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Establishment, Characterization and Evaluation of a Reference Field in a MNSR Reactor for Calibration Neutron Dosimeters

Asgari, Afrouz | 2025

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 58026 (46)
  4. University: Sharif University of Technology
  5. Department: Energy Engineering
  6. Advisor(s): Hosseini, Abolfazl; Choopan Dastjerdi, Mohammad Hossein; Mokhtari, Javad
  7. Abstract:
  8. The purpose of this research is to establish a neutron reference field for the calibration of neutron dosimeters based on the beam of the neutron radiography system in the Isfahan miniature neutron source reactor (MNSR). The aim is to address the several limitations, including the inability to achieve a wide range of dosimetry, high uncertainty due to variations in distance from the source, significant scattering, and non-uniformity. To overcome these issues, a linear, stable, and wide-range dosimetry field based on MNSR have been established, characterized and evaluated by using the variations of the reactor power. In the first step of establishing the calibration field, the linear and reliable range of the reactor power display instrument was determined to evaluate the feasibility of using reactor power variations to create a wide range of the dosimetry. In this study, three methods were used: investigating the kinetic behavior of the reactor core, active and passive out-of-core tests. The results showed the fission chamber response at the power of 1.5 W to 30 kW is linearly accurate. A wide range of the dosimetry was examined using both active and passive dosimeters. The results showed the beamline outlet can be used for calibrating dosimeters from 0.5 μSv/h to 10 mSv/h, which complies with the ISO 8529 requirements. One of the most important challenges of the neutron calibration system is analyzing the contribution of scattered neutrons in that field. In this research, the contribution of scattering was determined in the output of the beam as the calibration field. Three methods were employed: generalized, semi-empirical, and reduced fit methods, along with two approaches: simulation and experimental to separate direct and scattered neutrons. The results were in good agreement with each other, and the maximum contribution of scattered neutrons was less than 14%, which complies with the of ISO 8529 requirements. The characteristics of the reference field were determined using experimental and simulation. The neutron spectrum was determined using the neutron activation method and spectrum unfolding with SANDII, genetic algorithms, and simulation. The thermal and total neutron fluxes were determined to be 1.12E+05 ± 9.86E+02 n.cm-2.s-1 and 1.25E+05 ± 1.05E+03 n.cm-2.s-1, respectively. The thermal and total neutron doses were 43.4 mSv/h and 47.10mSv/h. The uniformity, TNC and flux stability were over 92%, 89%, and 98% in the reference field. The results showed that the MNSR beam is a suitable choice for calibration and maintains its positive characteristics as the distance from the output increases. According to the results, the optimal point is at the closest distance to the beam output. The effect of reactor core temperature changes on the characteristics and stability of the field was investigated. The temperature correction factor was measured 0.00156 (mSv/h)/(℃×kW). The gamma spectrum and gamma dose at each reactor power level were measured to investigate the effect of gamma noise on the calibration of the gamma-sensitive neutron dosimeters. The results show a linear relationship between gamma dose and power changes. To verify traceability, the calibration coefficient of the test instrument was measured using the predetermined field method and the reference instrument of the SSDL, which confirmed the traceability results
  9. Keywords:
  10. Linear Response ; Reactor Cores ; Scattering ; Characterization ; Genetic Algorithm ; Traceability ; Gamma Noise ; Reactor Core Temperature ; Reactor Core Kinetics ; Miniature Neutron Source Reactor ; Neutron Reference Calibration Field

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