[cobrausn]

The bottom of the ocean is generally a very good 'containment system'. That's not to say there won't be environmental damage, but we do have some examples to go off of to indicate how much.

http://en.wikipedia.org/wiki/USS_Thresher_(SSN-593)

http://en.wikipedia.org/wiki/List_of_sunken_nuclear_submarin...

[mpyne]

Not to mention all the sunken Soviet/Russian submarines (most of which have two nuclear reactors instead of one).

[gabriel34]

None of these examples were worst case scenario. How well does the ocean absorbs the ionizing radiation from a meltdown or some other extreme event? Would part of the ocean's water become radioactive and spread through currents? Are the oceans so voluminous that the effect would be only a minor increase in radiation levels?