Introduction
Ever since the development of the atomic bomb during the Second World War, it has always been a road of progress regarding nuclear fission and energy. A major stride was taken back in the year 2000 when the international Generation 4 initiative was developed (Liou, Shao, & Liu, 2011). The development particularly focused on the development of a new generation of nuclear reactors. A major aspect of the development of this system is the significant progress that it will bring compared to the generation 3 reactors, considering the former has both the reactor and the associated fuel facilities. With the progress and development that has been witnessed on these reactors, commercial deployment is expected by the year 2030 while other systems are expected by the year 2023 (Liou, Shao, & Liu, 2011).
The generation 4 initiative was reinforced by the United States back in the year 2006 when the country called for a global energy nuclear partnership, the core to identify proper methods and policies that would develop the research necessary to deliver a Sustainable and proliferation-resistant fuel cycle. With this goal in focus, the members of this initiative identified six reactor systems which include: very high-temperature gas cooler reactor, gas-cooled fast reactor, sodium-cooled fast reactor, lead-cooled fast reactor, molten salt reactor and finally the supercritical water cooled reactor.
The feasibility of the six generation 4 reactors was investigated by the various members of the global energy nuclear partnership but the main focus was on the activities of either two or three of the stated. In the United Kingdom, the VHTR, SFR, and GFR had been identified as the most prioritized of the systems and the British nuclear Fuels body provided the necessary funds until the year 2006.Another body that participated in the development of this research is the Engineering and physical science research council. The support of this U K based council was withdrawn in the year 2006.
Considering the very high-temperature nuclear reactor, it has been regarded as an extension of the high-temperature reactor. The latter was proposed in the 1950s and various prototypes and experimental setups were conducted mainly to observe the characteristics of a temperature reactor. The very high-temperature nuclear reactor obtains its uniqueness from the properties associated with its development. The tiny coated fuel particles are contained in a graphitic matrix that is located in a graphitic core particularly cooled by helium of which the refractory nature enables the core to achieve very high gas temperatures (Harp, Hawari, & Bourham, 2007). In this, the electricity generated is enough for some processes that may include the production of hydrogen using thermo-chemical cycles. As a matter of fact, the concept that the very high-temperature nuclear reactor is based on is a thermal system operating a once-through fuel cycle, although closed cycles using thorium fuel are possible, as are burner ‘cores that can efficiently transmute plutonium. (Sawa, 2012)\
In this aspect, the development of the very high-temperature fuel nuclear reactors can be regarded as a giant step towards the full understanding and development in the nuclear field. The very high-temperature fuel nuclear reactor basically uses a TRISO microsphere as the fundamental unit in the core. This fuel is irradiated in a controlled environment that resembles anticipated operating conditions. Onto the composition aspect of the TRISO fuel, it is about 1mm in diameter and composed of a UCO kernel. The kernel is surrounded on the inner side by a pyrolytic graphite and a silicon carbide layer while on the outside, it is surrounded by another graphite pyrolytic layer. The coating is particularly for environmental protection against radiation and for the containment of the energy released.
All the characteristics stated of this fuel particle just indicate how complicated the fabrication process is.The major concern is the coating process because this entails environmental protection and a simple miscalculation can have tragic results. This tristructural isotropic coated particle is mainly produced by vapor deposition in a device called the spouted bed. As per the safety concerns and the need to meet the criteria for producing particles that are risk-free, there is the need to understand the fluidization phenomena that occurs in the spouted bed. On this regard, the fundamental cannon is to ensure that there is proper distribution of fuel particles to ensure uniform deposition which consequently means that there will be sufficient contact of particles with the reactive gas.
However, regardless of the technological advancement and know how behind this technique, it is still a mystery how the movement of particles occurs. It has, in turn, required an understanding of solid flow pattern and distribution because this is a necessary criterion in the design of a spouted bed, particularly because all the trajectories that the particles follow must meet the process requirements. However, this may be a little more difficult considering a large number of particles that should be observed in the device. More to this is the fact that the manufacturing process usually occurs at very high temperatures and this, in turn, limits the ability to accurately measure the temperature as very few devices are heat resistant. Furthermore, considering the complex design of the system, an indirect method of measurement may be used. The system consists of cooling water system, heating system, heat removal system and the thermal insulation system and as such even the direct measure of the pressure signal inside.
Shifting the main focus to the design of the system, the sprouted bed can be divided into two parts of which the first part particularly focuses on gas distribution and contains gases such as argon, hydrocarbon gases which include ethyne and propyne, hydrogen and methyl trichlorosilane vapor supply device, flow control system and gas distribution system. The other part of the device is known as the conical sprouted bed coating system and comprises the following: sprouted bed, cooling water system, heating system, the thermal insulation system, the gas airtight device and the temperature control device. In this regard, the conical sprouting system design is the most critical because the density, diameter, and geometry of the particles determine the very critical aspects of the very-high-temperature reactor. It is thus important to consider some of the characteristics of the sprouted bed when designing the system. Some of the characteristics include: cone included angle, inlet diameter, column diameter and the height of the conical part. These are the determinants of the properties that the fuel particles: particle diameter, static bed height, particle density (Liou, Shao, & Liu, 2011). It is thus a correlation because these properties determine the sprouted bed hydrodynamics.
In the manufacture, the spherical kernel particles are fluidized by the fluidization gas and coated by the reactive gas in the conical sprouted bed coating furnace. In this regard, the coating technologies for Pyc and Syc are the most critical because they determine the integrity of the coated kernel particle. As a precautionary measure, it has always been stated that the failure of these coating layers has a direct impact on the failure of the reactor and hence requires the very best in its handling. The amount of kernel corresponds to 3 kg of uranium per coating for the fuel used by the Japanese high-temperature engineering reactor (Liou, Shao, & Liu, 2011).
Considering that the temperature in the device constantly changes, at the desired point, the reactants are inserted into the coater which subsequently results in the production of a coating layer of the particles that are fluidized. The coating is meant to achieve the desired size and when this is attained, the gas supply is replaced with argon of which the particles that have attained the right size are removed. Finally sorting is done by means of a vibrating table whereby the particles with the desired shapes are separated from those that have odd shaped, as in the case of the Japanese high-temperature engineering test reactor.
In the design and manufacture of the particles, the primary cause of failure is the strong mechanical shocks that are subjected to the particles resulting from violent particle fluidization which occurs in the coater. Furthermore, the unloading procedure of the particles can induce violent shocks on the particles which can subsequently lead to particle failure. In this, various developments have occurred in the manufacture some of which include optimizing the process of particle fluidization and also by allowing the process to be smooth without the unloading and loading procedures at the intermediate coating process.
The second aspect of the reactor is the graphite matrix. To begin with, natural graphite powder, electrographite and a binder are mixed with which fine grinding results into a graphite matrix. The matrix is used to coat the manufactured particles and later on warm pressed to make an annular cylinder of green compacts. The green compacts are produced in a very complicated procedure which involves weighing, preheating, loading and the unloading of overcoated particles.
 
References
Harp, J. M., Hawari, A. I., & Bourham, M. A. (2007). Simulation of-ray spectrometry of failed TRISO fuel. Nuclear Instruments and Methods in Physics Research.
Liou, M., Shao, Y., & Liu, B. (2011). Pressure analysis in the fabrication process of TRISO UO2-coated fuel particle. Nuclear Engineering and Design.
Sawa, K. (2012). TRISOFuelProduction.