12.43. A containment structure enclosing the reactor primary system acts as the final backup barrier to fission product release to the environment in the defense-in-depth design of the reactor safety systems. Although containment design in LWRs is traditionally based on holding the pressure resulting from the release of the primary coolant in a loss-of coolant ac­cident (LOCA) (§12.37) and to withstand the impact of internally gener­ated missiles, margins are such that substantially higher pressures can be accommodated before failure. As we shall see later (§12.95), additional challenges are provided by so-called severe accidents in which the core is degraded. Fission product transport into the containment can be modeled analytically with various uncertainties involved. Therefore, it is useful to view the containment as the essential physical envelope which will indeed prevent the fission products from entering the environment despite the modeling uncertainties that may be present.[22]

12.44. Containments for PWRs are large cylindrical or spherical pres­sure vessels designed for pressures on the order of 345 kPa(g) (50 psig). Since the PWR containment must be large enough anyway to enclose all of the components of the entire primary system, which may include four steam generators and a pressurizer in addition to the reactor pressure vessel itself, it is practical to achieve the necessary pressure reduction merely by allowing steam formed by coolant flashing to vent into the large volume. On the other hand, the BWR primary system is contained essentially within the pressure vessel with much less building volume needed for housing it. Therefore, BWRs use means to condense the released steam in pressure suppression pools so that the containment building need not be much larger than that required to hold the system.