Pressurized water reactors use a reactor pressure vessel (RPV) to contain the nuclear fuel, moderator, control rods and coolant. They are cooled and moderated by high-pressure liquid water (e.g. 16MPa). At this pressure water boils at approximately 350°C (662°F). Inlet temperature of the water is about 290°C (554°F). The water (coolant) is heated in the reactor core to approximately 325°C (617°F) as the water flows through the core. As it can be seen, the reactor has approximately 25°C subcooled coolant (distance from the saturation).
The reactor pressure vessel is the pressure vessel containing the reactor core and other key reactor internals. It is a cylindrical vessel with a hemispherical bottom head and a flanged and gasketed upper head. The bottom head is welded to the cylindrical shell while the top head is bolted to the cylindrical shell via the flanges. The top head is removable to allow for the refueling of the reactor during planned outages.
The body of the reactor vessel is constructed of a high-quality low-alloy carbon steel, and all surfaces that come into contact with reactor coolant are clad with a minimum of about 3 to 10 mm of austenitic stainless steel (e.g. 304L) in order to minimize corrosion.
Low-carbon steel, also known as mild steel is now the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications. Low-carbon steel contains approximately 0.05–0.25% carbon making it malleable and ductile. Mild steel has a relatively low tensile strength, but it has high toughness and it is easy to form. Special requirements for materials of reactor vessel include low activation capability (especially due to Co-60 formation). Examples of high-quality low-alloy carbon steels:
- SA-508 Gr.3 Cl.2 (low-alloy ferritic steel)
- 15Kh2NMFA (low-alloy ferritic steel)
Pure iron is too soft to be used for the purpose of structure, but the addition of small quantities of other elements (carbon, manganese or chromium for instance) greatly increases its mechanical strength. The synergistic effect of alloying elements and heat treatment produces a tremendous variety of microstructures and properties. The four major alloying elements are:
- Chromium. In these steels chromium increases hardness and strength. Generally speaking, the concentration specified for most grades is approximately 2%. This level appears to result in the best balance between hardness and toughness. Chromium plays an important role in the hardening mechanism and is considered irreplaceable. At higher temperatures, chromium contributes increased strength.
- Nickel. Nickel does not form any carbide compounds in steel, it remains in solution in the ferrite, thus strengthening and toughening the ferrite phase.
- Molybdenum. Molybdenum (about 0.50-8.00%) when added to a steel makes it more resistant to high temperature. Molybdenum increases hardenability and strength, particularly at high temperatures due to the high melting point of molybdenum. Molybdenum is unique in the extent to which it increases the high-temperature tensile and creep strengths of steel.
Austenitic stainless steels, which is used as a corrosion-resistant clad, contain between 16 and 25% of chromium and can also contain nitrogen in solution, both of which contribute to their relatively high corrosion resistance. The best known grade is AISI 304 stainless, which contains both chromium (between 15% and 20%) and nickel (between 2% and 10.5%) metals as the main non-iron constituents. 304 stainless steel has excellent resistance to a wide range of atmospheric environments and many corrosive media. These alloys are usually characterized as ductile, weldable, and hardenable by cold forming.
Type 304L stainless steel, which is widely used in nuclear industry, is an extra-low carbon version of the 304 steel alloy. This grade has slightly lower mechanical properties than the standard 304 grade, but is still widely used thanks to its versatility. The lower carbon content in 304L minimizes deleterious or harmful carbide precipitation as a result of welding. 304L can, therefore, be used “as welded” in severe corrosion environments, and it eliminates the need for annealing. Grade 304 has also good oxidation resistance in intermittent service to 870 °C, and in continuous service to 925 °C. Since grade 304L does not require post-weld annealing, it is extensively used in heavy gauge components. Examples of used stainless steels:
- Type 304L stainless steel
- Type 08Kh18N10T stainless steel
The reactor pressure vessels are the highest priority key components in nuclear power plants. The reactor pressure vessel houses the reactor core and because of its function it has direct safety significance. During the operation of a nuclear power plant, the material of the reactor pressure vessel is exposed to neutron radiation (especially to fast neutrons), which results in localized embrittlement of the steel and welds in the area of the reactor core. In order to minimize such material degradation radial neutron reflectors are installed around the reactor core. There are two basic types of neutron reflectors, the core baffle and the heavy reflector. Due to higher atomic number density heavy reflectors reduce neutron leakage (especially of fast neutrons) from the core more efficiently than core baffle. Since the reactor pressure vessel is considered irreplaceable, these ageing effects of the RPV have the potential to be life-limiting conditions for a nuclear power plant.
Radiation Damage to Metals
Materials in nuclear service are subjected to various types of radiation. Some of these can cause significant damage to the crystalline structure of materials. Nuclear radiation focuses large amounts of energy into highly localized areas. Damage is caused by the interaction of this energy with the nuclei and/or orbiting electrons.
As was written, charged particles with high energies can directly ionize atoms or they can cause excitation of surrounding electrons. The ionization and excitation dissipates much of the energy of heavier charged particles and does very little damage. This is because electrons are relatively free to move and are soon replaced. The net effect of beta and gamma radiation on metal is to generate a small amount of heat. Heavier particles, such as protons, alpha-particles, fast neutrons, and fission fragments, will usually transfer sufficient energy through elastic or inelastic collisions to remove nuclei from their lattice (crystalline) positions. This addition of vacancies and interstitial atoms causes property changes in metals.
In general, the effects of greatest interest can be described by the following groupings:
- Vacancies or Knock-ons. Vacancy defects result from a missing atom in a lattice position. The stability of the surrounding crystal structure guarantees that the neighboring atoms will not simply collapse around the vacancy. This can be caused by the direct interaction of a high energy neutron or a fission fragment. If a target or struck nucleus gains about 25 eV of kinetic energy (25 eV to 30 eV for most metals) in a collision with a radiation particle (usually a fast neutron), the nucleus will be displaced from its equilibrium position in the crystal lattice. During a lengthy irradiation (for large values of the neutron fluence), many of the displaced atoms will return to normal (stable) lattice sites (that is, partial annealing occurs spontaneously).
- Interstitials. Interstitial defects result from an impurity located at an interstitial site or one of the lattice atoms being in an interstitial position instead of being at its lattice position. An interstitial is formed when an atom, which is knocked out from its position, comes to rest at some remote point.
- Ionization. Ionization is caused by the removal of electrons from their electronic shells and has the effect of changing the chemical bonds of molecules. In metal, ionization does not cause dramatic changes in material’s properties. This is due to free electrons, which are typical only for metallic bond.
- Thermal and Displacement Spikes. Thermal and displacement spikes can cause distortion that is frozen as stress in the microscopic area. These spikes can cause a change in the material’s properties. This term identifies localized high temperature domains caused by the deposition of energy from neutrons and fission fragments. A displacement spike occurs when many atoms in a small area are displaced by a knock-on (or cascade of knock-ons). A 1 MeV neutron may affect approximately 5000 atoms, making up one of these spikes. The presence of many displacement spikes changes the properties of the metal being irradiated, such as increasing hardness and decreasing ductility.
- Impurity Atoms. The capture of neutrons and nuclear reactions induced by various radiations has the effect of transmuting an atom into an element which is foreign to the material.
- Radiation Induced Creep. In nuclear reactors, many metal components are subjected simultaneously to radiation fields, elevated temperatures and stress. Metal under stress at elevated temperature exhibits the phenomenon of creep, ie. the gradual increase in strain with time. Creep of metal components at reactor operating temperatures becomes faster when they are exposed to a radiation field.
Neutrons with sufficient energy can disrupt the atomic arrangement or crystalline structure of materials. The influence of structural damage is most significant for metals because of their relative immunity to damage by ionizing radiation. Pressurized-water reactors operate with a higher rate of neutron impacts and their vessels therefore tend to experience a greater degree of embrittlement than boiling-water reactor vessels. Many pressurized-water reactors design their cores to reduce the number of neutrons hitting the vessel wall. This slows the vessel’s embrittlement. The NRC’s regulations address embrittlement in 10 CFR Part 50, Appendix G, “Fracture Toughness Requirements” and Appendix H, “Reactor Vessel Material Surveillance Program Requirements.” Since the reactor pressure vessel is considered irreplaceable, neutron irradiation embrittlement of pressure vessel steels is a key issue in the long term assessment of structural integrity for life attainment and extension programmes.
Radiation damage is produced when neutrons of sufficient energy displace atoms (especially in steels at operating temperatures 260 – 300°C) that result in displacement cascades which produce large numbers of defects, both vacancies and interstitials. Although the inside surface of the RPV is exposed to neutrons of varying energies, the higher energy neutrons, those above about 0.5 MeV, produce the bulk of the damage. In order to minimize such material degradation type and structure of the steel must be appropriately selected. Today it is known that the susceptibility of reactor pressure vessel steels is strongly affected (negatively) by the presence of copper, nickel and phosphorus.
As was written, the distinction between brittleness and ductility isn’t readily apparent, especially because both ductility and brittle behavior are dependent not only on the material in question but also on the temperature (ductile-brittle transition) of the material. The effect of temperature on the nature of the fracture is of considerable importance. Many steels exhibit ductile fracture at elevated temperatures and brittle fracture at low temperatures. The temperature above which a material is ductile and below which it is brittle is known as the ductile–brittle transition temperature (DBTT), nil ductility temperature (NDT), or nil ductility transition temperature. This temperature is not precise, but varies according to prior mechanical and heat treatment and the nature and amounts of impurity elements. It can be determined by some form of drop-weight test (for example, the Charpy or Izod tests).
To minimize neutron fluence:
- Radial neutron reflectors are installed around the reactor core. Neutron reflectors reduce neutron leakage and therefore they reduce the neutron fluence on a reactor pressure vessel.
- Core designers design the low leakage loading patterns, in which fresh fuel assemblies are not situated in the peripheral positions of the reactor core.
If the metal is heated to elevated temperatures after irradiation (a form of annealing), it is found that the strength and ductility return to the same values as before irradiation. This means that radiation damage can be annealed out of a metal.
See also: Ductile-brittle Transition Temperature
See also: Irradiation Embrittlement
See also: Thermal Annealing