More Info

Thursday, November 13, 2008

Nuclear safety

Nuclear safety covers the actions taken to prevent nuclear and radiation accidents or to limit their consequences. This covers nuclear power plants as well as all other nuclear facilities, the transportation of nuclear materials, the use and storage of nuclear materials for medical, power, industry, and military uses. In addition, there are safety issues involved in products created with radioactive materials. Some of the products are legacy ones (such as watch faces), others, like smoke detectors, are still being produced.

Nuclear weapon safety, as well as the safety of military research involving nuclear materials, is generally handled by separate agencies than civilian safety, for various reasons, including secrecy.


Many nations utilizing nuclear power have special institutions overseeing and regulating nuclear safety.

Internationally the International Atomic Energy Agency "works for the safe, secure and peaceful uses of nuclear science and technology."

Civilian nuclear safety in the U.S. is regulated by the Nuclear Regulatory Commission (NRC). The safety of nuclear plants and materials controlled by the U.S. government for research, weapons production, and those powering naval vessels, is not governed by the NRC.

In the UK nuclear safety is regulated by the Nuclear Installations Inspectorate (NII) and the Defence Nuclear Safety Regulator (DNSR).

Key concepts

Nuclear safety imposes strict demands on the containment of toxic and/or radioactive materials. Contamination of surrounding communities and environment is regarded as a never events from the perspective of plant design. Due to the energetic nature of nuclear reactions, nuclear material in a chain reaction is not necessarily stable from an energy output perspective, often requiring active control mechanisms to impose artificial stability.

Systems are often designed with multiple redundant backups to preclude system failure, with each independent system often designed with a conservative factor of safety in an attempt to preclude failure of the primary system in the first place. Elimination of common mode failure mechanisms is integral to the design of nuclear facilities; preventing cascade failures.

Many facilities are designed around the defence in depth approach, with multiple active and passive systems designed around preventing catastrophic failure. At the core of such a system one finds the Reactor Protective System, with ionising radiation protection incorporated to protect facility crews and emergency responders in the event of an accident. The final layer of protection is typically a large containment building designed to prevent the release of nuclear material in the event that all active systems should be rendered inoperative.

Finally, beyond just technological means, human factors must also be taken into account. Elimination of conflict of interest is a key concern in regulatory strategy, and development of a safety culture to ensure that operator error does not allow avoidable errors to occur.


Nuclear power plants are some of the most complex systems ever devised, although much of that complexity is due to redundancy of systems, extensive backups, and the defense in depth strategy of design.

Failure modes of nuclear powerplants

There are concerns that a combination of human and mechanical error at a nuclear facility could result significant harm to people and the environment:

Operating nuclear reactors contain large amounts of radioactive fission products which, if dispersed, could pose a direct radiation hazard, contaminate soil and vegetation, and be ingested by humans and animals. Human exposure at high enough levels can cause both short-term illness and death, and longer-term deaths by cancer and other diseases.

Nuclear reactors can fail in a variety of ways. Should the instability of the nuclear material generate unexpected behavior, it may result in an uncontrolled power excursion. Normally, the cooling system in a reactor is designed to be able to handle the excess heat this causes, however, should the reactor also experience a loss-of-coolant accident, then the fuel may melt, or cause the vessel it is contained in to overheat and melt. This event is called a nuclear meltdown. Because the heat generated can be tremendous, immense pressure can build up in the reactor vessel, resulting in a steam explosion such as happened at Chernobyl.

Intentional cause of such failures may be the result of nuclear terrorism.

Hazards of nuclear material

Nuclear material and materiel may be hazardous if not properly handled or disposed of. Experiments of near critical mass sized pieces of nuclear material can pose a risk of a criticality accident. David Hahn serves as an excellent example of a nuclear experimenter who failed to develop or follow proper safety protocols. Such failures raise the specter of radioactive contamination.

Even when properly contained, fission by-products which are no longer useful generates radioactive waste, which must be properly disposed of. In addition, material exposed to nuclear material may become radioactive in its own right, or become contaminated with nuclear waste. Additionally, toxic or dangerous chemicals my be used as part of the plant's operation, which must be properly handled and disposed of.

Vulnerability of plants to attack

Nuclear power plants are generally (although not always) considered "hard" targets. In the US, plants are surrounded by a double row of tall fences which are electronically monitored. The plant grounds are patrolled by a sizeable force of armed guards. The NRC's "Design Basis Threat" criteria for plants is a secret, and so what size attacking force the plants are able to protect against is unknown. However, to scram a plant takes less than 5 seconds while unimpeded restart takes hours, severely hampering a terrorist force in a goal to release radioactivity.

Attack from the air is a more problematic concern. The most important barrier against the release of radioactivity in the event of an aircraft strike is the containment building and its missile shield. 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 addition, supporters point to large studies carried out by the US Electric Power Research Institute that tested the robustness of both reactor and waste fuel storage, and found that they should be able to sustain a terrorist attack comparable to the September 11 terrorist attacks in the USA. Spent fuel is usually housed inside the plant's "protected zone" or a spent nuclear fuel shipping cask; stealing it for use in a "dirty bomb" is extremely difficult. Exposure to the intense radiation would almost certainly quickly incapacitate or kill anyone who attempts to do so.

Risk assessment

* International Nuclear Events Scale
* Probabilistic risk assessment
o Severe Accident Risks: An Assessment for Five U.S. Nuclear Power Plants NUREG-1150 1991
o Calculation of Reactor Accident Consequences CRAC-II 1982
o Rasmussen Report: Reactor Safety Study WASH-1400 1975
o The Brookhaven Report: Theoretical Possibilities and Consequences of Major Accidents in Large Nuclear Power Plants WASH-740 1957

The AP1000 has a maximum core damage frequency of 5.09 x 10-7 per plant per year. The Evolutionary Power Reactor (EPR) has a maximum core damage frequency of 4 x 10-7 per plant per year. General Electric has recalculated maximum core damage frequencies per year per plant for its nuclear power plant designs:

BWR/4 -- 1 x 10-5
BWR/6 -- 1 x 10-6
ABWR -- 2 x 10-7
ESBWR -- 3 x 10-8

Enforcement organisations

* International Atomic Energy Agency
o International Nuclear Safety Advisory Group
* United States Atomic Energy Commission
o Nuclear Regulatory Commission (U.S.A)
* Canadian Nuclear Safety Commission
* Autorité de sûreté nucléaire, the French nuclear safety authority
* Radiological Protection Institute of Ireland
* Federal Atomic Energy Agency in Russia
* Radiation and Nuclear Safety Authority of Finland
* Nuclear Installations Inspectorate (UK)
o Defence Nuclear Safety Regulator (UK)
* Kernfysische dienst, (NL)
* Pakistan Nuclear Regulatory Authority
* Bundesamt für Strahlenschutz, (DE)
* Atomic Energy Regulatory Board (India)

No comments: