When or if the US sells a second-hand Virginia
SSN to Australia in 2033 Australia might get a nasty shock when it realises a
once fueled Virginia likely has a shorter operating life than a twice fueled Los
Angels SSN.
The amount of reactor usage is often quantified in
years of use which also correlates to the time the reactor is
fully active in port and also when it is underway. “Underway” might be measured
in the distance in nautical miles the sub has traveled and at what speeds in
knots.
Another way of measuring useful reactor life is
the amount of efficiently fissionable U235 remaining vs perhaps more
importantly the level of neutron poisons build-up that inhibits efficient reactor operation.