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Cube crusher set to squeeze nuclear costs

THE UK NUCLEAR industry is set to save over £20M during the next decade thanks to pioneering concrete research being undertaken at Sheffield University.

The university's civils department is developing a unique triaxial cube crusher to simulate the extremely high temperatures and pressures around reactor cores which will allow scientists to test the structural condition of the concrete jackets.

Chairman of the nuclear industry working group on power station safety Dr Michael Johnston told NCE: 'With this concrete inaccessible during use, we analyse the safety case using models based on finite element codes which do not fully allow for possible extremes of temperature and pressure. This vital research should help us provide a much more robust safety case for our ageing advanced gas cooled reactor power stations and help extend their working life.'

Codenamed mac2ET, the crusher is central to a £600,000 research project funded by the nuclear industry to establish the safe residual life of 30-year-old plus concrete protection at the heart of reactors.

Fatigue loading of ancillaries to Britain's ageing nuclear plants such as the pipes and ducts of the early AGRs have been the focus of attention so far in justifying extending the life of plants.

But there is little knowledge about the performance that might be expected from concrete buried deep inside the shielding, according to Sheffield pro vice chancellor Professor Peter Waldron. Samples cannot be withdrawn without compromising the integrity of structures and insitu testing is not practical.

Even before it has been completed, Sheffield's giant triaxial crusher is booked solid for a year of testing aimed at validating mathematical models being developed to analyse the behaviour inside these nuclear cores, says Waldron.

Every 10 years, Britain's seven operational AGRs, several completed in the early 1970s, must be subjected to an exhaustive safety analysis including numerous 'what if' scenarios. It is under these largely theoretical conditions that a rise in the normal pressure and temperature regime inside the prestressed concrete containment must be considered. And it is here that current mathematical modelling has its shortcomings.

'Extreme conditions could push the concrete beyond elastic limits, not fully considered in current codes, and the Sheffield tests will allow us for the first time to guarantee radiological safety during several of the more extreme 'what if' cases,' said Johnston. 'Just one day's shutdown of an AGR costs us over £500,000 and this research should save us many millions of pounds in ensuring continued life.'

The triaxial machine is designed to apply up to 350t force on each of three axes at right angles to the faces of a 100mm concrete cube and at the same time heat the specimen to 450C. The hefty triangular concrete base slab for the rig was cast in the laboratory last week.

Metal components for the massive reaction beams are being fabricated locally in the steel city. Final assembly and the first testing of specially cast cubes is scheduled for late May.

Critical technical problems which had to be overcome in design of the machine by Sheffield University involve allowing the loading rams on each axis to float laterally as the cube deflects in response to the other applied loads and the heating elements. Lasers are directed down the hollow axis of each loading ram to measure the deflections.

Mike Winney and David Hayward

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