Reactors

Background - June 27, 2006
All commercial nuclear reactors of the present generation are based on designs originally developed for military plutonium production or for the propulsion of nuclear submarines or other navy vessels. Although there are different ways of arranging the fuel, coolant and moderator in a reactor, they are no more than a variation on a theme. Nuclear power stations are glorified steam engines with basic designs dating from the 1950s.

No reactor is safe.

At the end of 2005 there were 443 nuclear power reactors, operating in 31 countries. The age, size and design type of all of these reactors varies considerably, but it is safe to say that all of them have very serious inherent safety flaws which cannot be eliminated by safety upgrading.

There is general consensus that the extension of the life of reactors is of the foremost importance today for the nuclear industry. Over the last two decades there has been a general trend against ordering new reactors. As a consequence, the average age of nuclear reactors around the world is increasing every year and is now 21.

At the time of their construction it was assumed that these reactors would not operate beyond 30 or 40 years. However, in order to maximise profits, lifetime extension offers an attractive proposition for the nuclear operators despite the danger. As the world’s nuclear power plants get older, there are efforts to play down the role of ageing. Those efforts include conveniently narrowing the definition of ageing.

Reactor types

The most prevalent reactor design in operation is the Pressurised Water Reactor (PWR), with 214 in operation around the world. It was originally conceived to propel military submarines, so the reactors are relatively small, but with a high-energy output. Consequently, the cooling water in the reactor’s primary circuit is at a higher temperature and pressure than other comparable reactor designs. These factors can accelerate the corrosion and cracking of components; in particular, the steam generators now frequently have to be replaced.

The most serious example discovered to date occurred at the Davis Besse reactor in Ohio, US. In this case the cracking had been allowed to continue expand for around a decade, despite routine checks. When discovered the crack had penetrated through the 160 mm thick pressure vessel with only the 5 mm steel lining of the vessel-which was bulging from the pressure - stopping a breach of the primary cooling system, the most important safety barrier.

Of similar design and history to the PWR is the Russian VVER reactor. There are currently 53 of these reactors deployed in seven countries in Eastern Europe in three main reactor designs. The oldest, VVER 440-230, has significant and serious design flaws, consequently the G8 and EU believe that they cannot economically be brought up to an acceptable safety standard. The lack of a secondary containment system and adequate emergency core cooling system are of particular concern.

The second most prevalent reactor design is the Boiling Water Reactor (BWR) (there are 90 in operation around the world), developed from the PWR. Modifications were
undertaken to increase the simplicity of the design and create higher thermal efficiency byusing a single circuit and generating steam within the reactor core. However, this modification has failed to improve safety. The result is a reactor that still exhibits most of the hazardous features of the PWR, while introducing a number of new problems.

The next most prevalent reactor currently deployed is the Pressurised Heavy Water Reactor, of which there are 39 currently in operation in seven countries. The main design is the Canadian CANDU reactor, which is fuelled by natural uranium and is heavy water cooled and moderated. The use of natural uranium significantly increases the volume of uranium in the core, which can lead to instabilities. The pressure tubes that contain the uranium tubes are subject to significant neutron bombardment. Experience in Canada has shown that they subsequently degrade and that expensive repair programmes have had to be undertaken, in some cases after only 20 years of operation.

The other design serialised in Russia was the RBMK reactor, which is a graphite moderated boiling water reactor and used at the Chernobyl station in Ukraine, which was the site of the world’s worst civilian nuclear power accident in 1986. The fundamental design flaws of these reactors have lead to the international community classifying these reactors as ‘non-upgradable’ and seeking their closure. Closure has occurred or will occur in Lithuania and Ukraine, but despite this, in Russia, efforts are underway to extend the lives of these reactors rather than retire them early.

The UK has developed from the plutonium production reactors two designs - the

Magnox - air-cooled, graphite-moderated naturaluranium reactor – and subsequently - the

Advanced GasReactor (AGR). The Magnox reactors with older steel pressure vessels have suffered from corrosion. These problems are aggravated by thermal ageing and material degradation caused by neutron-induced embrittlement. Brittle failure of the pressure vessel could lead to total loss of the primary coolant, and possibly large radioactive releases. For this and other reasons, a number of Magnox stations have already been shut down. Both reactor types have a high potential for large radioactive releases.

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