b'Figure 2. Comparison of the calculated fission gas Figure 3. Comparison of the thermal conductivity release for the standard UO 2fuel and for the UO 2 -5 vol %for the standard UO 2fuel and for the UO 2 -5 vol % diamond composite. diamond composite.Test Description ResultsA UO 2 -5 vol % diamond compositePeak fuel temperature rodlet containing eight fuel pellets andComparison of the peak fuel temper-two depleted uranium oxide insulatorature for the standard UO 2fuel and pellets was irradiated as part of theUO 2 -5 vol % diamond composite is ATF-1 experiment in a small I positionshown in Figure 1.In the beginning of the ATR operated by INL.This wasof life the peak temperature of the a non-instrumented drop-in test thatstandard UO 2fuel is 180C higher. utilized the double encapsulationAt the end of life, the peak tempera-concept to achieve prototypic claddingture of the standard UO 2fuel is temperatures.In this design, the test511C higher.Standard UO 2fuel rodlet is housed in a sealed helium- exhibits higher temperature due to filled stainless steel capsule. As the ATRits lower thermal conductivity and inlet coolant temperature is only 52Cdue to the increase of the thermal and the desired cladding temperatureresistance of the fuel-cladding gap is 360C, a significant thermalwhich occurs after the onset of the resistance between the rodlet andfission gas release.the coolant is required to achieve theFission gas release desired cladding temperature. DoubleComparison of the fission gas release encapsulation introduces a heliumfor the standard UO 2fuel and for the gap between the test rodlet and theUO 2 -5 vol % diamond composite capsule, thus providing the necessaryis shown in Figure 2.These results thermal resistance.2019|AFC ACCOMPLISHMENTS 157'