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are compared with those calculated in Step 2. If the distributions agree, then the reactor is critical and a consistent relation between pressure, temperature, fuel and coolant profiles, and energy production and absorption is determined. If the distributions do not agree, the pressure or the fuel and coolant profiles are modified and Steps 1 through 5 are repeated until a consistent critical configuration is obtained.


Radiant Energy Transfer

        The radiant energy emitted by the high temperature fissioning uranium can be absorbed by the uranium fuel, hydrogen coolant, or surrounding graphite walls. In order to obtain a high temperature propellant and also to prevent excessive heating of the graphite, it is desirable to have a large fraction of the energy absorbed by the hydrogen. If the uranium and hydrogen regions are extremely transparent, radiation will provide the means for transporting large amounts of energy to the graphite without heating the hydrogen to the desired temperature. If the uranium and hydrogen regions are made sufficiently opaque by optimizing the fuel and coolant density distributions or by seeding the coolant, radiation produced in the uranium region will travel small distances between the points of emission and absorption. The average distance