Fission product retention

The concepts presented in this paper emphasize the importance of diffusional phenomena during fallout formation and HTGR fission product retention. While other phenomena may be of considerable importance, these studies present a worthwhile position from which to view these processes. The advances resulting from those conceptual points of view have been considerable and it is believed are far from exhausted. 

TBP is a sufficiently powerful extractant for actinides that it may be used in diluted form. Dilution improves the hydrodynamic properties of the solvent, allowing more complete and rapid phase disengagement. Typically concentrations of 20-30 v/o TBP in OK are used in process flowsheets. Although TBP is relatively stable as an extractant, radiolysis does lead to the formation of some acidic phosphate esters, HDBP and H2MBP, which can impair process performance.289 An aqueous alkali wash of the recycled solvent is usually carried out to remove those by products. Radiolytic degradation of the diluent in the presence of nitric acid can result in the formation of hydroxamic acids,290 which can lead to fission product retention by the organic phase. Again the solvent wash is used to prevent the accumulation of such species. A comprehensive account of the industrial utilization of TBP has recently been published.291... 

The performance requirements for ceramic nuclear fuel elements include the following dimensional stability to high fuel burnups, fission product retention, corrosion resistance, high thermal performance, fabricability, economic advantage, inspectability, and chemical reprocessing and recycling. 

The fuels for fast breeder reactors include alloys such as U-Pu-Zr and the ceramic materials UO2-PUO2, UC-PuC, and UN-PuN, but the mixed oxides, UO2-PUO2, are the choice for prototype fast breeder fuel elements because of their high melting temperature, compatibility with cladding and coolants, and relatively good irradiation stability and fission product retention. The disadvantages are the relatively low metal density, the... 

Spherical fuel elements with TRISO coated particles are used, which have proven capability of fission product retention under 1 bOO C in accidents. 

The standard reactor modules and "safety-related" buildings, structures, systems, portions of systems, and components dedicated to assuring reactor shutdown, decay heat removal, fission product retention, and security, including new (unirradiated) fuel. 

The AHTR appears to have excellent safety attributes. The combined thermal capacity of the graphite core and the molten salt coolant pool offer a large time buffer to reactor transients. The effective transfer of heat to the reactor vessel increases the effectiveness of the RVACS and DRAGS to remove decay heat, and the excellent fission product retention characteristic of molten salt provides an extra barrier to radioactive releases. The low-pressure, chemically nonreactive coolant also greatly reduces the potential for overpressurization of the reactor containment building and provides an important additional barrier for fission product release. The most important design and safety issue with the AHTR may be the performance and reliability of the thermal blanket system, which must maintain the vessel within an acceptable temperature range. 

Fuel. The AHTR uses the same fuel as the GT-MHR. This high-temperature fuel has the same excellent high-temperature fission product retention capabilities. 

Beryllium oxide is used as a diluent for the UO2 to increase the diameter of the fuel rod and its thermal conductivity for improved thermal performance and also to improve fission product retention in the BeO-UO2 matrix over that possible with UO2 alone. 

The ceramic fuel and the multiple coating of the fuel kernels results in a micro pressure vessel capable of maintaining the integrity of the fuel and guaranteeing fission product retention in conditions much more severe than postulated accident conditions. 

C-SiC-C-coated fuel particles, mmntaining their integrity and fission product retention for long times at temperatures higher than those imposed during postulated extreme accidents... 

Design Basis Accidents Fission product retention Coated fuel particles Pressure essel unit Passive Passive/Active - Inherently safe fission product retention - Automatic closure fail-safe isolation valves - Unfiltered release below permissible limits (normal and disturbed operation) - Further reduetion by filtered venting of reactor building in case of disturbed operation... 

Water Air Ingress Severe Accidents Fission product retention Cooling water systems Same systems and design features as tor Design Basis Accidents Active/Passive Passive/Achve - Automatic isolation - valves - Low-temperature, low-pressure coolers - See design Basis Accidents - No accident with fuel heatmg above permissible limits... 

Fission Product Retention - Fuel element X C-SiC-C coated particle Leak-tight up to 1600°C... 

Fission Product Retention Coated particle Pressure vessel unit Passive Passive/active Inherently safe fission product retention within the fuel element For minimizing purposes Valves fail safe Further mitigation by filtered venting of the reactor building In case of non-isolatable leakages unfiltered release of coolant to environment via stack, no exceeding of limiting values of Art. 28.3 StrSchV. 

Fission product retention Same systems and design features as for design basis accidents Passive/active Same safety features as for design basis accidents... 

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