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.

Licensees (organizations applying for construction licenses or operating licenses for nuclear facilities) are required to show before the license is issued that they meet the requirements of the regulations.

Scope

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.

Nomenclature

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".

Safety of nuclear power plants

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 March 2011, nuclear experts told Congress that spent-fuel pools at US nuclear power plants are too full. They say the entire US spent-fuel policy should be overhauled in light of the Fukushima I nuclear accidents.

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.

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.

Geologic and seismic siting criteria

Geologic and seismic siting criteria are governed by federal regulation 10CFR100.23.

Nuclear power plants are designed to withstand the credible earthquakes ("Operating Basis Earthquake" and "Safe Shutdown Earthquake") with no damage to safety-related equipment per 10CFR100's Appendix A "Seismic and Geologic Criteria for Nuclear Power Plants." The pattern of the Earth's motion is considered as well as the strength of the vibrations.

Population criteria

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.

Protection from attack

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 shield

Main 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."

Procedures

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).

Accidents

Main 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.

Main article: Nuclear meltdown

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.

A "significant precursor" is an event that leads to a conditional core damage probability (CCDP) or increase in core damage probability (CDP) that is greater than or equal to 1 × 10^–3. In other words, given that the precursor event has occurred, the probability that a subsequent failure will cause core damage is ≥ 0.001.

As of 2005 the NRC reports that there have been 33 significant precursor events beginning in 1971 to 1979 (Three Mile Island). Since Three Mile Island, none have been reported.

No significant precursor events have been reported since then, though some groups claim other accidents have occurred.

Emergency Classifications

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 guides (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 Island

Main article: Three Mile Island accident

On March 28, 1979, in the USA, the Unit 2 nuclear power plant (a pressurized water reactor) on the Three Mile Island Nuclear Generating Station in Dauphin County, Pennsylvania near Harrisburg suffered a partial core meltdown. The Three Mile Island accident is considered to be the worst accident in American commercial nuclear power generating history, even though it led to no deaths or injuries to plant workers or members of the nearby community. Importantly, the reactor vessel did not rupture.

During the Three Mile Island accident, small amounts of radioactive gases were released. In addition to accidental release, radioactive gases were deliberately released into the atmosphere by the operators to relieve pressure on the primary system and avoid curtailing the flow of coolant to the core.

From a safety viewpoint, the system functioned as designed. Emergency Core Cooling Systems automatically turned on, and were turned off by the operators who had the mistaken belief that the reactor vessel was full of water (due to the faulty pressurizer reading caused by the stuck-open PORV). Finally, a fuel temperature check was done, revealing the problem. (Note: the vast majority of plants have direct measurements of water level in the reactor vessel, and do not rely on readings from the pressurizer.)

While the system functioned as designed, unfortunately, the design was found to be flawed. There is consensus that the accident was exacerbated by wrong decisions made because the operators were overwhelmed with information, much of it irrelevant, misleading or incorrect.

Extensive regulation and plant changes followed the accident. In addition to the improved operating training, improvements in quality assurance, engineering, operational surveillance and emergency planning have been instituted. Improvements in control room habitability, "sight lines" to instruments, ambiguous indications and even the placement of "trouble" tags were made; some trouble tags were covering important instrument indications during the accident.

List of accidents

See 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 (in millions 2006 $)
January 3, 1961	Idaho Falls, Idaho, US	Explosion at 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

Whistleblowers

Main 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.

Potassium iodide

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.

See also

• Nuclear safety
• 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

External links

• The US Nuclear Regulatory Commission supervises the US Nuclear industry
• Nuclear Power Safety and Security Information