WASHINGTON - For all his threats to fire tactical nuclear arms at Ukrainian targets, President Vladimir Putin of Russia is now discovering what the United States itself concluded years ago, US officials suspect: Small nuclear weapons are hard to use, harder to control and a far better weapon of terror and intimidation than a weapon of war.

Analysts inside and outside the government who have tried to game out Mr Putin's threats have come to doubt how useful such arms - delivered in an artillery shell or thrown in the back of a truck - would be in advancing his objectives.

The primary utility, many US officials say, would be as part of a last-ditch effort by Mr Putin to halt the Ukrainian counteroffensive, by threatening to make parts of Ukraine uninhabitable. The officials spoke on the condition of anonymity to describe some of the most sensitive discussions inside the administration.

The scenarios of how the Russians might do it vary widely. They could fire a shell 150mm wide from an artillery gun on Ukrainian soil, or a half-tonne warhead from a missile located over the border in Russia. The targets could be a Ukrainian military base or a small city.

How much destruction - and lingering radiation - would result depends on factors, including the size of the weapon and the winds. But even a small nuclear explosion could cause thousands of deaths and render a base or a downtown area uninhabitable for years.

Outcome will vary

Still, the risks for Mr Putin could easily outweigh any gains. His country could become an international pariah, and the West would try to capitalise on the detonation to try to bring China and India, and others who are still buying Russian oil and gas, into sanctions they have resisted.

Then there is the problem of prevailing winds: The radiation released by Russian weapons could easily blow back into Russian territory.

For months now, computer simulations from the Pentagon, US nuclear labs and intelligence agencies have been trying to model what might happen and how the US could respond.

It is no easy task because tactical weapons come in many sizes and varieties, most with a small fraction of the destructive power of the bombs the US dropped on Hiroshima and Nagasaki in 1945.

In a fiery speech last week full of bluster and menace, Mr Putin said those bombings "created a precedent".

The modelling results, one official familiar with the effort said, vary dramatically - depending on whether Mr Putin's target is a remote Ukrainian military base, a small city or a "demonstration" blast over the Black Sea.

Great secrecy surrounds Russia's arsenal of tactical arms, but they vary in size and power. The weapon Europeans worry the most about is the heavy warhead that fits atop an Iskander-M missile and could reach cities in Western Europe. Russian figures put the smallest nuclear blast from the Iskander payload at roughly one-third of the Hiroshima bomb's explosive power.

Much more is known about the tactical weapons designed for the US arsenal back in the Cold War. One made in the late 1950s, called the Davy Crockett after the frontiersman who died at the Alamo, weighed about 32kg; it looked like a large watermelon with four fins. It was designed to be shot from the back of a jeep and had about one-thousandth of the power of the bomb dropped on Hiroshima.

But as the Cold War progressed, both the US and the Soviets developed hundreds of variants. There were nuclear depth charges to take out submarines and rumours of "suitcase nukes".

Fact vs fiction

At one point in the 1970s, Nato had upward of 7,400 tactical nuclear weapons, nearly four times the current estimated Russian stockpile. By that time, they were also part of popular culture.

In 1964, James Bond defused a small nuclear weapon in Goldfinger, seconds before it was supposed to go off. In 2002, in The Sum Of All Fears, based on a Tom Clancy novel, a terrorist wipes out Baltimore with a tactical weapon that arrives on a cargo ship.

The reality, though, was that while the blast might be smaller than a conventional weapon would produce, the radioactivity would be long-lasting.

For deadly radiation, there is only one dramatic, real-life comparison on Ukrainian soil: what happened in 1986 when one of the four Chernobyl reactors suffered a meltdown and explosions that destroyed the reactor building.

At the time, the prevailing winds blew from the south and south-east, sending clouds of radioactive debris mostly into Belarus and Russia, although lesser amounts were detected in other parts of Europe, especially Sweden and Denmark.

Chernobyl, of course, was an accident. The detonation of a tactical weapon would be a choice - and likely an act of desperation. NYTIMES