A reactivity addition accident that involves high addition rates attaining prompt criticality might arise from rapid voiding of the central assemblies of the LMFBR, an ejection of a control rod in some way, fuel slumping following local melting, or even from mechanical compaction of the fuel in a high-pressure fast reactor depressurization.

The improbability of these initiators is discussed in Section 5.4. It is sufficient to say that each could occur mechanistically, and therefore should be the subject of further study. These events could give reactivity addition rates in the range of from $ 5/sec to $ 200/sec (see Table 5.6).