Sunday, 21 September 2014
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A look inside the Dounreay Fast Reactor

The first images from inside the Dounreay Fast Reactor since it was constructed in the 1950s have been obtained. In 1955, The Engineer reported on the concepts behind the reactor, and how it was built to be safe — but not to be dismantled.

Dounreay is one of the strangest places in Britain. It’s literally on the edge of things, isolated in bleakly beautiful landscape on the Caithness cliffs at the extreme north of Scotland, where the mainland starts to fray into the islands of the Orkneys, the 1950s high-tech of its buildings and technology make it seem more like something from science fiction than reality: a James Bond villain’s lair or an early Doctor Who view of the future. It seems like a future which didn’t happen, although as we explain in this issue, breeder reactors whose design is partly based on the Dounreay facilities may be about to return to the UK to handle the country’s nuclear legacy.

The decommissioning of the breeder reactors of Dounreay has now reached a fascinating stage: the engineers have inserted nitrogen-cooled robotic video cameras into the first of the breeders to become functional — the Dounreay Fast Reactor, which is housed within the site’s most iconic structure, the ‘golf ball’ dome. The video cameras are providing the first images for over half a century of the internals of the reactor: ghostly pictures of the breeder elements which are jammed in place, their steel swollen by the neutron radiation, a problem unanticipated when the reactor was built.

Inside DFR

The remote cameras show the swollen and stuck breeder elements inside DFR

The Engineer reported on the early days of the Dounreay Breeder project back in 1955, with an article contributed by two of the senior engineers, JM Kendall and TM Fry. In it, they explain the thinking behind the project: that breeding a fissile fuel — plutonium — by exposing non-fissile uranium-238 to the high-energy neutrons emitted by fission of uranium-235 is a more efficient way to use uranium than to separate out the U-235 and discard the rest. ‘The cost of fissile material is measured in thousands of pounds per kilogram,’ they say, ‘so the initial charge represents a considerable capital investment. If it is to be used as a catalyst for deriving electrical power from U238, each kilogram of fissile material must yield hundreds of kilowatts of electricity. We cannot consider the fast reactor an economic source of power until ratings of this order have been demonstrated.’

In the end, economics was the end of Dounreay. The end of the nuclear arms race in the 1980s caused the price of fissile uranium — which had been in demand for making weapons — to fall, and it no longer made sense to breed nuclear fuel.

Building the DFR was always seen as an experiment. ‘No engineering experience is available on which to design a core with the rating, outlet temperature and burn-up required,’ Fry and Kendall said; the best way to proceed was therefore to design the most flexible plant possible. ‘As failures are replaced by better components, it is hoped to demonstrate that a fast reactor can be a sound economic proposition for the production of industrial power.’

DFR

The DFR reactor vessel is lowered in through the sphere

Material choice and construction methods were chosen carefully, and based on those used in highly-radioactive chemical plants which used concentrated nitric acid and had to be made leak-proof. The reactor vessel itself — referred to as ‘the pot’ — along with the primary and secondary coolant circuits, were made from stainless steel which was butt-welded and the welds radiographically inspected along almost their entire length. Sodium-potassium alloy (NaK) was chosen for both coolant circuits, as it could handle the heat from the nuclear reaction; the secondary coolant, which transferred the reaction heat across an exchanger into water to raise steam, had to be chemically inert to the primary coolant, so it was decided to use the same material.

The article lists the many safety precautions which were built into the reactor. The golf ball, 135ft in diameter and an inch thick, was designed to contain any fission products which might escape if the reaction vessel were breached. It was also designed to contain the effects of a sodium fire, in the event of a reactor accident which evaporated the coolant. In this event, Kendall and Fry say, all the oxygen in the sphere could be consumed, creating a partial vacuum inside and allowing the atmospheric pressure outside to begin to crush the sphere. ‘The most pessimistic estimate is that, at 5lb per square inch external pressure, a shallow dimple some 18ft in diameter would appear at the top,’ they said. ‘This would not raise stresses in the steel greater than 1dwt per square inch’.

DFR Sphere

The Golf Ball sphere under construction

Nowhere in the article do the authors mention decommissioning the reactor at the end of its life — an omission which had made the task at the plant such a difficult one. DFR closed down in 1977; its successor, the Prototype Fast Reactor in a neighbouring building, closed in 1994. You can read more about the project to decommission the site here. Most of the DFR’s fuel, apart from one rod, and almost all of the NaK has now been removed from the reactor and destroyed by reacting it with water vapour in a purpose-built plant inside the sphere, but 977 breeder elements remain. The elements remain radioactive, and the decommissioning engineers are now trying to work out how to remove the swollen, split metal safely.

‘Being able to see inside the reactor for the first time in half a century is a historic moment and a milestone in the DFR decommissioning programme,’ commented DFR senior manager John Smith. ‘The images and information captured are now enabling the Dounreay team to prepare accurate plans for the safest and most efficient approach for the removal of the remaining fuel and disassembly of the vessel’s internal structures.  They are also assisting in the detailed planning now underway for the cleansing and/or destruction of the residual NaK remaining in the pipes and vessels of the DFR.’

Once the reactor has been disassembled and removed, the sphere itself will be demolished; this is scheduled to happen in 2025. At that point, 70 years after Kendall and Fry explained the project, the Caithness landscape will return to its pristine state.


Readers' comments (4)

  • I remember the Dounreay Fast Breeder Reactor programme (to give its original name) most clearly. It was exciting times for the Nuclear Industry, and helped prompt me to take the Physics route (put off at University by a disillusioned Tutor - and I won't forgive him for that - his comments were completely wrong !).

    It is such a shame that long term Vision could not predict the future for Dounreay, also that Energy-Efficient Fusion has proved to be such a difficult goal to attain.

    But that is what comes through Innovation. Would we, for example, have any of the modern so-called high-tech commercial devices if it were not for (a) the Space Race and (b) the Cold War ? I sincerely doubt it.

    In a way, the Internet Millionnaires owe Science most of their Profit. But these 'get rich quick' people care little for the background that made their wealth possible, and once again, the Taxpayer has to 'Foot the Bill' for such as the De-Commissioning of Dounreay FBR.

    As a Taxpayer, Scientist, Engineer (semi-retired) and long-time Observer of Human Cultures, I would be delighted if some Taxes from the later Tiers who have benefited was paid into a Science Research, Development, Production, Operation and De-Commissioning Fund.

    We have probably less than 10,000 years before the next Magnetic Pole 'Switch' occurs, and at that time, it is conceivable that the Human Race might become extinct - if we don't "De-Planet". To be able to achieve "De-Planetisation" and "Colonisation" of another world will require MASSIVE Funding and Sciences and Technologies to be considerably advanced.

    Fifty years ! And it seems like only yesterday (to me). So something like 200 Dounreay Lifetimes are left to us !

    Let us remember just how far we have come in only 200 years. We CAN do it. And Dounreay stands, to me, as a symbol of our Courage and Capability.

    I care not for the Critics - Hindsight is easy. Let us learn from our Great Enterprises, and improve our Foresight.

    P L Hartley, MSc, CAET, MInstP, MIET

    P.S. RIP, Dounreay - and Good Luck to the beautiful Caithness coast (for the next 10,000 years, I hope).

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  • A fascinating article and certainly a tribute to those who would look forward beyond their lifetimes. Mr Hartleys' critique says it all. Hopefully the government will support the need to concentrate resources on the "STEM" subjects as previously reported in The Engineer and that we can aspire to further great enterprises.

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  • Dounreay and Sellafield are damining testaments to the indifference the nuclear industry had to human health. These people knew the health risks and potential environmental damage ..and ignored them....Cockrofts filters turned out to be anything but a "folly" on pile 1...our nuclear scientists and engineers should be posthumously disgraced for knowingly leaving such a toxic leagacy.

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  • Have I not read that amongst Scottish academics those trained and operating in the sciences, technology and Engineering are almost 100% against the split [rather patronizingly the 'meja' suggested that this was because they applied logic, rational analysis, economic models and so on] whilst those of alternative persuasions 'the Arts' -who being superior to us simple souls- have allowed their hearts to overcome common sense. So nothing new there. I suppose as usual 'we' will have to pick-up-the pieces.

    Mike B

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