In a conventional reactor, a moderator is needed to slow down the neutrons so that they are more likely to fission atoms.
This makes it possible to increase the yield of any given fusion weapon by the simple expedient of adding layers of cheap natural (or even depleted) uranium.
Fast fission of uranium-238 provides a large part of the explosive yield, and fallout, in many designs of hydrogen bomb.
With thermal neutrons, yields of fission products with mass between the peaks, such as 113mCd, 119mSn, 121mSn, 123Sn, 125Sb, 126Sn, and 127Sb are very low.
The higher the energy of the state that undergoes nuclear fission, the more likely a symmetric fission is, hence as the neutron energy increases and/or the energy of the fissioning atom increases, the valley between the two peaks becomes more shallow; for instance, the curve of yield against mass for 239Pu has a more shallow valley than that observed for 235U, when the neutrons are thermal neutrons.