Moata research reactor

Moata hall, photographed by Max Dupain

Moata an aboriginal word meaning fire-stick or gentle-fire was one of a group of reactors designed by the Argonne National Laboratory in the USA in the mid 1950s.

The 100 kW Argonaut reactor first went critical in 1961 and was acquired by ANSTO (then the Australian Atomic Energy Commission). Its purpose was to train scientists in reactor control, neutron physics and accumulate experimental nuclear data on fuel and moderator systems. While it was initially used as a research tool and training reactor, the scope of Moata's operation was extended in the mid seventies to include activation analysis and neutron radiography.

After 34 years of successful operation, the reactor was shutdown as its continued operation could no longer be economically justified. It is now in the final stages of dismantling.

Moata in 1970s

Reactor use

Moata provided accurate measurements of nubar (the number of neutrons emitted per fission) for thermal neutron fission of U233, U235, Pu239 and Pu241. Nubar is possibly the most important parameter in the design of nuclear systems.

ANSTO's work made a significant contribution to reactor science by setting the benchmark for measurements of U235. Commercially, Moata provided rapid, accurate assays for uranium ore processing. Its neutron radiography rig was world class, attracting around 15 per cent of the world market in neutron radiography for jet engine turbine blades at the time. Several other measurement techniques developed from Moata are still used at ANSTO today.

Used fuel

Moatas highly-enriched uranium (HEU) fuel was installed in two assemblies, each comprising six fuel elements. Each element consisted of 12 aluminium clad fuel plates containing around 22g of U235 in an aluminium and uranium alloy. The fuel was unloaded in 1995 and shipped to the US in 2006, where it remains.

Historical view inside Moata

The reactor core

The reactor core was contained within a monolithic concrete bioshield, approximately 5.8 m wide, 6.4 m long and 3.3 m high. The core cavity measured approximately 1.2 m wide, 1.5 m long and 1.6 m high.

The first, or innermost, 300mm of concrete around the Moata core was composed of heavy density (HD) concrete. Approximately 50 per cent of its volume consisted of iron slugs, which effectively absorb neutrons and intense gamma radiation produced during reactor operation. It was in this material that the vast majority of activation occurred.

The surrounding normal density concrete outside this inner region had very low levels of activation, which fell rapidly as the distance from the HD concrete increased. At about 200mm from the HD concrete, the activation levels corresponded approximately with release criteria. At 500mm no activation was measured.

Knowledge sharing

Moata is the first reactor to ever be decommissioned in Australia. As such, it has attracted the interest of specialists groups around the world.

In 2009, ANSTO hosted an International Decommissioning Network (IDN) workshop entitled Support in Planning for the Decommissioning of Small Reactors and Nuclear Facilities. The workshop coincided with the preliminary dismantling phase of Moata, allowing participants observe the work being undertaken.

3D model of Moata

Project planning

Prior to dismantling, extensive project planning and investigations were undertaken. These included a thorough safety analysis, work method planning and plans for waste management. The reactor materials were characterised to determine their precise radionuclide constituents and the resultant radiological doses likely to be received by staff engaged in the project.

The various plans and assessments became the basis of internal ANSTO safety committee reviews and submissions to the Australian Radiation Protection and Nuclear Safety Authority (ARPANSA) for its approval to undertake the dismantling.

3D modelling

To assist the project team, a 3D modelling expert was engaged early to develop an accurate model of the reactor structure. The concrete-cutting proposals were incorporated into the electronic 3D model.

From this, a scaled-down plastic replica of Moata incorporating accurate pieces was produced (see image). This model provided invaluable guidance to the various tenderers and was used by the successful tenderer in developing its plans, methods and sequence for the dismantling.

Members of the decommissioning team

Dismantling process

Preliminary dismantling of Moata involved the removal of internal components in July 2009 by a highly-trained internal team. The dismantling process began in March 2010 with the demolition and removal of the concrete bio-shield by a contractor who had radiation safety training.

The outer shielding material was removed for potential release to landfill. The HD concrete, and 200mm of surrounding concrete is being stored at ANSTO until a national repository is built. Dismantled concrete sections were scanned by an In-Situ Object Counting System (ISOCS) a high precision spectrometer - which analyses gamma radiation emitted by an object. Because the ISOCS cannot measure beta emitters, these had to be analysed separately.

In order to do this, the concrete samples were sent to a laboratory in Denmark where the beta and gamma contents were analysed. These results, used in conjunction with the ISOCS scan data, allowed ANSTO's Waste Operations to determine which materials would go to landfill or be retained at ANSTO (see the waste information paper).

Material destined for landfill was assessed against a variety of criteria developed by the IAEA and Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). This criterion is based on radioactivity content per unit mass (bequerels/gram etc) of material, irrespective of quantity. The NSW Department of Environment and Climate Change (DECC) also has criteria based on the total amount of such material in a given disposal.

To ensure the protection of workers, the whole area was contained within a large 12 x 24 x 7m (h) tent. The work was undertaken with local extract ventilation, which was then filtered through two stages of HEPA filtration.

Radioactivity and decay

Nearly all concrete contains granite aggregates. Granites and some sands contain significant amounts of naturally occurring radioactive materials (NORMs) of the Thorium and Uranium series. When using simple dose-rate measurements, these materials do not discriminate the actual nuclides present, and give the appearance that the material may contain man-made radioactivity, when this is not the case.

Moata contains low levels of these materials. The activated concretes gamma emitters are Co6o, Eu152 and Eu154. The beta emitters are H3 and Fe55. The half lives for all these isotopes is less than the period that the reactor has been shut down.

All the nuclides of interest in the HD concrete will have decayed within a few decades to levels where they would be almost impossible to measure and no longer be of concern.