1955 ARC and Modelling Results While the central focus was the isothermal test data, the ARC provided further evidence that supported the conclusions from the isothermal results. The first test that was conducted was a heat-waitsearch (HWS) test, used to quickly screen different mixtures for thermal safety. The test starts at ambient temperature (25 oC) and cycles through 3 modes: 1) Heat mode ramps the temperature to the next preset increment, generally 5 oC 2) Wait modes allows some time for the sample temperature to equilibrate 3) Search mode tracks the sample temperature and if self-heating is detected, the instrument matches the sample temperature to prevent heat loss. Thermal runaway onset is the temperature where the self-heating rate exceeds 0.2 oC per minute. This is normally followed by an escalating temperature, termed thermal runaway. Figure 4 shows pure AN, AN + Pyrite and the reactive ground system of AN, Pyrite and naturally occurring weathering products exhibit progressively lower thermal runaway onset temperatures of 226 oC, 116 oC and 45 oC, respectively, demonstrating a significant reduction in thermal stability. As noted earlier, the ARC simulates a worst-case scenario, like blast holes, where heat and gases cannot easily escape. In the ARC testing, the onset of thermal runaway was 10 oC lower than in the isothermal test standard temperature of 55 oC demonstrating that self-heating and thermal runaway initiate at lower temperatures under conditions where heat dissipation is suppressed. The HWS method is primarily intended as a rapid screening tool to rank relative thermal stability and identify onset temperatures, rather than to determine time-to runaway starting from a target temperature (Stoessel, 2008). To investigate the time to thermal runaway in the reactive ground system and an inhibited system the iso-aging mode was used. Iso-aging is a single cycle of the heat wait search test that is used to determine time to thermal runaway from an initial set temperature.
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