Reactor pressure vessel

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The reactor vessel used in the first US commercial nuclear power plant, the Shippingport Atomic Power Station. Photo from 1956. Shippingport LOC 135430pu.jpg
The reactor vessel used in the first US commercial nuclear power plant, the Shippingport Atomic Power Station. Photo from 1956.

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.

Contents

Classification of nuclear power reactors

A typical RPV Reactorvessel.gif
A typical RPV

Russian Soviet era RBMK reactors have each fuel assembly enclosed in an individual 8 cm diameter pipe rather than having a pressure vessel. Whilst most power reactors do have a pressure vessel, they are generally classified by the type of coolant rather than by the configuration of the vessel used to contain the coolant. The classifications are:

Of the main classes of reactor with a pressure vessel, the pressurized water reactor is unique in that the pressure vessel suffers significant neutron irradiation (called fluence) during operation, and may become brittle over time as a result. In particular, the larger pressure vessel of the boiling water reactor is better shielded from the neutron flux, so although more expensive to manufacture in the first place because of this extra size, it has an advantage in not needing annealing to extend its life.

Annealing of pressurized water reactor vessels to extend their working life is a complex and high-value technology being actively developed by both nuclear service providers (AREVA) and operators of pressurized water reactors.

Components of a pressurized water reactor pressure vessel

A reactor vessel body and vessel head being shipped to Dresden Generating Station HD.6B.276 (11842858626).jpg
A reactor vessel body and vessel head being shipped to Dresden Generating Station

All pressurized water reactor pressure vessels share some features regardless of the particular design.

Reactor vessel body

The reactor vessel body is the largest component and is designed to contain the fuel assembly, coolant, and fittings to support coolant flow and support structures. It is usually cylindrical in shape and is open at the top to allow the fuel to be loaded.

Reactor vessel head

A reactor vessel head for a pressurized water reactor Reactor Vessel head.jpg
A reactor vessel head for a pressurized water reactor

This structure is attached to the top of the reactor vessel body. It contains penetrations to allow the control rod driving mechanism to attach to the control rods in the fuel assembly. The coolant level measurement probe also enters the vessel through the reactor vessel head.

Fuel assembly

The fuel assembly of nuclear fuel usually consisting of uranium or uranium–plutonium mixes. It is usually a rectangular block of gridded fuel rods.

Neutron reflector or absorber

Protecting the inside of the vessel from fast neutrons escaping from the fuel assembly is a cylindrical shield wrapped around the fuel assembly. Reflectors send the neutrons back into the fuel assembly to better utilize the fuel. The main purpose though is to protect the vessel from fast neutron induced damage that can make the vessel brittle and reduce its useful life.

Materials

The RPV provides a critical role in safety of the PWR reactor and the materials used must be able to contain the reactor core at elevated temperatures and pressures. [1] [2] The materials used in the cylindrical shell of the vessels have evolved over time, but in general they consist of low-alloy ferritic steels clad with 3–10 mm of austenitic stainless steel. The stainless steel cladding is primarily used in locations that come into contact with coolant in order to minimize corrosion. [2] Through the mid-1960, SA-302, Grade B, a molybdenum-manganese plate steel, was used in the body of the vessel. [2] As changing designs required larger pressure vessels, the addition of nickel to this alloy by roughly 0.4-0.7 wt% was required to increase the yield strength. [2] Other common steel alloys include SA-533 Grade B Class 1 and SA-508 Class 2. Both materials have main alloying elements of nickel, manganese, molybdenum, and silicon, but the latter also includes 0.25-0.45 wt% chromium. [2] All alloys listed in the reference also have >0.04 wt% sulfur. [2] Low-alloyed NiMoMn ferritic steels are attractive for this purpose due to their high thermal conductivity and low thermal expansion, properties that make them resistant to thermal shock. [3] However, when considering the properties of these steels, one must take into account the response it will have to radiation damage. Due to harsh conditions, the RPV cylinder shell material is often the lifetime-limiting component for a nuclear reactor. [1] Understanding the effects radiation has on the microstructure in addition to the physical and mechanical properties will allow scientists to design alloys more resistant to radiation damage.

In 2018 Rosatom announced it had developed a thermal annealing technique for RPVs which ameliorates radiation damage and extends service life by between 15 and 30 years. This had been demonstrated on unit 1 of the Balakovo Nuclear Power Plant. [4]

Radiation damage in metals and alloys

Due to the nature of nuclear energy generation, the materials used in the RPV are constantly bombarded by high-energy particles. These particles can either be neutrons or fragments of an atom created by a fission event. [5] When one of these particles collides with an atom in the material, it will transfer some of its kinetic energy and knock the atom out of its position in the lattice. When this happens, this primary "knock-on" atom (PKA) that was displaced and the energetic particle may rebound and collide with other atoms in the lattice. This creates a chain reaction that can cause many atoms to be displaced from their original positions. [5] This atomic movement leads to the creation of many types of defects. [5] The accumulation of various defects can cause microstructural changes that can lead to a degradation in macroscopic properties. As previously mentioned, the chain reaction caused by a PKA often leaves a trail of vacancies and clusters of defects at the edge. This is called a displacement cascade. [6] The vacancy-rich core of a displacement cascade can also collapse into dislocation loops. Due to irradiation, materials tend to develop a higher concentration of defects than is present in typical steels, and the high temperatures of operation induce migration of the defects. This can cause things like recombination of interstitials and vacancies and clustering of like defects, which can either create or dissolve precipitates or voids. Examples of sinks, or thermodynamically favorable places for defects to migrate to, are grain boundaries, voids, incoherent precipitates, and dislocations.

Radiation-induced segregation

Interactions between defects and alloying elements can cause a redistribution of atoms at sinks such as grain boundaries. The physical effect that can occur is that certain elements will be enriched or depleted in these areas, which often leads to embrittlement of grain boundaries or other detrimental property changes. This is because there is a flux of vacancies towards a sink and a flux of atoms away or toward the sink that may have varying diffusion coefficients. The uneven rates of diffusion cause a concentration of atoms that will not necessarily be in the correct alloy proportions. It has been reported that nickel, copper and silicon tend to be enriched at sinks, whereas chromium tends to be depleted. [6] [7] The resulting physical effect is changing chemical composition at grain boundaries or around voids/incoherent precipitates, which also serve as sinks.

Formation of voids and bubbles

Voids form due to a clustering of vacancies and generally form more readily at higher temperatures. Bubbles are simply voids filled with gas; they will occur if transmutation reactions are present, meaning a gas is formed due to the breakdown of an atom caused by neutron bombardment. [6] The biggest issue with voids and bubbles is dimensional instability. An example of where this would be very problematic is areas with tight dimensional tolerances, such as threads on a fastener.

Irradiation hardening

The creation of defects such as voids or bubbles, precipitates, dislocation loops or lines, and defect clusters can strengthen a material because they block dislocation motion. The movement of dislocations is what leads to plastic deformation. While this hardens the material, the downside is that there is a loss of ductility. Losing ductility, or increasing brittleness, is dangerous in RPVs because it can lead to catastrophic failure without warning. When ductile materials fail, there is substantial deformation before failure, which can be monitored. Brittle materials will crack and explode when under pressure without much prior deformation, so there is not much engineers can do to detect when the material is about to fail. A particularly damaging element in steels that can lead to hardening or embrittlement is copper. Cu-rich precipitates are very small (1-3 nm) so they are effective at pinning dislocations. [6] [8] It has been recognized that copper is the dominant detrimental element in steels used for RPVs, especially if the impurity level is greater than 0.1 wt%. [8] Thus, the development of "clean" steels, or ones with very low impurity levels, is important in reducing radiation-induced hardening.

Creep

Creep occurs when a material is held under levels of stress below their yield stress that causes plastic deformation over time. This is especially prevalent when a material is exposed to high stresses at elevated temperatures, because diffusion and dislocation motion occur more rapidly. Irradiation can cause creep due to the interaction between stress and the development of the microstructure. [6] In this case, the increase in diffusivities due to high temperatures is not a very strong factor for causing creep. The dimensions of the material are likely to increase in the direction of the applied stress due to the creation of dislocation loops around defects that formed due to radiation damage. Furthermore, applied stress can allow interstitials to be more readily absorbed in dislocation, which assists in dislocation climb. When dislocations are able to climb, excess vacancies are left, which can also lead to swelling. [6]

Irradiation assisted stress corrosion cracking

Due to the embrittlement of grain boundaries or other defects that can serve as crack initiators, the addition of radiation attack at cracks can cause intergranular stress corrosion cracking. The main environmental stressor that forms due to radiation is hydrogen embrittlement at crack tips. Hydrogen ions are created when radiation splits water molecules, which is present because water is the coolant in PWRs, into OH− and H+. There are several suspected mechanisms that explain hydrogen embrittlement, three of which are the decohesion mechanism, the pressure theory, and the hydrogen attack method. In the decohesion mechanism, it is thought that the accumulation of hydrogen ions reduces the metal-to-metal bond strength, which makes it easier to cleave atoms apart. [6] The pressure theory is the idea that hydrogen can precipitate as a gas at internal defects and create bubbles within the material. The stress caused by the expanding bubble in addition to the applied stress is what lowers the overall stress required to fracture the material. [6] The hydrogen attack method is similar to the pressure theory, but in this case it is suspected that the hydrogen reacts with carbon in the steel to form methane, which then forms blisters and bubbles at the surface. In this case, the added stress by the bubbles is enhanced by the decarburization of the steel, which weakens the metal. [6] In addition to hydrogen embrittlement, radiation induced creep can cause the grain boundaries to slide against each other. This destabilizes the grain boundaries even further, making it easier for a crack to propagate along its length. [6]

Designing radiation-resistant materials for reactor pressure vessels

Very aggressive environments require novel materials approaches in order to combat declines in mechanical properties over time. One method researchers have sought to use is introducing features to stabilize displaced atoms. This can be done by adding grain boundaries, oversized solutes, or small oxide dispersants to minimize defect movement. [5] [6] By doing this, there would be less radiation-induced segregation of elements, which would in turn lead to more ductile grain boundaries and less intergranular stress corrosion cracking. Blocking dislocation and defect movement would also help to increase the resistance to radiation assisted creep. Attempts have been reported of instituting yttrium oxides to block dislocation motion, but it was found that technological implementation posed a greater challenge than expected. [5] Further research is required to continue improving the radiation damage resistance of structural materials used in nuclear power plants.

Manufacturers

Because of the extreme requirements needed to build large state-of-the-art reactor pressure vessels and the limited market, as of January 2020 there are only a handful of manufacturers in the world including: [9]

See also

Related Research Articles

<span class="mw-page-title-main">Nuclear reactor</span>

A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nuclear fission is passed to a working fluid, which in turn runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. As of 2022, the International Atomic Energy Agency reports there are 422 nuclear power reactors and 223 nuclear research reactors in operation around the world.

<span class="mw-page-title-main">Pressurized water reactor</span> Type of nuclear reactor

A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants. 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, high pressure water then flows to a steam generator, where it transfers its thermal energy to lower pressure water of a secondary system where steam is generated. The steam then drives turbines, which spin an electric generator. In contrast to a boiling water reactor (BWR), pressure in the primary coolant loop prevents the water from boiling within the reactor. All light-water reactors use ordinary water as both coolant and neutron moderator. Most use anywhere from two to four vertically mounted steam generators; VVER reactors use horizontal steam generators.

<span class="mw-page-title-main">Boiling water reactor</span> Type of nuclear reactor that directly boils water

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.

<span class="mw-page-title-main">Nuclear meltdown</span> Reactor accident due to core 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 United States 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.

<span class="mw-page-title-main">Neutron radiation</span> Ionizing radiation that presents as free neutrons

Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new nuclides—which, in turn, may trigger further neutron radiation. Free neutrons are unstable, decaying into a proton, an electron, plus an electron antineutrino. Free neutrons have a mean lifetime of 887 seconds.

<span class="mw-page-title-main">Loss-of-coolant accident</span> Form of nuclear reactor failure.

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.

<span class="mw-page-title-main">Light-water reactor</span> Type of nuclear reactor that 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.

Passive nuclear safety is a design approach for safety features, implemented in a nuclear reactor, that does not require any active intervention on the part of the operator or electrical/electronic feedback in order to bring the reactor to a safe shutdown state, in the event of a particular type of emergency. Such design features tend to rely on the engineering of components such that their predicted behaviour would slow down, rather than accelerate the deterioration of the reactor state; they typically take advantage of natural forces or phenomena such as gravity, buoyancy, pressure differences, conduction or natural heat convection to accomplish safety functions without requiring an active power source. Many older common reactor designs use passive safety systems to a limited extent, rather, relying on active safety systems such as diesel-powered motors. Some newer reactor designs feature more passive systems; the motivation being that they are highly reliable and reduce the cost associated with the installation and maintenance of systems that would otherwise require multiple trains of equipment and redundant safety class power supplies in order to achieve the same level of reliability. However, weak driving forces that power many passive safety features can pose significant challenges to effectiveness of a passive system, particularly in the short term following an accident.

<span class="mw-page-title-main">Supercritical water reactor</span> Concept nuclear reactor whose water operates at supercritical pressure

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

<span class="mw-page-title-main">Lead-cooled fast reactor</span> Type of nuclear reactor cooled by molten lead

The lead-cooled fast reactor is a nuclear reactor design that use molten lead or lead-bismuth eutectic coolant. These materials can be used as the primary coolant because they have low neutron absorption and relatively low melting points. Neutrons are slowed less by interaction with these heavy nuclei so these reactors operate with fast neutrons.

<span class="mw-page-title-main">Zirconium alloys</span> Zircaloy family

Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance. One of the main uses of zirconium alloys is in nuclear technology, as cladding of fuel rods in nuclear reactors, especially water reactors. A typical composition of nuclear-grade zirconium alloys is more than 95 weight percent zirconium and less than 2% of tin, niobium, iron, chromium, nickel and other metals, which are added to improve mechanical properties and corrosion resistance.

<span class="mw-page-title-main">Sodium-cooled fast reactor</span> Type of nuclear reactor cooled by molten sodium

A sodium-cooled fast reactor is a fast neutron reactor cooled by liquid sodium.

The Whiteshell Reactor No. 1, or WR-1, was a Canadian research reactor located at AECL's Whiteshell Laboratories (WNRL) in Manitoba. Originally known as Organic-Cooled Deuterium-Reactor Experiment (OCDRE), it was built to test the concept of a CANDU-type reactor that replaced the heavy water coolant with an oil substance. This had a number of potential advantages in terms of cost and efficiency.

<span class="mw-page-title-main">Embrittlement</span> Loss of ductility of a material, making it brittle

Embrittlement is a significant decrease of ductility of a material, which makes the material brittle. Embrittlement is used to describe any phenomena where the environment compromises a stressed material's mechanical performance, such as temperature or environmental composition. This is oftentimes undesirable as brittle fracture occurs quicker and can much more easily propagate than ductile fracture, leading to complete failure of the equipment. Various materials have different mechanisms of embrittlement, therefore it can manifest in a variety of ways, from slow crack growth to a reduction of tensile ductility and toughness.

Radiation damage is the effect of ionizing radiation on physical objects including non-living structural materials. It can be either detrimental or beneficial for materials.

A nuclear reactor coolant is a coolant in a nuclear reactor used to remove heat from the nuclear reactor core and transfer it to electrical generators and the environment. Frequently, a chain of two coolant loops are used because the primary coolant loop takes on short-term radioactivity from the reactor.

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.

Radiation materials science is a subfield of materials science which studies the interaction of radiation with matter: a broad subject covering many forms of irradiation and of matter.

Neutron embrittlement, sometimes more broadly radiation embrittlement, is the embrittlement of various materials due to the action of neutrons. This is primarily seen in nuclear reactors, where the release of high-energy neutrons causes the long-term degradation of the reactor materials. The embrittlement is caused by the microscopic movement of atoms that are hit by the neutrons; this same action also gives rise to neutron-induced swelling causing materials to grow in size, and the Wigner effect causing energy buildup in certain materials that can lead to sudden releases of energy.

<span class="mw-page-title-main">Organic nuclear reactor</span> Nuclear reactor that uses organic liquids for cooling and neutron moderation

An organic nuclear reactor, or organic cooled reactor (OCR), is a type of nuclear reactor that uses some form of organic fluid, typically a hydrocarbon substance like polychlorinated biphenyl (PCB), for cooling and sometimes as a neutron moderator as well.

References

  1. 1 2 Zinkle, Steven J. (2009). "Structural materials for fission & fusion energy". Materials Today. 12 (11): 12–19. doi: 10.1016/S1369-7021(09)70294-9 .
  2. 1 2 3 4 5 6 "Assessment and management of ageing of major nuclear power plant components important to safety: PWR pressure vessels". International Atomic Energy Agency. 1999.
  3. ↑ Blagoeva, D.T.; Debarberis, L.; Jong, M.; ten Pierick, P. (2014). "Stability of ferritic steel to higher doses: Survey of reactor pressure vessel steel data and comparison with candidate materials for future nuclear systems". International Journal of Pressure Vessels and Piping. 122 (122): 1–5. doi:10.1016/j.ijpvp.2014.06.001.
  4. ↑ "Rosatom launches annealing technology for VVER-1000 units". World Nuclear News. 27 November 2018. Retrieved 28 November 2018.
  5. 1 2 3 4 5 "Development of Radiation Resistant Reactor Core Structural Materials". International Atomic Energy Agency. 2009.
  6. 1 2 3 4 5 6 7 8 9 10 11 Was, Gary S. (2007). Fundamentals of Radiation Materials Science: Metals and Alloys. Springer. ISBN   978-3-540-49471-3.
  7. ↑ "Fact Sheet on Reactor Pressure Vessel Issues". NRC: Fact Sheet on Reactor Pressure Vessel Issues. United States Nuclear Regulatory Commission.
  8. 1 2 Hoffelner, Wolfgang (2013). Materials for Nuclear Plants: From Safe Design to Residual Life Assessment. Springer. ISBN   978-1-4471-2914-1.
  9. 1 2 "Heavy Manufacturing of Power Plants - World Nuclear Association". www.world-nuclear.org.
  10. ↑ "Reactor Pressure Vessel Is Installed in Hualong One". www.yicaiglobal.com.
  11. ↑ "Framatome forge raises replacement parts production : Corporate - World Nuclear News". www.world-nuclear-news.org.
  12. ↑ "Le Creusot to resume manufacture of forgings - World Nuclear News". www.world-nuclear-news.org.
  13. ↑ Mishra, P.K; Shrivastav, Vivek. "Heavy steel forgings for Pressurised Water Reactor program" (PDF). BARC Newsletter. 377 (July–August 2021): 38. Retrieved 30 August 2021.
  14. ↑ Sarkar, Apu; Kumawat, Bhupendra K.; Chakravartty, J.K. (2015). "Ratchetting behavior of 20MnMoNi55 reactor pressure vessel steel". Journal of Nuclear Materials. 467: 500–504. Bibcode:2015JNuM..467..500S. doi:10.1016/j.jnucmat.2015.09.010.
  15. ↑ Reporter, B. S. (26 July 2009). "L&T plays crucial role in building India's first nuclear-powered submarine". Business Standard India. Retrieved 10 April 2021.
  16. ↑ "12 companies selected for the prestigious National Technology Awards 2020 for commercialization of innovative indigenous technologies". pib.gov.in. PIB Government of India.
  17. ↑ "Toshiba Gives IHI Contract For Pressure Vessel For First ABWR In U.S. - News - Nuclear Power News - Nuclear Street - Nuclear Power Plant News, Jobs, and Careers". nuclearstreet.com.
  18. ↑ "Toshiba, IHI to dissolve nuclear power venture". Reuters. October 19, 2018 – via www.reuters.com.
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