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Coincidence γ-ray spectrometers are promising candidates for future radionuclide monitoring systems, due to their improved minimum detectable activity (MDA) compared to most single-detector systems. To evaluate the performance of future detector designs, a simulation approach to determining MDA using the Geant4 toolkit is proposed in this work. In particular, a phenomenological γ-ray background model was developed and validated against γ-ray coincidence measurements in unshielded and shielded environments. The background model performs well in the unshielded case, where single γ-ray background dominates, but underestimates coincidence spectra in the shielded case. An example of how MDA can be calculated as a function of a detector design parameter is presented to facilitate future detector optimization work. 

The POSEIDON project activities between March 2024 and February 2025 are summarized. Monte Carlo simulations show the clear advanta-ge of large CZT detectors compared to the presently used small detectors for PGET of spent nuclear fuel was demonstrated: the efficiency to detect the full energy of the gamma rays from the decay of 137Cs is more than 20 times larger. This justifies the further development of the large CZT detec-tors. The results of a first measurement campaign combining a tomogra-phic setup at Uppsala University with a detector from IDEAS in Oslo and a collimator from HIP in Helsinki demonstrates the feasibility of PGET measurements using state-of-the-art large position-sensitive CZT detec-tors, showing that such detectors are a viable choice for future develop-ment of the PGET method. This experimental work is being complemen-ted with Monte Carlo simulation of PGET of real spent nuclear fuel. First results, using a somewhat simplified gamma ray emission source, de-monstrate that the Monte Carlo framework performs well. A measurement campaign using two commercial gamma ray imagers, the CZT-based H420 and the germanium-based GeGI, in a realistic nuclear waste setup at Svafo and a real-life decommissioning situation at TSL, has resulted in valuable practical experience with both imagers. The conclusion is that both imagers perform similarly, with the H420 being somewhat easier to handle and operate. The activities have increased knowledge and exper-tise in the Northern Countries on several topics related to gamma ray imaging. A brief outlook of the continuing activities over the next year is presented.

Small Modular Reactors (SMRs) are gaining popularity due to several economic and safety benefits, as well as their desirable load-following capability, which allows them to complement intermittent renewable energy sources. However, the impact of load-following operation on fuel performance remains under-explored. This study employs the fuel performance code, TRANSURANUS, to assess the effects of such operations on Pressurised Water Reactor (PWR) fuel performance, given the design similarities between many SMRs and large Light Water Reactors (LWRs). Hypothetical load-following operations were simulated by varying linear heat rate levels, neutron flux, and coolant flow rate in accordance with a parametrised load-following operation. Most fuel performance parameters remained below safety limits. However, in some cases, the PCI-SCC model (SPAKOR) predicted cladding cracks and failures that were not observed during regular reference operation. A sensitivity analysis of load-following parameters indicated minimal deviations in fuel performance, apart from the PCI-SCC related issues.

In 2021, the cross-disciplinary Alva Myrdal Centre for Nuclear Disarmament (AMC) was inaugurated at Uppsala University. This moment appears almost time-defining: nuclear weapons were returning to the centre of world politics, though not in ways that suggested new opportunities for disarmament. Yet times inevitably change, and disarmament will eventually return to the global agenda. When it does, the tools required for effective and credible verification must be ready.

In this talk, I will give an overview of the verifiable pathways to nuclear disarmament, and in addition I will provide a brief description of our ongoing research projects within the Technical Verification and Monitoring working group at AMC. Our research portfolio will be presented, including the development of active neutron techniques for verifying absence/presence of fissile material, nuclear archaeology, and coincidence gamma-ray spectrometry for detection of nuclear test explosions.

This data set contains raw timestamped list-mode data obtained using an array of HPGe detectors for the purpose of testing coincidence spectrometry in the context of measurement on air filter samples. Data from one air-sampling station managed by the Swedish Defense Research Agency (FOI) is made available. This air-sampling station is located in Umeå, Sweden (Latitude 63.85°N, Longitude 20.34°E, 46 m above sea level). In addition to the air filter sample, data from a blank filter as well as a filter that was spiked with a known activity of radionuclides is made available in this data set. The detector setup used to collect this data set is made up of five individual HPGe detectors, with one of them surrounded by an active BGO Compton suppression shield. The data set provides a testing ground for investigating the use of multi-fold coincidence spectrometry as a tool to lower the minimum detectable activity of anthropogenic radionuclides in air filter samples. Access to this data set allows researchers to explore and evaluate analysis methodologies for coincidence γ-ray spectrometry on real samples. 

A key to the verification of nuclear weapon dismantling is the identification of presence respectively absence of fissile materials in items, specifically weapons grade plutonium and/or high enriched uranium. In the case of plutonium, spontaneous fission of minority isotopes enables its detection through emitted neutrons, making passive use of neutrondetectors an attractive path. In the case of high enriched uranium, the emission of spontaneous fission neutrons is negligible, making its detection difficult. However, using active interrogation where an external neutron source irradiates the item under investigation, induced fission neutrons are emitted from high enriched uranium as well as from weapons grade plutonium. Liquid organic scintillation detectors are a promising route to detect fission neutrons for verification of nuclear disarmament. These detectors are sensitive to fast neutrons, which are characterized by low self-attenuation in most materials. In addition, while the detector is sensitive to gamma radiation, it can be effectively discriminated by pulse shape analysis. This work details the assessment of the applicability of a liquid organic scintillator for fast neutron detection for use in verification of nuclear weapon dismantling. The assessment was performed as a part of the BeCamp2 measurement campaign organized by SCK-CEN, where a delegation from the Alva Myrdal Centre on Nuclear Disarmament of Uppsala University participated. As a part of the campaign, 19 items with unknown content were assessed, with three different aims: template matching, determining the absence of nuclear material, and a technology challenge using active interrogation In this first stage, where the content of the items is still not disclosed, it was concluded that the equipment was able to identify the presence of spontaneous fission content in items, using passive interrogation mode, within the measurement time constraints. Gamma and neutron spectrum comparison was a valuable tool for template matching, and active interrogation measurements enabled the detection of fissile content in interrogated items. Our preliminary assessment is that liquid organic scintillator detectors have the potential to be part of the toolbox that will support the technical verification of nuclear weapons dismantlement. Lessons learned from the campaign are discussed with focus on advantages and disadvantages of the technique, and possibilities for further development of the analysis.