2017 | AFC ACCOMPLISHMENTS 113 The Multi-SERTTA irradiation vehicle will enable high throughput nuclear-heated transient safety research on ATF specimens, surrounded by hot pressurized water, in the TREAT facility. Test Apparatus (Multi-SERTTA). Multi-SERTTA is capable of static liquid water conditions representing pressurized water reactors (280°C, 15.5 MPa), enables high throughput testing, and offers a respectable instrumentation package suitable for Reactivity Initiated Accident (RIA) type testing of ATF fresh fuel samples. A larger single-vessel version (Super- SERRTA) will offer enhanced natural convection, increased instrumentation options, and structural capability for RIA and Loss of Coolant Accident (LOCA) testing as needed for pre- irradiated fuel specimens. Finally, full PWR forced convection capabilities, in addition to all of Super-SERTTA’s capa- bilities, will be available in theTREAT Water Environment Recirculating Loop (TWERL).These water-bearing irradiation vehicles will enable a full suite of safety testing capabilities on water-reactor fuels at any point in their development life cycle (Figure 1).The Multi-SERTTA design has been the primary focus of recent design and engineering efforts in order to enable timely testing of ATF specimens under the ATF-3-1 campaign. Owing to Multi-SERTTA’s novel layout, a full size out-of-reactor prototype was needed to qualify its design and ensure its success.A modified Multi-SERTTA design was created to enable nuclear calibration measurements (referred to as -CAL). SinceTREAT had no historic examples of core-to-specimen power coupling factors (PCF) measured in the presence of Pressurized Water Reactor (PWR)-condition water, this first-of-a-kind -CAL design had to both provide equivalent reactivity worth for trial core operations, and enable PCF measurements on fission- able dosimeters surrounded by hot pressurized water.The principle design modification performed for -CAL involved placing a small inner tube through each Multi-SERTTA vessel so that dosimeter could be surrounded by the water, but not immersed in it, thus enabling timely extraction, dosimeter radiation measurement, and radiochemical analysis in order to determine PCFs (Figure 2). Accomplishments: In order to support the Multi-SERTTA prototype fabrication and testing effort, a detailed prototype testing plan was developed and complete to accomplish a level 3 milestone and prototype fabrication work commenced. Simultaneously, the -CAL design underwent detailed engineering, performance predic- tions, and safety analyses in prepara- tion for a design review.This design review was performed in mid FY17