Neuigkeiten aus dem Hotlabor

Chalk River Unidentified Deposits (CRUD) are dissolved and suspended solids, product of the corrosion of structural elements in water circuits of nuclear reactors.  

The chemical composition of CRUD is variable as it depends on the composition of the reactor’s structural material, as well as the types of refueling cycles.  Recent internal investigations have found unexpected but significant Si-amount in CRUD. The chemical composition of CRUD holds key information for an improved understanding of CRUD formation and possible impact in fuel reliability and contamination prevention.

The standard analytical methods available in the hot laboratory did not allow an easy quantitative determination of the Si-amount in CRUD. A new innovative procedure has been developed and tested with synthetic CRUD name Syntcrud.

The adapted flex-fusion digestion method presented here is able to provide reliable concentrations of several elements within CRUD, including Si, which was not possible in methods used previously for ICPMS measurement.

PSI has a unique (worldwide) environment for the investigation of highly radioactive / toxic materials:

> Materials (different fuel types, very high burn-up, different cladding materials, materials activated in SINQ).

> The hot lab with advanced tools for microsample analysis and preparation.

> The large-scale equipment for advanced material analysis.

This unique combination at PSI allows us to meet the needs of our industrial partners to improve plant safety / efficiency, up to fundamental research.

The quantitative distribution of fission products over the cross-section of a pellet with a shielded electron probe microanalyzer (EPMA) used for verification analysis of the material behavior to validate the model. In this context, Xe behavior during transients/failure (LOCA, RIA) is an important safety parameter that can’t be measured with the EPMA at the periphery. Microstructural EBSD investigations on a microsample extend the information horizon, which is deepened at the microXAS beamline by detailed X-ray analyses.

The fuel used for nuclear energy production is normally enclosed in zirconium-based cladding tubes that constitute the first barrier between the radioactive material and the environment. In water-moderated reactors, cladding tubes tend to corrode, generating hydrogen as side product. The study of the hydrogen embrittlement in zirconium alloys is of high relevance for the industry.

Depending on temperature, local hydrogen concentration, and local stress conditions, different hydrogen-induced embrittlement mechanisms can be active in the cladding material: in certain conditions hydrogen in solid solution might cause material softening through a mechanism known as hydrogen enhanced localized plasticity (HELP).

With the goal of determining the conditions necessary to activate the HELP effect in zirconium alloys, samples have been evaluated by different micro-mechanical and macro-mechanical techniques. Results highlight the importance of the interplay between solid solution hydrogen and hydrides on the hardness and yield point of the tested materials.

This highlight presents a successful, in-house developed integration of an Electron Beam Ion Source (EBIS) able to ionize gases to high charge states with a customized commercial MC-ICP-MS. The successful joining of the two ion flight paths is a milestone towards comprehensive routine analyses of solids, liquids, and gases using THE SAME MASS SPECTROMETER, the latter analyses free from atmospheric contamination. After implementation of an introduction system for gas mass spectrometry, routine analyses will comprise isotope ratio and relative abundance determinations of fission gases in used nuclear fuel. In addition to the unique versatility of the MC-EBIS-ICP-MS, inclusion of the EBIS furthers opens the little-studied field of mass spectrometry of highly charged ions.

A safe, economical and environmental friendly disposal of used nuclear fuel represents an essential objective of relevance for all. This guides the approach under development at the laboratory for reactor physics and thermal-hydraulics. Establish higher resolution simulation methods to gain more detailed knowledge on the content of each single nuclear fuel rod ever irradiated in a reactor. Thereafter, use this knowledge to explore optimization approaches that could potentially enlarge the range of disposal options allowing to fulfill the highest level of safety standards while reducing economical costs and geological footprints at the same time.

Die Untersuchung von Kernbrennstoff bei sehr hohem Abbrand ist entscheidend für die Bewertung der Sicherheitsmarge des untersuchten Brennstoffs sowohl unter normalen als auch unter Unfallbedingungen. Das PSI ist eines der wenigen heißen Labore, die Zugang zu bestrahltem UO₂-Brennstoff mit sehr hohem Abbrand aus kommerziellen Reaktoren haben. Der Einsatz relevanter Werkzeuge zur Untersuchung, Handhabung und Analyse dieser hoch bestrahlten Materialien unterstreicht die erforderliche Expertise.

Die Implementierung von fokussierten Ionenstrahl-(FIB)-Instrumenten in Materialforschungslaboren im letzten Jahrzehnt hat nicht nur die Präparation sehr dünner Proben für das Transmissionselektronenmikroskop (TEM), insbesondere an Grenzflächen, erheblich verbessert, sondern auch zur Entwicklung neuer Analysemethoden direkt im Instrument selbst geführt. Es hat sich zu einem leistungsstarken Instrument für die Analyse hochradioaktiver Materialien entwickelt, da es die Herstellung und Untersuchung sehr kleiner Proben ermöglicht, die anschließend mit hochsensiblen Detektoren analysiert werden können, ohne dass es zu starken Störungen durch das Strahlungsfeld der Probe selbst kommt.

The formation and growth of cracks by stress corrosion cracking (SCC)in reactor internals and recirculation pipes due to the highly oxidising environment is a serious issue in boiling water reactors. At first, SCC mitigation was attempted by injecting H₂ into the feed water, where the injected H₂ recombines with the H₂O₂ and O₂ to water and reduces the electrochemical corrosion potential, and consequently the SCC susceptibility. Several disadvantages of the injection of high amounts of H₂, have led to the development of noble metal additions to the reactor feed water. With injection of a much smaller amount of H₂, the noble metal particles of a few nanometres in size, formed in-situ, work as catalysts for the efficient reduction of the oxidizing species formed by radiolysis, and thus lower the ECP and SCC susceptibility.

An interdisciplinary study conducted at different PSI laboratories (LES, AHL, LRS, SYN) in collaboration with Studsvik AB (Sweden) demonstrates that selenium originating from fission in light water reactors is tightly bound in the crystal lattice of UO₂. This finding has positive consequences for the safety assessment of high-level radioactive waste repository planned in Switzerland, as it implies (contrary to previous assumptions) that the safety-relevant radionuclide ⁷⁹Se will be released at extremely low rates during aqueous corrosion of the waste in a deep-seated repository.