Hopefully in a hundred years your childrens children will be able to
say the same thing after seawater has had more time to eat away at the
housings of nuclear torpedoes containing plutonium.



Plutonium can be dreadful stuff if it's in a highly bioavailable
chemical state and/or a physical form that gets into the body,
especially the lungs, and lodges there. Whether macroscopic hard
integral shapes of plutonium metal are nearly as big a deal is
another question (especially deep underwater, rather than exposed to
the atmosphere.

The worry quotient depends also upon where they are in the ocean:
depth, hydrologic and geologic activity, and proximity to land.
Remember, the various nuclear-armed nations have actually fired quite
a number of weapons for test purposes, some of them quite large
(multimegaton), under and just above the ocean. This in addition to
all the chemical dumping, agricultural runoff, oil spills, etc. that
have occurred and continue to occur. If only man had stunk up the
sea so *little* that the innards of the odd "broken arrow" made a big
contribution!

Anyway, quite a bit of thought has gone into the problem of disposing
of high-level waste, and much of it is applicable to intact or broken
weapons shapes from the arms control effort and thus is first cousin
to lost weapons. Nonproliferation as well as short- and long-term
environment, health, and safety issues are considered there.


One of these methods is sub-seabed disposal in hydrologically and
geologically quiet areas where the bottom is covered with deep mud.
(This should not be confused with an older, apparently abandoned idea
involving deliberate introduction of the materials into deep-ocean
subduction zones.) Admittedly these ideas envision either drilling
or kinetic penetration so as to emplace the materials some meters into
the mud; and also some form of containment. See for instance page 200
et seq. of
http://books.nap.edu/html/plutonium/0309050421.pdf
However, it goes to show how the deep ocean can under some
circumstances be regarded in this context.

See also
http://www.llnl.gov/csts/publications/sutcliffe/
regarding what happens to plutonium in water (albeit with an emphasis
on relatively shallow fresh water; salt water under many many
atmospheres of pressure might be different).


Finely divided plutonium and/or plutonium dispersed in a fire is a
much bigger deal to those who get caught in the plume -- in the
atmosphere. The reason is that plutonium has the interesting property
of being a lot more reactive in small pieces than large. This is part
of the reason why physically energetic weapons accidents (e.g., bombs
lost from aircraft) present a special risk, and why insensitive
explosives were eventually developed for use in nuclear weapons.

Anyway, the reactor plant found in most of the same subs and ships is
probably a lot more significant as a hazard than are the weapons pits,
as long as the stuff is presumptively out of the reach of terrorist
organizations and aspiring nuclear states. On that subject, the
ability to find and salvage such an item in deep water far from land,
even if you start with a decent general idea of where it is, is not a
technically or economically trivial task.

In my personal opinion, if you are so unfortunate as to lose custody
of either a weapon or a reactor, losing it intact in the deep ocean
seems not nearly as bad as some of the other possibilities.

Cheers,
--Joe