Reactor vessels are the protective containment surrounding the nuclear fission core in a nuclear reactor. The vessel which is the central component of the reactor coolant system has a cylindrical shell with a spherical bottom head and a ring flange at the top. A closure head is bolted to the flange. It is typical for the reactor vessels to be made with stainless steel usually of the ferritic type that is well suited for welding and with the high strength and toughness but low porosity under neutron irradiation. When the reactor is operated, neutron radiation from the reactor core causes embrittlement of the reactor pressure vessel materials making it less tough and less capable of maintaining its structural integrity.
Reducing the rates of embrittlement in reactor vessels
- Pressurized water reactors (PWRs) are considered to be more susceptible to embrittlement than the boiling water reactors (BWRs) since the BWR vessels generally experience less neutron irradiation and therefore less embrittlement. Many utilities make use of core designs that will reduce the number of neutrons that will reach the vessel wall and will minimize the rate of embrittlement on the reactor vessels.
- Another factor that contributes to a certain extent towards the embrittlement of the reactor vessels is the kind of steel used. Steels with a higher proportion of copper and nickel tend to be more susceptible to embrittlement than steels with a lower proportion of the 2 elements.
- Another reason for PWR reactor vessels embrittlement is pressurized thermal shock that can occur under some accidents scenarios when cold water is introduced to the reactor vessel while it is still pressurized. The introduction of cold water can cause the vessel to cool rapidly resulting in large thermal stresses to steel. These thermal stresses along with the high internal pressure and an embrittled reactor vessel could lead to cracking that will ultimately lead to failure of the vessel. This can be a potential safety issue which is the focus of inspections on the reactor vessel structural integrity.
Metallurgy and fabrication of stainless steel reactor vessels
- Reactor vessels for light water cooled reactors are usually manufactured from low carbon alloy steel to secure strength. The reactor pressure vessel is the most critical pressure-boundary component as far as safety and plant life is concerned which requires that the material for pressure retaining component should have sufficient strength and fracture toughness for the assurance of structural integrity.
- The reactor vessels are typically lined with stainless steel for corrosion prevention against water when used as a reactor coolant. Stainless steel contains sufficient amounts of chromium to form a passive film of chromium oxide that prevents surface corrosion by blocking oxygen diffusion into the steel surface and blocks corrosion from spreading into the internal structure of the reactor vessel.
- The manufacture of reactor vessels is difficult, time consuming and expensive due to the absolute necessity of near perfection in forming, welding and machining. Parts are forged, heat treated, quenched and tempered before they are subjected to ultrasonic inspection, tangible tensile tests, impact tests, bend tests and finally magnetic particle inspection.