Gianni Petrangeli

Defence in depth

This chapter describes the Defense in Depth (DID) concept in nuclear safety. It provides multiple independent protections against the occurrence of accidents and their progression. The object of independent barriers in totality is only an objective. The definition of DID can be defined on a general defense principle. It can be implemented through design and operation provisions. The decision to create DID in the plants can be taken at the start of nuclear energy development. It also indicates a remarkable farsightedness. History has demonstrated that it has been the best defense against the uncertainties of the technology. The DID concept is based on four principal barriers against the external release of radioactive products. At the same time, it is also based on five defence levels for using the barriers in a unique way. The actual implementation of DID needs the support of some base requirements. These requirements descend from the technical principles of nuclear safety and can lead to the specific measures.

Nuclear safety research

Nuclear safety research has been explained in this chapter. These activities have been supported by a significant research effort. Research can be made on reactivity accidents. The safety research budget based on worldwide basis is increasing day-by-day. During the period of high investment, the cost of the safety research in terms of nuclear energy production has been only a few units. The majority of accident situations studied is extremely rare. They are even beyond any practical possibility of happening. These situations can be reproduced in laboratories or in experimental facilities, sometimes on a large scale. There are numerous subjects, which can be considered by safety research. In transients and accidents, reactor physics plays a vital role. To severe accidents, physical phenomena are specific. Safety systems are of advanced type. These systems include primary system depressurization and containment of molten masses on the containment floor. Probabilistic methods are used for safety studies. Research on advanced concepts can be made for the future reactors.

The effects of nuclear explosions

This chapter is primarily concerned with the effects of nuclear explosions. The effects of the explosion of nuclear weapons are primarily concerned with the safety of nuclear installations. Thermonuclear fission–fusion bombs with an energy output of up to many tens of megatons can vary from case-to-case. Scaling laws can be used to evaluate the consequences of the energies. In fusion bombs, the rapid compression of the fusion material can be obtained by conventional explosives. The neutron bombs are always clean and those based on fission-fusion are “dirty”. Initial nuclear radiation can be emitted by the nuclear reaction in the first minute after the explosion. It essentially comprises gamma and neutron radiation, and propagates at velocities equal or close to the velocity of light. The destructive shock wave can be caused by the blast and by its reflections on solid walls. It is the highest proportion with reference to the others such as nuclear radiation energy and thermal energy. The propagation velocity is slightly higher than the velocity of sound.

Nuclear safety criteria

This chapter aims at defining a set of general safety and radiation protection criteria for nuclear plants. The “trial and error” approach has been adopted for many other types of industrial undertakings and for other activities. The other activities include the fire protection of buildings and plants. The first collection of internationally accepted safety criteria is given in the “General design criteria for nuclear plants.” The General Design Criteria (GDC) are regulatory that are established by the central national institutions for protecting the population. The fundamental assumptions in the GDC have withstood the test of time and no substantial modifications are required. A series of regulatory standard includes more specific documents than the GDC or similar compilations. The regulatory guides can be described as an acceptable way to satisfy the various requirements of the GDC. The degree of technical detail is high. It goes down to the definition of numerical values of key parameters for the analytical demonstrations.

Chapter 6 – The dispersion of radioactivity releases

This chapter illustrates some simple and quick methods for the evaluation of the dispersion in the environment of gaseous releases. The gaseous releases can be used for the preparation of short-term emergency plans. The radioactive products released to the environment are mainly gaseous and have high velocity. Liquid releases can also be taken into account under some circumstances. The prevailing accidental release can be the release of flammable or toxic liquids. The radioactive isotopes can be released during an accident from a nuclear power station. Plutonium and tritium can be considered for specific plants and accidents. Some physical–chemical properties of the isotopes are important for studying the consequences of accidents. Xenon, krypton, iodine, cesium, and tritium can be considered volatile. Strontium has an intermediate position, approaching that of a non-volatile element. Iodine can be formed in a very modest proportion. Cesium and iodine can be easily removed. In case of an accident, the releases outside the plant occur by slow infiltration through the leakage paths of the containment system.

Underground location of nuclear power plants

This chapter focuses on underground location of nuclear power plants. Plants can be built at ground level with external surfaces of the vital parts. These vital parts are covered with soil or special material. Plants can also be located in deep excavation. The location of the turbine–generator system at depth is close to the reactor cavern. Various feasibility studies have been made in recent times in Sweden, Germany, and Switzerland. The consequences of possible severe accidents can provide better mitigation. The underground location has been the most effective solution, even against extreme attacks. A sub-surface location can be implemented in a unique way to resist any conventional weapon and nuclear bombs. The increase in cost is also significant and many evaluations have been made. The increase in cost is also because of the increase in the construction time. High cost and long construction times can be weighed against the potential benefit objective of improving the resistance against severe accidents.

Health consequences of releases

This chapter focuses on the principles of health protection and safety. A practice that entails exposure to radiation helps to yield sufficient benefit to the exposed individuals to out-weigh the radiation detriment that it causes. Radiation sources and installations can be provided with the best available protection and safety measures under the prevailing circumstances. Radiation exposures, which are not a part of a practice, can be reduced by intervention. At the same time, the intervention measures can be optimized. Defensive measures can be incorporated into the design and operating procedures for radiation sources, to compensate for potential failures in protection and safety measures. Protection and safety can be ensured by sound management as well as good engineering. It can also be ensured by quality assurance, training, and qualification of personnel. Hereditary effects can manifest themselves in the descendants of the exposed individual. The radioactivity of a sample is the number of disintegrations per second. Somatic effects can manifest themselves in an exposed individual.

Inventory and localization of radioactive products in the plant

This chapter focuses on the amount and the normal location of the radioactive products present within a plant. All the radioactive products contained in used fuel can be stored at the plant, in spent fuel pools or dry containers for temporary storage. The factors responsible for the release evaluation are the volatility of the element and their chemical or physical properties. Each conceivable accident has specific aspects, which alter some indicative percentages, to provide an average measure of the natural release potential of various isotopes. The radioactive products contained in the fuel are normally located in the sinterized uranium dioxide of reactor fuel. The primary coolant contains a certain amount of radioactivity because of nuclides formed by the irradiation in the core of elements dispersed in the coolant. The concentration of radioactive products in the water depends on the entity of fissures and the effectiveness of the primary water purification system. Radioactive products, which are present in decay storage tanks for gases, can be extracted from the primary water before their release to the atmosphere.

Nuclear facilities on satellites

This chapter focuses on nuclear facilities on satellites. Radioisotope-powered thermoelectric generators can be used for the electric loads on board. On the other hand, radioisotope-powered heat generators can be used to guarantee the suitable thermal conditions for the equipment on board during a mission. The reason for the use of radioisotopes is that space missions require absolutely reliable sources of electric energy and heat. Radioisotopes are capable of operating for years in severe environmental conditions. Plutonium can be used in oxide form that is more robust in accident conditions. The General Purpose Heat Source (GPHS) thermoelectric generator is designed to withstand a variety of accident events, including an unforeseen re-entry to the earth. Plutonium dioxide can be protected by a graphite shield. The impact of the GPHS module on a hard surface can cause the release of plutonium, either at stratospheric elevations or on the ground. The plutonium can be released in part as vapor or as breathable particles of a diameter at high altitude.

Erroneous beliefs about nuclear safety

This chapter discusses some beliefs, which are prevalent in the field of nuclear safety. A large part of the risk of a nuclear plant is related to plant situations of shutdown or low power. A plant is shutdown for inspection and periodic maintenance. In solid system conditions, the pressure-resisting structures of the primary system are in effect. During the Three Mile Island (TMI) accident, the operators block the operation of the safety injection system. Other protections, such as safety valves, exist against the over-pressurization of the primary system. Pouring large amounts of cold water on an overheated core can produce large quantities of hydrogen by metal–water reactions. The judgment operator can decide the way by which the cooling operation has to be performed. Timely action is beneficial as it limits the possibility of unforeseen aggravating phenomena. The spraying of water causes the condensation of the steam in the containment. All the diagnostic and intervention means are required for taking the correct decision in any situation.