Heavy Water Reactors

The use of heavy water as a moderator is highly efficient for maintaining a neutron economy, enabling the use of natural uranium. This eliminates the need for domestic enrichment capabilities, which can be an advantage for certain national energy programs. While natural uranium (~0.7% U-235) is the standard, modern heavy water reactors frequently use slightly enriched uranium (~0.9% to 1.2% U-235) to improve efficiency. HWRs typically feature a horizontally oriented design, which allows for on-line refuelling – the adding of new fuel and removing spent fuel without shutting down the reactor. This unique feature also allows them to simultaneously produce radioactive isotopes, for example Cobalt-60 and Molybdenum-99. Supply chain considerations include the production of high-purity heavy water and specialised zirconium alloy pressure tubes. Logistical challenges involve managing the tritium produced in the heavy water and the volume of spent fuel (the use a natural uranium means that the fuel is used up and must be replaced more often, thereby producing ~ 4 to 5 times the volume of spent fuel per unit of energy compared to LWRs).

The nuclear safety risks of HWRs are related to direct exposure to radiation and contamination through the release of radioactive materials into the environment. This can be due to malfunctions or accidents, or the mismanagement of radioactive waste or spent fuel. Robust safety systems and regulations are essential to minimise these risks. The modular pressure tube design and large heat sink provided by the cool moderator offer distinct safety characteristics. However, management of tritium and the potential for a positive void coefficient in certain designs require specific safety systems.

Stringent security measures are essential to ensure proper control and prevent, detect or respond to theft, sabotage, unauthorised access, illegal transfer or other malicious acts involving the nuclear fuel, other radioactive material or the nuclear facility itself.

The applicable IAEA safeguards agreement is implemented, involving, for example, inspections, nuclear material accounting, and containment and surveillance measures, to verify the peaceful use of nuclear materials. The online refuelling capabilities require diligent monitoring to ensure all fuel movements are accounted for and authorised.