IRA FLATOW: This is Science Friday. I’m Ira Flatow.

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[? SINGING VOICES: ?] Please duck and cover. Duck and cover. We did what we all must learn to do. You and you and you and you. Duck and cover.

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IRA FLATOW: If you’re of a certain age, you may recall the practice of duck and cover. As schoolchildren, we were told to duck under our desks to seek cover as the nuclear bombs fell. The government also issued detailed instruction booklets on how to build a home fallout shelter.

But fallout, of course, was a secondary concern. Because the immediate effects of the atom bomb, the one that killed 100,000 in Hiroshima, that’s an immediate effect. And there was up to 80,000 dead in Nagasaki and roughly half of those people perished on the days the bombs dropped.

Now with the threats of nuclear war being tossed back and forth between the US and North Korea, I thought it might be instructive to review the basics of nuclear war. How the bombs work, how they exact such a terrible toll, and the lasting dangers like radiation they leave behind.

Not because we’re predicting imminent nuclear war, but because if anything does happen, the magnitude of the tragedy will no doubt crowd out this important scientific conversation. So how much have we learned about nuclear bomb damage and radiation exposure in the seven decades since the bombs fell on Japan? Here to school us on the ABC’s of nuclear weaponry are my guests.

Alex Wellerstein, assistant professor of science and technology studies at the Stevens Institute of Technology. Laura Grego, senior scientist in the global security program at the Union of Concerned Scientists. Steve Simon, radiation health physicist at the National Cancer Institute. Welcome to all of you. Thanks for joining us today.

STEVE SIMON: Afternoon.

ALEX WELLERSTEIN: Good to be here.

LAURA GREGO: Thank you.

IRA FLATOW: Alex, by now we have several generations of people growing up without ever feeling the threat of a nuclear attack. And what’s the lesson of history? What should we be carrying into the future related to nuclear warfare?

ALEX WELLERSTEIN: I think people want to believe that it would never occur. And that’s very optimistic of people. But even at the end of the Cold War and even into the present, the chance of it happening is never zero. As long as the weapons are around, as long as they’re out there, we should have some part of our brain assigned to the idea that this is a possibility.

IRA FLATOW: Let’s go through some of the ABC’s of a nuclear bomb. Give me a time lapse, from the bomb dropping to the blast to days later. What happens?

ALEX WELLERSTEIN: So a nuclear bomb, you can think of it as a very complicated invention. It’s a lot of little pieces that all come together in just the right way to produce this explosion. And that coming together takes about 1/10 of a second.

It’s a very fast assembly, as they would say of this radioactive material, this fuel for the weapon, all of these other little parts. And so what you’re going to get is this just intense fireball that forms. And it forms in like 1/10 of a second or so.

And it’s hotter than anything you’re going to have on the surface of the Earth, it’s going to be brighter and whiter than anything you’ve ever seen. Much worse than any kind of chemical explosion. And it’s just this hot radiating ball that is then going to be expanding outward.

So you’re going to have three main effects coming in. One is this blast wave, you’re going to have this superheated air and that’s going to have the effect of knocking down buildings, breaking in windows, knocking over trees, knocking over people, whatever.

You’re also going to have this intense heat coming off of the fireball itself that might ignite some fires. It will burn skin if it’s exposed to it. And then you’re also going to have this radiation, which is of course the effect that people often focus on because it’s the weirdest effect. It’s the least understandable.

And that isn’t something you’re going to see or feel but if you’re too close to the bomb, you’re going to get a death sentence without realizing it. If you’re further out, you might be increasing your cancer risk down the line. So those are the three sort of immediate effects. And all of those are going to be happening within the first few seconds or few minutes of the bomb going off.

IRA FLATOW: When you say radiation, does that include the fallout? What is fallout?

ALEX WELLERSTEIN: Fallout is the radioactive cloud. So when you think of the mushroom cloud, think of that as being filled with lots of radioactive particles.

If that mushroom cloud, if that fireball has gone off in a way that sucks up dirt into the cloud, the radioactivity in there is going to attach itself to this dirt. And so as this cloud blows with the wind, these heavy particles that are now radioactive are going to fall out of the cloud.

IRA FLATOW: I got you, I got you.

ALEX WELLERSTEIN: And so this is a delayed effect. For a small bomb, all of the worst of that might fall out in an hour or two. For a really large bomb, it might take a couple of days for it all to fall out. But that’s going to be spreading contamination downwind of the explosion.

IRA FLATOW: Steve, how is this fallout absorbed by the body?

STEVE SIMON: Sure, Ira, I’m happy to explain that. The phenomena by which the radiation is absorbed by the body is a sequence of steps that happen very, very quickly. We call this kind of radiation ionizing radiation. That is, it has enough energy to ionize atoms that it interacts with. And ionize means to release electrons from those atoms.

So these gamma rays, they’re a lot like x-rays but they’re much more powerful, much more energetic. They impact the cells of the body, the bones, of the skin, anything that it might pass through. And it has enough energy to knock electrons out of the atoms of that material.

So this is the process called ionization and it’s actually those electrons that do the damage to the cells of the body. It’s not the gamma rays directly, it’s those electrons. So it’s a sequence of steps. And so when you say absorbed by the body, it’s this process that happens very, very quickly where the atoms are ionized.

IRA FLATOW: Would it not get into our food system also?

STEVE SIMON: The fallout can definitely enter our food system. And it does that through environmental processes that we’re all familiar with. We know how plants grow, we know how irrigation happens, we know the processes of farming and growing plants. And so that is an avenue, we call it a pathway, it’s an avenue or a pathway to contamination. So it’s not the radiation that contaminates the plants but it is the fallout material.

It’s the debris, it’s this radioactive material. Once it enters the food chain, then it can travel. It can go through the food chain in steps. It can be diluted or can be concentrated depending on the kind of food. And eventually can be consumed by man or animals.

IRA FLATOW: Laura Grego, let’s say that North Korea does launch a missile. What happens with our muscle defense system at that point? Do we have one? And give us a hypothetical scenario.

LAURA GREGO: Yeah. Well, the United States has very fortunate geography. We have huge oceans to the east and the west and we have friendly neighbors to the north and south. Not every country is in such a peaceful neighborhood.

So we don’t worry about short range missiles so much. We worry about the kinds of missiles that can carry nuclear weapons from that far distance. To do that, they have to go thousands of miles. And they’re called intercontinental ballistic missiles. And that’s what we worry would carry a nuclear weapon.

Intercontinental describes how far they go and ballistic means that after launch, they’re in free fall, just subject to the forces of gravity rather than like a cruise missile or an airplane that’s under powered flight. So in the case that North Korea decided to launch an attack against the United States, the first thing that would happen is these powerful rocket would ignite.

And they do look quite a bit like a space launch vehicle. So if you’ve seen a picture of that, you kind of know what it looks like. It’s a really bright signal. So we have satellite-based sensors that would pick that up and see that very quickly. And that boosting missile would take about three to five minutes to get going really quickly.

And it goes up, arcs through the vacuum of space. And when that’s detected, and we also have radars in the vicinity, that would cue the missile defense system. The one that we have that’s meant to defend the continental or the 50 United States is called the Ground-Based Midcourse System.

So those radars would detect it and fire control would launch one or a few interceptors from ground-based silos in either Alaska or California. Those look a lot like big space launch vehicles too. So after the nuclear missile from the adversary burns out, it releases what’s called– well, the nuclear weapon, which would be encased in a re-entry vehicle.

It’s a hardened shell that’s meant to protect that weapon from the heat and stress of a high speed re-entry through the atmosphere. That looks like a large cone sort of roughly the height of a human. And the interceptor from California or Alaska also gets up going to speed.

And it releases what’s called a kill vehicle. And that’s about the size of an office file cabinet. And the idea is that that file cabinet-sized kill vehicle would maneuver itself and try to run into the incoming nuclear weapon and destroy it with the force of impact.

IRA FLATOW: So how successful have we been in testing this?

LAURA GREGO: Yeah. Well, it’s a tough problem. It’s one of the most complicated, complex systems the Pentagon has ever taken on. There have been 18 tests, intercept tests of the system. And it’s succeeded about half the time.

And it’s important to note that these tests have been really under conditions that are scripted for success. They don’t have decoys that can try to confuse the defense. They’re not under the most stressing conditions. Not the types of conditions you’d expect it to work in or you’d want it to work in real life. So it’s been a very challenged system.

IRA FLATOW: So it’s 50-50 right now.

LAURA GREGO: It’s 50-50 right now under the most simplified and rosy conditions.

IRA FLATOW: And how does a hydrogen bomb compare to the atom bombs that were dropped on Hiroshima and Nagasaki in terms of strength and chemistry? How do they compare?

ALEX WELLERSTEIN: So the bombs dropped on Nagasaki and Hiroshima, what we call atomic bombs, they work by nuclear fission. So splitting of heavy atoms, enriched uranium or plutonium. And a hydrogen bomb takes one of those bombs. So you take basically the Nagasaki bomb. And instead of using its energy to just directly destroy a city, you use it to ignite a fusion reaction similar to how the sun works.

And that amplifies its power by a lot. So just to give you a rough sort of numerical approach, the Hiroshima bomb was 15 kilotons, 15,000 tons of TNT equivalent. That’s about enough to destroy a big chunk of, say, midtown Manhattan.

If one of those went off right now in midtown Manhattan, it would probably kill about 300,000 people. So that’s pretty bad. The kinds of hydrogen bombs that North Korea has or claims to have are about 10 times more powerful than that. 150 kilotons. So they’ve multiplied it by 10. And so that raises the number of dead.

If that went off in the same spot, in midtown Manhattan, you’d get something like 900,000 dead. And you’d be destroying basically the entire southern tip of the island to some degree and parts of New Jersey and parts of Brooklyn and parts of Queens.

You can make them as big as you want. They just become difficult to put on a missile. They become big and heavy. So the biggest one ever made was more like 100 million tons of TNT. That’s a bus the size of– I mean, excuse me, that’s a bomb the size of a school bus.

That’s not an easy thing. So 100 to 500 kilotons has been the sort of sweet spot for a bomb that’s relatively easy to put on a missile. So it’s not as big as some of the Cold War sort of monster bombs were. But it’s big enough to ruin your day.

IRA FLATOW: Laura, we were talking about the missile defense system. Have people been proposing new ideas for how we might improve? Better than 50-50? I mean, you launch two missiles, you’ve got 50 chance that one is going to hit.

LAURA GREGO: Yeah, it’s tricky. I mean, certainly the idea of defense sounds great. And the practice of it is much, much more difficult. It’s very difficult to get it to save the day. So one of the things I didn’t mention is that the adversary would release this nuclear weapon in a reentry vehicle.

But it also might release, at the same time, decoys that look a lot like that weapon. And they could be as light as a Mylar balloon that you’d get at a birthday party. Because in the vacuum of space, everything travels, you know, there’s no air resistance.

So a balloon would travel at the same rate as the heavy warhead. So this issue of trying to intercept the weapon while it’s up in the vacuum of space makes it really tricky. It’s prone to this mid-course– we call it discrimination problem. Where we can’t tell which is a real one and which is a fake one. You have to attack them all.

So there have been some ideas to try to get the launching missile just as it’s launching. And that’s tricky because, as I mentioned earlier, that launch phase is really only three to five minutes. You need to be really close. So all of the options are really pretty tricky.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from PRI, Public Radio International. Talking about the ABC’s of missile, of ICBMs and bombs. And also about radiation. Now, I understand, Steve– I don’t mean Steve.

I mean, yeah, I do mean Steve. That you were actually– I’m sorry. You’re actually in the Marshall Islands. When the explosions were there. I don’t mean you were on the island yourself, but after the explosion.

STEVE SIMON: Well, Ira, I was not there when the explosions occurred. I came there much later.

IRA FLATOW: I would hope so.

STEVE SIMON: Yeah. Much, much later. The Marshall Islands are a big– or a small island country. But they’re spread over a tremendous area of ocean. And so various islands where the tests had taken place had been monitored in the early years for contamination.

But the rest of the country had never been monitored. Over 90% of the people, 95% of the people lived. So many years later, I did actually live in the Marshall Islands for over five years and monitored the entire country for residual contamination.

IRA FLATOW: And what did you find?

STEVE SIMON: Well, we found a huge grading of contamination, as one might expect. As the other speakers described, you have a lot of fallout potentially very near to the detonation site. And that becomes diluted with the increasing distance.

So we found a very large grading of contamination from levels on small islands where you would not want to live to very low level contamination that tapered all the way down to the background level depending on exactly which island that you were measuring.

IRA FLATOW: Right, so what’s the one thing that’s changed about our knowledge of radiation since we started studying it 70 years ago after the bombs were dropped?

STEVE SIMON: Well, I think we’ve learned many things. But from my own perspective as a health physicist, I think one of the lessons that we’re now more acutely aware of is the long-term cancer risk associated with low level exposure. I mean, we’ve always had an appreciation that a high dose of radiation delivered very quickly would inflict tremendous damage on the human body.

But we learned over time, not just from studies of nuclear explosions, but from all different kind of populations where exposure took place that low level exposure among a large population can result in a continuing small cancer risk over many, many years. But if the population is large, that adds up among that whole population.

IRA FLATOW: [INAUDIBLE] Steve. Alex, you’re shaking your head in agreement with what he’s saying.

ALEX WELLERSTEIN: It’s one of these things that people have a very hard time conceptualizing. Which is that it’s not like you get exposed and you’re going to get cancer. But if you get exposed and your cancer goes up by 1%, for any individual person, that’s not much.

But if you have 100 million people and you have 1% more cancers, that adds up to a lot more people. And so I was shaking my head because this is a very good explanation of the trickiness of talking about radiation and risk is that people want to sort of see it as you got it or you don’t.

And it’s really more like you’re adding dice to your life. Your fatal cancer risk in the United States is around 20% just as a baseline. So add a couple percentage to that and then imagine that across a big number and that adds up.

IRA FLATOW: We’re going to take a break and continue our discussion about nuclear weapons. We have some listeners and callers tweeting us. So we’ll see if we can get to some of those tweets, so many interesting questions. Stay with us, we’ll be right back after this break.

This is Science Friday. I’m Ira Flatow. We’re talking this hour about the science of nuclear warfare. From bomb chemistry to radiation fallout to missile defense with my guests Alex Wellerstein, Laura Grego, and Steve Simon. A lot of people are on the phone. Let’s see if we can get to– oh, here’s a question that a lot of people have been asking. I’m going to go to Peter in Berkeley, California for it. Hi, Peter.

PETER: Hi, Ira. Thank you so much for this discussion. It’s critical. And I think more people need to know about the work of Alan Robock, the climatologist at Rutgers and his colleagues, who have been warning for years and years that the real danger is nuclear winter.

First popularized by Carl Sagan and his colleagues. But Robock is saying that even in the neighborhood of 50 bombs going off, which could easily happen, say, between India and Pakistan, for example–

IRA FLATOW: All right, all right. Let me get the question in because we’re running out of time. Let me ask Alex, what about nuclear winter? How feasible is that?

ALEX WELLERSTEIN: Yeah, it’s a really interesting topic. And it’s one that’s generated a lot of scientific and political controversy over the year. Just to put it basically, the idea is in a full nuclear exchange where your weapons are not, say, going off in a desert or something, they’re going off on cities or prairies or what have you.

A lot of fires are going to be started. And those fires are going to put a lot of smoke, just regular old soot, into the atmosphere. And if put enough smoke into the atmosphere, you’ll reflect a lot of sunlight.

And if you reflect too much sunlight, you won’t have any sun and the temperatures will dip and your crops will all fail and you’ll starve to death. And so this is the nuclear winter hypothesis. Carl Sagan famously put it out there in 1983. Alan Robock has been working on it for years. People who have continued this work, more and more fine grained simulations.

What makes it difficult and controversial is that, fortunately, we don’t have anything like this ever occurring. It’s not an experiment we can run. And so there’s a lot of parameters. How much smoke will be put into the atmosphere? How many fires will be started? Will that smoke dissipate or will it reflect or will it not?

And so depending on the parameters you choose, you either come up with the answer of– something like the Robock answer, which is it’s actually pretty easy to imagine this occurring, at least on some level. Maybe not the full ice age but enough to affect crop failures. Or you get some people who say, I don’t think we have enough nuclear weapons to possibly do it.

It’s one of these areas that I find interesting because how do you react to that uncertainty? Do you assume the worst case or the best case? What’s the appropriate one? It appears the military assumes the best case. And that’s interesting to think about why that might be.

IRA FLATOW: We’re running out of time. They’re so many questions I want to ask. Steve Simon, you’re a radiation specialist at NCI. What factors would determine how long before you could come out of a fallout shelter if you happen to be in one and it’s keeping the radiation out? We’re all assuming that’s all happening.

STEVE SIMON: Well, that’s a good question, Ira. Let’s talk about the reality of a fallout shelter. And maybe we should use the phrase a sheltering in place. Because the idea of a fallout shelter that we had when we were kids, not many of those exist any longer.

So more likely, people will be advised to stay in their homes, in their apartment buildings, in buildings of substantial structure that have withstood the blast and still have all their integrity to them. So how long do they need to stay in there? Well, it’s not a simple answer.

It depends to a degree on the intensity of the contamination at that place. Right? That would make sense. It would also depend on what kinds of activities and how long a time you were going to spend outside. So the length of time that you would need to spend inside would depend on your occupation, your age.

In other words, emergency workers, medical workers, police, firemen. They could possibly, due to the urgency for them to perform their duties, could leave earlier. Earlier that is then families and children.

IRA FLATOW: So you’re saying don’t depend on a fallout shelter. Or so as you– a different word– to actually shield you from all the radiation?

STEVE SIMON: Oh, what exactly is your question?

IRA FLATOW: Well, I’m saying even though you’re in a fallout shelter, don’t expect that to shield you from all the radiation. It’s not hermetically sealed.

STEVE SIMON: Absolutely, absolutely. Especially a home, when you’re sheltering in place. It’s not an underground bunker. Very few of us would have that luxury. But you will be afforded the protection of the walls, the thickness of the walls will attenuate the radiation.

Whether they will prevent contamination from creeping in through air ducts and those kinds of systems would all depend on the design of your home and how you would fortify that after the emergency. So there’s a lot of variables there.

IRA FLATOW: And we could talk forever about this because–

STEVE SIMON: We could.

IRA FLATOW: We used to talk about it forever in the 50s. I want to thank you all for taking time to be with us today. Steve Simon, radiation health physicist at the NCI. Laura Grego, senior scientist in global security program at the Union of Concerned Scientists.

Alex Wellerstein, assistant professor of science and technology studies at Stevens Institute of Technology in New Jersey. Thank you for taking time to be with us all today.

LAURA GREGO: Thank you.

STEVE SIMON: You bet, thank you.

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