In a typical pressurised water reactor with a large dry containment, a substantial containment volume surrounds a spatially distributed reactor coolant system. In boiling water reactors with Mark I or Mark II containment systems, steam is routed to a water pool, where condensation limits the pressure rise. The difference is not an architectural peculiarity. It is a consequence of the reactor design, the postulated release of mass and energy, and the selected method of controlling pressure under accident conditions.