Since most diagnostic tasks are based on the inversion of the reactor transfer function, the prior estimation of this transfer function is required. The first project’s strategic objective is to develop simulation tools that will be dedicated to modelling the effect of stationary fluctuations in power reactors with a high level of fidelity. This includes the following specific objectives.

  • To develop modelling capabilities that allow the determination of the fluctuations in neutron flux resulting from known perturbations applied to the system. Existing low-order computational capabilities will be consolidated and extended. Simultaneously, advanced methods based on deterministic neutron transport and on probabilistic (i.e. Monte Carlo) methods will be developed so that the transfer function of a reactor core can be estimated with a high resolution in space, angle and energy.
  • To classify the possible types of perturbations that may exist in commercial nuclear reactors – such perturbations need to be expressed as either fluctuations of macroscopic cross-sections (based on expert opinion) or in more physical terms. In the latter case, emphasis will be put on developing models that reproduce vibrations of reactor vessel internals due to Fluid-Structures Interactions (FSIs), and on determining the corresponding noise source in terms of fluctuations in macroscopic cross-sections.
  • To evaluate the uncertainties associated with the reactor transfer function estimation and to perform sensitivity analyses targeted at neutron noise calculations.

Although the tools that allow to estimate the reactor transfer function can be verified against analytical or semi-analytical solutions for simple systems and configurations, validation using reactor experiments specifically designed for noise analysis applications is essential. The project will target the following objectives.

  • To set up and perform neutron noise measurements corresponding to a so-called absorber of variable strength. This will allow the validation of the neutronic modelling tools and of the uncertainty analysis framework developed in the project.
  • To set up and perform neutron noise measurements corresponding to a so-called vibrating absorber. This will allow the validation of the structural mechanics modelling tools developed in the project.
  • To develop and test a new type of detector that allows to measure the scalar neutron flux and the neutron current. Recovering higher moments of the angular neutron flux gives additional information about the gradient of the neutron density, which itself can be used to better localise possible perturbations.

The modelling capabilities for reactor transfer function estimation and the advanced signal analysis techniques need to be combined into a set of tools that can be used directly for analysing plant data and performing core diagnostics. Being able to identify and characterise anomalies before they have an adverse effect on plant availability is of utmost importance. More specifically, the following objectives will be targeted.

  • To develop and use advanced signal processing that extracts the relevant and meaningful fluctuations from measured signals. Emphasis will be put on how to deal with non-stationary and intermittent signals.
  • To develop and use machine learning data analysis methodologies for inverting the reactor transfer function and recovering the anomaly responsible for the observed fluctuations. Emphasis will be put on situations where the in-core and ex-core instrumentation is very scarce.

Most of the diagnostic tasks require an inversion or unfolding procedure, i.e. a backtracking of the driving perturbation (not measurable) from the induced neutron noise (measurable at some discrete locations throughout the core). Advanced signal analysis and machine learning techniques will be combined with reactor transfer function modelling capabilities to perform an inversion. The following objectives will be targeted.

  • Using actual plant data, to demonstrate the applicability, usefulness and importance of the proposed and developed methodology to the nuclear industry. The possibility of classifying the detected anomalies depending on their safety impact will be emphasised. A large variety of reactor types will be used. In addition, data
    (either simulated or from actual measurements) with known “anomalies” will provide the opportunity to test the methods.
  • To make recommendations about in-core and ex-core instrumentation that maximise the applicability of the techniques developed. This not only includes recommendations about the number and location of detectors, but also about possible new types of detectors better suited for core diagnostics.
  • To disseminate the knowledge gathered in the project to relevant stakeholders in the nuclear industry, so that the methodology can be retrofitted to the current fleet of nuclear reactors and used in future units.

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