The era of nuclear energy (and weapons) dates back to 2 December, 1942 when Enrico Fermi brought the first nuclear reactor to criticality.
Since that time the use of nuclear energy has proliferated across the world and while some of us remember the more recent disasters such as Chernobyl and Three Mile Island, there have been literally hundreds of incidents involving nuclear reactors on land and at sea. Many of these involved deaths or exposure to dangerous levels of radioactivity and most were caused by human error, poor control systems and faulty instrumentation. These incidents occurred in the USA, the UK, Japan, the old Soviet Union, France, Australia, Belgium, Finland, Germany, Canada, Spain, South Korea and India. This list is probably incomplete as certainly in the early nuclear era such accidents often went unreported to the public. For the record the first reported nuclear incident occurred on 12 December, 1952, at the Chalk River power plant in Canada. Here a technician opened valves that led to the melting of some uranium fuel and nearly demolished the reactor core.
The first major incident occurred at Britain's Windscale plutonium production reactor where a fire burned 11 tons of uranium and killed at least 32 people. In an attempt to expunge the records the plant was later renamed Sellafield. Windscale was established following World War 2 as although Britain was involved in the US Manhattan Project the Americans refused to transfer the technology. In great secrecy massive effort went into creating a British nuclear weapon and the first plutonium was produced at Windscale in 1952. In the process used, the graphite had to be heated up to anneal it as otherwise neutrons striking the graphite caused a distortion of the crystal structure.
On 8 October, 1957, a technician was heating the reactor up, but because of the inadequacy of the temperature measurement instrumentation the control room staff thought the reactor was cooling down and needed an extra boost of heating. In fact temperatures were abnormally high when the control rods were eventually withdrawn for a routine start to the plant's chain reaction. A canister of lithium and magnesium (being used to create tritium) were probably the first to catch fire and this coupled with igniting uranium and graphite caused temperatures to soar above 1300°C.
It was only four days later that indication of radioactive releases through the off gas stack were noticed and finally on the fifth day the reactor was flooded with water and the fire was extinguished. By this time a dangerous cloud of fallout was carried over the North of England but authorities decided that the only environmental action required was to ban milk distribution over an area of 200 square miles. The reactor itself was destroyed and the final conclusion was that operator error and defective procedures were the cause. Windscale itself remained as a nuclear facility and numerous other radiation related incidents were reported during the seventies, including one in 1979 when another fire resulted in six people being exposed to radiation. Little wonder that a name change was required for the plant.
The next event to make headlines occurred on 28 March, 1979, when as a result of a series of malfunctions, mistakes and misinterpretations the worst nuclear incident in the USA took place at Three Mile Island, nearly resulting in the worst of all disasters, namely total meltdown.
This incident seems to be the result initially of the opening of a pressure relief valve that provided the indication that it was closed. When the emergency injection water system (EIW) was automatically activated operators observed that the water level in the primary system was still rising (but was actually decreasing) and they turned the EIW off. The steam and water escaping through the primary release valve created a 'loss of coolant accident' (LOCA), but the instrument that checks for radiation levels did not register an alarm. The water level continued to drop and the top of the core was soon exposed with further release of hydrogen and radioactive gases.
After more than two hours the next shift operator came on duty and noticed that the primary relief valve was excessively hot. He shut the secondary valve but more than a quarter of a million gallons of radioactive cooling water had already been released. The water within the primary loop however continued to boil away causing more damage to the core. After two and a half hours the operators received the first indication that radiation levels were rising and 15 minutes after this, half of the core was uncovered. Even after operators pumped water back into the system, pressure was still too high and the pressure relief valve was again opened. Nine hours after the start of the incident, hydrogen exploded within the containment structure causing pressure spikes on gauges and an audible thud. These were ignored as being just an electrical malfunction with the thud being explained away as a ventilation damper. It was only after nearly 16 hours that adequate water was once again pumped around the core bringing the temperature under control although the core itself was half melted and part of it disintegrated. Some reports indicate that The Three Mile Island accident had many precedents with this same valve failure having occurred in similar reactors.
Chernobyl was not the first or the last nuclear incident in the USSR but it created the most impact and is still recorded as the world's worst nuclear power accident. Chernobyl had four RBMK reactors and as Reactor Four was to be shut down for maintenance it was decided on 25 April 1986 to take advantage of this to run an experiment. The test was to check whether in the event of a shutdown enough electrical power would be available to operate emergency equipment and the core cooling pumps before the diesel generators came on line.
Note should be taken that the RBMK reactor found at Chernobyl had inherent design faults and it is the only nuclear reactor that uses water as a coolant and graphite as a moderator that results in instability at low power and sometimes rapid, uncontrollable power increases. (The RBMK was derived from a design intended for plutonium production) As the reactor shutdown proceeded it was down to approximately half power when the electric load dispatcher refused to allow further shutdown. As part of the test the emergency core cooling system was switched off and the reactor continued at half power. Late in the evening of 25 April further power reduction was granted. For the test the reactor was supposed to be stabilised at 1000 MW prior to shutdown but as a result of an operational error the power fell to about 30 MW leading to excess steam pockets (which resulted in additional heating). The operators tried to raise power again using manual control and at 13h00 on 26 April stability was achieved at 200 MW. At this time the operator reduced the flow of feed water to maintain steam pressure, while pumps that were powered by the slowing turbine provided less and less cooling. This resulted in additional steam in the cooling channels and the operators could not control a power surge estimated to be 100 times nominal power. The sudden increase in temperature caused the fuel to rupture and fuel particles reacted with the water creating an explosion that destroyed the reactor core. A second explosion occurred almost immediately and a fireball blew off the reactor's heavy steel and concrete lid. The rest is history with more than 30 people being killed instantaneously and more than 135 000 being evacuated. Note that while standard operating procedure required a minimum of 30 rods to retain control, the test was carried out with only eight rods. Estimates are that at least a further 2500 people died as a result of exposure to radiation while there has been a marked increase in the incidence in the region of certain forms of cancer. There are still about 14 RMBK reactors still operating but they have been extensively modified and use new control procedures to prevent a repeat of Chernobyl, including ensuring that the reactor cannot be operated at low power where the instability is introduced.
While it did not receive anywhere near the same press coverage, Japan suffered its worst nuclear accident on 30 September, 1999 at Tokaimuru. This was a nuclear processing plant commissioned to purify uranium. Instead of using automated machines the plant used manual labour to mix 5,2 pounds of uranium with nitric acid. Pushed by management to increase productivity the workers involved poured 35 pounds of uranium into the tank and a nuclear fission chain reaction followed. The nuclear reaction was eventually controlled but reports indicate that over 80 people were exposed to dangerous levels of radiation. The three workers mixing the uranium exhibited severe radiation sickness symptoms and records do not specify whether they actually survived.
Besides incidents on land there have been numerous accidents at sea on nuclear powered submarines or vessels carrying nuclear weapons. While most of the submarine incidents were Russian, two American nuclear vessels (USS Thresher and USS Scorpion) also sank. One commentator recently wrote that the world's sixth largest nuclear power is 'the deep blue sea'.
So much for the negative side. All of these incidents (and the many others) prompted major investigations and while there are still many decaying reactors operating in the old Soviet Union, Western help has been provided to improve safety and control measures. Three Mile Island prompted the US to increase safety requirements including regulations in regard to operator training, testing and certification. Automation and control systems as well as instrumentation have also progressed significantly over the last two decades and newer plants will inherently be much safer than was the case in the past, although there is still the human factor. However, data interpretation software has been introduced to determine the state of the various plant systems and to ensure that information is conveyed in a clearly understandable format to the operator. While Chernobyl occurred during this period of introspection it was acknowledged that if the RMBK reactor had a US-style containment structure, none of the radioactivity would have escaped. New reactor designs such as the local PBMR and other US developments incorporate passive safety features.
Like many others I personally believe that nuclear power must remain part of the electrical power generation portfolio, which is being fuelled by the world's need for sustainable energy. The danger in the future probably lies in those countries that are shunned by the West but are developing their own nuclear programmes for power or weapon capability. Without assistance in recognising the lessons learned in the rest of the world their inexperience could see another Chernobyl or worse.
Dr Maurice McDowell has many years' experience as a technical journalist, editor, business manager and research scientist. His third party analyses of world-class companies and processes, as well as his insight into industry and technology trends are well respected.
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