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The long time-frame of the necessary safe enclosure of radioactive waste in containers gains interna-tionally an increasing attention. As well extended storage periods as a usually necessary subsequent transportation after storage have to be evaluated.
In Germany, the concept of dry interim storage in dual purpose metal casks before disposal is being pursued for spent nuclear fuel (SNF) and high level waste (HLW) management. The initially planned and established dry interim storage license duration of up to 40 years will be too short and its exten-sion will become necessary since there is no repository available in due time.
The main safety relevant components are the thick-walled dual purpose metal casks. These casks consist e.g. of a monolithic cask body with integrated neutron shielding components (polymers, e.g. polyethylene) and a monitored double lid barrier system equipped with metal and elastomer seals. The metal seals of this bolted closure system guarantee the required leak-tightness whereas the elastomer seals allow for leakage rate measurement of the metal seals.
At BAM we focus on the long-term performance of the sealing system and neutron shielding materials as their application conditions have to be concerned as rather demanding.
Therefore, BAM began to investigate the overall function and long-term behavior of elastomer and metal seals and also the irradiation influence on neutron shielding materials.
Recently a new project was initiated to expand our investigations to get more and in some context better applicable data.
This paper describes the approach of our new research project with the aim of further closure of existing knowledge gaps and presents an overview of investigations concerning the low temperature behavior of elastomer seals, of running long-term tests on elastomer seals at different temperatures under static conditions and the influence of gamma irradiation on neutron shielding materials.
Overview of ongoing research and future prospects on polyethylene neutron shielding materials at BAM
(2023)
The extension of the interim storage period of radioactive waste before disposal will cause additional challenges for the nuclear waste management in Germany, so that an extensive knowledge of the long-term performance of casks, including their components and inventories, will be required for future extended storage licenses.
Ultra-high and high molecular weight polyethylenes ((U)HMW-PE) are used for neutron shielding purposes in casks for storage and transport of spent fuel and high-level waste due to their extremely high hydrogen content. During their service life of several decades as cask components, the PE materials are exposed to neutron and gamma radiation from the radioactive inventory of the casks, mechanical assembling stresses and temperature. All these combined effects affect the material properties of such components which in turn may be crucial for some possible accident scenarios.
At the Bundesanstalt für Materialforschung und -prüfung (BAM), the effects of high temperature exposure in combination with subsequent or previous irradiation were investigated with a comprehensive aging program including thermal aging at 125 °C for different aging periods up to 5 years and irradiation with doses ranging from 50 to 600 kGy.
This contribution provides an overview of the ongoing research related to the structural changes of (U)HMW-PE induced by gamma irradiation and high temperature exposure and focuses on current research perspectives at BAM with regard to the prediction of the dynamic behavior of the material during extended interim storage in case of an accident scenario. First results of the coupled effect of temperature, radiation and mechanical loading will be presented. The effect of microstructural changes induced by gamma irradiation and high temperature on the mechanical behavior of (U)HMW-PE will be assessed.