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This paper presents the results of the fission chambers (FC) behavior under intensive neutron and gamma radiation at the research reactors. The FC detector unit (DU) was designed for the International Experimental Thermonuclear Reactor neutron diagnostic “divertor neutron flux monitor.” Two mockups of the DU with FCs containing 1 g and 100 mg of 235U were tested. The test complex is based on the FC signal processing unit and the FC parameters’ monitoring system. The processing unit contains several measuring chains and acquire FC signal over a wide range of neutron flux variation. The FC parameters’ monitoring system records signal shapes, acquires pulse-height amplitude spectra, and checks electrical parameters of the neutron detector. The radiation tests were carried out at the research reactors IRT MEPhI and VVR-c. The structure and characteristics of the test complex as well measurement techniques and tools for FC characterization are discussed. The results of reactor experiments to determine characteristics of the DU mockup and wide-range neutron flux monitoring channels are presented. [ABSTRACT FROM AUTHOR]

In order to ensure the quality and the relevance of irradiation programs in the future Jules Horowitz Reactor (JHR), the French Alternative Energies and Atomic Energy Commission (CEA) has significantly increased its research and development effort in the field of in-pile instrumentation during the last decade. Major progress has been achieved in the capability to perform accurate in-pile measurements using reliable and updated techniques. A significant part of this effort has been conducted in the framework of the Joint Instrumentation Laboratory between the CEA and the Belgian Nuclear Research Centre (SCK $\cdot$ CEN). In order to improve measurement techniques for neutron flux assessment, a unique system for online measurement of fast neutron flux has been developed and recently qualified in-pile. The fast-neutron-detection-system (FNDS) has been designed to monitor accurately high-energy neutron flux (E > 1 MeV) in typical material testing reactor (MTR) conditions, where neutron flux levels can be as high as $10^{15}\,\,\text {N}\cdot \text {cm}^{-2}\cdot \text {s}^{-1}$ and are generally dominated by thermal neutrons. Moreover, the neutron flux is coupled with a high gamma flux of typically a few $10^{15} \gamma \cdot \text {cm}^{-2}\cdot \text {s}^{-1}$ , which can be highly disturbing for the online measurement of neutron fluxes. The patented FNDS system is based on two detectors allowing the simultaneous detection of both thermal- and fast-neutron flux. Thermal neutrons can be measured using a self-powered neutron detector or a 235U miniature fission chamber, while fast neutron detection requires a miniature fission chamber with a special fissile material presenting an energy threshold near 1 MeV, which can be 242Pu for MTR conditions. Fission chambers are operated in Campbelling mode for an efficient gamma rejection. FNDS also includes a specific software that processes measurements to compensate online the fissile material depletion and to adjust the sensitivity of the detectors, in order to produce a precise evaluation of both thermal and fast neutron flux even after long-term irradiation. FNDS has been validated through a two-step experimental program. A first set of tests was performed at BR2 reactor operated by SCK $\cdot$ CEN in Belgium. Two FNDS prototypes were operated in-pile during nearly 1000 h. These tests exhibited the consistency of the measurement of thermal to fast neutron flux ratio with Monte Carlo neutron and photon transport calculations, as well as the right compensation of fissile material depletion. This paper describes a second test recently completed at ISIS reactor operated by CEA in France. For this irradiation, FNDS signal was compared to reference thermal- and fast-neutron flux measurements using activation dosimeters analyzed under COFRAC Quality Certification. During this latter test, FNDS proved its ability to measure online the fast neutron flux with an overall accuracy better than 5%. FNDS is now operational and is assumed to be the first and unique acquisition system able to provide an online measurement of the fast neutron flux in MTR conditions. This system will be used to perform spectral neutron characterization of JHR channels, but it may also be implemented in future irradiation experiments, for a better and real-time evaluation of the fast neutron flux received by material and fuel samples. [ABSTRACT FROM AUTHOR]

Neutron and gamma flux measurements in designated positions in the test blanket modules (TBMs) of ITER will be important tasks during its campaigns. Investigations on self-powered detectors (SPDs), a class of reactor flux monitors are undertaken in the framework of an ongoing project on development of nuclear instrumentation for European ITER TBMs. This paper reports the findings of experiments performed with an SPD in flat sandwich-like geometry. A detector with vanadium emitter is chosen for preliminary studies. Its irradiation in a thermal neutron field gives a proof of the principle of flat SPDs. It is further irradiated in the mixed neutron–gamma field of a 14-MeV neutron generator and a bremsstrahlung photon field. The test SPD signals are proportional to the incident fluxes, deeming it suitable for flux monitoring. Whereas both neutrons and gammas can be detected with appropriate optimization of geometries, materials, and sizes of the components, the present design is more sensitive to gammas than fast neutrons. Based on the measured sensitivities of the SPD, its response under TBM conditions is predicted. [ABSTRACT FROM AUTHOR]

Neutron diagnostic “Divertor Neutron Flux Monitor” is being under the development for the ITER project. This diagnostic is a primary tool for total neutron yield and fusion power measurements. Diagnostic consists of the three detector modules and the enhanced data acquisition (DAQ) systems. Each module contains three fission chambers (FCs) with 235U and three with 238U. The choice of the FC types and sensitivities is determined by the project requirements for the neutron flux measurement range, error, and time resolution. The DAQ system performs preliminary amplification and processing of the fission chamber signals, and the calculation of the neutron flux at the module location to calculate the total neutron yield and fusion power. The DAQ system transfers measured data to the upper level of the control system with a time resolution of 1 ms and provides raw data archiving. The hardware and software of the DAQ system were developed taking into account the requirements of the ITER project. The data processing system mockup of a single detection module was manufactured. This mockup passed laboratory and reactor tests. The system has passed the preliminary design approval. Preparation for the manufacturing of the system prototype is carried out at this moment. The architecture of the DAQ system and its components are shown in the report, and the test results of the system mockup are discussed. [ABSTRACT FROM AUTHOR]

The JSI TRIGA reactor features several in-core and ex-core irradiation facilities, each having different properties, such as neutron/gamma flux intensity, spectra and irradiation volume. A series of experiments and calculations was performed in order to characterise radiation fields in irradiation channel thus allowing users to perform irradiations in a well characterised environment. Since 2001 the reactor has been heavily used for radiation hardness studies for components used at accelerators such as the Large Hadron Collider (LHC) at CERN. Since 2010 it has been extensively used for testing of new detectors and innovative data acquisition systems and methods developed and used by the CEA. Recently, several campaigns were initiated to characterise the gamma field in the reactor and use the experimental data for improvement of the treatment of delayed gammas in Monte Carlo particle transport codes. In the future it is planned to extend the testing options by employing pulse mode operation, installation of a high energy gamma ray irradiation facility and allow irradiation of larger samples at elevated temperature.