Recently, the leakage incident at the Kakrapar Atomic Power Station (KAPS) got wide media coverage. The apparent delay in lifting the plant emergency triggered the fertile imagination of a few. “However, my concerns are more for the workers in the plant,” a perennial critic of nuclear power said.
He may not know the robust steps in place to restrict radiation doses to workers in nuclear power plants to less than prescribed limits during their normal operation and to acceptable levels during abnormal events. In KAPS incident, health physics specialists could keep the workers' doses within prescribed limits.
An important barrier broke for no known reason, but safety systems worked as per design.
Was there any delay in handling the KAPS incident? NPCIL developed special tools to grab and pull out 12 fuel assemblies, one at a time, applying minimum force, from Q5, the stricken coolant channel. Operators confirmed the overall health of each assembly after transferring them to the inspection bay. After inspecting the special tooling, they used it to retrieve the next assembly.
Experts from BARC and NPCIL continuously reviewed the progress keeping contingency options ready in case the next assembly removal posed any constraint. Operators recovered the last assembly on March 21 and arrested the leak by installing the necessary hardware to isolate the channel from the rest of the cooling system. They normalized the reactor coolant system, before the management lifted the plant emergency.
The entire procedure involved mock up trials by dedicated crew, prompt and orderly issuance of work permits and enforcing their conditions, provision of fool- proof protective accessories to workers among other steps.
NPCIL's operating and maintenance staff is experienced and capable to handle emergency situations involving radiation exposure. Shri G.R Srinivasan, former Vice Chairman, Atomic Energy Regulatory Board (AERB) recalled that radiation exposures were generally satisfactory during en masse coolant channel replacement (which is a good portion of decommissioning) at various plants and during, after and until restart of units after various incidents — end shield repair at Rajasthan Atomic Power Station (RAPS), fire at the Narora Atomic Power Station (NAPS), flooding at KAPS etc. There are not really many incidents in India over 400 reactor-years of operation.
In India's nuclear power plants, the doses were low and within AERB limits to all of over 16,000 to 17,000 workers annually, during 2000-2015.
How was this achieved? In pressurised heavy water reactors, tritium formed by neutron irradiation of heavy water is an important source of internal radiation as the gas leaks from the system as tritiated water vapour
When heavy water spillage occurs, NPCIL staff mops it up from sumps. Driers continuously collect tritiated water vapour. Both help to reduce internal dose to workers. The designers reduced the number of valves and made primary transport system in later PHWRs valve-less to reduce leakage
NPCIL reduced activation products in the system by choosing materials containing reduced level of impurities such as cobalt. A discussion with Shri Sanatkumar, former Executive Director, NPCIL revealed several steps taken to reduce radiation levels in TAPP 3&4, the 540 MW PHWRs.
During early 1990s when AERB started enforcing lower dose limits for workers in a phased way, doses As Low As Reasonably Achievable (ALARA) became NPCIL's working mantra.
Actions such as addition of change room facilities, zoning, access control, ventilation arrangements; remote handling and decontamination facilities and hot-spot management helped to achieve the ALARA objective. Judicious use of time, distance and shielding helps to control radiation exposure to workers.
NPCIL and BARC specialists depended on the priceless experience and expertise acquired during normal operation and abnormal events to handle the KAPS incident.
Delay was justifiable. Hurry may have been costly. Recovery effort should not obliterate the possible cause of the event. Let us wait for the results of the root cause analysis.
K.S. Parthasarathy
ksparth@yahoo.co.uk
(The writer is a former Secretary, Atomic Energy Regulatory Board)
