The high-level committee tasked with reviewing the safety of Indian nuclear power plants against external events of natural origin found that “the design, operating practices and regulations followed in India have inherent strengths… to deal with natural events and their consequences,” India’s Atomic Energy Regulatory Board (AERB) said in a press release.
The committee noted that India’s 18 pressurized heavy water reactors (PHWRs) were particularly well equipped to deal with natural events. It also outlined the interim safety measures that have recently been taken to enhance the safety of two older boiling water reactors at Tarapur. These measure have included work on offsite and onsite power supplies to provide cooling in the event of a prolonged station blackout as well as preparatory work for inerting the containment.
It also pointed out that design provisions to cool the core of a reactor during an extended station blackout were tested in 1993, when a turbine hall fire at Narora 1 caused a 17-hour blackout on the site. However, the committee still recommended the provision of a back-up system.
Unlike the Fukushima case the simultaneous occurrence of a strong earthquake and a tsunami at Indian NPPs in not foreseen, the committee concluded.
The committee also said that: “The requirements for siting and design of NPPs with respect to postulated design basis natural events, as specified in AERB safely regulation found to be appropriate and sufficiently conservative.”
“However in the light of Fukushima experience it is considered prudent to further advance this conservatism through better treatment of uncertainties in data and computational processes,” it added.
AERB will now review the report in detail and will follow-up with Nuclear Power Corporation of India for the implementation of the recommendations.
In the long term, the Nuclear Regulatory Commission may require US plant design changes based on lessons learned. Changes could include emergency diesel generator installations, spent fuel pool integrity or offsite grid connections. Secondary design changes could address ways to charge water to the reactor in a station blackout event. All will probably address levels of defence-in-depth. Emergency planning for beyond-design-basis-events (BDBE) may also require changes to address general area site support, evacuation and financial considerations.
In addition, Generation 1 through 3 operating nuclear plants should be reviewed for defence-in-depth hardening against natural events. We should reconsider the question of spent fuel storage and its ultimate disposal, particularly in the US where it remains stored onsite. We should re-examine potential for radioactive water to flow out of containment. Especially after natural events like seismic loading, cracks initiated may allow radioactive water from containment flooding to leak outside via common drains, pipe chases and cable conduits. That compromises secondary containment. Lastly, we should re-evaluate logical multiple-event scenarios. While the combined tsunami flood was well beyond predictions, the combination of events was not a surprise.
JK August, VP, operations, CORE, [email protected] The author is also chair of the American Society of Mechanical Engineers/American Nuclear Society (ANS) Committee 28, Gas Reactors