President Joe Biden warned last week that the war in Ukraine could devolve into a nuclear “armageddon” after Russian President Vladimir Putin renewed his nuclear threats.

“We have not faced the prospect of Armageddon since (President) Kennedy and the Cuban Missile Crisis,” Biden told a crowd at a fundraiser Thursday night.

The world held its collective breath 60 years ago this month when the United States and the Soviet Union squared off over Soviet missiles in Cuba.

And now, Biden raises deep concern as Putin threatens the use of nuclear weapons while casting the war in Ukraine as a geopolitical battle of Russia against the West.

How likely is a nuclear incident?

No one wants to give up nuclear weapons first and put themselves into a weaker position, says Pablo Calderon Martinez, assistant professor in politics and international relations at Northeastern University–London. He does not believe that a disarmament is possible in the current geopolitical situation.

Nine countries currently possess close to 13,000 nuclear weapons, which is significantly less than about 60,000 weapons during the Cold War. These countries are the United States, Russia, France, China, the United Kingdom, Pakistan, India, Israel and North Korea.

Those countries consider nuclear weapons indispensable for their security, Calderon Martinez says.

“It’s a state of mind,” he says. “It really does depend on the security calculations of individual states, and how they perceive their security.”

Some countries that don’t have nuclear weapons, like Germany, don’t view nuclear weapons as necessary for their security, Calderon Martinez says, while other countries like South Africa, Brazil and Ukraine decided to get rid of their nuclear capabilities altogether. But the perceived need for nuclear weapons is increasing among certain countries, Calderon Martinez says.

Since the world saw the scale of destruction in Hiroshima and Nagasaki during World War II, the international community considers the use of nuclear weapons or nuclear materials in warfare a taboo. Most countries unilaterally draw a red line at Russia’s threats of using nuclear weapons, Calderon Martinez says, because of the fear of escalation.

“From the practical and strategic point of view it doesn’t make any sense for Russia to deploy nuclear weapons,” he says.

The U.S. and NATO are overwhelmingly more powerful, according to Calderon Martinez. As soon as Russia uses nuclear weapons, indiscriminately killing civilians, the West will be forced to act by either launching a nuclear counterattack or sending troops to fight Russia, Calderon Martinez says.

Sending American or European troops to kill Russian troops could lead to World War III, he says. If the West responds with nuclear weapons, that would lead to further escalation of the nuclear exchange and the traditional warfare won’t matter anymore.

However, a prevailing opinion among international relations experts, Calderon Martinez says, is that “nuclear holocaust” is more likely to happen as a result of an accident, miscommunication or miscalculation.

“The U.N. is the only forum we have where people can have discussions, conversations and negotiations to avoid such misunderstandings,” he says.

What happens when a nuclear weapon is used

Pran Nath, Matthews distinguished university professor in physics, agrees that nuclear wars are not winnable.

“This is not a scenario that you contemplate, and hopefully will never happen,” he says.

Nuclear weapons are a deterrent and if used will cause mass destruction, Nath says.

Smaller nuclear weapons are called tactical weapons, but there is no universal definition of what they are. They can probably destroy a part of the city or eliminate a battalion, Nath says. Both the U.S. and the Soviet Union developed dozens of types of tactical nuclear weapons during the Cold War, such as nuclear artillery shells, nuclear anti-aircraft missiles and nuclear anti-tank rounds.

The use of tactical nuclear weapons would increase the risk of nuclear war, which can quickly spiral and lead to the use of the larger nuclear weapons—strategic nuclear weapons.

Strategic nuclear devices can range from 20 kilotons to several megatons of trinitrotoluene explosive (TNT), Nath says. The bombs used in Hiroshima and Nagasaki were 15 and 21 kilotons and killed 140,000 and 74,000, respectively, in the blasts. Many more died from radiation in the following weeks, months and years.

When a strategic nuclear weapon explodes, it first creates an intense fireball, which is as hot as the surface of the sun and arrives at the destination within microseconds. That creates a shockwave, a combination of a blast and fireball, that travels with a very high speed, from 600 to 800 miles per hour, and carries a lot of pressure. The shockwave can crash anything within several miles from where the bomb falls, Nath says.

“There will be almost 100% destruction in this part of the blast area,” Nath says.

The explosion creates alpha, beta and gamma radioactive particles. Gamma rays are the most harmful—they can go through thick materials and quickly deposit energy into the body. The body temperature on the surface can rise to 50 degrees Celsius, which will heat up the body and cause severe burns.

The third stage of the nuclear bomb explosion is the fallout. The nuclear blast creates a mushroom cloud, which can reach up to about 10 miles in the atmosphere. It is very rich in radioactive isotopes with a lifetime of a few seconds to thousands of years, Nath says. That’s the reason why the radioactivity in the blast zone and around it is very high in the first few days. People who survive the blast should not venture out for a few days until the radioactivity has fallen significantly, Nath says.

The mushroom cloud can be dispersed by winds, spreading the fallout for hundreds of miles, and can cause cancer and skin diseases.

How to prepare for a nuclear attack

“It’s important for individuals to just know what to do, even though the likelihood that you may actually have to use that knowledge is exceedingly low,” says Neil Maniar, who directs Northeastern’s Master of Public Health in Urban Health program in the Department of Health Sciences in the Bouve College of Health Sciences.

It is important for individuals, families and communities to be prepared and have a plan just like in the case of any major weather event or a major public health emergency, he says.

Maniar recommends checking out a reliable online resource like ready.gov for detailed instructions. At minimum, people should have their prescription medication on hand, a flashlight, a crank or battery powered radio, packaged food in the event that they lose power, water, and extra batteries for devices that they might need.

“Realistically, you probably want to have a supply that can get you through a week or two weeks,” Maniar says.

To survive high levels of radiation, people would need to find shelters that are made of 3 feet of concrete, 3 to 4 feet of soil or a foot of steel, Nath says. After the attack, any windows should remain closed.

“If you have to venture out, you should come back and have a shower, if the shower is still working, to remove the radioactive material from your body,” Nath says.

The COVID-19 pandemic showed that there is still room to significantly improve the U.S. infrastructure to prepare for disasters and emergencies, Maniar says. That infrastructure includes both communications and overall public health infrastructure to fully meet the needs of the residents in different communities.

This article originally appeared on News@Northeastern.

Photo by Spencer Platt/Getty Images.