Radioactive Decay Processes Research Articles

CURRENT RADIOLOGICAL STATUS OF CONTAMINATED MEADOWS AND PASTURES AND ASSESSMENT OF THE POSSIBILITY OF THEIR RETURN TO USE

Meadows and pastures are the fodder base for milk and meat production. The rehabilitation and return to economic use of the radionuclide-contaminated meadows and pastures is an important and urgent national economic task. The criterion of returning radionuclide-contaminated meadows and pastures to economic use may be individual doses to residents who consume milk and meat produced on these lands. Values that determine the doses are the terrestrial density of soil contamination and permissible levels of radionuclides in food products (PL-2006). Considering abandoned agricultural lands, it is necessary to note the time that passed after the accident. The contamination of the territory by 137Cs and 90Sr decreased in more than 2 times due to radioactive decay only. The decreasing due to radioactive decay and self-remediation processes (fixation of radionuclides by soil, their redistribution in the root layer of the soil, etc.) resulted in declining of radioactive contamination of products. This allows us to consider the issue of returning reclaimed lands to use in accordance with the current radiological criteria and existing legal acts. Agricultural lands are a territorial resource and the main means of agricultural production, therefore the rehabilitation of the lands will contribute to the development of both individual producers and settlements as a whole. After the Chornobyl accident, in accordance with the Law of Ukraine "On the Legal Regime of the Territory Exposed to Radioactive Contamination Due the Chornobyl Catastrophe", the territory exposed to radioactive contamination had been divided into zones. However, the belonging of the lands to a zone does not guarantee that the products produced there will meet the requirements of PL-2006. Therefore, in 1998, taking into account the type of soil, the UIAR assessed the maximum permissible levels of soil contamination which ensure the fulfillment of the requirements of PL-2006 in terms of the content of 137Cs in produced products. However, in the 25 years that have passed since the establishment of these levels and the cesium transfer factors into plants have decreased significantly. Therefore, the values of the maximum permissible levels of soil contamination are conservative and their use leads to conservative conclusions. The current radiological state of meadows and pastures contaminated with radionuclides and the possibility of returning them to use were assessed. It is shown that most of the considered abandoned lands actually belong to the third zone of radioactive contamination. Some of the lands belong to the second zone of radioactive contamination - radiation-hazardous lands. It was found that the restrictions set in the recommendations for conducting agriculture in the conditions of radioactive contamination for the production of milk and meat are very conservative. Assessment of the possibility of using contaminated meadows and pastures for milk and meat production requires more precise and targeted forecasting methods.

Decay rate changes in radioactive gamma emission as affected by 18 MeV proton cyclotron

Previous efforts to investigate changes in the decay constants of radioactive nuclides discovered that solar flares can temporarily alter radioactive decay rates. Thus, discerning whether external factors affect radioactive decay rates is vital for understanding nuclear processes. This study sought to explore the effect of neutrinos on radioactive nuclei by constructing a gamma radiation detection system that employs a radioactive source in front of a neutrino emission system. Responding to cyclotron operations, each of the four detection systems registered gamma count rate decreases. The results of this study confirm that rises in neutrino flux affected the decay rates of the examined radioactive nuclides. Here we provide significant evidence that neutrinos affect the radioactive decay process. Neutrino detection is challenging due to the minuscule absorption in a stable nucleus. However, the study found a greater probability of radionuclides interaction with the neutrino.

A dentist’s perspective on scintigraphy about its applications as a diagnostic tool for a myriad of diseases affecting oral health

A "branch or specialty of medicine & medical imaging that uses radionuclides and relies on the process of radioactive decay in the diagnosis and treatment of disease" is known as nuclear medicine. Assessing physiologic change, which follows biochemical changes, is made easier with the use of radionuclide imaging, which is sometimes referred to as a functional imaging approach. Scintigraphy is a diagnostic procedure used in nuclear medicine where radioisotopes are administered internally into the body in liquid or gaseous forms, and the distinctive radiation that emerges is captured by external detectors known as gamma cameras, producing two-dimensional pictures. Bone scintigraphy, lymphoscintigraphy, salivary gland scintigraphy, and radio-immuno-scintigraphy are a few examples of scintigraphy techniques.Nuclear imaging has the advantage of giving extremely high levels of diagnostic sensitivity, which is successful in identifying even the smallest pathophysiological changes that are highlighted in the process of diagnosing early-stage to progressive diseases, making it the preferred diagnostic modality.

Optimized Monte Carlo simulations for voxel-based internal dosimetry

Objective. The scientific community has considered internal dosimetry by the Monte Carlo method the gold standard. However, there is a trade-off between simulation processing time and the statistical quality of the results that makes it a challenge to obtain accurate absorbed dose values in some situations, such as dose estimation in organs affected by cross-irradiation or limited computing power. Variance reduction techniques are used to reduce computational processing time without impairing the statistical quality of the results, such as tracking energy cutoff, secondary particle production threshold, and parallelism of different types of emissions from radionuclides. Approach. In this work, GATE Monte Carlo code and its variance reduction techniques were evaluated to calculate S values of organs from the international commission on radiological protection (ICRP) report 110 male phantom for the lutetium-177, iodine-131, yttrium-90, and radium-223 radionuclides. The results are compared with the data from the OpenDose collaboration. Main results. A cutoff of 5 MeV for local electron deposition and 2.0 mm of secondary particle production range resulted in a computational efficiency increase of 7.9 and 1.05 times, respectively. Simulation of ICRP 107 spectra-based source proved to be about 5 times more efficient when compared to a decay simulation using G4RadioactiveDecay (Geant4-based radioactive decay processes). Track length estimator (TLE) and split exponential track length estimator (seTLE) techniques were used to calculate the absorbed dose of photon emissions, resulting in computational efficiency up to 29.4 and 62.5 times higher when compared to traditional simulations, respectively. In particular, the seTLE technique accelerates the simulation time by up to 1426 times, achieving a statistical uncertainty of 10% in volumes affected by cross-irradiation. Significance. The variance reduction techniques used in this work drastically reduced the simulation time and maintained the statistical quality of the calculated absorbed dose values, proving the feasibility of the use of the Monte Carlo method in internal dosimetry under challenging situations and making it viable for clinical routine or web applications.

Время достижения радиологической эквивалентности РАО и природного уранового сырья при реализации ЗЯТЦ на базе РБН с учётом основных факторов неопределённости расчёта радиационных рисков

One of the most important problems in the large-scale development of nuclear energy is the management and disposal of radioactive waste (RW). The potential biological hazard of RW for human health considerably exceeds the health hazard from uranium feedstock. Due to the natural processes of radioactive decay, the potential biological hazard of RW decreases over time. However, long-term storage and disposal of long-lived RW leads to an increase in the health and environmental hazard of nuclear energy. Recently, to reduce the hazard effects, the novel ap-proach based on the principle that hazard of RW should not exceed the hazard of natural urani-um raw materials has been used. This principle is called as "radiation equivalence" if the hazards levels are assessed by the radiation dose magnitude, and "radiological equivalence" is used if the hazards are assessed by the magnitudes of lifetime radiation risks of malignant neoplasms development. Earlier papers have already given point estimates of the time to reach “radiation” or “radiological” equivalence when analyzing a scenario based on a closed nuclear fuel cycle. This article proposes a method for estimating the uncertainties in the time to achieve radiation and ra-diological equivalence. The results of these estimates are presented for a model scenario for the development of nuclear power, in which thermal neutron reactors are gradually being replaced by fast neutron reactors by 2100. The numerical simulation method is used. Modern radiation risk models proposed by the International Commission on Radiological Protection (ICRP) and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) are applied. The determination of equivalent radiation doses to organs and tissues was based on dose factors for radioactive substances provided for wide use by the US Environmental Protection Agency (EPA). If we assume that the maximum relative error of the initial values of the potential biologi-cal hazard of RW is about 246%, then the 95% confidence limit for achieving radiological equiva-lence was less than 340 years. The results of the study of the scenario for the development of nuclear energy in Russia, calculations show that since the integrity of containers with RW is main-tained for 1000 years, the safety of RW for future generations will be ensured.

ОЦІНКА МОЖЛИВОСТІ РЕГУЛЮВАННЯ ВОДНОГО РЕЖИМУ ОКРЕМИХ ТЕРИТОРІЙ ЧОРНОБИЛЬСЬКОЇ ЗОНИ ВІДЧУЖЕННЯ

The characteristics of the natural features of the Chernobyl Exclusion Zone (CEZ), in particular the nature of the relief, soil cover, main plant groups, and landscape characteristics, are provided. The hydrographic network of the region is detailed, represented by rivers, lakes, ponds, dams of the Pripyat and Uzh rivers, the reservoir of the Kyiv HPP, reclamation channels, water mirrors in front of filtration dams. It was noted that the main processes that form the current level of radioactive contamination of water systems in the CEZ are natural processes of radioactive decay, vertical sinking and geochemical fixation of radionuclides. Analysis of water pollution monitoring data in rivers and canals of the exclusion zone shows that by regulating water levels in most areas of the catchment areas of the CEZ, it is impossible to significantly influence the reduction of water pollution in them compared to the effects of their natural self-purification. Therefore, it is impractical to continue large-scale works on water flow regulation by hydrotechnical structures in order to minimize the removal of radioactive substances beyond the exclusion zone. It is advisable to leave in operation only those systems that allow maintaining increased moisture in peatlands, reducing the risks of fires. The studies carried out on the reclamation system “Buryakivka” in the basin of the Sakhan River, as an experimental training ground for the regulation of the regime of floodplains of the CEZ, are covered in detail. The use of the method of remote sensing of the Earth (DSS) to control the water regime and monitor the transformation of reclamation systems into wetlands is substantiated. In particular, spectral analysis of image pixels in different parts of the spectrum using vegetation indices. A positive nature protection effect from the increase in the area of wetlands in the Exclusion Zone was noted in the context of the prospects of climate change and in the context of the probability of increasing cases of dry periods in the territory of Polissia.

Radio-species transport model for coupled fluid dynamics-neutron activation calculations

A coupled methodological approach was developed for the modelling of fluid activation on ITER relevant components, using Computational Fluid Dynamics (CFD). Based on ANSYS Fluent's general transport mathematical model, built-in equations (UDS) were customized to represent radioactive species' activation and decay processes, fully embedded in fluid transport equations. Such model, so called Radioactive Species Transport Model (RSTM) was benchmarked against previous approaches on this field. Additionally, two applications relevant for ITER In-Vessel design are reported: (1) a complete FW06 and (2) a complete FW18 with its Shielding Block (SB18). These results provide new sensibility on the effects of flow stagnation and recirculation under real activation rate fields for In-Vessel components. This implementation shows that previous approaches may have overestimated the activation levels expected on downstream FWs by the order of 20–40%, depending on the characteristics of the FW panel.

Программный модуль оценки достижения радиологической эквивалентности (РОЗА-РАО)

The main problem of the large-scale development of nuclear energy in the world is the safe dis-posal of accumulated radioactive waste (RW). The hazard of RW at the time of their formation to human health far exceeds the hazard of the source natural uranium ore, after the use of which these wastes were formed. Due to the natural processes of radioactive decay, the hazard of radi-oactive waste decreases over time. The hazard of waste can also be reduced by artificial trans-mutation or incineration of the radionuclides that make up RW composition. For uranium nuclear fuel cycles, the natural reference level of RW hazard is the hazard level of the uranium ore, upon reaching which decisions can be made on the final disposal of RW. The article presents a meth-od for estimating the time of achieving radiological equivalence of the mass of accumulated RW of nuclear energy and the source mass of natural uranium raw materials used in this case. A de-scription of the software module "Radiological support of protection – radioactive waste" (PM ROZA-RAO), which implements this method, is also presented. Radiological equivalence herein refers to the equality of the corresponding radiation risks for the population. The methodology used is determined by the current recommendations of the International Commission on Radio-logical Protection (ICRP) and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) on the calculation of human radiation risks from internal exposure based on equivalent doses in organs and tissues. The determination of equivalent doses to organs and tissues was based on dose coefficients of radioactive substances provided for widespread use by the United States Environmental Protection Agency (EPA). Calculations by means of the PM ROZA-RAO showed that within the framework of the most likely scenario for the development of two-component nuclear energy system in Russia, for radioactive waste accumulated by 2100, the time of onset of radiological equivalence is less than 100 years (98.8 years). With the help of the developed software module, for the first time in the nuclear industry, the parameters of the processing of exposed nuclear fuel (the composition and activity of radionuclides for controlled storage, combustion and transmutation) and the management of RW can be determined, minimizing the potential radiation risks of the population, which corresponds to the best practice of developed countries in the field of radiation protection.

Prospects for searching new Double Beta Decay physics with KamLAND-Zen 800

Two-neutrino double beta decay (2υββ) is a rare second-order weak radioactive decay process. It has been observed in neutrinoless double beta decay (0υββ) search experiments. Precise observation of 2υββ is essential to reduce the theoretical uncertainty in the calculation of nuclear matrix elements required to obtain the effective Majorana mass from a lifetime of 0υββ. Also, 2υββ itself is interesting because new physics could be hiding in the energy spectrum, such as for example, Majoron emission mode 0υββ and 2υββ with neutrino self-interaction. KamLAND-Zen 800 is an experiment to search for 0υββ of 136Xe with a large ultra-pure liquid scintillator detector, KamLAND. KamLAND-Zen 800 has been observing 745 kg of xenon gas at 91% enriched in 136Xe since 2019, providing a high statistics 2υββ decay sample. We describe the potential for new physics searches in 2υββ with KamLAND-Zen 800.

APERTRACK: A particle-tracking model to simulate radionuclide transport in the Arabian/Persian Gulf

A Lagrangian rapid-response model for simulating the transport of radionuclides in the Arabian (or Persian) Gulf is described. The model is based on a tide model including five constituents, which was solved in advance, and baroclinic circulation was obtained from HYCOM operational ocean model. The radionuclide model includes physical transport (advection and diffusion), radioactive decay and geochemical processes (interactions of radionuclides between water and sediments, described in a dynamic way). The model can lead with instantaneous or continuous releases. Some hypothetical releases from a coastal nuclear power plant were simulated. Results show that the moment of release affects the fate of radionuclides due to the temporal variability of baroclinic currents. Also, comparing results for releases of Cs and Pu, it was seen how the geochemical behaviour of the radionuclide clearly affects the further radionuclide distributions. It is easy to setup the model for a particular release and it provides a fast response; thus the present model is an appropriate tool to support decision-making after a nuclear accident.

Review of techniques, challenges, and new developments for calcium isotope ratio measurements

Calcium isotope meaurements apply to problems spanning the ‘cosmos to benthos.’ Fractionation, source mixing, radioactive decay, and nucleosynthetic processes cause variations in Ca isotope ratios, which can be measured using different types of mass spectrometers. Early measurements employed thermal ionization mass spectrometry (TIMS), which continues to be the most common and precise reference technique. An increasing number of studies use multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Both TIMS and MC-ICPMS can yield high-quality data for δ44/40Ca and δ44/42Ca, respectively, but wide ranges in precision exist. Sample size requirements are lower for TIMS, but MC-ICPMS offers faster throughput. Both techniques require purifying Ca by ion-exchange chromatography when analyzing samples with complex matrices. In-situ techniques, like laser ablation MC-ICPMS (LA-MC-ICPMS) and secondary ion mass spectrometry (SIMS), do not require purification. In-situ techniques provide high spatial resolution, but LA-MC-ICPMS and (nano)-SIMS suffer from relatively poor precision compared to TIMS and MC-ICPMS, and the methods lack well-characterized standards, which are essential for comparing datasets.The past few years have seen advances in Ca purification techniques, including automated methods that accelerate sample throughput. The next generation of Ca purification techniques should focus on complete purification and quantitative separation of low-Ca samples. Application of collision-cell technology for suppressing Ar-related molecular isobaric interferences will improve quantification of 40Ca using certain MC-ICPMS models. Similarly, development of new amplifier technology will foster high-precision Faraday cup measurements of low abundance Ca isotopes. Adopting common reference materials, including newly developed standards with complex matrices, would signficantly aid inter-laboratory comparisons, method development, and data compilation efforts. In general, at least two widely-used standards should be measured along with unknown samples to confirm data quality, as well as improve utilization of published Ca isotope datasets by a broader community.

Моделювання вторинного радіоактивного забруднення повітря в Україні внаслідок вітрового під-йому радіонуклідів.

The paper presents the results of the simulation of secondary air pollution scenario in Ukraine due to the wind lift of anthropogenic radionuclides during a dust storm in April 2020. A variant of the Bagnold formula was used to parameterize the intensity of radionuclide resuspension. To set the initial pollution of the territory of Ukraine, the reconstruction of meteorological conditions and fallout of Cs-137 after the disaster at the Chernobyl nuclear power plant was carried out through the use of the RODOS nuclear emergency response system and the WRF meteorological model. For the normalized root-mean-square error of the calculated total fallout in the 50-km zone around the Chernobyl NPP the value NMSE=4,5 was obtained. A decrease in the levels of pollution of the Earth's surface during the time after the accident due to radioactive decay and other environmental processes was estimated. The distribution of contamination of the Earth's surface obtained in this way was used to assess the intensity of wind rise and atmospheric transport of radionuclides during a dust storm on April 16–18, 2020. The calculations were carried out using the CALPUFF model. The input meteorological data were the results of the WRF-Ukraine weather forecasting system. In the calculation of secondary contamination, the effect of fires in the Chernobyl Exclusion Zone (ChEZ) was not taken into account. The calculated average daily concentrations of Cs-137 in the air were maximum for the first day of the dust storm (April 16), when the wind speed was maximum (13 m/s with gusts up to 19 m/s). Average daily concentrations on April 16 ranged from the background values (5,8·10-6 Bq/m3 in Kyiv) to 2,2·10-3 Bq/m3 (ChNPP). The obtained estimates are much less than the permissible concentrations (0.8 Bq/m3). At the same time consideravle exceedance of background values were predicted in a large part of Ukraine – from Rivne NPP (2,2·10-5 Bq/m3) to Kharkiv (1,3·10-5 Bq / m3). In the vicinity of the ChEZ in the cities of Chernihiv and Slavutich, the obtained estimates of daily average concentration were 1,6·10-4 Bq/m3 and 3,2·10-4 Bq/m3 respectively.

Transcriptional Changes in the Ovaries of Perch from Chernobyl.

Fish have been highly exposed to radiation in freshwater systems after the Chernobyl Nuclear Power Plant (NPP) accident in 1986 and in freshwater and marine systems after the more recent Fukushima NPP accident in 2011. In the years after the accident, the radioactivity levels rapidly declined due to radioactive decay and environmental processes, but chronic lower dose exposures persisted. To gain insights into the long-term effects of environmental low dose radiation on fish ovaries development, a high-throughput transcriptomic approach including a de novo assembly was applied to different gonad phenotypes of female perch: developed gonads from reference lakes, developed/irradiated from medium contaminated lake, and both developed/irradiated and undeveloped from more highly contaminated lakes. This is the most comprehensive analysis to date of the gene responses in wildlife reproductive system to radiation. Some gene responses that were modulated in irradiated gonads were found to be involved in biological processes including cell differentiation and proliferation (ggnb2, mod5, rergl), cytoskeleton organization (k1C18, mtpn), gonad development (nell2, tcp4), lipid metabolism (ldah, at11b, nltp), reproduction (cyb5, cyp17A, ovos), DNA damage repair (wdhd1, rad51, hus1), and epigenetic mechanisms (dmap1). Identification of these genes provides a better understanding of the underlying molecular mechanisms underpinning the development of the gonad phenotypes of wild perch and how fish may respond to chronic exposure to radiation in their natural environment, though causal attribution of gene responses remains unclear in the undeveloped gonads.

An in-situ electron microscopy study of dual ion-beam irradiated xenotime-type ErPO4

Rare-earth phosphates adopting the xenotime (REPO4; RE = Tb − Lu & Y, Sc) structure are proposed as a potential matrix for the confinement of minor actinides. Minor actinides (e.g., Np, Am, Cm) undergo a radioactive decay process in which high-energy recoil atom (70–100 keV) and energetic alpha particles (4.5–5.8 MeV) are produced. In this study, the impact of these energetic decay products on the structure of xenotime-type ErPO4 has been investigated via high energy dual ion-beam irradiation of ErPO4 ceramics. Au2+ (1.5 MeV) and He+ (160 keV) ions were used to simulate the effects of recoil atom and α-particles, respectively. Multiple experiments were carried out in which the Au2+ and He+ ions with varying ion-fluences (ions/cm2) and ion-flux (ions/cm2/s) were implanted sequentially (Au2+ followed by He+ irradiation) and simultaneously (Au2+ + He+ irradiation) into ErPO4 ceramics. Sequential ion-irradiation experiments have shown that the xenotime structure was amorphized by Au2+ ions at a relatively lower ion-fluence (5 × 1013 ions/cm2) in comparison to the monazite structure. Upon irradiation of the amorphous ErPO4 with He+ ions, recrystallization of the amorphous xenotime due to α-particles was not observed. However, simultaneous ion-irradiation experiments on ErPO4 showed that the amorphization of the xenotime structure was prevented upon deposition of higher amounts of electronic energy (Eelectronic) in the lamella. Likewise monazite samples, the α-healing mechanism was also experimentally demonstrated in synthetic xenotime samples.

Preliminary investigation of olimpi field, Mediterranean Sea, using in-situ and laboratory radio-tracing methods

In-situ and laboratory data of radioisotopes were acquired and interpreted for the Olimpi field, Mediterranean Sea. Standard oceanographic parameters and in-situ radon progeny profiles were analysed for four different sites, utilizing a state-of-the-art gamma-ray spectrometer optimized for deep sea research. In addition, a sediment core recovered from the Gelendzhik mud diapir plateau was analysed by means of high resolution laboratory gamma-ray methods providing vertical distribution of excess lead 210Pb, potassium 40K, and caesium 137Cs. Water column profiles of radon progenies revealed higher concentrations above mud volcano areas/structures in comparison to the selected reference site. This increase is likely attributed to higher emanation rates of radon gas in areas which are affected by the local diapirism. In all cases the concentration of radon progeny almost linearly decreased toward the surface due to radioactive decay and dilution processes, however differences in the concentration slopes between sites indicates a continuous release of fluids in the Moscow and Milano sites. The profile of the excess 210Pb in the Gelendzhik sediment core revealed two different accumulation rates. Using two local maxima of 137Cs, two dating points were used to suggest that an intense mud flow event took place during 1963–1986. Also, as the activity concentration of 40K increased by almost 30% during this period, and the chemical constituent of the accumulated material differed from the underneath layers, a material of different origin is presumed. Moreover, in the period 1986–2016 the accumulation rate was much lower indicating the absence of other, more recent, intense mud flow events in the neighbouring area of the core sampling.

Algorithms and algebraic solutions of decay chain differential equations for stable and unstable nuclide fractionation

Radioactive decay processes, such as alpha decay, produce decay chains where the mass numbers of nuclides decrease as larger nuclides expel energetic particles to form smaller nuclides. Under these conditions, the coefficient matrix that describes the differential rate expressions for radioactive decay can be made lower triangular. With this special structure, formulating an algebraic solution to the decay chains can be done by first formulating the eigenvectors that make up the coefficient matrix, which can then be solved using forward substitution for a lower triangular matrix. This work details the derivation of algebraic solutions for decay chains of any number of stable and unstable nuclides with any number of branching based on this eigenvector analysis. A prototype computational code was developed to validate and compare this methodology against a number of other methods for solving similar systems. A two-phase sorting algorithm yielding the lower triangular matrix structure was established to apply the developed algebraic solutions for decay chains involving beta-emitting radionuclides transformed into daughter nuclides without change in their mass number. The methodologies produced in this work provide an efficient way to estimate nuclide fractions from natural decay processes.

Investigation of pre-service chemistry teachers’ understanding of radioactive decay: a laboratory modelling activity

The first purpose of this study was to introduce a laboratory modelling activity focusing on teaching the concepts of radioactive elements/atoms, radioactive decay, and half-life. The second was to investigate pre-service chemistry teachers’ understanding of these concepts. Fifteen pre-service chemistry teachers who had enrolled in a Physical Chemistry Laboratory course participated in the study. The participants simulated the radioactive decay process of an imaginary radioactive element called Cornium during the laboratory modelling activity. In this way, pre-service chemistry teachers were able to visualize and model what is meant by radioactive decay and half-life. Data sources included responses given to open-response conceptual questions, participants’ drawings about radioactive decay and half-life, semi-structured interviews, activity worksheets, and observation notes. Qualitative analysis of data revealed that the pre-service chemistry teachers had misconceptions about the definition of radioactive elements/atoms, radioactive decay, and the half-life process before the activity. The role of the modelling activity in exposing and eliminating these misconceptions was discussed.

Large-scale energy production with thorium in a typical heavy-water reactor using high-grade plutonium as driver fuel

Thorium can be introduced into the energy vector in combination with high-grade plutonium (HG-Pu). Excellent neutron economy of heavy-water moderator allows use of mixed ThO2/HG-PuO2 fuel in heavy-water reactors with high efficiency, leading to the exploitation of large world thorium reserves and extending the availability of the nuclear energy by two orders of magnitude. In the present work, the criticality calculations have been performed with the code MCNPX 3-D geometrical modeling of a typical heavy-water reactor, where the structure of all fuel rods and bundles is represented individually. In the course of time calculations, nuclear transformation and radioactive decay of all actinide elements as well as fission products are considered. Five different fuel compositions have been selected for investigations: (1) 97% thoria (ThO2) + 3% PuO2; (2) 96% ThO2 + 4% PuO2; (3) 95% ThO2 + 5% PuO2; (4) 94% ThO2 + 6% PuO2; and (5) 92% ThO2 + 8% PuO2. The behavior of the criticality k∞ and the burn-up values of the reactor have been pursued by full power operation at 640 MWel (2180 MWth) for approximately 7 years. Time calculations have been conducted with MCNPX and CINDER codes under consideration of all nuclear transformation and radioactive decay processes on the actinide isotopes and fission fragments. As the reactor allows fuel recharging at on-power operation mode, the reactor criticality has been followed down to keff,end = ~1.05. The corresponding burn-up values and operation periods for the investigated modes are (1) 18 GWd/MT and 780 days; (2) 27 GWd/MT and 1200 days; (3) 35 GWd/MT and 1560 days; (4) 44 GWd/MT and 1940 days; and (5) 60 GWd/MT and 2640 days. Among the investigated four modes, 94% ThO2 + 6% PuO2 seems a reasonable choice under consideration of the high price of the HG-Pu as driver fuel. The mixed fuel has the potential of an extensive exploitation of thorium resources. Reactor will run with the same fuel charge for approximately 5 years and allow a fuel burn-up approximately 44 GWd/MT, comparable with conventional light-water reactors (LWRs). Plutonium component of the mixed fuel will become nonprolific after few months of plant operation through the accumulation of even isotopes. Addition of few percent natural uranium to the initial mixed fuel charge will keep the 233U component below 22% and hence at nonprolific level over the entire plant operation period. Replacement of 4% ThO2 with 4% nat-UO2 will practically not change main technical parameters of the reactor.

Non-Uniformity of the Indoor Radon Concentration under a Convective Mixing Mechanism.

This paper focuses on the results of computational fluid dynamics (CFD) modeling of radon concentration distribution in living areas within residences. The COMSOL Multiphysics® 5.3 software package has been employed for solving coupled momentum and species transport problems together with pseudo-reaction term modeling of the radon radioactive decay process. The reliability and verification of the simulation model was tested by comparing with available experimental data. The obtained results show the existence of stagnant zones where the concentration of radon is substantially higher than the average values. The impact of factors such as wind velocity, air tightness, and incoming radon flux were taken into consideration.

SUDOQU, a new dose-assessment methodology for radiological surface contamination

A new methodology has been developed for the assessment of the annual effective dose resulting from removable and fixed radiological surface contamination. It is entitled SUDOQU (SUrface DOse QUantification) and it can for instance be used to derive criteria for surface contamination related to the import of non-food consumer goods, containers and conveyances, e.g., limiting values and operational screening levels. SUDOQU imposes mass (activity)-balance equations based on radioactive decay, removal and deposition processes in indoor and outdoor environments. This leads to time-dependent contamination levels that may be of particular importance in exposure scenarios dealing with one or a few contaminated items only (usually public exposure scenarios, therefore referred to as the ‘consumer’ model). Exposure scenarios with a continuous flow of freshly contaminated goods also fall within the scope of the methodology (typically occupational exposure scenarios, thus referred to as the ‘worker model’). In this paper we describe SUDOQU, its applications, and its current limitations. First, we delineate the contamination issue, present the assumptions and explain the concepts. We describe the relevant removal, transfer, and deposition processes, and derive equations for the time evolution of the radiological surface-, air- and skin-contamination levels. These are then input for the subsequent evaluation of the annual effective dose with possible contributions from external gamma radiation, inhalation, secondary ingestion (indirect, from hand to mouth), skin contamination, direct ingestion and skin-contact exposure. The limiting effective surface dose is introduced for issues involving the conservatism of dose calculations. SUDOQU can be used by radiation-protection scientists/experts and policy makers in the field of e.g. emergency preparedness, trade and transport, exemption and clearance, waste management, and nuclear facilities. Several practical examples are worked out demonstrating the potential applications of the methodology.