Open Engineering
Environment and Development.

Reactor Risks

Following the disaster at the Fukushima nuclear power plant in Japan, what is the safety of the UK nuclear plants. While we do not expect earthquakes or tsunamis in the UK (though a tsunami has occurred in the past in the Bristol channel), designers of any plant will try design it to withstand the failures that they think are likely: the problem is that unlikely events do occur, witness the volcano that grounded much of Europe's air traffic in 2010 and the Buncefield Oil Depot explosion in 2005 (the largest explosion in Europe since the war). The greatest risk probably comes from the spent fuel which is stored outside of the reactor and must be kept safe thousands of years into the future. As of 2007, the United States had accumulated more than 50,000 metric tons of spent nuclear fuel from nuclear reactors.

Reactor Types

The reactor at Fukushima is a Boiling Water Reactor (BWR). BWRs are one of two types of Light Water Reactor (LWR), the other being the Pressurised Water Reactor (PWR). Most UK reactors are not LWRs but a British design which is gas cooled. However worldwide LWRs are the most common, and starting with Sizewell B, Britain has decided to build LWRs in the future.

LWRs suffer from a safety risk which was identified 40 years ago; See this 1972 article from the New Scientist on Walter Patterson's website : Walter Patterson. The problem is also extensively discussed in Walter Patterson's classic Pelican book "Nuclear Power" published in 1976.

Summary of the LWR Risk

These details are paraphrased from the above article and from wikipedia's entry on the PWR.

In the core of an operating reactor heat comes from:

Emergency shut-down arrangements, can bring the fission chain reaction 'a' to a swift halt; but the heating from 'b' the decay of the intensely radioactive fission products, cannot be stopped. If a broken pipe were to allow the cooling water to escape from the core of a light water reactor, the fission product heating would cause a rapid temperature rise which could lead to an uncontrollable meltdown accident, with appalling consequences. Accordingly, both forms of light water reactors have emergency core cooling systems, but it is difficult to know if these will work in a real accident as their design relies mainly on computer models not on real experiment.

The fission products continue to generate decay heat at initially roughly 7% of the reactor's full power level, which requires 1 to 3 years of water pumped cooling. If cooling fails during this post-shutdown period, the reactor can still overheat to above 2200 degrees centigrade where separation of water in to its constituent elements Hydrogen and Oxygen occurs. In this event there's a high danger of hydrogen explosions, threatening structural damage and/or the exposure of highly radioactive stored fuel rods in the vicinity outside the plant in pools (approx 15 tons of fuel is replenished each year to maintain normal PWR operation).

Could a Disaster Happen in the UK?

All nuclear reactors contain a toxic brew of highly radioactive fission products so while there is any risk that this material can escape, no operating reactor is 100% safe. Britain's most modern reactor Sizewell B, is a PWR and thus shares the same risk as the Fukushima reactors had of meltdown should cooling fail. The design solution to this is to install various safety systems and carry out a 'Probabilistic Safety Study' to estimate the risk of a core meltdown - it claimed that the risk was less than once in a million years. An Australian statistical journal published a detailed analysis of this study and concluded "that there is no reason at all for me to accept their final probablility values as having any value". It also comments tellingly that the probabilities calculated are not compared to the behaviour or real-world PWRs and reactor accidents. [Reference: Probabilistic Risk Assessment for the Sizewell B PWR (PDF) by T.P. Speed]

Some of the latest reactor designs are claimed to have very few things to go wrong. In that context it is interesting to note that at Sizewell B "the pipework alone would stretch from London to Birmingham, and the cables used simply for instrumentation and control from London to Brighton. Nearly 37,000 valves act to control the flow of the coolant water and safely shut off the reactor in the event of a malfunction." Independent 31.1.1995] Clearly there is very little to go wrong!

The UK Doesn't Have Earthquakes, So What Could Go Wrong ...

But it does have seaweed, floods, unstable shingle beds, terrorists and all sorts of cock-ups! In 2006, Torness nuclear power station near Edinburgh suffered a complete blockage by seaweed, resulting in supplies of main cooling water being lost for a period; in 1999 a RAF Tornado crashed into the North Sea less than 1 km from the power station following an engine failure. In 2002 NUCLEAR inspectors considered a safety review of Sizewell B nuclear power station after corrosion was found to have almost eaten away the vessel that holds the hot nuclear heart of a related American nuclear reactor. In 2010 Sizewell B was shut down over 6 months for emergency repairs inside the reactor building. During the shutdown it also caught fire: about 50 firefighters were at the scene for seven hours. Dungeness power stations are built on open shingle; a fleet of lorries is used to continuously maintain shingle sea defences for the plant as coastal erosion would otherwise move shingle away at an estimated rate of 6 m per year. Bradwell nuclear power station in the UK leaked radioactive effluent for 26 years. In 2009, a nuclear leak which could have caused a disaster at the Sizewell-A nuclear power station in the UK was only averted by chance.

The Next Reactors - Better or Worse?

The new reactors planned for Britain will be also be LWRs of the pressurized water reactor (PWR) type, so they will have the same problem of meltdown if coolant is lost as the reactors at Fukushima have. The manufacturers argue that they have put in more safety features to prevent coolant loss and to contain the nuclear material within the reactor should a meltdown nevertheless occur. One of the designs is called EPR (Europe European Pressurized Reactor) and has been developed mainly by Framatome (now Areva NP), Electricité de France (EDF) in France, and Siemens AG in Germany.

As of 2010, four EPR units are under construction. The first two, in Finland and France, are both facing costly construction delays. Construction commenced on two additional Chinese units in 2009 and 2010.

A key development goal was to "To further improve economic performance" (i.e. make it cheaper). To this end, the reactor features:"

At the same time the manufacturers claim improved safety:

What can we make of these claims? The only one that can be verified at the moment is the 'Shorter construction period of 48 months'. Four EPR units are under construction. The first two, in Finland and France, are both facing costly construction delays. Construction commenced on two additional Chinese units in 2009 and 2010. Work began on the Finnish Olkiluoto EPR in 2005, but is now at least three and a half years behind schedule and more than 50 percent over-budget.

The claims of improved safety make it plain that the risk of meltdown is still there. We are unlikely ever to know whether the measures are effective in practice until an accident occurs because full scale tests are too expensive and dangerous to carry out, i.e. they have not put a reactor into meltdown or crashed a jumbo jet into one.

The Higher fuel utilization claim has been examined and appears DANGEROUS. It relates to 'High Burn-Up Fuel' which means that more-enriched uranium is used as reactor fuel to increase burn-up rate. The fuel is left in the reactor for longer and becomes hotter and more radioactive than conventional spent fuel. This preseumably increases the amount of emergency cooling required if the reactor is shut down. [Reference: High Burn-up Radioactive Spent Fuel (PowerPoint) by Dr Paul Dorfman of the Nuclear Consultation Group]

High Burn-Up Fuel increases the problems of managing the waste fuel once it is removed from the reactor:

Richards (2008) from the UK based Nuclear Consultation Working Group that fuel produced by the EPR reactor “is more demanding at every stage of the nuclear cycle from the reactor itself, subsequent cooling in ponds, through drying and storage in dry casks to eventual burial. It will increase potential worker and public exposure to radiation." Furthermore, the uncertainties about management and long-term storage of high burn-up spent fuel are so large, that allowing its generation "would expose the future taxpayer to the risk of huge uncovered liabilities."

Finally we might note that many pundits have commented that the new reactors will be safer than the Fukushima reactors, essentially because they are 'new' 'modern' technology, not the primitive systems installed at Fukushima 40-years ago. This sits oddly with the claim of a 60-year lifespan for the new EPRs! Everything was 'state-of-the-art' once and the the digital instrumentation & control systems installed in the EPR will probably be a maintainance nightmare in 50 years time when its designers are long retired or dead.


New Reactors Work DifferentlyAll fission reactors work the same way: 80 tonnes or so of uranium or plutonium fuel generate heat by nuclear fission and become highly radioactive.FAIL
New Reactors can't fail the way Fukushima didPWRs and EPRs have the same weakness as Fukushima - if the cooling fails they will melt down.FAIL
New Reactors are saferEPRs have added safety mechanisms to compensate for the higher pressures, radioactivity levels and output, and to try to contain the core if it does melt. Generally these systems have not been tested experimentally.DOUBTFUL
New Reactors won't meltdownThe probability of meltdown has been calculated with mathematical models instead of basing it on real reactor experience. The latter suggests accidents are many thousands of times more probable than is claimed.DOUBTFUL
New EPR Reactors produce less wasteOnly because they burn up the fuel more, producing more intensely radioactive waste which is harder to handle though less in volume.FAIL
The waste problem is solvedOn the contrary, EPRs will make it worse with intensely radioactive waste that must be cooled for a century before it can be placed in longer term storage. No permanent storage site suitable for the hundreds of thousands of years that the waste must be kept, has been created.FAIL

UK Fallout Map - This interactive map shows how fallout from different reactors could affect the UK. Sizewell B and Dungeness are only 60 miles from London.