Or is the reason Australia is getting second hand boats is that the reactor material of those boats will be sufficiently degraded that extracting and chemically recovering enough material for nuclear weapon use would be extremely difficult or impossible?
Ah no, nuclear physics doesn’t work like that. Here let me explain;- Pu comes in two main varieties (or isotopes) Pu239 and Pu240. Nuclear weapons are made from Pu239 and not Pu240. If you leave Pu239 in a reactor for more than about 30 days it turns (transmutes) into Pu240.
Pu239 and Pu240 are extremely difficult to separate when mixed (unfeasible really), so even if the reactor initially starts with a high degree of Pu239, after 30 days of running, it’s turned into a mixture of 239/240.
Although these burnable moderator reactors are a lot more complex in their element conversions the above holds true.
The question was more about whether there's enough fissile material in a second-hand reactor to remove enough to make a weapon/s while still retaining enough reactor life to cover up the fact that you have removed enough material to make a weapon/s, this would be the IAEAs worry. But thanks for the explanation.
You’d have to cut through the cladding for the nuclear fuel to remove the fissile material - and unless the fuel was fresh and unused you would have to problem of separating the Pu239 from the mix, and dealing with the radioactivity (and heat) of the actinides produced by the fission reaction.
Then you’d have to replace or repair the cladding - which is built to very fine tolerances, fit it back in the reactor, and hope the fiddling about has not thrown anything out of whack, or that the tinkered-with fuel rod can cope with the extreme environment in the reactor - because if it doesn’t it will show up in the reactor chemistry checks.
Then there’s the issue of the US and UK personnel who are going to be floating around the boats, and the shipyard workers who will be blabbing about any odd procedures going on on the boats.
