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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.

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.

In the event of a far-reaching nuclear disarmament, nuclear weapon states would cease pro-duction of fissile materials for nuclear weapons, and also place their pre-existing plutonium andhigh enriched uranium (HEU) under international inspection. In this case, it can be anticipated that sizes of stockpiles are declared, with detailed records on how they were produced. Forexample, if the core geometry and operational history of a military reactor would be declaredby the owner state, the plutonium production of the core can be reconstructed using reactorphysics codes, which can provide confidence in the declared plutonium stockpiles.Challenges include that any data on fuel cycle operations provided by the state cannot beassumed to be truthful. This leads to the additional need to verify the declared operationalhistory by consistency checks within records and when possible with inspections. A further complication is that a long time might have passed since the fissile material production tookplace, and records can therefore be expected to be incomplete or inaccurate, and the responsiblepersonnel may be retired or even deceased. In this case, there may be a need to be able toreconstruct lost information on fuel cycle operations.

We consider a test and training case of the Swedish pressurised heavy water reactor (PHWR) in Ågesta, which was in operation between 1963 and 1974. In this case study, we explore available archive material in order to obtain information such as the core loadings of the reactor, its operational history and the fuel designs used. Primarily we explore the use of records from the tritium monitoring for consistency checks with operational data records. Tritium production occurs via neutron capture in deuterium; a main component of heavy water, and since heavy water is an expensive asset, the consumption of it is closely monitored, for example by measuring tritium release. Moreover, the tritium production is proportional to the neutron flux in thereactor which is also a crucial component for calculating plutonium production

Postirradiation examination of nuclear fuel is routinely performed to characterize the important properties of current and future fuel. Gamma emission tomography is a proven noninvasive technique for this purpose. Among various measurement elements of the technique, a gamma-ray detector is an important element whose spectroscopic abilities and detection efficiency affect the overall results. Finding a combination of high detection efficiency and excellent energy resolution in a single detector is often a challenge. We have designed a novel planar segmented high-purity germanium detector that offers simultaneous measurement in six lines of sight with excellent energy resolution. The simultaneous detection ability enables faster data acquisition in a tomographic measurement, which may facilitate achieving higher spatial resolution. In this work, we have demonstrated the first use of the detector by performing a full tomographic measurement of mockup fuel rods. Two methods of detector data analysis were used to make spectra, and the images (tomograms) were reconstructed using the filtered back projection algorithm. The reconstructed images validate the successful use of the detector for tomographic measurement. The use of the detector for real fuel measurement is being planned and will be performed in the near future.

Radionuclide monitoring is a proven means of non-intrusive verification of the nuclear test ban treaty. In addition to that, the potential use of radionuclide monitoring spans beyond the detection of nuclear test explosions, since radionuclides can also be released and detected from operations of nuclear fuel cycle facilities, such as the reactor operation and nuclear reprocessing of plutonium production.

In this work, we consider the use of coincidence and anticoincidence techniques as a means to increase the sensitivity in radionuclide monitoring, in terms of improved minimum detectable amount for radionuclides of interest to filter stations used in radionuclide monitoring. In particular, a multi-detector setup is currently being prepared for the evaluation of the technique, and to provide validation data for a coincidence detector simulation codes.

In this presentation, we will describe the multi-detector setup assembled for enabling the evaluation of various types of spectrometry, including 1) gamma-gamma coincidence (from dual detectors and up to five High Purity Germanium (HPGe) detectors), 2) anticoincidence using BGO active shield with single HPGe detector, as well as use of multiple detectors in add-back mode, i.e. simply using the combined detector volume for increased efficiency of single gamma rays. We will present the results of measurements of a calibration sample, and provide a discussion on the advantages and disadvantages of the tested techniques in the context of radionuclide monitoring.