High Temperature Feedback effects

The assumption is that once the feedback associated with temperature is known the negative feedback coefficient for the reactor can be estimated. This will determine if sufficient excess reactivity is present for the desired mission lifetime. The dimensions of the reactor when heated to the operating temperatures of 1300 K for peak fuel temperature and 860 K in the pressure vessel are shown in Table 5-2  

This ensures that while the volumes of components in the core are changing, the mass of each material remains constant. Finally, the cross sections of the materials being used in the core were corrected to those at operating temperature The vast majority of the materials in the core do not have readily available cross sections at the desired temperature. The NJoy code was used to generate cross sections for the different materials in the reactor [MacFarlane, 1994]. The temperatures used were the peak values from the FEPSIM model. These temperatures are probably somewhat high, but are a much better approximation than using room temperature cross sections. This run resulted in a k-effective of 1.022 0.001. The net reactivity swing of the system was 2. With a total excess reactivity of 5.7 this leaves another 3.7 for bumup and other losses. 

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Figure 5-3 Flux Profile Vs Positiou for 3 differeut reflector positious 5.3 High Temperature Feedback effects ... 

The AHTR has the potential to provide a highly robust safety case because of various inherent and passive safety characteristics. Inherent safety characteristics include a low-core-power density, high-heat-capacity core, and high-temperature-margin fuel. Other inherent safety characteristics of the AHTR include atmospheric pressure operation and efficient liquid-coolant heat transfer. Reactor physics for the AHTR are similar to other graphite based, coated-particle fuel systems (GT-MHR) where negative feedback comes from the high-temperature Doppler effect within the fuel. 

Since feedbacks may have a large potential for control of albedo and therefore temperature, it seems necessary to highlight them as targets for study and research. Besides the simple example above of cloud area or cloud extent, there are others that can be identified. High-altitude ice clouds, for example, (cirrus) have both an albedo effect and a greenhouse effect. Their occurrence is very sensitive to the amount of water vapor in the upper troposphere and to the thermal structure of the atmosphere. There may also be missing feedbacks. 

There is a potentially important influence of ice and snow at the surface, for ice and snow have high albedos. Ice-albedo feedback may increase the sensitivity of the climate system. That is, low temperature causes more ice and a higher albedo, allowing less absorption of sunlight and therefore causing a still lower temperature. This temperature effect is included by... 

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