control drum is about 10 cm across and contains l0B as a neutron absorber in a 120° sector. This control technique allows reactor shutdown from normal operating conditions. The value of Keff changes from 1.05 to 0.934 with rotation of the drums and the corresponding shut-down margin is 0.071. Thermal and nuclear constraints determine the core diameter. A cylindrical core with a ratio of height to diameter of one was assumed. The maximum allowable fuel element size was determined by the upper limit of fuel temperature. The transient heat conduction program HEATING5-JR was used to calculate the temperature distribution in the fuel elements. Due to the low thermal conductivity of UO2 (<0.08 W/cm/K) axial TZM fins welded to the heat pipe and fuel element must be incorporated to keep the fuel temperature below 2070 K. These fins improve the overall thermal conductivity to 0.16 W/cm/K and limit the maximum fuel temperature to 2021 K. Fig. 4 shows the relationship between maximum fuel temperature and fuel element pitch for various core heat pipe diameters. Fuel element pitch is limited by criticality requirements. Beginning of life Keff was set at 1.05 for 0.05 excess reactivity. Fuel element pitch was determined using DOT3.5 so as to make Keff equal to 1.05 while staying within the limits set by thermal constraints. Fig. 5 shows Keff versus module pitch for various heat pipe diameters. In order to guarantee safety under accident conditions measures should be taken to ensure the reactor remains subcritical. A 12 cm diameter 10B plug in the core centre would limit Keff to 0.96 even if all of the control drums inadvertently turned to their reflecting positions. In the case of a launch accident where all of the core heat pipes become filled with water the boron plug would have to be 29.8 cm in diameter to keep the core subcritical due to the moderating effect of water. The present design incorporates a 29.8 cm boron plug in the core which would be replaced by a BeO plug of the same dimensions after orbital insertion.
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