A Channel for Neutron Flux Measurement

The design principles of a channel for measuring fluxes of thermal neutrons in a nuclear-reactor control and protection system are considered. The channel consists of a fission chamber, a set of boron-containing gas-filled ionization chambers, and an electronic unit that ensures a counting rate of fission pulses of up to 1 × 10⁶ s^–1. It is shown that the widest linear range is achieved in channels equipped with a fission chamber and a gas-filled ionization chamber with electrodes coated with natural boron. The channel allows for measurements of the thermal-neutron flux density in a range of 0.5 to 5 × 10¹¹ cm^–2 s^–1.     

New measurement system for on line in core high-energy neutron flux monitoring in materials testing reactor conditions

Flux monitoring is of great interest for experimental studies in material testing reactors. Nowadays, only the thermal neutron flux can be monitored on line, e.g., using fission chambers or self-powered neutron detectors. In the framework of the Joint Instrumentation Laboratory between SCK-CEN and CEA, we have developed a fast neutron detector system (FNDS) capable of measuring on line the local high-energy neutron flux in fission reactor core and reflector locations. FNDS is based on fission chambers measurements in Campbelling mode. The system consists of two detectors, one detector being mainly sensitive to fast neutrons and the other one to thermal neutrons. On line data processing uses the CEA depletion code DARWIN in order to disentangle fast and thermal neutrons components, taking into account the isotopic evolution of the fissile deposit. The first results of FNDS experimental test in the BR2 reactor are presented in this paper. Several fission chambers have been irradiated up to a fluence of about 7 × 10(20) n∕cm(2). A good agreement (less than 10% discrepancy) was observed between FNDS fast flux estimation and reference flux measurement.     

A conceptual project for a divertor monitor of the neutron yield in the ITER

The operating conditions of a neutron diagnostic system responsible for measuring the neutron yield in the ITER tokamak reactor are analyzed. Based on results of physical calculations and analysis of suitable methods for measuring the neutron yield, an original concept of a system for measuring neutron fluxed in the divertor zone of the ITER is proposed. The design for the neutron flux monitor located in the divertor zone of the tokamak is selected in view of the requirements specified for the neutron diagnostic system of the ITER and its operating conditions. Four fission chambers with different sensitivities and radiator materials are used as sensitive elements of the monitor. This system is capable of measuring neutron fluxes over the entire dynamic range of the neutron yield in the ITER with an error of ≤10% and a time resolution of 1 ms that are necessary for studying the physical mechanism of thermonuclear plasma ignition and burning. Several possible variants for housing the detector unit inside the divertor assembly and integrating it in the existing project are proposed. The problems of carrying out efficiency calibration of the divertor neutron monitor with the aim of determining the absolute value of the neutron yield in the ITER tokamak reactor are discussed.     

Characteristics of Pulses Used in Ionization Fission Chambers

Characteristics of the pulse amplitude distribution in the ionization fission chambers (IFC) used in the protection control system of the RBMK nuclear reactor were studied experimentally. The obtained results were used to optimize IFC parameters and for the interoperational monitoring of IFC.     

A Multichannel Neutron Collimator for the ITER Tokamak

A concept for the multichannel neutron collimator of the ITER is presented. The design of the collimator is based on the use of a 12-collimator radial neutron camera that was developed earlier and two complementary compact neutron cameras. It is proposed that the compact neutron cameras be placed inside the shielding blocks located in the equatorial (nine collimators) and diverter diagnostic (seven collimators) ITER ports. The plasma would thereby be fully covered in the vertical direction, and the total number of collimator channels would be 21 (12 channels in the radial camera and nine channels in the compact camera). The collimator length and diameters, as well as the optimal materials for the shielding blocks and the inner walls of the collimators, were determined using the MCNP code. It is shown by simulation that, for an adequate collimation of neutron fluxes to be achieved, the collimators should be 1.0–1.5 m long, have an i.d. of 4–5 cm, and be enclosed in a water–iron shielding. It is proposed that threshold fission chambers based on ²³⁸U and natural- and CVD-diamond detectors be used as the sensors. The computations are presented to demonstrate that this composition and arrangement of the channels in the radial and compact cameras makes it possible to measure the two-dimensional spatial distribution of the neutron source and the total thermonuclear power to an accuracy of 10%.     

Neutron analysis of the ITER vertical neutron camera

The neutron fields in the collimators of a new design for the vertical neutron camera (VNC) of the ITER have been calculated for a standard isotropic bulk DT neutron source. The neutron and gamma-ray spectra and flux densities at the neutron detector sites have been calculated. The signal-to-background ratio of VNC detectors (²³⁸U-based fission chambers and diamond detectors) has been estimated. The signal-to-background ratios versus the threshold energy were calculated for the diamond detectors operating in the threshold counter mode. The effect of background Γ-ray radiation on the performance of the diamond detectors in the VNC environment has been estimated. The radiation heating of the VNC structural components has been calculated. The serviceability of the VNC with the proposed design has been demonstrated.     

decrease in the dead time of proportional counters of neutrons after exposure to pulsed fluxes of ionizing radiation

During irradiation of a proportional counter with a high-power bremsstrahlung flux, a high density of charged particles arises in the interelectrode gap owing to primary and secondary ionization. Charge separation in the interelectrode gap induces an internal electric field that leads to a decrease in the gas-amplification factor to a level below the neutron-recording threshold. The delay time of neutron recording (dead time) by the counter after a bremsstrahlung burst may exceed 100 µs. A device developed for switching a high voltage applied to the counter for the bremsstrahlung emission time is described, as are experiments on detection of prompt fission neutrons emitted after irradiation of the fissile substance with a pulsed bremsstrahlung flux. The results show that, when this device is used, the neutron-recording delay time decreases severalfold.     

A Freon-filled bubble chamber for neutron detection in inertial confinement fusion experiments

Neutron imaging is one of the main methods used in inertial confinement fusion experiments to measure the core symmetry of target implosions. Previous studies have shown that bubble chambers have the potential to obtain higher resolution images of the targets for a shorter source-to-target distance than typical scintillator arrays. A bubble chamber for neutron imaging with Freon 115 as the active medium was designed and built for the OMEGA laser system. Bubbles resulting from spontaneous nucleation were recorded. Bubbles resulting from neutron-Freon interactions were observed at neutron yields of 10(13) emitted from deuterium-tritium target implosions on OMEGA. The measured column bubble density was too low for neutron imaging on OMEGA but agreed with the model of bubble formation. The recorded data suggest that neutron bubble detectors are a promising technology for the higher neutron yields expected at National Ignition Facility.     

A Controlled Energy Spectrum of a High-Intensity Neutron Field on a BR-K1 Reactor

A method for discrete control of the neutron energy spectrum at the center of the core of an aperiodic fast reactor is described. The neutron spectrum at the center of the reactor core was measured by the neutron activation method using the a priori spectrum as a superposition of partial fission and evaporation spectra.     

A method for determining the intensity of concomitant neutron source D(d, n)3He when studying the characteristics of delayed neutrons from nuclear fission induced by neutrons from reaction T(d, n)4He

A factor that may be responsible for the discrepancies in the total delay neutron yields, the relative yields of separate groups of delayed neutrons, and half-lives of their precursors has been investigated. These discrepancies are shown to be attributable to the effect of a concomitant neutron source—reaction D(d, n)³He. Such a source is unavoidably present in an experiment where reaction T(d, n)⁴He on a solid-state target is used as a neutron source. A method has been developed to calculate the contribution of neutrons from the reaction D(d, n)³He to the measured total and relative yields of delayed neutrons and the half-lives of their precursors in heavy nuclei fission induced by neutrons with energies of 14–18 MeV. The energy dependence of the parameters of delayed neutron groups from the neutron-induced fission of ²³⁸U nuclei in the energy range of 14.2–17.9 MeV is presented.     

Studying the spectrum of neutrons emitted from the LSDS-100 using a multiwire proportional fission chamber

A fast multiwire proportional fission chamber capable of detecting ∼10⁶ fission fragments per second was produced. Heptane vapor at a pressure of 15 mbar was used as a working gas. The full width at the base of the pulse from a fission fragment was ≤0.2 μs. The spectrum of the neutron flux density on the spectrometer surface was measured by the neutron slowing-down time in the lead of the LSDS-100 at neutron energies ranging from 18 eV to 11.3 keV.     

A Two-Section Gas-Filled Ionization Chamber for Neutron Flux Measurements

A promising design of a -type two-section gas-filled ionization chamber is described for the first time. A relationship between the design parameters, gas pressure, and characteristics of the material is determined, under which full compensation of the background currents from the -radiation and the measurement of thermal neutron fluxes in a range of 400 to 7 ×10⁹cm^–2s^–1at a load characteristic with a 5% nonlinearity are provided. Test results of the chamber are presented.     

High-Intensity Neutron Fields at the Center of the Metallic Core of Fast Reactors as the Primary Standard Neutron Fields

It is proposed to use high-intensity neutron fields with an average neutron energy of 0.7–1.5 MeV at the center of the metallic core of fast reactors as the primary standard neutron fields along with the known standard fields of fission neutrons with average energies of 1.93 and 2.13 MeV, respectively, from ²³⁵U and ²⁵²Cf nuclei, which are used in the world practice of taking neutron measurements. It is also proposed to create standard fields of nuclear fission neutrons with an average energy of 2 MeV by the bombardment of ²³⁵U converters with a beam of fast monoenergetic neutrons with energies of 2.5 and 14 MeV and 50-MeV protons.     

Experimental Study of the Deformation of Instantaneous Neutron Spectra from the Fission of <Superscript>235</Superscript>U Nuclei in Fast-Reactor Core Materials

The results from studies of the deformation of the instantaneous neutron spectra resulting from the fission of ²³⁵U nuclei in the material of structural elements of a fast reactor core are presented. The neutron spectra at and near the center of the reactor core were measured using neutron activation and fission neutron detectors.__________Translated from Pribory i Tekhnika Eksperimenta, No. 4, 2005, pp. 15–25.Original Russian Text Copyright © 2005 by Sevast’yanov, Koshelev, Maslov.     

A New Method for Measuring Specific Energy Losses of Fission Fragments

A method for measuring the specific energy losses of fission fragments is described. It is based on the digital processing of signals from a double Frisch-grid ionization chamber. The energy losses of ²⁵²Cf fission fragments were measured for masses and energies varying over wide ranges. A method is proposed for a more correct account of fission-fragment energy losses in the targets and entrance windows of spectrometers.     

A method for nondestructive assay of nuclear materials in facilities with a pulse neutron generator and a digital technology for discrimination between neutrons and photons

A method for determining the ²³⁵U concentration in fuel assemblies of a high-power channel-type PBMK reactor is described. The measure of ²³⁵U content of an analyzed sample is the number of neutrons from thermal-neutron fission of ²³⁵U, normalized to the number of γ quanta produced in thermal neutron capture by hydrogen nuclei in the scintillator or by ¹⁰B in the glass of a photomultiplier tube. A pulse neutron generator based on DT reaction is the neutron source, and an organic scintillator with the pulse shape discrimination between neutrons and γ rays with the aid of the digital technology is a detector. The scintillator is also used as a neutron moderator. Simulation of the method shows that the ²³⁵U content of the analyzed sample can be determined for 1 min with an accuracy of 1% or better. The efficiency of the method has been confirmed by experimental investigations on a model of the setup.     

Gas-Discharge Detectors for Monitoring the Total Neutron Yield in the International Thermonuclear Experimental Reactor

The design of two detector units made integral with their communications links is described. The detector unit composed of a KHT23-1 fission chamber and an assembly of CHM67 neutron counters is used to detect slow neutrons, while the detector unit based on a KHT23-8 threshold fission chamber is intended for fast neutrons. The parameters of these detector units along with their communications links are presented.     

Irradiation tests of ITER candidate Hall sensors using two types of neutron spectra

We report on irradiation tests of InSb based Hall sensors at two irradiation facilities with two distinct types of neutron spectra. One was a fission reactor neutron spectrum with a significant presence of thermal neutrons, while another one was purely fast neutron field. Total neutron fluence of the order of 10(16)?cm(-2) was accumulated in both cases, leading to significant drop of Hall sensor sensitivity in case of fission reactor spectrum, while stable performance was observed at purely fast neutron spectrum. This finding suggests that performance of this particular type of Hall sensors is governed dominantly by transmutation. Additionally, it further stresses the need to test ITER candidate Hall sensors under neutron flux with ITER relevant spectrum.     

A Method for Rapid Measurements of Working-Gas Properties in a Pulse Ionization Chamber

A method for measuring the properties of a working gas (the electron-drift velocity, the electron diffusion, and the probability of electron capture by electronegative impurities) is described. The method is based on the digital processing of the anode and cathode pulses of an ionization chamber after passing through a waveform digitizer. The measured values of the drift velocity, the diffusion coefficient, and the electron-capture probability are presented for various argon–methane mixtures. The results may be used in designing, adjusting, and operating ionization chambers, as well as in carrying out original investigations of the phenomena occurring during the drift of electrons in various gas mixtures.