March 16, 2016 2:03:30 am
Three days after one of the two 220 MWe units at the Kakrapar Atomic Power Station (KAPS) in Gujarat had to be shut down after leakage of heavy water from its coolant system, the cause of the incident has been pinpointed to one of the coolant channel assemblies of the reactor. The pressure tubes of the coolant channels in this reactor were replaced with ones made from “improved material” five years back in 2011, as part of a pressure tube ageing management programme. The source of the leak was identified after operators of the unit entered the affected reactor building for investigations.
How serious is this leak? The Atomic Energy Regulatory Board (AERB) has stated categorically that there are “no major safety concerns” and that KAPS Unit 1 is “stable” and the “reactor is in cold shutdown state”. There are counterviews, though. A Gopalakrishnan, a former Chairman of the AERB from 1993 to 1996 and someone who had publicly criticised India’s nuclear establishment for its excessive secrecy and lack of accountability, had, just a day after the leak, alluded to the country’s nuclear power operator — the Nuclear Power Corporation of India Ltd (NPCIL) — and the AERB possibly relaxing their adherence to a Pressure Tube Aging and Integrity Management Programme as a pointer to the cause of the incident. Under this programme, the pressure tube material was progressively changed to Niobium-stabilised Zircalloy over the nineties for the country’s mainstay pressurised heavy water reactor fleet in the wake of an incident that happened at the Pickering Heavy Water Reactor in Canada in August 1983. Gopalakrishnan’s assessment of the incident was posted on online nuclear resource DiaNuke.org.
NPCIL did not respond to a detailed questionnaire sent on the issue. The AERB responded to a query sent by The Indian Express through a statement on Monday that provisionally rated the incident at KAPS Unit-1 at “level – I” in the IAEA International Nuclear and Radiological Event (INES) scale, which corresponds to an “anomaly” in the plant.
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Kakrapar, though, is not the only leak in recent months. The first 1,000 MWe unit Kudankulam Nuclear Power Plant, the country’s first nuclear reactor unit built with Russian assistance, is learnt to have been briefly shut down recently to attend to a leak noticed in the conventional system of the station recently. The date of this leak could not be ascertained. The Russian VVER-1000 reactor unit is learnt to have been restarted after “necessary rectification” and the incident, officials said, had “no radiological safety implications” and therefore did not call for any safety and security audit.
Prior to these two incidents, the most recent documented instance of a leak was four years ago, when there were two instances of tritium uptake (or exposure) of workers at the Rajasthan Atomic Power Station at Rawatbhata during maintenance work. While the first instance occurred in Unit-5 on June 23, 2012, the second instance took place in Unit-4 on July 19, 2012. Both the instances were localised to small areas and there was no release of radioactivity or radiation to the environment or public domain.
The Unit-5 episode occurred due to rise in tritium levels in a localised area of the containment building, following the opening of the moderator cover gas line where the welding jobs were performed. In the second instance, a localised leak of tritiated heavy water from the moderator pump seal led to tritium uptake of workers in the area.
In case of the latest leak at KAPS, which employs around 350 people, the AERB noted that “the reactor is being continuously cooled and at present there are no major safety concerns. There has been no radioactivity release exceeding the specified daily limits for normal operation, from March 11, 2016, till date”. The regulator also asserted that there has also not been any case of workers receiving abnormal radiation exposures.
An independent detailed Environmental Monitoring carried out in the vicinity of the plant up to a distance of around 20 kms, the AERB said, showed “that there is no increase in the background radiation levels and/or radioactive contamination, corroborating that no abnormal releases have taken place”. The AERB observers who were rushed to the site on Friday have since returned and the information has been confirmed by their assessment as well.
Currently cooling of the reactor is continuing and planning and preparations for isolating the leak and other recovery operations is in progress, as reported by the station. AERB, the regulator said, “is obtaining regular updates from the plant and is continuously monitoring the safety status”.
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