Fission gas release

The models and material property data for predicting fission gas release from heavy metal contamination and failed particles are described in Refs. 2 and 3. These models give the release-rate-to-birthrate ratio (R/B) from contamination and failed particles as a function of chemical element, isotope half-life, temperature, and burnup. In addition, the effect of fuel... 

The peak fissile particle failure for fission gas release was predicted to be 0.00085 percent the peak fissile failure for fission metal release was predicted to be 0.0063 percent. The difference between these two predicted failure rates reflects the fact that particles with failed SIC coatings but... 

Total failure for fission gas release Total failure for fission metal release... 

Two types of FP traps have been installed in JOYO. One is a cesium trap installed in the primary coolant sodium purification system to capture cesium released from failed fuels. An open pore, foam-like glassy carbon that consists of thin struts of Reticulated Vitreous Carbon (RVC) is used as a material for collecting cesium. The capacity of this trap is designed to be 7.4E+12 Bq. The other trap is a Cover Gas Clean-up System (CGCS) to collect and store the noble fission gas released from failed fuels. Although it is planned that only one failed fuel pin will be in the core at any time, the CGCS is designed to handle the releases of up to twelve failed fuel pins. 

Highly heat-conducting fuel rods - for reducing the maximum operating fuel temperature to T < 1,000°C, which provides a small (less than 7%) fission gas release from fuel and its low pressure on claddings, as well as a relatively small amount of thermal energy in fuel. 

Alternatively, enrichment could be used to flatten the channel and bundle power distributions in the core without increasing reactor power. Using CANFLEX, for example, as the carrier for SEU, peak linear element ratings could be reduced to below 40 kW/m, resulting in virtually no fission gas release during normal operation, and increased operational and safety margins. 

Optimization of the chamfer shape, pellet length-to-diameter ratio (L/D), and pellet dish. The chamfer shape at the ends of the pellets influences the size of the interpellet, circumferential sheath ridges, as does the L/D ratio. The chamfer, as well as the dishes at the pellet ends, also provide space to accommodate fission gas release. 

Optimization of radial and axial clearances provision of plenum voidage at the ends of the element to provide volume to accommodate fission gas release. Optimization of endcap shape to avoid stresses in that region. 

Use of large-grain pellets to reduce fission gas release into the free inventory. 

In addition to the WR 1 irradiations, AECL also irradiated four 19-element bundles in the NPD reactor between 1977 and 1987. Two of these bundles contained 2.6 wt% UO2 (enriched to 93 wt% U) in Th02 the other two contained 1.45 wt% UO2 in Th02- The irradiation was done at low powers (<30 kW/m) to discharge burnups of approximately 40 MW d/kg HE. The fission-gas release from these fuels was typically <1%, demonstrating good performance at low powers. 

Floyd, M.R., J. Novak, and P.T. Truant. 1992. Fission Gas Release in Fuel Performing to Extended Burnups in Ontario Hydro Nuclear Generating Stations, Proceedings of the IAEA Technical Committee Meeting on Fission Gas Release and Fuel Rod Chemistry related to Extended Burnup, (also Atomic Energy of Canada Limited, Report AECL-10636). 

Truant, R, A.J. Hains, and J.H.K. Lau. 1988. Generation Maneuvering at Bruce NGS-B Fuel Fission Gas Release Results, International Atomic Energy Agency, Vienna, IWGFPT/28, IAEA-TC-624/11. 

The fission gas bubbles present at the grain boundaries can be liberated by mechanical impact. Ruhmann et al. (1987) reported that by impact testing of typical LWR fuel pellets (irradiated to a burnup of 33 MWd/kg U) at ambient temperature with a specific load of about 1000 N m/g, 6 to 7% of the total Kr inventory of the pellet was released. Since these investigations were directed to the study of fission gas release from fuel caused by accidental mechanical impact, the analytical technique used did not aim at a quantitative liberation of the gases present at the grain boundaries. For this reason, the reported values are assumed to represent only a small fraction of the total grain boundary inventory. 

Due to the dependence of fission gas release on the fuel temperature, the advanced fuel assembly configurations (PWR 18 x 18, BWR 9x9 arrays) show only low fission gas release to the plenum when operated at correspondingly low heat ratings, as compared to the older 16 x 16 or 8 X 8 assembly configurations. 

At the same operating temperature and otherwise comparable conditions, mixed-oxide fuels rods show about the same fission gas release as UO2 fuel. This can be clearly seen from Fig. 3.17., where fractional fission gas release from the fuel pellets to the rod free volume is shown. The data also demonstrate the effect of rod power in the current or in the preceding fuel cycle (Goll et al., 1993). To be... 

Figure 3.17. Fission gas release of UO2 and mixed-oxide fuel rods as a function of the average rod power...

The APIOOO fuel rod design considers effects such as fuel density changes, fission gas release, clad creep, and other physical properties that vary with bumup. The integrity of the fuel rods is provided by being designed to prevent ... 

A bumup-dependent fission gas release model has been used to determine the internal gas pressure as a function of irradiation time (Section 4.2.3.1.2 of Reference 6.1). This information has been used to ensure that the plenum volume of the fuel rod has been designed such that the maximum internal pressure of the fuel rod will not exceed the value which would cause fuel damage. Void volume and clearances are provided within the fuel rods to accommodate fission gases released from the fuel, as well as differential thermal expansion between the clad and the fuel, and fuel density changes during irradiation. In addition, the ends of the fuel pellets themselves are dished slightly to allow greater axial expansion at the pellet centreline and to increase the void volume for fission gas release. 

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