Detector for separate detection of neutrons and γ rays

A method for separate dosimetry of neutrons and γ rays is described. The neutrons are detected according to the recoil protons and γ rays are detected according to the secondary electrons. The results of an experimental check of the possibility of using for this purpose two identical detectors with the same total KBr(Tl) scintillator mass but different surface area in contact with a hydrogen-containing liquid as well as the computed values of the sensitivity are presented. Scintillators which are suitable for such systems are listed. 5 references. St. Petersburg State Technological Institute. Translated from Atomnaya énergiya, Vol. 88, No. 2, pp. 149–152, February, 2000.     

Determination of radiation attenuation coefficients of heavyweight- and normal-weight concretes containing colemanite and barite for 0.663 MeV γ-rays

Accurate measurements have been made to determine radiation transmission of concretes produced with barite, colemanite and normal aggregate by using beam transmission method for 0.663 MeV γ-rays energy of ¹³⁷Cs radioactive isotopes by using NaI(Tl) scintillation detector. Linear and mass attenuation coefficients of thirteen heavy- and four normal-weight concretes were calculated. It was determined that the linear attenuation coefficient (μ, cm⁻¹) decreased with colemanite concentration and increased with barite concentration in both type of the concretes. Mass attenuation coefficient values of our concretes were compared with the values proposed by the United States National Institute of Standards and Technology (NIST).     

Determination of energy absorption in a mixed flux of fast neutrons and γ-rays by an ionization method

The possibility of separate determinations of the energy absorption of fast neutrons and -rays in the mixed radiation flux from a reactor has been studied with ioni/ation chambers. Three chambers with different hydrogenous fillers were used: polyethylene with an ethylene filler; graphite with a CO₂ filler and a chamber made from aerion, a conducting plastic, which was filled with a mixture of ethylene and CO₂. Calculations have been carried out to ascertain the sensitivity of these chambers to neutrons with energies ranging from 0.2–8 Mev. Variation of the neutron spectrum over wide limits has no effect on the accuracy in the determination of the absorbed dose in the hydrogenous substrates. A calculation shows that the error in the determination of the absorbed energy for fast neutrons is approximately 15% and is a weak function of the relative doses of neutrons and -rays.In conclusion the authors wish to express their gratitude to Yu. F. Chernilin for help in this experiment and T. B. Radzievskii for discussion of the results.     

Development and validation of a fast reactor core burnup code – FARCOB

A large fast breeder reactor requires the accurate estimation of power produced in different parts of the reactor core and blanket during any operating condition for a safe and economic operation through out reactor life time. A fast reactor core simulation code FARCOB based on multigroup diffusion theory has been developed in IGCAR for core simulation of PFBR reactor under construction. FARCOB uses centre mesh differencing scheme with triangular meshes in the X–Y plane. Steady state solution results match exactly with those of other reputed codes DIF3D and VENTURE for SNR-300 benchmarks. For burnup simulation, core is divided into radial and axial burnup zones and burnup equations are solved at constant power. Burnable fuel and blanket number densities are found and stored for each mesh, so that the user can alter burnup zones and core geometry after a burnup step. For validation, results of FARCOB has been compared with results of other institutes in two burnup benchmarks (ANL 1000 MWe benchmark and BN-600 hybrid core benchmark). It is found that FARCOB results match well with those of the other institutes.     

Measurement and analysis of the energy–angular distribution of secondary neutrons for Be at 5.9 and 6.4 MeV incident neutrons

The energy-angle double-differential neutron emission cross-sections of beryllium have been measured using the time of flight technique for 5.9 and 6.4 MeV incident neutrons, respectively, at 10 laboratory angles between 25° and 150°. The measured results are compared with model calculations based on the LUNF code and those of other authors and the ENDF/B-VII data. The estimation of the inelastic scattering neutron cross-sections leaving ⁹Be^∗ at the low-lying level states are also theoretically analyzed by the LUNF code. The experimental and calculated results indicated that the lowest (1.68 MeV) level still contributes to the (n, 2n) reaction with cross-sections of several 10 mb. The angular distributions and the angle-integrated elastic scattering cross-sections are also presented in comparison with other ones, these being in good agreement with the ENDF/B-VII data.     

Transport of low-level NORM – a case for consistency and practicality

Abstract

In 1996 the International Atomic Energy Agency (IAEA) adopted a system for exemption of lowlevel radioactive material from transport regulations based on the principle that exemption values should be commensurate with the risk posed by the material as represented by the maximum potential radiation dose to individuals. For many naturally occurring radionuclides the derived dose-based, radionuclide-specific exemption concentrations were substantially lower than the previous radionuclide-independent definition of radioactive material (70 Bq g^–1) [1900 pCi g^–1] due to the stringent dose criterion applied. It was recognised that this would bring large quantities of previously unregulated naturally occurring radioactive material (NORM) handled in industry into the scope of the transport regulations. To minimise the economic impact of the dose-based values, a special provision was included to provide for a 10-fold increase in exemption values for radionuclides in natural material provided the material is not intended to be, and has not previously been, processed for recovery of its radionuclides (the wording regarding previous use was added in 2003). This '10 times' or '10×' provision for certain natural material reflects a second concept underlying IAEA guidance, namely, that a dose criterion may be relaxed within cautious bounds to achieve a balance between practical issues and radiological concerns. On the other hand, restriction of the provision on the basis of past or intended use of the material is inconsistent with the basic principle underlying the Transport Regulations in that there is no risk basis for assigning different exemption values to identical materials on the basis of their past or anticipated use. In fact, the same material can move in and out of the scope of regulatory control as its anticipated use changes. As a practical matter, safety guidelines for potentially hazardous material should be based on measurable properties of the material and not the whims of human intentions. To improve the practicality as well as the consistency of the Transport Regulations as applied to NORM, the 10× provision should be revised to apply to all natural materials, regardless of their intended use.     

γ-Ray spectra excited in inelastic scattering of fast neutrons on manganese,aluminum, iron,copper, tin,and antimony

The study of inelastic scattering of fast neutrons is an important problem of both theoretical and practical interest. From the theoretical point of view the importance of this work lies in the possibility of obtaining data concerning levels in stable nuclei. The practical value arises in connection with the important role played by inelastic scattering of neutrons in fast-neutron reactors as well as the fact that the extension of reactor theory to fast-neutron reactors requires data on the spectra of inelastically scattered neutrons [1, 2]. In this connection the necessity for developing a neutron spectrometer for fast neutrons and-spectroscopy for inelastic neutron scattering is obvious. In the last 5–7 years a great deal of work has been devoted to this problem.The present work reports on measurements of-ray spectra excited in inelastic scattering of 2.8 Mev neutrons by manganese, aluminum, iron, copper, tin and antimony. The measurements were carried out with a scintillation spectrometer consisting of an NaI(Tl) crystal, a FEU-1B photomultiplier and a 50-channel pulse-height analyzer with a magnetic-drum memory. The spectrometer resolution was 6.5–7% for-rays from Co⁶⁰.-Rays of the following energies (Mev) were found: in manganese 0.97, 1.41, 1.92, 2.3; in aluminum 0.84, 1.00, 1.80, 2.16; in iron 0.84, 1.25, 1.46, 1.70; in copper 0.63, 0.78. 0.96, 1.12, 1.38, 1.46, 1.72, 2.03; in tin 0.84, 1.16, 1.50, 1.80, 2.16; in antimony 1.04, 1.50, 1.84, 2.16.Abbreviated version of a paper appearing in the Ukrainian Journal of Physics.The authors wish to take this opportunity to thank L. M. Beliaev and G. F. Dorbrzkanskii of the Institute of Crystallography, Academy of Sciences, USSR for making the NaI(TI) crystal and for kndly allowing us to use it in carrying out the present work.     

A scenario of core disruptive accident for Japan sodium-cooled fast reactor to achieve in-vessel retention

As the most promising concept of sodium-cooled fast reactors, the Japan Atomic Energy Agency (JAEA) has selected the advanced loop-type fast reactor, so-called JSFR. The safety design requirements of JSFR for Design Extension Condition (DEC) are the prevention of severe accidents and the mitigation of severe-accident consequences. For the mitigation of severe-accident consequences, in particular, the In-Vessel Retention (IVR) against postulated Core Disruptive Accidents (CDAs) is required. In order to investigate the sufficiency of these safety requirements, a CDA scenario should be constructed, in which the elimination of power excursion and the in-vessel cooling of degraded core materials are evaluated so as to achieve IVR. In the present study, the factors leading to IVR failure were identified by creating phenomenological diagrams, and the effectiveness of design measures against them were evaluated based on experimental data and computer simulations. This is an unprecedented approach to the construction of a CDA scenario, and is an effective method to objectively investigate the factors leading to IVR failure and the design measures against them. It was concluded that mechanical/thermal failures of the reactor vessel due to power-excursion/thermal-load could be avoided by adequate design measures, and a clear vision for achieving IVR was obtained.     

Studies on absorption coefficients of dual-energy γ-rays and measuring error correction for multiphase fraction determination

In tiffs article, principle and mathematical method of determining the phasc fractions of multiphase flows by using a dual-energy γ-ray system have been described. The dual-energy γ-ray device is composed of radioactive isotopes of ^241Am and ^137Cs with γ-ray energies of 59.5 and 662 keV, respectively. A rational method to calibrate the absorption coefficient was introduced in detail. The modified arithmetic is beneficial to removing the extra Compton scattering from the measured value. The result shows that the dual-energy γ-ray technique can be used in thrce-phase flow with average accuracy greater than 95%, which enables us to determine phase fractions almost independent of the flow regime. Improvement has been achicved on measurement accuracy of phase fractions.     

Measurements of fast—neutron capture cross sections for ^159Tb and ^169Tm

The neutron capture cross sections for ^159Tb and ^169Tm relative to the ^197Au (n,γ)^198Au reaction are measured at neutron energies of 0.57,1.10 and 1.60 MeV by using the activation method.The activities of the products are measured with a high resolution HPGe detector gamma-ray spectrometer.The errors of the present work are 5-6% for Tb,6-7% for Tm.The recommended data in energy region of 0.4-3.0MeV are given as compared with other data published previously.     

Oxidation behaviour of zirconium alloys and their precipitates – A mechanistic study

The precipitate oxidation behaviour of binary zirconium alloys containing 1 wt.% Fe, Ni, Cr or 0.6 wt.% Nb was characterised in TEM on FIB prepared transverse sections of the oxide and reported in previous studies [1], [2]. In the present study the following alloys: Zr1%Cu, Zr0.5%Cu0.5%Mo and pure Zr are analysed to add to the available information. In all cases, the observed precipitate oxidation behaviour in the oxide close to the metal-oxide interface could be described either with delayed oxidation with respect to the matrix or simultaneous oxidation as the surrounding zirconium matrix. Attempt was made to explain these observations, with different parameters such as precipitate size and structure, composition and thermodynamic properties. It was concluded that the thermodynamics with the new approach presented could explain most precisely their behaviour, considering the precipitate stoichiometry and the free energy of oxidation of the constituting elements.The surface topography of the oxidised materials, as well as the microstructure of the oxide presenting microcracks have been examined. A systematic presence of microcracks above the precipitates exhibiting delayed oxidation has been found; the height of these crack calculated using the Pilling–Bedworth ratios of different phases present, can explain their origin. The protrusions at the surface in the case of materials containing large precipitates can be unambiguously correlated to the presence of these latter, and the height can be correlated to the Pilling–Bedworth ratios of the phases present as well as the diffusion of the alloying elements to the surface and their subsequent oxidation. This latter behaviour was much more considerable in the case of Fe and Cu with Fe showing systematically diffusion to the outer surface.     

Total,scattering and γ-ray-production cross sections for few-MeV neutrons on 54Fe

Cross sections for the following neutron interactions were obtained for ⁵⁴Fe in the few-MeV incident-energy range with resolutions of 50–100 keV: total interaction from ∼0.5 to 4 MeV, elastic scattering from 1.2 to 4 MeV, inelastic scattering from 2 to 4 MeV for excitation energies up to 3 MeV, and inelastic-scattering γ-ray production from 1.4 to 3.6 MeV for transitions corresponding to the deexcitation of levels up to 3 MeV. The results are mutually consistent within experimental errors. A spherical optical-statistical model describes the energy-averaged behavior of these experiments reasonably well, although slight evidence for direct-reaction strength is exhibited by the angular distributions of neutrons scattered to the 1.408 MeV, 2⁺, level.     

Measurements of neutron spectra from iron and boron-in-polyethylene bombarded with 14 MeV neutrons

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Spatial distribution of γ-rays and moderated neutrons in the graphite column of the RFT reactor

Measurements have been made of the spatial distributions of neutrons of various energies and the -rays in the graphite thermal column of the RFT reactor. The fluxes of thermal, resonance, and fast neutrons were measured using the activity induced in various indicators. The decay in the intensity of the /gg-radiation was determined by small condenser type ionization chambers. At distances from 80 to 160 cm the fluxes of resonance and fast neutrons fall off approximately exponentially with a decay length of /R~ 13 cm and 15.7 cm, respectively. At large distances the fluxes of fast and resonance neutrons are in equilibrium. The flux of slowed-down neutrons in this region falls off exponentially with a decay length of 18.2 cm as determined by the penetrating fast-neutron component with an energy of approximately 6.6 Mev. The intensity of the -radiation in the graphite column falls off in an almost exponential manner with an attenuation coefficient = 3.78 · 10^–2 cm^–1. The theoretical predictions are found to be in satisfactory agreement with the experimental data.