TRIGA

TRIGA is a swimming pool reactor that can be installed without a containment building, and is designed for research and testing use by scientific institutions and universities for purposes such as undergraduate and graduate education, private commercial research, non-destructive testing and isotope production.

The TRIGA reactor uses uranium zirconium hydride (UZrH) fuel, which has a large, prompt negative fuel temperature coefficient of reactivity, meaning that as the temperature of the core increases, the reactivity rapidly decreases. Because of this unique feature, it has been safely pulsed at a power of up to 22,000 megawatts.^[1] The hydrogen in the fuel is bound in the uranium zirconium hydride crystal structure with a vibrational energy of 0.14eV.^[2] These levels fill when the fuel is hot, and transfer energy to thermal neutrons making them more energetic and, therefore, less likely to cause a fission. TRIGA was originally designed to be fueled with highly enriched uranium, but in 1978 the US Department of Energy launched its Reduced Enrichment for Research Test Reactors program, which promoted reactor conversion to low-enriched uranium fuel.^[3][4]

The concept of the TRIGA reactor was invented in the summer of 1956 when the president of the newly founded company General Atomics, Frederic de Hoffmann, invited a group 30–40 nuclear scientists (many of them alumni of the Manhattan Project) to consider what types of reactors would be commercially promising. The exploratory work focused on three concepts: a "Ship Reactor" (to propel merchant ships), a "Test Reactor" (for experimentally irradiating parts of nuclear power reactors), and a "Safe Reactor" (which eventually became TRIGA). During June-September the scientists worked out designs for the three reactors, and at the end of the summer de Hoffmann chose TRIGA for commercial development.^[5]

The "Safe reactor" working group was led by Edward Teller, who proposed that General Atomics should break into the market by developing a reactor that was much safer than its competitors. In Teller's words the reactor should be safe "even in the hands of a young graduate student"^[6], or in the words of Freeman Dyson "even in the hands of an idiot clever enough to by-pass the entire control system and blow out the control rods with dynamite."^[5] The working group comprised ten physicists, chemists, and engineers, including Freeman Dyson (who worked on the mathematical theory) and Massoud T. Simnad (who developed the fuel chemistry). At the end of the summer the preliminary design was handed over to a smaller team of General Atomics scientists, Ted Taylor, Stan Koutz, and Andrew McReynolds, for detailed development. The prototype for the TRIGA nuclear reactor (TRIGA Mark I) was commissioned on 3 May 1958 on the General Atomics campus in San Diego and operated until shut down in 1997. It has been designated as a nuclear historic landmark by the American Nuclear Society.

Mark II, Mark III, and other variants of the TRIGA design have subsequently been produced, and a total of 33 TRIGA reactors have been installed at locations across the United States. Those that remain operational continue to be upgraded or modernized.^[7] A further 33 reactors have been installed in other countries. Many of these installations were prompted by US President Eisenhower's 1953 Atoms for Peace policy, which sought to extend access to nuclear physics to countries in the American sphere of influence. Consequently, TRIGA reactors can be found in a total of 24 countries, including Austria, Bangladesh, Brazil, Congo, Colombia, United Kingdom, Finland, Germany, Taiwan, Japan, South Korea, Italy, Indonesia, Malaysia, Mexico, Morocco, Philippines, Puerto Rico, Romania, Slovenia, Thailand, Turkey, and Vietnam.

TRIGA International, a joint venture between General Atomics and CERCA [fr]—then a subsidiary of AREVA of France—was established in 1996. Since then, all TRIGA fuel assemblies have been manufactured at CERCA's plant in Romans-sur-Isère, France.

Some of the main competitors to General Atomics in the supply of research reactors are Korea Atomic Energy Research Institute (KAERI) of South Korea and INVAP of Argentina.

The TRIGA Power System (TPS) is a proposed small power plant and heat source, based upon the TRIGA reactor and its unique uranium zirconium hydride fuel, with a thermal power output of 64 MW producing 16 MW of electricity.^[8][9]

Edward Teller's goal was an "inherently safe" reactor, meaning that even if all the control rods are abruptly withdrawn, the reactor will settle to a stable state before it is damaged by the heat. To achieve this it needs to have a large negative temperature coefficient, i.e. the amount of reactivity should decrease quickly as the reactor gets hotter.

Most nuclear reactors exhibit a negative temperature coefficient to some extent. As fuel temperature rises, thermal expansion reduces the fuel density, lowering reactivity. In addition, the neutron absorption of U-238 (the non-fissile uranium isotope) increases with temperature due to Doppler broadening, increasing neutron capture and thereby reducing the number of neutrons available to sustain the fission chain reaction.

The TRIGA design additionally makes use of effects related to the energy of thermal neutrons.^[10] Like most reactors, TRIGA is a thermal-neutron reactor in which fast neutrons produced by fission are slowed by a neutron moderator until they reach thermal equilibrium with the surrounding material. The U-235 fission cross section is much larger for low-energy neutrons than for higher-energy ones. As the temperature of the moderator increases, the average energy of thermal neutrons also increases, reducing the probability that they will induce fission and increasing the likelihood that they will escape from the fuel (the "leakage effect") and be absorbed elsewhere. An even larger negative coefficient can be obtained by incorporating neutron absorbers such as erbium into the fuel^[11], which preferentially absorb higher-energy thermal neutrons.

The challenge of exploiting the leakage effect was to ensure that the temperature of the neutrons tracks the temperature of the reactor. In general the thermal energy of the neutrons will always correspond closely to the temperature of the moderator. But in a conventional reactor the moderator is separate from the fuel; for example a light-water reactor uses water next to the fuel rods as a neutron moderator. If there is a power surge in such a reactor, the fuel rods can heat up and melt long before the water starts to heat up. Therefore, the TRIGA designers set out to make a "homogeneous reactor", where the fissionable material and the moderator are combined in a homogeneous mixture.^[10]

Several candidate materials were investigated as potential moderators that could be combined with uranium fuel, including beryllium, beryllium oxide, graphite, lithium hydride, sodium hydride, and zirconium hydride (ZrH_x). Zirconium hydride was ultimately selected because of its low chemical reactivity, low toxicity, good thermal conductivity, and low cost. However, little was known in 1956 about the metallurgy or neutronics of ZrH, so extensive experimental and theoretical work was needed.^[12]

The TRIGA fuel consists of a homogeneous alloy of uranium-zirconium hydride. The zirconium matrix can hold a large number of hydrogen atoms (with a H/Zr ratio around 1.6^[13]), and the hydrogen is a good moderator. Water surrounding the fuel provides additional moderation and cooling. Because the uranium is dispersed inside the zirconium hydride matrix, the temperature of the hydrogen and hence the thermal neutrons instantly track the rest of the fuel. Using standard TRIGA fuel, 50% of the temperature coefficient of the reactor is due to the thermal spectrum of the neutrons.^[11]

• List of commercial nuclear reactors
• List of nuclear research reactors
• Hydrogen-moderated self-regulating nuclear power module