Gas As a Reactor Coolant

Gas cooling was originally chosen for thermal-spectrum reactors because it provided a means of removing heat from a reactor core without 

From a very fundamental point of view, a coolant must be capable of removing the heat from a reactor with a reasonable expenditure of pumping power. The heat removal rate per unit frontal area of coolant is simply the product of the coolant density, velocity, specific heat, and the temperature rise across the core, i.e., pvCp AT. Typical values of these parameters for water, helium, and sodium coolants are illustrated in Table 11. The coolant-gas velocities are generally consistent with a pumping power of 5 to 10% of the electrical output of the nuclear plant. 

Although the density of the gas coolant is considerably smaller than that of the water or sodium coolant, the allowable temperature rise and coolant velocity can be substantially larger. In addition, of course, the coolant cross-sectional area can be made larger if necessary, with some sacrifice to the power density but without neutron absorption. The temperature rise for the water coolant is generally limited by the amount of boiling allowable, whereas the temperature rise for the sodium coolant is generally limited by thermal-stress problems that can arise in metal 

Typical Ccxjlant Characteristics for Water-, Gas-, and Sodium-Cooled Reactors 

For the coolant conditions shown in Table II, the cross-sectional coolant channel area required for helium cooling would be about the same as for water and only about twice that required for sodium. By allowing a coolant pressure of 70 atm (about 1000 psia), the helium coolant is superior to pressurized water at 1500 psi and approximately equivalent to sodium in its heat removal capacity per unit coolant area. The capacity for heat removal should not, of course, be confused with the conductivity of the coolant. In this respect, the sodium coolant is superior to the high-pressure gas, although it can be argued that the high conductivity of the sodium coolant is not particularly necessary and indeed creates some problems in thermal stresses. 

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