Firstly, this is not necessarily a blog about Building Science, but was interesting enough to post up.
Schmieman, a civil engineer from Pacific Northwest National Laboratory in the US, is currently senior technical advisor on what might well be one of the challenging and impressive engineering projects in the world today, the €1.5bn international effort to clean up the remains of mankind’s worst nuclear accident.
Known as the Shelter Implementation Plan (SIP), the project is funded through the European Bank for Reconstruction and Development (EBRD) by 46 different countries and organizations
But the SIP’s crowning glory – is, without a doubt, the construction of the New Safe Confinement (NSC) – an immense steel arch, designed to last for 100 years, which will protect the sarcophagus from the elements, and enable engineers to safely and methodically tidy up a nuclear legacy that has troubled the world for the past 27 years.
Standing 110m high, 250m wide, 150m long and weighing in at 30,000 tons, the NSC is currently being assembled 600m away from the damaged reactor where, thanks to the remediation work of the past two decades, the relatively low ground-level radiation dose levels enable engineers to work for up to 40 hours a week. If all goes to plan, at some point in 2015 the shelter will briefly become one of the largest moving structures on land as engineers begin carefully sliding it along vast tracks to its final resting place.
Given that Ukrainian authorities have deemed the 30km exclusion zone around Chernobyl won’t be completely safe for another 20,000 years it’s perhaps surprising to learn that it’s safe to work at the site. But according to Schmieman the dose levels in the so-called “erection zone” are lower on the ground than those that he would experience in his office back in Washington state, where the local geology is responsible for far-higher doses of background radiation.
Nevertheless, Chernobyl remains an exceptionally hazardous place. Many areas within the facility are completely sealed off, and dose rates close to the reactor are extremely high. ‘The hazard increases as you get closer to the source and it’s a deadly hazard if you get close enough,’ said Schmieman. Little wonder that a remote assembly technique was viewed as so attractive.
Work is now well under way on the structure. Last November, the first segment of the arch was lifted to an interim height of 22m and with the end of the harsh Ukrainian winter now in sight, Chernobyl’s international team of engineers is preparing to return.
This spring, further sets of legs will be added until half of the arch is complete. Work will then begin on the identical second half of the arch, and ultimately the two halves will be bolted together.
With the shelter designed to remain in place for at least 100 years, a host of monitoring and control systems will help to keep it safe and reduce the need for regular maintenance. And Schmieman’s team has devised a particularly elegant solution to the problem of corrosion.
‘There are quite a few steel structures in the world that have already lasted 100 years,’ he said. ‘The one that comes instantly to mind for most people is the Eiffel Tower. They’re continually repainting the Eiffel tower because that’s the cheapest thing to do, but we can’t do that because of the hazard of the radiation dose rates up close to the reactor.’
Instead, the outside of the shelter’s tubular steel structure is protected from corrosion by an air-conditioning system that circulates through the 12m space between the shelter’s tubular steel structure and its stainless steel cladding.
This air is blown into the gap through large desiccant dryers, which remove moisture for the air and maintain it at less than 40 per cent humidity, a condition under which carbon steel will not corrode. This system recirculates around 45,000 m3 of air per hour, at a pressure around 50 Pascal’s higher than the outside air to prevent areas of stagnation developing inside.
The system also heats the air in the annular space to a temperature around 3°C warmer than the air inside the shelter. Incredibly, the NSC is one of a handful of buildings that will enclose a volume of air large enough to create its own weather. But by maintaining a temperature difference between the upper surfaces and the air within, condensation, and the prospect of “rain” falling on the shelter’s radioactive contents, will be avoided.
Note – See corresponding video on other blog posts
Excell, Jon. “Building Chernobyl’s New Safe Confinement.” The Engineer. Centaur Communications Ltd, 11 02 2013. Web. 25 Nov 2013. <http://www.theengineer.co.uk/in-depth/the-big-story/building-chernobyls-new-safe-confinement/1015479.article>.