ENGINEERS began powering up Australia's new $400 million nuclear reactor yesterday, 10 months after a manufacturing fault forced it to be shut down.

However, with the Lucas Heights reactor not expected to be fully operational until September, hospitals will have to continue rationing nuclear medicines, postponing medical procedures for thousands of patients, for four more months.

The Argentinian-designed OPAL reactor was shut down last July, three months after it was officially opened, when 13 uranium plates in the fuel core came loose. Replacement parts manufactured in France were delivered in December, but it took until this week for the nuclear regulator to approve their installation.

While the reactor was built for biological, material science and environmental research, one of its most important jobs is producing the 500,000 doses of nuclear medicine used here each year.

The shutdown has meant nuclear medicine has had to be flown in from South Africa, Canada and Europe. The chief executive of the Australian Nuclear Science and Technology Organisation, Ian Smith, said flights have been delayed, for various reasons, on average every 2½ weeks.

Each delay has affected up to 1500 patients, forcing hospitals to ration imports. Children with cancer are a priority. "It's a nerve-racking business," Dr Smith said. "You get heart-wrenching stories [from parents] of children who needed a dose."

The delays would continue because it would be September before regulatory authorities could approve the reactor's return to commercial pharmaceutical production. Dr Smith blamed the shutdown on "a manufacturing flaw" in the core's fuel assemblies that hold the uranium plates.

Asked if they were covered by a warranty, he replied: "We are having discussions with the Argentinians, but it has not been a priority. The priority has been getting the reactor working."

New fuel assemblies were ordered from France because Lucas Heights needed the approval of a very cautious nuclear watchdog before powering up the reactor. "We felt that would be easier if we went to an established manufacturer in France," he said.