Heavy water reactors (HWRs) at the beginning of 2001 represented about 8% by number and 4.7% by capacity of all operating power reactors. With many years of operating experience Canada has developed the 700 MWe CANDU-6, which has been built in several countries outside Canada. India has also built a series of 220 MWe HWRs. Work on evolutionary HWRs is ongoing in Canada, India and Russia and is briefly described below.

The new Canadian evolutionary Heavy Water Reactor11 is the 935 MWe CANDU-9. Canada is also working on a 400 — 650 MWe Next Generation CANDU. The NG CANDU design features major improvements in economics, inherent safety characteristics and performance. It optimises the design by utilizing SEU fuel to reduce the reactor core size, which minimizes the amount of heavy water required for moderation, and allows light water to be used as the reactor coolant. It is expected that the potential for offsite releases of radioactive material in NG CANDU will be sufficiently low that a target of “no evacuation” can be achieved. In June 2002, Atomic Energy of Canada renamed the NG as Advanced Candu Reactor (ACR) and announced that the ACR-700 will be “market-ready” by 2005.

In India, a continuing process of evolution of HWR design has been carried out. In 2002 construction began on two 500 MWe units at Tarapur which incorporate feedback from several indigenously designed and built 220 MWe units. The Advanced HWR (AHWR), under development in India, is a 235 MW heavy water moderated, boiling light water cooled, vertical pressure tube reactor with its design optimised for utilization of thorium for power generation. The conceptual design and the design feasibility studies for this reactor have been completed and the detailed design is in progress. The design incorporates a number of passive systems and the overall design philosophy includes achievement of simplification to the maximum extent.

A reactor design concept for an ‘Ultimate Safe’ reactor with 1000 MW output is being developed by the Russian Institute ITEP, in conjunction with other Russian organizations12. The prototype for this conceptual design is the KS150 reactor in Bohunice in the Slovak Republic. Low temperature heavy water is used as the moderator, and the design incorporates gaseous coolant, either CO2 or a mixture of CO2 and helium, and low fissile content fuel. The entire primary system, including main gas-circulators, steam generators and intermediate heat exchangers are contained within a multi-cavity, pre-stressed concrete pressure vessel. The design is said to be super safe, for example, accidental withdrawal of all control rods will add a relatively small amount of reactivity to the system compensated by the negative reactor power coefficient.