Yu.N. Pepyolyshev

Pattern recognition application for surveillance of abnormal conditions in a nuclear reactor

Abstract The system to monitor abnormal conditions in a nuclear reactor, based on the noise analysis of the reactor basic parameters such as power, temperature and coolant flow rate has been developed. The pattern recognition techniques such as clustering, cluster analysis, feature selection and cluster visualization methods form the basis of the software system. Apart from non-hierarchical clustering procedures applied earlier, the hierarchical one is recommended. The system application for IBR-2 Dubna reactor diagnostics is shown.

Predicting of slow noise and vibration spectra degradation in the IBR-2 pulsed neutron source using a neural network simulator

Employing a neural net model of the noise state of the IBR-2 reactor (JINR, Dubna, Russia) and a model of the vibration state of movable reflectors in the reactor we have predicted slow degradation in reactor noises. Operative diagnostics and prediction of the reactor noise behavior with time involves separating of the reflector degradation trend in power noises. We investigate two neural models. The first concerns the vibrations of the reflectors and the second is a simplified reactor noise model. The predicted results are close, in character, to the experimental data. They show that it is the movable reflectors that are mainly responsible for the degradation of power noises.

Influence of automatic regulator parameters on power transition processes of the IBR-2 reactor

Abstract Using a model of the IBR-2 reactor based on the block structure with z-transformation of constants and experimentally determined feedback parameters, power transition processes have been evaluated at different values of the parameters of the automatic power regulator (APR). It has been shown that, at regular reactivity excitations, the best transition processes correspond to higher APR rates at the elimination of the APR smoothing unit. The recommendations are given for choosing the APR parameters when at normal operation of the IBR-2 reactor there are random reactivity excitations.

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.

Some Experimental and Monte Carlo Investigations of the Plastic Scintillators for the Current Mode Measurements at Pulsed Neutron Sources

Scintillation detector (SD) is widely used in neutron and gamma-spectrometry in a count mode. The organic scintillators for the count mode of the detector operation are investigated rather well. Usually, they are applied for measurement of amplitude and time distributions of pulses caused by single interaction events of neutrons or gamma’s with scintillator material. But in a large area of scientific research scintillation detectors can alternatively be used on a current mode by recording the average current from the detector. For example,the measurements of the neutron pulse shape at the pulsed reactors or another pulsed neutron sources. So as to get a rather large volume of experimental data at pulsed neutron sources, it is necessary to use the current mode detector for registration of fast neutrons. Many parameters of the SD are changed with a transition from an accounting mode to current one. For example, the detector efficiency is different in counting and current modes. Many effects connected with time accuracy become substantial. Besides, for the registration of solely fast neutrons, as must be in many measurements, in the mixed radiation field of the pulsed neutron sources, SD efficiency has to be determined with a gamma-radiation shield present. Here is no calculations or experimental data on SD current mode operation up to now. The response functions of the detectors can be either measured in high-precision reference fields or calculated by a computer simulation. We have used the MCNP code [1] and carried out some experiments for investigation of the plastic performances in a current mode. There are numerous programs performing simulating similar to the MCNP code. For example, for neutrons there are [2-4], for photons - [5-8]. However, all known codes to use (SCINFUL, NRESP4, SANDYL, EGS49) have more stringent restrictions on the source, geometry and detector characteristics. In MCNP code a lot of these restrictions are absent and you need only to write special additions for proton and electron recoil and transfer energy to light output. These code modifications allow taking into account all processes in organic scintillator influence the light yield.

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

A promising design of a КНК-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 γ-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.

A Method for Determining the Average Charge inside a Fission Chamber

A procedure for determining the main characteristic of gas-filled ionization chambers, which are intended for neutron flux measurements in reactors, critical assemblies, and other neutron sources, is provided for the first time. The results of an estimation of this characteristic for chambers of various designs are presented. A semiempirical correlation between the design parameters of the fission-chamber and the average charge value appearing in the ionization chamber per one nucleus fission reaction under neutron irradiation is obtained. The method makes it possible to determinate the main characteristic of gas-filled fission chambers and their maximum sensitivity to thermal neutrons to an error of no more than 7 and 10%, respectively.

Control of reactor pulses by external neutron injection

Abstract It is shown that it is possible to regulate the energy of each pulse of a powerful pulsed periodic reactor using an injector of relatively low power. The target in the reactor core generates the neutron pulses forced by the injector. The correlations for determination of the moment of injection are obtained. The equation connecting the mean reactor power, the intensity integral of the target and the scatter multiplicity of the pulse energy without injection is obtained. In addition to its function as a regulator, the injector plays the role of an auxiliary emergency unit. It is shown that using an injector provides a regime in which the reactor can generate power pulses in the form of periodic packets.

The dynamic method for time-of-flight measurement of thermal neutron spectra from pulsed sources

Abstract A time-of-flight method for measurement of thermal neutron spectra in pulsed neutron sources with an efficiency more than 10 5 times higher than the standard method is described. The main problems associated with the electric current technique for time-of-flight spectra measurement are examined. The methodical errors, problems of special neutron detector design and other questions are discussed. Some experimental results for spectra from the surfaces of water and solid methane moderators obtained at the IBR-2 pulsed reactor (Dubna, Russia) are presented.