- Nuclear safety in the United States
Nuclear safety in the U.S. is governed by federal regulations and continues to be studied by the Nuclear Regulatory Commission (NRC). The safety of nuclear plants and materials controlled by the U.S. government for research and weapons production, as well those powering naval vessels, is not governed by the NRC.
Following the Fukushima Daiichi nuclear disaster, according to Black & Veatch’s annual utility survey that took place after the disaster, of the 700 executives from the US electric utility industry that were surveyed, nuclear safety was the top concern. There are likely to be increased requirements for on-site spent fuel management and elevated design basis threats at nuclear power plants. License extensions for existing reactors will face additional scrutiny, with outcomes depending on the degree to which plants can meet new requirements, and some of the extensions already granted for more than 60 of the 104 operating U.S. reactors could be revisited. On-site storage, consolidated long-term storage, and geological disposal of spent fuel is "likely to be reevaluated in a new light because of the Fukushima storage pool experience".
In October 2011, the Nuclear Regulatory Commission instructed agency staff to move forward with seven of the 12 safety recommendations put forward by the federal task force in July. The recommendations include "new standards aimed at strengthening operators’ ability to deal with a complete loss of power, ensuring plants can withstand floods and earthquakes and improving emergency response capabilities". The new safety standards will take up to five years to fully implement.
The topic of nuclear safety covers:
- The research and testing of the possible incidents/events at nuclear facilities,
- What equipment and actions are designed to prevent those incidents/events from having serious consequences,
- The calculation of the probabilities of multiple systems and/or actions failing thus allowing serious consequences,
- The evaluation of the possible timing and scope of those serious consequences (the worst-possible result in extreme cases being a release of radiation),
- The actions taken to protect the public during a release of radiation,
- The training and rehearsals performed to ensure readiness in case an incident/event occurs.
- Accidents that have occurred.
In the following, the names of federal regulations will be abbreviated in the standard way. For example, "Code of Federal Regulations, Title 10, Part 100, Section 23" will be given as "10CFR100.23".
More than a quarter of U.S. nuclear plant operators "have failed to properly tell regulators about equipment defects that could imperil reactor safety", according to a Nuclear Regulatory Commission report.
In February 2011, a major manufacturer in the nuclear industry reported a potential "substantial safety hazard" with control rods at more than two dozen reactors around the USA. GE Hitachi Nuclear Energy said it had discovered extensive cracking and "material distortion," and recommended that the boiling water reactors using its Marathon control rod blades replace them more frequently than previously told. If the design life if not revised, it "could result in significant control blade cracking and could, if not corrected, create a substantial safety hazard and is considered a reportable condition," the company said in its report to the NRC.
Radioactive waste storage
The Fukushima nuclear disaster has reopened questions about the risks of U.S. nuclear reactors, and especially the pools that store spent fuel. In March 2011, nuclear experts told Congress that spent-fuel pools at US nuclear power plants are too full. A fire at a spent-fuel pool could release cesium-137. Experts say the entire US spent-fuel policy should be overhauled in light of Fukushima I.
With the cancellation of the Yucca Mountain nuclear waste repository in Nevada, more nuclear waste is being loaded into sealed metal casks filled with inert gas. Many of these casks will be stored in coastal or lakeside regions where a salt air environment exists, and the Massachusetts Institute of Technology is studying how such dry casks perform in salt environments. Some hope that the casks can be used for 100 years but cracking related to corrosion could occur in 30 years or less. Robert Alvarez, a former Department of Energy official who oversaw nuclear issues, said dry casks would provide safer storage until a permanent nuclear repository was built and loaded, a process that would take decades.
About one third of reactors in the US are boiling water reactors, the same technology which was involved in the Fukushima Daiichi nuclear disaster in Japan. There are also eight nuclear power plants located along the seismically active West coast. Twelve of the American reactors that are of the same vintage as the Fukushima Daiichi plant are in seismically active areas. 
Earthquake risk is often measured by "Peak Ground Acceleration", or PGA. The following nuclear power plants have a two percent or greater chance of having PGA over 0.15g in the next 50 years: Diablo Canyon, Calif.; San Onofre, Calif.; Sequoyah, Tenn.; H.B. Robinson, SC.; Watts Bar, Tenn.; Virgil C. Summer, SC.; Vogtle, GA.; Indian Point, NY.; Oconee, SC.; and Seabrook, NH.
GE Mark 1 reactor design
Experts have long criticized General Electric's Mark I reactor design, because it offered a relatively weak containment vessel. Three GE scientists resigned 35 years ago in protest of the design of the Mark I containment system. David Lochbaum, chief nuclear safety officer with the Union of Concerned Scientists, has repeatedly questioned the safety of the Fukushima I Plant's GE Mark 1 reactor design, which is used in almost a quarter of the United States' nuclear fleet.
Aging of nuclear reactors
An important concern in the nuclear safety field is the aging of nuclear reactors. Researchers at Pennsylvania State University will use ultrasonic waves to look for cracks and other defects in hot metal parts, in order to identify “microscale” defects that lead to big cracks.
Population-criteria for siting U.S. nuclear power plants is covered under federal regulation 10CFR100.11.
Minimum distances must be set for an exclusion area (which is typically inside the Protected Area's fence), a low population zone and a population center distance. To calculate the minimum assured distances for each of these, a maximum possible amount of radioactivity release (called a "source term") must be assumed and worst-case wind conditions must be assumed.
Nuclear power plants in their licensing submittals so far have used extremely conservative fallout inputs from the somewhat antiquated WASH-1400 study. The NRC has disavowed the assumptions and thus the results of WASH-1400 as being far too pessimistic (see NUREG-1150), and is in the process of generating a new state-of-the-art study (see SOARCA).
A bounding calculation using a source term from WASH-1400 typically calculates a minimum Emergency Planning Zone (EPZ) of about 5 miles (8.0 km) from the plant, which in practice is rounded up to 10 miles (16 km) for actual implementation.
After 9/11, it would seem prudent for nuclear plants to be prepared for an attack by a large, well-armed terrorist group. But the Nuclear Regulatory Commission, in revising its security rules, decided not to require that plants be able to defend themselves against groups carrying sophisticated weapons. According to a study by the Government Accountability Office, the N.R.C. appeared to have based its revised rules "on what the industry considered reasonable and feasible to defend against rather than on an assessment of the terrorist threat itself".
The Protected Area
The Protected Area encloses the Exclusion Zone (as defined in 10CFR100.3 ). It also serves as a security zone, within which only trusted individuals are allowed to walk unescorted.
The Protected Area is surrounded by a double fence, and the gap in between the fences is electronically monitored. There are very few gates, and those are well guarded. Numerous other security measures are in effect.
The missile shieldMain article: Containment building
The missile shield protecting the containment structure was originally intended to protect only from natural forces, such as tornadoes. For example, it usually is designed to withstand the impact of a telephone pole flying at 60 miles per hour (100 km/h) and hitting end-on. One plant, Florida's Turkey Point NGS, survived a direct hit by Category 5 Hurricane Andrew in 1992, with no damage to the containment.
No actual missile shield has been subjected to an aircraft impact test. However, a highly similar test was done at Sandia National Laboratories and filmed (see Containment building), and the target was essentially undamaged (reinforced concrete is strongly resistant both to impact and to fire). The NRC's Chairman has said "Nuclear power plants are inherently robust structures that our studies show provide adequate protection in a hypothetical attack by an airplane. The NRC has also taken actions that require nuclear power plant operators to be able to manage large fires or explosions - no matter what has caused them."
In the U.S., the Operating License is granted by the government and carries the force of law. The Final Safety Analysis Report (FSAR) is part of the Operating License, and the plant's Technical Specifications (which contain the restrictions the operators consult during operation) are a chapter of the FSAR. All procedures are checked against the Technical Specifications and also by a Transient Analysis engineer, and each copy of an approved procedure is numbered and the copies controlled (so that updating all copies at once can be assured). In a U.S. nuclear power plant, unlike in most other industries, approved procedures carry the force of law and to deliberately violate one is a criminal act.
Reactor Protective System (RPS)Main article: Reactor Protective System
Design Basis Events
"Design Basis Events [DBE] are defined as conditions of normal operation, including anticipated operational occurrences, design basis accidents, external events, and natural phenomena for which the plant must be designed to ensure functions (b)(1)(i) (A) through (C)" of 10CFR50-49. These include (A) maintaining the integrity of the reactor coolant pressure boundary; (B) maintaining the capability to shut down the reactor and maintain it in a safe shutdown condition; OR (C) maintaining the capability to prevent or mitigate the consequences of accidents that could result in potential offsite exposures. The normal DBEs evaluated are Station Blackout (where all offsite and onsite AC power is lost for a specified duration) and loss-of-coolant accident (LOCA).
As the Fukushima I nuclear accidents showed, external threats — such as earthquakes, tsunamis, fires, flooding, tornadoes and terrorist attacks — are some of the greatest risk factors for a serious nuclear accident. Yet, nuclear plant operators have normally considered these 'beyond design basis events' so unlikely that they have not built in complete safeguards.
Assessments of risks
The NRC (and its predecessors) have over the decades produced three major analyses of the risks of nuclear power: a fourth, all-encompassing one (the State-of-the-Art Reactor Consequence Analyses, or SOARCA, study) is in generation now. The new study will be based on actual test results, on probabilistic risk assessment (PRA) methodology, and on the evaluated actions of government agencies.
The existing studies (all now disavowed by the NRC and to be replaced by SOARCA) are:
- NUREG-1150 (1991)
- CRAC-II (1982) (based on WASH-1400 results)
- WASH-1400 (1975)
- WASH-740 (1957) (not PRA-based)
Comparisons of risks of nuclear power plants
Reactor vendors now routinely calculate probabilistic risk assessments of their nuclear power plant designs.
General Electric has recalculated maximum core damage frequencies per year per plant for its nuclear power plant designs:
- BWR/4 — 1 × 10–5 (a typical plant)
- BWR/6 — 1 × 10–6 (a typical plant)
- ABWR — 2 × 10–7 (now operating in Japan)
- ESBWR — 3 × 10–8 (submitted for Final Design Approval by NRC)
The AP1000 has a maximum core damage frequency of 5.09 × 10–7 per plant per year. The European Pressurized Reactor (EPR) has a maximum core damage frequency of 4 × 10–7 per plant per year.
AccidentsMain article: Nuclear accidents
Nuclear irradiation accidents have occurred in the United States. There are several types of accidents, and catastrophic meltdown is only one type. Criticality accidents are unsustained bursts of nuclear radiation which occur when too much fissile material (a substance capable of sustaining a nuclear fission chain reaction, e.g., nuclear fuel) is brought together, leading to a nuclear chain reaction for a very brief period of time. This usually results in a blue flash. Close proximity to such an event can cause radiation sickness and death if the reaction was sufficiently large.
A nuclear meltdown is a term for a nuclear reactor accident which results in the overheating and melting of the reactor core. This is a problem because it opens the potential for the containment building to fail, resulting in release of radioactive material into the atmosphere and environment. It should be noted that reactors are designed in such a way that if there is a meltdown, the reactor will not go supercritical and cause a nuclear explosion.
The NRC established a classification scale for nuclear power plant events to ensure consistency in the communications and response.
Unusual Event – This is the lowest of the four emergency classifications. This classification indicates that a small problem has occurred. No radiation leak is expected and federal, state and county officials are notified.
Alert –Events are in process or have occurred which involve an actual or potential substantial degradation in the level of safety of the plant. Any releases of radioactive material from the plant are expected to be limited to a small fraction of the Environmental Protection Agency (EPA) Protective Action Guide for Nuclear Incidents (PAGs)
Site Area Emergency – Involves events in process or which have occurred that result in actual or likely major failures of plant functions needed for protection of the public. Any releases of radioactive material are not expected to exceed the EPA PAGs except near the site boundary.
General Emergency – The most serious emergency classification and indicates a serious problem. A general emergency involves actual or imminent substantial core damage or melting of reactor fuel with the potential for loss of containment integrity. Emergency sirens will be sounded and federal, state and county officials will act to ensure public safety. Radioactive releases during a general emergency can reasonably be expected to exceed the EPA PAGs for more than the immediate site area.
Three Mile IslandMain article: Three Mile Island accident
On March 28, 1979, equipment failures and operator error contributed to loss of coolant and a partial core meltdown at the Three Mile Island Nuclear Power Plant in Pennsylvania. The mechanical failures were compounded by the initial failure of plant operators to recognize the situation as a loss-of-coolant accident due to inadequate training and human factors, such as human-computer interaction design oversights relating to ambiguous control room indicators in the power plant's user interface. In particular, a hidden indicator light led to an operator manually overriding the automatic emergency cooling system of the reactor because the operator mistakenly believed that there was too much coolant water present in the reactor and causing the steam pressure release. The scope and complexity of the accident became clear over the course of five days, as employees of Met Ed, Pennsylvania state officials, and members of the U.S. Nuclear Regulatory Commission (NRC) tried to understand the problem, communicate the situation to the press and local community, decide whether the accident required an emergency evacuation, and ultimately end the crisis. The NRC's authorization of the release of 40,000 gallons of radioactive waste water directly in the Susquehanna River led to a loss of credibility with the press and community.
The World Nuclear Association has stated that cleanup of the damaged nuclear reactor system at TMI-2 took nearly 12 years and cost approximately US $973 million. Benjamin K. Sovacool, in his 2007 preliminary assessment of major energy accidents, estimated that the TMI accident caused a total of $2.4 billion in property damages. The health effects of the Three Mile Island accident are widely, but not universally, agreed to be very low level. The accident triggered protests around the world.
List of accidentsSee also: Nuclear power plant accidents in the United States
Nuclear power plant accidents in the U.S.
with multiple fatalities or more than US$100 million in property damage, 1952-2010
Date Location Description Fatalities Cost
January 3, 1961 Idaho Falls, Idaho, US Criticality Steam Explosion at SL-1 National Reactor Testing Station 3 $US22 March 28, 1979 Middletown, Pennsylvania, US Loss-of-coolant and partial core meltdown, see Three Mile Island accident and Three Mile Island accident health effects 0 US$2,400 September 15, 1984 Athens, Alabama, US Safety violations, operator error, and design problems force six year outage at Browns Ferry Unit 2 0 US$110 March 9, 1985 Athens, Alabama, US Instrumentation systems malfunction during startup, which led to suspension of operations at all three Browns Ferry Units 0 US$1,830 April 11, 1986 Plymouth, Massachusetts, US Recurring equipment problems force emergency shutdown of Boston Edison’s Pilgrim Nuclear Power Plant 0 US$1,001 March 31, 1987 Delta, Pennsylvania, US Peach Bottom units 2 and 3 shutdown due to cooling malfunctions and unexplained equipment problems 0 US$400 December 19, 1987 Lycoming, New York, US Malfunctions force Niagara Mohawk Power Corporation to shut down Nine Mile Point Unit 1 0 US$150 March 17, 1989 Lusby, Maryland, US Inspections at Calvert Cliff Units 1 and 2 reveal cracks at pressurized heater sleeves, forcing extended shutdowns 0 US$120 February 20, 1996 Waterford, Connecticut, US Leaking valve forces shutdown Millstone Nuclear Power Plant Units 1 and 2, multiple equipment failures found 0 US$254 September 2, 1996 Crystal River, Florida, US Balance-of-plant equipment malfunction forces shutdown and extensive repairs at Crystal River Unit 3 0 US$384 February 16, 2002 Oak Harbor, Ohio, US Severe corrosion of control rod forces 24-month outage of Davis-Besse reactor 0 US$143 February 1, 2010 Montpelier, Vermont, US Deteriorating underground pipes from the Vermont Yankee Nuclear Power Plant leak radioactive tritium into groundwater supplies 0 US$700
Experts have disagreed about whether an accident as serious as the Chernobyl disaster could occur in the USA. In 1986, Commissioner Asselstine testified before Congress that:
While we hope that their occurrence is unlikely, there are accident sequences for U.S. plants that can lead to rupture or by-passing of the containment in U.S. reactors which would result in the off-site release of fission products comparable or worse than the releases estimated by the NRC to have taken place during the Chernobyl accident.
WhistleblowersMain article: Nuclear power whistleblowers
In 1976 Gregory Minor, Richard Hubbard, and Dale Bridenbaugh "blew the whistle" on safety problems at nuclear power plants in the United States. The three nuclear engineers gained the attention of journalists and their disclosures about the threats of nuclear power had a significant impact.
According to the Nuclear Regulatory Commission, 20 states in the USA have requested stocks of potassium iodide which the NRC suggests should be available for those living within 10 miles (16 km) of a nuclear power plant in the unlikely event of a severe accident. Iodine is a fission product in a nuclear reactor, and in the event of a severe accident a fraction of that iodine is expected to leak from the fuel and out of the containment building. If ingested, this iodine would tend to be accumulated by a person's thyroid. Potassium Iodide (KI) is an over-the-counter drug that may reduce the amount of radioactive iodine absorbed by the body’s thyroid gland. KI offers a degree of protection only to the thyroid gland and only in cases when the release contains radioactive iodine. KI would be supplemental to evacuation and sheltering. In cases where the public may be exposed to certain types of radioactivity, state and local health officials may advise the public to take Potassium Iodide (KI) tablets.
Radioactive Iodine (radioiodine) is one of the products that can be released in a serious nuclear power plant accident. Potassium Iodide (KI) is a non radioactive form of iodine that may be taken to reduce the amount of radioactive iodine absorbed by the body’s thyroid gland. When taken before or shortly after a radiological exposure, potassium iodide blocks the thyroid glands ability to absorb radioactive iodine. KI is a secondary protection for evacuation and sheltering are the primary means of protection.
Potassium Iodide should be taken by the public during an emergency only when directed by public health officials. A TV and radio Emergency Alert System (EAS) message will be broadcast and public health officials will inform the public when to take KI. Potassium iodide is available to persons within 10 miles of the plant though the county health department. During an emergency, KI is available to the general public at evacuation Reception Centers.
- Nuclear safety
- Protective Action Guide for Nuclear Incidents
- Nuclear power
- Nuclear power in the United States
- List of nuclear reactors
- Institute of Nuclear Power Operations
- Nuclear Power 2010 Program
- List of civilian nuclear accidents
- List of civilian radiation accidents
- List of military nuclear accidents
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- The US Nuclear Regulatory Commission supervises the US Nuclear industry
- Nuclear Power Safety and Security Information
- Nuclear safety
- Nuclear energy in the United States
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