Australia's only nuclear reactor is back in full operation but continued problems have forced multi-million dollar maintenance years ahead of schedule. The $430 million Open Pool Australian Lightwater (OPAL) reactor has been plagued with problems since it was first powered up in late 2006.

The reactor, at Lucas Heights in Sydney's south, produces medicines and imagery services used in the treatment and diagnosis of human diseases including cancer. But a series of hairline cracks has been discovered at the outset in an inner wall of the reactor.

The molecularly altered heavy water at the core is being slowly diluted by the surrounding ordinary water used to moderate the reactor's temperature.

The 7,500 litres of heavy water were expected to need replacing about every 10 years, but the cracks have increased the pace of dilution, which forced the heavy water to be changed recently, acting chief executive Ron Cameron of the Australian Nuclear Science and Technology Organisation (ANSTO) said.

The problem was not a safety risk and the $3 million cost would be borne by the reactor's Argentinian manufacturer, INVAP. "It's their responsibility to come up with the fix for the thing but obviously our guys are quite expert in a number of these areas," Dr Cameron told AAP.

"And we're fairly happy that with the current heavy water we'll be able to keep going for a quite a few months if not a year." Dr Cameron and his team will be meeting with INVAP before the end of the year to discuss the best of a few options for fixing the cracks.

"We don't want to rush it because it's important to get it right," he said. "And since the reactor is operating reasonably normally, there's no great rush to it either." The reactor was shut down for 10 months until May of this year because of displacement of fuel plates in the reactor's core.

(The Age, 10/11/2008)