Yes and no.
For example, the Tsar Bomba was designed to have a maximum yield of 100MT, when using a U-238 tamper. In a surprising moment of wisdom the USSR tested it without the Uranium tamper, substituting some other heavy metal like Tungsten. This reduced it’s yield to “only” 50MT, making it the cleanest weapon per MT ever. Still, it released vast amounts of fallout. If they had used the Uranium tamper, it would have released 1/4 of the total fallout of all the tests up to that point.
When the Hydrogen Bomb was developed, one of the scientists noted that it’s primary utility was the ability to use cheap (essentially free) U-238 as fuel, making an atomic arsenal of vast size and destructiveness possible.
I see from comments that I was too vague (which I was.) It was a somewhat general question but I did have a specific fictional scenario in mind - whereby someone uses a tiny nuclear weapon to destroy several ships at sea all at once and pass it off as a big conventional explosion that did them in (in the circumstances, stacking a lot of conventional explosives together and detonating them just isn’t what’s done, but there is a nearby nuke handy.) The perpetrators don’t want people to realize that a nuke went off, and hope that the ocean will wash away the fallout and that satellites won’t be looking out for the double-flash signature.
For that, no chance. The area will be awash with very specific isotopic residues that would not just identify the explosion as a nuke, but identify exactly what sort.
Ok - but if the explosion is weak and small enough, maybe it wouldn’t occur to people that it was something non-conventional, and they wouldn’t send out a radiological investigation team?
Don’t you think someone would be a mite curious how someone got their hands on thousands of tons of conventional explosives and how they got those explosives near those ships, and they might concier alternate explosives?
You either have a small explosion or you have something that can sink several ships at once. Those two criteria are mutually exclusive.
I’m curious about the utility of convincing people that it was conventional explosives instead of a nuclear weapon. I’m imagining something like a carrier strike group gets sunk in the western Pacific. We’re not going to be appreciably less pissed at the perpetrators if they tell us “it’s OK, we only used conventional explosives.” Are you imagining a scenario where the actual perpetrators blame someone else for the deed?
In a weird turn of events, the “cleanest” bombs tend to be the biggest ones, in that you can use a relatively small fission primary (0.3 kilotons is the one that I think the modern bombs use) to set off a BIG fusion explosion- on the order of megatons.
So while it’s not “clean” in the sense of no fallout, the percentage of non-fusion energy can be truly tiny relative to the size of the bomb.
Prompt radiation (the neutrons) aren’t that long-ranged, and in general are inside the blast/heat radius anyway.
In fact, a big bomb can even get less, total (not just relative to the blast size) of some sorts of fallout, because you get less incompletely-fissioned fissionable material.
OTOH, as pointed out upthread, you can’t avoid fission child products, most of which are damned hot and all of which more distinctive than fingerprints.
After a period of months or years, the “worst” of the child nuclides will have decayed away into something less virulent, but right away, the best you can hope for is that no one thinks to check for them.
I may be wrong, but I’m pretty sure that the minimum amount of fissile material needed for a critical mass puts a lower-bound on the explosion size. As in, you have to have some minimum amount of U235/Pu241 to get the bomb to go off, and that means your bomb is going to be some minimum size, which I think is still somehwere around 10-20 tons of yield.
That’s the equivalent of 10-20 tons of TNT. There’s no way on Earth that anyone’s going to mistake that for anything other than a nuclear detonation, unless it’s confused with something like that fertilizer explosion in West, Texas a few years back (it was actually rated at about 10 tons equivalent).
And that’s a REALLY big bang- as in wrecked a big chunk of a small town big. A bigger bang on the order of 1-3 kilotons gets you something like the Halifax Explosion or the Texas City Disaster.
On the lower end, that’s also in the same ballpark as the WWII “Grand Slam” bombs the British used, and the MOAB bomb the US used in Afghanistan.
One more thing about those little nukes; they’re kind of the opposite of the larger weapons, in that their prompt radiation effects far outstrip the blast and heat effects- ISTR that the Davy Crockett (10-20 ton weapon) has a radiation kill radius something like 1.5 times the blast/heat radius. Larger weapons go the other direction- with a 10 megaton bomb, the heat and blast destruction radii are far in excess of the prompt radiation radius.
The only real advantage I can see with a really small nuke would be that it would be much smaller to deliver than a MOAB or “Grand Slam” bomb, which do literally weigh 10 tons or so.
I think the point about the equivalent mass of the conventional bomb is a critical point. If you are taking out a small fleet of ships you are looking at an explosion of Halifax equivalent force. How is the perpetrator supposed to have snuck a ship sized bomb into the middle of the fleet in order to blow it up? A very large submarine that is 80% bomb perhaps? Trying to claim you blew up a fleet of ships with a few thousand tons of explosives isn’t easily supported. No plane can carry enough explosive. An Antonov 124 had a payload of 150 tons.
Taking TNT as the example (since that is how nuclear weapon yields is measured), TNT weighs 1.65 grams per cc, or 103 lbs per cubic foot. A single kilo (short) ton of TNT would take up around 19,400 cubic feet, or a cube around 27 feet per direction. That’s around 14 standard 20-foot shipping containers for each kiloton. Or around 70 containers for a Hiroshima. (All numbers rounded with wild abandon.) Kind of tricky to sneak around.
You did I was really trying to support the points made above - that any attempt to explain away the weapon as conventional has to pass some very difficult logistic credibility tests.
And now the question is what type of ships are destroyed at what distance to determine how much bang is needed to do it. This boat-baker comes to mind…
They probably wouldn’t need to send out a radiological investigation team. The US (and, in all probability, many other nations) has equipped a network of satellites with bhangmeters.
Bhangmeters use photodiodes to pick up on the double-flash signature of nuclear weapons. The time between the flashes can be used to take a SWAG at nuclear yield.
The US has equipped even GPS satellites with bhangmeters, so the odds are slim when it comes to setting off a nuclear weapon, even a small one, without it being detected by a bhangmeter-equipped satellite.
I was really trying to support the points made above - that any attempt to explain away the weapon as conventional has to pass some very difficult logistic credibility tests.