Fuel Sphere overview

The German High Temperature Reactor Fuel Development Program successfully developed, licensed and manufactured thousands of uranium and thorium based spherical fuel elements that were used to power the experimental AVR and the commercial THTR reactors. The main objective was to prove intrinsic safety of the plant under normal operation and all design basis accidents. In the 1970s, this program extended the performance envelope of HTGR fuels by developing and qualifying the TRISO-coated particle system. Irradiation testing in real-time AVR tests and accelerated MTR tests demonstrated the superior manufacturing process of this fuel and its irradiation performance.

The fuel kernel, at the centre of the TRISO fuel kernel, is the primary power source for the HTMR100 reactor and produces almost all of the fission products. The fuel kernel also serves as a significant barrier to radionuclide release by immobilizing many of the fission products as stable oxide compounds and delaying the diffusive release of others, allowing them to decay into more stable isotopes. These processes substantially reduce fission product release from particles.

• The buffer layer bonds to the fuel kernel and is a low-density, porous carbonaceous layer. The buffer layer provides void volume for the accumulation of gaseous fission products released from the fuel kernel, accommodates fuel kernel swelling, and serves as a sacrificial layer for fission fragments.
• The inner pyrocarbon (iPyC) layer, between the buffer and SiC layer, is a gas-tight coating that protects the kernel from hot gaseous chlorine compounds during SiC decomposition and provides a smooth substrate for SiC deposition. The iPyC completely retains fission gases xenon, krypton, and iodine and also serves as a diffusion barrier to metallic fission products. During irradiation this layer shrinks and the contraction helps to reduce tensile stresses on the SiC.
• The third layer is a near-theoretical density SiC layer which serves as the pressure bearing component of the particle and the primary metallic fission product diffusion barrier as well as retains all gaseous fission products. The SiC layer is the primary load bearing layer of the particle.
• The fourth and outer layer is another high-density, isotropic layer, called the outer pyrocarbon (oPyC). This layer serves as a further diffusion barrier for gaseous and metallic fission products, and like the iPyC layer, it too contracts during irradiation helping to reduce tensile stress on the SiC. The oPyC also protects the SiC during particle handling and sphere/compact formation and provides a bonding surface to the carbon matrix in the Fuel Element.

The fuel element (FE) for the HTMR100 is a 60 mm diameter sphere, known as a pebble, consisting of a spherical fuel zone of approximately 50 mm diameter, in which the TRISO-coated particles are randomly distributed in the graphitic matrix material. A fuel-free shell of graphite matrix of about 5 mm in thickness is then moulded to the fuel zone.

The matrix material consists of a carbonized organic binder and nuclear-grade graphite material that acts as a fission neutron moderator, heat transfer medium, and protection against external forces. The graphitic matrix material exhibits high density, high thermal conductivity, high mechanical strength, low thermal expansion, low anisotropy, low Young's modulus, good corrosion resistance, good dimensional stability under neutron irradiation, and a very low concentration of impurities.

HTMR Fuel project

The primary goal of the HTR fuel development programme is to produce fuel spheres containing mixtures of uranium and thorium and only uranium for irradiation testing in the short term as well as thorium and plutonium in the long term.

The fuel will be based on the original German manufactured fuel with the exception that the kernel will be made up of only uranium dioxide (UO₂) and also a homogeneous mixture of uranium dioxide (UO₂) and thorium dioxide (ThO₂) while the long term fuel will consist of a homogeneous mixture of plutonium dioxide (PuO₂) and thorium dioxide (ThO₂).

This involves the development of the Kernel, Coated Particle and Fuel Sphere laboratories processing (UO₂) and (Th,U)O₂ powders as well as (Th,Pu) O₂ to produce fuel to specification. The Quality Control methods and testing equipment required shall also be developed.

After development of the fuel and in the longer term, Fuel Sphere manufacturing plants will be designed to produce required quantities of released fuel spheres per annum.

The fuel plants could be built on licensed sites in the country that has a HTR development programme while the (Th/Pu) fuel plant could be built on a licensed site in a country in close proximately to a secured supply of plutonium feed. The Fuel plants could be operated by local licensed operators and the plants will be licensed by the country's nuclear regulators.

Kernel Process Overview
The Kernel Facility would produce (Th,U)O₂ or (Th,Pu)O₂ kernels (sintered microspheres) 0,5 mm in diameter from a casting solution prepared from mixtures of uranium nitrate (U(NO₃)₄) and or plutonium nitrate (Pu(NO₃)₄) and thorium nitrate (Th(NO₃)₄) as well as an alcohol additive to modify the viscosity of the solutions.
The casting solutions are fed through vibrating nozzles, where the liquid stream is broken up into spherical droplets. As the spherical droplets pass through an ammonia gas curtain they solidify enough to withstand the impact on entering the liquid precipitation medium. The gelled spheres are then aged, washed, dried, calcined and sintered; defective kernels are removed by sieving and sorting. The product is then sampled and after analyses and release, batches are combined into a lot and then portioned into batches for the Coated Particle Facility.

Coated Particle Process Overview
The CVD Facility pyrolytically deposits layers of carbon and an intermediate layer of silicon carbide (SiC) on 0,5 mm diameter kernels. The carbon buffer layer is deposited from acetylene (C₂H₂) in argon, the inner and outer isotropic pyrolytic carbon (PyC) layers from a mixture of acetylene and propylene (C₃H₆) in argon, and the SiC layer from Methyltrichlorosilane (CH₃SiCl₃, MTS) using hydrogen as carrier gas. The MTS is heated in an evaporator while H₂ gas sweeps the vapour into the CVD reactor. A gas feed system feeds process gasses into the reactor at controlled rates at a high accuracy. An off-gas handling and combustion burner system scrubs the HCl produced in the reaction and removes soot from the gas. Unburnt hydrocarbon gasses are also combusted in a controlled way to remove the risk of build-up of explosive gasses in the extraction system.

The coated particles are then sieved to reject oversized or undersized particles. The sieved particles are then transported to sorting tables where good particles (spherical) are sorted from odd shaped or broken particles. The individual batches are sampled using a percentage sampler and then sent for chemical and physical analyses. After quality release, the batches are combined (homogenised) and then split into the quantities required for overcoating.

Fuel Sphere Process Overview
The Fuel Sphere Manufacturing Facility produces spherical fuel elements by means of assembling the two fuel sphere components i.e. coated particles and matrix graphite powder. Over-coated particles are produced by coating coated particles with a thin layer of graphite in an over-coater dish. The particles are dried, sieved, sorted and portioned into representative batches.

Over-coated particles and graphite are mixed and pressed into a fuel core; a fuel free zone is then pressed onto the fuel core to produce a green fuel sphere. The pressing performed during both pressing stages is done in a silicone rubber mould to deliver a semi-isostatic pressing action. The pressed fuel spheres are machined to a specific shape and dimension. Heat treatment is performed in two steps i.e. carbonising and annealing. The carbonising process converts the phenolic resin into carbon under an inert gas atmosphere. The annealing process is a purification step to drive off certain volatiles. Annealing is performed at high temperature under a vacuum. The final heat treated spheres are then examined for defects by means of visual-, dimensional- and X-Ray inspection.

Fuel spheres that passed the in-line quality inspections are loaded into Fresh Fuel Containers. Only quality released fresh fuel are released to the reactor for usage.