Great job of reporting here by the New York Times:

BRUSSELS — The White House has quietly assembled a team of national security officials to sketch out scenarios of how the United States and its allies should respond if Putin...unleashes his stockpiles of chemical, biological or nuclear weapons.

The Tiger Team, as the group is known, is also examining responses if Putin reaches into NATO territory to attack convoys bringing weapons and aid to Ukraine, according to several officials involved in the process. Meeting three times a week, in classified sessions, the team is also looking at responses if Russia seeks to extend the war to neighboring nations, including Moldova and Georgia, and how to prepare European countries for the refugees flowing in on a scale not seen in decades.

So impressed with the Bidens in the run-up to and throughout the war. This is thorough preparation. Joe Biden was made for this moment.

Those contingencies are expected to be central to an extraordinary session here in Brussels on Thursday, when President Biden meets leaders of the 29 other NATO nations, who will be meeting for the first time — behind closed doors, their cellphones and aides banished — since Mr. Putin invaded Ukraine.

Holy cow! That is an extraordinary "extraordinary session." I don't know, but I reasonably believe, that the Times' headline is inaccurate--this is the purpose, not to "rally" allies.

...today, from the White House to NATO’s headquarters in Brussels, a recognition has set in that Russia may turn to the most powerful weapons in its arsenal to bail itself out of a military stalemate.

NATO’s secretary general, Jens Stoltenberg, underscored the urgency of the preparation effort on Wednesday, telling reporters for the first time that even if the Russians employ weapons of mass destruction only inside Ukraine, they may have “dire consequences” for people in NATO nations. He appeared to be discussing the fear that chemical or radioactive clouds could drift over the border. One issue under examination is whether such collateral damage would be considered an “attack” on NATO under its charter, which might require a joint military response.

The current team was established in a memo signed by Jake Sullivan, Mr. Biden’s national security adviser, on Feb. 28, four days after the invasion began, according to the officials involved in the process, who spoke on the condition of anonymity to discuss sensitive planning. A previous iteration had worked for months, behind the scenes, to prepare the U.S. government for the likelihood of a Russian invasion of Ukraine.

My goodness, look at that. "Tiger Team" created four days after Putin began his war. Amazing preparation by the Bidens. I hope that someone is preparing a book on this like Graham Allison's Essence of Decision on the Cuban Missile Crisis.

...
Stoltenberg, sounding far more hawkish than in the past, [Jens always had a beak to his nose as I recall.] said he expected “allies will agree to provide additional support, including cybersecurity assistance and equipment to help Ukraine protect against chemical, biological, radiologic and nuclear threats.”

As Biden flew to Europe on Wednesday, both he and Stoltenberg warned of growing evidence that Russia was in fact preparing to use chemical weapons in Ukraine.

...few of the leaders set to meet in Brussels on Thursday ever had to deal with those scenarios [One for sure has]— and many have never had to think about nuclear deterrence [One has.] or the effects of the detonation of battlefield nuclear weapons, designed to be less powerful than those that destroyed Hiroshima. The fear is that Russia is more likely to use those weapons, precisely because they erode the distinction between conventional and nuclear arms.

Well, and that tactical nukes are like in the Kremlin's handbooks and have been since the breakup of the S.U.
...
One major issue the Tiger Team is looking at is the threshold that could prompt the alliance to use military force in Ukraine. Biden has made clear that he is enormously reluctant to to do so, fearing that direct confrontation with Russia could escalate the conflict beyond control. “That’s World War III,” he noted recently.
...
Officials believe the chances that Mr. Putin will resort to detonating a nuclear weapon are small. 

But biochem--that is an entirely different matter I bet this article goes on to say. And a "dirty" nuclear bomb, like deliberately degrading the many of Ukraine's nuclear power plants that Russia now has in its control. You don't get the blast damage of course but the cloud of radiation wafting over Poland, the Baltics--remember Chernobyl--is the same, in invisible to the naked eye and comes with little to no warning even when it is purely accidental.

Several officials said the White House and Pentagon have had some tension over how much detail the Defense Department is willing to share on its highly secretive war planning — especially concerning responses to any use of nuclear weapons — even in the classified setting of the Tiger Team.

There's that "willingness to share" bullshit again. What is up with that? Share with whom? The Pentagon works for this president, not the other way around. Are "Pentagon" and "Defense Department" being used synonymously? The DefSec reports to the president, not vice versa. Who is not sharing with whom? Goddamn it, I wish somebody would explain this.
...
If Putin did strike a NATO country intentionally, he would not only bring the force of the military alliance to bear on Russia, but also probably find himself facing NATO troops inside Ukraine...

Well, duh! What if it's accidentally on purpose as in the damaging of a nuclear reactor and the release of the radioactive cloud? There's the rub! What if it is a sure-fire tactical nuclear bomb explosion?  Bees and Gees, an intentional detonation of a nuclear bomb breaches an immense psychological barrier: a deafening, blinding explosion, the mushroom cloud, a weapon that has not been used but twice previously and that 77 years ago. If Putin does that, even in the middle of Ukraine let's say, as far from foreign borders as possible, and even if no radiation wafts over the border, I could see the U.S. considering that beyond the pale and leading immediately to full-scale nuclear war. I know that this is an article on what some U.S. officials told the Times reporters and not the U.S. officials saying directly. However, this section of the article does begin with "the chances that Putin will resort to detonating a nuclear weapon are small." My fear is that Tiger Team is deliberating on the response scenarios to the obvious black-white scenarios--exploding a nuclear bomb and intentionally striking NATO--because those are the easy calls to make. The gray issues, a nuclear reactor meltdown, "Oopsies", or a biochem attack are the hard calls and it is on those that are this crisis' essence of decision. 

Senator Angus King of Maine, an independent and a member of the Senate Intelligence and Armed Services Committees, visited the Polish-Ukrainian border over the weekend...King said that as Russian forces struggle to make headway, Mr. Putin could try [1] to strike a diplomatic agreement, [2] intensify his bombardment of Ukrainian cities and level them, or [3] lash out against the West with a cyberattack.

“...[4] is escalate to de-escalate, which is a tactical nuclear weapon,”...in which [Russia] would employ a nuclear weapon as a warning — and then negotiate.

No sir. That is not what a tactical nuclear weapon is. Either Senator King or the reporters, which include the incomparable David E. Sanger, are mistakenly conflating two very different things. A tactical nuclear weapon is a weapon, it is used to kill people, it is not a "warning." There has been talk of Russia firing a warning shot, exploding a nuke somewhere in a remote region or a teeny-tiny nuke that wouldn't create danger of a radiation cloud or inflict death. "My button's bigger than your button--watch." That's a warning, not a weapon.

Do you not see what I mean? Why are you restricting the worst-case reasonably plausible scenario to tactical nukes when "the chances are small"? What about biochem? There should be a: "[5] biochemical attack." Putin has done that before guys! In Syria! We didn't assemble a "Tiger Team" for that? If you're not thinking through the more plausible, harder, scenarios, then you're not thinking it through.