How safe will Rooppur be?
Monday, May 31, 2010
Point CounterpointHow safe will Rooppur be?
Nuclear reactors are much safer nowadays.
Abdul Matin
It is reported that Bangladesh and Russia signed a framework agreement on May 21 on cooperation for peaceful uses of nuclear energy. It covers many areas, including construction of two nuclear power reactors at Rooppur, each of 1000 MW capacity.
The news was welcomed by the people as it is likely to materialise a demand that originated during the pre-liberation period. It will also add a feather to Prime Minister Sheikh Hasina's cap if she succeeds in bringing this agreement to a fruitful conclusion when all previous governments failed.
While welcoming the idea of building nuclear power plants with Russian assistance, one cannot forget the catastrophic accident that occurred on April 26, 1986 at Chernobyl Nuclear Power Plant (unit 4). Many people are asking: "How safe will Rooppur be? Will it be like another Chernobyl"?
A direct answer is: "No, it will be safe, and can't be like another Chernobyl," but this is unlikely to satisfy an inquisitive mind. One would naturally like to know under what circumstances the Chernobyl accident occurred and how far the situation at Rooppur will be different so as to prevent a recurrence of a similar accident.
The Chernobyl accident occurred because of both design faults and serious operational errors.
When the accident occurred at Chernobyl, the reactor was not in routine operation. Some engineers were performing an experiment on the reactor to see if the inertia of the turbine could be used to generate enough electricity to operate the emergency supply for a short period prior to the start of its standby emergency diesel generators.
The people performing the experiment were not familiar with normal reactor operation procedures or its safety systems. They also lacked basic knowledge on reactor physics and engineering.
A nuclear reactor generates substantial heat even after shutdown due to a high level of radioactivity inside the core. All power reactors are provided with normal and emergency core cooling systems to remove this heat, known as residual heat, after the reactors are shut down. Normally storage batteries and emergency diesel generators are used to supply power to the emergency core cooling systems in absence of any external power supply.
While performing the experiment the reactor operators switched off many of the reactor safety systems, including the automatic shutdown mechanism and the emergency core cooling system, in violation of normal reactor operating procedures. During the experiment the reactor power began to rise, producing voids (steam bubbles) as the power generated by the turbine was insufficient to maintain the core cooling system.
This in turn increased the reactor power, producing more voids, thus leading to a sudden power surge before the reactor could be shut down. The power surge damaged the core, thus releasing highly radioactive substances. The resulting high steam pressure caused a pipe to explode, rupturing the roof of the building. It was followed by more explosions.
Eventually, radiation leaked to the atmosphere. It should be made clear that there was no nuclear explosion at Chernobyl. The explosions were conventional, caused by high steam pressure inside the pipes.
Nuclear reactors, except those built in the then Soviet Union, are housed inside what is known as a containment building. This is a dome-like heavy structure built around the reactor to prevent any release of radioactivity, which may accidentally leak out of the reactor core to the atmosphere.
The pressure inside a containment building is kept lower than the atmospheric pressure, thus the air from inside cannot leak out. The Chernobyl reactor did not have a containment building, as a result the radioactivity that leaked from the core could escape to the atmosphere. Modern Russian reactors are built with containment buildings.
The effectiveness of containment buildings was demonstrated in the Three Mile Island Unit 2 accident in 1979, when virtually all radiation was retained inside the containment building, despite considerable damage to the core.
It may be recalled that the Russians proposed to build a 400 MW nuclear reactor without any containment building at Rooppur in late 1960's. When we asked them why the reactor did not have a containment building, they proudly replied that their reactors did not have accidents to justify one. Years later, the Chernobyl accident shattered their pride.
Rooppur, on the other hand, will have pressurised water reactors (PWR), most probably of generation-III type, with reliable and tested safety features. The core of a PWR consists of slightly enriched uranium as fuel, and water as both moderator and coolant, a combination that makes the void coefficient negative.
A negative feedback stabilises a system. If voids are formed in the core for any rise in power level, the negative void coefficient will bring the power down. No power surge, like the one in Chernobyl unit 4, is possible in a PWR. This feature makes a PWR reactor inherently safe.
The reactor at Rooppur will be built with containment buildings. It is possible to build double containments around reactors at a nominally extra cost, which is recommended for the Rooppur reactor.
A PWR incorporates multiple barriers to prevent the release of radioactivity to the atmosphere. The first barrier is the ceramic fuel pellet where nuclear fissions take place and energy is released. The pellet retains most of the fission products, the main source of radioactivity.
The second barrier is a sealed metal tube called the cladding that contains the fuel pellets. The cladding retains any gaseous radioactive material that may leak out of the fuel pellets.
The third barrier is the closed primary cooling water system that circulates through the core and carries the heat to the steam generator. The cooling system will contain any radioactivity that may leak out of the cladding.
The last barrier is the containment building designed to contain any radioactivity that may leak out of the primary cooling system through any accidental rupture.
Generation-III reactors are compact, more economical and safer versions of generation-II reactors, which form the bulk of the reactors (60%) now in operation. Generation-III reactors incorporate passive safety systems that come into operation without any action by the operator.
The driving force for the core cooling system is provided by gravity, temperature difference (convection flow) and gas accumulators. All these features make these reactors inherently safer than generation-II reactors.
The Rooppur operation team will consist of a select group of engineers specially trained in reactor technology, reactor operation and maintenance. They will undergo a rigorous program of training at home and abroad and will be required to pass a rigorous test for qualifying as reactor operators at the end of their training program.
Only the holders of operation licences will be allowed to operate the reactor at Rooppur. It may be mentioned here that some engineers of the original Rooppur operation team, recruited in early 1960's, built, commissioned and operated reactors in Pakistan, Argentina, Romania and South Korea.
The Rooppur nuclear power plant will be purely a power producing facility, and no experimentation of any kind on the reactor will be permitted.
Lessons learned from the Three Mile Island accident, the Chernobyl accident and other minor incidents helped to improve both reactor safety features and operational procedures over the years.
As a nuclear power plant operator, Bangladesh will be a member of the World Association of Nuclear Operators (WANO), formed in 1989. This organisation ensures adherence to a high level of safety by setting international benchmarks applicable to all power reactors around the world. Rooppur will be obliged to maintain the same safety standards.
It is to be noted that no accident involving the release of any significant amount of radioactivity to the atmosphere has occurred since the Chernobyl accident.
Improved design features, reliable and tested safety standards incorporating both active and passive safety systems of the PWR reactors and the best possible training programs for the operators will ensure the safe operation of the Rooppur nuclear power plant. An accident like that of Chernobyl unit 4 at Rooppur will be the most unlikely event under any circumstances.
Abdul Matin is a former Chief Engineer of Bangladesh Atomic Energy Commission.
