Pressurized heavy-water reactor
A pressurized heavy-water reactor (PHWR) is a nuclear reactor that uses heavy water (deuterium oxide D2O) as its coolant and neutron moderator.[1] PHWRs frequently use natural uranium as fuel, but sometimes also use very low enriched uranium. The heavy water coolant is kept under pressure to avoid boiling, allowing it to reach higher temperature (mostly) without forming steam bubbles, exactly as for a pressurized water reactor. While heavy water is very expensive to isolate from ordinary water (often referred to as light water in contrast to heavy water), its low absorption of neutrons greatly increases the neutron economy of the reactor, avoiding the need for enriched fuel. The high cost of the heavy water is offset by the lowered cost of using natural uranium and/or alternative fuel cycles. As of the beginning of 2001, 31 PHWRs were in operation, having a total capacity of 16.5 GW(e), representing roughly 7.76% by number and 4.7% by generating capacity of all current operating reactors.
Nuclear proliferation[edit]
While prior to India's development of nuclear weapons (see below), the ability to use natural uranium (and thus forego the need for uranium enrichment which is a dual use technology) was seen as hindering nuclear proliferation, this opinion has changed drastically in light of the ability of several countries to build atomic bombs out of plutonium, which can easily be produced in heavy water reactors. Heavy-water reactors may thus pose a greater risk of nuclear proliferation versus comparable light-water reactors due to the low neutron absorption properties of heavy water, discovered in 1937 by Hans von Halban and Otto Frisch.[5] Occasionally, when an atom of 238U is exposed to neutron radiation, its nucleus will capture a neutron, changing it to 239U. The 239U then rapidly undergoes two β− decays — both emitting an electron and an antineutrino, the first one transmuting the 239U into 239Np, and the second one transmuting the 239Np into 239Pu. Although this process takes place with natural uranium using other moderators such as ultra-pure graphite or beryllium, heavy water is by far the best.[5] The Manhattan Project ultimately used graphite moderated reactors to produce plutonium, while the German wartime nuclear project wrongfully dismissed graphite as a suitable moderator due to overlooking impurities and thus made unsuccessful attempts using heavy water (which they correctly identified as an excellent moderator). The Soviet nuclear program likewise used graphite as a moderator and ultimately developed the graphite moderated RBMK as a reactor capable of producing both large amounts of electric power and weapons grade plutonium without the need for heavy water or - at least according to initial design specifications - uranium enrichment.
239Pu is a fissile material suitable for use in nuclear weapons. As a result, if the fuel of a heavy-water reactor is changed frequently, significant amounts of weapons-grade plutonium can be chemically extracted from the irradiated natural uranium fuel by nuclear reprocessing.
In addition, the use of heavy water as a moderator results in the production of small amounts of tritium when the deuterium nuclei in the heavy water absorb neutrons, a very inefficient reaction. Tritium is essential for the production of boosted fission weapons, which in turn enable the easier production of thermonuclear weapons, including neutron bombs. This process is currently expected to provide (at least partially) tritium for ITER.[6]
The proliferation risk of heavy-water reactors was demonstrated when India produced the plutonium for Operation Smiling Buddha, its first nuclear weapon test, by extraction from the spent fuel of a heavy-water research reactor known as the CIRUS reactor.[7]