Many research reactors were built in the 1960s and 1970s. 1975 saw the peak number of operating research reactors with 373 in 55 countries.
These reactors are primarily designed to produce neutrons, activate radioactive or other ionizing radiation sources for scientific, medical, engineering or other research purposes including teaching and training. Many of them are located on university campuses.
According to IAEA, no new research nuclear reactors were added to the list of more than 240 operation research power reactors around the world in 2009. Many of these reactors are used for materials testing and the production of isotopes for medicine and industry. As older reactors are retired and replaced by fewer more multipurpose reactors, the number of operational research reactors is expected to drop to between 100 and 150 by 2020.
The figure 3-1 presented above illustrates that Russia has the highest number of research reactors, followed by USA, Japan, France, Germany and China. Many developing countries also have research reactors, including Algeria, Bangladesh, Colombia, Ghana, Jamaica, Libya, Thailand and Vietnam. The trends reveal that even though many research reactors are under-utilized and many older ones will be shut down and subsequently undergo decommissioning, the need for research reactors is not waning. Presently, seven new research reactors are under construction and nine more are planned. Some of these new reactors are innovative reactors designed to produce high neutron fluxes and will be either multipurpose reactors or dedicated to specific needs.
These reactors are relatively smaller than power reactors whose primary function is to produce heat to generate electricity. Their power is designated in megawatts or kilowatts thermal (MWth or MWt), but a common practice is to use MW or KW for megawatts or kilowatts. 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 the other hand, their fuel requires more highly enriched uranium, typically up to 20 percent U-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 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.
1. TYPES OF RESEARCH NUCLEAR REACTORS:
Because of a wide range of research covered by these reactors, a much wider array of designs are used for research reactors whereas 80 percent of the world’s nuclear plants are of two similar types. They also have different operating modes, producing energy that may be steady or pulsed. The common designs for research nuclear reactors are divided into the following three categories:
1.1 The Pool Type Research Nuclear Reactors:
A common design is the pool type reactor where the core is a cluster of fuel elements sitting in a large pool of water. Between the fuel elements are control rods and empty channels for experiments. In one particular design (Material Testing Reactor), a fuel element comprises several curved aluminium-clad fuel plates in a vertical box. The water 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.
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 called Materials Testing Reactor (MTR) type fuel elements; aluminium clad, curved plates of fuel arranged in long rectangular boxes which are arranged between grid plates to form the core. 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 pumping pool 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 aluminium 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 pressure drop across the core of the pumped coolant water flow. Again, in the United States, aluminium clad fuel plates are usual.
1.2 The Tank Type Research Nuclear Reactor:
This type of research reactors are similar except that cooling is more active.
1.3 The TRIGA Type Research Nuclear Reactor:
The core of this type of research nuclear reactor 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.
Perhaps the most interesting reactor design of the common types, from a technical and safety perspective, is the TRIGA, developed in the 1950s by General Atomic. Its unique fuel and core design concept has a very large and very prompt negative temperature coefficient, 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 in addition 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 more prompt negative temperature coefficient because it has a strong resonance: Absorption at about 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. It is 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 peak power levels, during a few millisecond pulse, of up YO 10 GW.
Cooling is by natural convection of light water for power levels up to two MW. At higher power levels forced flow is used, but the high fuel temperature tolerance and negative reactivity coefficients mean that pony motors are not needed for shutdown cooling following a loss of the primary coolant Dumps.
Other designs are moderated by heavy water 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.
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, 2 prototypes 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 have research 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.
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, advanced materials development, drug design and nuclear medicine.
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 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.
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 nuclear 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 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, but internationally unsupervised, research nuclear reactor.
The next chapter is dedicated to the Conventional Nuclear Power Reactors.