The University of Manchester Greg BlackThesis submitted for the degree of Doctor of EngineeringIrradiated Graphite Waste: Analysis and Modelling of Radionuclide Production with a View to Long Term Disposal23rd June 2014The UK has predominantly used graphite moderator reactor designs in both its research and civil nuclear programmes. This material will become activated during operation and, once all reactors are shutdown, will represent a waste legacy of 96,000 tonnes [1]. The safe and effective management of this material will require a full understanding of the final radiological inventory. The activity is known to arise from impurities present in the graphite at start of life as well as from contamination products transported from other components in the reactor circuit. The process is further complicated by radiolytic oxidation which leads to considerable weightloss of the graphite components. A comprehensive modelling methodology has been developed and validated to estimate the activity of the principle radionuclides of concern, 3H, 14C, 36Cl and 60Co. This methodology involves the simulation of neutron flux using the reactor physics code WIMS, and radiation transport code MCBEND. Activation calculations have been performed using the neutron activation software FISPACT. The final methodology developed allows full consideration of all processes which may contribute to the final radiological inventory of the material. The final activity and production pathway of each radionuclide has been researched in depth, as well as operational parameters such as the effect of changes in flux, fuel burnup, graphite weightloss and irradiation time. Methods to experimentally determine the activity, and distribution of key radionuclides within irradiated graphite samples have been developed in this research using a combination of both gamma spectroscopy and autoradiography. This work has been externally validated and provides confidence in the accuracy of the final modelling predictions. This work has been undertaken as part of the EU FP7 EURATOM Project: CARBOWASTE, and was funded by the Office for Nuclear Regulation.

Le sujet de cette thèse concerne la région des transfermiums. Il s'agit des noyaux de numéro atomique supérieur à 100 qui sont intéressants pour de nombreuses raisons. Tout d'abord, cette région est encore peu connue. En effet, les noyaux deviennent de plus en plus difficiles à produire lorsque qu'ils deviennent riches en nucléons. A fur et à mesure que l’étude progresse vers les noyaux les plus lourds, il devient difficile de les produire à plus de quelques unités, voire simplement de trouver des réactions permettant de les synthétiser. C'est pourquoi ces noyaux sont, aujourd'hui, cantonnés à des recherches très minutieuses. En outre, les modèles nucléaires prédisent l’existence d’un îlot de stabilité, encore inaccessible expérimentalement, pour les noyaux super-lourds vers Z~114–126, N~184. A contrario, les études dans la région des transfermiums nous apportent des informations sur cet îlot de stabilité. Cette thèse traite principalement de la structure nucléaire de deux isotopes impairs en proton de mendélévium (Z=101) : le ²⁴⁹Md et le ²⁵¹Md. Ces deux isotopes sont déformés, ce qui nous permet d’avoir accès à des états quantiques communs avec des noyaux sphériques beaucoup plus lourds pouvant appartenir à l’îlot de stabilité. Les noyaux ont été créés à l'université de Jyväskylä en Finlande par réaction de fusion-évaporation à l'aide d'un faisceau de ⁴⁸Ca et de cibles de ²⁰³⁻²⁰⁵Tl. Les noyaux sont extraits de l’important bruit de fond des réactions parasites à l'aide du séparateur à gaz RITU grâce à la technique de corrélations génétiques. Afin d'extraire la structure nucléaire de ces noyaux, les expériences exploitent les techniques de spectroscopie gamma et électron : détecteurs Jurogam II et SAGE. Grâce à ces instruments, une partie de la structure des noyaux peut être connue. Nous nous focalisons sur des structures collectives comme les bandes rotationnelles, mais aussi d'états métastables (les isomères), ou encore les transitions à une particule. Ce travail a permis d’extraire deux bandes rotationnelles du ²⁵¹Md. Nous avons pu pour la première fois réaliser la spectroscopie électron de ce noyau ce qui nous a permis de contraindre l’assignement des orbitales nucléaires : il s’agit des têtes de bande 1/2⁻ et 7/2⁻. Nous avons également découvert un isomère du ²⁵¹Md et pu observer des transitions l’alimentant et le désexcitant. Ce travail a également permis d’ébaucher une structure collective de ²⁴⁹Md ainsi que de mesurer des propriétés de l’état fondamental. Finalement une estimation de la section efficace de production de ²⁴³Es (⁴⁸Ca + ¹⁹⁷Au) a aussi été faite afin d’évaluer la possibilité d'une future expérience. L’ensemble des données spectroscopiques est confronté à de nouveaux calculs de champs moyen de type Hartree-Fock-Bogoliubov utilisant les forces de Skyrme et Gogny. Cette thèse prolonge les recherches sur la région des noyaux lourds et vient compléter les données spectroscopiques de cette région encore largement inconnue. / The subject of this thesis is the study of the transfermium nuclei region. These are nuclei having an atomic number larger than 100, which are interesting for several reasons. First of all, this region is still poorly known. Indeed, nuclei are more and more difficult to produce as soon as the number of nucleons they are made of increases. When studies move towards the heaviest elements, it becomes difficult to produce more than a few atoms, the reactions production sometimes being simply impossible. That is why these nuclei as still restricted to very thorough studies. Furthermore, nuclear models predict an island of stability for super-heavy elements with Z~114–126, N~184, which are however experimentally still out of reach. Conversely, studies in the transfermium region can provide information of this island of stability. The thesis is mainly related to the nuclear structure of two proton-odd mendelevium (Z=101) isotopes: ²⁴⁹Md and ²⁵¹Md. These isotopes are deformed, which provides access to quantum states also involved in heaviest spherical nuclei from the predicted island of stability. Nuclei were produced at the University of Jyväskylä with fusion-evaporation reactions using a ⁴⁸Ca beam on ²⁰³⁻²⁰⁵Tl targets. Mendelevium nuclei were selected from the large background of parasitic reactions using the RITU gas-filled separator and the genetic correlations technique. The nuclear structure is deduced from the gamma and electron spectroscopy. The SAGE and Jurogam II arrays have been used. These devices provide new insight into nuclei structure: we focussed on the collective structure revealed through rotational bands, on metastable states (isomers) or on single-particle transitions. In this work, two ²⁵¹Md rotational band could be highlighted. We have been able to perform for the first time the electron spectroscopy of this nucleus, which provides a constrain for the nuclear orbitals assignment. The 1/2⁻ and 7/2⁻ band-heads were assigned. We furthermore observed for the first time a ²⁵¹Md isomer with several feeding and de-exciting transitions. In this work, we could also sketch the collective structure of ²⁴⁹Md and measure some of its ground-state properties. Finally, the cross section for the ²⁴³Es production (⁴⁸Ca + ¹⁹⁷Au) was measured in order to estimate the feasibility of a future spectroscopy. Spectroscopic data are compared to new mean-field calculations. Hartee-Fock Bogoliubov calculations using the Skyrme and Gogny forces were made. This thesis is part of the ongoing research program on heavy nuclei; it provides new spectroscopic data in a region where much remains to be discovered.

Master’s thesis focuses on the history and current progress in research of molten salt reactors around the world, with an emphasis placed on the properties of molten salts and the problems associated with their use. In relation to the practical part, one chapter is devoted to the creation of input file in the MCNP software. The practical part deals with neutron activation analysis of graphite prism experiment, which is filled with powder NaCl salt. This experiment is focused on the effect of salt on neutron flux with different energy levels. The whole problem was also simulated in the MCNP environment along with the experiment. At the end of the thesis, the individual methods are compared and evaluated.

Radiation is defined as ionizing if it has enough energy to remove electrons from atoms or molecules when it passes through or collides with matter. This ability implies potentially detrimental effects on living tissue. Ionizing radiation shielding is therefore a discipline of great practical importance. The thesis builds upon the author's previous work on the topic and widens the scope of discussion with theoretical and practical issues of advanced shielding calculations. The theoretical part of the thesis describes several approaches to calculating fluence or absorbed dose at an arbitrary point in space. Point-kernel methods provide sufficiently accurate results for simpler shielding problems. In many practical cases, however, calculations based on the transport theory are necessary. There are two basic types of transport calculations: deterministic transport calculations in which the linear Boltzmann equation is solved numerically, and Monte Carlo calculations in which a simulation is made of how particles migrate stochastically through the problem geometry. Advantages and disadvantages of both methods are discussed. In the practical part are the results of radiation shielding calculations performed with a major Monte Carlo code - MCNP6, compared with those obtained in the experiments, which were carried out at the Ionizing Radiation Laboratory at Department of Electrical Power Engeneering, FEEC BUT. The experiments consisted of placing a cobalt-60 radioisotope source at three different positions inside a lead collimator, and counting pulses with two different scintillation detectors positioned in front of the opening of the collimator, alternately with or without lead shield located between the source and the used detector. Agreement of the calculations and the data from the measurements is reasonable, given the inherent uncertainties of the experimental set-up. Performed sensitivity analysis shows relative importances of different parameters used as inputs in simulations, such as densities of materials, or dimensions of the scintillation crystals. Annotated MCNP input files used for simulation are also part of the thesis.

Dissertation

Les nombres magiques de nucléons, qui se manifestent par de grandes écarts en énergie dans le modèle en couches sphériques, s'érodent et émergent dans les systèmes nucléaires asymétriques. L'évolution de ce comportement est étudiée dans deux régions sur, et près, des nombres magiques de neutrons N = 20 et N = 28 dans les isotopes de néon et d'aluminium riches en neutrons. La collectivité et la structure des états excités est étudiée par la spectroscopie gamma en-ligne des réactions de knock-out d'un et deux nucléons, et la diffusion inélastique. Les expériences ont eu lieu au RIKEN Nishina Center à Tokyo, Japon. Les rayons gamma prompts ont étés observées dans le détecteur DALI2 et la structure nucléaire des isotopes de néon et d'aluminium riches en neutrons ont étés déduites. Entre outre, les sections-efficaces inclusives et exclusives des états liés induites par le knockout d'un ou deux protons, en combinaison avec les calculs du modèle en couche et la théorie réactionnelle, élargissent les tendances systématiques de la région. Les deux régions riches en neutrons à N = 20 et N = 28 ont été discutés dans les contexte d'un grand îlot de déformation réuni par la chaîne isotopique du magnésium. Ces observations élargissent la cartographie de cette région et contribue à la compréhension des forces motrices derrière sa formation. / Magic numbers of nucleons, which appear as large energy gaps in the spherical nuclear shell model, have been known to erode and emerge in asymmetric nuclear systems. The evolution of this behaviour is studied in two regions at, and near, magic neutron numbers N=20 and N=28 in neutron-rich neon and aluminium nuclei. Collectivity and excited state structure is studied through in-beam gamma-ray spectroscopy of one- and two-nucleon knockout reactions, and inelastic scattering. Experiments were carried out at the RIKEN Nishina Center in Tokyo, Japan. Prompt gamma-rays were observed in the DALI2 detector and the nuclear structure of neutron-rich neon and aluminium isotopes were deduced. In addition, inclusive and exclusive cross sections to bound states from one- and two-proton knockout reactions, in combination with shell model calculations and reaction theory, extends systematic trends in the region. The two neutron-rich territories of N=20 and N=28 have been discussed as an extended island of deformation, joined-up through the magnesium isotopic chain. These observations enhance the mapping of this region and contribute to understanding the driving forces behind its formation.

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Independent yield distributions of high-energy neutron-induced fission are of importance to achieve a good understanding of fission. Even though the mass and charge yield distributions of thermal neutron-induced fission are well known, there are few experimental data for high-energy neutron-induced fission. In addition to basic research on the fission process, independent yield distributions of high-energy neutron-induced fission play a key role in the development of Generation IV fast nuclear reactors. To facilitate measurements of independent fission yields of high-energy neutron-induced fission, a dedicated ion guide and a proton-neutron converter were developed and put to use in experiments at the isotope separator facility IGISOL in Jyväskylä. In parallel, a simulation model of the system was developed in order to optimize the collecting efficiency of fission products in the ion guide. The model uses the Monte Carlo code MCNPX to simulate the neutron production, the fission model code GEF to simulate the fission process, and GEANT4 for ion transportation. In order to benchmark the simulation model, metal foils were inserted in the ion guide with the purpose of collecting fission products. At the same time, nickel, cobalt and indium foils were located between the pn-converter and the ion guide to record the neutron flux from the pn-converter. After the beam was turned off, and after several days of cooling, g-ray spectroscopy measurements of the foils were conducted using a well shielded HPGe detector. Based on the identified g-ray transitions in the spectroscopy data, the productions of corresponding fission products and neutron activation products were calculated, and then used to benchmark the transportation and collection of fission products, as well as neutron production, in the simulations. The conclusion from the benchmark is that the transportation of fission products in the helium gas, as simulated by GEANT4, agrees very well with the measurement, while the transportation of fission products in the uranium targets agrees with the measurement within 10%. The neutron flux at the high-energy part of the neutron spectrum is overestimated by about 40%.Thanks to the benchmark it has been shown that the predictive power of the model is satisfactory and sufficient for the purpose of modeling the ion guide. Furthermore, the parameters involved in the simulations, such as neutron production, distance between the neutron source and the ion guide, volume of the ion guide and so on, play an important role in the optimization of the setup. However, the lower than expected fission rate suggests that the optimization on these parameters may not be enough to achieve a sufficiently high intensity of fission products, especially for nuclei far from the stability line. To achieve a sufficiently high intensity, an electric field guidance, similar to the RF structure of the CARIBU gas catcher presented in G.Savard et al. Nucl. Inst. Meth. B, 376: 246, 2016, to collect fission products is considered.

Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: See full text for abstract / AFRIKAANSE OPSOMMING: Sien volteks vir opsomming