Investigations into the safe operation of research reactors. AP 2 Aging behavior of technical equipment in research reactors

This report summarizes the results of the work on the aging behavior of technical equipment in research reactors. The focus of the investigations was on passive mechanical components, structural structures and experimental facilities or their components from research reactors operated in Germany. In particular, those safety-relevant devices were examined that differ significantly from those in power reactors in terms of the materials used, construction and operating conditions. A significant difference in the design conditions in research reactors compared to power reactors is the use of aluminum alloys for experimental facilities as well as for passive mechanical components in the area close to the core (e.g. reflector housing in the FRMZ, moderator tank in FRM II, sections of the central channel in the FRM II). Another significant difference between research reactors and power reactors are some operating conditions, in particular pressure and temperature, the values of which are much lower than in power reactors. This results in significantly lower stresses, which means that the occurrence of some aging mechanisms is very unlikely (including mechanical or thermal fatigue) or can even be ruled out (e.g. stress corrosion cracking). Damage caused by corrosion in medium-carrying systems or experimental facilities, as well as in power reactors, is avoided through strict water chemistry specifications and appropriate monitoring and the use of cleaning systems to ensure compliance with the values. The main aging mechanism for passive mechanical components and experimental equipment in research reactors is radiation-induced damage to aluminum alloys. Irreversible changes in the mechanical properties of aluminum alloys are caused by the transmutation of aluminum to silicon. The mechanism is known and accordingly precautions are taken for more heavily irradiated facilities (replacement after set operating times) to ensure safe operation. With the help of recurring tests (visual inspections, pressure tests), the scope and frequency of which are based on the specifications that apply to power reactors in the KTA rules, aging-related damage to passive mechanical components and experimental equipment can be detected at an early stage and appropriate countermeasures can be taken. The stresses on structures in research reactors do not differ from those in power reactors and are quantitatively lower in some places, such as the reactor pool, due to the lower output. The same aging phenomena also occur here. To prevent undesirable changes caused by aging phenomena, visual inspections are carried out regularly, e.g. B. detect cracks early and take appropriate countermeasures.