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Any old iron?

Energy - Political indecision over the building of new nuclear reactors means UK engineers must give extra life to existing plant. Jon Young reports

Fears of an energy crisis are growing. Fossilfuelled and nuclear power plants are nearing the end of their lives and investment in a new generation of reactors remains far from guaranteed.

To try and bridge the gap - albeit temporarily - a bunch of very clever people are spending a great deal of time in the UK's largest commercial radiological laboratory.

Hidden away on the outskirts of Warrington is Amec's Birchwood Laboratories, the home of the largest collection of PhD radiation chemists in the country - six.

Amec NNC (the former National Nuclear Corporation, bought by Amec in 2005) offers a consultancy service to the energy industry. Chief among its tasks is to examine what happens to the materials used in a reactor when the life of a nuclear plant is extended.

Oxidation is the key degrader of metals inside a reactor.

When CEGB - now British Energy - built its Advancedcooled Gas Reactors (AGR) it sent samples of the materials used to Birchwood Laboratories.

NNC took the samples and using autoclaves, commonly referred to as 'top hats', mimicked the conditions found in the reactors.

This was achieved by creating a pressure of about 40bar inside the autoclaves, filling them with carbon dioxide and heating them to between 400°C and 500°C.

Every six to 12 months the samples are removed and weighed. The process of oxidation adds weight, the increase in which gives an indication of the level of oxidation and therefore the rate of degradation.

'The original thinking [behind the research] was probably not life extension but to give an advance warning of how things will behave, ' says Amec NNC technology services manager Mike Smith.

Ten different materials are being tested in the autoclaves.

The core of a nuclear reactor is made up of 2,000 to 3,000 hexagonal graphite bricks penetrating the reactor floor.

In the centre of each brick is a circular hole. In an alternating pattern either fuel strings or control rods are dropped into the holes.

When a chain reaction is under way neutrons bounce from fuel stringer to fuel stringer creating immense heat to power electricity generating turbines.

This is regulated by lowering heat-control rods, made of a neutron-absorbent material such as boron, into the reactor, thereby slowing down the reaction.

As the graphite blocks are repeatedly heated and cooled, cracks can form. If two cracks form on either side of a single brick then it may shift and block the control rod.

Cracked bricks have been spotted at several reactors recently during scheduled maintenance shutdown periods, raising concerns about life expectancy.

Amec NNC has been able to calculate the number of damaged bricks that a reactor can lose and still function.

But it is still working on an inspection method that could detect the extent of cracking remotely.

Among the gadgetry at Birchwood is a replica 10m long stainless steel control rod with deformed graphite bricks into which it can be dropped.

A cell load sensor is fitted to the end of the control rod and this measures the force required to lift out the rod.

This gives an indication of the severity of the deformity in the bricks.

The project has been running for a few months and will be completed early next year.

British Energy commissioned the research the investigate the value of fitting the sensors to all its control rods.

On a sister project, cracks, are being measured by fitting a cell load sensor to one end of a fuel stringer.

A wire brush is fitted to the other end. The theory is that as the stringer is removed, parts of the brush may catch on the cracks giving an idea of the presence and extent of the cracking.

This will causea spike in the force needed to remove the fuel stringer, and this can then be measured by the sensor.

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