Publications by Oskari Ville Pakari

Welch Method and Bootstrapping Applied to Subcritical Gamma Noise

Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand

We measured the prompt neutron decay constant

\alpha

alpha of the CROCUS zero-power reactor at the Swiss Federal Institute of Technology Lausanne using cross-power spectral density (CPSD) analysis of gamma-gamma correlations from two trans-stilbene org ...

TAYLOR & FRANCIS INC2025

Development of an Event Tracking Feature in OpenMC for Neutron Spectroscopy and Scatter Camera Systems

Andreas Pautz, Oskari Ville Pakari

This paper presents the development and application of an event tracking feature within the OpenMC Monte Carlo code, designed for advanced neutron spectroscopy and scatter camera system simulations. The newly implemented feature enables the tracking of ind ...

American Nuclear Society2025

CORTEX experiments – Part I: Modulation campaigns in AKR-2 & CROCUS for the validation of neutron noise codes

Nicolas Weiss, Daniel Godat, Pavel Frajtag, Andreas Pautz, Adolfo Rais, Oskari Ville Pakari, Vincent Pierre Lamirand, Mathieu Hursin, Fanny Vitullo, Thomas Jean-François Ligonnet, Axel Guy Marie Laureau

We present the experimental campaigns – namely, three per facility – carried out between 2018 and 2021 in the AKR–2 and CROCUS zero power reactors within the framework of the Horizon 2020 European project CORTEX. Their purpose was to produce high-quality a ...

2025

Measurement of the Prompt Neutron Decay Constant in a Zero-Power Reactor Using a Novel 3D Detector System

Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand, Mathieu Hursin, Fanny Vitullo

Analyzing neutron noise in zero-power reactors provides information about integral parameters, such as the prompt decay constant

\alpha

alpha. This information can be used to validate codes that predict said parameters, or time-dependent behavior, part ...

TAYLOR & FRANCIS INC2025

Comparison of high-fidelity transport code predictions against the NECTAR intra-pin reaction rate measurements

Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand, Mathieu Hursin, Thomas Jean-François Ligonnet, Tom Mager

The advancement of high-fidelity nuclear reactor simulation methods is essential for reducing conservatism in safety margins and optimizing reactor design and operation. This work evaluates the predictive capabilities of the deterministic solvers GeN-Foam ...

2025

Gamma noise to non-invasively monitor nuclear research reactors

Pavel Frajtag, Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand, Tom Mager

Autonomous nuclear reactor monitoring is a key aspect of the International Atomic Energy Agency’s strategy to ensure nonproliferation treaty compliance. From the rise of small modular reactor technology, decentralized nuclear reactor fleets may strain the ...

Nature Research2024

Gamma-ray Spectroscopy in Low-Power Nuclear Research Reactors

Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand

Gamma-ray spectroscopy is an effective technique for radioactive material characterization, routine inventory verification, nuclear safeguards, health physics, and source search scenarios. Gamma-ray spectrometers typically cannot be operated in the immedia ...

MDPI2024

CORTEX experiments, Part II: Postprocessing of neutron noise time series to determine reliable mean and uncertainties

Andreas Pautz, Adolfo Rais, Oskari Ville Pakari, Vincent Pierre Lamirand, Mathieu Hursin, Axel Guy Marie Laureau

In this paper we present the methodology developed to analyze the detector response of neutron noise experiments performed at EPFL CROCUS and TUD AKR-2 reactors in the framework of the H2020 CORTEX project. The noise, or modulation, is induced by controlle ...

2024

Generative AI tools in the nuclear engineering community: A survey-based evaluation of the current adoption and impacts

Alessandro Scolaro, Oskari Ville Pakari

Generative AI tools (often abbreviated genAI or GAI), such as chatGPT, have arguably great potential to induce a paradigm shift in knowledge work.Herein, we present the results of a survey conducted within the nuclear engineering professional community to ...

American Nuclear Society2024

Fast simulation of neutron noise using the Transient Fission Matrix approach and validation on the CROCUS reactor

Oskari Ville Pakari, Vincent Pierre Lamirand

This article presents the development of a neutron noise calculation technique with a reduced calculation time based on the TFM (Transient Fission Matrix) approach. The latter contains different information such as the system transfer function (fission and ...

EDP Sciences2024

Intra-pin Reaction Rate Measurements in the CROCUS Experimental Reactor

Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand, Mathieu Hursin, Thomas Jean-François Ligonnet, Tom Mager

In response to the need for validating high-fidelity deterministic neutronics solvers capable of providing pin-resolved neutron flux distributions, intra-pin reaction rates measurements were performed in the CROCUS experimental reactor. A specially designe ...

2024

Neutron-Gamma Noise Measurements in a Zero-Power Reactor Using Organic Scintillators

Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand

Noise measurements in light water reactor systems aid in generating validation data for integral point kinetic parameter predictions and generating monitoring parameters for reactor safety and safeguards. The CROCUS zero-power reactor has been used to supp ...

Ieee-Inst Electrical Electronics Engineers Inc2024

Measurement of the Prompt Decay Constant in a Zero Power Reactor Using a Novel 3D Detector System

Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand

Analyzing neutron noise in zero power reactors is a valuable tool for gaining insights into integral parameters, such as the prompt decay constant α. This information can be used to validate codes that predict said parameters, or time-dependent behavior, p ...

American Nuclear Society2024

Millimetric fuel rod displacements: an experimental study of the impact on local thermal neutron flux in the CROCUS reactor

Daniel Godat, Pavel Frajtag, Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand, Fanny Vitullo, Thomas Jean-François Ligonnet

The impact on the thermal neutron flux due to a few mm displacement of a single fuel rod in the periphery lattice of the CROCUS zero-power reactor was measured. These high-resolution experiments were carried out using a miniature neutron detector positione ...

American Nuclear Society2022

High accuracy measurement of the prompt neutron decay constant in CROCUS using Gamma Noise and bootstrapped uncertainties

Pavel Frajtag, Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand, Tom Mager

We experimentally demonstrate the advantages of gamma detection for noise measurements to deter-mine the prompt neutron decay constant a of a nuclear reactor using the power spectral density (PSD) method, coupled to a new uncertainty estimation scheme base ...

PERGAMON-ELSEVIER SCIENCE LTD2022

Towards the Validation of Noise Experiments in the CROCUS Reactor Using the TRIPOLI-4 Monte Carlo Code in Analog Mode

Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand

Intrinsic neutron noise experiments offer a non-invasive manner to measure the prompt decay constant or reactivity of fissile systems. Using the fluctuations in the density of fission chains, one can infer the kinetics parameters via correlation analysis s ...

2021

Analysis of the first COLIBRI fuel rods oscillation campaign in the CROCUS reactor for the European project CORTEX

Daniel Godat, Pavel Frajtag, Andreas Pautz, Adolfo Rais, Oskari Ville Pakari, Vincent Pierre Lamirand, Mathieu Hursin, Carlo Fiorina, Axel Guy Marie Laureau

The Horizon2020 European project CORTEX aims at developing an innovative core monitoring technique that allows detecting anomalies in nuclear reactors, such as excessive vibrations of core internals, flow blockage, or coolant inlet perturbations. The techn ...

2021

Validation of axial void profile measured by neutron noise techniques in CROCUS

Pavel Frajtag, Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand, Mathieu Hursin

Recently a joint project has been carried out between the Paul Scherrer Institut, the Ecole Polytechnique Federale de Lausanne and swissnuclear, an industrial partner, in order to determine the axial void distribution in a channel installed in the reflecto ...

2021

CORE SIM+ simulations of COLIBRI fuel rods oscillation experiments and comparison with measurements

Daniel Godat, Paolo Vinai, Pavel Frajtag, Andreas Pautz, Adolfo Rais, Oskari Ville Pakari, Vincent Pierre Lamirand, Mathieu Hursin, Carlo Fiorina, Axel Guy Marie Laureau

At EPFL, the CROCUS reactor has been used to carry out experiments with vibrating fuel rods. The paper presents a first attempt to employ the measured data to validate CORE SIM+, a neutron noise solver developed at Chalmers University of Technology. For th ...

2021

First in-core gamma spectroscopy experiments in a zero power reactor

Pavel Frajtag, Andreas Pautz, Oskari Ville Pakari, Vincent Pierre Lamirand, Axel Guy Marie Laureau

Gamma rays in nuclear reactors, arising either from nuclear reactions or decay processes, significantly contribute to the heating and dose of the reactor components. Zero power research reactors offer the possibility to measure gamma rays in a purely neutr ...

E D P SCIENCES2021