Pressurised heavy water reactor

From Wikipedia, the free encyclopedia

A pressurised heavy water reactor (PHWR) is a nuclear power reactor that uses unenriched natural uranium as its fuel and heavy water as a moderator (deuterium oxide D₂O). The heavy water is kept under pressure in order to raise its boiling point, allowing it to be heated to higher temperatures and thereby carry more heat out of the reactor core. While heavy water is expensive, the reactor can operate without expensive fuel enrichment facilities thus balancing the costs.

The original commercial PHWRs are the CANDUs, a Canadian design built by AECL. Marketed world-wide, some 29 are in use, or under refurbishment. The Nuclear Power Corporation of India Limited (NPCIL) has built and operates 11 PHWR units. Initially these indigenously built reactors were reverse-engineered from the CANDU design, but later models have diverged significantly.

The current installed examples of pressurised heavy water reactors are to be superseded by more advanced designs in the future such as The Advanced Heavy Water Reactor (AHWR) being researched at BARC in India and by the Advanced CANDU Reactor under development by AECL in Canada.

In the CANDU-based design, heavy water is contained in a large tank called a calandria. Several hundred horizontal or vertical pressure tubes form channels for the fuel penetrate the calandria, which contain the nuclear fuel and are a part of the primary heat transport loop. The heat transport fluid flowing through the pressure tubes (usually heavy water, but also light water or oil in the past) and the heavy water in the calandria are separate and do not mix. As in the pressurised light water reactor, the primary coolant generates steam in a secondary circuit to drive the turbines. The pressure tubes containing the fuel rods can be individually opened, and the fuel rods changed without taking the reactor out of service. This reactor has the least down-time of any known type.

[edit] Purpose of using heavy water

See nuclear reactor physics and nuclear fission and heavy water for complete details.

The key to maintaining a nuclear reaction within a nuclear reactor is to use the neutrons being released during fission to stimulate fission in other nuclei. With careful control over the geometry and reaction rates, this can lead to a self-sustaining chain reaction, a state known as "criticality".

Natural uranium consists of a mixture of various isotopes, primarily ²³⁸U and a much smaller amount (about 0.72% by weight) of ²³⁵U. ²³⁸U can only be fissioned by neutrons that are fairly energetic, about 1 MeV or above. No amount of ²³⁸U can be made "critical", however, since it will tend to parasitically absorb more neutrons than it releases by the fission process. ²³⁵U, on the other hand, can support a self-sustained chain reaction, but due to the low natural abundance of ²³⁵U, natural uranium cannot achieve criticality by itself.

The "trick" to making a working reactor is to slow some of the neutrons to the point where their probability of causing nuclear fission in ²³⁵U increases to a level that permits a sustained chain reaction in the uranium as a whole. This requires the use of a neutron moderator, which absorbs some of the neutrons' kinetic energy, slowing them down to an energy comparable to the thermal energy of the moderator nuclei themselves (leading to the terminology of "thermal neutrons" and "thermal reactors"). During this slowing-down process it is beneficial to physically separate the neutrons from the uranium, since ²³⁸U nuclei have an enormous parasitic affinity for neutrons in this intermediate energy range (a reaction known as "resonance" absorption). This is a fundamental reason for designing reactors with discrete solid fuel separated by moderator, rather than employing a more homogeneous mixture of the two materials.

Water makes an excellent moderator; the hydrogen atoms in the water molecules are very close in mass to a single neutron, and thus have a potential for high energy transfer, similar conceptually to the collision of two billiard balls. However, in addition to being a good moderator, water is also fairly effective at absorbing neutrons. Using water as a moderator will absorb enough neutrons that there will be too few left over to react with the small amount of ²³⁵U in the fuel, again precluding criticality in natural uranium. Instead, light water reactors first enhance the amount of ²³⁵U in the uranium, producing enriched uranium, which generally contains between 3% and 5% ²³⁵U by weight (the waste from this process is known as depleted uranium, consisting primarily of ²³⁸U). In this enriched form there is enough ²³⁵U to react with the water-moderated neutrons to maintain criticality.

One complication of this approach is the requirement to build an uranium enrichment facility, which are generally expensive to build and operate. They also present a nuclear proliferation concern; the same systems used to enrich the ²³⁵U can also be used to produce much more "pure" weapons-grade material (90% or more ²³⁵U), suitable for producing a nuclear bomb. This is not a trivial exercise, by any means, but simple enough that enrichment facilities present a significant nuclear proliferation risk.

An alternative solution to the problem is to use a moderator that does not absorb neutrons as readily as water. In this case potentially all of the neutrons being released can be moderated and used in reactions with the ²³⁵U, in which case there is enough ²³⁵U in natural uranium to sustain criticality. One such moderator is heavy water, or deuterium-oxide. Although it reacts dynamically with the neutrons in a similar fashion to light water (albeit with less energy transfer on average, given that heavy hydrogen, or deuterium, is about twice the mass of hydrogen), it already has the extra neutron that light water would normally tend to absorb.

The use of heavy water moderator is the key to the PHWR system, enabling the use of natural uranium as fuel (in the form of ceramic UO₂), which means that it can be operated without expensive uranium enrichment facilities. Additionally, the mechanical arrangement of the PHWR, which places most of the moderator at lower temperatures, is particularly efficient because the resulting thermal neutrons are "more thermal" than in traditional designs, where the moderator normally runs hot. This means that the CANDU is not only able to "burn" natural uranium and other fuels, but tends to do so more effectively as well.