General Atomics' well known TRIGA® nuclear reactor program is completing fifty years of success in the design and operation of its reactors. TRIGA, the most widely used research reactor in the world, has an installed base of over sixty-five facilities in twenty-four countries on five continents. Now the only remaining supplier of research reactors in the United States, General Atomics continues to design and install TRIGA reactors around the world, and has built TRIGA reactors in a variety of configurations and capabilities, with steady state power levels ranging from 20 kilowatts to 16 megawatts. The TRIGA reactor is the only nuclear reactor in this category that offers true "inherent safety," rather than relying on "engineered safety."

The idea of such a safe reactor was originally conceived by Dr. Edward Teller when a team of scientists was assembled in the "Little Red Schoolhouse" in San Diego in the summer of 1956¹. The mandate to this distinguished group, working under Dr. Teller, was to "design a reactor so safe … that if it was started from its shut-down condition and all its control rods instantaneously removed, it would settle down to a steady level of operation without melting any of its fuel." In other words, "engineered safety," or the prevention of catastrophic accidents by engineering the reactor control and safety system, was not good enough and the challenge was, therefore, to design a reactor with "inherent safety," guaranteed by the laws of nature. This way, the safety of the reactor would be guaranteed even if the engineered features were by-passed and the control rods, which contain the poison materials for shutting down an operating nuclear reactor, were rapidly removed.

In meeting this challenge, the idea of the "warm neutron principle" was introduced as a first step towards the design of an inherently safe reactor. In a water-cooled reactor, the general result from suddenly removing the control rods is a catastrophic accident, leading to a melting of the fuel. This is because the neutrons from the fission reaction remain "cold" from interacting with the cold water around the fuel and maintain their ability to cause further fissioning of uranium atoms in the fuel. This in turn results in the temperature of the fuel continuing to increase rapidly until it finally melts. However TRIGA is no ordinary light water reactor because much of its "moderation" of neutrons is due to the hydrogen that is mixed in with the fuel itself. Therefore, as the fuel temperature increases when the control rods are suddenly removed, the neutrons inside the hydrogen-containing fuel rod become warmer than the neutrons outside in the cold water. These warmer neutrons inside the fuel cause less fissioning in the fuel and escape into the surrounding water. The end result is that the reactor automatically reduces power within a few thousandths of a second, faster than any engineered device can operate. In other words, the fuel rods themselves act as an automatic power regulator, shutting the reactor down without engineered devices.

In the 1950s, General Atomics pioneered the manufacture of fuel rods containing hydrogen. GA metallurgists perfected the process of making fuel rods containing high concentrations of hydrogen by using an alloy of uranium and zirconium metal. The resulting alloy was as tough and as corrosion resistant as stainless steel. Thus, the unique uranium-zirconium-hydride (UZrH) nuclear fuel was developed at General Atomics and the use was extended in the 1980s by designing and developing proliferation resistant (low-enriched uranium) UZrH fuels for use in higher power regimes where newer TRIGA research reactors were being designed to operate. This fuel design provides the highest degree of safety against nuclear accidents regardless of power level. While there have been rare instances of safety related accidents at a few research reactors, such incidents have never, can never, and will never occur at a TRIGA reactor based on the simple physical principles of UZrH fuel.

The demonstrated advantages of TRIGA fuel over other fuel used in research and test reactors include:

• The warm neutron principle utilized in the UZrH fuel gives the reactor a "prompt negative temperature coefficient of reactivity" versus a delayed coefficient for other types of research reactors utilizing aluminum clad plate-type fuel. This allows TRIGA reactors to safely withstand events that would completely destroy plate-fueled reactor cores.
  
• UZrH is chemically stable. It can be safely quenched at 1200°C in water, while destructive and unsafe exothermic metal-water reactions take place with aluminum of plate-type fuel at 650°C.
  
• High-temperature strength and ductility of the stainless steel or Alloy 800 fuel cladding provides total clad integrity at temperatures as high as 950°C. The aluminum cladding on plate-type fuel melts and fails at about 650°C.
  
• The UZrH fuel material has far superior retention of radioactive fission products compared with aluminum-clad, plate-type fuel. These plate-type fuels will melt at about 650°C, releasing nearly all of the volatile fission product inventory in the fuel. At the same temperature, UZrH retains more than 99% of these fission products, even if all the cladding were to be removed.

The prototype TRIGA (Training, Research, Isotopes, General Atomics) nuclear reactor was commissioned on General Atomics' then new site on May 3, 1958. Known as the TRIGA Mark I reactor, it was originally licensed to operate at a power level of 10 kilowatts, but was soon upgraded to 250 kilowatts. This little reactor, because of its inherently safe features, could also be rapidly "pulsed" to power levels of over 1000 megawatts after which (and without any outside intervention) it would return, in a few thousandths of a second, to a safe low power as a result of the effect of the ubiquitous warm neutrons. This original TRIGA, designated as a nuclear historic landmark because it pioneered the use of unique, inherently safe capabilities in nuclear reactors, operated successfully until 1997, when it was permanently shut down because of its age. The pulsing feature of UZrH fueled reactors, first demonstrated in this prototype TRIGA at General Atomics, are standard among many TRIGA reactors, and special designs of pulsed TRIGA's in use today routinely achieve power levels of 22,000 MW to test the safety of fuels for nuclear power reactors.

In times of increasing public concern with the perceived hazards of nuclear facilities, the safety advantages of TRIGA type reactors in themselves justify the use of this technology. The unique safety of the UZrH fuel makes unnecessary the expensive pressure containment building required by present safety regulations for research reactors with aluminum clad plate-type fuel. In contrast, many TRIGA reactors are located in existing buildings on university campuses and even in hospitals.

Additionally, there are perceived environmental hazards associated with temporary storage of the spent fuel at the reactor facility, transportation of the fuel and its final disposal. The unique design of UZrH fuel allows it to be used for a significantly longer time in the reactor, typically three to four times as long as other types of fuel. Therefore in a given period of time, there will be only one-third to one-fourth as much spent fuel discharged from a TRIGA reactor. The results are far less spent fuel stored at the reactor site, less fuel to be transported over public highways, and less fuel to be put into permanent high level waste storage or reprocessed.

General Atomics' TRIGA system is, therefore, truly more safe, robust and environmentally friendly than any other system in use today. It is the only research reactor that does not rely on external controls for safety, provides nearly complete retention of volatile fission products, requires far less restrictive siting criteria, and presents significant advantages in the storage, transportation and disposal of the spent nuclear fuel.

TRIGA reactors were being installed less than three years after the first gathering of Dr. Teller's group in "The Little Red Schoolhouse." Originally designed to meet requirements for educational programs, operator training and nuclear research programs, TRIGA's utilization has been expanded to meet requirements of large scale medical and industrial applications, including the production of radioisotopes, production of pure silicon, cancer therapy with neutrons, and real-time nondestructive testing. Extensive use has been made of TRIGA reactors for the development and testing of power reactor fuels in the United States, Japan and Romania. The worldwide acceptance of this technology has allowed GA to compete successfully worldwide against major suppliers of reactors such as Siemens of Germany, Technicatome of France and AECL of Canada. In 1997, General Atomics was awarded a turnkey contract to build a large Nuclear Research Center in Thailand with a 10 MW TRIGA reactor as the centerpiece. This facility is scheduled to be commissioned in 2001. TRIGA reactor projects have just been, or are in the process of being completed in Colombia and Indonesia. The Kingdom of Morocco has contracted with General Atomics to construct a medium power TRIGA reactor. Many others are actively considering TRIGA facilities to support their programs for peaceful uses of nuclear energy, university education, research and development programs, as well as for medical and industrial applications.

¹ Freeman Dyson, "Little Red Schoolhouse," in Disturbing the Universe, Basic Books, 1979.

® TRIGA (Training, Research, Isotopes, General Atomics) is a registered trademark of General Atomics.