On the basis of calculations performed on the results of experiments conducted in reactors with volumes up to 10 m(3) for so-called vapour-pressure producing (runaway) reaction systems, it is found that ail simplified or steady-state methods regularly afford overdimensioned relief cross-sections. As a consequence, unnecessarily large mass flows leave the reactor to enter the containment system. A balanced design of both plants components is possible only with the aid of transient methods. In the case of gas generating reactions simplified methods also lead to excessively large pressure relief cross-sections. Design with the aid of a transient method for such a reaction system is rendered difficult by the fact that a conservative submodel cannot be used a priori for description of the expansion behaviour. A parametric study is then required.