Pressurised heavy water reactor 3056828 222839464 2008-07-01T10:56:18Z SmackBot 433328 Date the maintenance tags or general fixes {{mergeto|Heavy water reactor|date=July 2008}} A '''pressurised heavy water reactor''' (PHWR) is a [[nuclear power]] reactor that uses unenriched [[natural uranium]] as its fuel and [[heavy water]] as its [[Neutron moderator|moderator]] ([[deuterium oxide]] D<sub><small>2</small></sub>O). 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 first commercial PHWRs were a Canadian design built by [[AECL]], the [[CANDU]]s. Marketed world-wide, 29 are in use or under refurbishment. The [[Nuclear Power Corporation of India Limited]] (NPCIL) has built and operates 13 PHWR units. Initially these indigenously built reactors were reverse-engineered from the CANDU design, but later models have diverged significantly. [[Siemens]] has also offered a PHWR design in the past, completing one unit in [[Argentina]] and partially completing a larger version at the same site. == 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 [[Nuclear fission|fission]] to stimulate fission in other nuclei. With careful control over the geometry and reaction rates, this can lead to a self-sustaining [[nuclear chain reaction|chain reaction]], a state known as "[[critical mass|criticality]]". Natural uranium consists of a mixture of various [[isotope]]s, primarily [[uranium-238|<sup><small>238</small></sup>U]] and a much smaller amount (about 0.72% by weight) of [[uranium-235|<sup><small>235</small></sup>U]]. <sup><small>238</small></sup>U can only be fissioned by neutrons that are fairly energetic, about 1 [[MeV]] or above. No amount of <sup><small>238</small></sup>U can be made "critical", however, since it will tend to parasitically absorb more neutrons than it releases by the fission process. <sup><small>235</small></sup>U, on the other hand, can support a self-sustained chain reaction, but due to the low natural abundance of <sup><small>235</small></sup>U, 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 <sup><small>235</small></sup>U 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 neutron]]s" and "thermal reactors"). During this slowing-down process it is beneficial to physically separate the neutrons from the uranium, since <sup><small>238</small></sup>U 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 <sup><small>235</small></sup>U in the fuel, again precluding criticality in natural uranium. Instead, [[light water reactor]]s first enhance the amount of <sup><small>235</small></sup>U in the uranium, producing [[enriched uranium]], which generally contains between 3% and 5% <sup><small>235</small></sup>U by weight (the waste from this process is known as [[depleted uranium]], consisting primarily of <sup><small>238</small></sup>U). In this enriched form there ''is'' enough <sup><small>235</small></sup>U 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 <sup><small>235</small></sup>U can also be used to produce much more "pure" [[weapons-grade]] material (90% or more <sup><small>235</small></sup>U), 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 <sup><small>235</small></sup>U, in which case there ''is'' enough <sup><small>235</small></sup>U 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. ==Advantages and Disadvantages== 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 UO<sub><small>2</small></sub>), 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. However, the use of natural uranium fuel does have a drawback: increased volumes of spent fuel relative to other reactor types using enriched fuel. ==References== *[http://www.cised.org/EconomicsofNuclearPowerfromHeavyWaterReactors.pdf Economics of Nuclear Power from Heavy Water Reactors] *[http://www.dae.gov.in/publ/ar0203/chap1.pdf Nuclear Power Program - Stage1 - Pressurised Heavy Water Reactor] ==See also== *[[List of nuclear reactors]] *[[Heavy water reactor]] *[http://www.aecl.ca/ Official website of AECL] [[Category:Nuclear power reactor types]] [[fr:Réacteur à eau lourde pressurisée]] [[ru:Тяжеловодный ядерный реактор]]