2018 | ANNUAL REPORT 105 evaluate the effects of fuel form (i.e. annular vs solid) and fuel-design variables on pin performance.The test assembly was irradiated for 312 equivalent full power days at peak power of 45.6 kW/m, peak burnup of 65.2 GWd/tHM and peak fast fluence of 9.9 × 1022 n/cm2. Initial post-irradiation examination (PIE) of the overall assembly and selected pins showed good and predictable perfor- mance of MOX with HT-9 cladding . However, data remain limited to two studies of single pins [6,7]. A more systematic study of the perfor- mance of MOX annular fuel pins has been undertaken in this project, with the specific objective both to ameliorate the understanding of the effects of irradiation temperature and burnup on the fuel microstructural evolution and to provide supporting data for validation of fuel-perfor- mance models. Project Description In the FO-2 assembly, there were 12 different fuel variants.Ten variants were solid MOX pellets and 2 were annular MOX pellets.This work focuses only on the annular variants. Four pins of two different types were selected: two of the L-type and two of the K-type. Design parameters of each pin are summarized inTable 1.The two types of pins have the same nominal design data, except for the initial Pu content, which is 26 wt% for the L-type and 22 wt% for the K-type. Design parameters K-type L-type Cladding Material HT-9 HT-9 Outer diameter (mm) 6.858 6.858 Cladding thickness (mm) 0.533 0.533 Plenum volume (cm3 ) 23.6 23.6 Fuel Pellet Material MOX MOX Outer diameter (mm) 5.59 5.59 Inner diameter (mm) 1.397 1.397 Pellet length range (mm) 6.27 to 8.66 6.27 to 8.66 Gap width (mm) 0.203 0.203 Smeared density (%TD) 80±1 80±1 Pellet density (%TD) 91.7 91.7 Oxygen-to-metal ratio 1.96±0.010 1.96±0.010 Pu content (Pu/(Pu+U) wt%) 22 26 Table 1. Design parameters of the FFTF pins.