Why not just use man portable device?

Or nuclear armed sub launched cruise missile.

Because you couldn't get the surprise and the megatonnage of hollowing out a ship and filling it with nukes.

JamesD, I dont know how much you know about nuclear weapons so bear with me if you already know this. Larger yield is actually less efficient than small yield. The only reason old nukes were so big was to compensate for a large CEP. Todays nukes, ICBMs SLBMs, are not far behind in accuracy when compared to early GPS guided munitions. Order of magnitude at most. And in the case of cruise missiles the are just as accurate. So you can literally put a nuke through a window these days. Another consideration is fall out. A bomb like the one you propose was actually conceptually designed by the Soviets during the cold war. The purpose was to end all life on Earth if the Soviets were on the losing side of a nuclear conflict. When a nuclear weapon explodes it creates a fireball. For a typical 475kt Trident D-5 II this fireball will be about a kilometer in diameter. EVERYTHING in it will get vaporized. Actually it turns into fine particles more popularly known as fallout. This fallout will eventually settle to the ground depending on winds and can travel for thousands of kilometers. If you detonate a bomb that big 100+ megaton in a ship. YOu will irradiate huge amounts of sea water. Salt water fallout is the worst kind of fallout. It is notoriously difficult to remove. The ecological consequenses are enormous. If the explosion is large enough, 100+ megaton, you will get a fireball about 8+ kilometer diameter. Thats 134+ million cubic kilometers of vaporized water plus the ship and any thing for 4 kilometers around the blast. Thats an unprecidented amout of salt water fallout. You will kill far more people in other countries, including your own, than in the target country. Within a 25 km diameter anything of value will be destroyed. And there will be so much death and destruction for approximately 50km to 100km diameter. Health services will collapse and there are some theories that suggest that much fallout could kill a significant portion of the world population. So if its your goal to end the world or a part of it, go ahead. But if you want to destroy a harbor, the two most clandestine and surprising ways to do it is SADM or cruise missile. Even an ICBM will have suprise. No vessel not already underway can be made ready in 30 minutes before RV come screamind in at mach 25. Even less time for SLBM. Another thing is most nuclear weapons will actually not hit their target before exploding. They will detonate many hundreds of meters above and destroy with blast effects. Against a soft target like a port this is more effective as the blast radius is larger for an air burst. This also helps if the fireball does not touch the ground as the local fallout is minimal. The two bombs over Japan were exploded in the air. The only reason a nuclear device would be detonated on the ground is for hard targets like underground command centers and bunkers. You could think of it this way. The destruction area caused by a 100 megaton is 3400+ sq km limited to the vicinity of the blast. The destruction area for a 12 MIRV Trident D-5 II is 475kt warhead is approx 1200 sq km. So with one MIRVed SLBM you get 1/3rd of the destruction with about 18 times less yield and less ecological damage. And it can be on target in minutes. One such SLBM would completely destroy a Naval Base like Severomorsk Naval Base in Russia. There would be almost no escape for any units in the blast area. Oh and with the threat of dirty bombs these days, A ship filled with fissionable material would be likely to be detected by anti terrorism sensors.

I was led to believe that most of the fallout was the result of the type of primary and tamper used, which in this case if it were only a normal sized primary with only fusion chambers using lead tampers, would not be that dirty in a proportional sense. A version of the Tsar Bomba with a lead tamper created 97% of it's power from fusion. I wasn't aware that sea water was a carrier of fallout, but I'll see if I can find out more about this. I was led to believe that that was the reason tactical sea nukes were considered the nuclear weapons most likely to be used in the future of any nuclear weapons. The whole idea with the ship, which I should have explained, was that it would be untraceable for a long amount of time. The nuke ship would go off, and a large city would be destroyed, but it might take months to find out who sent the ship in, if they can find it out at all. A ship like that also probably wouldn't have to go into the port, just near it, and then detonate. Thank you for your information. The helpful and informed attitude you and I are discussing in is a reason I decided to join this forum.

Another thing I forgot to tell you. Nuclear weapons are not anonomous. If you detonate one, there will be residual material left over. Molecular analysis will identify the source of the material. The reason nukes are popular at sea are for three reasons. 1) Carriers are so strong defensively, the only way to ensure their rapid destruction is nuclear explosion. 2) Submarines are so dangerous and stealthy that the only way to rapidly clear the area of any with certainty was to detonate a nuke within a few kilometers to crush the hull. 3) To attack land targets. Self explanitory. P.S. The fallout only gets real bad when the fireball touches something. Airburst prevent this and cause more wide stread destruction. The subsurface blast are dirty as hell but thats just how it is.

I know they'll find it eventually, but that could be years from then, after they've only begun to rebuild. And you'd still have an initial measure of surprise that satellite surveillance might deny you if you want to use ICBM's. The Fireball stuff is true, but again, that's how it is. This method was intended to deliver a bomb of 50-100 Mt magnitude by surprise. I think in that it would be successful, but any bigger probably would destroy the world. I do think that trying to launch an attack this way, and then following it up with a conventional or more traditional style nuclear attack, would be highly successful.

Well I have to disagree with you twice. 1) I never really believe in all the so called end of the world scenarios short of an astroid the size of Texas. 2) And as for suprise. It doesnt matter for non-nuclear power. But a nuclear foe, most of them, has a sort of sh*t list of countries they would attack if they got suprised attacked. And remeber all your destroying is one port. You could sink a large tanker in most ports and disable it for considerable time. Also a more difficult suprise attack to foil would be to detonate a natural gas tanker. Huge explosion. And it would take days to find out otherwise. To move the amount of fissile material you are talking about is like calling every major intelligence agency and asking them to pretend not to notice. Then your ship gets captured but SOF while at sea. A better way to achive suprise would be SLBM. 5 to 15 minute flight time and not a damn thing could be done to stop it. Also the destruction would be more wide spread because of the MIRVs. If absolute not warning before detonation was required then you have to use man portable SADM or stealthy SLCM.

This is almost the original idea - see Einstein's letter to Roosevelt. And there was a post war concept in the USSR - sponsored by Beriev - to do this with baby submarines which were basically nuclear weapons. I sure don't want to be on the crew of that ship! This is a very bad idea, as Stalin explained to Beriev.

I wonder what is the point of a "clean nuclear weapon" - which by the way your idea is not? It is illegal and immoral to attack a port in the first place. Why do you want to do it?