Reconstruction of Missing Height-Sensor Readings on the Basis of Archival Neutron Field Monitoring Data

Abstract

Constant monitoring of the state of the core ensures safe operation of a nuclear reactor. This article examines the reconstruction of the energy release in the core of a nuclear reactor on the basis of the indications of height sensors. Situations where some sensors fail are not rare. Any reduction in the number of sensor indications increases the error. However, the missing measurement information can be reconstructed by mathematical methods, and replacement of the failed sensors can be avoided. The simplest algorithm for reconstruction missing indications consists in approximating the height distribution by harmonic functions. However, the coeffi cients of these functions are found to be correlated. It is suggested that a set of natural functions determined by means of statistical estimates obtained from archival data be constructed. The procedure proposed makes it possible to reconstruct the fi eld even with a signifi cant loss of measurement information.Safe and effi cient operation of a nuclear reactor is secured by constantly monitoring the state of the core, including the energy release, by means of sensors used for in-reactor monitoring. In VVER, direct-charge sensors (DCS) are united into neutron-measurement channels with seven sensors uniformly arranged along the height in each channel [1]. Radial and height sensors comprised of four extended sensitive sections for monitoring the energy release (SMER) are installed in RBMK [2]. Situations where some in-reactor sensors fail are not rare in the practice of operating nuclear reactors. If a sensor fails, then the use of its indications is prohibited. For example, if more than one section of a RBMK sensor fails, then the detector is considered to be 'prohibited'. A neutron measurement channel in VVER is prohibited upon failure of more than two sensors.A reduction in the number of in-reactor monitoring sensor indications on the basis of which the energy release in the reactor core is reconstructed results in a larger determination error and requires modifi cation of the standard reconstruction algorithm. We note that in VVER the sensors can be replaced only between runs while in RBMK they can be replaced 'on the fl y' without stopping the reactor, but this complicates the work because not just the sensor but the entire fuel assembly must be replaced so that actually unplanned refueling is performed.The missing measurement information can be restored by mathematical methods and premature replacement of the failed detectors can thereby be avoided, which is also expedient economically. For this, it is proposed that the missing sensor indications be calculated by fi tting the known points using functions determined on the basis of archival data. We shall clarify this for the following example.

On the Refinement of the Algorithm for the Reconstruction of Missing ICMS-sensor Readings

“…For this, it is proposed that missing sensor indications be calculated by approximation the known points. Approximation is suggested to be made as an expansion to the series of functions, adjusted in a certain way using the archive data [3]. The proposed technique enables to use accumulated data from an archive to restore the neutron field even after a serious loss of measurement information.…”