This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).
Research reactors comprise a wide range of civil and commercial nuclear reactors that are generally not used for power generation. The primary purpose of research reactors is to provide a neutron source for research and other purposes. Their output (neutron beams) can have different characteristics depending on use. They are small relative to power reactors whose primary function is to produce heat to make electricity. Their power is designated in megawatts (or kilowatts) thermal (MWth or MWt), but here we will use simply MW (or kW). Most range up to 100 MW, compared with 3000 MW (i.e., 1000 MWe) for a typical power reactor. In fact the total power of the world's 283 research reactors is little over 3000 MW.
Research reactors are simpler than power reactors and operate at lower temperatures. They need far less fuel, and far less fission products build up as the fuel is used. On the other hand, their fuel requires more highly enriched uranium, typically up to 20% uranium-235 (235U), although some older ones use 93% 235U. They also have a very high power density in the core, which requires special design features. Like power reactors, the core needs cooling, and usually a moderator is required to slow down the neutrons and enhance fission. As neutron production is their main function, most research reactors also need a reflector to reduce neutron loss from the core.
Types of research reactors
There is a much wider array of designs in use for research reactors than for nuclear power reactors, where 80% of the world's plants are of just two similar types. They also have different operating modes, producing energy that may be steady or pulsed.
A common design (67 units) is the pool type reactor, where the core is a cluster of fuel elements sitting in a large pool of water. Among the fuel elements are control rods and empty channels for experimental materials. Each element comprises several (e.g., 18) curved aluminium-clad fuel plates in a vertical box. The water both moderates and cools the reactor, and graphite or beryllium is generally used for the reflector, although other materials may also be used. Apertures to access the neutron beams are set in the wall of the pool. Tank type research reactors (32 units) are similar, except that cooling is more active.
The TRIGA reactor is another common design (40 units). The core consists of 60-100 cylindrical fuel elements about 36 mm diameter with aluminium cladding enclosing a mixture of uranium fuel and a zirconium hydride moderator. It sits in a pool of water and generally uses graphite or beryllium as a reflector. This kind of 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 rapid increase in power is quickly cut short by a negative reactivity effect of the hydride moderator.
Other designs are moderated by heavy water (12 units) or graphite. A few are fast reactors that require 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.
Research 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, advanced materials development, drug design and nuclear medicine.
The International Atomic Energy Agency (IAEA) lists several categories of broadly classified research reactors. They include 60 critical assemblies (usually zero power), 23 test reactors, 37 training facilities, two prototypes and even one producing electricity. But 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 reactors (62), followed by USA (54), Japan (18), France (15), Germany (14) and China (13). Many small and developing countries also have research 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 research reactors were built in the 1960s and 1970s. The peak number operating was in 1975, with 373 in 55 countries.
Neutron beams are uniquely suited to studying the structure and dynamics of materials at the atomic level. Neutron scattering is used to examine samples under different conditions such as variations in vacuum pressure, high temperature, low temperature and magnetic field, essentially under real-world conditions.
Using neutron activation analysis, it is possible to measure minute quantities of an element. 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 activation is also used to produce the radioisotopes, widely used in industry and medicine, by bombarding particular elements with neutrons. For example, yttrium-90 microspheres to treat liver cancer are produced by bombarding yttrium-89 with neutrons. 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 neutrons and then separating the molybdenum from the other fission products in a hot cell.
Research reactors can also be used for industrial processing. Neutron transmutation doping makes silicon crystals more electrically conductive for use in electronic components. In test reactors, materials are subject to intense neutron irradiation to study changes. For instance, some steels become brittle, and alloys which resist embrittlement must be used in nuclear reactors.
Like nuclear power reactors, research 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, but internationally unsupervised, research reactor.
Fuel assemblies are typically plates or cylinders of uranium-aluminium alloy (U-Al) clad with pure aluminium. They are different from the ceramic UO2 pellets enclosed in zircaloy cladding used in power reactors. Only a few kilograms of uranium are needed to fuel a research reactor, albeit more highly enriched (compared with perhaps a hundred tonnes in a power reactor).
Highly-enriched uranium (HEU - >20% uranium-235 (235U)) allowed more compact cores, with high neutron fluxes and longer times between refuelling. Therefore, many reactors up to the 1970s used it, and in 2004 more than 60 civilian research reactors still did so.
Since the early 1970s, security concerns have grown, especially since many research reactors are located at universities and other civilian locations with much lower security than military weapons establishments where much larger quantities of HEU exist. Since 1978, only one reactor, the FRM-II at Garching in Germany, has been built with HEU fuel, while 21 have been commissioned on LEU fuel in 16 countries.
The question of enrichment was a major focus of the UN-sponsored International Nuclear Fuel Cycle Evaluation in 1980. It concluded that to guard against weapons proliferation from the HEU fuels then commonly used in research reactors, enrichment should be reduced to no more than 20% 235U. This followed a similar initiative by the USA in 1978 when its program for Reduced Enrichment for Research and Test Reactors (RERTR) was launched.
Most research reactors using HEU fuel were supplied by the USA and Russia, hence efforts to deal with the problem are largely their initiative. The RERTR program concentrates on reactors over 1 MW, which have significant fuel requirements.
These programs have led to the development and qualification of new, high-density, low enriched uranium (LEU) fuels. The original fuel density was about 1.3-1.7 g/cm3 uranium. Lowering the enrichment meant that the density had to be increased. Initially this was to 2.3-3.2 g/cm3 with existing uranium-aluminum fuel types.
To late 2004, 38 research reactors (11 in USA) either have been or are being converted to low-enriched uranium silicide fuel, and another 36 are convertible using present fuels. 31 more, mostly Russian designs, need higher-density fuels not yet available. The goal is to convert 105 reactors by 2013. Some other HEU reactors are expected to close down by then. US exports of HEU declined from 700 kg/yr in the mid-1970s to almost zero by 1993.
The Soviet Union made similar efforts from 1978, and produced fuel of 2.5 g/cm3 with enrichment reduced from 90 to 36%. It largely stopped exports of 90% enriched fuel in the 1980s. However, no Russian research reactor has yet been converted to LEU.
The first generation of new LEU fuels used uranium (U) and silicon (Si) (U3Si2-Al—uranium silicide dispersed in aluminium), at 4.8 g/cm3. There have been successful tests with denser U3Si-Al fuel plates up to 6.1 g/cm3, but US development of these silicide fuels ceased in 1989 and did not recommence until 1996.
An international effort is underway to develop, qualify and license a high density fuel based on uranium-molybdenum (U-Mo) alloy dispersed in aluminium, with a density of 6-8g/cm3. The principal organizations involved are the US RERTR program at Argonne National laboratory (ANL) since 1996, the French U-Mo Group (CEA, CERCA, COGEMA, Framatome-ANP and Technicatome) since 1999 and the Argentine Atomic Energy Commission (CNEA) since 2000. This development work has been undertaken to provide fuels that can extend the use of LEU to those reactors requiring higher densities than available in silicide dispersions and to provide a fuel that can be more easily reprocessed than the silicide type. Approval of this fuel was expected in 2006 but tests since 2003 have failed to confirm performance due to unstable swelling under high irradiation, and the target is now 2010.
In Russia, a parallel Russian RERTR program funded by the Russian Ministry of Atomic Energy (MINATOM) and the US RERTR program has been working since 1999 to develop U-Mo dispersion fuel with a density of 2-6 g/cm3 for use in Russian-designed research and test reactors. However, this too has not fulfilled expectations.
In a further stage of U-Mo fuel development that has now become the main priority, ANL, CEA and CNEA are testing U-Mo fuel in a monolithic form, instead of a dispersion of U-Mo in aluminium. The uranium density is 15.6 g/cm3 and this would enable every research reactor in the world to convert from HEU to LEU fuel without loss of performance. The target date for availability is 2010.
All fuel is aluminium-clad.
U-Al fuels can be reprocessed by Cogema in France, and U-Mo fuels may also be reprocessed there. U-Si and TRIGA fuels are not readily reprocessed in conventional facilities. However, at least one commercial operator has confirmed that U-Si fuels may be reprocessed in existing plants if diluted with appropriate quantities of other fuels, such as U-Al.
To answer concerns about interim storage of spent research fuel around the world, the USA launched a program to take back US-origin spent fuel for disposal and nearly half a tonne of uranium-235 from such HEU fuel has been returned. By the time the program was to end with fuel discharged in 2006, U-Mo fuel was expected to be available. Due to the slippage in target date, the US take-back program has now been extended by ten years.
Disposal of high-enriched or even 20% enriched fuel needs to address problems of criticality and requires the use of neutron absorbers or diluting or spreading it out in some way.
In Russia, a parallel trilateral program involving IAEA and the USA is intended to move 2 tonnes of HEU and 2.5 tonnes of LEU spent fuel to the Mayak reprocessing complex near Chelyabinsk over the ten years to 2012. This Russian Research Reactor Fuel Return Program (RRR FRP) envisages 38 shipments (of both fresh and spent fuel) from ten countries over 2005-08, then 8+ shipments from six countries to remove all HEU fuel discharged before reactors are converted to LEU or shut down. Seventeen countries have Soviet-supplied research reactors, and there are 25 such reactors outside Russia, 15 of them still operational. Since Libya joined the program in 2004, only North Korea objects to it.
- WNA paper on Research reactors
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