A neutron physics analysis of the modified PACER concept was conducted to assess the required liquid zone thickness of which the volume fraction is 25% in the form of Li2BeF4 (Flibe) jets and 75% as void. These liquid jets surround a low-yield nuclear fusion explosive and protect the chamber walls. The neutronic calculations assumed a 30-m-radius underground spherical geometry cavity with a 1-cm-thick stainless steel liner attached to the excavated rock wall. Achievement of tritium breeding ratios of1.05 and LIS requires a Flibe thickness of 1.6 and 2.0 m, respectively, which results in average energy densities of 24 900 and 19085 J/g. Our calculations show that for a Flibe zone thickness > 2.5 m, the activation of the steel liner and rock would be low enough after 30 yr of operation that the cavity would satisfy the U.S. Nuclear Regulatory Commission's rules for “shallow burial” upon decommissioning, assuming other sources of radioactivity could be removed or qualified as well. This means that upon decommissioning, the site could essentially be abandoned, or the cavity could be used as a shallow burial site for other qualified materials.