India's 18 pressurized heavy water reactors have inherent strengths that would help them to cope with the consequences of unexpected natural events, but its two boiling water reactors will need further safety enhancements, according to a report prepared for India's nuclear regulator.
The Atomic Energy Regulatory Board (AERB) set up a high-level committee in March to review the safety of Indian nuclear power plants in the event of natural disasters, following the earthquake and tsunami that hit the Fukushima Daiichi plant in Japan earlier that month. The committee has now submitted its report for review by the AERB.
The two BWRs at Tarapur are India's oldest operating nuclear reactors, and the AERB committee points out that operator Nuclear Power Corporation of India Ltd (NPCIL) has already taken interim safety measures to enhance the units' safety. These include provisions for continuous reactor cooling under prolonged station blackout, taking into account the loss of both on- and off-site power, plus preparatory work for 'inerting' the containment with nitrogen gas in order to avoid hydrogen explosions.
In the event of a station blackout, the BWRs would be able to maintain core cooling for up to eight hours before make-up water would need to be supplied to the emergency condenser, the report found. Safety systems including back-up power supply systems would also need upgrading to meet revised flood levels, and the committee called for a detailed study to identify design improvements with corrective actions to address the deficiencies completed at the earliest opportunity.
The design provisions of the PHWRs to provide reactor core cooling in station blackout conditions had already been demonstrated during a 17-hour blackout caused by a turbine hall fire at Narora 1 in 1993, but the committee also recommended that provision should be made for a reliable back-up to supply water to the primary heat transport system. However, the committee found that the used fuel in storage ponds at Indian plants could be assured for at least a week under such conditions.
As the submarine faults that are capable of generating tsunamis are located at very long distances from the Indian coasts, Indian plants are not considered to be at risk from the occurrence of a simultaneous strong earthquake and tsunami. After the Indian Ocean tsunami of 2004, flood levels for the Kalpakkam nuclear power plant site on India's south-eastern coast have been undergoing reassessment and it is likely that maximum postulated flood levels along that coast will be revised upwards, the committee reported. This will mean that design improvements will have to be considered for the two power plants at Kalpakkam, also known as Madras 1 and 2 (or MAPS 1 and 2). The prototype fast breeder reactor (PFR) under construction at Kalpakkam is unlikely to need any revisions as its defences are already sufficiently high.
Another recommendation from the committee was the creation of an emergency facility at each nuclear power plant site that would remain functional even in circumstances when the plant might find itself physically isolated from outside help for a considerable period of time. The committee also notes that the "extremely remote" possibility of an accident leading to partial or total melting of fuel in a reactor's core under unforeseen circumstances should be taken into account in accident management planning, with relevant design provisions and revisions to operating procedures applied to plants under construction as well as to those already in operation.
The report will be reviewed by the AERB, before its recommendations are applied by NPCIL, although the regulator noted NPCIL's own measures to bolster safety and preparedness. The operator filed its own report including recommendations to boost safety levels, and a roadmap for their implementation, in July.
Researched and written
by World Nuclear News