no effort is made to flatten it. The ratio of high to low power density is 3.04 if it is uncontrolled. Also, the heat pipe load near the core centre would exceed the designed capacity (with a safety factor of 2.0). Measures such as the addition of burnable poisons to the core or spatial fuel density variations should be used to flatten the core power distribution. Core Heat Pipes. Heat pipes are excellent heat transport devices due to their high conductivity, low heat capacity and lack of active elements [10, 11]. Liquid lithium was selected as the working fluid due to its large liquid transport factor over the operating temperature range. The heat pipe material is TZM (Mo-0.5Ti-0.lZr- 0.02W) which is chemically compatible with lithium. An artery wick design was used because it is most suitable for the high rate of heat transport required in this application. The heat transport capacity of a heat pipe is limited by the following five constraints: [12] • capillary force limit; • entrainment limit; • nucleate boiling limit; • viscosity limit; • sonic limit. Zero gravity performance was calculated using HPAD [17], The capillary force and sonic limits were the most restrictive in this application. HPAD can be used to find the minimum heat pipe cross section consistent with the sonic limit, working fluid inventory, and allowable radial and axial temperature drops. HPAD makes the following assumptions about the heat pipe: • negligible radial and axial pressure drop; • constant heat pipe cross section; • vapor momentum loss is ignored; • liquid flow in the wicks is laminar. Parametrics for the heat transport capability are shown in Fig. 6. Heat pipe wall thickness was determined by the lithium vapor pressure. It would be desirable to reduce the wall thickness in order to lower the temperature drop. The temperature drop across the wall in both the evaporator and condenser was 6.2 K, while the axial vapor temperature drop was negligible. A 400 mesh size artery with 0.7 porosity was used here, giving an effective pore radius of 4.0 E —5 m and a permeability of 3.0 E —7 m2. The thermal conductivity of the wall was 96 W/m/K and that of the liquid filled wick was 77 W/m/K. We optimized the following design variables to achieve the minimum systems mass: wall thickness; effective pore radius; operating temperature; evaporator length; condenser length; adiabatic section length; pipe ID; and artery diameter. Shielding. Basic conditions to be considered in the shielding calculations are as follows: • keep the fast neutron dose below 5.0 E 12 nvt for 5 year life; • keep gamma dose below 5.0 E 6 rad for 5 year life; • shape the shield to minimize mass; • keep the shield temperature below 600 K. Use of the reactor for unmanned missions is presumed in this study.
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