Pichlmaier, A. (Corresponding author)MLZ* ; Gerstenberg, H.MLZ* ; Kastenmüller, A.MLZ*

2015

Report No.: IAEA-CN-231

Abstract: The FRM II is Germany’s most modern research reactor. It became critical for the first time on March 2nd, 2004. Sinceroutine operation started in May 2005, it has just completed its first ten years of operation. The FRM II is a multi-purposereactor and operates at nominal power of 20 MW. It is light water cooled and heavy water moderated. It is usedpredominantly for neutron scattering experiments in a wide range of applications from fundamental physics to materialand life sciences but also runs a prompt gamma neutron activation analysis facility, a tomography beam line, a positronsource and a medical irradiation facility. Important fields of activity include also isotope production and Silicon doping.Most of the core components of the FRM II are made from Aluminium (EN AW-5754, AlMg3). This material undergoesembrittlement under neutron irradiation, mainly by Silicon formation in the Aluminium by capture of thermal neutrons.Since only few data on highly irradiated Aluminium are available worldwide the FRM II runs its own irradiationprogramme. This is also a requirement of the licensing procedure. Based on the concept of tensile strength samples of thesame Aluminium are irradiated during the operation. After accumulation of certain neutron fluences the samples areremoved and the remaining tensile stress as well as fracture mechanics parameters are determined. The concept of usingtensile stress is the only one accepted for the FRM II so far. Although it is a well-established procedure, it is in the case ofthe FRM II core components very conservative and not necessarily in line with the latest knowledge in science andtechnology. Consequently, it may lead to the requirement of exchange of some components before they reach their trueend of lifetime. This, in turn, unnecessarily reduces the availability of the FRM II due to additional maintenance breaksand generates avoidable radiation dose for the personnel. Therefore together with external fracture mechanics experts, theexpert organization and the licensing authorities a different concept has been developed: The proof of sufficient stabilityshall now be performed using fracture mechanics. To this end, detailed calculations on the mechanical stress on theaffected components have been carried out. These components are mainly the central channel that houses the fuelelement, the moderator tank itself, the beam tubes as far as they are located in high neutron flux areas and othercomponents made form Aluminium that reach into the moderator tank and into areas with high neutron flux. Maximumundetected fracture size and fracture growth have been assumed in application of the relevant codes. Detailed neutronfluence calculations have been carried out and parameters on the material properties after irradiation in the light offracture mechanics were deduced from the samples irradiated at the FRM II. Finally all these results were put togetherleading to a detailed picture of required material properties – as necessary for safe operation – and existing materialproperties – after a certain irradiation was accumulated – which lead to a prediction of the remaining lifetime of the corecomponents made from Aluminium.Although the final result has not been approved in every detail by the licensing authority already now a premature exchange of some components could be avoided. The complete new concept for the lifetime of core components madefrom Aluminium

Keyword(s): Key Technologies (1st) ; Others (2nd)

1. Nukleare Entsorgung und Reaktorsicherheit (IEK-6)

1. No specific instrument