Fuel-Cycle Coat

Reactor core fuel -type - enrichment by 235U, %, maximum -average burn-up, MWday/kg -fuel cycle, eff. days, minimum - number of refuellings Based on UO2 w/multi-layer coatings 20 125 900 3... 

A once-through fuel cycle is planned until fuel reprocessing of coated particle fuel can be effectively and economically achieved. Once available, centralized or regional reprocessing would be expected. 

Both open and closed fuel cycle options are possible for the VKR-MT. Open fuel cycle scheme may be essentially the same as for the VVER and VK-300 [X-1] reactors. The specific feature of a VKR-MT closed cycle is that the SiC outer coating of micro fuel elements is resistant to nitric and sulphuric acids. Therefore, the mechanical removal of these coatings in ball mills should be added to a process chart [X-3]. The remaining layers of pyrolythic graphite are removed through the heating of micro fuel elements in air at 800 C. After that, a conventional aqueous method could be applied to reprocess the uranium dioxide fuel. 

The standard fuel cycle option for the CHTR would depend upon the technology development for reprocessing of TRISO coated particle fuel and fuel compacts. In case of the development of a reprocessing technology, a closed nuclear fuel cycle option would be adopted. Fresh fuel for the reactor would be made from and Th recovered from the spent fuel. Alternately, it would be a once though fuel cycle without reprocessing. The objective then would be to achieve the highest possible fuel bum-up. 

Fuel cycle option, basic Once-through fuel cycle U dioxide fuel in TRISO coated particles within graphite spheres on-line refuelling Once-through fuel cycle U dioxide fuel in TRISO coated particles within graphite fuel compacts... 

Once-through fuel cycle with the uranium dioxide fuel in TRISO coated particles is identified as basic for all HTGR designs considered. 

As an alternative, closed fuel cycles with MOX or hybrid U-Th fuel are considered for the GT-MHR. An option to apply fuel reprocessing is mentioned for the HTR-PM and the GTHTR-300 for the latter it has already been investigated. In case of TRISO coated particles, aqueous reprocessing methods (e.g., PUREX) cannot be applied directly. Specifically, the problem is with the SiC coating layers, which are not dissolved in mixtures of acids and, therefore, require a mechanical treatment to be removed. 

An option to use thorium fuel (GT-MHR) and an option to operate in a closed fuel cycle with the reprocessing of the TRISO coated particle fuel (GT-MHR, GTHTR300, HTR-PM) for the GTHTR300 it is indicated that the feasibility of TRISO fuel reprocessing has already been investigated. 

The intrinsic proliferation resistance features of the AHTR are essentially identical to that of the gas cooled reactors using graphite matrix coated particle fuel (see Section 4.5.2), because the same fuel and fuel cycle are used. 

The currently targeted discharge bum-up of the LEU spherical fuel elements is 80 000-MWd/t U. Spherical fuel elements with coated particles have the potential of reaching a much higher bum-up, as it has been demonstrated in previous high temperature gas cooled reactors. It is expected that high fuel bum-up will contribute to the reduction of fuel cycle costs. 

The AHTR uses the same fuel and has the same fuel cycle options as gas cooled reactors with coated particle fuels. As a consequence, the provisions for sustainability and waste management are essentially identical to those for high temperature gas cooled reactors. 

The VHTR has two typical reactor configurations, namely the pebble bed type and the prismatic block type. Although the shape of the fuel element for two configurations are different, the technical basis for both configuration is same, such as the TRISO-coated particle fuel in the graphite matrix, foil ceramic (graphite) core structure, helium coolant, and low power density, in order to achieve high outlet temperature and the retention of fission production inside the coated particle under normal operation condition and accident condition. The VHTR can support alternative fuel cycles such as U—Pu, Pu, mixed oxide (MOX), and U—thorium (Th). 

Pebble bed and prismatic reactor are the two major design variants. Both are in use today. In either case, the basic fuel construction is the TRISO-coated particle fuel. Uranium, thorium, and plutonium fuel cycle options have been investigated and some have been operated in the reactors. Spent fuel may be direct disposed or recycled. The unique constmction and high bumup potential of the TRISO fuel enhances proliferation resistance. 

The fuel for a gas reactor uses tiny particles of uranium or plutonium oxide coated with carbon and silicon carbide. The particles create a barrier to the release of fission products and can withstand maximum attainable accident temperatures. The GT-MHR has a three-year operating fuel cycle—half of the fuel in the reactor core is replaced every 18 months while the reactor is shut down. By contrast, the PBMR has continuous refueling with the reactor in operation. Both designs use inert helium gas as a coolant. 

Current power reactors primarily use onee-through fuel cyeles. Uranium mines, ehemieal eonversion faeilities, and uranium enriehment plants already exist. The only commercial-scale eomponent of the fuel eyele that does not exist for these reactors is fuel fabrication of the coated-particle fuel. The basic fuel fabrication technology, however, exists. Thus primary requirement is to develop and demonstrate the fabrication technology on a commercial scale. The SNF from these reactors can be directly disposed of. Consequently, the fuel cycle status of these two reactors is given a high rating. 

Monolithic refractory coatings have been applied to metallic components in furnaces for fuel ash corrosion control. Results have been less than satisfactory because of the large thermal expansion mismatch between the metal and refractory. Failure usually occurs upon thermal cycling which causes cracking, eventual spalling of the refractory, and direct exposure of the metal to the effects of the fuel ash. 

The nozzle of original design was fabricated from a niobium alloy coated with niobium silicide and could not operate above 1320°C. This was replaced by a thin shell of rhenium protected on the inside by a thin layer of iridium. The iridium was deposited first on a disposable mandrel, from iridium acetylacetonate (pentadionate) (see Ch. 6). The rhenium was then deposited over the iridium by hydrogen reduction of the chloride. The mandrel was then chemically removed. Iridium has a high melting point (2410°C) and provides good corrosion protection for the rhenium. The nozzle was tested at 2000°C and survived 400 cycles in a high oxidizer to fuel ratio with no measurable corrosion.O l... 

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