Median fission energies

The calculations showed that Pu-238 systems aie about as fast in neutron energy (the median fission energy is about 1.3 MeV) as those of Pu-239. Figure 1 presents the calculated critical masses of bare Pu-238 spheres. These results agree well with Hanford Monte Carlo calculations which used the Butler cross-section measurements. In addition, subcritical and critical material-replacement experiments, and fission cross-section measurements have been performed on the Jezebel assembly at LASL. Their results Indicate that the reactivity worth of Pu-238 is at least as niuch as that of Pu-239 (Ref. 7). Figure 1 shows that the Pu-238 critical mass is ioughly one-half that for Pu-239 (Ref. 8). 

The FCX is a cylindrical twb-zoned reactor.. The central zone is essentially a graphite enriched uranium mixture and the outer zone is a graphite reflector. The core region Of this reactor has an intermediate energy spectrum typical of NERVA reactors with a median fission energy of 300 eV. 

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