The reactor was shut down last July when a routine inspection found fuel plates had come loose during operation.

ANSTO blamed the problem on the fuel design and manufacturing process and applied to nuclear watchdog ARPANSA to use a different fuel.

Last week ARPANSA gave the green light to a restart and ANSTO CEO Dr Ian Smith said that process would begin immediately.

ANSTO said Dr Smith had turned down an extension to his contract and would finish on May 16. He plans to return to New Zealand with his family and resume a non-executive career.

Speaking about the reactor restart, Dr Smith said the downtime had been frustrating for everyone involved.

``It was important that these painstaking steps were taken to ensure that OPAL can recommence operating,'' he said.

``We are keen to get back to business (and) are hoping that full power of 20 megawatts will be achieved in a few weeks time, but this has to be done gradually. It's not just a case of flicking a switch.''

A special start-up core which consists of new fuel assemblies manufactured to the modified design and a neutron-emitting source are needed to begin the nuclear reaction and the power is then increased gradually over a period of time.

``The process of recovering from the fuel displacement took a number of months,'' Dr Smith said. ``Following the investigation, which began as soon as the problem was discovered, a solution to the problem had to be developed. This included design changes to the fuel assemblies and new ones being manufactured.''

Not everybody is as thrilled with the restart as Dr Smith. Friends of the Earth spokesman, Dr Jim Green, said there was no need for Australia to have a nuclear reactor.

Dr Green said the ANSTO operation accounted for 90 per cent of Australia's nuclear waste when measured by radioactivity.

``My biggest issue with it has always been weapons proliferation,'' he said. ``We just set such a bad example by even having a reactor.

``Australia could still be involved in international discussions on nuclear issues without operating a reactor. We have to pretend there are no health or environmental effects.''