The Special Treatment Required for Hydrogen

The practical advantages of the use of water as a moderator and as a coolant make it necessary to consider methods of improving the age-diffusion approximation. 

The more accurate treatment required for hydrogenous systems can be separated into two classifications  

The essentials of the age-diffusion method may be retained, but various parameters of the calculation may be chosen to give better results for hydrogenous reactors (a) The group diffusion models discussed in Chap. 8 may be applied with particular choices of the group constants designed to yield improved accuracy. These constants are not necessarily calculated in any prescribed manner but are obtained from meas- 

In general, a particular method of the second class will give useful results only for a limited range of types of reactors thus, in undertaking a new design analysis, considerable effort must be applied to find the appropriate constants. 

The most important of these is the second, and in many cases, to require that the second moment be adequately represented is regarded as sufficient. In the age-diffusion approximation, the second moment is proportional to the age, and it was shown previously that 

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