CANDU The original CANDU design used
heavy water as both the
neutron moderator and the coolant for the primary cooling loop. It was believed that this design would result in lower overall operating costs due to its ability to use
natural uranium for fuel, eliminating the need for enrichment. At the time, it was believed there would be hundreds and perhaps thousands of nuclear reactors in operation by the 1980s, and in that case the cost of enrichment would become considerable. Further, the design used both pressurized and unpressurized sections, the latter known as a "calandria", which it was believed would lower construction costs compared to designs that used highly pressurized cores. In contrast to typical light water designs, CANDU did not require a single large pressure vessel, which was among the more complex parts of other designs. This design also allowed it to be refuelled while it was running, improving the
capacity factor, a key metric in overall performance. However, the use of natural uranium also meant the core was much less dense compared to other designs, and much larger overall. It was expected this additional cost would be offset by lower capital costs on other items, as well as lower operational costs. The key trade-off was the cost of the fuel, in an era when enriched uranium fuel was limited and expensive and its price was expected to rise considerably by the 1980s. In practice, these advantages did not work out. The high expected fuel costs never came to be; when reactor construction stalled at around 200 units worldwide, instead of the expected thousands, fuel costs remained steady as there was ample enrichment capability for the amount of fuel being used. This left CANDU in the unexpected position of selling itself primarily on the lack of need for enrichment and the possibility that this presented a lower
nuclear proliferation risk.
ACR ACR addresses the high capital costs of the CANDU design primarily by using
low-enrichment uranium (LEU) fuel. This allows the reactor core to be built much more compactly, roughly half that of a CANDU of the same power. Additionally, it replaces the heavy-water coolant in the high-pressure section of the calandria with light water. This greatly reduces the amount of heavy water needed, and the cost of the primary coolant loop. Heavy water remains in the low-pressure section of the calandria, where it is essentially static and used only as a moderator. The reactivity regulating and
safety devices are located within the low-pressure moderator. The ACR also incorporates characteristics of the CANDU design, including on-power refueling with the
CANFLEX fuel; a long
prompt neutron lifetime; small reactivity holdup; two fast, independent, safety shutdown systems; and an emergency core cooling system. The fuel bundle is a variant of the 43-element CANFLEX design (CANFLEX-ACR). The use of LEU fuel with a neutron absorbing centre element allows the reduction of
coolant void reactivity coefficient to a nominally small, negative value. It also results in higher burnup operation than traditional CANDU designs. ==Safety systems==