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This lecture shows an overview of cross-cutting topics whithin the dynamic sector of hydrogen technologies including European and other international strategies, legal and policy frameworks, networks and relevant stakeholders, research and demo-projects, regulations, codes & Standards as well as initiatives and associations concerning hydrogen safety and education.
The German nuclear waste management strategy consists of dry interim storage of spent fuel inside dual purpose casks and its subsequent direct final disposal in a deep geological repository. Interim storage is limited to 40 years so far, but will have to be extended until repository site selection and operation is concluded, potentially up to 100 years. In this context, research on the long term performance of safety relevant cask components and fuel rod integrity during such extended periods of time is of outmost importance. The barrier function of the fuel cladding depends on its integrity under operational and accidental conditions along with storage and subsequent transportation and is mainly affected over time by altering of the cladding microstructure and by thermo-mechanical conditions during reactor operation and subsequent wet and dry storage.
This paper focuses on failure mechanisms regarding cladding embrittlement caused by physical phenomena concerning hydrogen dissolution and precipitation of hydrides in the zirconium matrix. Within this context, potential cladding integrity issues under extended interim storage conditions followed by transportation are emphasized.
Delayed hydride cracking (DHC) is a well-known time-dependent temperature-activated phenomenon failure mechanism of the fuel cladding during reactor operation. The phenomenon consists of hydrogen diffusion and hydride precipitation at the tip of an initial crack and the propagation of the crack to an approximate distance equal to the length of the hydride. This failure mechanism is temperature-activated phenomenon and enhances along with thermal cycling of the fuel cladding, e. g. during reactor shutdowns. DHC shows limited relevance under quasi static storage conditions with continuously lowering temperatures. However, the literature points that DHC may occur when the temperature during storage decreases to a certain level after some decades and in combination with stress concentrations in the cladding material.
When spent fuel is transferred to dry storage by vacuum drying, the cladding temperature raises up to peak temperatures of ~370°C. Under these conditions, hydrides dissolute into the zirconium matrix up to the solubility limit defined by the peak temperature and this amount of hydrogen can precipitate in a radial re-oriented fashion due to the temperature decrease during storage. Cladding materials with radially precipitated hydrides show significant embrittlement. Radially oriented hydrides are perpendicularly oriented to hoop stresses representing the most critical configuration. Brittle failure of the cladding then occurs by crack propagation through radially oriented hydrides and finally through the zirconium matrix. The susceptibility to radial hydride precipitation depends on cladding material type and microstructure, hydrogen content, pre-drying hydride distribution, irradiation conditions, and temperature and stress histories during drying and storage operations.
This paper addresses major considerations concerning spent fuel cladding embrittlement due to hydride precipitation with regard to the boundary conditions of dry spent fuel storage in Germany. Analytical, numerical, and experimental approaches are to be discussed by BAM in order to identify the specific needs for future R&D work in that area with the purpose to provide the necessary data base for proper safety demonstration and evaluation along with future extended storage licensing procedures.
The current German waste management strategy consists of dry interim storage of spent fuel inside dual purpose casks at various sites and its subsequent direct final disposal in a deep geological repository. Interim facilities are licensed for up to 40 years of operation but this storing period will have to be extended until repository site selection and operation is concluded, potentially up to 100 years. In this context, research on long term mechanical behavior of fuel cladding and cask components during extended interim storage and transportation to a final repository are of outmost importance.
Barrier properties of the fuel cladding depend on its integrity which is affected over time by thermo-mechanical loads and by microstructure altering including potential embrittlement caused by re-oriented hydride precipitation. Experimental and numerical investigations have been performed by the international research community in order to predict the degree of cladding embrittlement against storage time. Results have shown that embrittlement of cladding materials is governed by the concentration, orientation and size of precipitated hydrides. Nevertheless, the existing variety of fuel materials and assemblies available on the market and the many parameters effecting hydride precipitation do not allow sufficient reliable safety predictions in the long term without further investigation.
Thus, knowledge on cladding integrity over storage time and during subsequent transportation is to be gained taking into consideration the relevant fuel and cladding material types as well as cladding temperatures along with peak values during cask drying and cooling rates during storage periods. Expected research results will increase the accuracy of cladding safety analysis. This paper summarizes the current German status regarding extended interim storage and how BAM is going to identify and address potential cladding integrity issues in the long term.
The presentation explains the requirements, process and challenges of the container design testing procedures for the Konrad repository, where low and intermediate level radioactive waste is planned to be disposed of beginnung end of 2022. BAM performs container design testing for more than 30 years whether analytically or experimentally. The challenges of the complex Konrad requirements concerning their proper interpretation and application is discussed in this presentation.
BAM is part of the “Quality Infrastructure Digital” (QI Digital) initiative of the German Federal Ministry for Economic Affairs and Climate Action (BMWK). This initiative aims at coordinating a large cooperation project, “QI Digital” for making German and European quality management systems fit for the digital transformation. It is a fact that the digitization of new products and services is occurring much faster than quality management systems can be accordingly updated. Thus, the digital transformation of quality management is necessary to preserve the competitiveness of the German and European industries that stand for quality and safety in the global market. The Use Case “Reliable Hydrogen Refueling Station” of QI Digital aims at creating new solutions that are transferable to other technical systems and at further developing standards and technical rules as a contribution to technology transfer and innovation.
H2Safety@BAM
(2023)
The presentation provides an overview about the understanding of fuel cladding failure mechanisms dominated by hydride embrittlement during long term dry storage. A currently launched research project focuses on the development of a fracture mechanics approach to describe brittle failure of fuel cladding during long term interim storage (BRUZL)“. Finally, the BAM contribution to the current European Joint Program (EJP) on radioactive waste management is illustrated.
The Bundesanstalt für Materialforschung und -prüfung (BAM) in Germany has gathered extensive expertise over decades in the fields of design testing and safety assessment of transport and storage containers for radioactive materials. On behalf of the German commission for the disposal of highly radioactive waste, BAM had been assigned the elucidation of requirements for high level waste disposal containers specifically in terms of their retrievability from deep geological formations during repository operation and recovery after its closure over a period of time between 500 to 1000 years. The output of the initial assessment is presented taking into account the frame of the current German repository site selection procedure together with the definition of repository conditions corresponding to the potential deep geological formations to be considered for disposal.
This presentation addresses the spectrum of relevant stress factors for disposal containers and reflects their relevance and consequences during the repository operation phase including the emplacement and their potential retrieval. In addition, further aspects and consequences for container recovery after repository closure are discussed taking into consideration potential repository conditions of different geological formations.
Finally, potential disposal container concepts are briefly addressed comparing the consequences of an improvement of existing dual purpose casks versus completely new disposal canister designs adjusted to the specific geological conditions of the host rock formation. Pros and cons of these two concepts are depicted in order to address major challenges. However, precise container design concepts cannot be consolidated before repository conditions and derived specific container requirements are designated.