My experience with a year and a half old MX-6 has been just fine. No evidence of pump out detected and paste consistency and temps remain just fine. Shelf life seems pretty good.
How much carbon is emitted in the mining, refining, distributing and disposing of the fissionable material used in these "zero emission" reactors?
Like EVs and their batteries and charging sources, there's more than just what's involved in the engine / reactor.