We (the US) managed to pack one into an 6" artillery shell that weighed 118 lbs (or 128lbs for the second version) - the W48 [1].
That's much smaller than a suitcase. Heavy, sure, but quite a bit smaller than the carry-on suitcase I was lugging around last week, and I've got a small one compared to the monstrosities that are considered to be carry-on sized now.
> During the Cold War, the United States and Soviet Union both deployed nonstrategic nuclear weapons for use in the field during a conflict. While there are several ways to distinguish between strategic and nonstrategic nuclear weapons, most analysts consider nonstrategic weapons to be shorter-range delivery systems with lower yield warheads that might be used to attack troops or facilities on the battlefield. They have included nuclear mines; artillery; short-, medium-, and long-range ballistic missiles; cruise missiles; and gravity bombs. In contrast with the longer-range “strategic” nuclear weapons, these weapons had a lower profile in policy debates and arms control negotiations, possibly because they did not pose a direct threat to the continental United States. At the end of the 1980s, each nation still had thousands of these weapons deployed with their troops in the field, aboard naval vessels, and on aircraft.
> In 1991, the United States and Soviet Union both withdrew from deployment most and eliminated from their arsenals many of their nonstrategic nuclear weapons. The United States now has approximately 760 nonstrategic nuclear weapons, with around 200 deployed with aircraft in Europe and the remaining stored in the United States. Estimates vary, but experts believe Russia still has between 1,000 and 6,000 warheads for nonstrategic nuclear weapons in its arsenal. The Bush Administration quietly redeployed and removed some of the nuclear weapons deployed in Europe. Russia, however seems to have increased its reliance on nuclear weapons in its national security concept. Some analysts argue that Russia has backed away from its commitments from 1991 and may develop and deploy new types of nonstrategic nuclear weapons.
> The Soviet Union produced and deployed a wide range of delivery vehicles for nonstrategic nuclear weapons. At different times during the period, it deployed devices that were small enough to fit into a suitcase-sized container, nuclear mines, shells for artillery, short-, medium-, and intermediate-range ballistic missiles, short-range air-delivered missiles, and gravity bombs. The Soviet Union deployed these weapons at nearly 600 bases, with some located in Warsaw Pact nations in Eastern Europe, some in the non-Russian republics on the western and southern perimeter of the nation and throughout Russia. Estimates vary, but many analysts believe that, in 1991, the Soviet Union had more than 20,000 of these weapons. The numbers may have been higher, in the range of 25,000 weapons in earlier years, before the collapse of the Warsaw Pact.
"Suit-case sized container" does not imply that it will actually fit into anything that looks like suitcase or otherwise be inconspicuously transportable, which is what all the scenarios of danger of suitcase nukes presume.
Even not taking the physical dimensions into account, the physics packages tend to be very heavy (which is mostly dictated by physics involved and trying to make the enclosure smaller seems to make it heavier, which makes sense given the physics).
In all I don't see how such weapon would be relevant for terrorist tactics, because using similar amount of effort one could just place large enough chemical explosive charge to cause significant (although several orders of magnitude smaller than from small nuclear warhead) damage.
Solving this problem doesn't require a huge amount of imagination. In fact the solution is rather obvious.
Let's just say that portable nukes are a very, very bad thing. A single 1kT nuke in the centre of a city would be very bad news on its own, and would make 9-11 look like a footnote. (1kT is a realistic yield for the ultra-portable Mk-54 warhead.)
Strategically, tiny nukes in small numbers are far more worrying than ICBMs, because unlike ICBMs they don't come with a built-in warning time or a sender's address.
I could walk around Amsterdam all day with a 6" cross section piece of tubular metal and nobody would bat so much as an eye. And I have multiple suitcases large enough to put a 1 meter segment of that pipe in (inches, meters...).
Carting around a 120lb cylinder 6" in diameter and 33" long seems eminently doable in a rolling suitcase. You'll want to get a sturdy one and make sure your suicide bomber is well built, but I don't see what's so disbelievable about it.
Does this skepticism come from thinking "briefcase" rather than "suitcase," or...?
A chemical weapon is as effective as a portable nuclear device from a terroristic point of view (they tend to exploit vulnerabilities in the defence system to instill fear in the population), the main difference is probably the damage caused by the radiation.
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That's much smaller than a suitcase. Heavy, sure, but quite a bit smaller than the carry-on suitcase I was lugging around last week, and I've got a small one compared to the monstrosities that are considered to be carry-on sized now.
[1] https://en.wikipedia.org/wiki/W48