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Off the north-east coast in the North Sea stand the towering rigs that drilled for oil and gas. Many of these structures are at, near or beyond the lifetimes they were designed to last, giving the country a difficult choice: carry out engineering work to allow them to continue for longer, or take them out of service, dismantle them and find a safe way to dispose of their components. In the case of the nuclear technologies, a complicating factor is that they were not built with dismantling in mind.
Gaps in knowledge
In some cases in the nuclear sector, science when facilities were built had gaps. When the earliest reactors were built, it was not known that the neutron radiation resulting from nuclear fission would harden and embrittle steel and make it swell. Some effects of neutron irradiation of graphite were known in the 1940s, but are still not well understood more than half a century later, which caused problems when the graphite core of the Windscale AGR at Sellafield was dismantled in the 1990s.
Design deficiencies
Other legacy issues from early reactors included removing the liquid-metal coolant – a mixture of sodium and potassium – from breeder reactors at Dounreay on Scotland’s north coast which had been used to transmute uranium into plutonium. The original engineers did not design in any system to drain the liquid: a valve system would have been useful, but no such tap existed.
One of the world’s most important sites for developing technology to decommission nuclear facilities is RACE (Robotic Applications in Challenging Environments), a centre operated by the UK Atomic Energy Authority at Culham in Oxfordshire. RACE develops and tests robots, from enormous arms designed to grasp and move large and heavy components, to mobile crawling and walking robots that can work on and move smaller equipment.
RACE develops robots to work in the irradiated conditions of conventional nuclear fission reactors and handle the radioactive waste products, but in recent years much of its work has been concerned with nuclear fusion, as the largest experimental fusion reactor, JET (Joint European Torus), is also at the Culham site, and its work is informing the construction of ITER (International Thermonuclear Experimental Reactor), a much larger reactor modelled closely on JET, being built near Marseilles. A great deal of the building of ITER uses robots, and RACE has been developing these.
JET operated for 40 years, mostly researching how to handle plasma compressed and accelerated by magnetic fields with no nuclear reactions, but late last year it ran its last set of reactions using a plasma of deuterium and tritium, achieving the longest-ever sustained fusion reaction of 5.2 seconds. Now JET is no longer in use, the first-ever decommissioning of a fusion reactor is about to start, a process that will take 12 years.
IMechE's Managing Ageing Nuclear Assets and Decommissioning 2024 conference takes place 22-23 May in Manchester. Find out more and book now.
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