Reactor Noise Techniques Research Articles

Delayed neutron effect in time-domain fluctuation analyses of neutron detector current signals

In the previous study, the theoretical formulae of two time-domain reactor noise techniques, i.e., the auto-covariance and the variance-to-mean function techniques, were derived (Kitamura et al., 2018). These two techniques analyse the temporal fluctuation in continuous neutron detector current signals arising from ionization chambers such as the fission chamber. They are hence inherently insensitive to the count-loss effect that sometimes brings serious difficulties to the conventional time-domain techniques that analyse the number of detector pulse signals. With regard to these two techniques, the experimental conditions under which they successfully measure the subcriticality through determination of the prompt neutron decay constant were clarified. However, for mathematical simplicity and the sake of insight, the previous study was performed on the basis of a theoretical model neglecting delayed neutrons. The formulae of these two techniques are hence re-derived by taking delayed neutrons into consideration. Using the formulae thus derived, the delayed neutron effect in these two techniques is discussed.

Determination of prompt neutron decay constant by time-domain fluctuation analyses of detector current signals

The conventional time-domain reactor noise techniques analyse the number of neutron detector pulse signals, so that they sometimes encounter serious difficulties owing to the count-loss effect due to the dead time of detector systems. To avoid all the difficulties coming from the count-loss effect, a novel time-domain technique was recently proposed (Pál and Pázsit, 2015). This technique analyses the auto-covariance function of continuous current signals arising from ionization chambers such as the fission chamber, so that it is inherently insensitive to the count-loss effect. In the present study, a different time-domain technique that analyses the integral values of current signals is proposed. With regard to these two techniques, the experimental conditions under which they successfully measure the subcriticality through determination of the prompt neutron decay constant are clarified.

On the limitations of linear power reactor noise analysis: A point kinetics approach

A novel method is introduced which allows the higher order perturbative solution of a linear operator with a time variant coefficient. This method employs a form of raising and lowering operators which generate higher and lower order harmonics from a given harmonic. The analysis has been applied to the point kinetics equations with a monotone oscillatory reactivity to place bounds on the relative error of the linearization method frequently employed in the power reactor noise techniques. As a result, the maximum permissible reactivity amplitude for a given reactor as a function of frequency has been obtained such that regular power reactor noise methods remain accurate enough. Three benchmarks in subcritical and critical configurations show the accuracy of this method.

The Reactor Noise Threshold

Time-domain noise analysis techniques such as the Rossi-[alpha], the variance-to-mean, and the interval-distribution methods can be used to measure fundamental reactor parameters in a wise variety of reactor systems, provided the power level of the system is not too high. Simple expressions have been derived that define the maximum power level (i.e., the reactor noise threshold) above which time-domain reactor noise techniques are likely to fail in subcritical, critical, and supercritical systems.

An application of reactor noise techniques to neutron transport problems in a random medium

Neutron transport problems in a random medium are considered by defining a joint Markov process describing the fluctuations of neutron population and the random changes in the medium. Backward Chapman-Kolmogorov equations are derived which yield an adjoint transport equation for the average neutron density. It is shown that this average density also satisfies the direct transport equation as given by the phenomenological model.

Reactor noise standards in the U.S.A.

The American Society of Mechanical Engineers (ASME) has fostered the development of codes and standards to be used by all facets of the general industry. The ASME Committee on Operation and Maintenance of Nuclear Power Plant (O&M Committee) currently has responsibility of 20 ASME nuclear power standards addressing areas such as “Requirements for Inservice Performance Testing of Pressure Relief Devices” (OM-01) to “Inservice Testing of Diesel Drives in Nuclear Power Stations” (OM-16). The Subcommittee on Vibration Monitoring (SCVM) is responsible for several O&M Committee standards, two of which utilize reactor noise techniques: OM-05, “Inservice Monitoring of Core Support Axial Preload in Pressurized Water Reactors”, and OM-12, “Vibration Monitoring and Diagnostics of Light Water Reactor Loose Parts”. This paper presents an outline of the O&M Committee standards and briefly describes the highlights of the two SCVM standards which utilize Reactor Noise techniques.

Observation of in-core instrument tube vibrations in a boiling water reactor by evaluating reactor noise data

Using reactor noise techniques, a vibrating in-core instrument tube in the Mühleberg Boiling Water Reactor could be detected. The normalized auto power spectral density (NPSD) of the current fluctuations of a mini in-core chamber, fixed within the instrument tube, has been analyzed before and after repair. There was a strong evidence of a peak at the resonance frequency of the vibration at 2.6 Hz in the NPSD. It vanished promptly after the bypass-streaming at the lower core plate, being the physical origin of the vibrating instrument tube, was re-arranged.

A study of two-phase flow characteristics using reactor noise techniques

Recent work at the University of Washington has demonstrated the potential for studying two-phase flow characteristics using neutron noise techniques. This method offers the advantage of measuring global characteristics of two phase phenomenon without disturbing the flow or requiring large numbers of sensors. Not only can the presence of the injected air into the flow loops be seen in the neutron spectrum but also the neutron spectrum reflects the type of flow by producing a characteristic signature.

Survey of recent applications of reactor noise techniques at Argonne National Laboratory

Recent applications of reactor noise techniques to the improvement of subcritical reactivity and β eff measurements in fast critical assemblies and to fast power reactor surveillance by neutron/acoustic noise cross correlation are reviewed. The Inverse Kinetics Rod Drop (IKRD) technique has been the primary subcriticality measurement technique applied at ANL because of its relative insensitivity to counter efficiency and prompt lifetime changes and the fact that a near-critical reference is not needed. Estimates of the random errors in IKRD reactivity measurements resulting from noise effects have been developed by simulation studies and theoretical analysis. Relative standard deviations of 1–20% are indicated by these methods for initial Δk from −0.0005 to −0.035, respectively, under conditions typical of the Fast Test Reactor (FTR) mockup experiments in the ZPR-9 Fast Critical Facility. Space-dependent kinetics calculations have been applied to the IKRD technique to study systematic errors that are functions of detector location as well as changes in kinetics parameters and effective source strength in the course of a rod drop. In various situations, systematic errors from these causes may be as low as several parts per hundred for detectors located outside the axial reflector due to cancellation of errors. For other detector locations, the principal error components might be in the same direction, thereby amplifying the resultant error. Noise measurements have been used as cross checks on IKRD results. The break frequency derived from cross power spectral density or polarity correlation has been found not to be a good reactivity estimator in fast power reactor critical assemblies presumably because wide disparities between neutron lifetimes in the core and reflector produce deviations from point kinetics in the range of the break frequency. Various two-detector mid-frequency polarity coherence function methods have produced good reactivity estimates, although these methods depend on near-critical reference measurements and are perturbed by differences in detector efficiency between the reference and measured systems. Correction methods to account for detector efficiency and β eff changes have been developed. The accuracy of fast critical experiment measurements of Δk k are directly affected by the accuracy with which β eff is known. Recent conflicting experimental data on β i , λ i has been the incentive for use of a noise-based integral β eff measurement technique. The technique is a modified detector noise variance-to-mean method using sampling intervals from 8 to 200 sec; β eff has been measured to a precision of ±2.9% in the FTR mockup critical assembly. Exploratory work on the use of neutron/acoustic noise cross correlation as a tool for surveillance of fast power reactor systems has been carried out using the Experimental Breeder Reactor II. Cross correlation of neutron signals from in-core detectors with acoustic signals from accelerometers mounted on various mechanical components connected into the core structure has been studied up to several thousand hertz. Flow-induced subassembly vibrations at about 10 hertz have been observed.

Canadian experience in the area of reactor noise

This report outlines the reactor noise and dynamics program and reviews some Canadian experience in applying reactor noise measurement techniques to the development of the CANadian Deuterium Uranium (CANDU) pressure tube reactor system. Our general objective is to develop technical competence in the area of reactor noise and dynamics and apply it to the further development of the CANDU reactor system. To achieve this objective, work is directed towards the following areas: Development Application Expansion Advancement. Application of reactor noise analysis techniques in the CANDU-BLW power reactor, Gentilly-1, is discussed since the work has demonstrated the role of noise measurements in commercial stations. Gentilly-1 with Boiling Light Water (BLW) coolant in the channels and a positive void feedback provided an ideal opportunity for assessment of the use of reactor noise techniques in power reactor systems. Consequently programs using both (1) inherent and (2) imposed excitation were carried out. Results from both programs are presented and their significance in satisfying program objectives is discussed.

Kinetic Parameters in a Plutonium Lattice Containing Water Regions

Reactor noise techniques have been used to determine how the quantity ²eff/I changes when water regions of several sizes and shapes are introduced into a light water lattice of plutonium-enriched r...

Precision of Shutdown Margin Measurements Using the Two-Frequency Reactor Noise Technique in an LMFBR

The precision of reactor noise techniques for measuring subcritical reactivity can be related to detector efficiency and parameters of the measuring instrumentation. Using a two-detector cross-correlation technique, we have determined that such measurements can be made in a certain fast reactor (the FFTF) to -3 to -5 dollars with acceptable statistics and measurement times.