To begin, let’s consider the history of nuclear accidents around the world.

The British Windscale Reactors

The British Windscale reactors were military reactors, built solely to produce material for atomic weapons. Their initial purpose was to create Plutonium-239 from Uranium-238. As the reactor was operated, cartridges of mined uranium were inserted into the front of the reactor and slowly pushed through, falling out the other side. The plutonium could then be extracted. Britain used this plutonium to build an atomic bomb. Schematic drawing of a windscale reactor

Schematic drawing of the windscale reactor Schematic drawing of the windscale reactor

Technology then advanced and the need to develop thermonuclear weapons arose. This required a different atomic isotope known as tritium. So the British added cartridges containing lithium (to produce the tritium) amongst the uranium cartridges in the Windscale reactors.

Diagram showing disaplacement of atoms in graphite in a nuclear reactor Diagram showing disaplacement of atoms in graphite in a nuclear reactor

The body of the nuclear reactor was made from a large block of graphite. When graphite is exposed to a large number of neutrons (which are present in the processes going on inside the reactor), the graphite atoms get knocked out from their regular pattern. Over time, this dislocation gradually builds up energy within the graphite. The British nuclear scientists realized that this stored energy could cause problems so they devised a protocol called annealing to allow the reactor to overheat, releasing the energy in a controlled manner. The problem was that the reactor was never designed for this process, so the temperature monitors were not in the right positions and did not have a high enough range.

On one occasion, an annealing process was started and the reactor began to heat up. The problem was that the reactor heated up unevenly, which went unnoticed due to the sparse spacing of the temperature monitors. A small area of the reactor overheated, rupturing one of the lithium capsules - which started to burn violently. This fire spread quickly to the graphite core (graphite is essentially a form of coal), setting the centre of the reactor on fire. A drastic solution to the problem was sought; the decision to flood the reactor with water was made. This may sound like a sensible solution, but in the extreme temperatures of a nuclear reactor it is explosive. An official investigation placed the blame squarely on the operators and neglected the political pressures contributing to the accident, which rather angered the operators. All the while, radioactive particles (soot, smoke and bits from the burning reactor) went up the chimney. Fortunately, the majority of these were caught by filters placed at the top of the chimney.

Three Mile Island, USA

The United States experienced its own nuclear accident in 1979 at Three Mile Island, essentially caused by three factors: 1. Insufficient/improper instrumentation, 2. Poor communication and 3. A lack of failure planning on behalf of the reactor designers and operators.

Three Mile Island pressurised water reactor

Three Mile Island (TMI) used a Pressurized Water Reactor (PWR). The reactor produced heat that was transferred to water coolant kept under extremely high pressure so that it wouldn’t boil. To keep the reactor coolant at these high pressures a special piece of equipment is added into the plumbing loop called a pressurizer, a metal cylinder which is heated and cooled to maintain a constant pressure inside.

It was the pressurizer that ultimately caused the accident at TMI. It sometimes had to release excess steam pressure through an automatic valve. On the day of the accident this valve opened up and when the pressure decreased the valve should have closed, but the valve had, in fact, jammed. There was no sensor in place to detect a jam, so the operators thought it was closed. The open valve caused water to be lost from the reactor. The operators were unaware of this as well. As the reactor core was exposed, it started to heat up beyond the normal temperature range. The nuclear fuel was held inside tubes made from Zirconium, which at high temperatures acts as a catalyst separating water into hydrogen and oxygen. This hydrogen gas can build up and cause an explosion that can rip a reactor apart, spreading radioactive material everywhere. At TMI, as the reactor heated up and became uncovered by the water, it started to melt. A nuclear melt-down is where the reactor core melts, flows down and forms a lump of metal at the bottom of the reactor vessel.

Chernobyl, USSR

The accident of 1986 at Chernobyl Reactor 4 in Ukraine (then in the Soviet Union) was a textbook case on how not to build and operate a nuclear power station. The reactor suffered from several design flaws which made the reactor hard to control, and increased the severity of any accident scenarios.

The reactor was undergoing a test at the time of the accident, a test to check emergency procedure. The reactors required cooling even when they had been shut down, and the test was to see how effective back-up mechanisms were. After the test, the control rods were lowered. Control rods are made of a neutron absorbing material, however the rods in the chernobyl reactor had tips (for the first meter) made of a material that increased the rate of reaction (for design reasons). A runaway chain reaction started, heating the reactor more. The heat warped the tubes the control rods passed through and they became stuck a third of the way in. The extreme heat caused hydrogen gas to form and build up. This gas ignited and blew the reactor open - there was no pressure containment in place to prevent this. The graphite in the reactor caught fire and sent radioactive material into the atmosphere - and across most of Europe.

Fukushima, Japan

Diagram of the Fukushima Daiichi power plant

The Fukushima Daiichi site was originally 35 meters above sea level, but during construction in the late 1960s this was lowered to only 10 meters. This was done for two reasons: firstly this allowed construction directly onto the bedrock to protect against earthquakes and secondly, to reduce the costs of pumping seawater coolant to the reactors. It was deemed that the 5 meter high seawall would be sufficient to protect against a tsunami, despite a 6 meter tsunami hitting the area 10 years previously.

The 2011 tsunami was 13 meters high. Initially, when the magnitude 9.0 earthquake hit, all of the reactors shut-down automatically and the cooling systems activated to keep the reactors at safe temperatures. Then the tsunami hit: it damaged the power lines connecting the site to the national grid which meant that the power to the cooling systems was cut. The tsunami also flooded all of the basements of the reactor and surrounding buildings. The back-up diesel generators were situated in basements of buildings by the seafront - all were flooded and damaged. Power was also lost in the control rooms, so operators were unable to control electronic systems. The reactors lost cooling and started overheating - eventually this caused several to go into meltdown. The earthquake cracked cooling ponds holding spent fuel, this also started to overheat and could have caught fire. The dramatic explosions seen were the result of hydrogen gas, formed by the superheated steam in the reactors separating into oxygen and hydrogen.

Some comments on nuclear waste

At Sellafield (a nuclear fuel reprocessing and nuclear decommissioning site in Cumbria) the worst of the nuclear waste is added to sand, poured into barrels, and heated - encasing the waste in a glass which means that it cannot spill and seep into water. The problem is what to do with these barrels full of radioactive waste. Finland is currently building a facility deep within a mountain to house their waste, eventually it will be backfilled with concrete and sealed.

So, taking all these events into consideration, is nuclear power dangerous?

The old Russian designs were inherently unsafe. Current designs have learnt from the accidents of the past and include more fail-safes and back-ups. They also benefit from research into user interface design, and the poor layouts of reactor control rooms should never be repeated. Also, it could be the case that nuclear reactors in Japan are a bad idea - it is probably the worst possible location to build such power stations due to frequent occurrences of earthquakes, tsunami, typhoons and Godzilla. The accidents of the past are now very unlikely to occur again.

The next wave of technologies poised to take over are designed to be inherently safe, and eliminate problems of conventional designs. Fusion power is particularly promising in terms of safety as it works with little material in the reactor at one time, reducing the material that could be involved in an accident.

Regarding the problem posed by nuclear waste: this is the legacy of mankind. It will remain dangerous for millions of years, and will need careful storage. If the pyramids are anything to go by, mankind has the remarkable ability to dig things up that older civilizations wanted to remain buried; nuclear waste could be a 20th century pharaoh’s curse. If we are unable to develop technology to ‘burn up’ the waste then the safe storage of the waste ultimately dictates the long-term safety of nuclear power.

Sources and further reading:

“Windscale: Britain’s Biggest Nuclear Disaster”, BBC (2007):

https://www.youtube.com/watch?v=vZ4vtUzG6sQ

“Britain’s Nuclear Secrets, Inside Sellafield”, BBC (2015):

https://www.youtube.com/watch?v=F6F6b-pV0ek

“Understanding the Accident of Fukushima Daaichi NPS”, IRSN (2013):

https://www.youtube.com/watch?v=JMaEjEWL6PU

“The Fukushima Nuclear Accident”, Australian Broadcasting Corporation (2011):

https://www.youtube.com/watch?v=25gLi5Y_PTM

“Seconds from Disaster - Fukushima”, Seconds From Disaster (2012):

https://www.youtube.com/watch?v=fyIBlygNlcc

“Into Eternity: A Film for the Future”, Michael Madsen (2010):

http://www.imdb.com/title/tt1194612/

“Pandora’s Box, A is for Atom”, BBC - Adam Curtis (1991):

https://www.youtube.com/watch?v=EviEN0ScOwg

“Three Mile island Pennsylvania Nuclear Power Station Meltdown”, History Channel:

https://www.youtube.com/watch?v=gWPnJanzvjI

“Surviving Disaster - Chernobyl Nuclear Disaster”, BBC:

https://www.youtube.com/watch?v=njTQaUCk4KY