Toughened radiation-resistant pipes show ‘small pox-like’ nodular corrosion at the Kakrapar Nuclear Power Plant in Gujarat
In a plot similar to a Bollywood thriller’s, scientists are burning the midnight oil to discover the reason behind the mysterious nuclear leak at the Kakrapar Nuclear Power Plant in Gujarat.
This 21st century atomic potboiler is actually unfolding through the hard work of scientists at the Bhabha Atomic Research Centre (BARC), whose laboratory actually shares a wall with the famous property where Raj Kapoor used to live. Here, they are working overtime to find out the real cause of the leaks at the twin reactors in southern Gujarat.
To avoid panic and further accidents, Indian nuclear watchdog Atomic Energy Regulatory Board (AERB) has shut down the affected plants till the cause has been found. Nuclear experts say the pipes, made from a rare alloy, have contracted what seems like small pox, and this contagion has spread all over the critical tubes in two Indian Pressurised Heavy Water Reactors (PHWR) at the Kakrapar facility. To make matters worse, more than a year into the investigation, the teams of scientists can’t figure out what has gone wrong.
It was on the morning of March 11, 2016, and as fate would have it, exactly five years after the Fukushima reactors in Japan began exploding, Unit Number 1 of the 220-MW PHWR at Kakrapar developed a heavy water leak in its primary coolant channel and a plant emergency was declared at the site.
The indigenously built nuclear plant had to be 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.
The atomic thriller really begins when experts were trying to find out why a leak recognition system failed, when it should have raised an alarm. “There is a leak detection system in place in all PHWRs, but in this case it failed to detect the leak on March 11, 2016,” confirms AERB Chairman SA Bhardwaj. The watchdog body 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. Nothing in the core of a nuclear reactor can be done in a jiffy, and several weeks after the first leak, the initial probe using a specially designed tool revealed four big cracks in a coolant tube had led to the massive leak.
The mystery unfolds
The discovery of the crack was only the beginning of the mystery. Further efforts to find the cause established that the outside of the tube, the part not exposed to high-temperature heavy water, was corroded due to unknown causes.
This was a stunning discovery, 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 tube. It is also very hard to access this part since the space is tiny in the annulus.
The AERB then ordered that all tubes made of a special zirconium-niobium alloy be checked on the outside. To their surprise, they discovered that the contagion of the nodular corrosion, ‘small pox-like’ in layman’s parlance, was widespread in many of the 306 tubes. Similar tubes from the same batch used at other Indian reactors continued to operate without corrosion.
The needle of suspicion now pointed to carbon dioxide, a gas known to be very stable in high-radiation environments. A further post mortem revealed that Unit-2, which is twin of the affected reactor, had also been affected by a similar leak on July 1, 2015. Investigations into Unit-2’s failure were made but no conclusive result had been found. This back-to-back failure of two fully functional nuclear reactors befuddled engineers.
BARC begins probe
Undaunted, AERB ordered that the entire assembly and not just the affected tube be safely pulled out and brought to BARC, India’s foremost nuclear laboratory, for detailed failure analysis.
In addition, since India operates another 16 similar nuclear plants, a full-fledged investigation was carried out on coolant channels at all atomic power plants. The investigating team found the ‘small pox-like’ corrosion was confined only to the two units at Kakrapar.
While NPCIL heaved a sigh of relief, the finding made it all the more difficult to discern the true cause of the leaks at Kakrapar. Mr. Bhardwaj says investigators are wondering if the carbon dioxide used in Kakrapar may have been contaminated, which caused the nodular corrosion.
The source of the carbon dioxide was traced backwards, and it seems only the Kakrapar plant was sourcing its gas from a Naptha cracking unit, where it was possibly contaminated by hydrocarbons.