Safety first

Print edition : July 31, 1999

AT the Kaiga Atomic Power Project, the watchword is safety. All activities are geared towards ensuring the safety of the reactor. "If anything, we are overcautious," says Dr. Ashok Mohan, Technical Advisor to Dr. R. Chidambaram, Chairman of the Atomic En ergy Commission.

Project Director V.K. Sharma said: "The crux of reactor safety is based on core cooling." All safety systems were aimed at preventing the dreaded eventuality of a Loss of Coolant Accident (LOCA). Emergency Core Cooling System (ECCS) is one of the safety systems adopted at Kaiga, which also boasts of a number of in-built safety measures, Sharma says.

Sometimes micro-cracks develop in the coolant tubes that house the natural uranium bundles. If these cut across the thickness of the coolant tubes, they can lead to a loss of coolant, that is heavy water, from the primary system and inadequate cooling of the fuel in the core. A LOCA may lead to a fire in the reactor.

Multiple barriers have been created to prevent radioactivity from escaping into the environment. To start with, the fuel pellet itself retains the fission products within its matrix. This is surrounded by the cladding of the fuel, which serves as the sec ond barrier. Any radioactivity released owing to a failure of the cladding is confined to the primary heat transport system (PHT) by the PHT boundary, which is the third barrier. The fourth and fifth barriers are the inner and outer containment of the do me over the building that houses the reactor. Besides, there is an exclusion zone of 1.6 km around the plant.

According to Sharma, once the reactor starts operating the fuel has to remain cool even if the reactor needs to be shut down. In other words, radioactivity has to be retained in the fuel itself. He said: "As long as you are able to cool the core, there i s no safety issue at all. All safety systems in the reactor address issues relating to core cooling."

The Project Director pointed out that natural uranium in the form of pellets were canned, aiming at low neutron absorption. As long as there was no rupture of the fuel, radioactivity remained inside the pellet and the cladding. Even if the shielding brok e, radioactivity would not escape because the structure of the fuel helped retain it, he said.

Sharma said: "The problem arises only when the cooling of the fuel core is lost. The dreaded scenario is the prospect of the entire cooling being lost and a significant failure of the fuel when the core melts down. That is what happened at Chernobyl (in the former Soviet Union) and Three Mile Island (in the United States). But there was no release of radioactivity into the atmosphere at Three Mile Island." Before the core meltdown took place, the fuel started losing shape. When the fuel touched the pres sure tube, the latter in turn came in contact with the calandria tube. The design of the Indian reactors was such that even if the pressure tubes and the calandria tubes came in contact with each other, the coolant would take care of it, Sharma explained .

Safety experts said that the RBMK-type reactor at Chernobyl used graphite, a burnable material, as moderator. The station operators there were conducting unauthorised experiments. Besides, Chernobyl had no containment dome to prevent radioactivity from e scaping.

A state-of-the-art control room. The control panels provide full information to reactor operators on the status of the plant.-

Sharma said: "Assuming that the entire core melts, the double containment (at Kaiga) is designed to withstand the pressures resulting from a LOCA and to contain the radioactivity inside the reactor without the plant personnel and people outside getting a ny significant dose.." A 100-metre-high stack with particulate filters filters radioactive emissions before letting them into the atmosphere. The stack height has been so designed that a person standing at the edge of the exclusion zone would receive rad iation within permissible limits.

As long as the primary circuit at Kaiga was intact, a LOCA would not occur, Sharma said. The reactor automatically shuts down when the power to the primary circuit goes off. This was one of the passive safety features, which was not based on outside inte rvention but on spontaneous reaction, he said. But the Nuclear Power Corporation of India Limited (NPC) was so obsessed with safety that it postulated an accident scenario when a double-ended rupture of this largest pipeline would take place without a wa rning, Sharma said. However, the steel used in the fabrication of the primary circuit would not permit crack-like defects to appear in it, he said. It was tested at minus 40C because steel tended to become brittle at lower temperature. But the material boasted of ductility.

Dr. Ashok Mohan said that the steel used in the making of the primary circuit was much stronger than multi-crystal and that there was no point where stress concentration would take place. Besides, quality checks were rigid, he added.

Station Director N. Rajasabai said that three stages of ECCS were provided in the reactor design. A rupture in the primary circuit and a fall in the pressure would be immediately detected and heavy water would be injected over the core by the opening of valves. The second stage involved the injection of light water (ordinary water) from the storage tanks. These would take care of the initial LOCA. Since light water would be of limited quantity, a third stage would come into operation when a massive quan tity of water - eight feet depth of water - available at the basement of the reactor building would do suppression cooling, Rajasabai said. Sharma pointed out that the philosophy was to use more and more water if a rupture took place in the primary circu it.

The fire incident in the first reactor at Narora made the NPC wiser. At the Kakrapar Atomic Power Station in Gujarat and at Kaiga, there is a separate routing of electricity cables to prevent a blackout, the kind of which engulfed the reactor building at Narora. Diesel and battery back-ups for the generation of electricity are available at Kaiga. The normal electricity cables, the cables from diesel generators and those from batteries were segregated physically by routing them in three different rooms. The cables were encased in metal trays too. Even if the wires leading from the batteries were damaged, the others would take over.

Sharma is convinced that a Narora-type blackout is not possible in Kaiga. Rajasabai added: "Even if a fire breaks out in one place, it will affect only one system of cables because the routes are different for the three systems of cables. So protection t o the reactor is still available. Physical separation of cables is an important safety feature in Kaiga."

This article is closed for comments.
Please Email the Editor