[en] In the present thesis, nuclear fuel particles are studied from the perspective of their characteristics, atmospheric transport and possible skin doses. These particles, often referred to as 'hot' particles, can be released into the environment, as has happened in past years, through human activities, incidents and accidents, such as the Chernobyl nuclear power plant accident in 1986. Nuclear fuel particles with a diameter of tens of micrometers, referred to here as large particles, may be hundreds of kilobecquerels in activity and even an individual particle may present a quantifiable health hazard. The detection of individual nuclear fuel particles in the environment, their isolation for subsequent analysis and their characterisation are complicated and require well-designed sampling and tailored analytical methods. In the present study, the need to develop particle analysis methods is highlighted. It is shown that complementary analytical techniques are necessary for proper characterisation of the particles. Methods routinely used for homogeneous samples may produce erroneous results if they are carelessly applied to radioactive particles. Large nuclear fuel particles are transported differently in the atmosphere compared with small particles or gaseous species. Thus, the trajectories of gaseous species are not necessarily appropriate for calculating the areas that may receive large particle fallout. A simplified model and a more advanced model based on the data on real weather conditions were applied in the case of the Chernobyl accident to calculate the transport of the particles of different sizes. The models were appropriate in characterising general transport properties but were not able to properly predict the transport of the particles with an aerodynamic diameter of tens of micrometers, detected at distances of hundreds of kilometres from the source, using only the current knowledge of the source term. Either the effective release height has been higher than reported previously or convective updraft may have influenced the transport. Models applicable to large particle dispersion in a turbulent atmosphere should be further developed. The health threat from large nuclear fuel particles differs from that of uniform contamination. In contact with human tissue such as skin, a highly active beta-emitting particle may cause a large but localised dose to the tissue, whereas at distances of more than about one centimetre from the source the dose is negligible. Large particles are poorly inhalable because of their size. They may be deposited in the upper airways but are not easily transported deep into the lungs. Instead, deposition onto the surface of skin is of more relevance with respect to acute deterministic health effects. In the present work, skin doses are calculated for particles of different sizes and different types by assuming the particles are deposited on the body surface. The deposition probability as a function of the number concentration of the particles in air is not estimated. The doses are calculated at the nominal depth of the basal cell layer and averaged over a square centimetre of the skin. Calculated doses are compared with the annual skin dose limit for the public (50 mGy at a depth of 0.07 mm and averaged over 1 cm'). After the Chernobyl accident the most active nuclear fuel particles detected in Europe, hundreds of kilometres from the source, would have been able to produce a skin dose exceeding this limit within one hour when deposited onto skin. However, the appearance of deterministic effects necessitates skin contact lasting more than one day. The health hazards of nuclear fuel particles must be taken into account in estimating the consequences of a severe nuclear accident and planning countermeasures to protect the rescue workers and the general public. (orig.)

[en] ORIGEN2 is a computer code that calculates nuclide composition and other characteristics of nuclear fuel. The use of ORIGEN2 requires good knowledge in reactor physics. However, once the input has been defined for a particular reactor type, the calculations can be easily repeated for any burnup and decay time. This procedure produces large output files that are difficult to handle manually. A new computer code, known as OTUS, was designed to facilitate the postprocessing of the data. OTUS makes use of the inventory files precalculated with ORIGEN2 in a way that enables their versatile treatment for different safety analysis purposes. A data base is created containing a comprehensive set of ORIGEN2 calculations as a function of fuel burnup and decay time. OTUS is a reactor inventory management system for a microcomputer with Windows interface. Four major data operations are available: (1) Build data modifies ORIGEN2 output data into a suitable format, (2) View data enables flexible presentation of the data as such, (3) Different calculations, such as nuclide ratios and hot particle characteristics, can be performed for severe accident analyses, consequence analyses and research purposes, (4) Summary files contain both burnup dependent and decay time dependent inventory information related to the nuclide and the reactor specified. These files can be used for safeguards, radiation monitoring and safety assessment. (orig.) (22 refs., 29 figs.)

[en] A Monte Carlo code is developed for simulating energy spectra of alpha particles from aerosol samples. Geometrical detection efficiency and energy loss of the alpha particles in the source itself and in the material between the source and detector are simulated. Different characteristics of the aerosol particles and medium material are taken into account in the computation. An excellent agreement with earlier results and measurements is found. The code can be applied for example in designing optimal aerosol sampling methods in direct alpha spectrometry

[en] Autoradiography was used to study radioactive particles that may be released from the nuclear fuel cycle. Autoradiography suits for pre-screening of the samples. Radioactive particles can be located accurately and detached from the sample for subsequent analyses. A digital scanner and tailored software allow to estimate the activity of the particles by a factor of two to three. High-energy beta emitters as well as pure beta emitters can be identified. A particle with activity of 0.1 Bq can be detected in five days of exposure. More than 10⁵ disintegrations are needed to detect a black spot on the autoradiography film. Total activity of beta active nuclides can be evaluated if the number of disintegrations is smaller than 10⁶. The diameter of the black spot is then below 1 mm. The edge of the black spot receives a beta dose of approximately 10 mGy. Particular emphasis was placed on method development for the point of view of in-field applications. (orig.) (12 refs., 17 figs., 5 tabs.)

[en] Highly radioactive particulate material may be released in a nuclear accident or sometimes during normal operation of a nuclear power plant. However, consequence analyses related to radioactive releases are often performed neglecting the particle nature of the release. The properties of the particles have an important role in the radiological hazard. A particle deposited on the skin may cause a large and highly non-uniform skin beta dose. Skin dose limits may be exceeded although the overall activity concentration in air is below the level of countermeasures. For sheltering purposes it is crucial to find out the transport range, i.e. the travel distance of the particles. A method for estimating the transport range of large particles (aerodynamic diameter d_a > 20 μm) in simplified meteorological conditions is presented. A user-friendly computer code, known as TROP, is developed for fast range calculations in a nuclear emergency. (orig.) (23 refs., 13 figs.)

[en] Large amounts of radioactive particles can be released in a severe nuclear accident. The particles that may cause a serious health risk via inhalation or deposition on the skin can be transported hundreds of kilometres via air flows. In the present study the transport range of uranium fuel particles of sizes between d_a=20-140 μm is estimated in simplified meteorological conditions. The analysis is applied to the particle transport in the Chernobyl accident. The results of the calculations are supported by the environmental findings of the particles. The wind speed and the initial plume rise have a crucial influence on the transport distance. A simple ballistic analysis is not adequate if the vertical air flow varies greatly during transport. In such weather conditions the analysis must be connected with three-dimensional trajectory calculations. (Author)

No abstract available

[en] Radiation dose to the skin caused by large nuclear fuel particles is calculated as a function on the particle size. The size range considered is 6-40 μm (aerodynamic diameter 20-140 μm). Air-tissue surface effects and self-absorption of the particles are taken into account in the dose estimation. The nuclide composition of the particles is estimated from the inventory of the Chernobyl reactor. When deposited on the skin the uranium fuel particle of size 40 μm can cause a dose of 1.6 Gy.cm^-2 to the basal cell layer in one day. The transport range calculations show that these particles may remain airborne tens of kilometres away from the power plant. (Author)

[en] An automated high-volume aerosol sampling station, known as CINDERELLA.STUK, for environmental radiation monitoring has been developed by the Radiation and Nuclear Safety Authority (STUK), Finland. The sample is collected on a glass fibre filter (attached into a cassette), the airflow through the filter is 800 m³/h at maximum. During the sampling, the filter is continuously monitored with Na(I) scintillation detectors. After the sampling, the large filter is automatically cut into 15 pieces that form a small sample and after ageing, the pile of filter pieces is moved onto an HPGe detector. These actions are performed automatically by a robot. The system is operated at a duty cycle of 1 d sampling, 1 d decay and 1 d counting. Minimum detectable concentrations of radionuclides in air are typically 1Ae10 x 10^-6 Bq/m³. The station is equipped with various sensors to reveal unauthorized admittance. These sensors can be monitored remotely in real time via Internet or telephone lines. The processes and operation of the station are monitored and partly controlled by computer. The present approach fulfils the requirements of CTBTO for aerosol monitoring. The concept suits well for nuclear material safeguards, too

[en] The potential use of direct high-resolution alpha spectrometry to identify the presence of trans-actinium elements in air samples is illustrated in the case when alpha-particle-emitting radionuclides are incorporated in nuclear fuel particles. Alpha particle energy spectra are generated through Monte Carlo simulations assuming a nuclide composition similar to RBMK (Chernobyl) nuclear fuel. The major alpha-particle-emitting radionuclides, in terms of activity, are ²⁴²Cm, ²³⁹Pu and ²⁴⁰Pu. The characteristics of the alpha peaks are determined by fuel particle properties as well as the type of the air filter. It is shown that direct alpha spectrometry can be readily applied to membrane filter samples containing nuclear fuel particles when rapid nuclide identification is of relevance. However, the development of a novel spectrum analysis code is a prerequisite for unfolding complex alpha spectra. (authors)