ChemicalEngine inherits up to FusionECU2, which sets lethalDistance to 2000 for radiation distance safety checks. The Halberd is a Rotation Detonation Engine, and I don't believe the KSP version was intended to emit radiation.

Not sure how many other engines this affects, but I created a MM patch as a workaround.

@PART[kspiHalberd]
{
	@MODULE[ChemicalEngine]
	{
		lethalDistance = 0.0001
	}
}