The build–up of 6Li and 3He, that is, the strong thermal neutron absorbers or the so called "neutron poisons", in the beryllium reflector changes the physical characteristics of the reactor, such as reactivity, neutron spectra, neutron flux level, power distribution, etc.; furthermore,gaseous isotopes such as 3H and 4He induce swelling and embrittlement of the reflector. The SAFARI–1 research reactor, operated by Necsa at Pelindaba in South Africa, uses a beryllium reflector on three sides of the core, consisting of 19 beryllium reflector elements in total. This MTR went critical in 1965, and the original beryllium reflectors are still used. The individual neutron irradiation history of each beryllium reflector element, as well as the impact of beryllium poisoning on reactor parameters, were never well known nor investigated before. Furthermore, in the OSCAR{3 code system used in predictive neutronic calculations for SAFARI–1, beryllium reflector burn–up is not accounted for; OSCAR models the beryllium reflector as a non–burnable, 100% pure material. As a result, the poisoning phenomenon is not accounted for. Furthermore, the criteria and hence the optimum replacement time of the reflector has never been developed. This study presents detailed calculations, using MCNP, FISPACT and the OSCAR{3 code system, to quantify the influence of impurities that were originally present in the fresh beryllium reflector, the beryllium reflector poisoning phenomenon, and further goes on to propose the reflector's replacement criteria based on the calculated fluence and predicted swelling. Comparisons to experimental low power flux measurements and effects of safety parameters are also established. The study concludes that, to improve the accuracy and reliability of the predictive OSCAR code calculations, beryllium re flector burn–up should undoubtedly be incorporated in the next releases of OSCAR. Based on this study, the inclusion of the beryllium reflector burn–up chains is planned for implementation in the currently tested OSCAR–4 code system. In addition to beryllium reflector poisoning, the replacement criteria of the reflector is developed. It is however crucial that experimental measurements on the contents of 3H and 4He be conducted and thus swelling of the reflector be quantifed. In this way the calculated results could be verified and a sound replacement criteria be developed. In the absence of experimental measurements on the beryllium reflector, the analysis and quantifcation of the calculated results is reserved for future studies. / Thesis (M.Sc. Engineering Sciences (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.

The build–up of 6Li and 3He, that is, the strong thermal neutron absorbers or the so called "neutron poisons", in the beryllium reflector changes the physical characteristics of the reactor, such as reactivity, neutron spectra, neutron flux level, power distribution, etc.; furthermore,gaseous isotopes such as 3H and 4He induce swelling and embrittlement of the reflector. The SAFARI–1 research reactor, operated by Necsa at Pelindaba in South Africa, uses a beryllium reflector on three sides of the core, consisting of 19 beryllium reflector elements in total. This MTR went critical in 1965, and the original beryllium reflectors are still used. The individual neutron irradiation history of each beryllium reflector element, as well as the impact of beryllium poisoning on reactor parameters, were never well known nor investigated before. Furthermore, in the OSCAR{3 code system used in predictive neutronic calculations for SAFARI–1, beryllium reflector burn–up is not accounted for; OSCAR models the beryllium reflector as a non–burnable, 100% pure material. As a result, the poisoning phenomenon is not accounted for. Furthermore, the criteria and hence the optimum replacement time of the reflector has never been developed. This study presents detailed calculations, using MCNP, FISPACT and the OSCAR{3 code system, to quantify the influence of impurities that were originally present in the fresh beryllium reflector, the beryllium reflector poisoning phenomenon, and further goes on to propose the reflector's replacement criteria based on the calculated fluence and predicted swelling. Comparisons to experimental low power flux measurements and effects of safety parameters are also established. The study concludes that, to improve the accuracy and reliability of the predictive OSCAR code calculations, beryllium re flector burn–up should undoubtedly be incorporated in the next releases of OSCAR. Based on this study, the inclusion of the beryllium reflector burn–up chains is planned for implementation in the currently tested OSCAR–4 code system. In addition to beryllium reflector poisoning, the replacement criteria of the reflector is developed. It is however crucial that experimental measurements on the contents of 3H and 4He be conducted and thus swelling of the reflector be quantifed. In this way the calculated results could be verified and a sound replacement criteria be developed. In the absence of experimental measurements on the beryllium reflector, the analysis and quantifcation of the calculated results is reserved for future studies. / Thesis (M.Sc. Engineering Sciences (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.

[ES] En el apoyo a la mejora de la calidad de medida en el Laboratorio de Radiactividad Ambiental (LRA) de la Universitat Politècnica de València (UPV), los códigos de Monte Carlo representan una potente herramienta para complementar las tareas relacionadas con la medida de la radiactividad ambiental, tales como la calibración en eficiencia de detectores de semiconductor, determinación de factores de corrección por coincidencia y caracterización de dosímetros de termoluminiscencia, entre otras. En la presente Tesis se desarrollan modelos de simulación en Monte Carlo a través de códigos y herramientas como MCNP6 y GEANT4. En primer lugar, se han realizado dos modelos de detector de semiconductor para espectrometría gamma, uno tipo HPGe (High Purity Germanium) y el otro BEGe (Broad Energy Germanium), ambos de alta pureza de germanio. Ambos detectores se emplean en las actividades y procedimientos rutinarios que se realizan en el LRA-UPV. Se detalla el procedimiento de caracterización geométrica de los detectores de semiconductor, así como del volumen activo del cristal de germanio hasta obtener un modelo geométrico optimizado. Por otro lado, se ha obtenido un tercer modelo de simulación, pero en este caso de un dosímetro de termoluminiscencia, en concreto de un TLD-100 LiF:Mg,Ti, modelo que se emplea en el servicio de dosimetría personal de la UPV. En el modelo de simulación se incluye una fuente puntual colimada de Rayos-X y el fantoma recomendado por la ISO 4037-3 (water slab phantom). Se obtiene la función de respuesta del dosímetro relativa a la energía del 137Cs y se estudia su comportamiento para diferentes condiciones de irradiación (calidad del haz de Rayos-X y ángulo de incidencia) así como para diversos materiales termoluminiscentes además del LiF. Los modelos de simulación para espectrometría gamma se han utilizado principalmente para la obtención de curvas de calibración en eficiencia para diferentes geometrías y matrices de medición, así como para el cálculo de factores de corrección por pico suma tanto para las series naturales del 238U y 232Th como para radioisótopos específicos empleados en la calibración experimental de los equipos. Por otro lado, se han aplicado los modelos de simulación en el contexto de respuesta en emergencias nucleares o radiológicas. En concreto, el modelo del detector BEGe se ha utilizado para desarrollar una metodología de optimización del proceso de medición de muestras radiactivas en matrices de agua de alta actividad. Esta metodología consiste en un procedimiento logístico que incluye un cribado o screening de emergencias soportado por simulaciones Monte Carlo, enfocado en elegir la configuración óptima de medición para obtener resultados fiables y precisos minimizando la manipulación de la muestra radiactiva. De este modo se reduce el tiempo de respuesta por parte del laboratorio, así como el riesgo de contaminación y exposición a dosis. / [EN] In support of the improvement of measurement quality at the Laboratorio de Radiactividad Ambiental (LRA) of the Universitat Politècnica de València (UPV), the Monte Carlo codes represent a powerful tool to complement the tasks related to the measurement of environmental radioactivity, such as the calibration in efficiency of semiconductor detectors, determination of coincidence summing correction factors and characterization of thermoluminescence dosimeters, among others. In the present thesis, Monte Carlo simulation models are developed using the MCNP6 code and the GEANT4 toolkit. Two semiconductor detector models for gamma spectrometry have been made, one type HPGe (High Purity Germanium) and the other one a BEGe (Broad Energy Germanium), both of high purity germanium. Both detectors are used in the routine activities and procedures carried out by the LRA-UPV. The geometric characterization procedure of the semiconductor detectors is detailed, as well as the active volume of the germanium crystal until an optimized geometric model is obtained. On the other hand, a third simulation model has been developed, but in this case from a thermoluminescence dosimeter, specifically from a TLD-100 LiF:Mg,Ti, a model used in the personal dosimetry service for the monitoring and assessment of the professionally exposed workers belonging to the UPV radioactive facility. The simulation model includes a collimated X-ray point source and the phantom recommended by the ISO 4037-3 (water slab phantom). The response function of the dosimeter relative to the energy of 137Cs is obtained and its behaviour is studied for different irradiation conditions (quality of the X-ray beam and angle of incidence) as well as for several thermoluminescent materials in addition to the LiF. The simulation models for gamma spectrometry have been used mainly to obtain efficiency calibration curves for different geometries and measurement matrices and to calculate true summing correction factors for both the 238U and 232Th natural decay series and for specific radioisotopes used in the experimental calibration of the equipment. On the other hand, simulation models have been applied in the context of nuclear or radiological emergency response. Specifically, the BEGe detector model has been used to develop a methodology for optimisation of the process of measuring radioactive samples in water matrices of high activity. This methodology consists of a logistic procedure that includes a screening for emergencies. This procedure is supported by Monte Carlo simulations, focused on determining the optimal measurement configuration to obtain reliable and accurate results, minimizing the manipulation of the radioactive sample. Therefore, the response time by the laboratory is reduced, as well as the risk of contamination and dose exposure. / [CA] En el suport a la millora de la qualitat de mesura en el Laboratori de Radioactivitat Ambiental de la Universitat Politècnica de València, els codis de Monte Carlo representen una potent eina per a complementar les tasques relacionades amb la mesura de la radioactivitat ambiental, com ara el calibratge en eficiència de detectors de semiconductor, determinació de factors de correcció per coincidència i caracterització de dosímetres de termoluminescència, entre altres. En la present tesi es desenvolupen models de simulació en Monte Carlo a través de codis i eines com MCNP6 i GEANT4. En primer lloc s'han realitzat dos models de detector de semiconductor per a espectrometria gamma, un tipus HPGe (High Purity Germanium) i l'altre BEGe (Broad Energy Germanium), tots dos d'alta puresa de germani. Aquests detectors s'empren en les activitats i procediments rutinaris que es realitzen en el Laboratori de Radioactivitat Ambiental (LRA) de la Universitat Politècnica de València (UPV). Es detalla el procediment de caracterització geomètrica dels detectors de semiconductor, així com del volum actiu del cristall de germani fins a obtindre un model geomètric optimitzat. D'altra banda, s'ha obtingut un tercer model de simulació, però en aquest cas d'un dosímetre de termoluminescència, en concret d'un TLD-100 LiF:Mg,Ti, model que s'empra en el servei de dosimetria personal de la UPV. En el model de simulació s'inclou una font puntual col·limada de Raigs-X i el fantoma recomanat per l'ISO 4037-3 (water slab phantom). S'obté la funció de resposta del dosímetre relativa a l'energia del 137Cs i s'estudia el seu comportament per a diferents condicions d'irradiació (qualitat del feix de Raigs-X i angle d'incidència) així com per a diversos materials termoluminescents a més del LiF. Els models de simulació per a espectrometria gamma s'han utilitzat principalment per a l'obtenció de corbes de calibratge en eficiència per a diferents geometries i matrius de mesurament així com per al càlcul de factors de correcció per pic suma tant per a les sèries naturals del 238U i 232*Th com per a radioisòtops específics utilitzats en el calibratge experimental dels equips. D'altra banda, s'han aplicat els models de simulació en el context de resposta en emergències nuclears o radiològiques. En concret, el model del detector BEGe s'ha utilitzat per a desenvolupar una metodologia d'optimització del procés de mesurament de mostres ambientals radioactives en matrius d'aigua d'alta activitat.. Aquesta metodologia consisteix en un procediment logístic que inclou un screening o cribratge d'emergències, suportat per simulacions Monte Carlo, enfocat a triar la configuració òptima de mesurament per a obtindre resultats fiables i precisos minimitzant la manipulació de la mostra radioactiva. D'aquesta manera es redueix el temps de resposta per part del laboratori, així com el risc de contaminació i exposició a dosi. / Finalmente, a la Universitat Politècnica de València por la financiación a través de la beca de Formación de Personal Investigador (FPI)-Subprograma 2 de la convocatoria de 2015 y a la Cátedra CSN-UPV Vicente Serradell / Ordóñez Ródenas, J. (2020). Desarrollo de Modelos de Simulación por Monte Carlo como Apoyo a la Medida de Radiactividad Ambiental en Operación Rutinaria y de Emergencias [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/152188 / TESIS