December 2007
Background
HIFAR (High Flux Australian Reactor), was for many years the only operating nuclear reactor in Australia. It was used for materials research, to produce radioactive materials for medicine and industry and to irradiate silicon for the high performance computer industry. It had the highest level of availability of any research reactor in the world.
HIFAR operated at Lucas Heights from 1958 to 2007. It was at the heart of almost all the research activities of the Australian Nuclear Science and Technology Organisation (ANSTO) and supports those of several other organisations on the same site near Sydney.
HIFAR was the first nuclear reactor in the southern hemisphere. It was installed initially in support of Australia developing a nuclear electricity generating capability.
However, research reactors are quite different from power reactors used to generate electricity. HIFAR's nominal maximum thermal power output was 10 megawatts and OPAL' is 20 megawatts, compared with an average power reactor's 3000 megawatts thermal. HIFAR's total fuel load was only 7 kilograms and OPAL's is also modest, while a power reactor may hold up to 190 tonnes of fuel.
The new research reactor, known as OPAL (Open Pool Australian Lightwater reactor), is a much more modern, powerful and effective neutron source. ANSTO says that OPAL will be one of the top three research reactors in the world.
It is a 20 MW open pool design using low-enriched fuel (less than 20% enriched). The core is submerged near the bottom of a 12.8 metre deep pool of demineralised water which provides cooling, moderation and a degree of containment. Unlike HIFAR, the moderator in OPAL is light water. The core is of compact design to maximise the neutron flux.
A high neutron flux is required for efficient radioisotope production, other irradiation services and neutron beam research. Surrounding the core is a reflector vessel containing heavy water which serves as a reflector to sustain the nuclear reaction. The neutron beam and irradiation facilities are located in the reflector vessel which provides a large zone of high thermal neutron flux where irradiation facilities can be located and neutron beams generated. OPAL's neutron beams will be more intense than HIFAR's and substantially free of gamma radiation.
Unlike HIFAR, OPAL has a cold neutron source that will allow it to produce cold neutron beams, to enable research on biological materials.
A wide range of research opportunities are provided by the irradiation and neutron beam facilities as well as materials testing and isotope production. These facilities have a greater production capability than HIFAR in terms of the range of neutron fluxes available and the total irradiation capacity.
How OPAL works
The key event in any reactor core is fission, in which a neutron hits the nucleus of a uranium atom and splits it. A great deal of energy is released, some of which is carried away by the two or three neutrons released in the process. These neutrons are what scientists and technologists use for materials bombardment or the development of new products.
The two or three neutrons produced when a uranium-235 atom splits are released at high speed. In order to split further atoms, the neutrons must be slowed down and moderated. In OPAL, that role is performed by the water surrounding the uranium fuel elements.
The core - 35 cm square and 60 cm high - comprises 16 square fuel assemblies containing low-enriched uranium silicide fuel plates with aluminium cladding. Water circulating through coolant channels between the fuel plates removes heat produced by the fission reaction.
There are five control rods with absorber plates. Four of the five control rods separate the core into four equal quadrants. The fifth control rod is cruciform shaped and is used for fine control of reactor power, due to its position in the centre of the core.
The reactor pool is linked to a service pool via a transfer canal, with a removable gate for isolating the pools from one another. The service pool is used for the loading and storage of silicon (for NTD, see below), radioisotopes and the storage of spent fuel, with a capacity for storing up to ten years of spent fuel.
The reactor is housed in a structurally reinforced building designed to withstand external events including a one in ten thousand year seismic event and also aircraft impact. In addition to providing structural integrity, the massive reinforced concrete forms the structural basis for the reactor containment.
The design of OPAL includes many safety characteristics. Its inherent safety is based on the open pool concept and the negative reactivity feedback coefficients of the core. It incorporates passive safety characteristics which rely on natural laws for their effectiveness. The range of protection systems incorporate a defence in depth approach to potential scenarios, including loss of power and loss of coolant incidents. It has several independent automatic shutdown systems.
Neutron scattering
A neutron is one of the fundamental particles that make up matter. They are only one-thousandth the size of the smallest atom, yet they can easily travel through centimeters of solid steel. This uncharged particle exists in the nucleus of a typical atom. OPAL is designed to be a neutron factory producing these for a variety of purposes.
Our understanding of many different materials can be improved by scientists using neutrons. Thermal neutrons generated in research reactors are scattered by atoms in the material being probed. The scattering pattern reveals the sample's molecular structure. This technique is called neutron scattering, the subject of the 1994 Nobel Prize for Physics.
Eight neutron beam instruments are served by OPAL. It will also will have the capacity for further expansion, including potential for a second neutron guide hall. A suite of equipment enables studies at different temperatures, pressures and magnetic fields.
Silicon irradiation
ANSTO has a strong reputation in the silicon irradiation business, with HIFAR having produced high quality, high conductivity silicon for the computer industry for many years. OPAL is designed to cater for growing demand for silicon irradiation, and this helps offset the operational costs of the reactor. Silicon doping in HIFAR has earned ANSTO some $2.5 million annually.
Silicon irradiation, or Neutron Transmutation Doping (NTD), changes the properties of silicon, making it highly conductive of electricity. Large, single crystals of silicon shaped into ingots, are air freighted to ANSTO where they were irradiated inside the HIFAR reactor. In the reactor the neutrons change one atom of silicon in every 1,000 million 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.
Irradiation is essential for certain components, such as high power discrete devices, transistors and memory chips, used in sophisticated computers, video cameras, and air conditioning units.
Radioisotopes
Most naturally occurring isotopes are not radioactive and do not emit radiation. Some, however, are unstable and throw off energy in the form of radiation. These radioactive isotopes are known as radioisotopes. Most radioisotopes are manufactured, either by subjecting elements to radiation inside a nuclear reactor, or by bombarding them using a particle accelerator.
OPAL will continue the important function of HIFAR in making radioisotopes that have medical, industrial and environmental applications.
On average, every Australian can expect at some stage in their life to have a procedure that uses radioisotopes for the diagnosis or treatment of illness. Nuclear medicine involves using small amounts of radiation from specially formulated radioisotopes (known as radiopharmaceuticals) to provide information about the functioning of specific organs and to detect and treat cancers. Researchers around the world are working constantly to develop new and more effective radiopharmaceuticals. Radiopharmaceuticals truly are 'designer drugs' - designed isotope doses for diagnosing and treating a particular condition. Specific designer drugs are used to treat specific tumors in different parts of the body. (see also Radioisotopes in Medicine)
In industry and agriculture radioisotopes are widely used for such things as checking the integrity of roads, bridges and aircraft engines; hardening wood and plastic composites; sterilizing insect pests; monitoring soil moisture and tracking ground water. (see also Radioisotopes in Industry)
The biggest customers for ANSTO's radioisotopes are the nuclear medicine departments of hospitals and clinics in Australia and overseas.
Discharges
Airborne emissions generated by ANSTO facilities are treated or filtered before being discharged to ensure that they pose no health hazard. Emissions are constantly monitored by ANSTO and than again independently monitored by the Australian Radiation Protection & Nuclear Safety Agency.
Liquid discharges are regulated by a trade waste agreement with Sydney Water. Any discharges are diluted on the Lucas Heights site in holding areas before release to the Sydney sewage system. Under the same agreement, discharges to the ocean near Cronulla, must be of drinking water quality. ANSTO has never breached its successive trade waste agreements with Sydney Water.
All discharges of radioactive effluent produce radiation exposures well below limits set by the International Commission on Radiological Protection.
Planning, building and fuelling OPAL
In 1997 it was announced that a new research reactor would be built at Lucas Heights near Sydney to replace the antiquated HIFAR which would reach the end of its design life around 2005. An 18-month environmental assessment of the proposal was then undertaken and the comprehensive Environmental Impact Statement (EIS) completed in January 1999. Government approval for the replacement project was then given.
Early in 1999 ANSTO selected four reactor vendors from the prequalification process to provide a 20 MW thermal multi-purpose pool-type reactor using low-enriched fuel as a replacement. AECL (Canada), INVAP (Argentina), Siemens (Germany) and Technicatome (France) were invited to bid for the main design and construction contract. The vendor had to maximise Australian industry involvement. In July 2000 INVAP was selected, in alliance with local engineering firms John Holland Construction and Engineering Pty Ltd and Evans Deakin Industries Limited, and a $278.5 million contract was signed.
The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) issued a licence to construct the reactor in April 2002 and construction of OPAL began in mid 2002.
The initial uranium silicide fuel load was provided by INVAP, from Argentina where the Atomic Energy Commission (CNEA) has world-ranking expertise in developing high-density research reactor fuels. It was made from uranium enriched by the US Department of Energy.
The contract for further supplies was won by Areva which will provide some 600 uranium silicide fuel elements per year from its CERCA subsidiary until a higher-density and more readily reprocessable uranium-molybdenum fuel is available. The contract also involves return of the spent silicide fuel for reprocessing.
Late in 2004 an agreement was signed with the US Department of Energy to take spent silicide fuel from OPAL if required over 10 years.
Individual system testing began in late 2005 and commissioning of the reactor, without nuclear fuel, began in February 2006. A licence to operate the reactor (with nuclear fuel) was issued by the Australian Radiation Protection & Nuclear Safety Agency in July 2006. ANSTO hailed the occasion as "taking us one step closer to a new era in Australian science", having persevered with its 1956-vintage HIFAR unit longer than comparable organizations overseas. Fuel was then loaded and the first self-sustaining critical reaction was achieved on 12 August 2006.
Following the licensing and the necessary test procedures, OPAL attained its full power of 20MW in November 2006. The new reactor, with ANSTO's $20 million cyclotron, will enable the rapidly-growing demand for medical isotopes to be met along with industrial uses and research.
However, defects with the CNEA fuel led to the reactor being shut down in July 2007, and as investigations proceeded ANSTO decided to replace it with fuel from CERCA. The new fuel - as start-up core rather than simply replacement fuel - had to be qualified by ARPANSA. Re-start is expected early in 2008.
HIFAR satisfied most of Australia's growing nuclear medicine needs, supported ANSTO's research Divisions and provided more than 7000 hours per year in neutron beam time to scientists and students from Australia and overseas. HIFAR has been a major research tool for Australian scientists.
HIFAR was used to make a range of radioisotopes for medicine, agriculture, industry and research. Commercially, it was used for neutron transmutation doping (NTD) of silicon.
Other uses included analysis of more than 30,000 samples per year. These include samples from exploration in the mining industry, for research by CSIRO and Australian universities - as well as ANSTO's own research projects.
Neutron beam facilities were installed around the 10 sides of HIFAR. The techniques lead to the creation of stronger, lighter, more heat resistant materials for industry, more useful chemicals and advanced pharmaceuticals.
How HIFAR worked
HIFAR is housed in a circular steel building 21 metres in diameter and 21 metres high, which is sealed. Access for people and equipment is through personnel and vehicle airlocks. The reactor is at the centre of the building within heavy shielding approximately 6 metres high and 7 metres across.
Twenty-five fuel assemblies were suspended vertically in a 2 metre diameter aluminium tank of heavy water. The fuel assemblies, each about 10 cm diameter, are spaced 15 cm apart and the uranium fuel in each assembly is 61 cm long. Thus the uranium core of the reactor fitted within an area 92 cm diameter and 61 cm high (about the same size as a domestic washing machine) located at the centre of the aluminium tank.
The quantity of uranium in one fuel element was 283 grams; the whole core therefore contained just over 7 kilograms of uranium. This was enriched to about 60% U-235 (compared with 3-4% in a power reactor and almost 20% in OPAL).
Each fuel element consisted of several concentric metal tubes through which heavy water was pumped at high speed for cooling. The 2 mm thickness of each tube was an enclosed 'sandwich', in which the 'meat' contained the uranium fuel. The aluminium cladding, the 'bread', protected the uranium from corrosion and prevents escape of radioactive fission products.
The aluminium tank, containing 6 tonnes of heavy water, was enclosed in a steel tank packed with graphite, which conserved neutrons by reflecting them back into the reactor. The steel tank was encased in a 10 cm thick lead shield and then in a 1.5 m thick biological shield of specially dense concrete. This absorbed neutrons and other harmful radiation, thus protecting the operators from radiation exposure.
Some 58 sealed tubes, both vertical and horizontal, penetrated the reactor, enabling scientists to place materials close to the core to study the effects on them of radiation and to produce radioisotopes.
Moderation, cooling and control
The heavy water which was basic to HIFAR's design had the important function of slowing down the neutrons which were liberated by fission in the reactor core.
Heavy water, which has deuterium atoms instead of ordinary hydrogen, is generally indistinguishable from ordinary water, but it is about 10% denser. It is not radioactive and occurs naturally in all water to the extent of about one part in 6,500. Its relative rarity means that pure heavy water is very expensive to produce. It sells for about $500 per litre.
The heavy water circulated through heat exchangers, which transferred the heat to ordinary water. It was then dissipated in cooling towers. HIFAR operated at atmospheric pressure and a maximum temperature of about 51° C - well below boiling point.
Control of all reactors is achieved by inserting materials, such as boron or cadmium, which absorb neutrons. In HIFAR there were six cadmium arms 1.45 metres long, shaped like railway signal arms, and which could be moved in an arc between the rows of fuel elements. Their position determined the reactor power. Since the number of neutrons can build up very quickly, the safety system has been designed to avert this by rapid insertion of the control arms.
Safety
HIFAR worked on a 28-day cycle - 24 days working, with four days shut down to change fuel elements and to maintain and check essential safety mechanisms. The control room was staffed 24 hours a day all year round, regardless of whether the reactor is operating.
In the event of any condition exceeding safe operating limits, the reactor protection system would automatically shut it down. This is done by the release of the cadmium control arms which drop between the fuel elements to absorb all of the free neutrons.
A number of cooling systems with back-up safety mechanisms installed in the reactor would operate if there were any likelihood of its fuel overheating.
The whole reactor was housed in a steel containment building in which the air is maintained at slightly less than atmospheric pressure. In the unlikely event of any release of radioactive material from the reactor, the building automatically seals to prevent any contamination of the environment.
Used Fuel
HIFAR's fuel was fabricated in the USA and the UK. In line with recommendations from several government inquiries, the used fuel has been sent overseas for either disposal or reprocessing, depending on the country of origin of the fuel. In 1963 and 1996, ANSTO shipped a total of 264 used fuel elements to the UK. In 1998, 240 were sent to the USA. A total of 1288 used fuel elements were sent to France for reprocessing in four shipments between 1999 and 2004. In 2006 a second shipment of 330 used fuel elements was sent to the USA. This left some 130 HIFAR used fuel elements destined for the USA.
HIFAR used fuel elements are stored on site and were accumulating at the rate of 38 per year. A sum of $88 million (1997 dollars) has been allocated for reprocessing UK-origin used fuel in Europe and shipping US-origin used fuel to the USA.
UK reprocessing was under a 1967 fuel purchase agreement, and the separated uranium was used in the UK for offsetting against ANSTO fuel purchases. The wastes from the reprocessing of the used fuel sent in the 1996 shipment will be held there for up to 25 years and then returned to Australia. It is classified as intermediate-level waste.
Decommissioning
The decommissioning process for HIFAR will be in four stages.
Stage one, which is currently underway, entails the removal of fuel and draining of heavy water from the facility. The spent fuel will eventually be shipped the USA and no waste will return to Australia. The heavy water will be sold for re-treatment.
Stage two, which will take place over approximately the next 10 years, involves maintaining the reactor whilst the decay of radioactive materials takes place within the reactor.
Stage three, to occur around 2016, will involve dismantling the reactor and removing all radioactive wastes to a national radioactive waste repository.
The fourth and final stage will be returning the site for unrestricted use.
MOATA is a low-power physics and training reactor, built at Lucas Heights in 1961 for experimental work on the physics of reactor core assemblies. It has not operated for some years and is being decommissioned.
MOATA was an "Argonaut" type of reactor, similar to others built overseas. Enriched fuel (around 80% U-235), as aluminium clad uranium-aluminium alloy plates, is arranged in two parallel tanks in a block of graphite. Water flows through the tanks, acting both as coolant and moderator. The graphite acts as a reflector.
Control is provided by four neutron-absorbing rods, adjacent to the core tanks. Complete shutdown is achievable by dumping the water, through a valve.
When originally constructed, the maximum thermal power of the reactor was 10 kilowatts. It was later modified to permit power up to 100 kW.
The low maximum power level of the reactor avoided excessive radiation levels after reactor shutdown. This enabled experimental equipment to be rearranged and reactor components to be maintained without the need for remote operations.
The Australian Synchrotron is a powerful (3 GeV) linear accelerator which complements OPAL in Sydney. It is being commissioned at Clayton, in the eastern suburbs of Melbourne, next to Monash University.
It accelerates electrons to almost the speed of light and contains these in a ring. As the electrons are deflected through magnetic fields they create electromagnetic radiation. The photons of radiation range in wavelength from infrared to hard x-rays. The intense radiation is channeled down beamlines to experimental workstations where it is used for research. Each of the initial 13 beamlines is set up for a particular wavelength or narrow range of wavelengths.
Main Sources:
ANSTO
AP Marks
Cameron, R. & Horlock, K. 2001, The Replacement Research Reactor, ANA conference paper.
Australian Synchrotron
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