As the hot fuel rods cracked they mixed with coolant water, causing a huge amount of high-pressure steam that lifted the lid off the core. The damage led to a steam explosion that spread radioactive fission products into the atmosphere. With much coolant now lost as steam, a more powerful second explosion shot burning fragments of radioactive material into the atmosphere.
This second explosion destroyed the core, and ended the chain reaction. The tsunami that hit Fukushima 25 years later caused flooding that disabled back-up generators, causing a failure in the cooling systems on the three operational reactors.
Although the reactors had shut down automatically when the earthquake struck, without continuous cooling the heat from radioactive decay in the fuel rods continued to build, eventually reaching the melting point of the fuel rods and causing complete core meltdown.
We acknowledge Aboriginal and Torres Strait Islander peoples as the First Australians and Traditional Custodians of the lands where we live, learn, and work. As the atoms split, they give off energy and, importantly, more neutrons. It is those neutrons that hit other uranium atoms, causing a chain reaction. More on:. Top Stories Celebrity cosmetic surgeon's 'barbaric' attempt to fix a tummy tuck under local anaesthetic. Live: 'I love you, Dad': Touching family tribute shared as entertainment 'master' Bert Newton farewelled.
Prime Minister says he does not believe he has told a lie in public life. The third order applied only to the 33 BWRs with early containment designs, and required 'reliable hardened containment vents' which work under any circumstances.
In Japan similar stress tests were carried out in under the previous safety regulator, but then reactor restarts were delayed until the newly constituted Nuclear Regulatory Authority devised and published new safety guidelines, then applied them progressively through the fleet.
Volcanic hazards are minimal for practically all nuclear plants, but the IAEA has developed a new Safety Guide on the matter. The Bataan plant in Philippines which has never operated, and the Armenian plant at Metsamor are two known to be in proximity to potential volcanic activity.
Nuclear plants are usually built close to water bodies, for the sake of cooling. The site licence takes account of worst case flooding scenarios as well as other possible natural disasters and, more recently, the possible effects of climate change. As a result, all the buildings with safety-related equipment are situated on high enough platforms so that they stand above submerged areas in case of flooding events.
Occasionally in the past some buildings have been sited too low, so that they are vulnerable to flood or tidal and storm surge, so engineered countermeasures have been built.
EDF's Blayais nuclear plant in western France uses seawater for cooling and the plant itself is protected from storm surge by dykes. However, in a 2. For security reasons it was decided to shut down the three reactors then under power the fourth was already stopped in the course of normal maintenance.
This incident was rated 2 on the INES scale. In the Kakrapar nuclear power plant near the west coast of India was flooded due to heavy rains together with failure of weir control for an adjoining water pond, inundating turbine building basement equipment.
The back-up diesel generators on site enabled core cooling using fire water, a backup to process water, since the offsite power supply failed.
Following this, multiple flood barriers were provided at all entry points, inlet openings below design flood level were sealed and emergency operating procedures were updated. Construction of the Kalpakkam plant was just beginning, but the Madras plant shut down safely and maintained cooling.
However, recommendations including early warning system for tsunami and provision of additional cooling water sources for longer duration cooling were implemented. Three of the six reactors were operating at the time, and had shut down automatically due to the earthquake. The back-up diesel generators for those three units were then swamped by the tsunami. This cut power supply and led to weeks of drama and loss of the reactors. The design basis tsunami height was 5. Tsunami heights coming ashore were about 14 metres for both plants.
Unit 3 of Daini was undamaged and continued to cold shutdown status, but the other units suffered flooding to pump rooms where equipment transfers heat from the reactor circuit to the sea — the ultimate heat sink. The maximum amplitude of this tsunami was 23 metres at point of origin, about km from Fukushima. In the last century there had been eight tsunamis in the Japan region with maximum amplitudes above 10 metres some much more , these having arisen from earthquakes of magnitude 7.
Those in and in were the most recent affecting Japan, with maximum heights This earthquake was magnitude 9. For low-lying sites, civil engineering and other measures are normally taken to make nuclear plants resistant to flooding. Lessons from Blayais and Fukushima have fed into regulatory criteria. However, few parts of the world have the same tsunami potential as Japan, and for the Atlantic and Mediterranean coasts of Europe the maximum amplitude is much less than Japan.
In any light-water nuclear power reactor, hydrogen is formed by radiolytic decomposition of water. This needs to be dealt with to avoid the potential for explosion with oxygen present, and many reactors have been retrofitted with passive autocatalytic hydrogen recombiners in their containment, replacing external recombiners that needed to be connected and powered, isolated behind radiological barriers. Also in some kinds of reactor, particularly early boiling water types, the containment is rendered inert by injection of nitrogen.
As of early , a few in Spain and Japan did not have them. Areva received in October a bulk order to supply its passive hydrogen recombiners to multiple Japanese units. This is beyond the capability of the normal hydrogen recombiners to deal with, and operators must rely on venting to atmosphere or inerting the containment with nitrogen. There is a lot of international collaboration, but it has evolved from the bottom, and only in s has there been any real top-down initiative.
In the aviation industry the Chicago Convention in the late s initiated an international approach which brought about a high degree of design collaboration between countries, and the rapid universal uptake of lessons from accidents.
There are cultural and political reasons for this which mean that even the much higher international safety collaboration since the s is still less than in aviation. International cooperation on nuclear safety issues takes place under the auspices of the World Association of Nuclear Operators WANO which was set up in In practical terms this is the most effective international means of achieving very high levels of safety through its four major programs: peer reviews; operating experience; technical support and exchange; and professional and technical development.
WANO peer reviews are the main proactive way of sharing experience and expertise, and by the end of every one of the world's commercial nuclear power plants had been peer-reviewed at least once. Following the Fukushima accident these have been stepped up to one every four years at each plant, with follow-up visits in between, and the scope extended from operational safety to include plant design upgrades. Pre-startup reviews of new plants are being increased. Its aim is to legally commit participating States operating land-based nuclear power plants to maintain a high level of safety by setting international benchmarks to which States would subscribe.
These obligations cover for instance, siting, design, construction, operation, the availability of adequate financial and human resources, the assessment and verification of safety, quality assurance and emergency preparedness. The Convention is an incentive instrument. It is not designed to ensure fulfilment of obligations by Parties through control and sanction, but is based on their common interest to achieve higher levels of safety.
These levels are defined by international benchmarks developed and promoted through regular meetings of the Parties. The Convention obliges Parties to report on the implementation of their obligations for international peer review. This mechanism is the main innovative and dynamic element of the Convention. Under the Operational Safety Review Team OSART program dating from international teams of experts conduct in-depth reviews of operational safety performance at a nuclear power plant.
They review emergency planning, safety culture, radiation protection, and other areas. The Convention entered into force in October As of August , there were 88 signatories to the Convention, 65 of which are contracting parties, including all countries with operating nuclear power plants. The plan arose from intensive consultations with Member States but not with industry, and was described as both a rallying point and a blueprint for strengthening nuclear safety worldwide.
It contains suggestions to make nuclear safety more robust and effective than before, without removing the responsibility from national bodies and governments. It aims to ensure "adequate responses based on scientific knowledge and full transparency".
Apart from strengthened and more frequent IAEA peer reviews including those of regulatory systems , most of the 12 recommended actions are to be undertaken by individual countries and are likely to be well in hand already.
Following this, an extraordinary general meeting of 64 of the CNS parties in September gave a strong push to international collaboration in improving safety. National reports at future three-yearly CNS review meetings will cover a list of specific design, operational and organizational issues stemming from Fukushima lessons. They include further design features to avoid long-term offsite contamination and enhancement of emergency preparedness and response measures, including better definition of national responsibilities and improved international cooperation.
Parties should also report on measures to "ensure the effective independence of the regulatory body from undue influence. However, in line with Swiss and EU intentions, "comprehensive and systematic safety assessments are to be carried out periodically and regularly for existing installations throughout their lifetime in order to identify safety improvements Reasonably practicable or achievable safety improvements are to be implemented in a timely manner.
An IAEA Design Safety Review DSR is performed at the request of a member state organization to evaluate the completeness and comprehensiveness of a reactor's safety documentation by an international team of senior experts.
It is based on IAEA published safety requirements. Therefore, it is neither intended nor possible to cover or substitute licensing activity, or to constitute any kind of design certification. In relation to Eastern Europe particularly, since the late s a major international program of assistance was carried out by the OECD, IAEA and Commission of the European Communities to bring early Soviet-designed reactors up to near western safety standards, or at least to effect significant improvements to the plants and their operation.
The European Union also brought pressure to bear, particularly in countries which aspired to EU membership. Modifications were made to overcome deficiencies in the 11 RBMK reactors still operating at the time in Russia. Among other things, these removed the danger of a positive void coefficient response.
Automated inspection equipment has also been installed in these reactors. The other class of reactors which has been the focus of international attention for safety upgrades is the first-generation of pressurised water VVER reactors. The V model was designed before formal safety standards were issued in the Soviet Union and they lack many basic safety features. Four are still operating in Russia and one in Armenia, under close inspection. Later Soviet-designed reactors are very much safer and have Western control systems or the equivalent, along with containment structures.
It comprises senior officials from the national nuclear safety, radioactive waste safety or radiation protection regulatory authorities from all 27 EU member states, and representatives of the European Commission. Several issues arise in prolonging the lives of nuclear plants which were originally designed for nominal or year operating lives.
Systems, structures and components SSC whose characteristics change gradually with time or use are the subject of attention, which is applied with vastly greater scientific and technical knowledge than that available to the original designers many decades ago. Some components simply wear out, corrode or degrade to a low level of efficiency.
These need to be replaced. Steam generators are the most prominent and expensive of these, and many have been replaced after about 30 years where the reactor otherwise has the prospect of running for 60 years.
This is essentially an economic decision. Lesser components are more straightforward to replace as they age, and some may be safety-related as well as economic. Fuel channel integrity is another limiting factor for Candu reactors, and mid-life inspection and analysis can extend the original , full-power operating hours design assumption to , hours. A second issue is that of obsolescence. For instance, older reactors have analogue instrument and control systems, and a question must be faced regarding whether these are replaced with digital in a major mid-life overhaul, or simply maintained.
Thirdly, the properties of materials may degrade with age, particularly with heat and neutron irradiation. In some early Russian pressurized water reactors, the pressure vessel is relatively narrow and is thus subject to greater neutron bombardment that a wider one. This raises questions of embrittlement, and has had to be checked carefully before extending licences. The graphite blocks cannot be replaced during the operating life of the reactors.
However, radiation damage changes the shape and size of the crystallites that comprise graphite, giving some dimensional change and degradation of the structural properties of the graphite. For continued operation, it is therefore necessary to demonstrate that the graphite can still perform its intended role irrespective of the degradation, or undergo some repair.
In Russia, after dismantling the pressure tubes, longitudinal cutting of a limited number of deformed graphite columns returns the graphite stack geometry to a condition that meets the initial design requirements. Leningrad 1 was the first RBMK reactor to undergo this over In respect to all these aspects, periodic safety reviews are undertaken on most older plants in line with the IAEA safety convention and WANO's safety culture principles to ensure that safety margins are maintained.
These SALTO missions check both physical and organizational aspects, and function as an international peer review of the national regulator. Equipment performance is constantly monitored to identify faults and failures of components. Preventative maintenance is adapted and scheduled in the light of this, to ensure that the overall availability of systems important for both safety and plant availability are within the design basis, or better than the original design basis.
Collecting reliability and performance data is of the utmost importance, as well as analysing them, for tracking indicators that might be signs of ageing, or indicative of potential problems having been under-estimated, or of new problems.
The use of probabilistic safety analysis makes possible risk-informed decisions regarding maintenance and monitoring programs, so that adequate attention is given to the health of every piece of equipment in the plant. This process is similar to that in other industries where safety is paramount, e.
Reliability centered maintenance was adapted from civil aviation in the s for instance, and led to nuclear industry review of existing maintenance programmes.
In the USA most of the about reactors are expected to be granted operating licence extensions from 40 to 60 years, with many to 80 years. This justifies significant capital expenditure in upgrading systems and components, including building in extra performance margins.
The IAEA has a safety knowledge base for ageing and long-term operation of nuclear power plants SKALTO which aims to develop a framework for sharing information on ageing management and long term operation of nuclear power plants. It provides published documents and information related to this. Knowledge management in relation to the original design basis of reactors becomes an issue with corporate reorganisation or demise of vendors, coupled with changes made over several decades.
While operators usually have good records, some regulators do not. Nuclear DKM addresses the specific needs of nuclear plants and organizations.
Its scope extends from research and development, through design and engineering, construction, commissioning, operations, maintenance, refurbishment and long-term operation LTO , waste management, to decommissioning.
Nuclear DKM issues and priorities are often unique to the particular circumstances of individual countries and their regulators as well as other nuclear industry organizations. Nuclear DKM may focus on knowledge creation, identification, sharing, transfer, protection, validation, storage, dissemination, preservation or utilization. Nuclear DKM practices may enhance and support traditional business functions and goals such as human resource management, training, planning, operations, maintenance, and much more.
There must always be a responsible owner of the DKM system for any plant. In most cases this will be the operator, however, based on a variety of changes such as market conditions, the responsible owner may change over time.
An effective nuclear DKM system should be focused on strengthening and aligning the knowledge base in three primary knowledge domains in an organization: people, processes and technology, each of which must also be considered within the context of the organizational culture. Knowledge management policies and practices should help create a supportive organizational culture that recognizes the value of nuclear knowledge and promotes effective processes to maintain it.
By the mids there was a divergence between drawings and modifications which had progressively been made, and also the operating company had not shared operating experience with the designer. Maintenance standards fell and costs rose. A detailed audit in showed that the design basis was not being maintained and that additional staff would be required to correct the situation at all Ontario Hydro plants, so the two A plants eight units were shut down so that staff could focus on the 12 units not needing so much attention.
From , six of the eight A units were returned to service with design basis corrected, having been shut down for several years — a significant loss of asset base for the owners. The scale runs from a zero event with no safety significance to 7 for a "major accident" such as Chernobyl. TMI rated 5, as an "accident with off-site risks" though no harm to anyone, and a level 4 "accident mainly in installation" occurred in France in , with little drama.
Another accident rated at level 4 occurred in a fuel processing plant in Japan in September Other accidents have been in military plants. Since the World Trade Centre attacks in New York in there has been increased concern about the consequences of a large aircraft being used to attack a nuclear facility with the purpose of releasing radioactive materials.
Various studies have looked at similar attacks on nuclear power plants. They show that nuclear reactors would be more resistant to such attacks than virtually any other civil installations — see Appendix. It concludes that US reactor structures "are robust and would protect the fuel from impacts of large commercial aircraft". The wingspan is greater than the diameter of reactor containment buildings and the 4.
Hence analyses focused on single engine direct impact on the centreline — since this would be the most penetrating missile — and on the impact of the entire aircraft if the fuselage hit the centreline in which case the engines would ricochet off the sides. In each case no part of the aircraft or its fuel would penetrate the containment.
Other studies have confirmed these findings. Penetrating even relatively weak reinforced concrete requires multiple hits by high speed artillery shells or specially-designed "bunker busting" ordnance — both of which are well beyond what terrorists are likely to deploy. Thin-walled, slow-moving, hollow aluminum aircraft, hitting containment-grade heavily-reinforced concrete disintegrate, with negligible penetration.
In Sandia National Laboratories in USA demonstrated the unequal distribution of energy absorption that occurs when an aircraft impacts a massive, hardened target. The test involved a rocket-propelled F4 Phantom jet about 27 tonnes, with both engines close together in the fuselage hitting a 3.
This was to see whether a proposed Japanese nuclear power plant could withstand the impact of a heavy aircraft. See also video clip. As long ago as the late s, the UK Central Electricity Generating Board considered the possibility of a fully-laden and fully-fuelled large passenger aircraft being hijacked and deliberately crashed into a nuclear reactor.
The main conclusions were that an airliner would tend to break up as it hit various buildings such as the reactor hall, and that those pieces would have little effect on the concrete biological shield surrounding the reactor. Any kerosene fire would also have little effect on that shield. In the s in the USA, at least some plants were designed to take a hit from a fully-laden large military transport aircraft and still be able to achieve and maintain cold shutdown.
The study of a s US power plant in a highly-populated area is assessing the possible effects of a successful terrorist attack which causes both meltdown of the core and a large breach in the containment structure — both extremely unlikely. It shows that a large fraction of the most hazardous radioactive isotopes, like those of iodine and tellurium, would never leave the site. Much of the radioactive material would stick to surfaces inside the containment or becomes soluble salts that remain in the damaged containment building.
Some radioactive material would nonetheless enter the environment some hours after the attack in this extreme scenario and affect areas up to several kilometres away. The extent and timing of this means that with walking-pace evacuation inside this radius it would not be a major health risk. However it could leave areas contaminated and hence displace people in the same way as a natural disaster, giving rise to economic rather than health consequences.
Looking at spent fuel storage pools, similar analyses showed no breach. Dry storage and transport casks retained their integrity. If another atom absorbs one of those neutrons, the atom becomes unstable and undergoes fission itself, releasing more heat and more neutrons.
The chain reaction becomes self-sustaining, producing a steady supply of heat to boil water, drive steam turbines and thereby generate electricity. How much electricity does nuclear power provide in Japan and elsewhere? With 54 nuclear reactors generating billion kilowatt-hours annually, Japan is the world's third-largest producer of nuclear power, after the U.
The Fukushima Daiichi station, which has been hit hard by the March 11 earthquake, houses six of those reactors, all of which came online in the s. Worldwide, nuclear energy accounts for about 15 percent of electricity generation; Japan gets nearly 30 percent of its electricity from its nuclear plants.
The U. About 20 percent of U. What fuels a nuclear reactor? Most nuclear reactors use uranium fuel that has been "enriched" in uranium , an isotope of uranium that fissions readily. Isotopes are variants of elements with different atomic masses. Uranium is much more common in nature than uranium but does not fission well, so fuel manufacturers boost the uranium content to a few percent, which is enough to maintain a continuous fission reaction and generate electricity.
Enriched uranium is manufactured into fuel rods that are encased in metal cladding made of alloys such as zirconium. Reactor No. How do you turn off a nuclear reaction? Sustained nuclear fission reactions rely on the passing of neutrons from one atom to another—the neutrons released in one atom's fissioning trigger the fissioning of the next atom.
The way to cut off a fission chain reaction, then, is to intercept the neutrons. Nuclear reactors utilize control rods made from elements such as cadmium, boron or hafnium, all of which are efficient neutron absorbers. When the reactor malfunctions or when operators need to shut off the reactor for any other reason technicians can remotely plunge control rods into the reactor core to soak up neutrons and shut down the nuclear reaction.
Can a reactor melt down once the nuclear reaction is stopped? Even after the control rods have done their job and arrested the fission reaction the fuel rods retain a great deal of heat. What is more, the uranium atoms that have already split in two produce radioactive by-products that themselves give off a great deal of heat.
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