NUCLEAR POWER REACTORS Dr. BC Choudhary Professor, Applied Science Department, NITTTR, Chandigarh-160019. Nuclear Reactors Ever since, the world’s first nuclear reactor by Fermi & coworkers in USA, 1942 A variety of nuclear reactors have been built, primarily to meet increasing demand of energy. Although, nuclear reactors are highly complex installations and great care has to be exercised in designing, they work on a very simple principle; Nuclear Fission . The energy released in the process of fission is dissipated into the surrounding environment as heat; . The heat generated in fission is removed by circulating a fluid, called ‘coolant’ around the fuel; . This heat is subsequently used to generate high pressure and high temperature steam; . The steam is fed to a turbine generator system to produce electricity. Reactor Classification Reactors have been built for a wide range of uses, from power generation to testing new reactor components. Design specifications vary widely depending upon the purpose for which the reactor is to be used. One can classify reactors in different ways and some of the criteria used for classification are: . Mean energy of neutrons causing the fission; . Material used in a reactor for fuel, moderator or coolant; . Geometrical structure; and . Purpose for which a reactor is meant. A. Mean Energy of Neutrons causing Fission Classified broadly into Three Types: • Fast: Most of the fissions are induced by neutrons having energies of the order of few hundred keV, or in some special type as low as 10 keV. • Intermediate: Mean energy of neutrons in the range 10-100eV. Intermediate reactors, sometimes also referred to as resonance reactors. • Thermal: Most of the fissions are induced by neutrons of energy around 0.025 eV. Extensively built for power generation and also for research purposes. B. Reactors according to the material used Classified according to the materials used for fuel, moderator or coolant and their physical state, Explicitly mention it by name along with the purpose it serves. For Example: . A fission reactor in which graphite is used as moderator, carbon dioxide (a gas) as coolant and natural uranium as fuel is referred to as the ‘Graphite moderated - gas cooled - natural uranium fueled reactor’. Reactors with ordinary water both as moderator and coolant with enriched uranium as fuel are referred to as ‘Light water moderated and cooled, enriched uranium reactors’ or simply ‘Light water reactors (LWR)’ Continued… Similarly one can have . ‘Heavy water reactor’(HWR, PHWR), . ‘Gas cooled heavy water moderated reactor’ . ‘Light water cooled heavy water moderated reactors’ C: Reactors by Structure From the point of view of their structure, thermal reactors can be further classified as . Homogeneous : Fuel and the moderator are in the same physical state or intimately mixed . Heterogeneous : Fuel is in the from of rods or plates which are regularly dispersed in the moderator, i.e. fuel and moderator are geometrically separated. Most of the present day reactors are of Heterogeneous type. D. Reactors According to Purpose Reactors can also be classified according to the purpose for which they are meant into the following categories: . Power generation . Conversion of one material into another Reactors built for basic research in various branches of science for testing new reactor designs or new reactor components, for producing radio-isotopes and for medical purpose (neutron therapy) are referred to as “Research reactors” . Cirus, Apsara & Purnima at Trombay are examples of research reactors. Usually a single research reactor simultaneously serves many of these purpose. Other Classification Features Reactors designed to convert a fertile isotope (232Th or 238U) into a fissile isotope (233U or 239Pu) “Convertors”. If the amount of newly produced fissile isotope is more than what is burnt in maintaining the chain reaction, ‘Breeders’. Reactors built to produce power are referred to as ‘Power reactors’. Today most of nuclear power comes from ‘Thermal reactors’ based on the fission of 235U. “Fast breeders” are expected to play an increasingly important role in future- one expects that they will provide a virtually unlimited source of energy. POWER REACTORS Power Reactors Thermal Fast Graphite moderated Light water Heavy water Thermal Breeders Liquid metal Gas cooled gas cooled Reactors Reactors fast breeder fast breeder (GCR) (LWR) (PHWR) (LMFBR) (GCFB) Pressurised Boiling water Molten salt Light water water reactors reactors breeder reactors breeder reactors (PWR) (BWR) (MSBR) (LWBR) Reactor Schematics All nuclear reactors consist of following basic components: . Reactor core, . Reflector, . Reactor vessel, . Radiation shield, . Structural materials, . Coolant loops and heat exchangers In fast reactors, a blanket is also placed between the core and the reflector. Core : The central region of a reactor where fission takes place, resulting in the release of energy • In fast reactors it contains a nuclear fuel, a coolant, control rods and structural materials. • In thermal reactors a moderator is also present. Usually fuel is in the form of a ceramic, i.e. either an oxide or a carbide or a nitride. In some cases, uranium in the metallic form is also used as fuel – fuel should have high melting point, high thermal conductivity, high resistance to radiation damage and chemically inert Fuel rods are covered with some protective material known as ‘Cladding or Canning’- should be highly resistive to corrosion, a poor neutron absorber, high melting point and good mechanical strength Zirconium, steel, aluminum, magnesium, nickel and some other similar materials have been proposed for this purpose. Zirconium is the best and one generally uses Zirconium alloy, known as Zircalloy-2(ZR-2), in thermal power reactors. In fast reactors, and sometimes in light water reactors, stainless steel is used. Aluminium is used mainly in research reactors A single cladded unit of fuel is known as the ‘fuel element’. Several such fuel elements when put together constitute a “fuel assembly or a fuel bundle”. Large number of such fuel assemblies are arranged in the form of a regular lattice. The lattice is usually square or hexagonal in the form Arrangement of square/hexagonal reactor lattice in a core. To remove fission heat from the core, necessary to circulate a fluid (liquid or gas) through the reactor called coolant- should have high thermal capacity, low neutron absorption, good radiation & thermal stability and compatible with fuel & clad. To slow down neutrons born in fission, a moderator is also present in the core of a thermal reactor - material of low mass number, large scattering cross- section and small absorption cross-section. Commonly used moderators are heavy water, light water and graphite. Fuel-coolant-moderator arrangement Lithium & Beryllium used in some in reactor core molten salt breeder reactors. In fast reactors, where no moderator is present, the same materials is used as coolant. To ensure safe operation of a reactor at desired power level, to start up and to shut down a reactor, fuel burn up and temperature effects, provision has to be made in the reactor core to control the multiplication factor. Achieved by having control rods (or plates) in the core - made of some highly neutron absorbing materials such as Boron, Cadmium, Hafnium, Gadolinium or their alloys. Control rods may sub-grouped as shim rods, regulating rods or safety rods, depending upon their function. To support fuel elements, for making coolant channels and for various other purposes, it is necessary to use some structural materials inside the core – similar properties as those of clad; stainless steel or Zircalloys. Blanket . Fast reactors are generally compact, hence a significant fraction of neutrons in the core leaks out of the system. To reduce this leakage and also to make proper use of the leaking neutrons, core in these reactors is surrounded by a region of fertile material (232Th or 238U) referred to as the ‘blanket’. Also serves as an additional neutron reflector as well as a shield. The neutrons absorbed in the blanket eventually leads to the production of fissile nuclei ; 233U or 239Pu. Reflector A region of non-fissionable material put next to the core (or blanket if present) to return back the neutron escaping from the core In fast reactors the material chosen is one of high mass number and low absorption cross-section so that the mean energy of neutrons returned back from this region is not much different from that of neutrons entering it. Ni, Cu and Mo are used as reflectors. In thermal reactors any good moderating material can be used as reflector. In LWRs and PHWRs, H2O and D2O are themselves used as reflectors. Graphite, Be or BeO may be used in gas cooled reactors. Reactor Vessel . The whole assembly is placed inside a vessel, called “Pressure or Reactor vessel” or “Calendria” Usually stainless steel is used to make the reactor vessel. For PWRs, which are designed to operate at high pressure, the wall of the pressure vessel is several inches thick. In BWR, the pressure is nearly one-half of that in a PWR so that in this case pressure vessel is thinner. Shielding . To protect the scientists and other personnel working around the reactor as well as the equipment placed around it from radiations emanating from the reactor core, the reactor vessel is encased inside thick concrete walls. • In some cases alternate layers of heavy and light elements such as concrete and polyethylene or concrete and water are also used. Further to reduce the heating effect of nuclear radiations and hence to prevent radiation damage of the pressure vessel a ‘thermal shield’ usually made of stainless steel, is placed next to the reflector. Reactor Building The entire structure, is placed inside a reactor building. It is air tight and is maintained at a pressure slightly lower than the atmospheric pressure so that no air leaks out of the building, except through the ventilation channels.