EXPERIMENTAL DETERMINATION OF THE ZERO POWER TRANSFER FUNCTION OF THE AKR-2

The transfer function is a basic characteristic of every nuclear reactor. It describes how a perturbation at a given place and time influences the neutron flux. In case of a known perturbation, the determination of characteristic reactor parameters is possible. The present paper shows an experimental method to determine the gain of the zero-power reactor transfer function (ZPTF) of the AKR-2 reactor at TU Dresden and the comparison to the theoretical shape of the ZPTF derived from kinetic parameters simulated with MCNP. For the experiments, a high-precision linear motor axis is used to insert an oscillating perturbation acting at frequencies smaller than the lower bound of the plateau region of the ZPTF. For higher frequencies, a rotating absorber is used. This device emulates an absorber of variable strength. The reactor response is detected with a He-3 counter. The data evaluation shows good agreement between measured and corresponding theoretical values of the gain of the ZPTF.

Review of kinetic modulation experiments in low power nuclear reactors

The safety improvement of nuclear reactors requires continuous efforts in understanding the fundamental physical quantities related to the fission process. In neutronic models, the reactor dynamics is covered by the kinetic parameters to characterize the temporal behavior of the neutron population subject to perturbations. The reactor transfer function is a frequency domain analogy of this temporal description. It can be measured experimentally through transfer function analysis via noise analysis or kinetic modulation, for the study of reactor stability and kinetic parameters. This paper summarizes the experimental measurements of reactor transfer function through kinetic modulation. Extensive work have been conducted experimentally, starting from the beginning of reactor physics research. An overview is given regarding various experimental designs and conducted analyses. The concepts of the modulation system are also discussed. The current work is limited to online contents and internal archives of CEA Cadarache due to difficulties in accessing references traced back to 1950s.

An Advanced Frequency-Domain Code for BWR Stability Analysis and Design

Density Wave Oscillations in BWRs are coupled with the reactor kinetics. A new analytical, frequency-domain tool that uses the best available models and methods for modeling BWRs and analyzing their stability is described. The steady state of the core is obtained first in 3D with two-group diffusion equations and spatial expansion of the neutron fluxes in Legendre polynomials. The time-dependent neutronics equations are written in terms of flux harmonics (nodal-modal equations) for the study of “out-of-phase” instabilities. Considering separately all fuel assemblies divided into a number of axial segments, the thermal-hydraulic conservation equations are solved (drift-flux, non-equilibrium model). The thermal-hydraulics are iteratively fully coupled to the neutronics. The code takes all necessary information from plant files via an interface. The results of the steady state are used for the calculation of the transfer functions and system transfer matrices using extensively symbolic manipulation software (MATLAB). The resulting very large matrices are manipulated and inverted by special procedures developed within the MATLAB environment to obtain the reactor transfer functions that enable the study of system stability. Applications to BWRs show good agreement with measured stability data.

Localization of a Noise Source in VVER-1000 Reactor Core Using Neutron Noise Analysis Methods

In this paper, localization of a noise source from limited neutron detectors sparsely distributed throughout the core of a typical VVER-1000 reactor is investigated. For this purpose, developing a 2-D neutron noise simulator for hexagonal geometries based on the 2-group diffusion approximation, the reactor dynamic transfer function is calculated. The box-scheme finite difference method is first developed for hexagonal geometries, to be used for spatial discretisation of both 2-D 2-group static and noise diffusion equations. The dynamic state is assumed in the frequency domain which leads to discarding of the time disrcetisation. The developed 2-D 2-group neutron noise simulator calculates both the discretised forward and the adjoint reactor transfer function between a point-like source and its induced neutron noise, by assuming the noise source as an absorber of variable strength type. Benchmarking of the mentioned neutron noise simulator revealed that it works satisfactorily. Finally, by using the inversion method of reconstruction, the location and values of a noise source of the type absorber of variable strength (or reactor oscillator) in VVER-1000 reactor cores are determined. Accuracy of this method is highly acceptable.