tubes made of the special zirconium-niobium alloy and discovered that the nodular corrosion was widespread in many of the 306 tubes in the reactor. While similar tubes from the same batch used at other Indian NPPs continued to operate without corrosion, further examination revealed that Kakrapar 2 had been affected by a similar leak in July 2015. AERB then ordered that the entire assembly, and not just the affected tube, be removed and transported to BARC for detailed failure analysis. A thorough examination of 16 similar nuclear plants had confirmed that the corrosion only affected the two units at Kakrapar. Bhardwaj says investigators then considered whether the carbon dioxide used at Kakrapar may have been contaminated. The source of the carbon dioxide was traced, and AERB found that only the Kakrapar plant was sourcing its gas from a Naptha cracking unit, where it had possibly been contaminated by hydrocarbons.