Transients without a loss-of-coolant are part of the DBAs. With the exception of containments with a pressure suppression system no major interaction with the containment atmosphere exist. Steam injected into the water pool of the suppression system is condensed in the pool and creates thus some circulation in the pool. There is experimental and operational evidence that the mixing in the pool is good; therefore, no major analytical efforts are undertaken to model this mixing.

Depending on the break size steam is injected into the containment atmosphere during loss of coolant accidents (LOCA) creating, in the short-term, injection driven flows and a pressure increase. Later condensation of steam on steel and concrete structures occurs. In the long term energy is removed through heat exchangers or to the outside of the containment pressure boundary if this boundary is designed as a steel shell. Some small amounts of hydrogen may be produced during a LOCA, due to the fuel cladding/steam reaction; recombiners in the containment will reduce a local accumulation as well as the total amount of hydrogen. During all these processes convection flows exist; usually the velocities are low.

Related computer codes have been developed since about 20 years. They are usually of lumped parameter type and can be assessed as being fully developed, in general. Well-known codes are CONTAIN (SANDIA) and COCOSYS (GRS).