A larger reactor will have either a higher power output, will be able to operate longer, or both. Each fission produces approximately 200 MeV in the form of gamma radiation and the kinetic energy of two fission fragments that are produced. This energy is dissipated in the fuel material as the fission fragments interact with surrounding atoms and slow down. The fission fragments are nearly always of uneven mass with varying probabilities of production. The distribution of fission products is characteristic for each fissionable nuclide and can be used to identify the fuel.
Some fission products have high thermal neutron capture cross sections.
These can accumulate and poison the chain reaction resulting in either lower power or increased neutron flux to compensate. They are included in the thermal utilization factor in the sixfactor formula. The approximate shape of the reactor zones is that of a compact mass of uranium oxide surrounded by porous rocks, which were presumably hydraulically connected to surface or ground water, allowing moderation and reflection of the neutrons produced by spontaneous fission or cosmicray induced fission. The relatively large size and spherical shape of the uraniumbearing region reduced buckling. When the surrounding porous rocks were saturated with water, the subsequent moderation and reflection allowed the reactor to achieve criticality.