Nuclear Power Plant Safety

(Transcript of the video commentary.)

Even though a nuclear power plant is a complex system of equipment working with hazardous radioactive materials, according to many scientific studies it is one of the safest ways to generate electricity. Reliable control and safety systems and a culture of following strict safety rules when it comes to handling nuclear materials are important aspects of nuclear energy. 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.

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. 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. 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 plants can never be 100 % safe. Though the risk of an accident is very small these days, it still exists, even at the most modern power plants. The simplest way to reduce the risk of a nuclear power plant accident is to use preventive safety measures. From a technology perspective, the design of the reactor as such uses built-in inherent safety elements, and critical reactor components always have several independent safety systems. Last but not least, the specialized skills and experience of operators also contributes to accident prevention.

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. Simpler, passive systems 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 circulation circuit. Barriers in a nuclear power plant are constantly being checked to ensure they are functional and free of leaks.

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. Along with operating diagnostics, active control and safety systems are capable of reliably handling situations where important parameters deviate from standard operating values, and through timely intervention return the reactor to a stable and safe state.
Modern nuclear power plants often utilize inherent safety features because they contribute to even safer reactor operation and help comply with increasingly stringent nuclear safety requirements. 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.

An important part of inherent safety is the use of natural circulation in the design of the cooling system, but probably the most important inherent safety system is a negative temperature coefficient. This important dependency ensures that an increase in the temperature of the reactor's core reduces the intensity of the fission reaction, thus also fuel temperature, and limit values are not exceeded.
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 movement 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 due to delayed radioactive transmutations, which must be safely conducted away from the core. Under normal conditions the reactor is cooled by circulating coolant; in emergency conditions related to coolant loss emergency core cooling systems take over. Their role, under all circumstances, is to quickly and effectively refill the lost coolant and conduct excess heat to an ultimate heat sink.
Modern power plants usually have emergency systems with triple redundancy. This means that there are three stand-alone independent physically systems, each of which is capable of performing all required safety functions on its own.
The last conditions for reactor safety is also related to cooling: containment of fission products. If steam containing radioactive substances escapes into the containment area, 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.

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

Analysis of accidents and safety incidents at nuclear facilities indicates that the human factor plays a big role. To minimize the danger due to the human factor, modern power plants are designed to not allow operators to make errors, or to be able to correct them safely. Great attention is of course paid to the psychological fitness and professional qualifications of nuclear facility operators.

A power plant, as an important element of energy infrastructure, can be considered a potential target for terrorism, and hence must be very carefully secured and protected. It is practically impossible for unauthorized personnel to enter 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 is capable of withstanding the impact of an aircraft.

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. 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.
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. This scale 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.

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 taken into account. So far, the seventh and highest level has only been used to designate the accidents at Chernobyl, Ukraine and Fukushima, Japan. A level 6 accident took place, for example, at the Mayak nuclear reprocessing 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.

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. Among the most important are the International Atomic Energy Agency, headquartered in Vienna, the World Association of Nuclear Operators, headquartered in London, and the relatively new World Institute for Nuclear Security.

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. When we understand the context, we should not be asking whether nuclear power plants are sufficiently safe, but rather what we can do to make them even safer.