Nuclear reactor safety

Diagram demonstrating the defense in depth quality of nuclear power
 plants.1st layer of defense is the inert, ceramic quality of the
uranium oxide itself.2nd layer is the air tight zirconium alloy
 of the fuel rod.3rd layer is the reactor pressure vessel
made of steel more than a dozen centimeters
thick.4th layer is the pressure resistant, air tight
containment building.5th layer is the reactor building or
 in newer powerplants a second outer containment building.

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, and the use and storage of nuclear materials for medical, power, industry, and military uses.

The nuclear power industry has improved the safety and performance of reactors, and has proposed new (but generally untested) “inherently” safe reactor designs but there is no guarantee that the reactors will be designed, built and operated correctly. Mistakes do occur and catastrophic scenarios involving terrorist attacks are still conceivable.

Nuclear weapon safety, as well as the safety of military research involving nuclear materials, is generally handled by agencies different from those that oversee civilian safety, for various reasons, including secrecy.

Agencies

IAEA headquarters in Vienna, Austria

Internationally the International Atomic Energy Agency "works for the safe, secure and peaceful uses of nuclear science and technology." Many nations utilizing nuclear power have special institutions overseeing and regulating nuclear safety.

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).

The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) is the Federal Government body that monitors and identifies solar radiation and nuclear radiation risks in Australia. It is the main body dealing with ionizing and non-ionizing radiation and publishes material regarding radiation protection.

Other agencies include:

Canadian Nuclear Safety Commission

Radiological Protection Institute of Ireland

Federal Atomic Energy Agency in Russia

Kernfysische dienst, (NL)

Pakistan Nuclear Regulatory Authority

Bundesamt für Strahlenschutz, (DE)

Atomic Energy Regulatory Board (India)

Complexity

Nuclear power plants are some of the most sophisticated and complex energy systems ever designed. Any complex system, no matter how well it is designed and engineered, cannot be deemed failure-proof. Stephanie Cooke has reported that:

The reactors themselves were enormously complex machines with an incalculable number of things that could go wrong. When that happened at Three Mile Island in 1979, another fault line in the nuclear world was exposed. One malfunction led to another, and then to a series of others, until the core of the reactor itself began to melt, and even the world's most highly trained nuclear engineers did not know how to respond. The accident revealed serious deficiencies in a system that was meant to protect public health and safety.

A fundamental issue related to complexity is that nuclear power systems have exceedingly long lifetimes. The timeframe involved from the start of construction of a commercial nuclear power station, through to the safe disposal of its last radioactive waste, may be 100 to 150 years.

Failure modes of nuclear power plants

Diagram demonstrating the defense in depth quality of nuclear power plants.

1st layer of defense is the inert, ceramic quality of the uranium oxide itself.

2nd layer is the air tight zirconium alloy of the fuel rod.

3rd layer is the reactor pressure vessel made of steel more than a dozen centimeters thick.

4th layer is the pressure resistant, air tight containment building.

5th layer is the reactor building or in newer powerplants a second outer containment building. 

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

Operating nuclear reactors contain large amounts of radioactive fission products which, if dispersed, can 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 death 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, which happened at Chernobyl. However, the reactor design used at Chernobyl was unique in many ways. It utilized a positive void coefficient, meaning a cooling failure caused reactor power to rapidly escalate. All reactors built outside the former Soviet Union have had negative void coefficients, a passively safe design. More importantly though, the Chernobyl plant lacked a containment structure. Western reactors have this structure, which acts to contain radiation in the event of a failure. Containment structures are, by design, some of the strongest structures built by mankind.

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

Hazards of nuclear material

Nuclear material 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, "The Radioactive Boy Scout" who tried to build a nuclear reactor at home, 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 byproducts which are no longer useful generate radioactive waste, which must be properly disposed of. In addition, material exposed to neutron radiation—present in nuclear reactors—may become radioactive in its own right, or become contaminated with nuclear waste. Additionally, toxic or dangerous chemicals may be used as part of the plant's operation, which must be properly handled and disposed of.

Vulnerability of nuclear plants to attack

Nuclear power plants are generally (although not always) considered "hard" targets. In the U.S., 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 of attacking force the plants are able to protect against is unknown. However, to scram (make an emergency shutdown) 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 an issue that has been highlighted since the September 11 attacks in the U.S. However, it was in 1972 when three hijackers took control of a domestic passenger flight along the east coast of the U.S. and threatened to crash the plane into a U.S. nuclear weapons plant in Oak Ridge, Tennessee. The plane got as close as 8,000 feet above the site before the hijackers’ demands were met.

The most important barrier against the release of radioactivity in the event of an aircraft strike on a nuclear power plant is the containment building and its missile shield. Current NRC Chairman Dale Klein 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 U.S. 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 U.S. 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.

In September 2010, analysis of the Stuxnet computer worm suggested that it was designed to sabotage a nuclear power plant. Such a cyber attack would bypass the physical safeguards in place and so the exploit demonstrates an important new vulnerability.

New nuclear technologies

The next nuclear plants to be built will likely be Generation III or III+ designs, and a few such are already in operation in Japan. Generation IV reactors would have even greater improvements in safety. These new designs are expected to be passively safe or nearly so, and perhaps even inherently safe (as in the PBMR designs).

Some improvements made (not all in all designs) are having three sets of emergency diesel generators and associated emergency core cooling systems rather than just one pair, having quench tanks (large coolant-filled tanks) above the core that open into it automatically, having a double containment (one containment building inside another), etc.

However, safety risks may be the greatest when nuclear systems are the newest, and operators have less experience with them. Nuclear engineer David Lochbaum explained that almost all serious nuclear accidents occurred with what was at the time the most recent technology. He argues that "the problem with new reactors and accidents is twofold: scenarios arise that are impossible to plan for in simulations; and humans make mistakes". As one director of a U.S. research laboratory put it, "fabrication, construction, operation, and maintenance of new reactors will face a steep learning curve: advanced technologies will have a heightened risk of accidents and mistakes. The technology may be proven, but people are not".

Safety culture and human errors

One relatively prevalent notion in discussions of nuclear safety is that of safety culture. The International Nuclear Safety Advisory Group, defines the term as “the personal dedication and accountability of all individuals engaged in any activity which has a bearing on the safety of nuclear power plants”. The goal is “to design systems that use human capabilities in appropriate ways, that protect systems from human frailties, and that protect humans from hazards associated with the system”.

At the same time, there is some evidence that operational practices are not easy to change. Operators almost never follow instructions and written procedures exactly, and “the violation of rules appears to be quite rational, given the actual workload and timing constraints under which the operators must do their job”. Many attempts to improve nuclear safety culture “were compensated by people adapting to the change in an unpredicted way”. For this reason, training simulators are used.

An assessment conducted by the Commissariat a` l’E´ nergie Atomique (CEA) in France concluded that no amount of technical innovation can eliminate the risk of human-induced errors associated with the operation of nuclear power plants. Two types of mistakes were deemed most serious: errors committed during field operations, such as maintenance and testing, that can cause an accident; and human errors made during small accidents that cascade to complete failure.

Nuclear and radiation accidents

Eivilian nuclear accidents, List of civilian radiation accidents, and List of military nuclear accidents

Serious nuclear and radiation accidents include the Chernobyl disaster, Mayak disaster, Soviet submarine K-431 accident, Soviet submarine K-19 accident, Chalk River accidents, Windscale fire, Three Mile Island accident, Costa Rica radiotherapy accident, Zaragoza radiotherapy accident, Goiania accident, Church Rock uranium mill spill and the SL-1 accident.

Risk assessment

International Nuclear Events Scale

Comparative Risk Assessment 

Probabilistic risk assessment

Severe Accident Risks: An Assessment for Five U.S. Nuclear Power Plants NUREG-1150 1991

Calculation of Reactor Accident Consequences CRAC-II 1982

Rasmussen Report: Reactor Safety Study WASH-1400 1975

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

(source:wikipedia)