FOR WANT OF A COMB!

Clearly, with combs you can get away with more. In today's world, some would say that you should not walk if you need the crutch. (Click on the picture to enlarge, then go back to get the next slide.)

My Kingdom for a Comb!

Opal fuel modifications approved
01 May 2008

Approval has been given to use a modified fuel design in Australia's Opal research reactor, altered to prevent dislodgement of nuclear fuel plates.

Opal at full power. The blue glow isCerenkov radiation caused byfaster-than-light particles travellingthrough cooling water (Image: Ansto)The 20 MWt Open Pool Australian Light-water (Opal) reactor began commissioning in 2006, reaching full power during November that year. It is owned and operated by the Australian Nuclear Science and Technology Organisation (Ansto) and was supplied by Invap of Argentina.

On 24 July, however, it was noticed during refuelling that three of the reactor core's 16 nuclear fuel assemblies each had one of their 21 fuel plates partially dislodged. This appeared to have been caused by the motion of coolant water, which flows from the bottom to the top of the tank in which the core sits and causes a certain amount of vibration.

Ansto's response was to shut down the reactor in order to analyse the problem. The organisation decided that a revised fuel assembly design was needed because the existing fuel was subject to "inadequate design and fuel manufacture techniques." Ansto said the roll-swaging process used for the faulted fuel was to blame.

The new fuel plate design incorporates what Ansto describes as 'stoppers' which limit longitudinal movement within the fuel assemblies. These 'defence in depth' features, which Ansto said prevent significant movement, take the form of two 4 mm plates of aluminium fixed by two screws to a side-frame of the fuel assembly and are held in position by the screws and the assembly handling pin.

This design was approved today by the Australian Radiation Protection and Nuclear Safety Agency (Arpansa), and Ansto is now free to restart Opal when ready.

Ansto said in a submission to Arpansa that it would improve video surveillance of fuel assemblies with in-core video inspection before and after fuel changes. Ansto will also inspect fuel destined for Opal at the point of manufacture and perform pull tests on the plates in a test sample of each batch of new nuclear fuel.

Nuclear reactor design 'flawed from start'
By Dani Cooper for ABC Science Online
Posted Mon May 5, 2008 6:49pm AEST
Map: Lucas Heights 2234
A flaw in the original design of Australia's only nuclear reactor is partly responsible for the shutdown of the facility in July last year, just months after it was officially opened.
Dr Greg Storr, who heads reactor operations for the Australian Nuclear Science and Technology Organisation (ANSTO), says an "oversight" at the commissioning stage of the $400 million Open Pool Australian Lightwater (OPAL) reactor left it vulnerable.
His comments come as ANSTO staff begin reloading fuel into the reactor, based at Lucas Heights south of Sydney, following approval today from the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) to restart the reactor.
Dr Storr says he expects the reactor to be operating at full power by the end of the month.
OPAL replaced Australia's first reactor, the 40-year-old High Flux Australian Reactor.
It was meant to produce four times the amount of radio-isotopes for nuclear medicine than its predecessor and expand the nation's capacity for nuclear medicine research.
But the OPAL reactor was shut down three months after it opened when staff discovered during routine maintenance that some fuel plates had become dislodged and were projecting above, but still attached to the fuel assembly.
Worst case scenario
Dr Storr says in the worst case one of the fuel plates - which are about 2.5 millimetres thick, about eight millimetres wide and 800 millimetres long - was above the fuel assembly by about 400 millimetres.
Unlike similar reactors around the world, the OPAL reactor design did not have a secondary mechanism in place to stop the fuel plates moving.
"Not only did we miss it in our review, the designer missed it, ARPANSA missed it and internationally people [who were also tendering for the job] missed it," Dr Storr said.
But he says the movement of the fuel plates cannot just be blamed on the design fault.
ANSTO investigators have concluded the plate movement was caused by three factors.
These are the original design fault, vibrations caused by the rapid flow of cooling water up through the plates and a fault in the manufacturing process for the fuel plates used by Argentine company CNEA.
Dr Storr says in the OPAL reactor core there are 16 fuel assemblies that each hold 21 fuel plates made up of aluminium and small amounts of uranium.
The two outside fuel plates are screwed in while the 19 internal plates are slid into grooves and "swaged" or crimped in place.
Readjusted
Dr Storr says the tool used in Argentina to swage the fuel plates had been readjusted and was slightly out.
And tests at the manufacturing site had also shown the vertical strength in the swaging of the fuel plates was "less than expected", he said.
Dr Storr says these factors meant because the plate was not held in place as well as it should have been, once the swaging bond was broken the vibrations caused the plates to move upwards.
To fix the problem, which has been estimated to have cost about $100,000 a week in lost revenue, ANSTO is now sourcing its fuel plates from a French-based manufacturer.
The design of the fuel assembly has also been changed to include a stopper to prevent further fuel plate movement.
Dr Storr says the new design was tested by leaving all but seven fuel plates completely "unswaged" for 33 days, which is the equivalent of one full operating program.
At the end there was only very slight damage to two fuel plates, he says.
Critics
Under the return-to-service approval ARPANSA chief, Dr John Loy has required ANSTO to develop a program to more fully understand the vibrational and other forces acting on the fuel plates; review the design of the modified fuel assemblies within two years; and regularly test the longitudinal strength of the fuel plates.
Nuclear campaigner for Friends of the Earth, Dr Jim Green, says many critics of the Lucas Heights reactor would have preferred it was never turned back on.
Dr Green, who completed his doctorate on medical isotope supply options, says Australia has no need to manufacture its own medical isotopes as there is surplus supply worldwide.
He says research and development funds should instead be directed towards developing a cyclotron facility that can also produce the necessary isotopes without the safety risks and nuclear waste issues.

Note: OPAL fuel is 19.7% enriched, is U3Si2.