OPAL and research reactors

About OPAL and other research reactors

What is a research reactor?

A research reactor is designed to achieve high performance in the production of neutrons. These neutrons can be used not only for neutron research, but also for the production of radioisotopes, and other radiation services, as occurs in ANSTO's research reactor OPAL.

How does OPAL differ to a nuclear power reactor?

Whereas the OPAL reactor is designed to produce a high neutron-flux, a nuclear power reactor is planned for the production of energy for electricity. The quantity of uranium fuel and heat generated in a power reactor is more than one hundred times greater than in OPAL.

How safe is OPAL?

The design of OPAL guarantees protection of reactor personnel, the public and the environment against radiation hazards. The design of fully meets requirements for research reactor safety established by the safety regulator ARPANSA (the Australian Radiological Protection and Nuclear Safety Agency) and IAEA (the International Atomic Energy Agency).

The inherent safety of the reactor is ensured by the open-pool concept and the design of the reactor core. An open pool means non-pressurised circuits, which reduces the possibilities of leaks or other types of pipe failure. The reactor core is designed to operate only when surrounded by "heavy water". Since the fuel in the core is not enough to sustain a chain reaction by itself, the reactor can be very easily and quickly shut down at the merest sign of abnormal operation.

The core cooling system uses water which in normal operation is pumped through but which will flow by natural convection in other circumstances. The reactor features natural safety features based on natural characteristics rather than on technical systems.

What is heavy water?

Heavy water is water with an isotope of hydrogen called Deuterium and is highly purified. It is held in a metal vessel around the outside of the reactor core at the bottom of the reactor pool. The reactor pool has normal water in it which has all impurities removed.

As neutrons are produced in the core and enter the heavy water they are slowed and diverted back to the core where they initiate further fission amongst the uranium fuel. Without the heavy water the fission process would stop and the reactor would naturally shut down.

What else can OPAL do?

OPAL is designed to achieve high performance in the production of neutrons. These neutrons are used for the production of radioisotopes, other radiation services and neutron research.

Advantages of the reactor design include a compact core which maximises neutron production, and the core is surrounded by a reflector vessel, which enables easy access to irradiation facilities.

What sort of fuel does the ANSTO reactor use?

The reactor core is comprised of sixteen fuel assemblies in a four by four array. The assemblies each contain 21 aluminium plates within which each has about 100 g of uranium fuel. The fuel comprises 19.7 % uranium-235 with the rest being uranium-238. The reactor core (which contains all the nuclear fuel) is about the size of a two-drawer filing cabinet.

Reactor Fuel

OPAL uses low enriched uranium (LEU) fuel over high enriched uranium (HEU) fuel. There is an important international effort that aims to increase research reactor reliance on LEU (less than 20% enriched U-235). HEU is a nuclear proliferation concern whereas LEU is not suitable for use in weapons.

What is the nuclear reaction in OPAL?

OPAL, as a nuclear research-reactor, is a structure in which a fission chain reaction can be maintained and controlled. The key event in the reactor is fission, in which a neutron hits the nucleus of a uranium atom and splits that atom.

Energy is released, some of which is carried away by neutrons released from the atom. These neutrons are what the scientists and engineers use for bombarding materials, or for the development of new products.

Two or three neutrons are produced when a uranium-235 atom fissions, and are released at high energy. In order for fission to support a chain reaction, the neutrons need to be moderated and reflected back into the fuel.

In OPAL, those tasks - moderation and reflection - are performed by the cooling water flowing through the fuel assemblies, and the heavy water contained in the reflector vessel surrounding the fuel.

How strong is the reaction in OPAL?

When operating at its maximum 20 megawatts, a strong blue glow, called Cherenkov radiation, is visible surrounding the core in the pool. Cherenkov radiation is the glow caused by electrons passing through a medium - in this case water - faster than light would pass through the medium.

How do the operators control reactions in OPAL?

There are five carefully positioned neutron absorbing control rods (or plates) of Hafnium between the fuel assemblies which enter from above and control the rate at which fission occurs. The rods also enable the reactor to be shutdown when required and are automatically pushed down into the core as well as being assisted by gravity.

What happens to neutrons that split from the fuel atoms?

A small cylindrical tank of heavy water inside the pool of lighter water surrounding the core is used as a neutron reflector and irradiation facility. This vessel is vital in operating the reactor as the heavy water reflects neutrons released from the reactor core back into the core.

The majority of neutrons enter this reflector vessel at high energies. As they pass further into the heavy water they lose energy and are eventually reflected back into the core. Meanwhile, neutrons are absorbed by irradiation targets located in the vessel, so not all the neutrons find their way back to the core to produce fission.

Some neutrons also find their way into neutron guides where they are channelled through mirror coated guides to various research instruments.

What is OPAL and why has it been given this name?

OPAL stands for Open Pool Australian Light-water reactor. It is a unique reactor because it incorporates the world's best safety and technological features. OPAL is a modern, open pool design, fuelled by low-enriched uranium and capable of generating 20 MW (megawatts) of thermal power. A key requirement for research reactors is the level of the neutron flux.

Some neutron beams will be at cold (approximately -250°C) temperatures. These beams are the type of beams used in state of the art research at the leading international nuclear research centres, particularly for soft matter science and structural biology.

For industrial applications, the supply of specialist industrial sources will be increased. For instance, the higher flux in OPAL will enable greater production of ytterbium, a high-quality isotope used in non-destructive testing of such materials as thin steel piping or structural components of plant systems.

What is neutron flux and why is it important?

Neutron flux is defined as the number of neutrons passing through an area in a given time. It is a measure of the intensity of neutron radiation. Neutron flux is very important in the applications of neutron radiation. For example, in the production of radioisotopes, which are used in cancer diagnosis and treatment, a sample is bombarded with neutrons to make it radioactive. The time taken to make the sample radioactive is directly related to the number of neutrons that hit the sample. So the higher the neutron flux on the sample, the quicker the process and the more effective and efficient it is.

What are the benefits to Australia of having a research reactor?

It is estimated that, on average, every Australian will have at least one nuclear medicine procedure using a reactor-produced radioisotope in their lifetime. This trend is likely to increase as current research is discovering more and more radioisotopes that have the potential to both diagnose and treat diseases such as cancer.

The capabilities of OPAL ensures that ANSTO researchers are at the cutting edge of research into new generations of radiopharmaceuticals, as well as ensuring that Australia has the means to produce, and ensure access to, these new medicines for all Australians.

ANSTO is able to support Australian manufacturing, minerals and agricultural industries. OPAL facilitates research and development relating to polymers, ceramics and other new materials. It also assists with problems in: life sciences and biotechnology, nanotechnology, understanding complex industrial processes, advanced therapeutic treatment with radiopharmaceuticals, and advanced environmental management processes. Thus it supports Australia's health system, as well as our manufacturing, minerals, petrochemical, pharmaceuticals and information science industries, among others.

OPAL also attracts regional and other foreign scientists to work in Australia and allows Australian scientists enhanced reciprocal access to complementary, first-class research facilities around the world.

What is happening with the two disused reactors at ANSTO?

HIFAR (High Flux Australian Reactor) was Australia's main research reactor and operated from 1958 until early 2007 when it was replaced by OPAL. It is in the early stages of decommissioning, a process which is expected to take about 10 years and return the site to clear ground which will be radiation free and able to be used for other facilities. Any irradiated materials will be managed like other radioactive waste and placed into long-term storage whilst radioactivity levels decrease over time. The main containment building and most of the internal equipment is not radioactive and will be able to be recycled or disposed of as normal waste.

Moata is a third and smaller research reactor ANSTO had which operated from 1961 to 1995. It is currently in the process of being decommissioned, with the process due for completion in late 2009.

What is spent fuel and what does ANSTO do with it?

Fuel elements that have been used to their limit are removed from the reactor and termed spent fuel. This is not waste, as it still holds material that can be extracted and used.

Most of the former spent fuel from HIFAR and Moata has left Australia. Some of this returned to the US where it will remain. Earlier spent fuel shipments went to Europe and Australia is obliged to take the processed waste back. The intermediate-level waste resulting from the overseas reprocessing of HIFAR spent fuel is due to return to Australia from around 2011.

The spent fuel from OPAL will be retained in the storage pool at OPAL for up to ten years and will then return to the US for reprocessing. None of this waste will be returned to Australia.

How safe are nuclear research reactors?

Research reactors are extremely safe. There are over 280 research reactors around the world. In all that time, there has never been an accident that affected the health or safety of people outside the reactor building.

What is the buffer zone about?

For historical reasons, the reactor is surrounded by a 1.6 km buffer zone. Such a buffer zone is not really required and far exceeds siting requirements for research reactors in other countries; in fact, many research reactors overseas are located within urban areas and on university campuses. In comparison, the ANSTO reactor is well away from suburban houses.

If a plane flew into the reactor, what would happen?

It is worth remembering that OPAL is a research reactor, not a power reactor (which are hundreds of times larger), and as such its design greatly limits any risk to community safety. OPAL is built within a strong containment building with very thick concrete walls. It was  designed to protect against severe impacts and from planes. There is also the added security of a Cesna shredder around the outside of the containment building for further protection from planes.

How would a large earthquake affect the reactor?

Reactors are designed to withstand severe seismic loads. These design loads are much higher than those used for other buildings and structures in Australia. OPAL is designed to meet the requirements of ARPANSA and international standards, and would withstand an extreme earthquake. Even in such an extreme events, OPAL would safely shut down.

Other questions you might have asked

1. ANSTO and local community
2. Radiation, radioactivity, uranium and fission
3. Storage and transport of radioactive waste
4. Nuclear medicines and radioisotopes
5. Neutron scattering and other research