Innovative reactor

A model of the Advanced Heavy Water Reactor at BARC.-V.V. KRISHNAN

The AHWR project enters a crucial phase with the regulatory board completing the pre-licensing appraisal of the reactor's design.

THE construction of India's futuristic Advanced Heavy Water Reactor (AHWR) is expected to begin by the end of 2007, according to Ratan K. Sinha, Director, Reactor Design and Development Group (RD&DG), Bhabha Atomic Research Centre (BARC). The first pour of the concrete will take place in a few months. The reactor will be powered by thorium, described as "the fuel of the future". The 300-MWe AHWR will signal the beginning of the third stage of India's three-stage nuclear power programme. It will use the naturally available thorium and the fissile material, uranium-233, as fuel. Boiling water will be the coolant and heavy water, the moderator.

According to Sinha, although the reactor was initially designed to generate 235 MWe, its capacity has been stepped up "through certain additional innovations and experimentation which helped in optimising the margins". The reactor will also produce 6,00,000 litres of desalinated water a day, which will meet the process requirements of demineralised water of the plant and the drinking water requirements of the plant staff. "This has been a gain in terms of the additional capacity we could envisage through innovations," he said.

The AHWR project has entered a crucial phase with the regulatory body, the Atomic Energy Regulatory Board (AERB), completing the pre-licensing appraisal of the reactor's innovative design. There is a mood of expectation at Engineering Hall No.7 where the RD & DG is located. It was in this hall that several important elements of India's ambitious atomic energy programme took shape. Today, this cavernous hall houses huge engineering facilities, massive robots called refuelling machines, computerised and control rooms, among other things. Keeping a direct tab on the development of the AHWR is Anil Kakodkar.

Homi J. Bhabha envisaged a three-stage programme. The first stage is in commercial domain with 15 PHWRs that use natural uranium as fuel for generating electricity. The second stage, which envisages construction of FBRs, has begun with the construction of the 500-MWe Prototype Fast Breeder Reactor pressing ahead at Kalpakkam. The FBRs will use plutonium-uranium mixed oxide as fuel. Four more FBRs with a capacity of 500 MWe will be built before 2020. In the third stage, reactors using thorium as fuel will be built.

Sinha, who is also the Director, Design, Manufacturing and Automation Group at BARC, argued that although the three-stage programme was conceived five decades ago, it was still valid. Only about 10,000 MWe of nuclear power can be generated with the limited quantity of natural uranium available in the country. Even at the current rate of electricity generation in the world, the known and reasonably assured resources of natural uranium in the world will not last more than 60 years. With a boom in population and the resultant increase in the country's energy requirements, "the bottom line is that we have to reach a thorium-utilisation programme fast", Sinha said.

Besides, "we have plenty of thorium available and that too of good quality," he said. India's three-stage programme with its step-by-step approach acquires relevance because thorium cannot be used straightaway in a reactor to generate electricity.

"We cannot use thorium alone as a fuel in a reactor like we do with natural uranium," explained Sinha. Unlike natural uranium, thorium does not have any fissionable isotopes. Unlike thorium, uranium-233 does not occur in nature as a constituent of natural uranium. Thorium has to be used in some other system (reactor) to convert it into uranium-233. The AHWR will use thorium as feed and convert it into fissile uranium-233, which will then undergo fission in situ to generate electricity. The AHWR will also use a small amount of plutonium.

But small quantities of plutonium reprocessed from the PHWRs alone will not be sufficient to support a big electricity-generation programme that will use large quantities of thorium. It is here that the relevance of the FBRs comes in. The breeder reactors will breed not only enough plutonium but can convert thorium into uranium-233.

"So we need the Fast Breeder Reactors... Since we cannot wait for the FBRs to produce uranium-233, we first want to use the plutonium produced from the PHWRs for the demonstration of new technologies relevant for the third stage. That is why we are going in for the AHWR," explained Sinha.

Its design has several innovative features. The AHWR will employ what are called passive safety features. According to a DAE note, these features will "achieve the conflicting goals of safety and economy".

The reactor will have no pumps and there will be no moving parts. "It neither depends on help from instrumentation nor relies on control mechanism," the note says. Passive safety depends on natural phenomena such as gravity, natural convection and stored energy.

The note says: "The AHWR will be one of the first-ever power reactors employing natural circulation, also known as thermo-siphoning, for cooling of the reactor core under all conditions."

Elimination of major equipment such as coolant pumps, driving motors and power equipment will cut down the plant cost. In the case of an accident, water stored in a huge overhead tank inside the reactor building will ensure core cooling for three full days, without any human intervention. Besides, the safety of the reactor will not depend on the operator's actions alone. The reactor will have three shut-down systems, including one to take care of postulated `insider threat' scenario.

Although the AHWR will be built in the near-term, the technologies in the reactor will be relevant for an entire era when thorium will be the fuel for a generation of reactors. Since the innovations will have a bearing on the safety of the reactor.

BARC felt it necessary to have the new safety features reviewed by the AERB under the pre-licensing safety reviews. Moreover, these safety features were not addressed by the existing system of regulatory documents, codes and guides that were prepared for the current generation of reactors.

Sinha said: "The design has many first-of-its-kind features. It has no pumps. The fuel itself is of a new kind. So we felt that our regulatory authorities should have a good look at some of these unique features and help us to know whether more validations are needed and whether we should do more R&D. For the past year and a half, the AERB has been having a good look at the design... Various safety features have been discussed. Deviations from the existing [safety] practices of the current generation of reactors have been examined in great detail."

The safety review committee concluded that the reactor's safety features were adequate for the reactor to go up for a formal licensing process.

The DAE is going ahead with further design validations of this reactor. A large experimental facility has been built in Engineering Hall No.7 so that RD & DG personnel could simulate, on a full scale, conditions that will prevail under various normal operations and postulated events of different kinds. This will help in validating the computer codes, which were originally used in designing such large-scale experiments. Such codes have been validated for small-scale experiments.