Nuclear power and safety issues

Print edition : September 25, 1999

Engineers at the Bhabha Atomic Research Centre are involved in an ongoing effort to redesign nuclear reactors, incorporate improved safety features and conduct extensive tests, all with the aim of ensuring total safety in the country's nuclear power g eneration system.

FROM the outside, Engineering Hall 7 at the Bhabha Atomic Research Centre (BARC), Trombay, near Mumbai, looks fairly nondescript. But inside, the cavernous hall is the centre of advanced research and development (R&D) in both engineering and safety in nu clear electricity generation. This R&D applies not only to the extant Pressurised Heavy Water Reactors (PHWRs) in the country but the planned Advanced Heavy Water Reactors. Inside Hall 7, experiments are conducted in laboratories and tests done on scaled -up models to ensure the safety of PHWRs.

The computerised control rooms for the two reactors of the Kaiga Atomic Power Project in Karnataka. The control rooms operate independent of each other, a provision intended to ensure greater safety.-M. MOORTHY

There are eight PHWRs in the country - two each at the Rajasthan Atomic Power Station (RAPS), the Madras Atomic Power Station (MAPS), the Narora Atomic Power Station (NAPS) and the Kakrapar Atomic Power Station (KAPS). The second unit (220 MW) at Kaiga i n Karnataka will join the list soon.

The design of PHWRs, which use natural uranium as fuel and heavy water as both moderator and coolant, has evolved over the years; there has also been a steady improvement in the technology itself. Among the improvements incorporated are double containmen t of the reactor building, changes in end-shields of reactors, introduction of a new concept of suppression tanks, the filling of the calandria vault with 400 tonnes of light water (ordinary water) and the use of zirconium niobium alloy in the manufactur e of pressure tubes.

Dr. V. Venkat Raj, Associate Director, Reactor Design and Development Group, BARC, said, "PHWRs are uniquely placed. We have multiple echelons of safety provisions to take care of severe accidents. Even if the emergency core cooling system fails in a los s of coolant accident (LOCA), fuel will not melt. We have a lot of systems and instruments to detect any such accident."

According to Dr. Anil Kakodkar, Director, BARC, "Reactor safety requires analysis of the behaviour of the full plant. We must have all protective mechanisms in place, and we must make several postulates and identify possible conditions for abnormal behav iour (of the reactor)." Kakodkar is the architect of the Advanced Heavy Water Reactor (AHWR) that will use uranium-233 as fuel and thorium as blanket.

Kakodkar said that the Department of Atomic Energy (DAE) had a comprehensive programme to deal with waste safety, and had studied the physical and chemical transformations that take place in waste, its dispersal, and so on. "They have become specialised branches of science," the BARC Director said. Kakodkar added: "We have done a lot of work on radiobiology - how radiation interacts with living matter."

Y.S.R. Prasad, Chairman, Nuclear Power Corporation of India Limited (NPC), which operates all the nuclear power stations in the country, said that safety depended on those who designed, operated and maintained the reactors. Apart from engineered safety f eatures and a variety of systems to minimise chances of accidents, the NPC had a training and retraining programme for its plant personnel.

THE crux of reactor safety is the cooling of the fuel core. All safety systems are aimed to prevent the eventuality of the core losing its cooling, the fuel melting down and losing its shape, and a fire breaking out. The emergency core cooling system (EC CS) is one of the safety systems that aims to ward off a LOCA. It was a LOCA that occurred at Chernobyl and Three Mile Island; in the latter case there was no release of radioactivity into the atmosphere.

During the operation of a reactor, hydrogen embrittlement, that is, micro-cracks, may sometimes develop in the coolant (pressure) tubes that house the uranium fuel bundles. If these cracks cut across the thickness of the coolant tubes, they may sag and t ouch the calandria tube, around which is the coolant heavy water. This can lead to a loss of coolant from the primary system, and inadequate cooling of the fuel in the core.

According to top NPC officials, three stages of ECCS were provided in the reactors. There were instruments and systems to sense a rupture in the primary circuit and a fall in pressure. Heavy water would be immediately injected over the core by opening va lves. The second stage involved the use of ordinary water from the storage tanks. These would take care of the initial LOCA. In the third stage several hundred tonnes of water would be available in the basement of the reactor building for suppression coo ling.

Venkat Raj explained the changes and improvements in design that had been incorporated over the years. The first unit at RAPS was built by Canada, and the second unit by the DAE. The RAPS units had a dousing tank on top of the reactor building, with a la rge quantity of light water to condense the steam in the primary coolant circuit. Since the primary coolant was under high pressure, pressure in the containment (the concrete dome over the reactor building that is intended to prevent radioactivity from e scaping into the atmosphere) would go up. But the RAPS reactor building was not designed to withstand high pressure.

In the event of a LOCA, water from the dousing tank would cascade and douse the steam. But if there was a spurious signal, water would come down needlessly. To avert such eventualities, said Venkat Raj, from MAPS onwards, the dousing tank was removed fro m the top of the building and the water was stored in a suppression tank in the basement of the reactor.

Dr. Anil Kakodkar, Director of the Bhabha Atomic Research Centre, Trombay.-SAGGERE RAMASWAMY

In the dousing tank, valves had to be opened to let the water down, and this needed time. The suppression tank did not require the opening of any valves. "This is an improvement we made," said Venkat Raj. Further, in the event of a LOCA, using the suppre ssion tank poses no threat to containment integrity.

ANOTHER improvement incorporated in the design was the building of double containment from Narora onwards. Kalpakkam had partial double containment - the cylindrical portion of the reactor building had two walls but the dome on top had a single wall. Fro m Narora, all reactors were provided with double containment, including the dome.

The inner (primary) containment was designed to withstand high pressure in the event of a LOCA. The large space between the inner containment and the outer (secondary) containment, called annular space, was maintained at below atmospheric pressure. Durin g normal operations, the space in the inner containment was kept at a pressure even lower than that in the annular space in order to prevent any radioactivity from escaping into the atmosphere. If an accident occurred, the inner containment would get pre ssurised, but whatever radioactivity reached the annular space would not escape because of the higher atmospheric pressure outside.

Changes were also introduced in the calandria vault and the end-shields. The calandria is a huge cylindrical stainless steel vessel with massive steel plates called end-shields on either side. The calandria has 306 coolant tubes housing the fuel bundles, surrounded by high-pressure, high-temperature heavy water which is the coolant. The calandria tube is surrounded by low-pressure, low-temperature heavy water which is the moderator.

From Narora onwards, the calandria vault was filled with about 400 tonnes of ordinary water (earlier, it was only air); this has an important bearing on safety. This is in addition to about 150 tonnes of heavy water, the moderator, in the calandria, as i n all other reactors. From the second unit at Kakrapar, the pressure tubes were made of zirconium niobium, which is less prone to hydrogen embrittlement effect unlike zircaloy, which was used earlier. The existing zircaloy tubes in all reactors in the co untry are now monitored periodically and are being replaced with tubes made of zirconium niobium.

Up to Kalpakkam, low-pressure moderator injection was used for ECCS. From Narora, this was improved to high-pressure, heavy water injection followed by medium-pressure light water injection and long-term circulation by pumps.

Computerised control systems have also been introduced to strengthen the safety aspects. For the two reactors at Kaiga, there are now two control rooms, situated side by side but totally independent of each other. This was consequent to a fire incident i n the first unit at Narora, which led to a black-out. Venkat Raj said: "We learnt our lessons from Narora. If the ventilation system is common, smoke entering one control room will invade the other room."

BARC engineers also simulated a LOCA. First they worked out a theoretical analysis, and to make sure that this was right, they did experiments with 1:10 scale models. Venkat Raj said, "These experiments validated both the theoretical analysis and the com puter code used for the calculation of the containment pressure rise."

Although an operating PHWR station has many systems and instruments to detect a LOCA, there is always the lurking fear that the instruments might fail. BARC therefore set up a test system called LOCA Environment Simulation Facility. The cables carrying e lectricity and signals, which might also fail, could be tested in this facility.

BARC had designed computer codes to analyse accidents such as LOCA. To validate these codes BARC built test facilities at a cost of crores of rupees to simulate the working of reactors and coolant systems. The predictions were first projected; subsequent ly the exercises were done. The test results were compared with the predictions to see whether the computer codes had been validated. Many cycles might be gone through in updating the computer codes.

Early this year, the Reactor Design and Development Group built a Facility for Integral System Behaviour Experiments (FISBE). This simulated PHWR coolant systems in a scaled manner. A feature of FISBE is that it simulated the full elevation - 40 metres - of the equipment as in a reactor.

Engineering Hall 7 also has a huge 3 MW Boiling Water Loop machine. Venkat Raj designed and constructed this about 15 years ago for heat transfer and safety-related experiments under conditions of reactor coolant flow, temperature and pressure. (Venkat R aj has now taken charge as Director, Health, Safety and Environment Group, BARC, in place of Dr. Umesh C. Mishra.)

LOCA simulation experiments demand acquisition of data very fast, every 100 microseconds. The fast data acquisition machine in the hall can scan 10,000 parameters every second. An Indian company developed the machine about ten years ago and it is still g oing strong.

The 3 MW Boiling Water Loop machine at the Reactor Engineering Division at BARC. It was designed for heat transfer and safety-related experiments under conditions of reactor coolant flow, temperature and pressure.-COURTESY: BARC

PHWRs which operate in the country have several safety systems to withstand earthquakes. For each nuclear power station site, the levels of earthquake are defined depending on the seismicity of the area. Two levels of earthquake are defined: the operatin g basis earthquake, and the safe shutdown earthquake. In the first level, all operating systems in a nuclear reactor are designed to withstand the seismic shocks, and the reactor will continue to operate. The level of a safe shutdown earthquake is higher than that of an operating basis earthquake, but the probability of its occurrence is lower. At this level, safety systems such as the ECCS, containment and reactor shutdown systems are so designed that the reactor can be shut down safely and the cooling maintained to remove heat from the fuel. In the event of an accident, any reactor should meet three important safety functions - it should shut down safely; there should be provision for continued core cooling to remove decay heat; and containment integ rity should be maintained. So long as these requirements are met, radioactivity will not escape into the atmosphere.

BARC engineers and scientists have undertaken complicated analyses to determine how earthquake shocks impacted on reactor equipment. Venkat Raj said, "We developed theoretical models. To confirm them with experiments, there are special test facilities ab road called shake tables, which would shake the equipment as an earthquake would." The engineers also mount the equipment on a diesel engine and run it on railway tracks; this has the same vibration effect as would be experienced in an earthquake.

The first article in a two-part series on issues of nuclear safety.

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