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7. Natural Circulation Flow and Decay Heat Removal One of the most difficult design and safety issues for a gas reactor is the loss-of-flow accident, which is an anticipated event. A loss-of-flow condition can occur due to a loss of power to the recirculating compressor or compressors, a fault such as a seizure in the compressor shaft bearings, or some other fault that disrupts the compressor operation. It is assumed that for this anticipated event, the plant protection system would shut down the reactor down by inserting control and safety rods. The pump coast-down and inertial flow of the coolant gas may allow for the removal of a significant fraction of the heat generated during the ~80-second reactor shutdown period. However, the decay heat generated due to the fission product decay must be continually removed to keep the reactor fuel and cladding from overheating and melting. Cooling the core can be achieved by a separate active auxiliary decay heat removal system or by removal of heat from the pressure vessel by conduction through the core structure. For a thermal gas reactor, the moderating material may allow for significant heat capacity and heat removal by conduction through the core radially to the boundary; however, the operating power density of the core must be limited such that the decay power can be removed without overheating the fuel. For a gas fast reactor, such as the SC-GFR concept, the moderating materials are non-existent and the structural materials are insufficient for conductive heat transfer to the boundary. Decay heat removal using natural circulation flow is achievable in water reactor systems, since a relatively large pressure head can be developed for a small change in temperature from the cold leg to the hot leg. It will be shown in this section of the report that natural circulation flow can also be established in an S-CO2 system, if the system remains pressurized. SNL has been operating two S-CO2 loops since 2008. Occasionally, very large flow rates have been observed well after termination of the experiment, even after all heaters and turbomachinery were shutdown. Because there were large temperature gradients still remaining in these loops, the flow has been attributed to a combination of condensation effects and natural circulation. These early observations prompted SNL to examine the natural circulation capability of S-CO2 by examining the Grashof number (Gr), which is a dimensionless number providing the buoyancy-to-viscous force ratio, Gr g2 Lc3 T 2 where g is the gravitational constant, is the coefficient of thermal expansion, is the density, Lc is the characteristic length, T is the temperature difference (wall to bulk), and is the viscosity. A comparison of Gr for CO2 and water is illustrated in Table 6, for g = 9.81 m/s2, Lc = 1 m, and T = 10°C. The table shows that at room temperature (300 K) and 7.69 MPa, Gr is 5.9e14 for CO2 and 7.5e10 for water. These values were calculated at temperatures and pressures near the operating conditions for the CO2 in the waste heat-rejection system of the cycle. For reference, these conditions are near the critical point of CO2 that occurs at 7.37 MPa and 304.1 K. This four order of magnitude increase in Gr for CO2 compared to water provides clear evidence of the tremendous potential for natural circulation flow and decay heat removal in S-CO2-cooled reactor systems. This would apply to both direct and indirect cycles. 40PDF Image | Supercritical CO2 Direct Cycle Gas Fast Reactor (SC-GFR)
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