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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. 

In this work we present the economic performance of equilibrium cycles for a low power-density small modular reactor for long fuel residence time. The results of the selected pressurized light water reactor, suggesting no advances in long cycle operation, are presented. In addition, we present the strategy that will be adopted for the successive in-core fuel management optimization.

Reports suggest that Pakistan's indigenous, military nuclear program faces a natural uranium shortage, limiting fissile material production. However, Pakistan has recently built three new plutonium-producing reactors and significantly expanded its reprocessing capabilities. The solution to this apparent contradiction could be an optimized fuel cycle, in which reprocessed uranium is reused to produce more fissile material from the same natural uranium stocks. To estimate Pakistan's highly enriched uranium (HEU) and plutonium production, this article combines a statistical framework with fuel cycle and reactor simulation codes. It explores different scenarios that Pakistan could use to efficiently allocate its limited uranium resources. The results indicate that Pakistan cannot support simultaneous HEU and plutonium production in a once-through fuel cycle, and that HEU production can only be sustained in alternative cycles. Calculations suggest plutonium stockpiles of 370 to 660 kg and, depending on the scenario, HEU stockpiles of 3,090 to 5,540 kg by the end of 2022.

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

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.

Since North Korea, or the Democratic People's Republic of Korea (DPRK) left the Non-Proliferation Treaty (NPT) in 2003, the DPRK nuclear fuel cycle has continued operation as well as further development, without transparency to international inspectors. A recent addition to the DPRK fuel cycle is the Experimental Light Water Reactor (ELWR), a 100 MWth light water reactor in Yongbyon, which, as appears to be in operation since October 2023.

Considering the non-NPT-signatory status of DPRK, it is of concern that the ELWR may be used to produce plutonium for nuclear warheads, in addition to or instead of its use for electricity production. In this work we explore the possibility to model the fissile material production in the ELWR, by modelling the core with Serpent2, and integrating information from available remote monitoring, such as satellite imagery of cooling water outlets from the facility.

Little openly available and verified information exists on the details of the ELWR, such as nuclear fuel design and core geometry. Therefore, a base case based on available statements is used to create a core model similar to conventional international LWRs. Multiple scenarios for core load options are based on either assumptions on 1) uniform enrichment optimised for low burnup and production of weapons-grade plutonium, 2) heterogeneous core with drivers of enrichment fuel and targets of natural uranium for plutonium production or 3) uniform enrichment for high burnup and electricity production. Plutonium production made possible using the ELWR is estimated per MWd, as well as per year of operation. Special consideration is given to cycle length constraints from requirement of weapons-grade quality of the produced plutonium, and implications on availability and stoppage frequency, which might be monitored remotely, unless online refuelling is a possibility.

Possible implications on the DPRK Pu stockpile are discussed, as well as attempts to identify unknown parameters of importance (i.e. attempting to make unknown unknowns into known unknowns).

We investigate variations in 137Cs activities, temperature and rainfall with time and predict how climate change will interact with existing 137Cs anomalies. We focus on several case-studies affected by distal fallout from Chernobyl; the Kymijoki watershed in Finland, Gävle in Sweden and Bavaria in Southeast Germany. In addition, we investigate 137Cs activities in Japan as a contrasting location proximal to the Fukushima nuclear power plant that was damaged during the tsunami in 2011. In Europe we find that 137Cs anomalies show up in moose, mushrooms and ground radiation levels in specific years that differ from place to place. There is little indication of direct relationships between weather conditions and 137Cs anomalies. However, environmental processes play critical roles in radionuclide behavior and may themselves be influenced by weather conditions and furthermore by changing climate. Erosion likely remobilizes subsurface 137Cs and transports it downstream where it may be deposited. Erosion is driven by heavy rainfall, snowmelt, and daily variations in temperature that alternatively freeze and thaw the ground. These insights have helped us interpret signs of erosion and deposition of soil contaminated with 137Cs in our case-study areas. Future variations in temperature, precipitation and extreme weather events will likely increase the occurrence of erosion and hence the redistribution of 137Cs in the environment. We have identified several areas sensitive to accumulation of and hence contamination by 137Cs in Europe. In most places the levels of 137Cs accumulation are rarely hazardous, however we recommend that local authorities assess the risks of new construction sites, agricultural practices and free-time activities such as hunting game and gathering mushrooms in areas that are sensitive to 137Cs accumulation.

A majority of the nuclear reactors in the world use low-enriched uranium as fuel, and will produce plutonium during operation. This will happen when neutrons undergo capture in U238 instead of causing fission of U235, which is a likely reaction as low-enriched uranium is composed of >95% U238.  While the plutonium can be used as fuel in the reactor, it is also a material highly desired by states producing nuclear weapons. Not all reactors produce plutonium of the same grade, which significantly impacts its usability in a nuclear weapon. For this reason, certain reactor technologies have been favored for military plutonium production. Heavy-water moderated reactors is one such family, that has been used in current or defunct nuclear weapons programmes by states such as India, Pakistan, Sweden, Switzerland and the United States.

A number of factors impact the rate and grade of plutonium production in a reactor. These include(but are not limited to) fuel design specifications (pellet radius, fuel density), operational temperature of the fuel as well as the coolant and moderator, and numerous other operational parameters such as specific power, cycle lengths and downtimes et cetera. The present study will look into investigating the relative sensitivity of plutonium production rate and grade towards these design and operational parameters. Based on the results from this evaluation, it is expected that we can better understand which parameters impact plutonium production quantity and quality the most. This will also help us understand the role and impact of uncertainties in these parameters and connect them to the plutonium content in the spent fuel produced by these reactors