2018 | AFC ACCOMPLISHMENTS 15 Figure 2. A schematic of the ATF-2 test train and fuel pin holders. insertion of the experiment into the ATR necessitated nuclear, thermal hydraulic, and structural analyses and a test in the ATR critical facility to satisfy the requirements specified in the ATR Safety Analysis Report (SAR). Given that the experiment represents the first loop in the ATR to simulate commercial light water reactor conditions and the difficulties in meeting the competing priorities of the vendor teams, the experiment challenged the entire irradiation testing infrastructure at Idaho National Laboratory (INL), including the experiment design and analysis groups, the engineering staff at ATR, and the fabrication facilities at North Holmes Labs, Materials and Fuels Complex (MFC) and ATR. Their efforts have led to the development of the first experimental apparatus in the United States dedicated to the steady- state testing of commercial light water reactor fuels. Accomplishments: Success of the ATF-2 experiment required a substantial engineering effort. Methods to control coolant chemistry, including boron, lithium hydroxide, and dissolved oxygen and hydrogen concentrations were developed. As one example, Figure 1 shows the boron addition system that was designed and fabricated that saves weeks of operation time for each ATR cycle. The holders and test train (see Figure 2) were designed so that the fuel pins could be removed after reaching a desired burnup and so the holder could be reconstituted with fresh fuel pins while sitting in the ATR canal during outages. The holders were fabricated either from stainless steel or from stainless steel with hafnium shrouds so that power levels were kept within specified bounds. Safety and programmatic analyses were performed to demonstrate that programmatic goals would be reached and ATR safety requirements were met. Difficulties in this regard included schedule pressures and design changes through the course of the project that caused significant amounts of re-analysis. Despite these challenges, the ATF-2 loop experiment was inserted into the ATR in June of 2018. The successful installation of the experiment is a testament to the dedicated teams at both the fuel vendors and INL.