Maybe someone can provide more precise figures, but I seriously doubt the whole premise that a submarine reactor can "melt down" -- simply because its operating power is typically fairly low (compared to power plants), and therefore the decay heating is low also. This difference is likewise indicated by the ~20-year service life of the core without refueling -- compared to about one year for a power plant.
On a similar note, reviews of a new movie called "K19: The Widowmaker" include statements like "a Chernobyl-style nuclear meltdown...." or " threatened to turn the North Atlantic Ocean into a radioactive gumbo...." or "The film stresses that had a meltdown occurred, it would have triggered a Chernobyl-like explosion that could have led to a Cold War confrontation...."
I suppose its Hollywood's nature to manufacture mythology, but unfortunately the viewing public doesn't know that that's what it is and takes it at face value....
Many people who do know a bit about nuclear science & technology, think its a waste of time to try to correct such misconceptions. But how else is the public ever to achieve some meaningful level of informed opinion, on which it can base important decisions on all nuclear-related matters ?
For now I will only point out that the total radioactive fission product inventory in a submarine reactor is a tiny fraction of that which is present in a powerplant like Chernobyl. The fission product inventory is roughly proportional to its operating power and duration of operation. The load-following feature of the submarine reactor means that it only operates at its full rated power a small fraction of time. This is particularly true of a sub like the K-19, the Soviet Union's first ballistic missile submarine -- these types of subs ("boomers"), in contrast to "hunter-killer" types, spend most of their time stationed at a particular ocean site, virtually undetectable, because their noisy engines & propellers are not running. Under these circumstances, the fission product inventory is likely to be hundreds of times less than Chernobyl. Plus of course there is no graphite to burn & spread the radioactivity in case of a meltdown, as at Chernobyl.
Jane's handbooks on submarines give figures on propulsion unit brake horse-power, maximum submerged speed, etc.
Maximum shaft power (full steam rating, single reactor) is on the order of 20 megawatts, with a corresponding thermal rating of about 70 MWth. Typical average operating power even for a hunter-killer sub like HMS Tireless is likely to be quite a bit lower - say around 40 MWth. This is nearly 100 x less than a modern nuclear power plant ( 1,200 MWe - 3,750 MWth). The corresponding decay heat following shutdown is also roughly a hundred times less - for example, one hour after shutdown, the decay heating would be about 0.5 MWth (instead of 50 MWth in the power plant). One day after shutdown its down to some 0.2 MWth.
The size and mass of the reactor is such that dissipating this much heat is unlikely to require temperatures anywhere near the melting point of the fuel or other components (for comparison's sake, a single electric kitchen stove heating element, when red hot, dissipates about 0.001 MWth or one kilowatt).
Any ex-submariners out there with more precise figures ?
Thanks.
Jaro