Neutron Spectrum Measurements in the Vertical Channel of the IR-8 Research Reactor

Experimental results of neutron spectrum and neutron flux measurements in a dry vertical channel of the IR-8 research nuclear reactor at the Kurchatov Institute Russian Research Center are presented. The data were obtained using five sets of various activation detectors. The spectrum was reconstructed using the KASKAD computational program used in the method of integral neutron detectors. The results obtained make it possible to optimally plan the experiments on the irradiation and study of the radiation stability of various elements of the diagnostic systems of the International Thermonuclear Experimental Reactor (ITER) in the new irradiation channel of the IR-8 research reactor.     

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 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.     

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.     

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.     

Radiation Hardness of a Fast Neutron Scintillation Spectrometer with a Stilbene Crystal

Neutron detectors with organic oscillators such as stilbene crystals are suggested for use in measuring the characteristics of neutron radiation in the International Thermonuclear Experimental Reactor (ITER). These detectors will be used as neutron-flux monitors in a multichannel neutron collimator for obtaining the spatial characteristics of a plasma source of thermonuclear neutrons with a 1-ms temporal resolution. In addition, operation in the spectrometric mode will ensure the measurement of the thermonuclear-neutron energy spectrum, which yields data on the fusion-plasma temperature and the ratio of the deuterium and tritium concentrations. During the operation of the facility, the detectors will be exposed to irradiation with a large fluence of fast neutrons. The first results obtained in studies of the characteristics of a stilbene crystal irradiated by fast neutrons with a fluence of up to 10¹⁴ neutrons/cm² are 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.     

Irradiation of cryogenic temperature sensors by gamma dose of 1 MGy

This article presents results of investigations of the cryogenic thermometers which can operate under high radiation environment. To clarify the temperature shifts due to gamma irradiation up to 1 MGy, the platinum (two types), carbon-glass, and TVO temperature sensors have been tested. Before the irradiation run, the selected sensors were calibrated and a series of experiments was carried out under irradiation of the sensors at 77.3 and 293 K when their readings were taken in situ. Afterwards, the postirradiation evolution (short and long time) of the sensors was measured at 4.2, 77.3, and 293 K. The results, depending on the type of the sensor, can be optimistic or pessimistic. Some comparisons with analogous data for fast neutron treatment have also been made.     

Progress in the conceptual development of the ITER vertical neutron collimator

Monte-Carlo calculations of the fluxes of forward and scattered neutrons at points of arrangement of the detectors for the analyzed design of the ITER multichannel vertical neutron collimator have been performed with the MCNP model. A concept of a vertical neutron collimator positioned in the diverter port is proposed, and the efficiency of its application for determining the spatial distribution of the ITER tokamak reactor’s neutron source is demonstrated with allowance for the calculation results and the possibilities of integrating the collimator into the ITER structure.     

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.     

Neutron spectroscopy results of JET high-performance plasmas and extrapolations to DT performance

In a fusion reactor with high energy gain, the fusion power will be mainly thermonuclear (THN). Measurements of the THN neutron rate are a good performance indicator of a fusion plasma, requiring neutron emission spectroscopy (NES) measurements to distinguish thermal and nonthermal contributions. We report here on recent NES results from JET high-performance plasmas with high fractions (about 65%) of THN emission. The analysis is made with a framework for analyzing NES data, taking into account THN reactions and beam-target reactions. The results are used to extrapolate to the equivalent DT rates. Finally, we discuss the applicability of using NES in the deuterium phase of ITER, both for the extrapolations to ITER’s future DT performance as well as for the measurements of confined energetic ions.     

New developments in the diagnostics for the fusion products on JET in preparation for ITER (invited)

Notwithstanding the advances of the past decades, significant developments are still needed to satisfactorily diagnose “burning plasmas.” D–T plasmas indeed require a series of additional measurements for the optimization and control of the configuration: the 14 MeV neutrons, the isotopic composition of the main plasma, the helium ash, and the redistribution and losses of the alpha particles. Moreover a burning plasma environment is in general much more hostile for diagnostics than purely deuterium plasmas. Therefore, in addition to the development and refinement of new measuring techniques, technological advances are also indispensable for the proper characterization of the next generation of devices. On JET an integrated program of diagnostic developments, for JET future and in preparation for ITER, has been pursued and many new results are now available. In the field of neutron detection, the neutron spectra are now routinely measured in the energy range of 1–18 MeV by a time of flight spectrometer and they have allowed studying the effects of rf heating on the fast ions. A new analysis method for the interpretation of the neutron cameras measurements has been refined and applied to the data of the last trace tritium campaign (TTE). With regard to technological upgrades, chemical vapor deposition diamond detectors have been qualified both as neutron counters and as neutron spectrometers, with a potential energy resolution of about one percent. The in situ calibration of the neutron diagnostics, in preparation for the operation with the ITER-like wall, is also promoting important technological developments. With regard to the fast particles, for the first time the temperature of the fast particle tails has been obtained with a new high purity Germanium detector measuring the gamma emission spectrum from the plasma. The effects of toroidal Alfven eigenmodes modes and various MHD instabilities on the confinement of the fast particles have been determined with a combination of gamma ray cameras, neutral particle analyzers, scintillator probe, and Faraday cups. From a more technological perspective, various neutron filters have been tested to allow measurement of the gamma ray emission also at high level of neutron yield.     

A comparative analysis of the sensitivity of CsI (Tl), ZnO(Ga), and YAG(Ce) scintillators to the plasma background radiation under operating conditions of the ITER tokamak reactor

Problems related to neutral particle flux measurements on the ITER tokamak reactor under intense plasma background radiation conditions are considered. The results of measuring a background sensitivity with respect to neutron and γ-radiation for the scintillation detector, which is based on three different crystals (CsI (Tl), ZnO(Ga), and YAG(Ce)), are presented. The scintillators are compared and conclusions about the possibility of their applications in detectors of neutral particle analyzers currently created at the Ioffe Institute for the ITER tokamak reactor, are drawn.     

Characteristics of the neutron field at the core center in the ИBB-2M water-moderated water-cooled reactor

The neutron spectrum has been measured at the center of the core in the ИBB-2M research water-moderated water-cooled reactor. A technique has been developed to measure the energy spectrum of neutrons in high-intensity fields (with a flux density of >10¹² cm^?2 s^?1) without recourse to fission detectors enclosed in boron shields.     

Asymmetric fission of <Superscript>235</Superscript>U nuclei induced by thermal neutrons and fast neutrons of pulsed nuclear reactors

The contributions that neutrons emitted from the neck of excited dumbbell-shaped fissioning ²³⁵U nuclei at the instant of their fission make to the prompt fission neutron spectrum, as well as the ratios of these contributions, have been measured for nuclear fission induced by thermal neutrons and fast neutrons of pulsed nuclear reactors.     

Effect of neutron irradiation on the electrodes of a vacuum photoemission detector

The effect of neutron irradiation on Ta-Be photocathodes of a vacuum photoemission detector (VPD) has been investigated. The VPD has been designed for tomography of plasmas using thermal X-ray radiation on the ITER international tokamak reactor. The exposure of VPD electrode samples to neutrons with a fluence of 1.6 × 10¹⁹ neutrons/cm² (E > 0.1 MeV) is shown to change the relative concentration of Ta and Be in the surface layer. As a result, the detector sensitivity in the DT operating mode of the ITER is inevitably altered. Some recommendations concerning selection of materials for the VPD electrodes are presented. The conclusion is drawn that sputtering by thermonuclear neutrons in the ITER may lead to deposition of a conductive film on the insulator surface and that a special shape of insulators must be therefore used to avoid formation of such a coating.     

ITER中子屏蔽横块的模态分析与谱分析研究

为避免真空室屏蔽层受地震的影响,根据ITER中子屏蔽层结构对其中子屏蔽单元模块进行了单点响应谱分析,将中子屏蔽结构的模态分析结果与法国某地的加速度谱进行了模态叠加。分析发现,应力极值在许可应力范围之内。     

ODS钢液态重金属环境腐蚀研究进展

铅基快堆(Lead-cooled fast reactor,LFR)采用液态重金属(铅/铅铋合金)作为冷却剂,具有高安全性、高经济性等特点,并且在核能可持续发展方面具有一定的优势,目前被列为第四代核能系统六种参考堆型之一。液态金属冷却剂在服役过程中具有高温、高流速和高密度等特点,会对堆芯结构材料产生严重的腐蚀破坏。纳米氧化物弥散强化钢(Oxide dispersion strengthened steel,ODS)具有优异的抗辐照性能和优良的高温力学性能,目前已经成为先进快中子反应堆堆芯候选结构材料研究热点,其抗液态金属冷却剂腐蚀的能力是影响反应堆安全运行的关键性能之一。对目前有代表性的ODS钢在液态重金属环境中的腐蚀研究进展进行了综述,并结合ODS钢在氧化性气体环境和超临界水中的氧化腐蚀研究进展,对影响ODS钢液态金属腐蚀行为的因素进行了讨论。     

Studying the dependence of the ratio of the prompt neutron yields in the symmetric and asymmetric fission of <Superscript>235</Superscript>U and <Superscript>238</Superscript>U nuclei on the energy of neutrons inducing this fission

The ratios have been measured of the prompt neutron yields in the symmetric and asymmetric fission of ²³⁵U nuclei induced by thermal neutrons and fast neutrons of pulsed nuclear reactors, as well as in the ²³⁸U fission induced by 14.7-MeV neutrons. The well-known measured integrated cross sections of the system of dosimetric nuclear reactions are compared to the cross sections calculated using the differential cross sections from the data libraries known worldwide, the three-component representation of fission neutron spectra from ²³⁵U and ²⁵²Cf nuclei, and the KASKAD spectrum reconstruction program.