 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Pressurized heavy-water reactor" – news ·newspapers ·books ·scholar ·JSTOR(May 2015) (Learn how and when to remove this message) |
Apressurized heavy-water reactor (PHWR) is anuclear reactor that usesheavy water (deuterium oxide D2O) as itscoolant andneutron moderator.[1] PHWRs frequently usenatural uranium as fuel, but sometimes also usevery low enriched uranium. The heavy watercoolant is kept under pressure to avoid boiling,[2] allowing it to reach higher temperature (mostly) without forming steam bubbles, exactly as for apressurized water reactor (PWR). Whileheavy water is very expensive to isolate from ordinary water (often referred to aslight water in contrast toheavy water), its low absorption of neutrons greatly increases theneutron economy of the reactor, avoiding the need forenriched fuel. The high cost of the heavy water is offset by the lowered cost of using natural uranium and/oralternative fuel cycles. As of 2025, 43 PHWRs were in operation, having a total capacity of 23.430 GW(e), representing roughly 11% by number and 6.5% by generating capacity of all current operating reactors.CANDU andIPHWR are the most common type of reactors in the PHWR family. Other designs include the German design PHWR KWU installed atAtucha Nuclear Power Plant inArgentina.
| This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(May 2015) (Learn how and when to remove this message) |
The key to maintaining anuclear chain reaction within anuclear reactor is to use, on average,exactly one of the neutrons released from eachnuclear fission event to stimulate another nuclear fission event (in another fissionable nucleus). With careful design of the reactor's geometry, and careful control of the substances present so as to influence thereactivity, a self-sustainingchain reaction or "criticality" can be achieved and maintained.
Natural uranium consists of a mixture of variousisotopes, primarily238U and a much smaller amount (about 0.72% by weight) of235U.[3]
238U can only be fissioned by neutrons that are relatively energetic, about 1MeV or above. No amount of238U can be made "critical" 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 of235U, natural uranium cannot achieve criticality by itself.
In aheavy water reactor, the trick to achievingcriticality using onlynatural or low enriched uranium, for which there is no "bare"critical mass, is to slow down the emitted neutrons (without absorbing them) to the point where enough of them may cause further nuclear fission in the small amount of235U which is available. (238U which is the bulk of natural uranium is also fissionable with fast neutrons.) This requires the use of aneutron moderator, which absorbs virtually all of the neutrons'kinetic energy, slowing them down to the point that they reach thermal equilibrium with surrounding material. It has been found beneficial to theneutron economy to physically separate the neutron energy moderation process from the uranium fuel itself, as238U has a high probability of absorbing neutrons with intermediate kinetic energy levels, a reaction known as "resonance" absorption. This is a fundamental reason for designing reactors with separate solid fuel segments, surrounded by the moderator, rather than any geometry that would give a homogeneous mix of fuel and moderator.
In alight water reactor, water makes an excellent moderator; theordinary hydrogen orprotium atoms in the water molecules are very close in mass to a single neutron, and so their collisions result in a very efficient transfer of momentum, similar conceptually to the collision of two billiard balls. However, as well as being a good moderator, ordinary water is also quite effective at absorbing neutrons. And so using ordinary water as a moderator will easily absorb so many neutrons that too few are left to sustain a chain reaction with the small isolated235U nuclei in the fuel, thus precluding criticality in natural uranium. Because of this, alight-water reactor will require that the235U isotope be concentrated in its uranium fuel, asenriched uranium, generally between 3% and 5%235U by weight (the by-product from this process enrichment process is known asdepleted uranium, and so consisting mainly of238U, chemically pure). The degree of enrichment needed to achievecriticality with alight-water moderator depends on the exact geometry and other design parameters of the reactor.
One complication of this light water approach is the need for uranium enrichment facilities, which are generally expensive to build and operate. They also present anuclear proliferation concern; thesame systems used to enrich the235U can also be used to produce much more "pure"weapons-grade material (90% or more235U), suitable for producing anuclear weapon. This is not a trivial exercise by any means, but feasible enough that enrichment facilities present a significant nuclear proliferation risk.
The solution to this problem, in aPHWR (pressurized heavy water reactor), is to use a moderator that doesnot absorb neutrons as readily as water. In this case potentially all of the neutrons being released can be moderated and used in reactions with the235U, in which case thereis enough235U in natural uranium to sustain criticality. One such moderator isheavy water, or deuterium-oxide. Although it reacts dynamically with the neutrons in a fashion similar to light water (albeit with less energy transfer on average, given that heavy hydrogen, ordeuterium, 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 as the moderator is the key to the PHWR (pressurized heavy water reactor) system, enabling the use of natural uranium as the fuel (in the form of ceramic UO2), which means that it can be operated without expensive uranium enrichment facilities. The mechanical arrangement of the PHWR, which places most of the moderator at lower temperatures, is particularly efficient because the resulting thermal neutrons have lower energies (neutron temperature after successive passes through a moderator roughly equals the temperature of the moderator) than in traditional designs, where the moderator normally is much hotter. Theneutron cross section for fission is higher in235
U the lower the neutron temperature is, and thus lower temperatures in the moderator make successful interaction between neutrons and fissile material more likely. These features mean that a PHWR can use natural uranium and other fuels, and does so more efficiently thanlight water reactors (LWRs). CANDU type PHWRs are claimed to be able to handle fuels includingreprocessed uranium or evenspent nuclear fuel from "conventional"light water reactors as well asMOX fuel and there is ongoing research into the ability of CANDU type reactors to operate exclusively on such fuels in a commercial setting. (More on that in the article on theCANDU reactor itself)
Pressurized heavy-water reactors do have some drawbacks. Heavy water generally costs hundreds of dollars per kilogram, though this is a trade-off against reduced fuel costs. The reduced energy content of natural uranium as compared to enriched uranium necessitates more frequent replacement of fuel;[citation needed] this is normally accomplished by use of an on-power refueling system. The increased rate of fuel movement through the reactor also results in higher volumes ofspent fuel than in LWRs employing enriched uranium. Since unenriched uranium fuel accumulates a lower density offission products than enriched uranium fuel, however, it generates less heat, allowing more compact storage.[4] While deuterium has alower neutron capture cross section thanprotium, this value isn'tzero and thus part of the heavy water moderator will inevitably be converted totritiated water. Whiletritium, a radioactive isotope of hydrogen, is also produced as a fission product in minute quantities in other reactors, tritium can more easily escape to the environment if it is also present in the cooling water, which is the case in those PHWRs which use heavy water both as moderator and as coolant. Some CANDU reactors separate out the tritium from their heavy water inventory at regular intervals and sell it at a profit, however.
While with typicalCANDU derived fuel bundles, the reactor design has a slightlypositiveVoid coefficient of reactivity, the Argentina designed CARA fuel bundles used inAtucha I, are capable of the preferred negative coefficient.[5]
While prior to India's development of nuclear weapons (see below), the ability to use natural uranium (and thus forego the need foruranium enrichment which is adual use technology) was seen ashindering 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 agreater risk ofnuclear proliferation versus comparablelight-water reactors due to the low neutron absorption properties of heavy water, discovered in 1937 byHans von Halban andOtto Frisch.[6] Occasionally, when an atom of238U is exposed toneutron radiation, its nucleus will capture aneutron, changing it to239U. The239U then rapidly undergoes twoβ− decays — both emitting anelectron and anantineutrino, the first one transmuting the239U into239Np, and the second one transmuting the239Np into239Pu. 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.[6] TheManhattan Project ultimately used graphite moderated reactors to produce plutonium, while theGerman 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). TheSoviet nuclear program likewise used graphite as a moderator and ultimately developed the graphite moderatedRBMK as a reactor capable of producing both large amounts of electric power andweapons grade plutonium without the need for heavy water or - at least according to initial design specifications -uranium enrichment.
239Pu is afissile material suitable for use innuclear weapons. As a result, if the fuel of a heavy-water reactor is changed frequently, significant amounts ofweapons-grade plutonium can be chemically extracted from the irradiated natural uranium fuel bynuclear reprocessing.
In addition, the use of heavy water as a moderator results in the production of small amounts oftritium when thedeuterium nuclei in the heavy water absorb neutrons, a very inefficient reaction. Tritium is essential for the production ofboosted fission weapons, which in turn enable the easier production ofthermonuclear weapons, includingneutron bombs. This process is currently expected to provide (at least partially) tritium forITER.[7]
The proliferation risk of heavy-water reactors was demonstrated when India produced theplutonium forOperation Smiling Buddha, its first nuclear weapon test, by extraction from the spent fuel of a heavy-water research reactor known as theCIRUS reactor.[8]
The high pressure prevents heavy water from boiling.
