Application of gamma-neutron ionization chambers for monitoring the distribution of the neutron flux density, energy release, and energy production in RBMK-1000

Abstract

A method developed for performing direct measurements of three-dimensional distributions of energy release and energy production in RBMK fuel assemblies is described. The method is based on performing measurements with a gamma-neutron chamber and comparing the neutron and gamma signals. The results of the measurements of the neutron flux density, energy release, and energy production are compared with the values obtained with the Prizma-M program of the Skala-micro information-measurement system. It is confirmed experimentally that the Prizma-M system can be used to monitor the distribution of not only the neutron flux density and energy release of fuel assemblies but also the energy production of off-loaded fuel assemblies.The need to perform periodic measurements of the thermal-neutron flux density distribution, energy release, and energy production in a RBMK-1000 core is due to the need for regular metrological certification, as provided in the operational documentation, of the Skala-micro information-measurement system and calibration of in-reactor detectors. This work is performed by scanning the core along the radius with Dt.6 hafnium detectors and scanning along the height of the core with Kt-19 fission chambers, However, when measurements of the energy release in fuel assemblies are performed the detectors do not provide the desired accuracy and do not permit determining the distribution of energy production in an operating reactor. The latter problem must be solved for complete certification of the Skala-micro system and, ultimately, to increase the safety of not only the reactor but also the storage facilities for spent nuclear fuel. The present work is devoted to solving these problems.Two-section triaxial Kt-18 ionization chambers were used to scan in an operating reactor fuel assemblies with assembly 49, specially provided for this purpose, with dry central tubes. One section of such a chamber is a fission chamber with 90% enrichment 235 U, and the other is a γ chamber. In chambers of this type, all electrodes are made of corrosion-resistant steel, and the insulation is made of compacted magnesium oxide; argon is the working gas [1]. The outer diameter of the chamber in the core is 6 mm, and the sensitive part of this section is 50 mm long; the distance between the centers of the sections is 85 mm. The ratio of the sensitive and total volume of the fission and γ chambers is ~0.047. In the course of the measurements, the chambers were moved with constant velocity along the height of a fuel assembly using a crane from the central hall. The measurements were performed at the No. 1 unit of the Kursk nuclear power plant on December 20, 2004 at 95% nominal power. Twenty five fuel assemblies were scanned. For technical reasons, some of these assemblies were scanned only along half of the core height. For this reason, we shall examine the value of any parameter only for the top half of a fuel

Development of RBMK in-reactor control

“…Simultaneous detection of the local thermal-neutron flux density by the sections makes it possible to determine the axial distribution of the power release in a working reactor and to calibrate the error in the SKALA-mikro information and measurement system on the basis of the function used to calculate the power release [11].…”