Monday 6 August 2007

Safety--Indian Nuclear Plants

Business India, March 14-27, 1994

Crying Wolf

Exaggerated fears about safety in the Indian nuclear power industry obscure the more pressing regulatory issues.

Shivanand Kanavi

A fortnight ago the American TV network, CBS, broadcast a film on India's nuclear power stations -Another Chernobyl? The film described the Indian nuclear programme as the most dangerous in the world and cited numerous 'instances' of unsafe operating practices at Indian nuclear power plants. Unfortunately, in its eagerness to titillate a jaded American TV audience, CBS overstated its case. Sensational they may be, but such exaggerated accounts, in fact, obscure and detract attention from the very real technical and safety problems that bedevil the nuclear industry everywhere.

Concern has, however, centred on what CBS correspondent Steve Kroft described as "the crack in the Rajasthan reactor.” Unit I of the Rajasthan Atomic Power Station, at Rawatbhatta, commissioned in 1972, has developed certain problems, which have worried nuclear engineers. It is important to understand the nature of these problems.

The crack in the 'endshield' was discovered in 1981. The endshield is an attachment to the main reactor vessel called the 'calandria’. The fuel bundles pass through the end shield and are defuelled or refuelled through it, on line. The endshield does not contain radioactive fuel or the heavy water coolant that is used both to draw the heat away from the nuclear reaction to the steam generator and as a moderator to control the chain reaction within the limits suitable for steady power production. Plain water is circulated through the endshield chamber. Unlike the heavy water, this water does not become radioactive. Due to a minute crack in the endshield some of this water was found to have trickled into the containment chamber. It did not cause any release of radioactivity inside the reactor or to the outside environment.

The reactor was immediately shut down, and investigations revealed that the crack had been caused by embrittlement due to neutron bombardment of the carbon steel plate. The reactor had been designed by Atomic Energy of Canada Limited. They too had faced a similar problem earlier at their plant at Douglas Point, in Ontario, Canada, and subsequently had changed over to another material for fabricating the endshield. But following the 1974 nuclear explosion at Pokhran, Canada had ceased all nuclear co-operation with India, and Indian nuclear engineers had to fall back on their own resources to deal with the problem.

The Atomic Energy Regulatory Board, the Indian watchdog body for safety in all installations handling radioactive materials, immediately stepped in. The AERB along with reactor engineers at BARC and the Nuclear Power Corporation decided that the crack would not increase nor would it lead to greater seepage if the reactor were operated at half its designed power level, of 220 MW. Since then RAPS-I has been operating at a power generating capacity of 100 MW and the crack is being continuously monitored.

The crack has caused more worry to the power engineers than to the safety personnel. Indium foil, which has the property of changing shape under pressure and seeping into cracks, thereby sealing them, has been pressed onto the endshield crack. The fuel channels near the crack have also been defuelled as abundant caution. Attempts are also on to design methods to reach the endshield and replace it with a new stainless steel one. These attempts will take some time to fructify but can then be used profitably at all reactors as they ‘age’.

Even in the worst case scenario of the end shield cracking open and all the water pouring out, there will be no radioactivity released, either inside or outside the station. The reactor will be immediately shut down and unless methods are developed to safely clean the leaked water and replace the endshield, the reactor will be permanently shutdown by the AERB.

The other problem that is being monitored in RAPS-I&II and MAPS-I is the sagging of the primary heat transfer tubes within the reactor. These are tubes containing pressurised heavy water which carries away the heat from the nuclear fission taking place inside the fuel bundles. These pressurised tubes are separated from the calandria tubes by a concentric gap of 8mm by garter springs placed at intervals. While the heavy water in the primary heat transfer tubes is at about 270 degrees Celsius, the calandria tubes are surrounded by cold heavy water, used as a moderator, at 70 degrees Celsius.

It has been observed that due to vibrations within the PHT tubes, the springs tend to shift from their positions leading to a lack of support at certain portions of the PHT tubes. The weight of the zircalloy PHT tube containing heavy fuel bundles makes it then sag. If the sag leads to contact with the colder calandria tube, a local cold spot which develops at the point of contact leads to the accumulation of deuterium in the zircalloy tube at that point, which can lead to blistering and even a possible rupture, leading to the coolant draining away from the PHT tube. Even in such an eventuality the heavy water is still not released outside the reactor. The reactor will have to be shut down and the particular tube isolated. The same loss of coolant in a boiling water reactor will lead to an extremely serious accident, as witnessed in the Three Mile Island, US. The Indian pressurised heavy water reactor is much safer in that respect.

When such blistering was observed in the Pickering reactor, in Canada, the Canadians switched over, in 1983, to a better material called niobium-stabilised zircalloy with a greater support of a larger number of more or less fixed garter springs. Again, due to the embargo on Indo-Canadian nuclear exchanges, the information reached India only in 1985. Since then all reactors are being periodically monitored for any sign of sagging using an ultrasonic probe developed by BARC. BARC and the NPC are also developing a complex retubing machine that can change the old tubes with the newly developed niobium-stabilised zircalloy tubes. The newer reactors at Kaiga and Kakrapar are fitted with these newer tubes.

Another worrisome problem is the corrosion of some valves that use cobalt alloys. Their corrosion, though very small, leads to cobalt being exposed inside the reactor to radiation which coverts into radioactive Cobalt-60. This isotope has a long half-life and poses an exposure risk to workers carrying out maintenance. However, a very positive development in this regard is the development of a chemical method using dilute organic acids to remove this CobaIt-60 from the system. It has been recently used successfully in MAPS-I; the results showing the reduction of radiation ranging from 50 to 97 per cent was reported in the last week of February at the International Conference on Operational Safety of Pressurised Heavy Water Reactors.

Conversion of small amounts of heavy water in the reactor to the deadly tritium is another problem that is monitored in all Pressurised Heavy Water Reactors, Recently BARC has also developed a method to remove tritium from the heavy water used in reactors. Its on-line application is still to be developed but has given a fillip to all those concerned with safety.

Dr A. Gopalkrishnan, a nuclear engineer from the University of California, at Berkeley, who worked on reactor designs and safety in various establishments in the US for 15 years, now heads the Atomic Energy Regulatory Board. While being proud of the achievements of our scientists and engineers regarding safety, he has a very clear agenda for higher safety standards in Indian nuclear power stations. Right now no individual worker is exposed to more than internationally accepted levels of radiation, but this is being achieved by using more manpower and rotating them so that the accumulated dosage per person is still at international levels.

But Gopalkrishnan is not satisfied with this arrangement. Most exposure happens not during the operation of the plant but while maintaining it or carrying out some repairs. He says, "More attention to local shielding, training repair crews using mockups prior to the job so that they spend the least amount of time in radiation zones, semi-automation of maintenance and inspection jobs, will definitely bring the total dosage of all workers in a plant to international levels.” Internationally acceptable radiation exposure levels are continuously being scaled down, leading to pressure on our own nuclear establishment to better safety practices.

Following the fire at the Narora Unit-l power station in March 1993, the AERB initiated a planned shut down of all nuclear power stations to check for faulty turbine blades. Cracks in a few blades in the General Electric-designed generator at Narora had led to the fire that caused damage worth over Rs.75 crore and considerable revenue loss due to plant shut down. Though over 50 such sets worldwide have shown cracks after prolonged usage, the sets used in India tend to break down earlier due to unstable conditions in the Indian power grid system. Due to wide load fluctuations, the frequency of the alternating current in the grid varies widely between 49 Hertz and 51.5 Hertz, leading to dangerous vibrations in the turbine.

The AERB' s insistence on checking all the turbines meant a loss of revenue for the NPC, but already the precaution has yielded results. Four blades with cracks were found in MAPS-l in Madras recently by the independent consulting group from the Central Mechanical Engineering Research Institute that carried out the tests. Though very new, the Kakrapar-I turbine is also being checked and, in fact, all blades are being replaced with design modifications made by BHEL. The Narora fire was not in the reactor but in the turbine and the safety systems of the reactor operated as designed. That is why although it was a financial disaster; the fire was classified on the Nuclear Events Scale - a sort of nuclear 'Richter scale' - as a level-3 incident and not an accident.

When asked why no official report has been published so far about Narora to allay public fears and permit informed debate, Gopalkrishnan says, "My aim is to make at least safety related information as publicly accessible as possible. Instead of leaving this to the individual initiative of the chairman of the AERB, it is better that it is statutorily recognised as well."

Regarding some of the questions raised by the critics about the radiation hazard at the thorium-rich, monazite sands in Kerala and the unusual number of deformities among people in villages around the Rawatbhatta power station, Gopalkrishnan says, "A highly rigorous epidemiological study is being conducted at Chavara, Kerala, by the regional cancer research institute. In another year the results will be available. Let us wait till then. Meanwhile, no such epidemiological work can be conducted in the villages around the Rawatbhatta plant since the sample size is too small to come to any conclusion. However, according to the suggestion of Dr Sanghamitra Ghadekar (an activist who has herself painstakingly collected the Rajasthan data and who is asking for a scientific investigation), I am going to constitute a panel of genetic and medical experts to analyse it from that angle and see. After all, as far as radiation from the Rajasthan power station is concerned, it has been continuously monitored and no dangerous level of radiation has been released outside."

Given the pressure on PSUs to become profit centres and the temptation on part of managements to take short cuts, the question of safety in nuclear power stations assumes new significance. Due to increased openness shown by the AERB, more individual workers, as well as unions, are directly approaching the AERB, often anonymously, with safety related complaints. The AERB is examining the complaints. However, there is a need to develop a second opinion regarding safety and nuclear engineering. In this respect, it is inexplicable that the NPC is not utilising the safety audit services offered by the International Atomic Energy Agency. After all, the safety services are independent of signing the nuclear non-proliferation treaty and, in fact, Indian experts have participated in such multinational teams to check safety in others' plants.

There is, however, no excuse for not developing nuclear engineering faculties in some academic institutions, like the IITs and the Indian Institute of Science, which can then provide the expertise for a second opinion as well. Besides, without questioning the authenticity of the data provided by the health physics division of BARC, which checks the release of radiation to the environment at all power stations and monitors the individual doses absorbed by each worker, it is clear that constituting the division as a separate body will enhance the credibility of the nuclear programme.

Lastly, while the developments in indigenous nuclear R&D have been impressive, particularly in the light of the embargo both in the supply of equipment and technical information to India, the weaknesses are there for all to see. For example, while all the reactors are working at lower than their rated capacity, the natural focus should be on carrying out well-focused R&D. At this stage the hurry to also develop advanced reactor systems, fast breeder prototype reactors, etc, shows that, without solving the problems at hand, there is a tendency to go on to the next prestigious project. In fact, it is most puzzling that BARC wants to have its hand in every scientific pie, whether parallel processing, superconductivity, or lasers, which can very well be done by other academic and R&D institutions. It would do well to concentrate instead on making our nuclear programme both safer and economically successful.

The International Nuclear Event Scale

Level and Examples

Major accident, 7 ,Chernobyl, USSR, 1986

Serious accident, 6, None

Accident with off-site risks, 5, Windscale, UK, 1957 and Three Mile Islands, USA,1979

Accident mainly in installation, 4 , Saint- Laurent, France, 1980

Serious incident, 3, Narora, India, 1993, Vendellos, Spain, 1989

Incident, 2 , None

Anomaly, 1, None

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