well, unless you bring a number of the inner contents of several such packs into very close proximity, at which point the reaction is going to be very much accelerated.
Warranty probably not valid if you get anywhere near criticality ;-)
Wrong isotope. The Pu238 used in RTGs is not fissile (does not undergo a chain reaction) - it decays by alpha decay. The fissile isotope used in weapons is Pu239.
Warranty probably not valid if you get anywhere near criticality ;-)