Here you go! the PLSM reaction multiplies though O_o

local function reaction(i, x, y, s, nt)
for r in sim.neighbors(x,y,1,1) do
local rtype = sim.partProperty(r, sim.FIELD_TYPE)
if rtype ~= tpt.el.plsm.id and rtype ~= tpt.el.fire.id and rtype ~= tpt.el.smke.id and rtype ~= tpt.el.lava.id then
sim.partCreate(-1, x+math.random(-1, 1), y+math.random(-1, 1), tpt.el.thrm.id)
end
end
end
elem.property(tpt.el.plsm.id, "Update", reaction)

This uses some newer functions that not every script uses. sim.neighbors can look in a certain radius around an element for things. Then you just compare it to a list of elements not to react with, and create THRM in a random spot around it (which instantly melts into lava though).

The -1 tells it to create a new particle. You can replace the existing particles instead though, which should make it multiply a little less, I think. Use 'i' there instead to replace the PLSM, and 'r' to replace the element it reacts with.