Core shroud

Last updated

A core shroud is a stainless steel cylinder surrounding a nuclear reactor core whose main function is to direct the cooling water flow. [1] The nuclear reactor core is where the nuclear reactions take place. Because the reactions are exothermic, cool water is needed to prevent the reactor core from melting down. The core shroud helps by directing this cool water towards the reactor core, providing stability to the nuclear reactions.

Nuclear reactor core portion of a nuclear reactor containing the nuclear fuel

A nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place and the heat is generated. Typically, the fuel will be low-enriched uranium contained in thousands of individual fuel pins. The core also contains structural components, the means to both moderate the neutrons and control the reaction, and the means to transfer the heat from the fuel to where it is required, outside the core.

Nuclear reaction Process in which two nuclei collide to produce one or more nuclides

In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce one or more nuclides that are different from the nuclide(s) that began the process. Thus, a nuclear reaction must cause a transformation of at least one nuclide to another. If a nucleus interacts with another nucleus or particle and they then separate without changing the nature of any nuclide, the process is simply referred to as a type of nuclear scattering, rather than a nuclear reaction.

Contents

Structure

The core shroud is composed of multiple cylindrical thermal shields stacked on top of each other. In between each thermal shield is a horizontal, stainless steel cylindrical plate that helps keep the thermal shields in place. The plates are then welded together with the thermal shields so that they create one solid structure.Vertical tie bolts are then used to reinforce each horizontal plate with their adjacent thermal shields, stabilizing the cylindrical core shroud. The thermal shields are needed because the core shroud exists near the nuclear reactor core where heat is constantly present. The thermal shields prevent heat from damaging the core shroud by absorbing or reflecting the heat.

A heat shield is designed to protect an object from overheating by dissipating, reflecting or simply absorbing heat. The term is most often used in reference to exhaust heat management and to systems for dissipation of heat due to friction.

Rail fastening system

A rail fastening system is a means of fixing rails to railroad ties or sleepers. The terms rail anchors, tie plates, chairs and track fasteners are used to refer to parts or all of a rail fastening system. Various types of fastening have been used over the years.

Core shroud walls are relatively thin, ranging from three to five centimeters in thickness. [2] This is because the core shroud is not built to withstand high amounts of pressure for long periods of time, so thicker walls would be unnecessary for the core shroud's function.

Function

Direction of Water Flow

The main function of the core shroud is to direct the current of water flow inside of the reactor pressure vessel. [2] Cold water is pumped into the reactor pressure vessel from an outside water source. [3] The cold water flows down in between the wall of the reactor pressure vessel and the outside wall of the core shroud where it meets the fuel assemblies. It is here that the cold water is heated, and the heated water flows back up the gap between the wall of the reactor pressure vessel and the outside wall of the core shroud. This creates steam, since the water is heated, which is then used to drive the steam turbine, which powers the generator and creates electricity. By directing the cold water into the reactor pressure vessel, and allowing the heated water to rise and evaporate, the core shroud will have successfully cooled the nuclear reactions.

Nuclear fuel material that can be used in nuclear fission or fusion to derive nuclear energy

Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.

Steam turbine type of turbine device which uses steam from a boiler to rotate the turbine blades

A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884.

Water Level

In order for the reactor core to remain cool, water is sometimes needed in greater amount, resulting in a flood like effect in the reactor vessel itself. [2] The core shroud must be able to maintain its strength when the vessel floods so that the reactor core does not melt down. The core shroud must be built to withstand the pressure of extra water because should it collapse, the fuel assemblies would not be able to cool properly.

Maintenance

In 1990, Stress Corrosion Cracking (SCC) was discovered in core shrouds, resulting in a heightened awareness of core shroud maintenance. [1] Routine core shroud inspections became mandatory in most nuclear power plants worldwide because if a core shroud were to collapse, a nuclear meltdown could occur. [4]

Nuclear meltdown severe nuclear reactor accident that results in core damage from overheating

A nuclear meltdown is a severe nuclear reactor accident that results in core damage from overheating. The term nuclear meltdown is not officially defined by the International Atomic Energy Agency or by the Nuclear Regulatory Commission. It has been defined to mean the accidental melting of the core of a nuclear reactor, however, and is in common usage a reference to the core's either complete or partial collapse.

Core shrouds crack because the heat from the nuclear reactions combined with the constant flowing water eventually wear out the steel plates. [5] One method used to fix this problem is reinforcing the core shroud plates. [2] This is done using an anchor bolt, which is used to attach additional steel plates to the core shroud surface, thereby reinforcing the structure. This is the most common method used to fix cracks in the core shroud since it is easy and relatively safe. Replacement of the core shroud is also an option, but it is not recommended because the cracked plates must be removed manually, leaving the laborers susceptible to radiation.

In the Fukushima Daiichi nuclear disaster, cracks had been discovered in their core shrouds. [5] In this specific event, however, core shroud cracking was not the cause of the nuclear disaster. Several other factors, such as the earthquake, tsunami, and equipment failures, caused the most damage to the nuclear power plant. It has been speculated that if a core shroud were to collapse due to cracking, it could result in a catastrophic nuclear meltdown. [4]

Related Research Articles

Pressurized water reactor nuclear reactor with a cooling system that operates under high pressure

Pressurized water reactors (PWRs) constitute the large majority of the world's nuclear power plants and are one of three types of light-water reactor (LWR), the other types being boiling water reactors (BWRs) and supercritical water reactors (SCWRs). In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated water then flows to a steam generator where it transfers its thermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spin an electric generator. In contrast to a boiling water reactor, pressure in the primary coolant loop prevents the water from boiling within the reactor. All LWRs use ordinary water as both coolant and neutron moderator.

Boiling water reactor type of light water nuclear reactor used for the generation of electrical power

A boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor (PWR), which is also a type of light water nuclear reactor. The main difference between a BWR and PWR is that in a BWR, the reactor core heats water, which turns to steam and then drives a steam turbine. In a PWR, the reactor core heats water, which does not boil. This hot water then exchanges heat with a lower pressure water system, which turns to steam and drives the turbine. The BWR was developed by the Argonne National Laboratory and General Electric (GE) in the mid-1950s. The main present manufacturer is GE Hitachi Nuclear Energy, which specializes in the design and construction of this type of reactor.

RBMK commercial nuclear reactor design

The RBMK is a class of graphite-moderated nuclear power reactor designed and built by the Soviet Union. The name refers to its unusual design where, instead of a large steel pressure vessel surrounding the entire core, each fuel assembly is enclosed in an individual 8 cm diameter pipe which allows the flow of cooling water around the fuel.

Loss-of-coolant accident mode of failure for a nuclear reactor

A loss-of-coolant accident (LOCA) is a mode of failure for a nuclear reactor; if not managed effectively, the results of a LOCA could result in reactor core damage. Each nuclear plant's emergency core cooling system (ECCS) exists specifically to deal with a LOCA.

Light-water reactor type of nuclear reactor uses normal water

The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator – furthermore a solid form of fissile elements is used as fuel. Thermal-neutron reactors are the most common type of nuclear reactor, and light-water reactors are the most common type of thermal-neutron reactor.

Steam explosion violent boiling or flashing of water into steam

A steam explosion is an explosion caused by violent boiling or flashing of water into steam, occurring when water is either superheated, rapidly heated by fine hot debris produced within it, or heated by the interaction of molten metals. Pressure vessels, such as pressurized water (nuclear) reactors, that operate above atmospheric pressure can also provide the conditions for a steam explosion. The water changes from a liquid to a gas with extreme speed, increasing dramatically in volume. A steam explosion sprays steam and boiling-hot water and the hot medium that heated it in all directions, creating a danger of scalding and burning.

Containment building reinforced steel or lead structure enclosing a nuclear reactor

A containment building, in its most common usage, is a reinforced steel or lead structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radioactive steam or gas to a maximum pressure in the range of 275 to 550 kPa. The containment is the fourth and final barrier to radioactive release, the first being the fuel ceramic itself, the second being the metal fuel cladding tubes, the third being the reactor vessel and coolant system.

Reactor pressure vessel Nuclear power plant component

A reactor pressure vessel (RPV) in a nuclear power plant is the pressure vessel containing the nuclear reactor coolant, core shroud, and the reactor core.

Supercritical water reactor

The supercritical water reactor (SCWR) is a concept Generation IV reactor, mostly designed as light water reactor (LWR) that operates at supercritical pressure. The term critical in this context refers to the critical point of water, and must not be confused with the concept of criticality of the nuclear reactor.

High Flux Isotope Reactor

The High Flux Isotope Reactor is a nuclear research reactor located at Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, United States. Operating at 85 MW, HFIR is one of the highest flux reactor-based sources of neutrons for condensed matter physics research in the United States, and it provides one of the highest steady-state neutron fluxes of any research reactor in the world. The thermal and cold neutrons produced by HFIR are used to study physics, chemistry, materials science, engineering, and biology. The intense neutron flux, constant power density, and constant-length fuel cycles are used by more than 500 researchers each year for neutron scattering research into the fundamental properties of condensed matter. HFIR has approximately 600 users each year for both scattering and in-core research.

This page describes how uranium dioxide nuclear fuel behaves during both normal nuclear reactor operation and under reactor accident conditions, such as overheating. Work in this area is often very expensive to conduct, and so has often been performed on a collaborative basis between groups of countries, usually under the aegis of the Organisation for Economic Co-operation and Development's Committee on the Safety of Nuclear Installations (CSNI).

A loss-of-pressure-control accident (LOPA) is a mode of failure for a nuclear reactor that involves the pressure of the confined coolant falling below specification. Most commercial types of nuclear reactor use a pressure vessel to maintain pressure in the reactor plant. This is necessary in a pressurized water reactor to prevent boiling in the core, which could lead to a nuclear meltdown. This is also necessary in other types of reactor plants to prevent moderators from having uncontrolled properties.

The three primary objectives of nuclear reactor safety systems as defined by the U.S. Nuclear Regulatory Commission are to shut down the reactor, maintain it in a shutdown condition and prevent the release of radioactive material.

Corium (nuclear reactor) lava-like material created in the core of a nuclear reactor during a meltdown accident

Corium, also called fuel containing material (FCM) or lava-like fuel containing material (LFCM), is a lava-like material created in the core of a nuclear reactor during a meltdown accident.

Gas core reactor rockets are a conceptual type of rocket that is propelled by the exhausted coolant of a gaseous fission reactor. The nuclear fission reactor core may be either a gas or plasma. They may be capable of creating specific impulses of 3,000–5,000 s and thrust which is enough for relatively fast interplanetary travel. Heat transfer to the working fluid (propellant) is by thermal radiation, mostly in the ultraviolet, given off by the fission gas at a working temperature of around 25,000 °C.

Boiling water reactor safety systems are nuclear safety systems constructed within boiling water reactors in order to prevent or mitigate environmental and health hazards in the event of accident or natural disaster.

Integral Molten Salt Reactor

The Integral Molten Salt Reactor (IMSR) is a design for a small modular reactor (SMR) that employs molten salt reactor technology being developed by the Canadian company Terrestrial Energy. It is based closely on the denatured molten salt reactor (DMSR), a reactor design from Oak Ridge National Laboratory, and also incorporates elements found in the SmAHTR, a later design from the same laboratory. The IMSR belongs to the DMSR class of molten salt reactors (MSR) and hence is a "burner" reactor that employs a liquid fuel rather than a conventional solid fuel; this liquid contains the nuclear fuel and also serves as primary coolant.

References

  1. 1 2 "Core Shroud Replacement Work". Archived from the original on 2014-08-15. Retrieved 2011-07-21.
  2. 1 2 3 4 "The Core Shroud Does Not Form a Barrier Function".
  3. ↑ "How Nuclear Power Works".
  4. 1 2 "Mechanism of Core Shroud and its Function". Archived from the original on 2013-11-13. Retrieved 2013-11-13.
  5. 1 2 "Study on SCC Growth Behavior of BWR Core Shroud" (PDF).
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.