Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1146 Fast-neutron tomography using a mobile neutron generator for assessment of steam-water distributions in two-phase flows PETER ANDERSSON ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-554-8947-2 UPPSALA urn:nbn:se:uu:diva-222459 2014 Dissertation presented at Uppsala University to be publicly examined in Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, Wednesday, 4 June 2014 at 09:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Bernhard Ludewigt (UC Berkeley). Abstract Andersson, P. 2014. Fast-Neutron Tomography using a Mobile Neutron Generator for Assessment of Steam-Water Distributions in Two-Phase Flows. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1146. 70 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-8947-2. This thesis describes the measurement technique of fast-neutron tomography for assessing spatial distributions of steam and water in two-phase flows. This so-called void distribution is of importance both for safe operation and for efficient use of the fuel in light water reactors, which compose the majority of the world’s commercial nuclear reactors. The technique is aimed for usage at thermal-hydraulic test loops, where heated two-phase flows are being investigated under reactor-relevant conditions. By deploying portable neutron generators in transmission tomography, the technique becomes applicable to stationary objects, such as thermal-hydraulic test loops. Fast neutrons have the advantage of high transmission through metallic structures while simultaneously being relatively sensitive to the water/void content. However, there are also challenges, such as the relatively low yield of commercially available fast-neutron generators, the tendency of fast neutrons to scatter in the interactions with materials and the relatively low efficiency encountered in fast-neutron detection. The thesis describes the design of a prototype instrument, FANTOM, which has been assembled and demonstrated. The main design parameters have been optimized to achieve maximal signal count rate in the detector elements, while simultaneously reaching an image unsharpness of ≤0.5 mm. Radiographic projections recorded with the assembled instrument are presented, and the performance parameters of FANTOM are deduced. Furthermore, tomographic reconstruction methods for axially symmetric objects, which is relevant for some test loops, have been developed and demonstrated on measured data from three test objects. The attenuation distribution was reconstructed with a radial resolution of 0.5 mm and an RMS error of 0.02 cm-1, based on data recorded using an effective measurement time of 3.5 hours per object. For a thermal-hydraulic test loop, this can give a useful indication of the flow mode, but further development is desired to improve the precision of the measurements. Instrument upgrades are foreseen by introducing a more powerful neutron generator and by adding detector elements, speeding up the data collection by several orders of magnitude and allowing for higher precision data. The requirements and performance of an instrument for assessment of arbitrary non-symmetric test loops is discussed, based on simulations. Keywords: Void distribution, neutron tomography, plastic scintillator, transmission measurements, neutron detection Peter Andersson, Department of Physics and Astronomy, Applied Nuclear Physics, Box 516, Uppsala University, SE-751 20 Uppsala, Sweden. © Peter Andersson 2014 ISSN 1651-6214 ISBN 978-91-554-8947-2 urn:nbn:se:uu:diva-222459 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-222459) List of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. Reprints were made with permission from the respective publishers. I Andersson, P., Sjöstrand, H., Jacobsson Svärd, S 11) Ef- fects of proton escape on detection efficiency in thin scintillator elements and its consequences for optimization of fast-neutron imaging. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associ- ated Equipment, -116. This paper describes the energy deposition distribution in thin plate-shaped plastic scintillators when exposed to DQG MeV neutrons. My contribution: I created the Monte Carlo scripts, performed the simulations and the analysis. I wrote the major part of the paper. II Andersson, P., Andersson Sundén, E., Jacobsson Svärd, S., 6M|VWUDQG + Correction for dynamic bias error in transmission measurements of void fraction. Review of Scien- tific Instruments9ROXPH This paper describes the systematic error in transmission meas- urements of void fraction in two-phase flows arising from the dynamic property of such systems. A correction procedure is suggested and demonstrated on artificial data. My contribution: I developed the method and performed the analysis. I wrote the major part of the paper. III Andersson, P., Valldor-Blücher, J., Andersson Sundén, E., Sjöstrand, H., Jacobsson-6YlUG6 ) Design and initial 1D radiography tests of the FANTOM mobile FAst-Neutron radi- ography and TOMography system. Accepted for publication, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. This paper describes the design optimization, the expected and experimentally determined performance of a portable fast- neutron radiography instrument. My contribution: I performed the optimization of the instrument design, assembled it and per- formed the measurements and the analysis. I wrote the major part of the paper. IV Andersson, P., Andersson Sundén, E., Sjöstrand, H., Jacob- sson-6YlUG6 Neutron tomography of axially symmet- ULFREMHFWXVLQJ0H9QHXWURQVIURPDSRUWDEOH neutron gen- erator. Submitted to Review of Scientific Instruments. In this paper tomographic reconstruction of axially symmetric objects using fast neutrons is demonstrated using five test ob- jects. My contribution: I developed the methods and the code used for the reconstruction and I performed the analysis. I wrote the major part of the paper. Additional papers by the author, not included in the thesis: Andersson P., Jacobsson-Svärd, S, Sjöstrand, H. 1Hu- tron tomography for void distribution measurements. ENC 2010 Transactions: Plant Operations. Rakopoulos V., Lantz M., Andersson P., Hjalmarsson A., Mat- tera A., Pomp S., Solders A., Valldor-Blücher J., Gorelov D., Penttilä H., Rinta-Antila S., Bedogni R., Bortot D., Esposito A., Gentile A., Passoth E., Prokofiev A. V., Introini M. V., Pola A., 7DUJHWWKLFNQHVVGHSHQGHQFHRIWKH S[Q QHXWURQHQHr- gy spectrum. EPJ Web of Conferences Mattera A., Andersson P., Hjalmarsson A., Lantz M., Pomp S., Rakopoulos V., Solders A., Valldor-Blücher J., Gorelov D., Penttilä H., Rinta-Antila S., Prokofiev A. V., Passoth E., Be- dogni R., Gentile A., Bortot D., Esposito A., Introini M. V., Po- la A., &KDUDFWHUL]DWLRQRID%( S[Q QHXWURQVRXUFHIRU fission yields measurments. arXiv:1304.0547 Contents 1. Introduction ........................................................................................... 9 Void distributions in light water reactors ............................................ 3. Transmission tomography ................................................................... 3.1. General principles ............................................................................. 6HWXSFRQILJXUDWLRQV .......................................................................... 3.3. Using neutrons as transmission probe ............................................... 17 ,QWHUDFWLRQVRIQHXWURQVZLWKPDWWHU .................................................. 19 5HFRQVWUXFWLRQDOJRULWKPV ................................................................. 19 3.6. Tomography of the two-phase flow .................................................. Components of a mobile setup for fast-neutron tomography .............. 1HXWURQJHQHUDWRUV ............................................................................ Collimators ........................................................................................ )DVW-neutron detectors ....................................................................... FANTOM, the FAst-Neutron TOMography system ........................... 7KHVHWXS ........................................................................................... 'DWDDFTXisition system ..................................................................... )$1720PRGHVRIRSHUDWLRQ .......................................................... ,QVWUXPHQWRSWLPL]DWLRQ .................................................................... 6. Experimental investigations ................................................................. 6.1. Measurements .................................................................................... 'DWDSURFHVVLQJ ................................................................................. 7. FANTOM results ................................................................................. 7.1. Radiographic imaging ....................................................................... 7RPRJUDSK\RQD[LDOO\V\PPHWULFREMHFWV ....................................... 8. Conclusions and discussion ................................................................. 8.1. A concept for neutron
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