Chapter 03: Research Nuclear Power Reactors

Many research reactors were built in the 1960s and 1970s. 1975 saw the peak number of operating research reactors with 220 in 19 countries as of March 2020.

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.

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.

According to World Nuclear Association, as of March 2020:

  • Many of the world’s nuclear reactors are used for research and training, materials testing, or the production of radioisotopes for medicine and industry. They are basically neutron factories;
  • These are much smaller than power reactors or those propelling ships, and many are on university campuses;
  • There are about 220 such reactors operating, in 53 countries; and
  • Some operate with high-enriched uranium fuel, and international efforts have substituted high-assay low-enriched fuel in most of these. Some radioisotope production also uses high-enriched uranium as target material for neutrons, and this is being phased out in favour of low-enriched uranium.

Nearly all of the world’s research reactors operate with thermal (slow) neutrons; Russia claims that its BOR-60 at Dimitrovgrad is the only fast neutron research reactor.* It started up in 1969 and is to be replaced after the end of 2020 with the MBIR, with four times the irradiation capacity. There is a world shortage of fast reactor research capacity, especially for fast neutron materials testing for Generation IV reactor developments. In February 2018 a bipartisan bill passed by the US House of Representatives authorized $2 billion for the construction of a “versatile reactor-based fast neutron source, which shall operate as a national user facility” by 2026. It will be a research reactor for “development of advance reactor designs, materials and nuclear fuels” of at least 300 MWt.

1. THE USED OF RESEACH NUCLEAR 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.

2. TYPES OF RESEACH NUCLEAR REACTORS:

2.1 Water-Cooled and Plate-Fuel Reactors:

According to Encyclopedia Britannica, these are the most common type of research reactor. Water-cooled, plate-fuel reactors use enriched uranium fuel in plate assemblies (see above Fuel types) and are cooled and moderated with water. They operate over a wide range of thermal power levels, from a few kilowatts to hundreds of megawatts. As the primary mission of research reactors is not electricity production, they are characterized not by the amount of electric power that they produce but rather by thermal power, neutron density, and nominal neutron energy within the core region. It is these parameters that help quantify a research reactor’s ability to perform specific research. The systems with the lowest licensed power levels are usually operated at universities and are used primarily for teaching, whereas those with the highest are used by research laboratories chiefly for materials testing and characterization as well as for general research.

A common form of the water-cooled, plate-fuel reactor is the pool reactor, in which the reactor core is positioned near the bottom of a large, deep pool of water. This has the advantage of simplifying both observation and the placement of channels, commonly referred to as beam ports, from which beams of neutrons can be directed and transported. At lower thermal power levels, no pumping is required, as the natural convection of the coolant past the fuel plates provides sufficient heat removal to maintain a safe operating state. A heat exchanger is usually located at or near the top of the pool, where the hottest water is stratified. At higher operating power levels, pumping becomes necessary to augment the natural circulation.

Most pool reactors use the water of the pool as a reflector (see above Reflectors), but some have blocks of a solid moderator (canned graphite or beryllium metal) around the core that serves as an inner reflector. The inclusion of graphite and beryllium provides a means for reflecting neutrons back into the core (or a useful location adjacent to the core) that would otherwise leak out of the core. Following interactions between the reflector components, neutrons tend to down-scatter quickly, producing relatively large local regions of thermal neutrons. These regions are often exploited for experimental use by transporting the neutrons down the beam ports and to the experimenter as required.

At higher power levels, it becomes more convenient to employ a tank-type reactor, because it is simpler to control the flow path of pumped water in such a system. Low-power educational reactors also are available in the tank form. The core and reflector arrangement and the position of these components within the tank are similar in both tank-type and pool-type systems. However, solid concrete shielding is increased in robustness around the sides of tank-type reactors, whereas pool-type reactors rely primarily on water as a biological shield.

2.2 TRIGA Research Nuclear Reactors:

The training, research, and isotope-production reactors–General Atomic (TRIGA) system is a popular variety of research reactor. It is another tank-type water-cooled system, but its fuel differs from that employed by the plate-fuel research reactors described above. The fuel element of the TRIGA reactor consists of stainless steel- or aluminum-clad rods containing mixed uranium and zirconium hydrides that are often doped with small concentrations of erbium.

Source: wikipedia

In contrast to thin plate-type fuel, TRIGA fuel elements are nominally 3.8 cm (1.5 inches) in diameter and in general approximately 67 cm (26 inches) in total length. A unique characteristic of this fuel is that it exhibits an extremely large negative power-reactivity coefficient—so large that the TRIGA reactor can be placed in an extremely supercritical state for an instant, causing its power to rise very rapidly, after which it quickly shuts itself down on the basis of the fuel’s inherent material composition and characteristics. The resulting power transient is referred to as a pulse and is useful for a number of dynamic experiments that require large bursts of neutrons over a short period of time. The total energy released in a pulse does not draw concern toward the safety of the reactor, since the automatic shutdown occurs very quickly and the energy release is proportional to both peak power and pulse duration.

Resources:

  1. World Nuclear Association – Research Reactors;
  2. IAEA – Research Reactors Worldwide;
  3. World Nuclear Association – Research Reactors; and
  4. SciTech: Research Reactors – An Overview.

  • This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on December 13, 2010;
  • The chapter was updated on November 27, 2015;
  • The chapter was updated on June 5, 2020

Chapter 04