Thermal group

Chase M W etal., 19S5,JANAP Thermochemical Tables, 3rd Edition, American Chemical Society and American Institute of Physics, for the National Bureau of Standards, Thermal Group, Dow Chemical, Midland, MI. 

Initial calculation with trial group structure consisting of the first 15 Hansen-Roach groups plus 10 thermal groups. Calculation was made to establish the critical dimensions lor the system. 

The first IS Hansen-Roach groups plus four thermal groups. 

The effect of using more energy groups (8 with S thermal groups) and higher Sn orders (S and SiJ in TW0TRAN tends to reduce keff toward unity, each by about 0.1 B 0.2%, which, although within the experimental uncertainty, does improve the accuracy in keff. 

This is the condition for the steady-state operation of a multiplying medium with only fission sources of neutrons and slowing down allow ed over the infinite lethargy range i.e., in the terminology of the next section, there is no thermal group. Condition (4.162) is called the criticality condition and it is a requirement on the nuclear characteristics of the multiplying medium. We will have more to say of this later. 

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