Neutrons striking the core of a uranium-235 atom split the atom’s nucleus, releasing two or three neutrons – a process called fission.

 

Some neutrons are guided down beamlines for research; others are reflected back to the core to maintain fission – a far from simple process.

 

As the reaction continues, the number of U-235 atoms drops, and eventually new fuel rods are required.

 

Inside the reflector vessel, irradiation ‘targets’ such as silicon ingots are placed alongside the reactor core, where they pick up stray neutrons.

 

Silicon develops an atomic impurity inside the reactor; this alters its conductive properties for use in electronics. Other irradiation targets include ‘molystrips’ – uranium–aluminium alloy plates that undergo fission and produce a useful radioisotope called molybdenum-99, crucial for nuclear medicine. 

 

 

OPAL’s reactor core comprises a 2.6 x 1.2 m2 reflector vessel made of tough, corrosion-resistant zirconium in a 13 m deep pool.

 

Protected in a high-security building with an aircraft-proof cage, the core has been compared in size to a bar fridge. Perhaps a more captivating analogy, however, would be 200,000 incandescent light bulbs: that’s roughly the equivalent heat energy – cooled by a constant flow of water – produced by the 16 fuel assemblies that comprise the reactor core, each made up of 21 uranium–aluminium alloy plates.

 

The fuel assemblies are surrounded by a reflector vessel filled with ‘heavy’ water (deuterium oxide or 2H2O), which ‘reflects’ the neutrons back to the uranium fuel source. Spent fuel from the reactor is sent to the U.S., where it is stored or reprocessed, and may eventually be reused.

 

 

 

 

Many Australians will require some sort of nuclear medicine procedure in their lifetime. With global supplies under threat, ANSTO is stepping up to deliver radiopharmaceuticals not just locally, but also worldwide.

 

WHEN John Kearley, a 48-year old Sydney-based doctor, suffered a heart attack in mid 2012, his physician, Louise Emmett, wanted to check how this episode might affect Kearley’s future health.
 

As part of a routine ‘stress test’ to survey the damage caused by the heart attack, Emmett, deputy director of diagnostic imaging at St Vincent’s Hospital, Sydney, arranged for Kearley to be injected with technetium-99m.

This radioisotope is used in more than 80% of the world’s 45 million nuclear medicine procedures every year.
 

The technetium-99m was combined with a tracer molecule called MIBI (methoxyisobutylisonitrile), which identifies healthy heart muscle cells, revealing where damage to Kearley’s heart tissue had occurred.
 

Gamma rays emitted as the technetium-99m decayed in Kearley’s bloodstream enabled Emmett to image Kearley’s heart at rest.

Kearley then took a different medication to artificially dilate his arteries, before jumping on a treadmill to work up a sweat and get his heart pumping for another, comparative scan.

While CT (computed tomography) scans can identify heart disease and artery blockages, “it doesn’t always mean it’s significant,” Emmett says.
 

The technetium-99m stress test helps doctors determine if the blockage is of concern – “whether it’s going to put the patient at an increased risk of another heart attack,” she adds.
 

After experiencing further chest pains, Kearley returned to St Vincent’s for another scan in early 2013.
 

“I kept getting weird and unexplained pain in my chest. That’s really scary,” he says. “The nuclear medicine scan I had tried to work out if there was lack of blood flow to parts of my heart muscle when I exercised – and there wasn’t. This information has removed a lot of my worries and helped me stay positive after a heart attack.”

Radioisotopes – varieties of radioactive elements – are used widely in nuclear medicine, primarily in diagnosing heart disease and cancer (including the extent of melanomas, for example), two of the leading causes of death in Australia, but also for imaging the lungs, bones, brain or kidneys.
 

Nuclear medicines play a key role in cancer therapy, targeting thyroid cancers and cancers of the bloodstream, among others.
 

Every year, ANSTO’s OPAL research reactor provides about 500,000 nuclear medicine doses to more than 200 hospitals around Australia. “It’s a fantastic imaging agent,” Emmett says.
 

IN SEPTEMBER 2012, the Australian Government announced plans to build an export-scale nuclear medicine facility at ANSTO, in response to a looming global shortage of molybdenum-99, the isotope used to produce technetium-99m (see ‘Reactorto-patient delivery’, next page). Such shortages have occurred before, and could happen again when older, existing reactors – particularly the National Research Universal Reactor in Ontario, Canada, and the High Flux Reactor in the Netherlands – are decommissioned in the coming years.
 

Worldwide, reactors responsible for 70% of the world’s molybdenum-99 are due to be decommissioned in the next few years.
 

Australia aims to step up as a key global supplier, building a new nuclear medicine production plant in Sydney and a co-located waste facility using a technology called Synroc (synthetic rock), which reduces waste volumes from nuclear medicine byproducts by 99% compared with methods such as cementation in cement–clay mixtures.

“Every so often, it gets to a crisis point when there’s a global shutdown and people have to wait for their therapy and their diagnosis,” says Emmett.
 

“In terms of treating our patients who are sick and really need treatment and diagnosis – it’s extremely important.”