Learning from Fukushima

March 21, 2011 12:25 am | Updated September 30, 2016 11:28 pm IST

Events over the last nine days at the crippled six-reactor Fukushima Dai-ichi Nuclear Power Plant (NPP) complex, since the unprecedented earthquake-tsunami combine struck Japan's north-eastern coast, seem to be slowly moving towards a stable situation. Barring the sporadic spikes during March 15-16, radiation levels at distances more than 20 km from the site have not been abnormal. The single event of radioactive iodine release on March 12 seems, however, to have found its way into the food chain. But in the unfolding story of this nuclear disaster, the third worst after Three Mile Island in 1979 and Chernobyl in 1986, there are a lot of unanswered questions. Evading safety requirements seems to be an endemic problem in the nuclear industry, as is a general lack of transparency on safety issues. For India post-Fukushima, the preparedness of the nuclear establishment to ensure the safety of its nuclear power plants, current as well as those planned, when faced with an extremely unlikely accident is a vital concern. While the Fukushima NPP structures withstood the impact of a shock of 9.2 magnitude 130 km from the site, the 10 metre high tsunami waves — 2.5 metres above the safety margin provided — exposed the ill-preparedness of the operator, the Tokyo Electric Power Company (TEPCO). A former Vice-Chairman of Japan's Nuclear Safety Commission, Kenji Sumita, wrote: “Every step TEPCO has taken is a day late and a dollar short. The release of information from TEPCO is even further behind.” If this could happen in Japan, an industrially advanced country with a high level of safety consciousness, one can well imagine the situation in India.

While the three operational boiling water reactors (BWRs) of General Electric's 1970s vintage Mark-1 design shut down automatically, as required, following the earthquake, TEPCO could not maintain the continued cooling necessary to remove the residual decay heat from their reactor vessels. Although this is only 3 per cent of the total power, it can last for over a month. This, it appears, was due to the absence of adequate back-up arrangements, which became clear following the station blackout caused by the tsunami. The emergency diesel generators that came on stopped functioning after an hour; the batteries packed up after eight hours; and the mobile generators that were rolled in failed to activate the pumps even as water levels in the reactors were falling, making a core meltdown distinctly possible. It appears that the flooding by the tsunami had also damaged the switchgears. Inadequate on-site water inventory also seems to have been a problem considering that sea water pumping was resorted to almost immediately. This also suggests that the reactors did not have appropriate passive cooling measures retrofitted for such a loss of coolant accident (LOCA). Indeed, in 1992 the United States Nuclear Regulatory Commission (NRC) highlighted several safety-related issues with the containment features of Mark 1 BWRs, which significantly included alternative water supply for reactor vessel injection and hydrogen control. The NRC specifically called for maintaining an inert nitrogen atmosphere in the containment during shutdown. The hydrogen explosions in two units suggest the absence of nitrogen-inerted containment at Fukushima. Further, one of the serious problems with Mark 1 reactors is the smaller containment volume to power ratio. This results in higher containment pressure, rendering the reactor more vulnerable to breach than later BWR designs or even the earlier Tarapur type pre-Mark 1 design.

In the Indian context, the issue of safety of the Tarapur nuclear power plant, which houses BWRs similar to Fukushima, has been raised. While containment itself is not an issue here, TAPS maintains a large inventory of water in huge reservoirs and has a much bigger suppression pool than those in Fukushima, which exploded in one of its units. Further, improved and redundant back-up power as well as passive cooling have been incorporated at TAPS. As regards the other mainstay reactors of the Pressurised Heavy Water (PHWR) kind, the safety features are much superior to earlier generation BWRs. Unlike Light Water Reactors (LWRs) — boiling or pressurised — PHWRs have separate coolant and (low temperature) moderator circuits and the reactor vessel calandria itself is maintained in a large water vault. The on-site water inventory for cooling in PHWRs is thus inherently very large. In addition, they have other mutually independent parallel means to remove the decay heat in case LOCA occurs. The more recent designs of PHWRs have added passive cooling features as well. The yet-to-be-built Advanced Heavy Water Reactor (AHWR) has a passive cooling feature that is intrinsic to the design. The new generation LWRs that are being imported are also expected to have passive cooling features. The Russian VVER-1000 of the kind being built at Kudankulam and Areva's EPR1600 have, in addition, a ‘core catcher' at the bottom of the reactor vessel to contain the core melt in case of a meltdown and prevent it from leaching into the soil below.

In light of the Fukushima accident, the lesson for India is not necessarily a roll-back on the nuclear energy option. It is that there is an imperative need to allow for the unimagined very low probability event with high impact potential in safety analyses. The current paradigm and methodology of Probabilistic Risk Assessment (PSA) may have to change once the details and the exact timeline of events at Fukushima become available. Notwithstanding the assurances given by the Department of Atomic Energy, the Atomic Energy Regulatory Board — which needs to be empowered — must carry out a thorough technical safety review of all the nuclear power plants, including those on the import list. This must be done against new benchmarks, and regular safety audits of each plant must be done and the results made public. Transparency in matters of nuclear safety is needed more than ever, post-Fukushima.

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