Different kinds of batteries suit different applications, and consequently several different chemistries exist. In order to better understand the limitations of low temperature performance, a Li-ion battery chemistry normally intended for room temperature use, graphite-Lithium Iron Phosphate, with 1 M LiPF6 ethylene carbonate:diethylene carbonate electrolyte, is here put under testing at -10°C and compared with room temperature cycling performance. Understanding the temperature limitations of this battery chemistry will give better understanding of the desired properties of a substitute using alternative materials. The experimental studies have comprised a combination of battery cycle testing, and surface analysis of the electrodes by Scanning Electron Microscopy and X-Ray Photoelectron Spectroscopy. Results showed that with low enough rate, temperature is less of a problem, but with increased charge rate, there are increasingly severe effects on performance at low temperatures. XPS measurements of low charge rate samples showed similar Solid Electrolyte Interface layers formed on the graphite anode for room- and low temperature batteries, but with indications of a thicker layer on the former. A section of the report handles specific low temperature battery chemistries. The conclusions- and outlook were made by comparing the results found in the study with earlier findings on low temperature Li-ion batteries and present possible approaches for modifying battery performance at lowered temperatures. / I detta projekt har litium-jon-batterier testats i avseende på sina lågtemperaturprestanda. Arbetet gjordes genom att testa och jämföra prestantda mellan prover vid -10°C och rumstemperaturprover. Med analytiska instrument studerades både den morfologiska och kemiska förändring som skett under användning. Vald batterikemi har varit av slaget grafit-litiumjärnfosfat med en typisk organisk elektrolyt. Denna batterikemi är inte på något sätt anpassad för lågtemperaturprestanda och med det hoppas kunna påvisas de effekter som en mer lämpligt lågtemperaturkemi åtgärdar, och förstå hur de gör det. Med låg temperatur uppkommer en större ’tröghet’ för de kemiska reaktioner som sker i ett batteri. Om designen inte är särskilt gjord för låg temperatur kan effekterna bli osäkra, rent av farliga. Risken ökar nämligen för plätering av metalliskt litium på den negativa elektroden, och skulle litiumdeponeringen växa i den riktning som kopplar samman batteriets poler så kortsluts systemet. Med den höga energidensitet som karaktäriserar litium-jon-batterier vore en kortslutning extra beklaglig då den organiska elektrolyten kan antändas, med en potentiell explosion som följd.Inom särskilda applikationer kan lågtemperaturmiljöer förväntas för ett batteri, till exempel för fordon. En elbil i skandinaviskt klimat skulle behöva fungera ohindrat även vintertid, då temperaturerna ofta når -10°C och lägre. Samtidigt får man påminnas om att litium-jon-batterierna är relativt moderna och ännu inte har fått något stort genomslag som framdrivningsmedel. Detta försätter bilindustrin i ett krafigt behov av omfattande forskning för att kunna ta strategiskt sunda beslut för att möjliggöra en ordentlig introducering av elbilar som trovärdig ersättare till de fossilt drivna bilarna. I linje med trenden att ständigt bygga säkrare bilar måste elbilarna kunna visa upp förutsägbarhet, och med detta pålitlighet och säkerhet. I detta arbetet erhölls resultat som visade på batterifunktion även vid den sänkta temperaturen, men med gränser för hur snabbt laddningöverföring kunde ske jämfört med i rumstemperatur. Bevis för bildande av skyddsfilm på anod efter 1.5 battericykler, snarlik komposition för -10°C - och rumstemperaturbatterier – men med vissa indikationer på ett tjockare bildat lager hos den senare. Därtill gjordes jämförelser med specifika lågtemperaturselektrolyter, där en skillnad i framförallt innehåll utav etylkarbonat (mindre andel vid lågtemperaturapplikationer) uppvisar stora förbättringar i kallare klimat. En sådan provblandning gjordes och uppvisade bättre prestanda vid -10°C än rumstemperaturbatterier med standardelektrolyt. Arbetet har utförts vid Institutionen för Kemi-Ångström vid Uppsala universitet.

The development of high temperature heat exchangers will play a vital part in the success of High Temperature Nuclear Reactors (HTRs). Manufacturing such heat exchangers from metals is becoming increasingly difficult as the operating temperatures keep rising. Above 1000'C most metals loose their strength and have high creep rates, while certain ceramic materials (including graphite, in the absence of oxygen) are able to operate at these temperatures. A literature study was done in order to identify the major problems regarding the use of graphite for heat exchanger construction as well as to investigate to what extent graphite has been used for heat exchanger construction in the past. Following from the literature survey, it was decided to design and manufacture a Printed Circuit Heat Exchanger (PCHE) from isotropic graphite to gain experience regarding the use of graphite as a heat exchanger material. This heat exchanger was then tested in order to learn about the operation of a graphite heat exchanger and to determine its effectiveness. A model ofthe heat exchanger was also constructed in order to determine what the performance of such a heat exchanger should theoretically be. It was found that the single greatest hurdle standing in the way ofgraphite being used as a heat exchanger material is its high gas permeability. This causes mixing between the two fluid streams as well as leakages to the environment, which have a negative effect on the heat exchanger's heat transfer capability. The methods used to establish a seal between the consecutive plates of the PCHE are also affected by the permeability of the graphite. Coatings on the surface of the graphite might be able to reduce its permeability and can also inhibit the high temperature degradation of graphite in the presence of oxygen. Manufacturing very small flow channels for the PCHE is limited by the availability of small enough end mills. Alternative manufucturing techniques is needed to economically construct a graphite PCHE. It was also found that the heat transfer effectiveness of the heat exchanger is influenced negatively by heat losses to the environment through the outer surface ofthe heat exchanger. Effective insulation around the heat exchanger or a graphite material :vith higher heat conductivity perpendicular to the flow direction might solve this problem. This study concluded that if diffusion bonding techniques, effective coatings and a graphite material with increased heat conductivity perpendicular to the flow direction are used, manufacturing a printed circuit heat exchanger from graphite is feasible. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010

The development of high temperature heat exchangers will play a vital part in the success of High Temperature Nuclear Reactors (HTRs). Manufacturing such heat exchangers from metals is becoming increasingly difficult as the operating temperatures keep rising. Above 1000'C most metals loose their strength and have high creep rates, while certain ceramic materials (including graphite, in the absence of oxygen) are able to operate at these temperatures. A literature study was done in order to identify the major problems regarding the use of graphite for heat exchanger construction as well as to investigate to what extent graphite has been used for heat exchanger construction in the past. Following from the literature survey, it was decided to design and manufacture a Printed Circuit Heat Exchanger (PCHE) from isotropic graphite to gain experience regarding the use of graphite as a heat exchanger material. This heat exchanger was then tested in order to learn about the operation of a graphite heat exchanger and to determine its effectiveness. A model ofthe heat exchanger was also constructed in order to determine what the performance of such a heat exchanger should theoretically be. It was found that the single greatest hurdle standing in the way ofgraphite being used as a heat exchanger material is its high gas permeability. This causes mixing between the two fluid streams as well as leakages to the environment, which have a negative effect on the heat exchanger's heat transfer capability. The methods used to establish a seal between the consecutive plates of the PCHE are also affected by the permeability of the graphite. Coatings on the surface of the graphite might be able to reduce its permeability and can also inhibit the high temperature degradation of graphite in the presence of oxygen. Manufacturing very small flow channels for the PCHE is limited by the availability of small enough end mills. Alternative manufucturing techniques is needed to economically construct a graphite PCHE. It was also found that the heat transfer effectiveness of the heat exchanger is influenced negatively by heat losses to the environment through the outer surface ofthe heat exchanger. Effective insulation around the heat exchanger or a graphite material :vith higher heat conductivity perpendicular to the flow direction might solve this problem. This study concluded that if diffusion bonding techniques, effective coatings and a graphite material with increased heat conductivity perpendicular to the flow direction are used, manufacturing a printed circuit heat exchanger from graphite is feasible. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010

Thesis (Ph.D.)--McMaster University (Canada), 1991. / Source: Dissertation Abstracts International, Volume: 54-02, Section: B, page: 0917.

Thesis (MSc.(Chemistry))--University of Pretoria, 2008. / Includes bibliographical references.

Thesis (Ph.D.)--University of Wollongong, 2007. / Typescript. Includes bibliographical references: leaf 165-177.

Thesis (Ph.D.)--Physics, Georgia Institute of Technology, 2008. / Committee Chair: de Heer, Walter; Committee Member: Chou, Mei-Yin; Committee Member: First, Phillip; Committee Member: Meindl, James; Committee Member: Orlando, Thomas

Thesis (Ph. D.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 156-160).

Graphite bricks make up a significant part of the core of an Advanced Gas-cooled Reactor (AGR). The graphite moderates the neutrons vital to the continuation of the fission chain reaction and provides support and stability for the entire core. During operation, the graphite can be oxidised due to the extreme conditions inside the core and so undergo weight loss. Differential shrinkage caused by neutron interaction throughout the brick can also cause radial cracking to occur. The effects of the oxidation, weight loss and cracking reduce the ability of the graphite to function as a moderator. The effects also have the potential of reducing the structural integrity of the brick, causing movement and structural instability of the entire core. It is, therefore, vital to monitor the condition of the graphite bricks and to understand how the changes in the graphite's properties and structure may affect the safe operation of the reactor. This report firstly looks briefly at the effect of irradiation on the graphite brick; the mechanisms leading to weight loss and cracking. The report then considers various methods which can be used to inspect the deterioration of graphite blocks within the cores of AGRs deriving quantitative and qualitative information on density and crack profiling. These methods will be considered for use both on small samples trepanned from the core and in-situ blocks within the reactor core, requiring non-destructive techniques. The inspection methods considered in this report are: Electrical Impedance Tomography (EIT); Four point probes; Eddy Current Tomography; and Electromagnetic Inductance Tomography (EMT).There are two main contributions of this thesis. First, the development an EIT methodology using outward facing probes, which were best suited to the geometry of the graphite bricks within the AGR. Proof of principle was established using both modelling and laboratory testing. The second contribution is the development of commercial grade EMT equipment, which can be used on-site to determine the conductivity of trepanned samples. The method was successfully demonstrated in the laboratory; however, further development will be required for use on-site, due to the sampling speed required.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Hitesh Bindra / Helium-cooled, graphite moderated reactors have been considered for a future fleet of high temperature and high efficiency nuclear power plants. Nuclear-grade graphite is used in these reactors for structural strength, neutron moderation, heat transfer and, within a helium environment, has demonstrated stability at temperatures well above HTGR operating conditions. However, in the case of an air ingress accident, the oxygen introduced into the core can affect the integrity of the fuel graphite matrix. In this work a combination of computational models and mixed effects experiments were used to better understand the air ingress process and its potential effects on the heat removal capabilities of an HTGR design following an air-ingress accident. Contributions were made in the understanding of the air-ingress phenomenon, its potential effects on graphite, and in experimental and computational techniques. The first section of this thesis focuses on experimental and computational studies that were undertaken to further the understanding of the Onset of Natural Convection (ONC) phenomenon expected to occur inside of an HTGR following an air ingress accident. The effects of two newly identified factors on ONC – i.e., the existence of the large volume of stagnate helium in a reactor's upper plenum, and the possibility of an upper head leak – were investigated. Mixed-effects experimental studies were performed to determine the changes induced in nuclear grade graphite exposed to high-temperature, oxidizing flow of varying flow rates. Under all scenarios, the thermal diffusivity of the graphite test samples was shown to increase. Thermal conductivity changes due to oxidation were found to be minor in the tested graphite samples – especially compared to the large drop in thermal conductivity the graphite is expected to experience due to irradiation. Oxidation was also found to increase the graphite's surface roughness and create a thin outer layer of decreased density. The effects of thermal contacts on the passive cooling ability of an HTGR were experimentally investigated. Conduction cool down experiments were performed on assemblies consisting of a number of rods packed into a cylindrical tube. Experimental conditions were then modeled using several different methodologies, including a novel graph laplacian approach, and their results compared to the experimentally obtained temperature data. Although the graph laplacian technique shows great promise, the 2–D Finite Element Model (FEM) provided the best results. Finally, a case study was constructed in which a section of a pebble bed reactor consisting of a number of randomly packed, spherical fuel particles was modeled using the validated FEM technique. Using a discrete elements model, a stable, randomly packed geometry was created to represent the pebble bed. A conduction cool down scenario was modeled and the results from the FEM model were compared to best possible results obtainable from a more traditional, homogeneous 1–D approximation. When the graphite in the bed was modeled as both oxided and irradiated, the homogeneous method mispredicted the maximum temperature given by the 3–D, FEM model by more than 100°C.