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Chapter03: Research Nuclear ReactorsEdited by Dr. Mir F. Ali

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Many research reactors were, built in the 1960s and 1970s. The peak number operating was around 1975, with 373 research reactors in 55 countries.

These reactors are, primarily designed to produce neutrons, activate radioactive or otherionizing radiation sources for scientific, medical, engineering or other research purposesincluding teaching and training. Many of them are located on university campuses. According to IAEA, there no new research nuclear reactors were, added to the list of morethan 240 operation research power reactors around the world in 2009. Many of thesereactors are, used for materials testing and the production of isotopes for medicine andindustry. As older reactors are retired and replaced by fewer, more multipurposereactors, the number of operational research reactors is expected to drop to between 100and 150 by 2020.

The figure 3-1 presented above illustrates that Russia has the highest number of researchreactors, followed by USA, Japan, France, Germany and China. Many developingcountries also have research reactors, including Algeria, Bangladesh, Colombia, Ghana,

Jamaica, Libya, Thailand and Vietnam. The trends reveal that even though many researchreactors are under-utilized and many older ones will be shut down and subsequently undergo decommissioning; the need for research reactors is not waning. Presently, 7 newresearch reactors are under construction and 9 more are planned. Some of these newreactors are innovative reactors designed to produce high neutron fluxes and will beeither multipurpose reactors or dedicated to specific needs.

http://www-naweb.iaea.org/napc/physics/ACTIVITIES/Research_Reactors_Worldwide.htm






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These reactors are relatively smaller to power reactors whose primary function is toproduce heat to generate electricity. Their power is designated in megawatts or kilowattsthermal (MWth or MWt), but a common practice is to use MW or KW for megawatts orkilowatts. Most of these reactors range up to 100 MW, compared with 3,000 MW (ie.1000 MWe) for a typical power reactor. These reactors operate at lower temperatures.

They need far less fuel, and far less fission products build up as the fuel is used. On theother hand, their fuel requires more highly enriched uranium, typically up to 20 percentU-235 (Uranium), although some older ones use 93 percent U-235. They also have a very high power density in the core, which requires special design features. Like powerreactors, the core needs cooling, and usually a moderator is required to slow down theneutrons and enhance fission. As neutron production is their main function, mostresearch reactors also need a reflector to reduce neutron loss from the core.

1. TYPES OF RESEARCH NUCLEAR REACTORS: Because of a wide range of research covered by these reactors, a much wider array of

designs in use for research reactors whereas, 80 percent of the world’s nuclear plants areof two similar types. They also have different operating modes, producing energy thatmay be steady or pulsed. The common designs for research nuclear reactors are, dividedinto thefollowing threecategories:

1.1 The PoolType ResearchNuclearReactors:

A commondesign is thepool typereactor, wherethe core is acluster of fuelelements sittingin a large pool of water. Betweenthe fuel

elements arecontrol rods andempty channelsfor experiments. In one particular design (Material Testing Reactor), a fuel elementcomprises several curved aluminum-clad fuel plates in a vertical box. The watermoderates and cools the reactor, and graphite or beryllium is generally, used for thereflector, although other materials may also be used. Apertures to access the neutronbeams are set in the wall of the pool.

http://www.world-nuclear.org/info/default.aspx?id=544&terms=research%20reactors






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The swimming pool reactor is very simple, and initially more than 40 such reactors were, built in the United States alone. The core is often made up of what are calledMaterials Testing Reactor (MTR) type fuel elements; aluminum clad, curved plates of fuelarranged in long rectangular boxes, which are arranged between grid plates to form thecore. Several positions in the grid are, not occupied by fuel elements, but by control rods,

beryllium reflectors, or experimental capsules. Cooling may be by natural convection of the pool water, although this is, augmented, for operation at higher power, by pumpingpool water through the core. This design led to the tank-in-pool reactor, similar to the

open-pool type but with the core contained in an aluminum tank. The cooling (light) water is, pumped through the core, but the pressure within the tank is only moderately elevated above that in the open pool, the pressurization being mostly due to the pressuredrop across the core of the pumped coolant water flow. Again, in the United States,aluminum clad fuel plates are usual.1.2 The Tank Type Research Nuclear Reactors: This type of research reactors, are similar, except that cooling is more active.

1.3 The TRIGA Type Research Nuclear Reactors: The core of this type of research nuclear reactors consists of 60-100 cylindrical fuel

elements about 36 mm diameter with aluminum cladding enclosing a mixture of uraniumfuel and a zirconium hydride moderator.

It sits in a pool of water and generally uses graphite or beryllium as a reflector. This kindof reactor can safely be pulsed to very high power levels (e.g., 25,000 MW) for fractions of a second. Its fuel gives the TRIGA a very strong negative temperature coefficient, and the

http://www.osti.gov/bridge/purl.cover.jsp?purl=/471422-hDVlCH/webviewable/





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rapid increase in power is, quickly cut short by a negative reactivity effect of the hydridemoderator.

Perhaps the most interesting reactor design of the common types, from a technical andsafety perspective, is the TRIGA, developed in the 1950s by General Atomic. Its uniquefuel and core design concept has a very large and very prompt negative temperaturecoefficient, the meat being a homogenized mixture of fuel and hydrogenous moderator,in the form of uranium-zirconium hydride. This provides prompt negative feedback,because there is no delay between fuel and moderator temperature variations. This is inaddition to the usual prompt Doppler Effect in U238 in reduced enrichment fuels.Beyond these effects, erbium can be, added as a burnable poison and adds even moreprompt negative temperature coefficient because it has a strong resonance: Absorption atabout 0.5 eV.

The fuel/moderator/poison has a design operating temperature of up to 750 C degree,and a safety limit of 1150 C degree, obviously much higher than aluminudfuel mixtures. Itis, formed into rods clad with stainless steel (Incoloy 800). With this combination of design features, very large reactivity insertions can be tolerated, and many TRIGA

research nuclear reactors are routinely and safely operated as pulsed reactors with peakpower levels, during a few millisecond pulses, of up YO 10 GW.

Cooling is by natural convection of light water for power levels up to 2 MW. At higherpower levels, forced flow is used, but the high fuel temperature tolerance and negativereactivity coefficients mean that pony motors are not needed for shutdown coolingfollowing a loss of the primary coolant Dumps.



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Other designs are, moderated by heavy water or graphite. A few are fast reactors thatrequire no moderator and can use a mixture of uranium and plutonium as fuel.Homogenous type reactors have a core comprising a solution of uranium salts as a liquid,contained in a tank about 300 mm diameter. The simple design made them popular early

on, but only five are now operating.

The IAEA has classified broadly research nuclear reactors into several categories. They include 60 critical assemblies (usually zero power), 23 test reactors, 37 training facilities, 2prototypes and even 1 producing electricity. However, most (160) are largely for research,although some may also produce radioisotopes. As expensive scientific facilities, they tend to be multi-purpose, and many have been operating for more than 30 years.Russia has the most research nuclear reactors (62), followed by USA (54), Japan (18),France (15), Germany (14) and China (13). Many small and developing countries also haveresearch nuclear reactors, including Bangladesh, Algeria, Colombia, Ghana, Jamaica,

Libya, Thailand and Vietnam. About 20 more reactors are planned or under construction,and 361 have been shut down or decommissioned, about half of these in USA. Many reactors were, built in the 1960s and 1970s. The peak number operating was in 1975, with373 in 55 countries.

2. THE USE OF RESEARCH UNCLEAR REACTORS:Research nuclear reactors have a wide range of uses, including analysis and testing of materials, and production of radioisotopes. Their capabilities are, applied in many fields within the nuclear industry as well as in fusion research, environmental science, advancedmaterials development, drug design and nuclear medicine.

Using neutron activation analysis, it is possible to measure minute quantities of anelement. Atoms in a sample are, made radioactive by exposure to neutrons in a reactor.The characteristic radiation each element emits can then be detected.

Neutron beams are uniquely suited to studying the structure and dynamics of materials atthe atomic level. Neutron scattering is, used to examine samples under differentconditions such as variations in vacuum pressure, high temperature, low temperature andmagnetic field, essentially under real-world conditions.

Neutron activation is, also used to produce the radioisotopes, widely used in industry and

medicine, by bombarding particular elements with neutrons. For example, yttrium-90microspheres to treat liver cancer are, produced by bombarding yttrium-89 withneutrons. The most widely used isotope in nuclear medicine is technetium-99, a decay product of molybdenum-99. It is, produced by irradiating uranium-235 foil with neutronsand then separating the molybdenum from the other fission products in a hot cell.

Research nuclear reactors can also be, used for industrial processing. Neutrontransmutation doping makes silicon crystals more electrically conductive for use in



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electronic components. In test reactors, materials are subject to intense neutronirradiation to study changes. For instance, some steels become brittle, and alloys, whichresist embitterment, must be used in nuclear reactors.

Like nuclear power reactors, research nuclear reactors are, covered by IAEA safety

inspections and safeguards, because of their potential for making nuclear weapons.India's 1974 explosion was the result of plutonium production in a large, butinternationally unsupervised, research nuclear reactor.

The next chapter is dedicated to the Conventional Nuclear Power Reactors.

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability ” onDecember 13, 2010: http://intuitech.biz/?p=7828

Resources:1. IAEA – Research Reactors Worldwide: http://www-

naweb.iaea.org/napc/physics/ACTIVITIES/Research_Reactors_Worldwide.htm 2. World Nuclear Association – Research Reactors: http://www.world-

nuclear.org/info/default.aspx?id=544&terms=research%20reactors 3. Research Reactors – An Overview:

http://www.osti.gov/bridge/servlets/purl/471422-hDVlCH/webviewable/471422.pdf

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