Direct lightning initiation of HMX

HMX can be detonated by an exploding bridgewire (EBW) or a slapper. Direct HMX detonation by an electrical arc has not been demonstrated. This paper shows that the shock from a 99% worst-case lightning current (200 kA and 500 ns rise time) can detonate standard-density HMX at elevated temperatures (i.e., HMX in the most sensitive $\delta$-phase at a temperature above 177$^\circ$C) and that the shock from a 200-kA peak current pulse with a 100 ns rise time can detonate HMX at ambient temperatures. Two necessary detonation conditions are used for these assertions. The relevant Pop-Plot for HMX is converted into an empirical detonation criterion, which is applicable to explosives subject to shocks of variable pressure. The minimum detonation spot size for detonation reported in the literature is used as the other condition. PBX-9501 and LX-04 have similar Pop-Plots to those of HMX; thus, the HMX result is directly applicable. Although PBX-9404 and PBX-9407 are HMX-based, they have somewhat lower Pop-Plot pressures than HMX and are more sensitive to shocks caused by electrical arcs than HMX. As the HMX thermal conductivity is low and the phase transition can be slow, the chance of having a sufficient amount of HMX converted to $\delta$-HMX during an accident is small; therefore, the threat of lightning shock-detonating HMX is very low. We recommend lightning simulation tests to be conducted for $\delta$-HMX. We also would like to point out that a lightning can ignite HMX, which, in turn, leads to detonation via the deflagration-to-detonation transition.