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In this paper the effect of changing from the traditional NTP coolant, hydrogen, to several alternative coolants were studied. Hydrogen is generally chosen as an NTP coolant, since its use maximizes the specific impulse for a given operating temperature. However, there are situations in which it may not be available or optimal. The alternative coolants which were considered are ammonia, methane, and carbon dioxide. A particle bed reactor (PBR) generating 200 MW and cooled by hydrogen was used as the baseline against which all the comparisons were made. Both 19 and 37 element cores were considered. The larger number of elements was found to be necessary in the case of carbon dioxide. The coolant reactivity worth was found to be directly proportional to the hydrogen coolant content. It was found that due to differences in the thermophysical proportions of the coolant that it would not be possible to use one reactor for all the coolants. The reactor would have to be constructed specifically for a coolant type.

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