Energy Correlation of Prompt Fission Neutrons

In all cases where neutron fluctuations in a branching process (such as in multiplicity measurements) are treated in an energy dependent description, the energy correlations of the branching itself (energy correlations of the fission neutrons) need to be known. To date, these are not known from experiments. Such correlations can be theoretically and numerically derived by modelling the details of the fission process. It was suggested earlier that the fact that the prompt neutrons are emitted from the moving fission targets, will influence their energy and angular distributions in the lab system, which possibly induces correlations. In this paper the influence of the neutron emission process from the moving targets on the energy correlations is investigated analytically and via numerical simulations. It is shown that the correlations are generated by the random energy and direction distributions of the fission fragments. Analytical formulas are derived for the two-point energy distributions, and quantitative results are obtained by Monte-Carlo simulations. The results lend insight into the character of the two-point distributions, and give quantitative estimates of the energy correlations, which are generally small

Comments on the stochastic characteristics of fission chamber signals

This paper reports on theoretical investigations of the stochastic properties of the signal series of ionisation chambers, in particular fission chambers. First, a simple and transparent derivation is given of the higher order moments of the random detector signal for incoming pulses with a non homogeneous Poisson distribution and random pulse heights and arbitrary shape. Exact relationships are derived for the higher order moments of the detector signal, which constitute a generalisation of the so-called higher order Campbelling techniques. The probability distribution of the number of time points when the signal exceeds a certain level is also derived. Then, a few simple pulse shapes and amplitude distributions are selected as idealised models of the detector signals. Assuming that the incoming particles form a homogeneous Poisson process, explicit expressions are given for the higher order moments of the signal and the number of level crossings in a given time interval for the selected pulse shapes

Neutron monitoring based on the higher order statistics of fission chamber signals

The work in this thesis corresponds to the safety aspect of Generation IV nuclear systems. One of the safety aspects concerns the enhancement of the performance of the in-vessel on-line core monitoring with neutron flux measurements. It was concluded earlier that fission chambers are the best candidate to provide in-vessel measurements in sodium cooled fast reactors. This thesis focuses on the performance of signal processing methods in order to unfold the count rate of fission chambers. The main goal is to investigate the possible application of processing methods based on the higher order statistics of the signal in order to provide accurate count rate estimation over a wide range both for stationary and transient signals. The work also consists of the study of self-monitoring capabilities in order to detect fission chamber malfunctions at an early stage. The investigation is based on analytic assessments, on simulation of fission chamber responses and signals, and on experimental application of processing methods.The thesis covers five main studies. The first part presents the theoretical description of fission chamber signals. The second part investigates the performance of the traditionally applied methods (pulse and Campbelling mode) through simulations. It is shown that these methods are not capable to cover the whole count rate range of the chamber. Therefore, the third part studies the possible application of methods based on higher order statistics of the signal through simulations and experiments. It is shown that these methods can provide accurate estimations over a wide count rate range. The fourth part covers the theoretical background of self-monitoring capabilities based on the spectral properties of the signal. Finally, the fifth part presents the implementation of the methods in a real-time neutron monitoring system based on a System on a Chip, which embeds a field-programmable gate array.By the methods elaborated in this thesis, a faster, more effective and more accurate monitoring of the reactor power is possible than with the methods used so far, even when the normal operating state is changing

pyFC: a TRIM-based fission chamber pulse shape simulator

This report presents the code system pyFC (python-based simulation of Fission Chambers) which simulates the pulse creation in fission chambers. The current pulses in a fission chamber are generated due to the ionization of the filling gas by the heavy ion emitted from the neutron induced fission in the fissile deposit. In pyFC the path of the heavy ion and the spatial distribution of the charges emerging from the ionization process are simulated with the TRIM code, and the parameters of the charge collection between the electrodes are computed with the BOLSIG software. The coupling of the codes is done in Python.The report presents the physical and geometrical considerations implemented in pyFC and the verification of the code through comparison with the results of Chester, a CEA code for simulation of fission chambers

pyFC: a TRIM-based fission chamber pulse shape simulator

This report presents the code system pyFC (python-based simulation of Fission Chambers) which simulates the pulse creation in fission chambers. The current pulses in a fission chamber are generated due to the ionization of the filling gas by the heavy ion emitted from the neutron induced fission in the fissile deposit. In pyFC the path of the heavy ion and the spatial distribution of the charges emerging from the ionization process are simulated with the TRIM code, and the parameters of the charge collection between the electrodes are computed with the BOLSIG software. The coupling of the codes is done in Python.The report presents the physical and geometrical considerations implemented in pyFC and the verification of the code through comparison with the results of Chester, a CEA code for simulation of fission chambers

Neutron monitoring based on the higher order statistics of fission chamber signals

The work in this thesis corresponds to the safety aspect of Generation IV nuclear systems. One of the safety aspects concerns the enhancement of the performance of the in-vessel on-line core monitoring with neutron flux measurements. It was concluded earlier that fission chambers are the best candidate to provide in-vessel measurements in sodium cooled fast reactors. This thesis focuses on the performance of signal processing methods in order to unfold the count rate of fission chambers. The main goal is to investigate the possible application of processing methods based on the higher order statistics of the signal in order to provide accurate count rate estimation over a wide range both for stationary and transient signals. The work also consists of the study of self-monitoring capabilities in order to detect fission chamber malfunctions at an early stage. The investigation is based on analytic assessments, on simulation of fission chamber responses and signals, and on experimental application of processing methods.The thesis covers five main studies. The first part presents the theoretical description of fission chamber signals. The second part investigates the performance of the traditionally applied methods (pulse and Campbelling mode) through simulations. It is shown that these methods are not capable to cover the whole count rate range of the chamber. Therefore, the third part studies the possible application of methods based on higher order statistics of the signal through simulations and experiments. It is shown that these methods can provide accurate estimations over a wide count rate range. The fourth part covers the theoretical background of self-monitoring capabilities based on the spectral properties of the signal. Finally, the fifth part presents the implementation of the methods in a real-time neutron monitoring system based on a System on a Chip, which embeds a field-programmable gate array.By the methods elaborated in this thesis, a faster, more effective and more accurate monitoring of the reactor power is possible than with the methods used so far, even when the normal operating state is changing

Energy correlations of prompt fission neutrons in the laboratory frame

Correlations between the energies and emission angles of prompt fission neutrons are of significance for all methods which use the statistics of detection events for determining subcritical reactivity in reactor cores or for non-destructive assay of nuclear materials for safeguards purposes. There is no experimental knowledge available on the existence or properties of such correlations. Therefore, recently increasing attempts are made to determine these correlations from the properties of the fission process. One possible reason of such correlations between fission neutron energies and angles in the laboratory system is the fact that the prompt neutrons are emitted from the moving fission targets, even if their energies and emission angles are independent in the moving frame of the fission fragment. In this paper this concept is investigated analytically and through numerical simulations. It is shown that such correlations are due to the random properties (energy and direction of motion) of the fission fragments, and the magnitude of the covariance depends on the second order moments of the fission fragment parameters. Preliminary numerical simulations show that the correlations in energy, generated this way, are rather small

The open-source toolbox of the nuclear safeguards data scientist

The poster presentation describes development of an open-source toolkit for the data scientist working in the field of nuclear safeguards. The work was presented at the Technical Meeting on Artificial Intelligence for Nuclear Technology and Applications, 25 - 29 October 2021. The key features, capabilities and the limitations of the toolkit were presented to the IAEA audience with main emphasis on the need for development of open-source toolkits and datasets for use by the wider scientific community

The open-source toolbox of the nuclear safeguards data scientist

The poster presentation describes development of an open-source toolkit for the data scientist working in the field of nuclear safeguards. The work was presented at the Technical Meeting on Artificial Intelligence for Nuclear Technology and Applications, 25 - 29 October 2021. The key features, capabilities and the limitations of the toolkit were presented to the IAEA audience with main emphasis on the need for development of open-source toolkits and datasets for use by the wider scientific community

Energy correlations of prompt fission neutrons in the laboratory frame

Correlations between the energies and emission angles of prompt fission neutrons are of significance for all methods which use the statistics of detection events for determining subcritical reactivity in reactor cores or for non-destructive assay of nuclear materials for safeguards purposes. There is no experimental knowledge available on the existence or properties of such correlations. Therefore, recently increasing attempts are made to determine these correlations from the properties of the fission process. One possible reason of such correlations between fission neutron energies and angles in the laboratory system is the fact that the prompt neutrons are emitted from the moving fission targets, even if their energies and emission angles are independent in the moving frame of the fission fragment. In this paper this concept is investigated analytically and through numerical simulations. It is shown that such correlations are due to the random properties (energy and direction of motion) of the fission fragments, and the magnitude of the covariance depends on the second order moments of the fission fragment parameters. Preliminary numerical simulations show that the correlations in energy, generated this way, are rather small