Pauli Peura

Bio: Pauli Peura is an academic researcher from Helsinki Institute of Physics. The author has contributed to research in topics: Neutron & Neutron moderator. The author has an hindex of 4, co-authored 7 publications receiving 27 citations. Previous affiliations of Pauli Peura include University of Helsinki.

Effect of Gamma-Ray Energy on Image Quality in Passive Gamma Emission Tomography of Spent Nuclear Fuel

Abstract: Gamma-ray images of VVER-440 and SVEA-96 spent nuclear fuel assemblies were reconstructed using the filtered backprojection algorithm from measurements with a passive gamma emission tomography prototype instrument at Finnish nuclear power plants. Image quality evaluation criteria based on line profiles through the reconstructed image are used to evaluate image quality for spent fuel assemblies with different cooling times, and thus different mixtures of gamma-ray emitting isotopes. Image characteristics at the locations of water channels and central fuel pins are compared in two gamma-ray energy windows, 600–700 and >700 keV, for cooling times up to 10 years for SVEA-96 fuel and 24.5 years for VVER-440 fuel. For SVEA-96 fuel, images in the >700-keV gamma-ray energy window present better water-to-fuel contrast for all investigated cooling times. For VVER-440, images in the >700-keV gamma-ray energy window have higher water-to-fuel contrast up to and including a cooling time of 18.5 years, whereas the water-to-fuel contrast of the images taken in the two gamma-ray energy windows is equivalent for a cooling time of 24.5 years. Images reconstructed from higher energy gamma rays such as those in the >700-keV energy window present better water-to-fuel contrast in fuel cooled for up to 20 years and thus have the most potential for missing fuel pin detection.

Measuring spent fuel assembly multiplication in borated water with a passive neutron albedo reactivity instrument

Abstract: The performance of a passive neutron albedo reactivity (PNAR) instrument to measure neutron multiplication of spent nuclear fuel in borated water is investigated as part of an integrated non-destructive assay safeguards system. To measure the PNAR Ratio, which is proportional to the neutron multiplication, the total neutron count rate is measured in high- and low-multiplying environments by the PNAR instrument. The integrated system also contains a load cell and a passive gamma emission tomograph, and as such meets all the recommendations of the IAEA’s recent ASTOR Experts Group report. A virtual spent fuel library for VVER-440 fuel was used in conjunction with MCNP simulations of the PNAR instrument to estimate the measurement uncertainties from (1) variation in the water boron content, (2) assembly positioning in the detector and (3) counting statistics. The estimated aggregate measurement uncertainty on the PNAR Ratio measurement is 0.008, to put this uncertainty in context, the difference in the PNAR Ratio between a fully irradiated assembly and this same assembly when fissile isotopes only absorb neutrons, but do not emit neutrons, is 0.106, a 13-sigma effect. The 1-sigma variation of 0.008 in the PNAR Ratio is estimated to correspond to a 3.2 GWd/tU change in assembly burnup.

Design of a novel instrument for active neutron interrogation of artillery shells.

Abstract: The most common explosives can be uniquely identified by measuring the elemental H/N ratio with a precision better than 10%. Monte Carlo simulations were used to design two variants of a new prompt gamma neutron activation instrument that can achieve this precision. The instrument features an intense pulsed neutron generator with precise timing. Measuring the hydrogen peak from the target explosive is especially challenging because the instrument itself contains hydrogen, which is needed for neutron moderation and shielding. By iterative design optimization, the fraction of the hydrogen peak counts coming from the explosive under interrogation increased from [Formula: see text]% to [Formula: see text]% (statistical only) for the benchmark design. In the optimized design variants, the hydrogen signal from a high-explosive shell can be measured to a statistics-only precision better than 1% in less than 30 minutes for an average neutron production yield of 109 n/s.