[en] Characterization of nuclear materials is equally important in nuclear safeguards (inventory verification) and in nuclear security (revealing illicit trafficking). Analysis of materials is a key issue in both fields. Natural (NU), depleted (DU), low-enriched (LEU), and high-enriched uranium (HEU) samples were analysed by high resolution gamma spectrometry (HRGS). Isotopic composition and total U-content of reactor fuel pellets and powder were determined. A unique HRGS method was developed for the first time for determining the production date of the material of unknown origin. Identifying reprocessed uranium proved to be possible by HRGS as well.

[en] For determining the origin of nuclear material out of regulatory control, information on the age of the material seems relevant. A new method for uranium age dating was developed by using high resolution gamma-spectrometry (HRGS) based on determining the daughter/parent activity ratio ²¹⁴Bi/²³⁴U by directly measuring the count rates of the relevant gamma peaks of ²¹⁴Bi and ²³⁴U. The method is non-destructive and does not require the use of reference materials of known ages. It works well first of all for high-enriched and aged material. The least enriched uranium sample dated by HRGS was a 5% enriched oxide material, the age of which was determined as 54 ± 7 y. The youngest sample was a 6.7 ± 0.7 y old metallic U of 90.8% enrichment. In order to extend capabilities and improve sensitivity and accuracy of the method, a higher efficiency (well-type) detector was provided. (author)

[en] For revealing illicit trafficking of nuclear materials, a neutron interrogation method was developed. A 4 MeV linear accelerator served as a pulsed neutron source for assay of highly enriched U samples. Produced in Be or heavy water by Bremsstrahlung, neutrons subsequently induced fission in the samples. A neutron collar was built for detecting delayed fission neutrons. A PC controlled multi scaler served as a time analyzer, triggering the detector startup by the beam pulse. A lower sensitivity limit of the order of 10 mg '2³'5U was determined in a 20 s measurement time with a mean electron current intensity of 10 μA

[en] Non-destructive methods for assaying sealed Pu-Be sources have been developed, by using neutron and gamma-spectrometric measurements. In the first method, a combination of gamma-spectrometric determination of Pu isotopic abundances and neutron counting were used. A passive neutron coincidence collar was designed with 14 ³He counters embedded in a polyethylene moderator surrounding the sources to be measured. The electronics consist of independent chains of pulse amplifiers and discriminators as well as a shift register. The neutron yield of the sources was determined by gross neutron counting, and the Pu content was assessed by adopting specific (α,n) reaction yields of individual Pu (and Am) isotopes, normalized to a calorimetric calibration. A pure gamma-spectrometric method was also developed, by determining photopeak areas of ²³⁹Pu lines, and applying absorption correction. In the third, pure neutron method, real coincidence count rates (R) were also recorded in addition to total (T) count rates, and an empirical correlation between the ratio R/T and Pu content was used. The results of individual methods agree well. In a series of measurements, determination of the Pu content of 75 Pu-Be sources resulted in a total Pu amount of 563 g in contrast with a declared nominal value 2050 g. (author)

[en] A NDA method for quantitative assay of Pu-Be neutron sources by HP-Germanium detector is described. Analyzing the structure of gamma spectrum in the 0-15000 keV energy range the value of around 1200 keV was obtained for the maximum energy of neutrons due to the effect of the interaction between neutrons and Germanium material via (n,γ) reaction. This effect and Compton tail of 4438 keV increase the background in the gamma spectra of Pu-Be neutron sources. Nevertheless, the commercial software MGA++ proved to be still reliable tool for analyzing the 0-600 keV energy range to derive the age and the isotopic composition of Pu material, although the part of its report about the existence of the Uranium is incorrect. Using three intensive gamma lines of Pu-Be, assuming far field assay of cylindrical sources the Pu content of the sources was determined with an uncertainty of up to 6%. In particular, the 375 and 413 keV lines were used for estimation of the Pu content and the 129 keV line for estimation of the dimensions of the sources. Combining this result with total neutron counting, the specific neutron yield can be verified. As a result of these measurements, it was shown that the claim of the supplier of the sources about two different sets of Pu-Be sources supplied to Hungary with different specific neutron yields is false. (author)

[en] It is shown that low background gamma spectrometry can be successfully used to identify uranium samples made from reprocessed uranium, by measuring the activity of ²³²U. It is demonstrated that ²³²U is always present in enriched or depleted uranium samples, but in reprocessed uranium the activity of ²³²U is at least 100 times higher than in other uranium samples of the same ²³⁵U enrichment. (author)

[en] There are about 200 Pu-Be sources, most of them out of use, in Hungary. Whereas it is a safeguards and safety issue, their Pu content was not provided upon delivery. The supplier declared neutron output only. Records on the Pu mass are still based on a rough estimate from it. Nominal Pu amounts estimated on the assumption of pure ²³⁹Pu content of the sources are regularly reported to the IAEA, and from 2004 onwards also to the EURATOM safeguards authorities. In order to create a more realistic situation, a development work commenced. A passive neutron coincidence well-counter was designed at the Institute of Isotopes and, using it together with a shift register provided by the IAEA, neutron measurements were carried out. Isotopic composition of Pu in the sources was determined by high resolution γ-spectrometry (HRGS). The Pu content was calculated from the neutron output and the isotopic composition. This combined neutron-γ method relies on (α,n) yields adopted for Pu isotopes and ²⁴¹Am. The method has been described in detail. Results show that nominal Pu masses have been overestimated even by an order of magnitude in some cases (where the ²³⁹Pu abundance is about 75 % or less of the total Pu content). Pu masses were also determined by pure HRGS, without neutron measurements. The method relies on absolute intensity measurements of 375 and 413 keV ²³⁹Pu photopeaks, applying attenuation correction, and taking into account the ²³⁹Pu abundance determined by γ- spectrometry, see the companion paper submitted to this symposium. In order to verify the results, heat output measurements of the Pu-Be sources were performed with a Small Sample Calorimeter provided by JRC IPSC, Ispra, Italy, combined with the isotopic composition determined by HRGS, relying on specific heat values from the literature. As a result, it has been established that the combined neutron-γ method provides Pu content data 20-30 % higher than calorimetry, but still much less than nominal values. On the basis of the calibration as above, more realistic (α,n) yields were established, with the aid of which, a much better agreement of the combined neutron-γ results with calorimetry was achieved. By using this method, a new inventory of the Pu content of Pu-Be sources is planned to be taken in the country. It has been tried to use pure neutron measurements as well, without γ-spectrometry at all. Based on gross (T) and coincidence (R) count rates, this so-called R/T method relies on a correlation established between the ratio R/T and the Pu content, using the graph or the corresponding empirical formula, fitted to the results of calorimetry

[en] This paper reports on the results of the experiments performed on spent VVER-440 fuel assemblies at the Paks Nuclear Power Plant (NPP), Hungary. The fuel assemblies submerged in the service pit were examined by high-resolution gamma spectrometry (HRGS). The assemblies were moved to the front of a collimator tube built in the concrete wall of the pit in the reactor block at the NPP, and lifted down and up under water for scanning by the refueling machine. The HPGe detector was placed behind the collimator in an outside staircase. The measurements involved scanning of the assemblies along their length of all the 6 sides, at 5-12 measurement positions side by side. Axial and azimuthal burnup profiles were taken in this way. Assembly groups for measurements were selected according to their burnup (10–50 GWd/tU) and special positions (e. g. control assembly, neighbour of control assembly). Burnup differences were well observable between assembly sides looking towards the center of the core and opposite directions. Also, burnup profiles were different for control assemblies and normal (working) fuel assemblies. The ratio of the measured activities of Cs-134 and Cs-137 was evaluated by relative efficiency (intrinsic) calibration. Measurement uncertainty is around 3 %. Taking into account irradiation history and cooling time (i. e.the time elapsed since the discharge of the assembly out of the core), the activity ratio Cs-134/Cs-137 shows good correlation with the declared burnup.

[en] Mixed convection flow in tubes is encountered in many engineering applications, such as solar collectors, nuclear reactors, and compact heat exchangers. Here, a numerical investigation has been conducted in order to determine the effects of variable properties on the flow pattern and heat transfer performances in laminar developing ascending flow with mixed convection for two cases: in case 1 the fluid is heated, and in case 2 it is cooled. Calculations are performed for air at various Grashof numbers with a fixed entrance Reynolds number of 500 using both the Boussinesq approximation (constant-property model) and a variable-property model. In the latter case, the fluid viscosity and thermal conductivity are allowed to vary with absolute temperature according to simple power laws, while the density varies linearly with the temperature, and the heat capacity is assumed to be constant. The comparison between constant- and variable-property models shows a substantial difference in the temperature and velocity fields when the Grashof number |Gr| is increased. The friction factor is seen to be underpredicted by the Boussinesq approximation when the fluid is heated (case 1), while it is overpredicted for the cooling case (case 2). However, the effects on the heat transfer performance remain negligible except for cases with reverse flow. On the whole, the variable-property model predicts flow reversal at lower values of |Gr|, especially for flows with opposing buoyancy forces. The deviation in results is associated to the difference between the fluid bulk and the wall temperature

[en] A laser nonlinear spectroscopy experiment has been designed and implemented to test the conjecture that enantiomers of chiral molecules may have different spectra because of the parity violation associated with neutral currents in the weak interaction between electrons and nuclei. We review the theoretical reasons that lead to this prediction and we outline the method currently used to calculate energy and frequency differences between mirror image molecules. Preliminary experimental tests have been conducted on hyperfine components of vibration-rotation transitions of CHFClBr in the 9.3 μm spectral range. The frequencies of saturation resonances of separated enantiomers have been compared and found identical within 13 Hz (Δν/ν <4.10^-13). (authors)