Theory of neutron fluctuations with applications in nuclear safeguards and noise diagnostics in research and power reactors

IMRE PÁZSIT
Abstract: Neutron fluctuations in both low power systems (“zero power reactor noise”) [1], [2] and power reactors (“power reactor noise”) [2], [3] carry important information about the system. The origin of the fluctuations is different in these two regimes, and consequently, they are described by different mathematical methods, as well as that they have different characteristics and application areas. At low power (critical assemblies and research reactors) the fluctuations are due to the branching process (multiple generation of several neutrons in fission); in power reactors it is the technological processes of random character, such as boiling of the coolant, vibration of mechanical structures etc., which induce the neutron noise. Common to them is that neutron fluctuations can be used to diagnose the state of the system, either by determining some of its parameters (reactivity coefficients, void content etc.), or by detecting beginning anomalies (excessive vibrations, flow anomalies etc.).
In this report the principles and selected applications of both zero power noise and the power reactor noise will be given. The current significance of research in power reactors is also indicated by some on-going collaborations, such as the large EU-supported co-ordinated research project, CORTEX, whose goal is the development and verification of core diagnostic methods [4], and the planned noise diagnostic option of the US DoE supported Versatile Test Reactor (VTR) Project [5]. A particular aspect of these projects is that the development and verification of calculational and experimental methods for power reactors is supported by dedicated experiments and measurements in zero-power and research reactors. In these latter, the perturbations need to be generated artificially; on the other hand, unlike in power reactors, determination of the parameters of the noise source as well as measurements of the induced neutron noise can be performed simultaneously under controlled circumstances.
Keywords: neutron noise, reactor diagnostics, safeguards, research reactors, power reactors.