FUEL AND FUEL CYCLE | HTR FUEL length of about 25 mm and 12 mm in diameter. It


contains about 1.2 g of uranium per compact (about 3,000 TRISO-coated kernels).


HTR fuel production The HTR fuel production process can be divided into four major fuel production process areas as well as two recycling areas for the recovery of uranium and other valuable materials from liquid process effluents, as well as out-of- specification solid fuel material. In the Kernel Production Facility, fresh U3


O8 powder is dissolved in nitric acid (HNO3) and mixed with special


chemicals to a viscous ammonium di-uranate (ADU) solution. This solution is drip-cast (vibro-dropped) to form microspheres from many small droplets, which are then gelled, dried and calcined to form UO3


. The UO3 is reduced to UO2 and sintered to a kernel. In the case of UCO kernels,


a similar process is utilised to partly form uranium carbide. Within the Coating Facility the kernels receive four


coatings using a chemical vapour deposition (CVD) process to produce the TRISO-coated particles. In the Fuel Compact (or Sphere) Production Facility, the


TRISO-coated particles are overcoated with a layer of matrix graphite powder (MGP). The MGP-overcoated particles are dosed into pressing moulds together with additional matrix graphite powder according to the desired packing fraction, which determines the volumetric fraction of TRISO-coated particles relative to the total fuel element volume. The resulting fuel element is then carbonised and annealed in two consecutive furnaces – and thereby significantly hardened. In the case of a fuel sphere, this is the final fuel element production step and it is now ready to be introduced into a fuel assembly. Fuel compacts are first inserted into rod-shaped openings within a prismatic graphite block to yield the final fuel assembly. Two recycling areas ensure that on the one hand almost no enriched uranium gets lost within the process and on the other hand the required chemicals are reused as often as possible. All traces of uranium from spent liquids are retrieved before they are discharged in the form of decontaminated waste-water. The liquid effluents from the production processes are


recycled and cleaned in the Effluent Treatment Facility. The main purpose is to recycle process liquids for reuse in the Kernel Production Facility. The scrap material from the different stages of the production process – including odd kernels, oddly coated kernels and off-specification fuel elements, as well as other uranium-containing materials – is recycled in the Uranium Recovery Facility to form U3


O8


development and adjacent commercial operation of the HOBEG fuel production plant. This unique know-how had a significant role in the revival of the HTR fuel technology within NUKEM.


NUKEM developed its up-to-date TRISO fuel production


process mainly during the design of the Pebble Bed Modular Reactor (PBMR) fuel Plant. The PBMR Fuel Plant (PFP), originally to be constructed near Johannesburg, was intended to fuel the first South African PBMR. The design of the reactor was based on the fuel specification and the equivalence of the fuel elements to the German fuel. This equivalence is important for the fuel qualification as the former NUKEM fuel has been long-term tested through irradiation tests in the German AVR reactor performed by the Research Centre Jülich in the 1980s. In the course of more recent fuel plant designs, including PMBR, the process was continuously upgraded in accordance with the most advanced international norms and standards. In general, the focus shifted from administrative criticality safety control to technical control, i.e., the application of safe geometry as far as possible. The implementation of geometrically-safe equipment is superior compared to administrative measures to prevent the occurrence of a critical configuration of fissile material. Safe maximum equipment dimensions are determined for certain worst case scenarios – these limits are kept throughout the geometrically-safe areas of a nuclear fuel production plant.


A lot of equipment of the former NUKEM/HOBEG was


redesigned with safe geometry considerations in place. The processes for the near-total recycling of uranium and chemicals, as well as for decontamination and purification of liquid and gaseous effluents were also developed in more recent fuel plant projects with respect to criticality safety and radiological protection. The important revival of the existing TRISO fuel


production know-how, the consideration of modern techniques and state-of-the-art safety requirements represents a challenging engineering task, which was accomplished by NUKEM at the end of the 2000s. The main target of the current century is to continuously replace fossil fuel generated, CO2-heavy process heat. This


can be achieved by building up a fleet of HTRs. Especially, the HT-SMR, which combines the advantages of the HTR with those of the SMR. SMRs can be deployed very flexibly in industrial cluster areas with high demand for process heat and NUKEM is ready to fuel the emerging HT-SMR fleet. ■


,


which is ready to be reused in the Kernel Production Facility. The HTR fuel element production plant operates as a closed loop system that is designed to approach a 100% overall uranium yield from the raw material U3


O8 Right:


The HTR fuel elements are carbonised and annealed in two consecutive furnaces where they are significantly hardened


to the


final fuel compact or sphere; and therefore, approaching zero emission. The installed quality control procedures ensure that only in-specification intermediate products (uranium kernels and TRISO-coated particles) are used to manufacture the final fuel compact or sphere which has to pass a final quality control step.


Improvements in the HTR production plant As it became evident in the early 2000s that there may be further interest in Pebble Bed Reactors, NUKEM reactivated the key personnel who were formerly responsible for the


24 | August 2023 | www.neimagazine.com


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