Heavy water reactors (HWRs) account for about 10.5% of all currently operating power reactors. However, in 2010, out of 60 new nuclear power units under construction, only 2 were with HWRs[22] [4.4].

There are only two vendors for this type of reactor, the AECL in Canada and the NPCIL in India. There are several HWR designs within the SMR range that are already available for deployment (see Chapter 3).

Conventional HWRs use an indirect energy conversion cycle. The primary coolant is heavy water and the primary moderator (separated from the coolant) is also heavy water. The secondary coolant is light water, and the Rankine cycle is used for energy conversion.

A conventional HWR has no pressure vessel and appears as a horizontally laid cylinder (the calandria) with low-pressure heavy water moderator penetrated by the horizontal pressure tubes — fuel channels containing fuel element bundles. Pressurised heavy water flows in each of the channels removing heat produced by the reactor. Heavy water coolant is distributed among the channels, and then collected, by a system of pipelines starting from the inlet headers and up to the outlet headers. The pressuriser is connected to the outlet header, while the pumps are connected to the inlet header. From the outlet headers the coolant is directed to steam generators where it passes the heat to the light water coolant of the secondary circuit. The fuel is UO2 with natural uranium. The reactivity control (in operation) is performed using several mechanisms, including absorber elements of different design and neutron poison addition to the moderator.

There is only one advanced SMR design in the HWR category — the Indian AHWR[23]. The basic characteristics of this AHWR are provided in Table 4.3.

This AHWR is different from the currently operated CANDU and PHWR reactors:

• it has boiling light water primary coolant and direct steam condensing cycle for energy conversion;

• it uses natural circulation of the coolant in all operating modes and, to boost it, it uses a vertical calandria and vertical pressure tube channels;

• it uses only mechanical control rods for reactivity control in operation;

• it uses fuel bundles of heterogeneous structure with Pu-Th or U-Th fuel.

Safety implications of the above mentioned design features are discussed in Section 8.4.

The use of mixed oxide thorium containing fuel is intended to involve thorium in power generation through 233U production and burning in-situ, without involving a complex chain with fast reactors and thorium fuel reprocessing. More details about the AHWR fuel design could be found in

[4.1].

The AHWR employs only passive systems for heat removal, which results in the large size of the containment (about 55×75 m), for a reactor of 30011 MWe.

The AHWR makes purposeful use of a part of the reject heat to run a seawater desalination plant. It also targets a 100-year lifetime for the plant, assuming all replaceable plant components are replaced periodically within this very long lifetime.

The indicated plant surface area is very small — 9 000 m2.