Research Reactors

(updated 24 May 2010)

• 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 250 such reactors operating, in 56 countries. 
• Some operate with high-enriched uranium fuel, and international efforts are underway to substitute low-enriched fuel. 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.
  

Research reactors comprise a wide range of civil and commercial nuclear reactors which 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% U-235, although some older ones use 93% 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.

As of mid 2009 there were 250 operating research reactors, one under construction, 248 shut down and 170 decommissioned. Two thirds of the operating ones were more than 30 years old.

Types of research reactors

There is a much wider array of designs in use for research reactors than for power reactors, where 80% of the world's plants are of just two similar types. They also have different operating modes, producing energy which 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 zirconium hydride (as 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, which 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 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 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.

Uses

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 so that the target nucleus has a neutron added. For example, yttrium-90 microspheres to treat liver cancer are produced by bombarding yttrium-89 with neutrons.

Neutron activation can result in fission.  The most widely used isotope in nuclear medicine is technetium-99m, a decay product of molybdenum-99*. It is produced by irradiating a target of U-235 foil with neutrons (for a week or so) and then separating the molybdenum-99 from the other fission products in a hot cell - teh Mo-99 being about 6% of the fission products.  Most Mo-99/Tc-99 production has been using HEU targets, but increasingly LEU is favoured and HEU is being phased out.

 * Technetium generators, a lead pot enclosing a glass tube containing the radioisotope, are supplied to hospitals from the nuclear reactor where the isotopes are made. They contain molybdenum-99, with a half-life of 66 hours, which progressively decays to technetium-99m, with a half-life of 6 hours. The Tc-99 is washed out of the lead pot by saline solution when it is required. It is then attached to a particular protein for administering to the patient. After two weeks or less the generator is returned for recharging, since it loses 22% of its product every 24 hours.

Research reactors can also be used for industrial processing.  Neutron Transmutation Doping (NTD), changes the properties of silicon, making it highly conductive of electricity. Large, single crystals of silicon shaped into ingots, are irradiated inside a reactor reflector vessel. Here the neutrons change one atom of silicon in every billion to phosphorus. The irradiated silicon is sliced into chips and used for a wide variety of advanced computer applications. NTD increases the efficiency of the silicon in conducting electricity, an essential characteristic for the electronics industry.

In test reactors, materials are also 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 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.

See also paper on Australian Reserach Reactors. 

Fuels

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 is needed to fuel a research reactor, albeit more highly enriched (compared with perhaps a hundred tonnes in a power reactor).  Research reactors typically operate at low temperatures (coolant below 100C), the operating conditions are severe in other ways. While power reactor fuel operates at power density of about 5 kW/cc, a research reactor fuel may be at 17 kW/cc in the fuel meat. Also burnup is very much higher, so the fuel must withstand structural damage from fission and accommodate more fission products.

Highly-enriched uranium (HEU - >20% U-235) allowed more compact cores, with high neutron fluxes and also longer times between refuelling.  Therefore many reactors up to the late 1970s used it, and most state-of-the-art reactors had 93% enriched fuel.

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 more than 20 have been commissioned on LEU fuel in 16 countries.  The Jules Horowitz reactor in France will start up in 2013 on uranium silicide fuel enriched to 27%, since the planned high-density U-Mo fuel will not be ready in time for it.)

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% U-235. 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. Overall 129 reactors out of the 207 using HEU in 2007 are targeted for conversion, and some 20 tonnes of HEU is involved. However, some are defence-related (mostly in Russia) or impractical for other reasons.  Some have lifetime cores which require no refueling, so there is little incentive to convert them.

In 2004 the US National Nuclear Security Administration (NNSA) set up the Global Threat Reduction Initiative (GTRI), which is congruent with RERTR objectives though it is mainly tackling the disposition of HEU fuel (fresh and used) and other radiological materials.  RERTR is now a major part of GTRI.  GTRI claims accelerated removal of Russian-origin fresh and used HEU fuel to Russia and of US-origin fuel to the USA, the total involved being nearly a tonne.  In particular, to January 2010, 915 kg of fresh and used HEU fuel has been returned to Russia from at least 11 countries including Hungary (155 kg), Serbia, Romania, Libya, Uzbekistan, Poland, Czech Republic, Latvia and Vietnam.  And to mid-January 2010, 1240 kg of US-origin HEU fuel has been returned from Europe, Israel, Turkey, Latin America, Japan and SE Asia.

After a hiatus of six years the US government late in 2008 had converted five university research reactors from using high- to low-enriched uranium fuel.*  It was reported in 2006 that worldwide, 40 remained to be converted under the RERTR scheme using currently-available fuels, and 19 more await development of high-density fuel.

* Texas A&M, University of Florida, Purdue, Oregon State and Washington State University reactors can now operate on fuel of less than 20% enrichment, and the University of Wisconsin reactor is to be converted in 2009.

These RERTR 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 U-Al fuel types.

To September 2009, 67 research reactors (17 in USA) had been converted to low-enriched uranium silicide fuel or shut down, including major reactors in Ukraine, Uzbekistan and South Africa. Another 34 are convertible using present fuels. A further 28, mostly Russian designs but including two US university reactors, need higher-density fuels not yet available. The goal is to convert or shut 129 reactors by 2018. US exports of HEU declined from 700 kg/yr in 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.  No Russian research reactor has yet been converted to LEU, and the Russian effort has been focused on its reactors in other countries.   However, Russia is now looking at the feasibility of converting six domestic reactors,* while others will require high-density fuels.  Another 68 Russian reactors fall outside the scope of the conversion program because they are defence-related or special purpose.

* IR-8, OR, and Argus at Kurchatov Institute, IRT-MEPHI at Moscow Engineering Physics Inst, IRT-T at Tomsk Polytechnic Inst, MIR at Dimitrovgrad Research Inst.

The first generation of new LEU fuels used uranium and silicon (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.  The presence of silicon makes reprocessing more difficult.

An international effort is underway to develop, qualify and license a high density fuel based on U-Mo alloy dispersed in aluminium, with a density of 6-8g/cm³. The principal organisations 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 which 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 jointly by Rosatom 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 which has become the main priority, ANL, CEA and CNEA are testing U-Mo fuel in a monolithic form - essentially pure metal, 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 was extended to 2013 but is in doubt.

All fuel is aluminium-clad.

Used fuel

U-Al fuels can be reprocessed by Areva 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 U-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 FRT) envisaged 38 shipments (of both fresh and used fuel) from ten countries over 2005-08, then 8+ shipments from six countries to remove all HEU fuel discharged before reactors 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.

Research Reactors originally with High-enriched Uranium (HEU) Fuel

Data from IAEA: Nuclear Research Reactors in the World, 2000.  NB some now are converted to LEU or closed.
MNSR = miniature neutron source reactor, Chinese copy of Slowpoke.
crit fast = very low power critical assembly for fast neutrons.

Sources:
IAEA, Nuclear Research Reactors in the World, reference data series #3, Sept 2000.
Research Reactors: an overview, by Colin West, ANS Nuclear News, Oct 1997.
IAEA, Research Reactor Facility Characteristics, 1985.
Research reactors under threat, by W.Krull, Nucl.Eng.Intl. Oct 2000.
O.Bukharin, 2002, Making fuel less tempting, Bull. Atomic Scientists, July-Aug 2002.
Travelli, A 2002, Progress of the RERTR program in 2001.
www.td.anl.gov/Programs/RERTR/RERTR.html
Travelli, A 2002, Status and Progress of the RERTR Program in the Year 2002, RERTR conference November 2002.
Snelgrove JL 2003, Qualification and Licensing of U-Mo Fuel, RRFM conference, March 2003.
NuclearFuel 17/3/03, 22/11/04, 26/3/07, 20/10/08.
 

Wachs, Daniel, 2010, Reactor Conversion, Nuclear Engineering International January 2010.

http://nnsa.energy.gov/