Confinement of radioactive material involves the fuel, the circuit and the containment systems, but in this section we will focus on the containment. LWRs generally have pressure-retaining reactor containment buildings. As was noted in 10.2, the coolant in an operating LWR is always radioactive. Thus the containment is provided both to contain the activity, which may be present during normal operation, as well as any fission products released as a result of a fault.

The containment must be designed to withstand the pressure and temperature generated by the steam produced by the release of the primary coolant into the containment following a hypothetical failure of one of the main coolant pipes. In

10.16 Schematic of BWR low pressure ECCS systems (Source: USNRC).

general containments can be divided into dry containment and pressure suppression containment.

Dry containments rely on their size and the heat sink provided by the structure of the containment and its contents to limit the peak pressure following a large LOCA. Pressure suppression containments reduce the peak pressure by forcing the steam released to flow through water in suppression pools or ice baskets. This allows a lower design pressure to be used for the containment structure. The design pressure for the structure is generally set to provide a margin above the peak pressure and is evaluated using conservative design rules. They are subjected to proof pressure tests at above the design pressure. Early PWRs and BWRs used dry containments but more modern BWRs have tended to use pressure suppression containments.