František Cvachovec

Measurement and calculation of fast neutron and gamma spectra in well defined cores in LR-0 reactor

A well-defined neutron spectrum is essential for many types of experimental topics and is also important for both calibration and testing of spectrometric and dosimetric detectors. Provided it is well described, such a spectrum can also be employed as a reference neutron field that is suitable for validating selected cross sections. The present paper aims to compare calculations and measurements of such a well-defined spectra in geometrically similar cores of the LR-0 reactor with fuel containing slightly different enrichments (2%, 3.3% and 3.6%). The common feature to all cores is a centrally located dry channel which can be used for the insertion of studied materials. The calculation of neutron and gamma spectra was realized with the MCNP6 code using ENDF/B-VII.0, JEFF-3.1, JENDL-3.3, ROSFOND-2010 and CENDL-3.1 nuclear data libraries. Only minor differences in neutron and gamma spectra were found in the comparison of the presented reactor cores with different fuel enrichments. One exception is the gamma spectrum in the higher energy region (above 8MeV), where more pronounced variations could be observed.

Experimental and Calculation Investigations of the Space-Energy Neutron and Photon Distribution in the Vicinity of Reactor Pressure Vessel and Surveillance Specimen Box of New Type in the WWER-1000 Mock-Up

The radiation field parameters in the vicinity of new type of WWER-1000 surveillance specimens boxes and in RPV simulator have been investigated in the WWER-1000 Mock-up in the reactor LR-0. The neutron and photon spectra have been measured with stilbene cylindrical crystals in the energy range approximately from 0.5 MeV to 10 MeV. The experimental values of integral neutron field parameters (attenuations of integral neutron flux densities between different points of mock-up as well as the spectral indices, i.e., ratios of integral neutron flux densities with different thresholds) were obtained also. The mock-up (LR-0 reactor) power monitoring during measurements was carried out with two fission chambers, one fixed, one moveable axially, so that four orders in power were covered. The experimental data (spectra, attenuation factors, and spectral indices) have been compared with the calculation results. The sensitivity to parameters of calculation model and method was evaluated also.

Quick algorithms for real-time discrimination of neutrons and gamma rays

Several new methods for the digital discrimination of neutrons and gamma-rays in a mixed radiation field are presented. The methods introduced discriminate neutrons and gamma rays successfully in the digital domain. They are mathematically simple and exploit samples during the life time of the pulse, hence appropriate for field measurements. All these methods are applied to a set of mixed neutron and photon signals from a stilbene scintillator and their discrimination qualities are compared.

Measurement of neutron spectra in a silicon filtered neutron beam using stilbene detectors at the LVR-15 research reactor

A well-defined neutron spectrum is an essential tool for calibration and tests of spectrometry and dosimetry detectors, and evaluation methods for spectra processing. Many of the nowadays used neutron standards are calibrated against a fission spectrum which has a rather smooth energy dependence. In recent time, at the LVR-15 research reactor in Rez, an alternative approach was tested for the needs of fast neutron spectrometry detector calibration. This process comprises detector tests in a neutron beam, filtered by one meter of single-crystalline silicon, which contains several significant peaks in the fast neutron energy range. Tests in such neutron field can possibly reveal specific problems in the deconvolution matrix of the detection system, which may stay hidden in fields with a smooth structure and can provide a tool for a proper energy calibration. Test with several stilbene scintillator crystals in two different beam configurations supplemented by Monte-Carlo transport calculations have been carried out. The results have shown a high level of agreement between the experimental data and simulation, proving thus the accuracy of used deconvolution matrix. The chosen approach can, thus, provide a well-defined neutron reference field with a peaked structure for further tests of spectra evaluation methods and scintillation detector energy calibration.

Digital Multiparameter System for Characterizing the Neutron-Gamma Field in the LR-O Experimental Reactor

Digital Multiparameter System for Characterizing the Neutron-Gamma Field in the LR-O Experimental Reactor.

The effect of biological shielding on fast neutron and photon transport in the VVER-1000 mock-up model placed in the LR-0 reactor

The paper is intended to show the effect of a biological shielding simulator on fast neutron and photon transport in its vicinity. The fast neutron and photon fluxes were measured by means of scintillation spectroscopy using a 45×45 mm(2) and a 10×10 mm(2) cylindrical stilbene detector. The neutron spectrum was measured in the range of 0.6-10 MeV and the photon spectrum in 0.2-9 MeV. The results of the experiment are compared with calculations. The calculations were performed with various nuclear data libraries.

Neutron and gamma spectrometry in the research reactor LR-0

The complex measurement system for the full-scale VVER models (mock-ups), created in the research reactor LR-0, is discussed. Fast neutron (energy from 0.5 to 10 MeV) and gamma spectra are measured in several representative points of the mock-ups by the two-parameter spectrometer with the cylindrical stilbene scintillation detectors. Measurements in the thermal and epithermal neutron region are carried out with the activation method using a broad set of activation monitors and with the 3He(n,p) counter. Activation measurements with threshold fast neutron detectors enlarge also the proton-recoil spectra measurements, such activation measurements are carried out especially in cases, when a spectrometer cannot be put in the necessary position. The core fission rate distribution is obtained by means of gamma-scanning of the fuel pins. The low noise precise power (flux density) monitoring system, independent of the operation control system, was developed in the LR-0 reactor for mock-up experiments.

Optimum filter-based discrimination of neutrons and gamma rays

An optimum filter-based method for discrimination of neutrons and gamma-rays in a mixed radiation field is presented. The existing filter-based implementations of discriminators require sample pulse responses in advance of the experiment run to build the filter coefficients, which makes them less practical. Our novel technique creates the coefficients during the experiment and improves their quality gradually. Applied to several sets of mixed neutron and photon signals obtained through different digitizers using stilbene scintillator, this approach is analyzed and its discrimination quality is measured.

Digital data processing of stilbene

Stilbene is a proven spectrometric detector for mixed fields of neutrons and gamma rays. By digital processing of shape output pulses from the detector it is possible to obtain information about the energy of the interacting neutron / photon and distinguish which of these two particles interacts in the detector. Another numerical processing of digital data can finalize the energy spectrum of both components of the mixed field. The quality of the digitized data is highly dependent on the parameters of the hardware used for digitization and on the quality of software processing. Our results also show how the quality of the particle type identification depends on the sampling rate and as well as the method of processing of the sampled data.

Neutron/gamma-ray discrimination through measures of fit

Statistical tests and their underlying measures of fit can be utilized to separate neutron/gamma-ray pulses in a mixed radiation field. In this article, first the application of a sample statistical test is explained. Fit measurement-based methods require true pulse shapes to be used as reference for discrimination. This requirement makes practical implementation of these methods difficult; typically another discrimination approach should be employed to capture samples of neutrons and gamma-rays before running the fit-based technique. In this article, we also propose a technique to eliminate this requirement. These approaches are applied to several sets of mixed neutron and gamma-ray pulses obtained through different digitizers using stilbene scintillator in order to analyze them and measure their discrimination quality.