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The successful evaluation of the safety hazards associated with fusion facilities require a thorough understanding of the material properties of the plasma facing components (PFC) during predicted accident scenarios. Current research by the Fusion Safety Program in this area is specifically directed towards addressing the consequences of steam reactivity with PFC materials during a Loss of Coolant Accident (LOCA). It is assumed that during this LOCA, water from the PFC coolant loop is injected into the torus and thus into the burning plasma.

The Steam-Reactivity Measurements System (SRMS) is used to study material properties for the above accident scenario. In particular, the SRMS measures hydrogen generation and tritium mobilization rates of irradiated specimens that are heated and exposed to steam. The experimental system consists of an integrated assembly that is housed inside of an inert-gas glovebox (shown in Figure 1).

Figure 1: Glovebox housing of SRMS system

The system is defined by the following components (refer to the schematic in Figure 2 and the zoomed photo in Figure 3):

• a gas manifold to provide an Ar or Ar-H2 mixture carrier gas,
• a compact steam generator that introduces steam into a quartz reaction chamber,
• a tube furnace that heats the reaction chamber from 27 °C to 1077 °C,
• two in-line Friedrich condensers operated at 4 °C to remove most of the exit steam the reaction chamber,
• an in-line trap operated at -23 °C for further drying of the carrier gas,
• an in-line ion chamber for measurement of the elemental tritium mobilization rates,
• a quadrupole mass spectrometer (QMS) for measurement of the composition of the carrier gas and mobilized gas species,
• an in-line oxidizer for converting the elemental tritium to the oxide form,
• an ethylene-glycol trap for collecting the oxidized tritium.

Figure 2: Schematic of experimental system

Figure 3: Close-up view of experimental system

Data from the ion chamber and post-test assay of the ethylene-glycol trap provide a measure of the tritium fraction that was mobilized in elemental form. Similarily, post-test assay of the water in the condensers provides a measure of the tritium fraction that is initially mobilized in the oxide form.

The ion chamber and QMS data are measured as a function of time so that the kinetics of the tritium mobilization and hydrogen generation rates are obtained. The total quantity of generated hydrogen, as deduced from the QMS data, is compared with the derived quantity from the measured weight gain of the specimen. The comparison provides an internal consistency check on the measured hydrogen generation rates (see Figure 4).

Figure 4: Hydrogen generation rate for several PFC candidate materials

Performance tests have been conducted to determine the QMS detection sensitivity for hydrogen and to establish an appropriate calibration method. System calibrations were performed using Ar-H2 gas mixture standards in which the H contents varied from 50 ppm to 5000 ppm. Based on these measurements, the detection sensitivity was determined to be lower than 20 ppm.