Lawrence Livermore National Laboratory, where we are right now, recently
declassified over a hundred film recordings of the above-ground nuclear
tests that the U.S. conducted between 1945 and 1962. These films are free and
available to the public on a playlist on our YouTube channel, and as you guys have
been watching them you've been asking some interesting questions regarding
some of the things that are happening in these films.
Lawrence Livermore nuclear physicist Greg Spriggs is here to help me answer
some of those questions today. We're going to use this laptop here to take a
look at some of our test films.
G: Fantastic
We stopped above-ground nuclear testing in 1962, and we stopped underground
nuclear testing in 1992 so aside from being an interesting relic of our
nuclear past, what do these test films help us do? The data that was obtained
from these detonations is used today to help validate our computer codes.
Since we stopped testing nuclear weapons and as a result we also stopped
designing nuclear weapons so now we do everything with big computer codes and
simulations. But the code is a computer code and it may give you a bogus answer
and you need to validate that against some real data and this benchmark data
that we obtained from the films is basically what we're normalizing our
computer codes to. So these are very, very important films. What new things can we
see now that the films are on a computer that we maybe couldn't see when they
were in physical form? The way they read the films back in the 1950s, they used a
device called a kodagraph which was basically just an image enlarger. And
it involved humans putting this film into this kodagraph, measuring, moving
things around and then trying to read it by eye. What we're doing out with the
computer though is we're able to read the optical density on the film and get
a much more accurate measure of the shock wave as a function of time and so
the uncertainty of the yield determinations has dropped from about
plus or minus 10% down to about plus or minus 1%. Sure not as much human error.
Yes. Instead of taking weeks to measure one particular film it now takes us just
a matter of minutes to do it, so we're able to analyze more films than
what was done back in the 1950s and we end up with much more accurate data and a
more complete record of what the data is on that film. So this is what the value
these films are now there are things that we're doing that they didn't even
think of doing back in the 50s or didn't have the technology to do. So
we've got more data, more accurate data, new techniques, and we're using all of
that to plug in to the computer codes that we use now to simulate nuclear
detonation. That's correct. From beginning to end, can
you sort of walk us through the big basics of what's happening in each of
these detonations. When the nuclear detonation goes off, you end up fissioning
uranium or plutonium some sort of nuclear fuel and it produces this huge
amount of energy in a very small volume. There are four effects
cause from a nuclear detonation, four major effects. The first one is prompt
radiation that's being emitted by the fission process, so you get prompt gammas
and prompt neutrons and neutrinos and so forth. The next effect that we see is the
thermal output. When the detonation occurs about 86% of the energy
that's released from the detonation goes into heating up the air, so the air
becomes very hot. The third effect is the shockwave--this high temperature plasma
that's formed in this primary, or in the weapon itself, starts to push on the
surrounding air and it creates a shockwave. That shockwave is moving very
very fast it's moving about Mach 100 and as a result it's very, very hot, so as the
shock wave expands it starts to cool, and in this film here as you watch it go
out you see it's bright initially but that's because the shockwave is very, very
hot, putting out lots of visible light, and in the next frame the intensity
comes down a little bit and the reason it comes down is because the shock wave
is considerably cooler, but it's still hot enough that it's opaque to the hot
gases on the inside. So we have this very hot ball of gasses but you can't see it
all you're really looking at is the surface temperature of the shockwave.
When the shockwave finally drops down below the Draper temperature of 525
degrees it disappears off of the film and it's completely transparent to light
and so as you continue out a couple more frames light intensity comes back up
again, and that's because the hot gases are now able to shine through. I see, so
it's almost like the shockwave is like the shell covering up the hot gas on the
inside and once that disappears as it cools off then you've got the hot gas on
the inside. Exactly, so there are two distinct pulses associated with the
nuclear detonation. You don't see that with a conventional weapon. The
reason is that the gases inside the conventional weapon don't get nearly as
hot as they do on a nuclear detonation, so this particular shot was out in
the Pacific, it was a tower shot. They put these on towers on this
particular island out here, and I believe this was Enwetok, the moisture
out in the Pacific was always on the order of almost 80 to 90%, most of the
time was close to 100%, so you get this Wilson cloud effect which obscures the
fireball. When we do air drops we don't have a Wilson cloud usually the air drop
is up high enough. In the Pacific we did it up high enough that
didn't have that much moisture or at the Nevada Test Site where the normal
surface humidity was very, very low and so you don't see this Wilson cloud on
most of the air drops. So this is the Wilson cloud as you go out a little bit
further the prompt radiation dies down, the Wilson cloud disappears and you can
now start to see what's on the inside of that Wilson cloud--you see the
bomb debris, the early cloud is actually starting to form at the very top up
there. In this particular case it brought coral into the fireball and that
coral got very hot and sort of got scorched and so you're looking at the
bomb debris, the smoke and burnt coral that got sucked up into the fireball. And
so this was, as you said, a tower shot? This was a tower shot. Okay, and then
there are they're just only two kinds, a tower shot and one that's dropped? They
did several different kind of shots out in the Pacific. One of them was called a
barge shot--these were the very high yield ones. They didn't want to drop them
out of a plane so they put them on a barge and they would float the barge out
into the deep water region around the Pacific. And then they did another series
of shots were air drops out in the Pacific, most of these were done during
Operation Dominic in 1962 and those really show the double pulse that I was
trying to explain earlier. At the Nevada Test Site they added one more category--
they had these helium balloons, they would put the bomb into the gondola underneath
the balloon and they would tether the balloon onto the surface, and they
did about 40 different balloon shots and they stopped doing it--the the top part
of the fireball was distorted because of the helium balloon and then they had
these lines, these tether lines, and you would see these spikes coming off on the
bottom of the fireball which was the cables that were actually vaporizing
during the detonation itself. Oh because that's one of the questions that we
often get on some of these I think I've got one of these, some people call
them like the protrusions or the legs that come off the fireball here...this is a
function of how they were carried to their detonation site? Right, so
that was a tower shot so if you go a little bit further and you can see those
spikes again but you'll also see the tower is vaporizing right below it and
these towers that were basically just a big elevator shaft, and again they had
cables that would tether this tower to keep it upright and you could see the
cables were starting to vaporize in addition to the just the tower itself
underneath there. These protrusions which are it's
tethers and the tower are, you know, lighting up faster than the sphere of
the explosion, why is that? The shockwave is very, very hot, it could be on the
order of five hundred thousand degrees...when it's very very hot, it puts out X-rays
and the X-rays go through the atmosphere and they interact with the
cable and the tower, so out in the air you don't see the interaction of the X-rays
with the air, but with the cable since it's a much higher density and
the X-rays interact with it and cause it to vaporize, and that's why you can actually
see the cables. So really on the outside of this fireball are a bunch of X-rays
that are being emitted by that shockwave surface because it's so hot it's just on
the cables you can see it and the tower you can see it. Right, so if anything else
solid were around this one would be able to see that too due to the X-rays? Yes, due to the
X-rays emitted by the surface. Well, let's then talk about this one because it's a
great example of another frequently asked question, which is the streaks in
the air behind some of these clouds, can you tell me what those were used for?
As the shock wave moves out further and further it gets cooler and cooler
and when it finally reaches 525 degrees centigrade, the Draper temperature, it no
longer emits visible light, and so on the film--which is only sensitive to visible
light--it looks as if the shock wave disappears off of the film.
I see. So they came up with this trick, they said, 'Hey you know when the shock
wave finally disappears, what we can do is we can put out these smoke trails
and we have at the shock wave front this very high density air region and
then on the outside the shock wave we have ambient air, and we can use the
optical illusion effect to illuminate where the shock wave front is', and it's
the same effect you go from a low density air above the straw to a
higher density region which is the water and it looks like the straw was broken
right so the straw comes in here and then once it hits the water it looks
like it goes down here so what happens is the shock wave as it moved away from
the detonation point, there was this optical illusion with these smoke trails
and it formed these little hooks in the shock--in these contrails, but it actually
didn't interact with the contrails it was just a little hook that was caused
from an optical illusion from that effect there. About five seconds before every
detonation, they would shoot up these rocket trails and these trails usually
were about halfway between the camera and the detonation point. In this particular
case there's another set of cameras on the other side which actually show the
hooks as the shock wave moves out. So they could follow the shock wave out
further and that would help them understand shock wave physics. For these
nuclear detonations they wanted to capture a larger portion of the light
output as a function time. Because you've got something that's hugely bright--very, very
bright. So they used a film called microfile which had silver halide crystals in
it that were actually on the order of about a micron, very fine-grained film. You
could actually blow that film up with an image enlarger maybe a hundred times, 200
times and still not see the grain. Wow. These particular films, the ones that
were done on the microfile film, the speed on it was 0.1 as opposed to
normal range film of 200 to 2,000, so it was very, very hard to expose unless
you had a nuclear detonation and the nuclear detonation was bright enough
that it would actually activate the film even during the high-speed photography
of it yeah. So they're very, very bright-- when a nuclear detonation occurs the
light output from a nuclear detonation changes by about 12 orders of magnitude.
The best film that Kodak made, which was the microfile film, only captures about
four orders a magnitude of that variation. And so that's what you're seeing on this
film is four orders of magnitude of light variation. And it was done very, very
early in the morning so there was no backlight to mess up the photography and
unfortunately if they didn't time it just perfectly it stayed dark a little
bit too long and so this particular film, it kind of disappeared off the film
because it stopped glowing. Since we're talking about the film, we've got these
numbers down at the bottom of most of these test films, what are these keeping track of?
When we render the films, the particular scanner that we're using is a
true frame by frame counter. It captures each
individual frame and we like to keep track of that so that when we measure
the data we have an absolute reference number here, so that we just simply put
the frame number down at the bottom. I see, ok. It doesn't refer to doesn't refer to
time or anything it's just a frame counter. I see are they
recording the explosion in real time are these slowed down at all?
The shock wave is moving at Mach 100, it's moving very, very fast
so the only way you can slow that down and
capture it you have to use a high-speed camera most cameras. The standard
Hollywood camera runs at 24 frames a second. When you're trying to capture the
shock wave as a function of time they wanted to run it at 2,400 frames a
second, about a factor 100. That was fast enough that they could capture this and
then when you play it back at normal speed of, on a video plays it back at 30 frames a
second it looks like it's running in a slow motion. So they had about 10 of
those kind of cameras that were capturing the early cloud and then they
had another series of cameras that were used to capture the late cloud. Well the
late cloud it takes about seven or eight minutes for the cloud to reach the top
and so they adjusted the speed on those cameras to one frame per second so that
was moving much much faster so it was a time-lapse. Interesting, so this is so we can get as much
scientific data from them as possible. Exactly. The last commonly asked question that we get
on a lot of these films is what happened to those places after the tests were
conducted and if we were to go to them now, would we be able to tell are there
any lasting impacts from the tests that were done there? At the Nevada Test Site
after the atmospheric testing was done they went underground and they started
making what were called subsidence craters, so you would detonate this several
thousand feet underground and it would vaporize a certain amount of soil
underneath which would then cause a cavity to form and then the weight of the
soil above would basically collapse into this cavity and it would cause a crater
on the surface those are called subsidence craters. We did about 800 of those
underground tests and we did 200 atmospheric shots. So if you went out to
the Nevada Test Site you might be able to find maybe a half a dozen different
areas where they had the original fallout plume where it went to the
ground. They usually have it fenced off so you can't get into it,
they monitor this constantly, they have remote solar powered monitors out there
so that nobody happens wandering into it and they can monitor it. They
do a lot of cleanup out there also, so if they had one site they wanted to clean
up they would basically go in there and scrape off the top four or five inches
of soil and put it in barrels and bury it. So the site out there is basically
pretty clean, so yeah, there's not very much out there anymore, it's just
they maintain it because it is a radiation area. In the Pacific they did a
lot of shots at the Bikini Atoll and Enewetok, and in those areas are
closed off to the public they actually went out and did scrape off some of the
coral sand that was on the surface that was contaminated and replaced it with
fresh sand but when you go out into the lagoon itself there was no way to
practically clean up all of that radioactive material that's underneath
the water so it's basically closed off to the public, so you really can't go out and see
that. We use these test films to help validate our nuclear models that we use
on our supercomputers but why would you say is that important Greg, why are we
doing that? We use the atmospheric test data and some of the underground test
data to basically validate our computer codes so that we can continue to monitor
the nuclear stockpile for the United States and certify that indeed if we
ever have to use it that it would work the way it was intended to work. Ok well, I
think that answers all of the frequently asked questions, Greg, thanks
for stopping by and talking to me about it, really appreciate it. Thank you for having me.
We learned a lot today, not only about our country's nuclear testing
past but also what we're still learning from these films decades later. It's all
part of this Lab's responsibilities around something called stockpile
stewardship and we have a video coming out soon on what exactly that means so
subscribe to this channel and stay tuned. This is something that's important to
learn about and understand today, so leave your questions about it down in
the comments below. Thanks for watching and we'll see you next time.