SSM perspective
Background
Radiation safety is of particular concern during power plant maintenance. Radionuclides produced by the neutron bombardment in the core during operation may accumulate on system surfaces such as the filter system or piping and contribute to the radiation field. Some ions that are prone to neutron capture often become trapped in the deposits on the fuel cladding surfaces, also called CRUD. It is important to understand how radionuclides become incorporated and released from the CRUD in order to increase radiation safety. Density functional theory (DFT) calculations have been used to gain a better understanding of CRUD formation and transformation in BWR and PWR.
Results
- Utilizing Ag+ adsorption and incorporation in CRUD as tracer element, a comprehensive possible understanding of CRUD formation and transformation in BWR and PWR emerges, i.e., ions become adsorbed as hydroxides and subsequent condensation reactions define the resulting oxide grain growth directions in the CRUD and Ag+ tracing H+.
- The concentration of Ag+ in early CRUD reflects the degree of generic supersaturation of metal ions in vicinity of the boiling zone. At later stages, the ions become integrated in the growing CRUD. Transformations in the deposit render surface-to-volume ratios reduced in the CRUD interior, and, in as much as the Ag+ ions are aliovalent to both NiO/NiFe2O4 (PWR) and Fe2O3/Fe3O4/NiFe2O4 (BWR) CRUD:s, consequently, silver ions is understood to become enriched on the resulting interfaces.
- At the high concentration limit, delafossite was shown to offer a viable structural motif for intercalation of the aliovalent Ag+ in the oxides corresponding oxides, more so in the BWR ferrites than in PWR NiO. The latter is consistent with observations of looser CRUD in PWR and more profound in BWR, as determined by the different boiling conditions experienced by the corresponding fuel surfaces.
- The aliovalent Ag+ is in contrast to Co2+ that is readily incorporated as (Fe,Co) Fe2O4 and (Ni,Co)Fe2O4 in magnetite and nickel ferrite.
- Enrichment of radionuclides in metastable PWR CRUD is understood to become problematic during shutdowns where the CRUD becomes subject to disintegration and CRUD particles become adsorbed to piping surfaces outside of the core, rendering in worst case maintenance delayed. Less problematic is BWR CRUD owing to its chemical stability.
- The fate of intercalated AgFeO2 follows from literature, where transformation into metallic silver particles results. Formation of spurious silver particles in the CRUD, and these originating from Ag+(aq), would comprise smoking-gun evidence for the proposed understanding. Such experimental undertaking is encouraged.
- The incorporation of Ag+ in CRUD is contrasted by the Antimony deposition on the fuel cladding. In case of the latter, we find Sb(III) to readily adsorb, albeit reversibly, to the cladding oxide scale comprising ZrO2. Antimony incorporation in the scale requires further oxidation to Sb(V), e.g., owing to radiolysis of water in vicinity of the fuel. This suggests that maintaining the oxidizing conditions on power plant shutdown would mitigate Antimony dissolution into the coolant. Thereby, possible subsequent deposition on piping surfaces would be avoided.
Relevance
Knowledge of how radionuclides become incorporated and released from the CRUD is essential in order to increase radiation safety at nuclear power plants in the long term. SSM has contributed to the development of models that increase the understanding of CRUD formation and transformation in both BWR and PWR. Through funding a group of researchers at Chalmers University of Technology, SSM has also contributed to the maintenance of national competence within nuclear radiation safety.
Need for further research
This report provides a theoretical foundation for further development of models that could help decrease the deposition on system surfaces in a long term.