A very superficial reaction, indeed

local function neighbours(x,y,r,type)
    local array = {}
    for dx=-r,r do
        for dy=-r,r do
            if tpt.get_property("type", x+dx, y+dy) == type then array[#array+1] = sim.partID(x+dx, y+dy) end
        end
    end
    return array
end

local ntrm = elements.allocate("ELEMENT", "NTRM")

elements.element(ntrm, elements.element(elements.DEFAULT_PT_PLUT))
elements.property(ntrm, "Name", "NTRM")
elements.property(ntrm, "Description", "Neutronium. Begins a chain reaction upon collision with neutrons.")
elements.property(ntrm, "Colour", 0x00AAAA)
elements.property(ntrm, "HighTemperature", 10001)
elements.property(ntrm, "HighPressure", 257)

local function ntrmUpdate(i,x,y,s,nt)

    local clife = tpt.get_property("life", i)
    local percent = 10
    local parts = neighbours(x, y, 1, elements.DEFAULT_PT_NEUT)

    if clife <= 0 then
        tpt.set_property("life", 3, i)
    end
    if clife > 0 and #parts > 0 and math.random(1, 1/(1-((1-percent/100)^(#parts)))) == 1 then
        local dx = math.random(-1,1)
        local dy = math.random(-1,1)
        if clife == 1 then
            sim.partCreate(-3, x+dx, y+dy, elements.DEFAULT_PT_NEUT)
            sim.partKill(i)
        else
            sim.partCreate(-3, x+dx, y+dy, elements.DEFAULT_PT_NEUT)
            tpt.set_property("life", clife-1, i)
        end
    end

end

elements.property(ntrm, "Update", ntrmUpdate)