Module 3: Nuclear Reactor Types PREPARED BY Samir Hamasha Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Types Module Objectives After the completion of this module, the student will be able to: • Recognize the importance of the nuclear reactor for the operation of the NPP. • Explain the main difference between NPP and steam power plants. • Explain the simple steam cycle. • Identify the functional requirements of the nuclear reactors. • Classify the nuclear reactors based on the type of the nuclear reaction, the moderator material and the use. • Explain the basic principle of operation of most common nuclear reactor types that are in use today.
Module 4: Nuclear Reactor Types Module Contents Topic 1. Introduction 2. The Simple Steam Cycle 3. Nuclear Reactors Technology 4. Classification of Nuclear Reactors 5. Typical Reactors and Current Technologies 6. Activities 7. References
1. Introduction • The nuclear reactor is the heart of any nuclear power plant. • It provides the thermal energy required to produce high pressure superheated steam which is needed to generate electrical power. • In normal steam power plants thermal energy is released from burning fossil fuels which is used as a heat source to generate electricity.
2. The Simple Steam Cycle • A schematic diagram of a simple steam power plant is shown in Fig. 3.1.
2. The Simple Steam Cycle How does it work? • High pressure superheated steam leaves the boiler which is also referred to as a steam generator and enters the turbine. • The steam expands in the turbine and in doing so, does work, which enables the turbine to drive the electric generator. • The low pressure steam leaves the turbine and enters the condenser, where heat is transferred from the steam (causing it to condense) to the cooling water. • Since large quantities of water are required, power plants are frequently located near rivers or lakes.
3. Nuclear Reactors Technology The nuclear chain reaction Review • It occurs when a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron and undergoes nuclear fission. • The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation and free neutrons. • These products of reaction are known as fission products.
3. Nuclear Reactors Technology The reaction can be controlled by: • Using neutron poisons(neutron absorbers),such as : Xenon-135 or samarium-149, which absorb excess neutrons, and • Using neutron moderators, which reduce the velocity of fast neutrons, thereby turning them into thermal neutrons (Fig. 3.3), which are more likely to be absorbed by other nuclei. • Increasing or decreasing the rate of fission has a corresponding effect on the energy output of the reactor.
3. Nuclear Reactors Technology In summary the basic functional requirements for a thermal reactor are: • a fuel such as Uranium -235 or Plutonium-239. • a moderator to change the fast neutrons to thermal ones. • a coolant to remove the heat. • a control system to control the number of neutrons. • a shielding system to protect equipment and people from radiation
4. Classification of Nuclear Reactors There are several methods to classify nuclear reactors. Some of these are: • Classification by type of nuclear reaction. • Classification by moderator/ coolant material. • Classification by use.
4.1. Classification by type of nuclear reaction There are two types of reactors based on the nuclear reaction; these are fission reactors and fusion reactors. 1. Fission reactors: • All commercial power reactors are based on nuclear fission. • They generally use uranium and its product plutonium as nuclear fuel. • Fission reactors can be divided into two classes, depending on the energy of the neutrons that sustain the fission chain reaction, namely thermal reactors and fast reactors.
4.1. Classification by type of nuclear reaction Thermal reactors • Thermal reactors use slowed or thermal neutrons. • Almost all current reactors are of this type. • These contain neutron moderator materials that slow neutrons until their kinetic energy approaches the average kinetic energy of the surrounding particles (Fig. 4.3).
4.1. Classification by type of nuclear reaction • Thermal reactors • The probability that thermal neutrons (speed 2200 meters per second) fissioning the fissile nuclei of uranium-235, plutonium-239, or plutonium-241 is higher compared to the faster neutrons that originally result from fission. • Using a moderator allows the use of low-enriched uranium or even natural uranium fuel. • The moderator is often also the coolant, usually water under high pressure to increase the boiling point.
4.1. Classification by type of nuclear reaction Fast reactors • use fast neutrons to cause fission in their fuel. • They do not have a neutron moderator, and use less-moderating coolants. • Maintaining a chain reaction requires the fuel to be more highly enriched in fissile material (about 20% or more ). • Fast reactors have the potential to produce less nuclear waste but they are more difficult to build and more expensive to operate. • Overall, fast reactors are less common than thermal reactors in most applications. • Some early power stations were fast reactors and also some Russian naval propulsion units.
4.1. Classification by type of nuclear reaction 2. Fusion reactors as explained earlier in this course fusion power is still experimental technology, generally with hydrogen as fuel.
4.2 Classification by moderator material Moderators are used by thermal reactors. There are four main categories based on the moderator materials: • Graphite moderated reactors: These reactors tend to reduce the kinetic energy of more fast neutrons converting them to thermal neutrons than light water reactors. Due to the extra number of thermal neutrons, these types can use natural uranium or un-enriched fuel.
4.2 Classification by moderator material 2. Water moderated reactors: • Light water reactors (LWRs): These reactors use ordinary water to moderate and cool the reactors. • Heavy water reactors (HWRs): Use deuterium oxide (D2O) which tends to increase the number of thermal neutrons and so these types can use natural uranium or un-enriched fuel.
4.2 Classification by moderator material 3. Light element moderated reactors. • Molten salt reactors (MSRs): Use a light elements such as lithium or berylliumas a moderator. • Liquid metal cooled reactors: Use a coolant that is a mixture of Lead and Bismuth and may use BeOas a moderator. 4. Organically moderated reactors (OMRs): Use biphenyl and terphenyl as moderator and coolant.
4.3. Classification by use The following are the most common uses of nuclear reactors: 1. Electricity • Nuclear power plants 2. Propulsion • Nuclear marine propulsion • Various proposed forms of rocket propulsion 3. Heat • Desalination plants • Heat for domestic and industrial heating • Hydrogen production for use in a hydrogen economy *(The hydrogen economy is a proposed system of delivering energy using hydrogen. The term hydrogen economy was coined by John Bockris during a talk he gave in 1970 at General Motors (GM) Technical Center. Hydrogen is therefore an energy carrier (like electricity), not a primary energy source (like coal)
4.3. Classification by use 4. Research reactors: These reactors are used for research and training, materials testing, or the production of radioisotopes for medicine and industry. These are much smaller than power reactors or those propelling ships, and many are on university campuses. Sharjah University has one of these. 5. Production reactors for transmutation(conversion) of elements such as the breeder reactors and reactors used to produce weapon grade plutonium.
5. Typical Reactors and Current Technologies In this section we will discuss some of the typical reactors used around the globe for electrical power generation. 5.1. Boiling water reactors (BWR) 5.2. Pressurized water reactors (PWR) 5.3. Pressurized heavy water reactors (PHWR)-CANDU . 5.4. Advanced power reactors (APR1400). 5.5. Gas-cooled reactor (GCR) 5.6. Advanced Gas-cooled Reactor (AGR) 5.7. Fast breeder reactor (FBR)
5.1. Boiling water reactors (BWR) In this reactor the enriched uranium oxide UO2 is used as the fuel and light water is used as the coolant and moderator. This is a thermal reactor (Fig. 4.4).
5.1. Boiling water reactors (BWR) How does it work? • The water is circulated by a pump and the water boils in the reactor vessel itself. • The steam produced is fed directly to turbine. • In BWR, the steam is generated in the core itself. • The reactor vessel has to be strong and is enclosed in concrete containment vessel to prevent hazard from the failure of the pressurized circuit. • The exhaust steam from turbine is condensed and the condensate is sent back to the reactor core through a feed pump. • Another pump is used for re-circulating the coolant in the reactor vessel before converting to steam.
5.2. Pressurized water reactors (PWR) • PWRs keep water under pressure so that it gets heat, but does not boil (Fig. 4.5).
5.2. Pressurized water reactors (PWR) • Water from the reactor and water in the steam generator that is turned into steam never mix. Because of this, most of the radioactivity stays in the reactor area (primary loop). • A primary characteristic of PWRs is a pressurizer, a specialized pressure vessel (Fig. 4.6). • During normal operation, a pressurizer is partially filled with water, and a steam bubble is maintained above it by heating the water with submerged heaters. • During normal operation, the pressurizer is connected to the primary reactor pressure vessel and the pressurizer "bubble" provides an expansion space for changes in water volume in the reactor. • This arrangement also provides a means of pressure control for the reactor by increasing or decreasing the steam pressure in the pressurizer using the pressurizer heaters. • The PWRs are the majority of current reactors, and are generally considered the safest and most reliable technology currently used in large scale deployment.
5.3. Pressurized heavy water reactors (PHWR)-CANDU • These reactors are pressurized water reactors. • These reactors are thermal reactors and fueled with natural uranium. • They use heavy water (Deuterium; D2O) as a moderator and they also use light water or heavy water as a coolant. • Instead of using a single large pressure vessel as in a PWR, the fuel is contained in hundreds of pressure tubes. • PHWRs can be refueled while at full power, which makes them very efficient in their use of uranium. • PHWR is also known as CANDU (CANada Deuterium-Uranium) because it is a Canadian design.
5.3. Pressurized heavy water reactors (PHWR)-CANDU • Fig. 4.7 shows the main components of this type of reactors. • These components are: 1. Fuel bundle 2. Calandria (reactor core) 3. Control rods 4. Pressurizer 5. Steam generator 6. Light water pump 7. Heavy water pump 8. Moderator (D2O) 9. Pressure tubes 10. Steam 11. Cold water from condenser 12. Concrete shielding.
5.4. Advanced power reactors (APR1400). • In early 2010, South Korea won its first export order; four APR-1400 reactors for the United Arab Emirates. The APR1400 is an advanced version of the OPR1000. • The power generation capacity of the APR1400 is 1400 MW. APR1400 is PWR and classified as thermal reactor that uses enriched U-235 as a fuel. • The design life of the major components such as the reactor vessel and the steam generator has been extended from 40 years to 60 years compared to the earlier version (OPR1000). • The reactor which consists of a vertical cylindrical shell, hemispherical lower head and hemispherical upper head manufactured by forging stainless steel
5.5. Gas-cooled reactor (GCR) • GCRs are nuclear reactors that use graphite as a neutron moderator and carbon dioxide or helium as coolant (Fig. 4.9). • There are many other types of reactor cooled by gas, however the term GCRs and to a lesser extent gas- cooled reactors are particularly used to refer to this type of reactor. • The GCR uses natural uranium as fuel, which enable the countries that developed them to fabricate their own fuel without relying on other countries for supplies of enriched uranium. • GCRs are designed for on-load refueling. • GCRs are nearly exclusively operating in Great Britain (Magnox reactors).
5.6. Advanced Gas-cooled Reactor (AGR) • These are the second generation of British gas-cooled reactors , using graphite moderator and carbon dioxide as coolant (Fig. 4.10).
5.6. Advanced Gas-cooled Reactor (AGR) • The fuel is uranium oxide pellets, enriched to 2.5-3.5%, in stainless steel tubes. • The carbon dioxide circulates through the core, reaching 650°C and then past steam generator tubes outside it, but still inside the concrete and steel pressure vessel. • Control rods penetrate the moderator and a secondary shutdown system involves injecting nitrogen to the coolant.
5.7. Fast breeder reactor (FBR) • FBRs have been designed not only to produce electricity, but also to generate (breed) fuel, namely plutonium out of uranium. • They use a plutonium (Pu) fuel rather than uranium (Fig. 4.11). • The Pu is surrounded by rods of U-238 which absorb neutrons and are transformed into Pu-239. • The principal reactor cooling system design of a FBR is similar to a PWR. The major difference is that the coolant in a FBR is liquid sodium instead of water. • There are 2 sodium circuits, where heat transfer is generated by a heat exchanger. • The secondary sodium circuits transfer the heat to a steam generator. • The function of the steam generator is similar to its function in other types of reactors. • Fig. 4.11: Fast Breeder Reactor (FBR) schematic.
5.7. Fast breeder reactor (FBR) • The FBR fuel is plutonium which needs fast neutrons to generate fission, therefore water cannot be used as a coolant, because of its moderating function. • The lack of a moderator to slow neutrons gives the word fast to the name