Safety of Nuclear Reactors

10 Key excerpts on "Safety of Nuclear Reactors"

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  eBook - ePub

  Safety, Reliability, Human Factors, and Human Error in Nuclear Power Plants

  • B.S. Dhillon(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  6   Safety in Nuclear Power Plants     6.1Introduction

  The International Atomic Energy Agency (IAEA) defines nuclear safety as “The achievement of proper operating conditions, prevention of accidents or mitigation of accident consequences, resulting in protection of workers, the public and the environment from undue radiation hazards” [1 ,2 ]. This covers nuclear power plants and all other nuclear-related facilities and areas.

  Over the years, due to the occurrence of various types of accidents in nuclear power plants, safety in nuclear power plants has become a very important issue, and it requires a continuing quest for excellence. All involved organizations and individuals need constantly to be alert to opportunities for lowering risks to the lowest practical level as possible. The nuclear power industrial sector has considerably improved the safety of reactors and other related systems and has proposed safer reactor and related systems designs. However, it is not possible to guarantee perfect safety because of potential sources of problems such as human errors and external events that can have a much greater impact than anticipated.

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  Applied Safety for Engineers

  Systems and Products

  • B.S. Dhillon(Author)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  6 Safety in Nuclear Power Plants

  DOI: 10.1201/9781003212928-6

  6.1 Introduction

  Nuclear safety defined by the International Atomic Energy Agency (IAEA) as “The achievement of proper operating conditions, prevention of accidents or mitigation of accident consequences, resulting in protection of workers, the public and the environment from undue radiation hazards” [1 –3 ]. This covers nuclear power plants as well as all other nuclear-related facilities and areas.

  Over the years, due to the occurrence of various types of accidents in nuclear power plants/facilities, safety in nuclear power plants has become a very important issue, and it requires a continuing quest for excellence. All involved individuals and organizations need constantly to be alert to opportunities for reducing risks to the lowest level possible. Over the years, the nuclear power industrial sector around the globe has considerably improved the safety of reactors and other associated systems and has proposed safer reactor and related systems designs. However, it is not possible for guaranteeing perfect safety because of potential sources of problems such as human errors and external events that can have a much higher impact than initially anticipated.

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  Double or Quits?

  The Future of Civil Nuclear Energy

  • Malcolm C. Grimston, Peter Beck(Authors)
  • 2010(Publication Date)
  • Taylor & Francis

    (Publisher)

  A further aspect of nuclear safety, distinct from concerns about design flaws, concerns the fate of reactors in politically or technologically unstable regions or times. Unlike many energy technologies, nuclear reactors, especially if the spent fuel has not been removed, represent a long-term health, environmental and security risk. The decommissioned Soviet submarines in the Barents Sea are a notable example, and it is believed that a number of reactors in the former Soviet Union pose similar risks. A related issue, which gained extra prominence following the events of 11 September 2001, is the potential vulnerability of nuclear installations to suicide terrorist attacks. The danger could be twofold – either the risk of radioactive releases or the risk of having to close down a number of large power plants indefinitely as a precautionary measure, with threats to secure supplies of electricity.

  Principles of Reactor Safety

  The safety of a nuclear reactor depends on its ability, under all circumstances, to prevent significant amounts of radioactive materials escaping into the environment. There are perhaps three primary safety imperatives that a reactor must obey:

  • to keep the core power down, by controlling the power level and stopping the fission chain reaction when necessary;
  • to keep the core cool, by maintaining adequate cooling to prevent core meltdown owing to decay heat in fission products after the reactor has been shut down;
  • to keep radioactive material contained as close as possible to the source, under all conditions.

  Three approaches, which are not mutually exclusive, can be taken to achieve these imperatives. They are the employment of passive safety systems, the employment of active (or engineered) safety systems and the development of an appropriate safety culture among operators. Passive safety relies on forces of nature such as changes in density at certain temperatures or gravity, while engineered safety relies more on technical features such as valves or pumps.

  Broadly speaking, it is easier to use passive safety features in smaller reactors than in larger ones. For example, the surface-area-to-volume ratio for a smaller reactor is greater than for a larger reactor of the same or similar type. As heat production is proportional to volume and heat dissipation is proportional to (surface) area, the ratio of passive cooling to heat production is higher in smaller reactors. This means that proportionally less cooling water is required in emergency scenarios, and it is easier to provide this water without requiring pumps. But now passive safety and simplified reactor design are being applied to new designs of larger reactors as well.

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  Nuclear Disasters & The Built Environment

  A Report to the Royal Institute of British Architects

  • Philip Steadman, Simon Hodgkinson(Authors)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  24 NUCLEAR SAFETY AND NUCLEAR RISKS outline the philosophy to be adopted at all stages of the design and construction processes, as well as during the operation of the plant (HSE, 1979). These give detailed consideration to the ways in which incidents, large and small, which could occur, should be avoided not only through careful design, but also in the fabrication, on-site workmanship, purchasing, handling, storage, inspection, and testing of materials and components; the on-site workmanship in the assembly of these compo-nents; and the keeping of records and carrying out of audits in all these areas. When the reactor is commis-sioned care is taken to establish, maintain and follow detailed operating, maintenance, testing, inspection and safety procedures. Particular care is taken to incorporate engineered safety features that are as foolproof as possible, and which can safely terminate almost any incident at the plant. Three principles are important in the design of these safety features: 1. Redundancy in the provision of safety systems: secondary and tertiary emergency systems are always built in so that adequate safety can be ensured even in the event of one or two safety systems failing concurrently. 2. Diversity in the types of safety systems installed: this is to prevent a set of safety systems all failing for the same reason (such as an electrical power failure). 3. Segregation of the safety systems, to guard against a single hazard such as a fire or flood knocking out all of the safety equipment at the same time. Duplicate safety systems are sited in different quadrants around the reactor. An example of the application of these principles is given by the proposed auxiliary feedwater system for the Sizewell ' PWR. Although only one working feedwater system is needed for safety, four are to be provided.

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  Fast Reactor Safety

  • John Graham(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Such a subject also involves siting policies as part of the safety considerations. However, the main objective is the design objective in which the plant (and of course the public) is protected against damage. The design objectives for any power plant will include safety, economical operation, reliability as well as flexibility, ease of operation, and compatibility with other power sources; but safety is a primary one, 1.1.3 THE TEXT This book therefore presents safety as an embryo science by outlining the analytical tools which are available to the safety engineer, by present-ing the experience which has been obtained using fast reactors to date, and finally by posing the questions which remain unanswered. This is done within the framework of designing a fast reactor power plant and of taking it through the regulatory process. Chapter 1 presents the methods of safety evaluation, Chapter 2 outlines possible disturbances to the system and their interactions. Chapters 3-6 take the reader through the design and licensing process, from the establish-ment of safety criteria, through special fast reactor considerations in safety, the barrier concept of safety, and the final presentation of the plant safety to the regulatory bodies and the public. For a detailed discussion of fast reactor technology, overall reactor safety, and a review of current safety practices, the reader is recommended to use the general references listed at the end of this chapter to supplement the present text. 1.2 Neutron Kinetics It is necessary to perform kinetic calculations on any reactor system throughout the design of that system from its initial conception through to its commissioning. In the initial assessment period calculations may 6 1 Safety Evaluation Methods necessarily be rather crude because of the uncertainties in most of the parameters.

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  World Energy Supply

  Resources - Technologies - Perspectives

  • Manfred Grathwohl, Mary Brewer(Authors)
  • 2018(Publication Date)
  • De Gruyter

    (Publisher)

  The purpose of reactor safety technology, for any type of reactor, is to prevent the release of radioactive substances into the environment in the event of reactor mal-function or accident. About 95% of the radioactive materials are in the fuel (includ-ing fission products); the rest are in the reactor coolant circulation, the storage basin for spent fuel elements, the (loaded) transport containers for fuel and in support facilities like the waste gas and waste water systems. In the intact facility, these fis-sion products are enclosed in several layers of containment structures, the fission-product barriers. Fig. 5-11 shows the arrangement of these barriers in a PWR (64). The first barrier is the crystal lattice of the fuel itself, which holds back about 95% of the fission products. It is surrounded successively by the fuel rod cladding, which is sealed with a gas-tight weld; the reactor pressure vessel, together with the completely closed reactor coolant circulation; and the gas-tight and pressure-resistant contain-ment vessel, which surrounds the reactor coolant system. The outer stressed concrete shell has only a limited containment function; it makes it possible to pump up cool-ant leaks from the containment vessel and protects the installation from external forces. Fission products cannot escape into the environment as long as the fuel elements are intact, which is why overheating must be avoided. Therefore the basic strategy for reactor safety is to prevent overheating of the fuel elements, i.e. a disequilibrium 360 5. Environmental impact and safety problems Fig. 5-11: Diagram of the fission product barriers in a pressurized water reactor 1 = Crystal lattice of the fuel; 2 = Fuel rod cladding; 3 = Reactor coolant circulation; 4 = Containment vessel; 5 = Stressed concrete shell. Source: Federal Ministry of Research and Technology (Ed.), German Reactor Safety Study, Bonn 1979. between the rates at which heat is produced and removed.

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  Fast Breeder Reactors

  An Engineering Introduction

  • A. M. Judd(Author)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  CHAPTER 5 Safety 5.1 INTRODUCTION The designer of a fast reactor, just like the designer of any other engineering enterprise, has to take into account what might happen if something goes wrong. He has to make sure that whatever happens the risk of injury, either to the operating staff or the general public, or of damage to property, is very slight. There are basically two ways of making a reactor safe. First the overall design concept is chosen so that it is inherently safe. That is to say that for a number of possible accidents the design is such that the reactor behaves safely and damage does not spread even if no protective action, automatic or deliberate, is taken. It is not possible to guard against all accidents in this way however well the overall design is chosen. The second way to make the reactor safe is to incor-porate protective systems. These are devices designed specifically to prevent the damaging consequences of accidents. A protective system can be active, like an automatic shutdown system, or passive, like a containment barrier. The overriding aim is to make sure that the risk to the public is extremely small, and to ascertain that the aim has been met the designer has to determine the response of the reactor with its protective systems to a range of accidents. To test the systems thoroughly it is often necessary to assume certain accidents happen even though no way is know by which they could actually take place. For this reason they are known as hypothetical accidents. The final step is to analyse all the accidents, whether hypothetical or not, and to ensure that the risks meet the criteria imposed by the authorities which regulate nuclear establishments. These criteria vary of course from country to country. This chapter starts by describing the inherent features of a sodium-cooled oxide-fuelled fast reactor which make for safety, and goes on to discuss some of the protective systems which can be used.

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  Sodium Fast Reactors with Closed Fuel Cycle

  • Baldev Raj, P. Chellapandi, P.R. Vasudeva Rao(Authors)
  • 2015(Publication Date)
  • CRC Press

    (Publisher)

  393 13 Safety Criteria and Basis 13.1 INTRODUCTION A nuclear reactor performs various operating conditions that are well covered in the design. They are called design basis conditions as defined in Table 13.1. Design requirements under these conditions are specified to assure the plant availability and integrity of the fuel elements and mechanical com-ponents. With this, a set of design criteria are established (see Table 13.2 for the core integrity and Table 13.3 for hot and cold pool components of the reactor assembly). Safety demonstration requires not only meeting these design criteria during normal and transient conditions but also meeting the criteria relevant to the safe shutdown state following transient events. The consolidated composi-tion of such criteria is termed as safety criteria. A few examples are given in Table 13.4. Thus, the safety criteria describe the design requirements for structures, systems, and components important to safety that shall be met for safe operation and also for the prevention or mitigation of the con-sequences of events that could jeopardize safety. Figure 13.1 shows the relationship between the categories, events, and safety targets [13.1]. The safety criteria are broadly classified into two categories: the first category covers the general safety requirements that are applicable to all types of reactors, while the second category covers requirements that are reactor system specific. Aspects covered under general safety principles are defense-in-depth requirements, safety functions, radiological dose limits to plant personnel and public, site boundary radiological requirements, emergency preparedness, etc. This chapter presents the generic features of fast reactors (FRs) as well as specific safety issues related to sodium to be addressed in the safety criteria.

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  Nuclear Engineering Handbook

  • Kenneth D. Kok(Author)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  Section III Introduction Related Engineering and Analytical Processes Kenneth D. Kok This section contains the description of related engineering and analytical processes that are used generally in nuclear engineering related to the design and operation of nuclear processes. Chapters 19 and 20 describe the safety evaluations that are used for nuclear facilities. Chapter 19 introduces the risk assessment and safety analysis process that is used for nuclear reactors that are licensed in the United States by the Nuclear Regulatory Commission (NRC). This process has evolved from a relatively simple safety analysis used in the 1950s to a detailed risk assessment process that is used today. Chapter 20 describes the process used in the United States by the Department of Energy for safety analysis of its facilities. It is more prescriptive and less probability and risk based than the process used by the NRC. Chapter 21 provides a discussion of criticality. Criticality occurs when sufficient fission-able material, such as U-235, is brought together under the appropriate conditions so that a self-sustaining reaction occurs. This reaction is required to make a reactor operate in a controlled manner, but is not desirable if it is not controlled. Chapter 22 “Heat Transfer, Thermal Hydraulic, and Safety Analysis” and Chapter 23 “Thermodynamics and Power Cycles” are analytical tools used by engineers to evaluate reactor and power-producing systems. Heat transfer and thermal hydraulics are not only important in the operation of nuclear reactors, they are also critical in the evaluation of how the systems will respond under upset conditions. The chapter on thermodynamics is included to show how the energy generated by the reactor is transferred by the reactor cooling system to the turbine power generating system used to produce electricity. 636 Nuclear Engineering Handbook Chapter 24 looks at some of the economic evaluations that can be applied to nuclear reac-tor systems.

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  Environmental Radioactivity and Emergency Preparedness

  • Mats Isaksson, Christopher L. Raaf(Authors)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  The critical parameters are the mass, volume and geometry of the uranium that is handled in processing. 6.3.3 Operation of Nuclear Power Plants In order to minimize the risks associated with the operation of nuclear power plants, they must first be properly identified, then assessed and managed. The identification of risks relies on previous experiences, research and scenarios. The use of probabilistic safety assessment (see below) is a valuable tool in identifying combinations of events that may have a negative impact on the operation of the plant. The risks identified must then be assessed to ensure their optimal management; see Section 6.1.1. Risk management consists of documents such as written instructions for normal operation and emergencies, activities such as education and training, maintenance, and a good “safety culture”. If, for example, core meltdown is not identified as a possible event, the risk of this event can be neither assessed nor managed. This was the case in the accident in 1979 at Three Mile Island (Harrisburg), which, however, led to only minor releases to the environment due to appropriate handling of the situation. At Chernobyl in 1986, the consequences of rapidly changing reactivity were identified and assessed, resulting in administrative and technical safety measures. However, these instructions and security systems were overridden, which rapidly led to total destruction of the reactor and reactor building. Finally, in the Fukushima accident in 2011, the risk of Tsunamis was identified, but the frequency and possible amplitude of such waves was not properly assessed. 6.3.3.1 Defence in Depth In order to deal appropriately with the risks associated with the operation of a nuclear power plant, the concept of defence in depth has been developed in which the first priority is to prevent accidents (IAEA 1996). If prevention fails, the strategy should limit the po- tential consequences, and prevent the accident from becoming more serious.

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