Nuclear Power Plant Safety

(Transcript of the lesson commentary.)

Safety — an important aspect of nuclear power

Even though a nuclear power plant is a complex system of equipment working with highly concentrated energy and radioactive materials, according to many studies it is one of the safest ways to generate electricity. Power plant control and safety systems and a culture of following strict safety rules when it comes to handling nuclear materials have been important aspects of the nuclear energy industry from the beginning. This is because there are fears that if the fission reaction got out of control, radioactive substances could escape into the environment and contaminate the plant's surroundings for many years to come.

A safely operated nuclear power plant produces no carbon dioxide, and so doesn’t contribute to the greenhouse effect, and basically has minimum impact on its surroundings. Waste created in a nuclear power plant is processed and effectively isolated from the environment. From the perspective of waste management, the nuclear power industry can serve as an example for other industrial sectors. Minimizing the risk of a nuclear accident, be it due to equipment failure or human error, is a fundamental nuclear energy safety principle and the main goal of nuclear safety.

One can see nuclear safety as a set of measures supporting the capability of the nuclear facility and its operators to ensure that the fission process is constantly under control and so radioactive substances created during fission are not released into the environment. Thanks to constantly improving safety systems and the responsible and professional approach of power plant designers and operators, every generation of nuclear reactors is not only more powerful, but above all, safer.

However, nuclear power plant safety can never be 100%, and the risk of an accident is small but not non-existent. The simplest way to reduce the risk of an accident is to use preventive safety measures. An emphasis is placed on the use of high-quality equipment during the construction phase, which is then checked and tested periodically. Critical reactor components often have several independent safety systems. The reactor’s design as such utilizes built-in inherent safety elements based on the laws of physics. Finally, accident prevention also includes technical training and testing of operators to ensure they are capable of operating the nuclear power plant safety.

The discovery of radioactivity and utilization of nuclear fission to generate electricity took humanity to a greater level of knowledge and technological development. In the small volume of a nuclear reactor, we have succeeded in controlling incredibly concentrated energy that at the same time, however, is a massive source of radioactivity. For the sake of comparison, a standard reactor is a source of radioactivity that is only about 10,000 times less than that contained in the entire Earth’s crust. When we realize the negative effects of radiation, the question of how safe such technology is immediately arises. Yes, the safety of nuclear power plants is constantly increasing and is at the forefront during all activities related to nuclear power. Therefore, we should not be asking whether nuclear power plants are sufficiently safe, but rather what we can do to make them even safer.

Safety elements and systems

The reliability of nuclear power plant safety systems is very important and is ensured through equipment quality, heterogeneity, strict physical segregation, and multiple levels of redundancy. Depending on their function, these systems are classified as either passive or active. Passive systems are simpler, as they usually do not have any moving parts, need no power or control signals, and operate solely on the basis of the laws of physics.

Passive systems are based on the concept of multiple barriers that prevent radionuclides from being released from nuclear fuel into the environment. The first barrier is a fuel matrix that keeps most fission products in ceramic pellets. Other barriers include cladding that protects the fuel pellets from contact with the coolant, a hermetic primary circuit, including the reactor vessel, and finally the large protective containment envelope that prevents the escape of any radionuclides that could be released, for example during an accident resulting in a leak in the cooling circuit.

Each barrier is important and even if one of them fails, other safety systems come into play, depending on the severity of the fault. The functionality and integrity of barriers in a nuclear power plant are checked constantly. Leaks in the cladding show up as increased activity in coolant, while a pressure drop in the primary circuit indicates leaks in the third barrier.

Under standard operating conditions the fission process at a nuclear power plant is controlled by active safety systems. These need electricity to operate and are directly controlled by the power plant’s operators or control system. Active systems must react correctly under all conditions to the current situation in the nuclear fuel. Together with operational diagnostics, they can even handle situations where important parameters deviate from optimum values, and through quick action return the reactor to a stable and safe state.

Modern nuclear power plants are increasingly making use of inherent safety features because they lead to even safer operation and help comply with nuclear safety requirements regardless of conditions. The best-known safety feature in this category involves shutdown rods suspended above the reactor’s core by electromagnets. In the event of power loss, they immediately drop into the core and stop the fission reaction.

Another crucial part of inherent safety is the use of natural circulation in the design of the cooling system. If all pumps fail, this ensures at least partial circulation of coolant, which cools the core through convection.

Probably the most important inherent safety feature of a nuclear power plant is a negative temperature coefficient, which ensures that an increase in the temperature of the reactor’s core reduces the intensity of the fission reaction, thus also temperature, and limit values are not exceeded. Thanks to inherent safety, the power plant can handle non-standard situations even without operator intervention, and the reactor returns to a safe state on its own.

Nuclear reactor safety conditions

A nuclear reactor is in a safe state if three key conditions are fulfilled: proper operation of nuclear fission control, adequate cooling of nuclear fuel, and reliable containment of fission products.

The fission reaction, and thus also reactor power, is controlled through the insertion and removal of control rods in the reactor’s core. The fission reaction is stopped completely and above all quickly using shutdown rods that undergo free fall or are otherwise quickly inserted into the reactor’s core. Like the control rods, the shutdown rods contain a neutron absorber and the fission reaction is stopped within several seconds.

Even though after a hazardous situation occurs the reactor is immediately shut down, large quantities of heat are still generated in the fuel, which must be safely conducted away from the core. Under normal conditions the reactor is cooled by coolant that is circulated through the main circuit using pumps. If there is a leak or malfunction in the primary circuit, emergency systems for cooling the core come on line. Their role, under all circumstances, is to quickly and effectively refill the lost coolant and conduct excess heat to an ultimate heat sink. If cooling towers are used, this can be the atmosphere, and in the case of direct cooling, water in a river or sea.

Modern power plants usually have emergency systems with triple redundancy. This means that there are three stand-alone independent physically systems, each of which can perform the required safety functions on its own. In the case of emergency cooling of the core, there are three systems capable of adding coolant to the reactor, emergency power systems have at least three independent ways of supplying electricity, etc.

The last conditions for reactor safety is also related to cooling: containment of fission products. If radioactive substances do leak into the coolant and enter the containment area as steam due to a leak in the primary circuit, sprinkler branches of the emergency cooling systems are triggered automatically, showering the internal space to condense steam and reduce pressure in the containment structure. This prevents the potential leakage of radioactive substances into the environment.

External forces affecting safety

Aside from failure of complicated equipment, nuclear power plant safety can also be affected by external factors such as intentional or unintentional human activity or natural phenomena.

Analysis of accidents and safety incidents at nuclear facilities indicates that the human factor plays a big role. Violation of safety rules or incorrect reactions and intervention by staff when responding to faults can cause a larger accident. To minimize the danger due to the human factor, modern power plants are designed to be able to correct operator error through built-in safety features. On the other hand, great attention is paid to staff training and to their psychological and technical ability to make responsible decisions under critical and unexpected conditions.

A power plant, as an important element of energy infrastructure, can be considered a potential target for terrorism and hence must be very thoroughly secured and protected. It is practically impossible for unauthorized personnel to enter it, especially nuclear areas, due to careful monitoring and physical and electronic security. The power plant is protected from large-scale attacks by a no-fly zone as well as containment vessel design that can withstand the impact of an aircraft. Inside the power plant, all fissile material is subject to inspections by experts from the International Atomic Energy Agency.

Natural phenomena represent a specific group of outside influences on safety. A nuclear power plant can be seriously damaged by an earthquake, hurricane or tsunami, or its equipment can be submerged during flooding. There are basically two ways of dealing with these risks. The simplest is to avoid building in locations where such natural phenomena occur. This is why power plants are usually built far enough away from watercourses that could flood them and are situated in seismically inactive locations. If natural conditions make it impossible to pick safer locations, power plants must be built and secured to withstand even the worst possible natural catastrophe in the given location.

Nuclear and radiation events and their evaluation

A fault or accident at a nuclear power plant is a very sensitive topic that receives great attention. The public needs to be informed clearly and quickly about all incidents, especially those where staff and the population could be exposed to radiation. To facilitate communication and understanding between the public, the media, and nuclear safety experts, in March 1990 the International Nuclear Event Scale was implemented. Jointly developed by the International Atomic Energy Agency (IAEA) and the Nuclear Energy Agency (NEA) of the Organisation for Economic Co-operation and Development (OECD). It classifies events into seven levels, with levels 1 to 3 being defined as incidents and levels 4 to 7 as accidents. Events that have no significance in terms of safety are called deviations from operating limits and are classified at level 0.

An incident or accident is always classified according to the country in which it occurred. Immediately following the event this classification may be temporary and may be revised after all circumstances and the extent of the event are clarified. The scale uses the same principle as the Richter scale — the severity of an accident in a given level is ten times as serious as the level below. When classifying a specific event, the extent of damage to the nuclear facility, irradiation of staff, and the extent of radioactive material leakage into the vicinity are considered.

So far, the seventh and highest level has only been used to designate the accidents at Chernobyl, Ukraine in 1986 and Fukushima, Japan in 2011. Accidents occur at reprocessing plants too, and a level 6 accident took place, for example, at the Mayak nuclear complex in the Russian town of Kyshtym in 1957. The most serious accident on the American continent, which took place at the Three Mile Island power plant in 1979, was classified as level 5 according to the INES scale.

Institutes monitoring the nuclear power industry

Every country that operates a nuclear power plant or handles radioactive material has a nuclear safety authority that supervises compliance with safety regulations of all nuclear operations in that country. Aside from these institutions there are also independent international organizations dealing with questions of safety that monitor all nuclear facilities in the world.

International Atomic Energy Agency (IAEA)

One of the most important ones is definitely the International Atomic Energy Agency in Vienna, founded by the UN in 1957. The task of this agency is to monitor the safe and peaceful use of nuclear energy so that it contributes to the health and prosperity of the whole world while preventing any misuse for military purposes. The agency supports help between member states, facilitates the exchange of technical and scientific information, and supervises compliance with safety regulations. The agency’s independent inspectors monitor the movement of every piece of fissile material.

World Association of Nuclear Operators (WANO)

The members of this independent non-profit organization with its headquarters in London are all companies and countries operating nuclear reactors, and its aim is to ensure maximum nuclear safety. The association supports the exchange of information and operating experience between all operators, organizes exchange programmes, workshops, and seminars, and provides technical support and independent inspection of nuclear facilities.

World Institute for Nuclear Security (WINS)

The World Institute for Nuclear Security was created in Vienna in 2008. Its primary goal is to improve the security of nuclear power plants and radioactive material to prevent any activity that could lead to its misuse for terrorism or other criminal activity. The organization offers specialized training in the area of nuclear security and issues recommendations, thus improving safe handling of nuclear material.

World Nuclear Association (WNA)

The World Nuclear Association was founded in London in 2001, based on the former Uranium Institute, as a forum for the nuclear fuel market. Its aim is to support the nuclear power industry and all related organizations, from uranium mining and manufacture of nuclear fuel to construction of nuclear power plants and disposal of radioactive waste. The association provides information regarding all aspects of the nuclear industry to the public.