The core of US research

Although 90% of the Advanced Test Reactor’s (ATR’s) programme is devoted to classified work for the US Navy’s nuclear propulsion programme, the remaining 10% supports a wide-ranging research and testing programme that includes development of fuels for future Generation IV reactors, testing the properties of materials in ageing nuclear power plants, and development of radiopharmaceuticals for treatment of cancer and other diseases.

At 250MW maximum core power, the ATR is the size of a small power reactor. Typical experimental runs, however, generally are below maximum power, usually in the 110-120MW range. The reactor burns highly enriched uranium (93%) fuel.

The pressurised, light water moderated and cooled reactor was commissioned in 1967, and is expected to remain operational at least until 2050. It was originally designed to provide the naval reactors programme with a test reactor for materials and nuclear fuels. The reactor has a virtually unlimited design life because the reactor vessel is positioned four feet away from the fuel, limiting embrittlement, Frances Marshall, manager of ATR irradiation test programmes, told NEI.

The stainless steel reactor vessel measures 3.65m in diameter and 10.67m high. The reactor core uses 40 highly enriched curved plate aluminum clad fuel elements, each 1.22m in both diameter and height. Peak unperturbed flux levels are in the range of 1x1015n/cm²s thermal and 5x1014n/cm²s fast neutrons.

The ATR is a workhorse, as well as a show horse. Along with its ambitious test programmes for exotic fuels and high-tech alloys for reactor components, it is used to produce a wide range of isotopes for medical, industrial, environmental, agricultural and research applications.

The reactor provides a large fraction of the Ir-192 used in commercial radiography applications in the USA, and high-specific activity Co-60 for medical applications. It provides irradiation services for government programmes, industry and university consortia.

Much of the ATR’s versatility as a test reactor comes from a unique serpentine fuel arrangement that offers nine high-intensity neutron flux traps and 68 additional irradiation positions inside the reactor core reflector tank, each of which can contain multiple experiments.

Two capsule irradiation tanks outside the core provide 34 additional low-flux irradiation positions.

The four flux traps in the corner lobes of the reactor core, as well as the core central position, are almost entirely surrounded by fuel. Four other flux traps between the lobes have fuel on three sides. The curved fuel geometry brings fuel closer on all sides of the flux trap positions than would be possible with a rectangular grid.

SPEEDING UP TIME

The unique core arrangement provides one of the ATR’s major attractions for fuel and power reactor studies: in a matter of months or even weeks it can duplicate years of radiation in a normal power reactor. In fuel studies, researchers can simulate 20 years of in-reactor operation in a year, Marshall explained. The ratio is slightly less for accelerated materials testing, she said.

About two years ago, the ATR conducted an extended test of graphite ageing in the United Kingdom’s Magnox reactors, said Marshall.

The ATR is one of only about seven reactors in the world capable of this kind of accelerated testing, she said, noting that one other US facility – HIFAR at the Department of Energy’s Oak Ridge National Laboratory in Tennessee – can do accelerated testing, as can test reactors in Japan, France, Germany, Belgium and Russia.

Another unique feature of the ATR is the control design, comprised of control cylinders or drums and neck shim rods, that permits large power shifts among the nine flux traps. The control cylinders rotate neutron absorbing hafnium plates and neutron reflecting beryllium plates towards and away from the core. The shim rods, which withdraw vertically, can be individually inserted or withdrawn to adjust power to individual flux traps. Researchers can independently control the power level in each of the four corner lobes.

Researchers also can simulate fast reactor conditions by putting thermal neutron absorbers on experiments.

The reactor has three major kinds of experiment facilities:

• Pressurised water test loops installed in some flux traps that replicate a variety of reactor conditions.
• Instrumented lead experiments that provide real-time measurements and temperature and atmosphere control in the experiment capsules.
• Simple drop-in capsule experiments in reflector or core irradiation positions.

NEW FUELS, NEW REACTORS

The unique capabilities of the ATR have given it a prominent supporting role in the development of both advanced reactors and advanced fuel cycles.

The ATR is the leading test facility for the US Advanced Fuel Cycle Initiative (AFCI), where it is being used to test the effect of combining actinides with fissile materials. Current AFCI tests contain metallic and nitride fuel test specimens composed of varying amounts of uranium, plutonium, neptunium, americium and zirconium, said Marshall. The tests seek to determine if different combinations of fissile materials can be burned to produce minimal waste products. “We are looking at both new fuel formulations and waste burning,” she said.

The AFCI tests now in the ATR, and scheduled for the next couple of years, are preliminary fuel form testing to support the two planned deployments of AFCI fuels. One is intended for use in a mixed oxide (MOX) with minor actinides in current commercial light water reactors. The other will be metallic or nitride fuel forms with minor actinides to be used in fast neutron spectrum reactors, designs for which have not been finalised.

As well as development of advanced reactor fuels, the ATR is being used to develop advanced structural materials for new reactors, beginning with the gas fast reactor (GFR). The GFR test contains fuel-related refractory ceramics, nickel-based 800H and MA754 alloys, and iron-based T122 and MA957 alloys.

In the future, Marshall expects the ATR to be used for more extensive fuel development for Generation IV reactors as well.

Two series of advanced gas reactor tests scheduled to start in autumn 2006 are intended to provide the groundwork for the proposed Next Generation Nuclear Plant (NGNP), an advanced reactor prototype that would be built at the Idaho National Laboratory (INL) and ultimately also would include a prototype nuclear hydrogen production facility. Although the Department of Energy has not made a final decision to proceed with NGNP construction, Congress did include funding for continued research in the fiscal year 2006 budget.

In one group of experiments, the ATR will be used to test the performance of particle fuel, small fuel pellets less than a quarter-inch in diameter. A second set of experiments will test graphite performance, particularly the phenomenon of graphite creep, in very high temperature reactors (VHTRs).

The US National Aeronautics and Space Administration (NASA) also has plans to use the ATR for space nuclear power and propulsion systems, including tests of both plutonium-238 fuelled space batteries, and reactor propulsion systems for deep space missions.

ADVANCED LEU FUELS

Another area where the ATR is proving to be a valuable research tool is the development of low-enriched uranium (LEU) fuels for research reactors around the world – work that is being carried out under the Reduced Enrichment for Research and Test Reactors (RERTR) programme, which is part of the Global Threat Reduction Initiative.

INL is working with the International Atomic Energy Agency to develop LEU fuels that will allow the current generation of research and test reactors to maintain their research capabilities, but remove the proliferation danger inherent in the use of HEU fuels, Marshall said.

Such advanced LEU fuels do not now exist, but INL and a handful of other facilities around the world are working to develop them as quickly as possible. One promising option being tested at INL is high-density molybdenum-uranium fuel.

Ultimately, the ATR itself will be converted to use LEU fuel sometime around 2014, Marshall said.

The ATR team also hopes to upgrade the ATR to make it more useful for isotope production, she said. In the near term, they are looking at installing a ‘rabbit’, a hydraulic tube system that will allow material to be quickly inserted into the reactor and removed within hours or days, rather than remaining for weeks until the end of a test run.

Current plans call for installation of the rabbit by 2010, but it “could happen sooner” if the funding became available, Marshall said.

Another possible new area of work is the investigation of fuel and cladding failures in commercial light water reactors. At least one industry association has approached the INL about the use of the ATR for these kinds of studies, but has not yet submitted a formal proposal.

One approach being considered would be to take samples of commercial fuel that already have reached full burnup lifetime, and put them in instrumented capsules, and burn them further in ATR until fission products are detected in the surrounding gases.

Marshall also noted that it is possible to simulate boiling water reactor conditions on five test positions within the loops, making it possible to test fuel cladding failure and corrosion in the current generation of BWRs, as well as conducting materials tests for next generation BWRs.

Finally, she said, there is now no wait for time on the ATR, and no bottlenecks have come about, even though 90% of the reactor’s test capacity is committed to navy research. However, pressure for research time and machine resources could become more intense in the future.

A number of factors could combine to put the squeeze on the ATR’s capacity. These include: development of a new generation of nuclear power plants –the so-called Generation III+ plants; development of advanced fuels; the longer-term development of the Generation IV plants; and the need to investigate plant and materials ageing issues as current generation nuclear plants are relicensed for 20 additional years of operation.

A second INL test reactor with capabilities that complement the ATR’s – and a greater capability to simulate fast reactor operations – is “a wish-list item” for the future. Marshall predicted that it would be “at least a decade” before formal proposals are prepared and designs are begun.