Introduction (Surveillance and Diagnostics of Next Generation Nuclear Reactors)

Kapitel i bok, 2025

Reactor diagnostics, in particular diagnostics based on neutron noise analysis, has been developed in the early 1960s and 1970s, and its application has become increasingly widespread to traditional light water and heavy water reactors [1 - 8]. These are the most commonly used commercial plants, belonging to Generation II (Gen-II). Experience shows the significant benefits of reactor diagnostics in terms of determining operational parameters in the normal and abnormal state, and in the detection, identification and quantification of incipient failures at an early stage. There are two relatively different areas of neutron noise diagnostics. At low power, such as in an educational or a research reactor, or in a power reactor during startup, the sources of neutron fluctuations of technological origin (vibration of core internals, boiling of the coolant, etc.) are absent, and the cause of the neutron fluctuations is solely the branching process, i.e. the neutron multiplication process in a constant medium. These types of neutron fluctuations are called zero-power reactor noise, and they are used mostly to determine the subcritical reactivity of the system while it is still driven by an external neutron source. The other area, called power reactor noise diagnostics, utilises the neutron fluctuations caused by processes of technological origin, for example, movement of control rods, fuel assemblies, two-phase flow, etc., called perturbations or noise sources. These processes are inherently random, and their effect will be to induce spatial and temporal fluctuations of the neutron flux, which is called power reactor noise. If the neutronic transfer between the cross-section fluctuations, represented by the perturbation at a certain point of the core and the induced neutron noise at some distance (at the detector position), is understood and can be quantified, then there is a chance that by analysing the detector response and in possession of the transfer function, one can detect, identify and quantify the noise source. The noise source/perturbation can either be present already in normal operation (such as two-phase flow in a boiling water reactor (BWR)) or may be due to an incipient failure, such as vibrations due to a mechanical failure. Power reactor diagnostics can be used to determine parameters of the processes already present (such as two-phase flow parameters), determine reactivity coefficients, detect changes in the value of these parameters and detect noise sources due to incipient failures at an early stage.