Containment B.Vessel & Steam Generator Vessel
ACCORDING TO ASME SEC.Ⅷ.DIV.Ⅰ
Steam generators are heat exchangers used to convert water into steam from heat produced in a nuclear reactor core. They are used in pressurized water reactors (PWR) between the primary and secondary coolant loops.
In typical PWR designs, the primary coolant is high-purity water, kept under high pressure so it cannot boil. This primary coolant is pumped through the reactor core where it absorbs heat from the fuel rods. It then passes through the steam generator, where it transfers its heat (via conduction through metal) to lower-pressure water which is allowed to boil.
Fabrication stage of the equipment
In commercial power plants, there are two to four steam generators per reactor; each steam generator can measure up to 70 feet (21 m) in height and weigh as much as 800 tons. Each steam generator can contain anywhere from 3,000 to 16,000 tubes, each about .75 inches (19 mm) in diameter. The coolant (treated water), which is maintained at high pressure to prevent boiling, is pumped through the nuclear reactor core.
Heat transfer takes place between the reactor core and the circulating water and the coolant is then pumped through the primary tube side of the steam generator by coolant pumps before returning to the reactor core.
A steam generator’s heat-exchange tubes have an important safety role, because they separate radioactive and non-radioactive fluid systems. (The primary coolant becomes briefly radioactive from its exposure to the core, and also has trace amounts of longer-lived radioactive isotopes dissolved in it, such as dissolved atoms of iron from pipes.) Because the primary coolant is at higher pressure, a ruptured heat-exchange tube would cause primary coolant to leak into the secondary loop. Typically this would require the plant to shutdown for repair.
To avoid such primary-secondary leaks, steam generator tubes are periodically inspected by eddy-current testing, and individual tubes can be plugged to remove them from operation. As with many nuclear components, mechanical engineers determine the inspection frequency using the known rates of corrosion and crack propagation in the material.