the halflife is is still the same it just the cell would be using the bare minimum amount of curries and as it ages its radioactivity entrophies making it produce less heat
so it would only produce enough heat to reach critical point for a decade after twenty years it would only get 60% of original activity 15 yrs would be the max on 20000 curies and it would be more expensive to have larger cells to compensate the loss still $720 a year for fuel just in a car is worth it
neil do u know the density of carbon nano tubes