Next generation aims to combine best of CANDU and PWR

5 February 2002

A new generation of nuclear technology is under development that combines heavy water moderation with light water cooling. The resulting power plant promises a number of attractive features.

Volatile energy markets, a growing consensus on the urgency of the need to combat climate change, and the emergence of energy security issues - all point to nuclear power generation as being an indispensable component of the world's energy future", according to Stephen Yu, Director, NG (next generation) CANDU Program, AECL (Atomic Energy of Canada Ltd).*

Hoping to benefit from this perceived potential future expansion of the nuclear energy market, AECL is now developing the NG CANDU nuclear power plant. The new plant "is designed to expand old markets and open new ones", according to Jerry Hopwood, Director, Advanced Reactor Development, AECL.

Essentially, the aim of the NG CANDU concept is to merge the best features of existing CANDU technology with the best elements of the pressurised water reactor (PWR) - the most commonly used reactor type around the world.

In particular the new design will combine the inherently-safe heavy water moderator of the CANDU with the compact pressurised light water cooling of the PWR.

The AECL approach is to use proven reactor design features so that the NG CANDU can be regarded as an evolutionary, rather than a revolutionary, product.

The aim is to achieve an overnight capital cost target of USD $1000/kWe, and be ready for market by the end of 2005. Levelised unit energy costs are projected to be around 30 US$/MWh, while availability is put at 90 per cent.

The reference NG CANDU design is envisaged as a medium-sized reactor with a capacity of 650 MWe, realising a capital cost reduction of 40 per cent relative to the current generation of plant.

Something of a boost to the CANDU cause came on 2 November 2001 when UK utility British Energy and AECL signed an agreement to begin a feasibility assessment of CANDU technology for possible introduction into the UK (which hitherto has used graphite-moderated gas cooled reactors, plus one PWR, at Sizewell B). An apparent convert to CANDU is Dr Robin Jeffrey, British Energy's executive chairman. "Since we took over Bruce we have gained a wealth of knowledge about these reactors. We are delighted with their performance..."

AECL has embarked on other commercial partnerships and co-ordinated licensing initiatives, in order to facilitate NG CANDU's market-readiness. On 9 November 2001, AECL signed a Memorandum of Agreement with Hitachi Limited of Japan, to work on development of NG CANDU. Hitachi is concentrating on the project's balance of plant, including the turbine generator, while AECL focuses on the nuclear steam plant. The companies are currently working together on the Qinshan CANDU project at the Qinshan Phase III site in China.

Pre-licensing is another part of NG CANDU's international market strategy and is being discussed with the Canadian Nuclear Safety Commission. Against the background of British Energy's interest in the concept, a co-ordinated licensing review with the UK regulator is also under consideration. An introductory licensing review with the United States regulator could lead to US pre-licensing, while the plant is also expected to be licensable in China.

Traditional strengths

AECL says its evolutionary approach accommodates significant design changes, while retaining the traditional strengths of CANDU reactors.

There are currently 23 CANDUs operating (plus eight units currently laid up, at Bruce A and Pickering A).

One such "traditional strength" is the CANDU's modular horizontal fuel channel core, which is economic to build.

Also retained is another key CANDU feature: automated on-power refuelling, with consequent high availability. The traditional CANDU fuel bundle design is also kept. The design simplicity of the fuel bundles makes their manufacture easy to localise in the project's host country.

As already noted, the NG CANDU design retains the low-temperature, low-pressure heavy water moderator that both provides passive safety and facilitates high fuel efficiency. CANDU reactors have always had exceptional "neutron economy" and the ability to use fuel with low fissile content remains a central feature of the advanced NG CANDU. But use of slightly enriched uranium (to 2 per cent) is envisaged rather than the natural uranium fuel (0.7 per cent fissile content) used in current CANDUs.

The use of slightly enriched uranium (SEU) extends fuel life in NG CANDU to three times that of natural uranium. Using SEU also boosts fuel burn-up, reducing the volume of spent fuel that will become waste by two thirds.

AECL's advanced 43-element CANFLEX fuel bundle - an evolution of the 37-element bundle - will give NG CANDU exceptional fuel cycle flexibility, says AECL. The CANFLEX bundle (which can also be backfitted to existing CANDUs) allows a wide selection of fuelling options, which, in addition to SEU, will include RU (recovered uranium), DUPIC (Direct Use of Spent PWR /BWR Fuel in CANDU), MOX (mixed oxide), and thorium.

Significant advances

Since light water replaces heavy water as the NG CANDU reactor coolant, the total heavy water requirement is decreased by 75 per cent, with a corresponding reduction in its capital expense. This major modification cuts the complexity of the heat transport system, further reducing capital and maintenance costs, as well as speeding up the commissioning process.

The NG CANDU, with an installed capacity of 650 MWe, compared with the 700 MWe class CANDU 6, has 60 per cent less core volume, with the diameter going from 760 cm to 500 cm. The reactor's compact footprint and scaled-down reactor building, reduce the size of the containment by one third. The number of fuel channels has been scaled-down from the 380 of the CANDU 6, to only 256.

Overall, less equipment and fewer systems are needed in the NG CANDU configuration. Two steam generators will replace the four that CANDU 6 plants currently use.

Also, the NG CANDU will operate at higher pressures than its predecessors, and has improved thermal efficiency, while the turbines in the NG CANDU have been downsized. The design simplicity of these alterations streamlines maintenance procedures - reducing operating costs.

The design also aims to achieve short construction times. A 48-month overall schedule is set for repeat units of NG CANDU - including 36 months for construction.

NG CANDU projects will employ open-top reactor construction. The dome of the reactor building is installed near the end of construction, allowing access to the interior of the reactor building from outside the perimeter walls. This significantly streamlines and shortens the construction schedule. Work sequences are more efficient and flexible because activities that previously had to be conducted sequentially can now be carried out independently of each other. By using a very-heavy-lift (VHL) crane, major component modules can be lifted through the open top of the reactor building, rather than being installed through the time-consuming horizontal method. Open-top construction facilitates the use of ready-built modules, which saves time and helps improve quality.

The open-top method has already been successfully implemented by AECL and the Third Qinshan Nuclear Power Company Limited (TQNPC) on the Qinshan Phase III project in China, which consists of two CANDU 6 units due to be completed in 2003. In addition, the Qinshan project demonstrated that 3D-CADD applications significantly reduce the amount of expensive custom field engineering needed to adapt the standard design.

Other advanced computer systems and controls that AECL is planning to use in NG CANDU include on-line real-time monitoring and control systems, such as the SMART CANDU ChemAND package for water chemistry control (which can also be backfitted to existing CANDUs, eg as is planned at the Gentilly 2 CANDU 6 reactor).

The NG CANDU will adopt a similar safety philosophy to existing CANDUs, with a dual safety system comprising two functionally and physically independent safety shutdown methods, but will have a simplified emergency core cooling system.

Also, with light water as the reactor coolant and heavy water only being used in the low-pressure moderator, potential tritium levels in containment will be lower.

Future markets

AECL says that on the basis of its current estimates for levelised costs, "the NG CANDU is competitive with even the best advanced gas-fired technology that will be available in the 2010 timeframe."

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