Swedish Radiation Safety Authority

Long-term safety assessment of deep geological repositories for spent nuclear fuel requires explicit evaluation of thermo-mechanical (TM) processes induced by decay heat and their influence on fractured host rock. A safety-relevant, though low-probability, scenario concerns shear reactivation of fractures intersecting deposition holes, which could compromise canister integrity if displacement exceeds design limits. This study presents a three-dimensional discrete element modelling approach to analyze the thermal evolution of the Forsmark repository (Sweden) and the associated long-term response of a discrete fracture network (DFN) during the post-closure phase. The model explicitly represents repository panel, deterministic deformation zones, and a stochastically generated fracture network embedded in a bonded particle assembly representing the rock for Particle Flow Code (PFC) numerical simulations. Time-dependent heat release from spent nuclear fuel canisters is implemented using a physically based decay power function. A deposition panel-scale heat-loading formulation accounts for deposition-hole and tunnel spacing. Two emplacement scenarios are analyzed: (a) a simultaneous all-panel heating scenario, used as a conservative bounding case, and (b) a sequential panel heating scenario representing staged emplacement and closure. The simulations show that temperature and thermally induced stress evolution are sensitive to the emplacement and closure sequence. Sequential heating produces a more gradual thermal build-up and lower peak temperatures than simultaneous heating, indicating that thermal and stress perturbations in the host rock can be influenced not only through repository design, but also by operational strategy. Thermally induced fracture shear displacement displays a systematic temporal response. Fractures located within the deposition panel footprint develop shear displacement rapidly during the early post-closure period, reaching peak values at approximately 200 years, followed by gradual relaxation as temperatures decline. The average peak shear displacement on fractures is on the order of 2–3 mm, while fractures outside the panel footprint show smaller early-time displacements and a more prolonged long-term response. All simulated shear displacements remain more than one order of magnitude below the commonly cited canister damage threshold for Forsmark of approximately 50 mm, even for the conservative simultaneous heating case. These results indicate that thermally induced fracture shear is unlikely to cause direct mechanical damage to canisters. At the same time, the persistence of residual shear displacement after heating implies permanent fracture dilation, which may influence long-term hydraulic properties and indirectly affect processes such as groundwater flow and canister corrosion. The modelling framework and results presented here were conducted for review purposes independently from the Swedish safety case, and provide a mechanistic basis for evaluating thermally induced fracture deformation in crystalline rock repositories and contribute to bounding the role of thermo-mechanical processes in the safety assessment of spent nuclear fuel disposal at Forsmark.

Objective.To evaluate the uncertainty of physical features of low-energy electron transport in liquid water due to the use of different Monte Carlo track-structure (MCTS) codes.Approach.Five MCTS codes developed specifically for liquid water, namely, Geant4-DNA, PHITS-TS, RITRACKS, NASIC, and PARTRAC are compared and used to calculate the electronic stopping power, pathlength and absorption range, dose-point-kernel, and the frequency-mean (y¯F) and dose-mean (y¯D) lineal energy for primary electron energies from 20 eV to 100 keV. The uncertainty of each calculated quantity is evaluated by the relative standard deviation (RSD) and the maximum relative difference (MRD) among the codes. The medium-to-high-energy (1-100 keV) performance of the codes is benchmarked against the stopping power and range data for liquid water reported in ICRU Report 90.Main Results.For energies above ∼1 keV theRSDis moderate and mostly between 5%-15%, but increases rapidly at lower energies, reaching 20%-100% at sub-100 eV energies. It is noteworthy that theMRDmay well exceed 100% below 100 eV, while remaining sizeable (>20%) even at relatively high energies (10-100 keV). Fairly good agreement between the MCTS codes and the ICRU data for the stopping power (1-100 keV) and range (10-100 keV) in liquid water is found with average deviations between 1%-16%, depending on the code.Significance.The present work reveals significant differences for low-energy electron transport among liquid water MCTS codes, especially below 100 eV. These differences potentially compromise the accuracy of nanoscale simulations where such electrons play a key role. The observed dispersion of results is a consequence of the limitations of the theoretical models used to calculate electron interaction cross sections and the lack of relevant experimental data for their validation and benchmarking. This highlights the need for further development of the physics models used in MCTS codes to reduce the uncertainties associated with low-energy electron transport calculations in liquid water.

This study investigates temporal and spatial variation in 137Cs activity concentrations in Swedish wild boar between 2011 and 2024 to assess long-term ecological impacts of radiocaesium of anthropogenic origin. Data from a total of 5844 meat samples collected across 39 municipalities in six counties were analysed. Average ground deposition of 137Cs in these municipalities in 1986 ranged from 1.84 to 51.55 kBq m-2. The average 137Cs concentration in wild boar meat was 1679 Bq kg-1, with 27.5% of samples exceeding the sales limit of 1500 Bq kg-1. Seasonal variation in 137Cs activity concentrations in wild boar meat was evident, with levels peaking in winter and early spring and declining nearly fivefold in late summer and early autumn, across all locations. On average, younger boars (≤1 year) exhibited 25% higher concentrations than older individuals, with the greatest differences observed in winter and no differences in autumn. No sex-specific differences in concentrations were detected. A weak but statistically significant correlation was observed between meat radioactivity and average ground deposition across all counties, with a slightly weaker correlation in Gävleborg and Uppsala. Overall, 137Cs deposition remained the primary factor explaining the observed levels of meat contamination. A twofold increase in 137Cs deposition resulted in an almost proportional increase in 137Cs activity concentrations in wild boar meat, whereas the 137Cs transfer factor (Tag) remained largely unchanged. The geometric mean Tag for wild boar was 0.052 m2 kg-1, while the effective half-life (Teff) of 137Cs varied widely among municipalities (2-3 to >30 years); no Teff could be estimated for the study area as a whole due to the absence of a consistent decline in 137Cs activity concentrations.

Introduction Targeted alpha therapies (TAT) are currently evaluated in several clinical trials, but the requirement of metrological traceability for the activity measurements of alpha-emitters are typically not met. For 211At (t½=7.2h) reference sources are not available. In anticipation of a new clinical trial involving 211At, a measurement using absolute 4pi-alpha liquid scintillation counting was performed [1] to establish metrological traceability to the equipment used in the hospital. The resulting activity measurements were then compared with those derived for a previous clinical trial [2] (without metrological traceability). Objectives The aim was to evaluate the impact of metrological traceability when measuring 211At for three radionuclide calibrators and three HPGe detectors. Materials and Methods A sample from an aqueous solution of 211At was measured using absolute 4pi-alpha liquid scintillation counting [1]. Samples from the same solution were then measured in three radionuclide calibrators and one (A) of three (A, B, C) HPGe detectors. Another comparison was made (using an identical 211At solution) between HPGe detector (A) and HPGe detectors (B, C) in different labs. HPGe detectors (A, B) used decay data from DDEP and detector (C) used data from NuDat3. Results For the three radionuclide calibrators, the difference in activity between the setting used for the previous clinical trial and the traceable calibration was 3.9%, 3.9% and -6.9%, respectively. The HPGe detectors differed compared to the traceable calibration 6.6%, -0.1% and 1.7%, resepectively. Had site (C) also used decay data from DDEP the difference would have been -4.5% (instead of 1.7%). Conclusion Before establishing metrological traceability, the radionuclide calibrators underestimated the activity of 211At by up to 7%. HPGe measurements rely on accurate data on the intensity of the 687 keV gamma associated with 211At decay. DDEP lists an uncertainty of 5% for this intensity, which is the largest contributor to the uncertainty of the measurements. There is a 6.5% difference in listed gamma emission intensity between DDEP and NuDat3. This highlights the need for better decay data for 211At if HPGe detectors are used for activity measurements. Funding Acknowledgements This work was supported by the Swedish Research Council, the Swedish Cancer Society, the King Gustav V Jubilee Clinic Research Foundation, the Swedish Radiation Safety Authority, and grants from the Swedish state under the agreement between the Swedish government and the county councils, the ALF-agreement.

For accurate activity measurements and to comply with EU regulations traceability is required. For alpha emitters of interest for emerging targeted alpha therapies (TAT), like 211At, standardised samples are not readily available. The use of gamma spectrometry can come with high uncertainties due to its reliance on decay data for gamma emissions. A new method for determining the activity of 211At in a clinical setting is proposed, i.e. on-site measurement, using absolute 4pi-alpha liquid scintillation counting (LSC) with thresholding on a portable TDCR counter. The method focuses on measuring only the alpha particles and not the electrons, which emission intensities come with a higher uncertainty. Using the absolute LSC method the relative expanded measurement uncertainty was 2.2 % (k = 2). A comparison using gamma spectrometry and the more readily available high purity germanium (HPGe) detector, the relative expanded uncertainty was 12 % (k = 2). The largest contribution to the much higher uncertainty of the gamma spectrometry comes from the uncertainties in the decay data. Using a portable TDCR-detector is a viable approach to establishing traceability for 211At, where transportation to a national metrology lab is not feasible. With better decay data the uncertainty of the gamma spectrometry can be reduced by at least one third, which would make it a better alternative when absolute measurements are not available.