There have been three major reactor accidents in the history of civil nuclear power – Three Mile Island, Chernobyl and Fukushima. The evidence over six decades shows that nuclear power is a safe means of generating electricity. In relation to nuclear power, safety is closely linked with security, and in the nuclear field also with safeguards. Safety focuses on unintended conditions or events leading to radiological releases from authorised activities. Security focuses on the intentional misuse of nuclear or other radioactive materials by non-state elements to cause harm. Safeguards focus on restraining activities by states that could lead to acquisition of nuclear weapons. In the 1950s attention turned to harnessing the power of the atom in a controlled way, as demonstrated at Chicago in 1942 and subsequently for military research, and applying the steady heat yield to generate electricity. A particular nuclear scenario was loss of cooling which resulted in melting of the nuclear reactor core, and this motivated studies on both the physical and chemical possibilities as well as the biological effects of any dispersed radioactivity. Chernobyl (Ukraine 1986) where the destruction of the reactor by steam explosion and fire killed 31 people and had significant health and environmental consequences.
Fukushima (Japan 2011) where three old reactors (together with a fourth) were written off and the effects of loss of cooling due to a huge tsunami were inadequately contained.
A table showing all reactor accidents, and a table listing some energy-related accidents with multiple fatalities are appended.
Apart from Chernobyl, no nuclear workers or members of the public have ever died as a result of exposure to radiation due to a commercial nuclear reactor incident. It should be emphasised that a commercial-type power reactor simply cannot under any circumstances explode like a nuclear bomb – the fuel is not enriched beyond about 5%, and much higher enrichment is needed for explosives. While nuclear power plants are designed to be safe in their operation and safe in the event of any malfunction or accident, no industrial activity can be represented as entirely risk-free.
It was not until the late 1970s that detailed analyses and large-scale testing, followed by the 1979 meltdown of the Three Mile Island reactor, began to make clear that even the worst possible accident in a conventional western nuclear power plant or its fuel would not be likely to cause dramatic public harm.
The decades-long test and analysis program showed that less radioactivity escapes from molten fuel than initially assumed, and that most of this radioactive material is not readily mobilized beyond the immediate internal structure.
It is the laws of physics and the properties of materials that mitigate disaster, as much as the required actions by safety equipment or personnel. A mandated safety indicator is the calculated probable frequency of degraded core or core melt accidents. However apart from these accidents and the Chernobyl disaster there have been about ten core melt accidents – mostly in military or experimental reactors – Appendix 2 lists most of them. The main safety concern has always been the possibility of an uncontrolled release of radioactive material, leading to contamination and consequent radiation exposure off-site.
Certainly the matter was severely tested with three reactors of the Fukushima Daiichi nuclear power plant in Japan in March 2011. A fundamental principle of nuclear power plant operation worldwide is that the operator is responsible for safety. With new reactor designs being established on a more international basis since the 1990s, both industry and regulators are seeking greater design standardisation and also regulatory harmonization.
It has long been asserted that nuclear reactor accidents are the epitome of low-probability but high-consequence risks. In passing, it is relevant to note that the safety record of the US nuclear navy from 1955 on is excellent, this being attributed to a high level of standardisation in over one hundred naval power plants and in their maintenance, and the high quality of the Navy's training program. To achieve optimum safety, nuclear plants in the western world operate using a 'defence-in-depth' approach, with multiple safety systems supplementing the natural features of the reactor core.
These can be summed up as: Prevention, Monitoring, and Action (to mitigate consequences of failures). The safety provisions include a series of physical barriers between the radioactive reactor core and the environment, the provision of multiple safety systems, each with backup and designed to accommodate human error. The barriers in a typical plant are: the fuel is in the form of solid ceramic (UO2) pellets, and radioactive fission products remain largely bound inside these pellets as the fuel is burned. The main safety features of most reactors are inherent - negative temperature coefficient and negative void coefficient. In the 1950s and 1960s some experimental reactors in Idaho were deliberately tested to destruction to verify that large reactivity excursions were self-limiting and would automatically shut down the fission reaction.
Beyond the control rods which are inserted to absorb neutrons and regulate the fission process, the main engineered safety provisions are the back-up emergency core cooling system (ECCS) to remove excess heat (though it is more to prevent damage to the plant than for public safety) and the containment.
Traditional reactor safety systems are 'active' in the sense that they involve electrical or mechanical operation on command.
The basis of design assumes a threat where due to accident or malign intent (eg terrorism) there is core melting and a breach of containment.
Nuclear power plants are designed with sensors to shut them down automatically in an earthquake, and this is a vital consideration in many parts of the world. In both the Three Mile Island (TMI) and Fukushima accidents the problems started after the reactors were shut down – immediately at TMI and after an hour at Fukushima, when the tsunami arrived. The Three Mile Island accident in 1979 demonstrated the importance of the inherent safety features.
Investigations following the accident led to a new focus on the human factors in nuclear safety. At Fukushima Daiichi in March 2011 the three operating reactors shut down automatically, and were being cooled as designed by the normal residual heat removal system using power from the back-up generators, until the tsunami swamped them an hour later. Nuclear plants have Severe Accident Mitigation Guidelines (SAMG, or in Japan: SAG), and most of these, including all those in the USA, address what should be done for accidents beyond design basis, and where several systems may be disabled.
In 2015 the Canadian Nuclear Safety Commission (CNSC) released its Study of Consequences of a Hypothetical Severe Nuclear Accident and Effectiveness of Mitigation Measures. The April 1986 disaster at the Chernobyl nuclear power plant in Ukraine was the result of major design deficiencies in the RBMK type of reactor, the violation of operating procedures and the absence of a safety culture.
The accident destroyed the reactor, and its burning contents dispersed radionuclides far and wide. The Chernobyl accident was a unique event and the only time in the history of commercial nuclear power that radiation-related fatalities occurred. The main positive outcome of this accident for the industry was the formation of the World Association of Nuclear Operators (WANO), building on the US precedent. An OECD expert report on it concluded that "the Chernobyl accident has not brought to light any new, previously unknown phenomena or safety issues that are not resolved or otherwise covered by current reactor safety programs for commercial power reactors in OECD Member countries.  In other words, the concept of 'defence in depth' was conspicuous by its absence, and tragically shown to be vitally important. There have been a number of accidents in experimental reactors and in one military plutonium-producing reactor, including a number of core melts, but none of these has resulted in loss of life outside the actual plant, or long-term environmental contamination. The well-publicized criticality accident at Tokai Mura, Japan, in 1999 was at a fuel preparation plant for experimental reactors, and killed two workers from radiation exposure. Accidents in any field of technology provide valuable knowledge enabling incremental improvement in safety beyond the original engineering. Aspects of nuclear plant safety highlighted by the Fukushima accident were assessed in the 143 nuclear reactors in the EU's 27 member states, as well as those in any neighbouring states that decided to take part. The Western European Nuclear Regulators' Association (WENRA) proposed these in response to a call from the European Council in March 2011, and developed specifications. In June 2011 the governments of seven non-EU countries agreed to conduct nuclear reactor stress tests using the EU model.
The reassessment of safety margins is based on the existing safety studies and engineering judgment to evaluate the behaviour of a nuclear power plant when facing a set of challenging situations.
The scope of the assessment took into account the issues directly highlighted by the events in Fukushima and the possibility for combination of initiating events. The documents had to cover provisions in the plant design basis for these events and the strength of the plant beyond its design basis.
Since the licensee has the prime responsibility for safety, they performed the reassessments, and the regulatory bodies then independently reviewed them. The European Commission adopted, with ENSREG, the final stress tests Report on April 26, 2012 and issued the same day a joint statement underlining the quality of the exercise.
The results of the stress tests pointed out, in particular, that European nuclear power plants offered a sufficient safety level to require no shutdown of any of them. The EU process was completed at the end of September 2012, with the EU Energy Commissioner announcing that the stress tests had showed that the safety of European power reactors was generally satisfactory, but making some other comments and projections which departed from ENSREG.
In the USA the Nuclear Regulatory Commission (NRC) in March 2012 made orders for immediate post-Fukushima safety enhancements, likely to cost about $100 million across the whole US fleet.
In Japan similar stress tests were carried out in 2011 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.
In addition to engineering and procedures which reduce the risk and severity of accidents, all plants have guidelines for Severe Accident Management or Mitigation (SAM). In mid-2011 the IAEA Incident and Emergency Centre launched a new secure web-based communications platform to unify and simplify information exchange during nuclear or radiological emergencies. The International Atomic Energy Agency (IAEA) has a Safety Guide on Seismic Risks for Nuclear Power Plants, and the matter is dealt with in the WNA paper on Earthquakes and Nuclear Power Plants. Volcanic hazards are minimal for practically all nuclear plants, but the IAEA has developed a new Safety Guide on the matter. 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. In 1994 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. In March 2011 the Fukushima Daiichi nuclear plant was affected seriously by a huge tsunami induced by the Great East Japan Earthquake. For low-lying sites, civil engineering and other measures are normally taken to make nuclear plants resistant to flooding.
In any light-water nuclear power reactor, hydrogen is formed by radiolytic decomposition of water.
There is a lot of international collaboration, but it has evolved from the bottom, and only in 1990s has there been any real top-down initiative. The IAEA Convention on Nuclear Safety (CNS) was drawn up during a series of expert level meetings from 1992 to 1994 and was the result of considerable work by Governments, national nuclear safety authorities and the IAEA Secretariat.
The obligations of the Parties are based to a large extent on the principles contained in the IAEA Safety Fundamentals document The Safety of Nuclear Installations. The IAEA General Conference in September 2011 unanimously endorsed the Action Plan on Nuclear Safety that Ministers requested in June. Following this, an extraordinary general meeting of 64 of the CNS parties in September 2012 gave a strong push to international collaboration in improving safety. In February 2015 diplomats from 72 countries unanimously adopted the Vienna Declaration of Nuclear Safety, setting out “principles to guide them, as appropriate, in the implementation of the objective of the CNS to prevent accidents with radiological consequences and mitigate such consequences should they occur” but rejected Swiss amendments to the CNS as impractical.
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. In relation to Eastern Europe particularly, since the late 1980s 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 other class of reactors which has been the focus of international attention for safety upgrades is the first-generation of pressurised water VVER-440 reactors. The main European safety collaboration is through the European Nuclear Safety Regulators Group (ENSREG), an independent, authoritative expert body created in 2007 by the European Commission to revive the EU nuclear safety directive, which was passed in June 2009.
Several issues arise in prolonging the lives of nuclear plants which were originally designed for 30 or 40-year operating lives. Thirdly, the properties of materials may degrade with age, particularly with heat and neutron irradiation.
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. The IAEA undertakes Safety Aspects of Long-Term Operation (SALTO) evaluations of reactors on request from member countries. Equipment performance is constantly monitored to identify faults and failures of components. 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.

The International Nuclear Event Scale  (INES) was developed by the IAEA and OECD in 1990 to communicate and standardise the reporting of nuclear incidents or accidents to the public. Since the World Trade Centre attacks in New York in 2001 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. In 1988 Sandia National Laboratories in USA demonstrated the unequal distribution of energy absorption that occurs when an aircraft impacts a massive, hardened target. As long ago as the late 1970s, 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 study of a 1970s 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. Similarly, the massive structures mean that any terrorist attack even inside a plant (which are well defended) and causing loss of cooling, core melting and breach of containment would not result in any significant radioactive releases.
However, while the main structures are robust, the 2001 attacks did lead to increased security requirements and plants were required by NRC to install barriers, bulletproof security stations and other physical modifications which in the USA are estimated by the industry association to have cost some $2 billion across the country. Switzerland's Nuclear Safety Inspectorate studied a similar scenario and reported in 2003 that the danger of any radiation release from such a crash would be low for the older plants and extremely low for the newer ones.
The designs for nuclear plants being developed for implementation in coming decades contain numerous safety improvements based on operational experience. One major feature they have in common (beyond safety engineering already standard in Western reactors) is passive safety systems, requiring no operator intervention in the event of a major malfunction. The main metric used to assess reactor safety is the likelihood of the core melting due to loss of coolant.
Concerns from those skeptical of nuclear energy were renewed two years ago after a tsunami hit Japan and devastated the Fukushima Daiichi nuclear power plant. Since 2011, in the post-Fukushima disaster world, the primary reaction in the United States has been a renewed outcry to close several nuclear plants, primarily those in regions subject to natural disasters, according to Paul Roland, treasurer of the UT Graduate Student Association. The free, public event is co-sponsored by the Graduate Student Association and the Department of Physics and Astronomy. Dave Lockbaum, director of the Nuclear Safety Program for the Union of Concerned Scientists, will speak on nuclear energy safety. In addition to Lockbaum’s 17 years of experience as a nuclear engineer, he has worked as an instructor for the Nuclear Regulatory Commission and has testified before Congress numerous times.
Lockbaum also will discuss how a disaster similar to Fukushima would play out in the United States and the unique role Toledo has with the Davis-Besse nuclear plant in Oak Harbor, Ohio, and the Fermi II nuclear power plant in Monroe, Mich. In over 16,000 cumulative reactor-years of commercial operation in 32 countries, there have been only three major accidents to nuclear power plants - Three Mile Island, Chernobyl, and Fukushima - the second being of little relevance to reactor design outside the old Soviet bloc. Of all the accidents and incidents, only the Chernobyl and Fukushima accidents resulted in radiation doses to the public greater than those resulting from the exposure to natural sources.
Most of the serious radiological injuries and deaths that occur each year (2-4 deaths and many more exposures above regulatory limits) are the result of large uncontrolled radiation sources, such as abandoned medical or industrial equipment. One of its functions was to act as an auditor of world nuclear safety, and this role was increased greatly following the Chernobyl accident. It prescribes safety procedures and the reporting of even minor incidents.
Incidents and accidents may happen, and as in other industries, will lead to progressive improvement in safety. Those improvements are both in new designs, and in upgrading of existing plants. These gave rise to a genre of dramatic fiction (eg The China Syndrome) in the public domain and also some solid conservative engineering including containment structures (at least in Western reactor designs) in the industry itself. Thus, even if the containment structure that surrounds all modern nuclear plants were ruptured, as it has been with at least one of the Fukushima reactors, it is still very effective in preventing escape of most radioactivity. The US Nuclear Regulatory Commission (NRC) specifies that reactor designs must meet a 1 in 10,000 year core damage frequency, but modern designs exceed this. The TMI accident proved the extent of truth in the proposition, and the molten core material got exactly 15 mm of the way to China as it froze on the bottom of the reactor pressure vessel. At Fukushima, cooling was maintained just long enough apparently to avoid testing the containment in this way.
A few of these are gases at normal temperatures, more are volatile at higher temperatures, and both will be released from the fuel if the cladding is damaged.
Cooling was lost after a shutdown, and it proved impossible to restore it sufficiently to prevent severe damage to the fuel. The national regulator is responsible for ensuring the plants are operated safely by the licensee, and that the design is approved. This double possibility has been well studied and provides the basis of exclusion zones and contingency plans.
No major design changes were called for in western reactors, but controls and instrumentation were improved significantly and operator training was overhauled. This was the result of research and analysis undertaken to address concerns raised during public hearings in 2012 on the environmental assessment for the refurbishment of Ontario Power Generation's (OPG's) Darlington nuclear power plant. Elsewhere (Safety of Nuclear Power info paper appendix) we tabulate these, along with the most serious commercial plant accidents. Many other such criticality accidents have occurred, some fatal, and practically all in military facilities prior to 1980. Cars and airliners are the most obvious examples of this, but the chemical and oil industries can provide even stronger evidence. These comprehensive and transparent nuclear risk and safety assessments, the so-called "stress tests", involved targeted reassessment of each power reactor’s safety margins in the light of extreme natural events, such as earthquakes and flooding, as well as on loss of safety functions and severe accident management following any initiating event.
WENRA is a network of Chief Regulators of EU countries with nuclear power plants and Switzerland, and has membership from 17 countries.
Armenia, Belarus, Croatia, Russia, Switzerland, Turkey and Ukraine signed a declaration that they would conduct stress tests and agreed to peer reviews of the tests by outside experts.
The exercise covered 147 nuclear plants in 15 EU countries – including Lithuania with only decommissioned plants – plus 15 reactors in Ukraine and five in Switzerland. Information was shared among regulators throughout this process before the 17 final reports went to peer-review by teams comprising 80 experts appointed by ENSREG and the European Commission. The first order requires the addition of equipment at all plants to help respond to the loss of all electrical power and the loss of the ultimate heat sink for cooling, as well as maintaining containment integrity.
These conspicuously came into play after the Fukushima accident, where staff had immense challenges in the absence of power and with disabled cooling systems following damage done by the tsunami. 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. EDF's Blayais nuclear plant in western France uses seawater for cooling and the plant itself is protected from storm surge by dykes. Three of the six reactors were operating at the time, and had shut down automatically due to the earthquake.
Lessons from Blayais have fed into regulatory criteria since 2000, and those from Fukushima will certainly do so. 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. In the aviation industry the Chicago Convention in the late 1940s initiated an international approach which brought about a high degree of design collaboration between countries, and the rapid universal uptake of lessons from accidents.
WANO peer reviews are the main proactive way of sharing experience and expertise, and by the end of 2009 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. 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.
It is not designed to ensure fulfillment of obligations by Parties through control and sanction, but is based on their common interest to achieve higher levels of safety. As of September 2009, there were 79 signatories to the Convention, 66 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. National reports at future three-yearly CNS review meetings will cover a list of specific design, operational and organizational issues stemming from Fukushima lessons. The V-230 model was designed before formal safety standards were issued in the Soviet Union and they lack many basic safety features. 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. Systems, structures and components (SSC) whose characteristics change gradually with time or use are the subject of attention. 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.
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. These SALTO missions check both physical and organizational aspects, and function as an international peer review of the national regulator. 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. This justifies significant capital expenditure in upgrading systems and components, including building in extra performance margins. 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. By the mid-1990s 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.
The scale runs from a zero event with no safety significance to 7 for a "major accident" such as Chernobyl. The wingspan is greater than the diameter of reactor containment buildings and the 4.3 tonne engines are 15 metres apart. 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. 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. Ironically and as noted earlier, with better understanding of what happens in a core melt accident inside, they are now seen to be not nearly as necessary in that accident mitigation role as was originally assumed. Other controls include physical shielding and limiting the time workers spend in areas with significant radiation levels. US utility requirements are 1 in 100,000 years, the best currently operating plants are about 1 in 1 million and those likely to be built in the next decade are almost 1 in 10 million.
The latter include iodine (easily volatalised, at 184°C) and caesium (671°C), which were the main radionuclides released at Fukushima, first into the reactor pressure vessel and then into the containment which in unit 2 apparently ruptured early on day 5. A second important concept is that a regulator’s mission is to protect people and the environment. Also there are a number of sets of mechanical codes and standards related to quality and safety. However, the physics and chemistry of a reactor core, coupled with but not wholly depending on the engineering, mean that the consequences of an accident are likely in fact be much less severe than those from other industrial and energy sources. As well as the physical aspects of safety, there are institutional aspects which are no less important - see following section on International Collaboration.
The second means that if any steam has formed in the cooling water there is a decrease in moderating effect so that fewer neutrons are able to cause fission and the reaction slows down automatically. Apparently during the Cold War neither Russia nor the USA targeted the other's nuclear power plants because the likely damage would be modest. The State-of-the-Art Reactor Consequences Analysis (SOARCA) showed that a severe accident at a US nuclear power plant (PWR or BWR) would not be likely to cause any immediate deaths, and the risks of fatal cancers would be vastly less than the general risks of cancer. However, this was not the prime cause of the Chernobyl accident. It once and for all vindicated the desirability of designing with inherent safety supplemented by robust secondary safety provisions.
It also caused radiation sickness in a further 200-300 staff and firefighters, and contaminated large areas of Belarus, Ukraine, Russia and beyond.
The list of ten probably corresponds to incidents rating 4 or higher on today’s International Nuclear Event Scale (Table 4).

Civil nuclear power has greatly improved its safety in both engineering and operation over its 55 years of experience with very few accidents and major incidents to spur that improvement. It then negotiated the scope of the tests with the European Nuclear Safety Regulators Group (ENSREG), an independent, authoritative expert body created in 2007 by the European Commission comprising 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.
The consequences of these – loss of electrical power and station blackout, loss of ultimate heat sink and the combination of both – were analysed, with the conclusions being applicable to other general emergency situations.
The final documents were published in line with national law and international obligations, subject only to not jeopardising security – an area where each country could behave differently. 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. However, in 1999 a 2.5 m storm surge in the estuary overtopped the dykes – which were already identified as a weak point and scheduled for a later upgrade – and flooded one pumping station.
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. Sea walls are being built or increased at Hamaoka, Shimane, Mihama, Ohi, Takahama, Onagawa, and Higashidori plants.
There are cultural and political reasons for this which mean that even the much higher international safety collaboration since the 1990s is still less than in aviation. These levels are defined by international benchmarks developed and promoted through regular meetings of the Parties.
It contains suggestions to make nuclear safety more robust and effective than before, without removing the responsibility from national bodies and governments. 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. If the DSR is for a vendor’s design at the pre-licensing stage, it is done using the Generic Reactor Safety Review (GRSR) module. It was preceded in 1999 by the Western European Nuclear Regulators' Association (WENRA), a network of Chief Regulators of EU countries with nuclear power plants and Switzerland, with membership from 17 countries.
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 raises questions of embrittlement, and has had to be checked carefully before extending licences. 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. There is widespread agreement that further extensions may be justified, to 80 years, and this prospect is driving research on ageing to ensure both safety and reliability in older plants. 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.
Three Mile Island 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 1980, with little drama. They show that nuclear reactors would be more resistant to such attacks than virtually any other civil installations – see Appendix 3. 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). But further (see Sept 2002 Science paper and Jan 2003 Response & Comments), realistic assessments from decades of analyses, lab work and testing, find that the consequence of even the worst realistic scenarios – core melting and containment failure – can cause few if any deaths to the public, regardless of the scenario that led to the core melt and containment failure. This was to see whether a proposed Japanese nuclear power plant could withstand the impact of a heavy aircraft. Any kerosene fire would also have little effect on that shield. In the 1980s 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. 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. Every country which operates nuclear power plants has a nuclear safety inspectorate and all of these work closely with the IAEA.
These are supported by continuous monitoring of individual doses and of the work environment to ensure very low radiation exposure compared with other industries. While this calculated core damage frequency has been one of the main metrics to assess reactor safety, European safety authorities prefer a deterministic approach, focusing on actual provision of back-up hardware, though they also undertake probabilistic safety analysis (PSA) for core damage frequency. When the 80 percent human error is broken down further, it reveals that the majority of errors associated with events stem from latent organizational weaknesses (perpetrated by humans in the past that lie dormant in the system), whereas about 30 percent are caused by the individual worker touching the equipment and systems in the facility.
Inherent or full passive safety design depends only on physical phenomena such as convection, gravity or resistance to high temperatures, not on functioning of engineered components.
Analysis of the accident showed the need for more intelligent siting criteria than those used in the 1960s, and the need for better back-up power and post-shutdown cooling, as well as provision for venting the containment of that kind of reactor and other emergency management procedures.
By way of contrast to western safety engineering, the Chernobyl reactor did not have a containment structure like those used in the West or in post-1980 Soviet designs.
All except Browns Ferry and Vandellos involved damage to or malfunction of the reactor core. They mobilized considerable expertise in different countries (500 man-years) under the responsibility of each national Safety Authority within the framework of the European Nuclear Safety Regulators Group (ENSREG). In accident scenarios, regulators consider power plants' means to protect against and manage loss of core cooling as well as cooling of used fuel in storage.
For severe accident management scenarios they must identify the time before fuel damage is unavoidable and the time before water begins boiling in used fuel ponds and before fuel damage occurs. The process was extended to June 2012 to allow more plant visits and to add more information on the potential effect of aircraft impacts. The third order applies only to the 33 BWRs with early containment designs, and will require 'reliable hardened containment vents' which work under any circumstances. As an example, French Safety Rules criteria for river sites define the safe level as above a flood level likely to be reached with one chance in one thousand years, plus 15%, and similar regarding tides for coastal sites.
In December 2004 the Madras NPP and Kalpakkam PFBR site on the east coast of India was flooded by a tsunami surge from Sumatra. 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. See also: paper on Cooperation in Nuclear Power Industry, especially for fuller description of WANO, focused on operation. 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. It aims to ensure "adequate responses based on scientific knowledge and full transparency".
IAEA Safety Standards applied in the DSR and GRSR at the fundamental and requirements level, are generic and apply to all nuclear installations.
The results of this monitoring and analysis are often shared Industry-wide through INPO and WANO networks.
Nuclear DKM may focus on knowledge creation, identification, sharing, transfer, protection, validation, storage, dissemination, preservation or utilization. A detailed audit in 1997-98 showed that the design basis was not being maintained and that 4000 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. A  thorough study was undertaken by the US Electric Power Research Institute (EPRI) using specialist consultants and paid for by the US Dept. This conclusion was documented in a 1981 EPRI study, reported and widely circulated in many languages, by Levenson and Rahn in Nuclear Technology.
It showed how most of the collision energy goes into the destruction of the aircraft itself – about 96% of the aircraft's kinetic energy went into the its destruction and some penetration of the concrete – while the remaining 4% was dissipated in accelerating the 700-tonne slab.
The extent and timing of this means that with walking-pace evacuation inside this radius it would not be a major health risk. It is still not certain how much of the core material was not contained by the pressure vessels and ended up in the bottom of the drywell containments, though certainly there was considerable release of radionuclides to the atmosphere early on, and later to cooling water**. Studies of the post-accident situation at Three Mile Island (where there was no breach of containment) supported the suggestion, and analysis of Fukushima is pending.
Clearly, focusing efforts on reducing human error will reduce the likelihood of occurrences and events." Following the Fukushima accident the focus has been on the organisational weaknesses which increase the likelihood of human error. All reactors have some elements of inherent safety as mentioned above, but in some recent designs the passive or inherent features substitute for active systems in cooling etc. Hence there is provision for relieving pressure, sometimes with a vent system, but this must work and be controlled without power. The principal conclusion is that existing resources and procedures can stop an accident, slow it down or reduce its impact before it can affect the public, but even if accidents proceed without such mitigation they take much longer to happen and release much less radioactive material than earlier analyses suggested. They also study means to protect against and manage loss of containment integrity and core melting, including consequential effects such as hydrogen accumulation.
The industry association, NEI, told the NRC that licensees with these Mark I and Mark II containments “should have the capability to use various filtration strategies to mitigate radiological releases” during severe events, and that filtration “should be founded on scientific and factual analysis and should be performance-based to achieve the desired outcome.” All the measures are supported by the industry association, which has also proposed setting up about six regional emergency response centres under NRC oversight with additional portable equipment.
Construction of the Kalpakkam plant was just beginning, but the Madras plant shut down safely and maintained cooling. Therefore, it is neither intended nor possible to cover or substitute licensing activity, or to constitute any kind of design certification.DSRs have been undertaken in Pakistan, Ukraine, Bulgaria and Armenia.
Lesser components are more straightforward to replace as they age, and some may be safety-related as well as economic. 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. 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. The maximum penetration of the concrete in this experiment was 60 mm, but comparison with fixed reactor containment needs to take account of the 4% of energy transmitted to the slab.
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. Caesium-137 has a half-life of 30 years, and is therefore potentially a long-term contaminant of pastures and crops. However, recommendations including early warning system for tsunami and provision of additional cooling water sources for longer duration cooling were implemented.
GRSRs have been done on AP1000 (USA & UK), Atmea1, APR1400, ACPR-1000+, ACP1000, and AES-2006 and VVER-TOI. This process is similar to that in other industries where safety is paramount, eg aviation. It concludes that US reactor structures "are robust and (would) protect the fuel from impacts of large commercial aircraft". Ideally any vent system should deal with any large amounts of hydrogen, as at Fukushima, and have minimum potential to spread radioactivity outside the plant. Filtered containment ventilation systems (FCVSs) are being retrofitted to some reactors which did not already have them, or any of sufficient capacity, following the Fukushima accident. The graphite was certainly incandescent as a result of fuel decay heat - sometimes over 1000°C - and some of it oxidised to carbon monoxide which burned along with the fuel cladding. 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. OSART missions are on request from the government, and involve staff from regulators, in these respects differing from WANO peer reviews. Reliability Centered Maintenance was adapted from civil aviation in the 1980s for instance, and led to nuclear industry review of existing maintenance programs. 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. The basic premise of a FCVS is that, independent of the state of the reactor itself, the catastrophic failure of the containment structure can be avoided by discharging steam, air and incondensable gases like hydrogen to the atmosphere. 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.

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