Pressurised heavy water reactor PHWR
A pressurised heavy water reactor (PHWR) uses heavy water as coolant and heavy water as moderator. 44 are operating worldwide, totalling 24.8 GW gross, out of 96 units of this type on record.
Operating
44
24.8 GW gross
Under construction
3
Planned
16
Retired
19
Heavy water absorbs so few neutrons that the reactor can run on unenriched natural uranium, which removes the enrichment step from the fuel chain entirely. The core is a horizontal tank pierced by hundreds of pressure tubes, so fuel bundles can be pushed through and replaced while the reactor runs. The CANDU line is the archetype; India's PHWR fleet is the largest by unit count. The trade is a large, expensive heavy-water inventory and a positive void coefficient that the design must manage.
Advantages
- [object Object]
- Refuels on load, so no shutdown outage and very high capacity factors are achievable
- Pressure tubes instead of one large vessel, so heavy forging capacity is not a constraint
- [object Object]
Drawbacks
- Large heavy water inventory is expensive to produce and must be kept for the plant's life
- Positive void coefficient, which the design must actively manage
- Lower burnup means more fuel bundles and more spent fuel volume per unit of electricity
- On-load refuelling machinery is complex and a proliferation concern, since fuel can be removed at any time
Design basis
| Coolant | Heavy water |
|---|---|
| Moderator | Heavy water |
| Fuel form | Natural uranium dioxide bundles |
| Typical enrichment | Natural uranium, 0.71% U-235 |
Where they operate
| India | 19 |
|---|---|
| Canada | 17 |
| South Korea | 3 |
| Argentina | 3 |
| Romania | 2 |
Model lines
| Model | Operating | Units on record |
|---|---|---|
| Horizontal Pressure Tube type | 17 | 22 |
| PHWR-700 | 2 | 16 |
| CANDU 6 | 7 | 12 |
| CANDU 850 | 4 | 4 |
| CANDU 750B | 4 | 4 |
| CANDU 500B | 4 | 4 |
| CANDU 500A | — | 4 |
| EPR | — | 3 |
| CANDU | — | 3 |
| UK SMR | — | 2 |
| PHWR KWU | 2 | 2 |
| CAP1000 | — | 2 |
Operating PHWR units
Sources
- Global Nuclear Power Tracker, Global Energy Monitor, August 2026 release — CC BY 4.0 · retrieved 2026-09-06