For nearly fifteen years, the behavior under boundary conditions and the safety of light water reactor components under near-operating loads and accident conditions has been examined experimentally and in accompanying analyses within the HDR Safety Program Project (PHDR). One important summary finding to be derived from these studies shows that ultimate verification of complex computer codes, e.g. those for accident analyses, is possible only by critical comparisons between calculations and realistic large scale tests. Only in this way will all effects of the actual sequence of physical events be simulated. The 1990 HDR Safety Program was focused on the behavior of hydrogen in the containment under major accident conditions with core melt-down, long term damage to pressurized components, the behavior of pipe systems under dynamic accident loads, and large fires of real materials.