M - Materials.
delta-phase plutonium-239-gallium alloy (inner sparkplug layer)
+ its plutonium-240/241 and americium parasite impurities
uranium-233 deuteride (outer sparkplug layer)
+ its uranium-232 parasite impurities
highly-enriched uranium-235 (tamper/pusher)
reactor-grade uranium-238/235 (radiation case liner)
deuterized polyethylene (sparkplug laying and shock barrier)
metal tritide-deuteride (DT-boosting and neutron tube)
cesium (neutron source, maybe)
germanium-impregnated FOGBANK plastic/"aerogel" (radiation channel filler)
beryllium (pusher/reflector)
lithium-6 deuteride (fusion fuel, but probably already tested before, cuz just salt)
tungsten, titanium, stainless steel - in assortment, just as a part of the assembly in whole
carbon fiber, silicon carbide (cuz spysat film capsules didn't need it for heatshield)
hafnium carbide, tantalum carbide (as an improvement of the previous item)
nickel, copper, gold, rhenium (liners, plutonium insulation, tamper admixture)
PBX-blah-blah-digits super-fine safe explosive (implosive sphere and initiation layer)
Probably, missed something.
In the assembly the materials are shielding each other, so it's not enough to just expose the plates.
And there are no evil Russkies with radiation detectors onboard.
For everything other there is ISS.