New Interpretation of Signals and In-Reactor Detector Calibration

Abstract

One way to increase the accuracy of monitoring general and distributed parameters of the core of a nuclear reactor is to calibrate the in-reactor neutron and γ-ray detectors. The calibration can be performed before the detectors are placed in the core as well as periodically during operation. The approaches used to calibrate detectors for different types of modern nuclear reactors are examined and the accuracy of some methods of calibration is presented. Calibration of in-reactor neutron and γ-ray detectors is needed in order to secure acceptable accuracy in monitoring general and distributed parameters of a reactor core, including the margin to the safe operating limit. It is well known that more accurate monitoring of the distributed parameters makes it possible to increase the average energy density of the fuel in the region of the ‘plateau’ in the power release distribution, decrease neutron leakage from the core and thereby increase fuel burnup [1]. The calibration can be done before the detectors are placed into the reactor as well as periodically in the course of operation. PWR, BWR. Movable in-reactor probes, such as small fission chambers, equipped with flexible communication lines and special drives, are the most commonly used means for performing periodic calibration of in-reactor detectors for PWR and BWR reactors. The chambers are moved in dry channels, passing through the bottom of the reactor vessel [2]. The number of probes ranges from 2 to 5, and all of them can scan in turn each of 43‐64 sleeves. KWU. A pneumatic ball system is used to scan the neutron flux density in the German KWU reactors. The system activates balls containing vanadium in 28 dry sleeves placed in the core. The balls are put in and removed under gas pressure. The activity of the balls is calculated outside the reactor vessel [2]. The neutron flux density in each stationary detector (fission chamber or emission detector) is reconstructed according to the indications of the probes or activity of the balls and the sensitivity is determined for the measurement time. The scan results can also be used to periodically monitor the error in the reconstruction of the neutron flux density distribution in the core. g-Sensitive Thermocouples for BWR. In BWR vessel reactors, thermocouples measuring the temperature differential between the sensitive element (heated by γ-radiation from the core) and the detector shell (cooled by the coolant) are being used once again to extend the service life of in-reactor detectors. To measure the γ-radiation dose rate, it is necessary to know its relation to the temperature difference, which depends on the thermal conductivity of the material (the thermal conductivity of the material changes as the fluence builds up) separating the heated elements and the detector casing.