These reactors are more economically to those nations that don’t produce enriched uranium because enrichment of uranium is very costly.
In this type of reactors, the natural uranium (0.7% U235) is used as fuel and heavy water as moderator. This type of reactor was first designed and developed in Canada.
The first heavy water reactor in Canada using heavy water as coolant and moderator of 200 MW capacity with 29.1% thermal efficiency was established at Douglas.
The coolant heavy water is passed through the fuel pressure tubes and heat exchanger. The heavy water is circulated in the primary circuit in the same way as with a PWR and the steam is raised in the secondary circuit transferring the heat in the heat exchanger to the ordinary water.
The control of the reactor is achieved by varying the moderator level in the reactor and therefore control rods are not required.
For rapid shutdown purpose, the moderator can be dumped through a very large area into a tank provided below the reactor.
Advantages :
The major advantage of this reactor is that the fuel need not be enriched.
The reactor vessel may be built to withstand low pressure, therefore, the cost of the vessel is less.
No control rods are required; therefore, control is much easier than other types.
The moderator can be kept at low temperature which increases its effectiveness in slowing down neutrons..
Disadvantages :
The cost of heavy water is extremely high
The leakage is a major problem as there are two mechanically sealed closures per fuel channel. Canadian designs generally are based or recovering high proportion of heavy water leakages as absolute leak-tightness cannot be assured.
Very high standard of design, manufacture inspection and maintenance are required.
The power density is considerably low (9.7 kW/liter) compared with PWR and BWR therefore the reactor size is extremely large.
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