JUDY WOODRUFF: The Trump administration confirmed
last night that Vice President Pence was scheduled
to meet secretly with top North Korean officials
during his recent trip to South Korea.
North Korean officials pulled out of the meeting
at the last minute.
It would have come after Pence sat near Kim
Jong-un's sister during the Olympics' opening
ceremony.
The Trump White House considers North Korea's
nuclear and missile programs the main threat
to American national security.
Last week, Miles O'Brien examined the North's
ability to make the material required for
nuclear weapons.
Tonight, Miles looks at what we know of the
devices themselves and how analysts gauge
North Korea's building progress.
It's for our weekly series on the Leading
Edge of science.
MILES O'BRIEN: North Korea has tested nuclear
weapons six times since 2006 at this mountainous
site in Punggye-ri.
No one from the outside has witnessed such
a test, but the experts believe this propaganda
movie produced by the regime offers an accurate,
albeit embellished, version of what happens
inside.
But how much is known about the design and
size of this secret arsenal?
JEFFREY LEWIS, Middlebury Institute of International
Studies at Monterey: This is actually a 3-D
model of North Korea's nuclear test site.
MILES O'BRIEN: Jeffrey Lewis is director of
the East Asian Nonproliferation Program at
the Middlebury Institute of International
Studies at Monterey.
JEFFREY LEWIS: So these black spheres are
actually the estimated size of the cavities
from the North Korea's nuclear explosions.
MILES O'BRIEN: He and his team used terrain
data gathered by satellites to build a 3-D
computer model of the Punggye-ri test site.
JEFFREY LEWIS: We estimated where the nuclear
explosions had occurred.
We were able to use satellite photographs
to see where the tunnels went into the mountains.
And when we finished that, one of the things
we realized is that that site could accommodate
a much larger nuclear explosion than we had
seen in the past.
MILES O'BRIEN: It led him and his team to
predict the North Koreans would soon test
a bomb with a few hundred-kiloton yield, much
larger than their previous tests.
The prediction came at the beginning of 2017.
In September, seismometers detected an explosion
about that big.
Even when testing occurs underground, some
gases can leak out.
The United Nations, the U.S., South Korea,
Japan, and China have surrounded the North
with sensors that can detect radioactive isotopes,
as well as so-called noble gases before they
decay.
The ratio of noble gases xenon 131 and 133
can offer a clue about the design of a bomb.
But the North Korean site is doesn't seem
to leak very much.
DAVID ALBRIGHT, President, Institute for Science
and International Security: The South Koreans
detected a little bit of xenon-133, but not
that much, and they didn't detect any other
isotopes.
MILES O'BRIEN: Physicist and former arms control
inspector David Albright is president of the
Institute for Science and International Security.
DAVID ALBRIGHT: Not what you would expect
from a large-yield weapon, so they're clearly
-- they're doing something, and it is an ideal
test site location.
The explosion is sealing the material inside.
MILES O'BRIEN: But the North Koreans do share
some tidbits about what their weapons might
look like, or so it seems.
SIEGFRIED HECKER, Former Director, Los Alamos
National Laboratory: They have actually shown
us what are presumed to be photographs of
their nuclear devices, and they have shown
that over the last few years' time frame.
This was two weeks after the first nuclear
test.
MILES O'BRIEN: Nuclear physicist Sig Hecker
is with the Center for International Security
and Cooperation at Stanford University.
He ran the Los Alamos National Laboratory
from 1986 through 1997 and has visited North
Korea seven times.
SIEGFRIED HECKER: This was 2007 in one of
their uranium processing laboratories.
MILES O'BRIEN: He believes the first weapon
they tested was akin to Fat Man, the bomb
that the U.S. dropped on Nagasaki.
In it, high explosives implode a plutonium
core, causing an instant nuclear chain reaction,
or explosion.
North Korea's first test in 2006 was an apparent
failure, but, in 2009, test number two was
a success.
Since then, the yields have grown steadily
larger.
And that leads experts to believe the North
Koreans have developed a so-called boosted
fission bomb.
SIEGFRIED HECKER: The boosted bomb is one
where you take highly enriched uranium or
plutonium, and you actually put the fusion
fuel inside to sort of help light more of
the plutonium or highly enriched uranium.
MILES O'BRIEN: In 1957, Great Britain tested
a one-stage boosted thermonuclear bomb.
Its yield, 720 kilotons, or about 50 Hiroshima
bombs.
DAVID ALBRIGHT: The trouble with them is,
as you increase the yield, it gets bigger
and bigger in diameter, and it uses an incredible
amount of weapon-grade uranium.
And so all the countries are motivated to
go to the two-stage weapon.
JEFFREY LEWIS: This is a 3-D model of North
Korea's thermonuclear weapon.
They released a series of pictures, and we
were able to assess its size.
MILES O'BRIEN: A two-stage, or hydrogen, bomb
allows for a much larger yield in a smaller,
lighter package.
It begins with a plutonium or highly enriched
uranium explosion, stage one.
That creates enough energy to squeeze hydrogen
and its isotopes, causing fusion, stage two.
The U.S. tested more than 100 hydrogen bombs
in the Pacific.
The largest was 15 megatons, or 15,000 kilotons,
more than 1,000 Hiroshima bombs.
After the last North Korean test, the regime
released a picture of a what looks like a
two-stage device.
SIEGFRIED HECKER: It looks as if this last
test, the yield, the explosive yield of that
is large enough that it was most likely a
two-stage bomb.
MILES O'BRIEN: But are these weapons small,
light and robust enough to be efficiently
delivered on a missile?
The experts believe the North Koreans have
designed their bombs with all of this in mind
from the outset.
SIEGFRIED HECKER: The first bomb was mostly,
I think they had to prove to themselves, essentially
a proof of principle that they can actually
make an efficient bomb.
And, from that point on, they were determined
towards making that bomb deliverable.
DAVID ALBRIGHT: So, they have to spend time,
I think, working on getting the yield up,
and not increasing the size too much, and
I think that, they have done.
MILES O'BRIEN: So it is likely the North Koreans
have bombs small enough to be delivered on
short-range missiles.
But weapons that are mounted on medium-range
or intercontinental ballistic missiles are
a much bigger challenge.
SIEGFRIED HECKER: They have to be smaller.
They have to be lighter.
They have to be even more robust.
They haven't demonstrated that they can do
that.
MILES O'BRIEN: And what about the missiles
themselves?
What can they deliver and where?
The rocket science in our next report.
I'm Miles O'Brien for the "PBS NewsHour."
