Nuclear safety in the United States

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Nuclear safety in the U.S. is governed by federal regulations and continues to be studied by the Nuclear Regulatory Commission (NRC).

It is important to note that 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.[1][2]

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.

Contents

[edit] 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.

[edit] 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".

An unofficial and unverified list of the relevant regulations, last updated in 2003, can be found at 2003 Code of Federal Regulations.

[edit] Safety of nuclear power plants

Nuclear power plants contain most of the non-military radioactive material in the U.S. NRC-generated safety regulation ballooned in the years immediately preceding the Three Mile Island accident, and surged again afterwards.

[edit] Geologic and seismic siting criteria

Geologic and seismic siting criteria are governed by federal regulation 10CFR100.23.[3]

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."[4] The pattern of the earth's motion is considered as well as the strength of the vibrations.

[edit] Population criteria

Population-criteria for siting U.S. nuclear power plants is covered under federal regulation 10CFR100.11.[5]

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.

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.

[edit] Protection from attack

Nuclear power plants are required to withstand the government-specified "Design Basis Threat" (DBT). The specifics of the DBT are a government secret.

[edit] The Protected Area

The Protected Area encloses the Exclusion Zone calculated above. 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.[6]

[edit] 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."[7]

[edit] 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.

[edit] Reactor Protective System (RPS)

Main article: Reactor Protective System

[edit] 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.[8] 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).

[edit] Assessments of risks of nuclear power plants

The NRC (and its predecessors) have over the decades produced three major analyses of the risks of nuclear power: a fourth, all-encompassing one 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) are:

General Electric has recalculated maximum core damage frequencies per year per plant for its nuclear power plant designs:[9]

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 (stated at this accuracy, one may question the analysis) per plant per year. The European Pressurized Reactor (EPR) has a maximum core damage frequency of 4 × 10–7 per plant per year.[10]

[edit] Accidents

Main article: Nuclear and radiation accidents

Dozens of radiation accidents have happened in the United States. They range from relatively small accidents where one person was exposed to radiation to what the NRC labels a "significant precursor" event.

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.

While no significant precursor events have been reported since then, other accidents continue to occur.[11]

[edit] 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 was 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.[12] 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.[12]

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.[citation needed])

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.

[edit] Chernobyl

The Chernobyl disaster was a major accident at the Chernobyl Nuclear Power Plant near Pripyat, Ukraine Soviet Union on April 26, 1986, consisting of an explosion at the plant and subsequent radioactive contamination of the surrounding geographic area. It is regarded as the worst accident ever in the history of nuclear power.[13]

The RBMK design used at Chernobyl could not have been built in the U.S., for a number of reasons.[14] First, it was impossible to build a full containment structure around it. Second, all U.S. reactors have a negative void coefficient, meaning that an increase in steam bubbles forming in the reactor core tends to decrease power, not increase it further — U.S. reactors must not be unstable against a power or temperature increase. Third, what existed of Chernobyl's Reactor Protective System was inadequate and not fail-safe, and a warning from a safety component was in fact ignored (in violation of the plant's procedures) without Scramming the plant. The Nuclear Regulatory Commission would not have issued a license to construct a plant such as an RBMK.

[edit] 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.[15] 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 iodine pills defeat this concentration of radioactive iodine by flooding the body with normal iodine, causing any additional iodine to be rejected.

[edit] References

  1. ^ About NRC, U.S. Nuclear Regulatory Commission, Retrieved 2007-6-1
  2. ^ Our Governing Legislation, U.S. Nuclear Regulatory Commission, Retrieved 2007-6-1
  3. ^ 10CFR100.23.
  4. ^ Standards definitions, from the American Nuclear Society
  5. ^ 10CFR100.11
  6. ^ Nuclear Power Plants Are Most Secure Industrial Facilities in U.S., NEI Tells Congress
  7. ^ STATEMENT FROM CHAIRMAN DALE KLEIN ON COMMISSION'S AFFIRMATION OF THE FINAL DBT RULE. Nuclear Regulatory Commission. Retrieved on 2007-04-07.
  8. ^ 10CFR50.49
  9. ^ Hinds, David; Chris Maslak (January 2006). Next-generation nuclear energy: The ESBWR (PDF). Nuclear News. Retrieved on 2008-05-13.
  10. ^ [1] (PDF)[dead link]
  11. ^ Madigan, Glasgow File Suit for Radioactive Leaks at Braidwood Nuclear Plant. Illinois Attorney General (2006). Retrieved on 2006-03-17.
  12. ^ a b Fact Sheet on the Three Mile Island Accident. U.S. Nuclear Regulatory Commission (February 20, 2007). Retrieved on 2008-05-15.
  13. ^ Geographical location and extent of radioactive contamination
  14. ^ University of Pittsburgh info on Chernobyl
  15. ^ Consideration of Potassium Iodide in Emergency Planning. U.S. Nuclear Regulatory Commission. Retrieved on 2006-11-10.

[edit] See also

[edit] External links