Thermonuclear weapon facts for kids

A thermonuclear weapon, also called a fusion weapon or hydrogen bomb (often called an H-bomb), is a very powerful type of nuclear weapon. It's much stronger than the first atomic bombs. These bombs can be smaller and lighter, or much more destructive, or both. They use a special process called nuclear fusion to create their huge power. This process means they can use materials like depleted uranium as their main fuel, which helps save rare materials like uranium-235 (Template:OptionalLink) or plutonium-239 (Template:OptionalLink).

The United States first tested a full-scale thermonuclear weapon in 1952. This test was called Ivy Mike. Since then, most countries with nuclear weapons use this design.

Modern fusion weapons have two main parts:

• A primary stage that uses nuclear fission (like an atomic bomb). This part is fueled by Template:OptionalLink or Template:OptionalLink.
• A separate secondary stage that uses nuclear fusion. This part contains special fuel, which is usually heavy types of hydrogen called deuterium and tritium, or a material called lithium deuteride. This is why they are often called hydrogen bombs.

A fusion explosion starts when the fission primary stage explodes. This explosion creates extreme heat, over 100 million kelvin, which makes it glow brightly with powerful X-rays. These X-rays fill the space between the primary and secondary parts, which are inside a strong container called a radiation case. This case traps the X-ray energy. The distance between the two parts is important. It makes sure that pieces from the primary explosion don't break apart the secondary before the fusion reaction can fully happen.

The secondary fusion stage has an outer layer, fusion fuel inside, and a central "spark plug" made of plutonium. The X-ray energy from the primary pushes on the outer layer of the secondary, squeezing the entire stage. This makes the plutonium spark plug inside much denser. The spark plug becomes so dense that it starts its own nuclear fission chain reaction. The heat from this reaction then ignites the very compressed fusion fuel around the spark plug, causing fusion reactions to start. In modern bombs that use lithium deuteride, the fissioning spark plug also releases neutrons. These neutrons hit the lithium, creating more tritium, which is needed for the fusion fuel.

The secondary's heavy outer layer (called a tamper) helps keep the fusion fuel from expanding too quickly. It also acts like a shield, stopping the fusion fuel from getting too hot, which would stop the compression. If this tamper is made of uranium, enriched uranium, or plutonium, it can also undergo fission when hit by fast neutrons from the fusion reaction. This adds to the bomb's total explosive power. Also, in many designs, the radiation case itself is made of a material that can fission when hit by these fast neutrons. Bombs like these are called two-stage weapons. The fission of the tamper and radiation case creates most of the bomb's total power and produces the most radioactive fallout.

Before the Ivy Mike test, the Operation Greenhouse tests in 1951 helped scientists understand the principles needed for thermonuclear weapons. Enough fission was achieved to boost a fusion device, and scientists learned enough to build a full-scale bomb within a year. Most modern thermonuclear weapons in the United States use a design called the Teller–Ulam configuration. This design was developed in 1951 by Edward Teller and Stanisław Ulam. Other countries like the Soviet Union, United Kingdom, France, China, and India also developed similar devices. The Soviet Union's Tsar Bomba was the most powerful bomb ever exploded. Since thermonuclear weapons are the most efficient way to get a very large explosion (over 50 kilotons of TNT (210 TJ)), almost all large nuclear weapons used by the main nuclear powers today are thermonuclear weapons based on the Teller–Ulam design.

How Thermonuclear Weapons Work

Primary and Secondary Stages

The main idea behind the Teller–Ulam configuration is to link different parts of a thermonuclear weapon in stages. The explosion of one stage provides the energy to start the next stage. At its simplest, this means there's a primary section, which is a small nuclear fission bomb (like a "trigger"), and a secondary section, which holds the fusion fuel. The energy from the primary explosion squeezes the secondary part through a process called radiation implosion. Once compressed, the secondary heats up and undergoes nuclear fusion. This process could even continue, with the secondary's energy igniting a third fusion stage. For example, the Soviet Union's AN602 "Tsar Bomba" is believed to have been a three-stage bomb (fission-fusion-fusion). In theory, by adding more stages, thermonuclear weapons could be made with extremely high explosive power. This is different from fission bombs, which have a limit to their power because you can only gather so much fission fuel in one place before it becomes too dangerous.

One possible version of the Teller–Ulam design

All these parts are surrounded by a container called a hohlraum or radiation case. This case temporarily traps the energy from the first stage. The outside of this radiation case is usually the bomb's outer casing. This is the only part of a thermonuclear bomb's design that is publicly visible in photographs.

The primary is thought to be a standard fission bomb that uses implosion. It likely has a core that is "boosted" by small amounts of fusion fuel (usually a mix of deuterium and tritium gas) to make it more efficient. This fusion fuel releases extra neutrons when heated and compressed, which causes more fission. When the bomb is fired, the Template:OptionalLink or Template:OptionalLink core is squeezed into a smaller ball by special layers of conventional high explosives. This starts the nuclear chain reaction that powers the "atomic bomb" part.

The secondary is usually shown as a column of fusion fuel and other parts wrapped in many layers. Around this column is a "pusher-tamper". This is a heavy layer of uranium-238 (Template:OptionalLink) or lead that helps squeeze the fusion fuel. If it's made of uranium, it can also undergo fission later. Inside this is the fusion fuel, usually a form of lithium deuteride. This dry fuel is easier to use in weapons than liquid tritium/deuterium gas. When neutrons hit this dry fuel, the lithium-6 produces tritium, a heavy type of hydrogen that can undergo nuclear fusion, along with the deuterium already in the mix. Inside the fuel layer is the "spark plug", which is a hollow column of fissile material (Template:OptionalLink or Template:OptionalLink). This spark plug, when squeezed, can undergo nuclear fission. If a third stage (tertiary) is present, it would be placed below the secondary and likely made of similar materials.

The Interstage: Connecting the Stages

Between the primary and secondary is the interstage. The primary fission explosion creates four types of energy:

1. Expanding hot gases from the explosives that squeeze the primary.
2. Superheated plasma from the bomb's fissile material.
3. Electromagnetic radiation (X-rays and gamma rays).
4. Neutrons from the primary's nuclear explosion.

The interstage's job is to carefully control how this energy moves from the primary to the secondary. It must direct the hot gases, plasma, radiation, and neutrons to the right place at the right time. If the interstage isn't designed perfectly, the secondary might not work at all, leading to a "fissile fizzle" (a partial explosion). The Castle Koon test is an example where a small flaw allowed neutrons from the primary to heat the secondary too early, weakening the compression and preventing fusion.

This is a secret paper by Teller and Ulam from March 9, 1951. It's about their new idea for staged implosion. This version has many parts blacked out because they are still secret.

There isn't much public information about how the interstage works. One of the best sources is a simple drawing of a British thermonuclear weapon. It was released by Greenpeace. This drawing shows the main parts and how they are arranged, but it lacks many details and likely has some intentional missing or incorrect information. It labels parts like "End-cap and Neutron Focus Lens" and "Reflector Wrap." The first part guides neutrons to the Template:OptionalLink/Template:OptionalLink Spark Plug. The "Reflector Wrap" is an X-ray reflector, usually a cylinder made of a material like uranium. It doesn't reflect like a mirror; instead, it gets very hot from the X-rays from the primary and then sends out its own, more evenly spread X-rays to the secondary. This is called radiation implosion. In the Ivy Mike test, gold was used as a coating over the uranium to improve this effect.

There's also a "Reflector/Neutron Gun Carriage." This reflector seals the space between the Neutron Focus Lens and the outer casing near the primary. It separates the primary from the secondary and works like the other reflector. About six neutron guns are attached to this carriage, pointing towards the center of the bomb. Neutrons from these guns pass through the neutron focus lens and are directed to the primary's center to help boost the initial fission of the plutonium. A "polystyrene Polarizer/Plasma Source" is also shown.

A U.S. government document released in 2004 mentioned the interstage. It said that a new design would replace "toxic, brittle material" and "expensive 'special' material" that need "unique facilities." The "toxic, brittle material" is widely believed to be beryllium, which fits the description and would also help control the neutron flow from the primary. Some material to absorb and re-emit X-rays in a specific way might also be used.

Candidates for the "special material" include polystyrene and a secret material called "Fogbank". Fogbank's exact makeup is classified, but some think it might be a type of aerogel. It was first used in thermonuclear weapons with the W76 warhead. Making Fogbank again for the W76 Life Extension Program was difficult because the original properties weren't fully documented. A huge effort was needed to figure out how to make it again. An impurity important to the old Fogbank's properties was left out of the new process. Only careful analysis of new and old batches revealed what that impurity was. The manufacturing process used acetonitrile as a solvent, which is flammable and toxic. This led to at least three evacuations of the Fogbank plant in 2006. Y-12 is the only place that makes Fogbank.

How the Secondary is Compressed

How exactly the energy is "transported" from the primary to the secondary has been a debated topic in public discussions. It's believed to be transferred through the X-rays and gamma rays released by the primary's fission. This energy then squeezes the secondary. The key question is how these X-rays create the pressure. There are three main ideas:

• Radiation pressure: This idea suggests that the pressure from the huge number of X-ray photons inside the closed casing is enough to squeeze the secondary. X-rays, like light, carry momentum and push on surfaces. While radiation pressure is usually too small to notice in everyday life, it's enormous inside a thermonuclear bomb. For the Ivy Mike test bomb and the modern W-80 warhead, the calculated radiation pressure was extremely high.

• Foam plasma pressure: This idea suggests that the X-rays turn the foam filler in the radiation channel (like polystyrene or "Fogbank" plastic foam) into a hot plasma. This plasma then pushes against the secondary's tamper, squeezing it tightly and starting the fission chain reaction in the spark plug. This then heats the fusion fuel, causing fusion. The high-energy neutrons from the fusion reaction then hit the Template:OptionalLink tamper (or the bomb casing), causing it to undergo a fast fission reaction, which provides about half of the total energy. This completes a fission-fusion-fission sequence. Fusion itself is relatively "clean" because it releases energy but not many harmful radioactive products. However, the fission reactions, especially the last ones, release a huge amount of radioactive fallout. If the last fission stage is removed (by using lead instead of uranium for the tamper, for example), the total explosive force is cut by about half, but the fallout is much lower. A neutron bomb is a type of hydrogen bomb designed with a very thin tamper to let as many fast fusion neutrons as possible escape.

How the foam plasma mechanism works.

1. The bomb before it explodes. The primary (fission bomb) is at the top, and the secondary (fusion fuel) is at the bottom. Both are surrounded by polystyrene foam.
2. The high explosives around the primary's core explode, squeezing the fissile material until it becomes super-critical and starts a fission reaction.
3. The fissioning primary releases X-rays. These X-rays bounce around inside the casing and hit the polystyrene foam.
4. The polystyrene foam turns into a hot plasma. This plasma pushes on the secondary, squeezing it. The plutonium spark plug inside the secondary starts to fission.
5. The squeezed and heated lithium-6 deuteride fuel creates tritium (Template:OptionalLink) and starts the fusion reaction. The neutrons released cause the Template:OptionalLink tamper to fission. A fireball begins to form.

• Tamper-pusher ablation: This is the most widely accepted idea. The outer layer of the secondary (the "tamper-pusher") gets extremely hot from the primary's X-rays. This causes its outer layers to expand violently and fly off (ablate). Because of momentum conservation, this flying-off material pushes the rest of the tamper-pusher inwards with immense force, crushing the fusion fuel and the spark plug. The tamper-pusher is strong enough to protect the fusion fuel from the extreme heat outside, which is important for the compression to work.

How the ablation mechanism works.

1. The bomb before it explodes. The nested spheres at the top are the fission primary. The cylinders below are the fusion secondary.
2. The primary's explosives have detonated and squeezed the primary's fissile pit.
3. The primary's fission reaction is complete. The primary is now millions of degrees hot and releasing gamma and hard X-rays. These heat up the inside of the hohlraum and the shield and secondary's tamper.
4. The primary's reaction is over and it has expanded. The surface of the secondary's pusher is now so hot that it is ablating (flying off), pushing the rest of the secondary (tamper, fusion fuel, and fissile spark plug) inwards. The spark plug starts to fission. (Not shown: the radiation case is also ablating and expanding outwards).
5. The secondary's fuel has started the fusion reaction and will soon burn up completely. A fireball begins to form.

Comparing these three ideas, the calculated ablation pressure is much greater than the other proposed pressures. This suggests that ablation is the main way the secondary is compressed.

History of Development

United States' Journey

The Castle Romeo test during Operation Castle

The idea of a fusion bomb ignited by a smaller fission bomb was first suggested by Enrico Fermi to Edward Teller in 1941. Teller spent much of the Manhattan Project trying to make this design work. After World War II, there wasn't much urgency to develop this "Super" bomb.

However, when the Soviet Union tested its first atomic bomb in August 1949, earlier than expected, the U.S. government debated whether to develop the much more powerful Super bomb. Despite some objections, President Harry S. Truman decided to go ahead with the new weapon's development on January 31, 1950.

Teller and other U.S. physicists struggled to find a working design. Stanislaw Ulam, a colleague of Teller, made two key breakthroughs:

1. Squeezing the thermonuclear fuel before heating it was a practical way to achieve the conditions needed for fusion.
2. The idea of "staging," or placing a separate thermonuclear component outside the fission primary and using the primary to compress the secondary.

Teller then realized that the gamma and X-ray radiation from the primary could transfer enough energy to the secondary to create a successful implosion and fusion reaction, if the whole assembly was wrapped in a hohlraum (radiation case).

The "George" test during Operation Greenhouse on May 9, 1951, was the first small-scale test of this basic idea. It was the first successful release of nuclear fusion energy, which made up a small part of the total explosion. This test made it almost certain that the concept would work.

On November 1, 1952, the Teller–Ulam design was tested at full scale in the "Ivy Mike" test. This happened on an island in the Enewetak Atoll. The bomb, called the Sausage, had a yield of 10.4 Mt (44 PJ) (over 450 times more powerful than the bomb dropped on Nagasaki). It used a very large fission bomb as a trigger and liquid deuterium as its fusion fuel. The device weighed about 80 short tons (73 t) in total, including 20 short tons (18 t) of cryogenic equipment to keep the deuterium liquid.

The liquid deuterium fuel of Ivy Mike was not practical for a weapon that could be deployed. The next step was to use solid lithium deuteride as fusion fuel. This was tested in 1954 in the "Castle Bravo" test (device code-named Shrimp). This test had a yield of 15 Mt (63 PJ), which was 2.5 times more than expected, and is the largest U.S. bomb ever tested.

The United States then focused on making smaller Teller–Ulam weapons that could fit into intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs). By 1960, megaton-class warheads were as small as 18 inches (0.46 m) in diameter and weighed 720 pounds (330 kg). Further improvements in miniaturization happened by the mid-1970s, allowing ten or more warheads to fit on a single MIRVed missile.

Soviet Union's Development

The first Soviet fusion design, called the Sloika (after a Russian layer cake), was developed by Andrei Sakharov and Vitaly Ginzburg in 1949. It was not a Teller–Ulam design. It used alternating layers of fissile material and lithium deuteride fusion fuel with tritium. While it could achieve nuclear fusion, it couldn't be scaled up to make arbitrarily large explosions like the U.S. designs. The fusion layer around the fission core could only moderately increase the fission energy (modern Teller–Ulam designs can increase it 30 times). Also, the entire fusion stage had to be squeezed by conventional explosives along with the fission core, requiring a lot more chemical explosives.

The first Sloika design test, RDS-6s, exploded in 1953 with a yield of 400 kt (1,700 TJ) (15-20% from fusion). Attempts to use the Sloika design for megaton-range results failed. After the U.S. tested "Ivy Mike" in November 1952, showing that a multi-megaton bomb was possible, the Soviets looked for a new design.

Sakharov's "Second Idea" was a proposal from Ginzburg in 1948 to use lithium deuteride in the bomb. When hit by neutrons, this material would produce tritium and free deuterium. In late 1953, physicist Viktor Davidenko made the first breakthrough: keeping the primary and secondary parts of the bombs separate ("staging"). The next breakthrough was discovered by Sakharov and Yakov Zel'dovich in early 1954: using the X-rays from the fission bomb to compress the secondary before fusion ("radiation implosion"). Sakharov's "Third Idea," which was the Teller–Ulam design in the USSR, was tested in the "RDS-37" shot in November 1955 with a yield of 1.6 Mt (6.7 PJ).

The Soviets showed the power of the "staging" concept in October 1961 when they exploded the huge Tsar Bomba. This 50 Mt (210 PJ) hydrogen bomb was the largest nuclear weapon ever developed and tested by any country.

United Kingdom's Efforts

Operation Grapple on Christmas Island was the first British hydrogen bomb test.

In 1954, work began at Aldermaston to develop the British fusion bomb, led by Sir William Penney. Britain's knowledge of how to build a thermonuclear fusion bomb was basic, and at the time, the United States was not sharing any nuclear information due to the Atomic Energy Act of 1946. However, the British were allowed to watch the U.S. Castle tests and used aircraft to collect samples from the mushroom clouds. This gave them clear evidence of how radiation implosion compressed the secondary stages.

Because of these difficulties, in 1955, British Prime Minister Anthony Eden agreed to a secret plan: if the Aldermaston scientists failed or were very delayed in developing the fusion bomb, they would use an extremely large fission bomb instead.

In 1957, the Operation Grapple tests were conducted. The first test, Green Granite, was a prototype fusion bomb, but it only produced about 300 kt (1,300 TJ), which was less than the U.S. and Soviet bombs. The second test, Orange Herald, was a modified fission bomb and produced 720 kt (3,000 TJ), making it the largest fission explosion ever. At the time, almost everyone thought this was a fusion bomb. This bomb was put into service in 1958. A second prototype fusion bomb, Purple Granite, was used in the third test, but it only produced about 150 kt (630 TJ).

A second series of tests began in September 1957. The first test was based on a "new simpler design: a two-stage thermonuclear bomb with a much more powerful trigger." This test, Grapple X Round C, exploded on November 8 and yielded about 1.8 Mt (7.5 PJ). On April 28, 1958, a bomb was dropped that yielded 3 Mt (13 PJ), which was Britain's most powerful test. Two final air burst tests in September 1958 dropped smaller bombs, each yielding around 1 Mt (4.2 PJ).

American observers were invited to these tests. After Britain successfully detonated a megaton-range device (showing they understood the Teller–Ulam design "secret"), the United States agreed to share some of its nuclear designs with the United Kingdom. This led to the 1958 US–UK Mutual Defence Agreement. Instead of continuing with its own design, Britain gained access to the design of the smaller American Mk 28 warhead and was able to make copies.

China's Nuclear Program

Mao Zedong decided to start a Chinese nuclear-weapons program during the First Taiwan Strait Crisis of 1954–1955. The People's Republic of China exploded its first hydrogen (thermonuclear) bomb on June 17, 1967. This was 32 months after they exploded their first fission weapon. The bomb had a yield of 3.31 Mt and took place at the Lop Nor Test Site in northwest China. China had received a lot of technical help from the Soviet Union to start their nuclear program. However, by 1960, the disagreement between the Soviet Union and China became so big that the Soviet Union stopped all assistance to China.

France's Development

The French nuclear testing site was moved to unpopulated French islands in the Pacific Ocean. The first test at these new sites was the "Canopus" test on the Fangataufa atoll in French Polynesia on August 24, 1968. This was France's first test of a multi-stage thermonuclear weapon. The bomb was exploded from a balloon at a height of 520 metres (1,710 ft). This test caused a lot of atmospheric pollution. Very little is known about France's development of the Teller–Ulam design, other than that France detonated a 2.6 Mt (11 PJ) device in the "Canopus" test. France reportedly had great difficulty at first with its Teller-Ulam design, but later overcame these challenges. It is now believed to have nuclear weapons as advanced as other major nuclear powers.

France and China did not sign or agree to the Partial Nuclear Test Ban Treaty of 1963, which banned nuclear test explosions in the atmosphere, underwater, or in outer space. Between 1966 and 1996, France carried out more than 190 nuclear tests. France's last nuclear test was on January 27, 1996. After that, the country dismantled its Polynesian test sites. France signed the Comprehensive Nuclear-Test-Ban Treaty that same year and agreed to it within two years.

One of France's Triomphant-class nuclear-armed submarines, Le Téméraire (S617)

In 2015, France confirmed that its nuclear arsenal has about 300 warheads. These are carried by submarine-launched ballistic missiles (SLBMs) and fighter-bombers. France has four Triomphant-class ballistic missile submarines. One submarine is always deployed in the deep ocean, but a total of three must be ready for use at all times. The three older submarines carry 16 M45 missiles. The newest submarine, "Le Terrible", was put into service in 2010. It has M51 missiles that can carry TN 75 thermonuclear warheads. The air fleet has four squadrons at four different bases. In total, there are 23 Mirage 2000N aircraft and 20 Rafales that can carry nuclear warheads. The M51.1 missiles are planned to be replaced with the new M51.2 warhead starting in 2016. This new warhead has a 3,000 kilometres (1,900 mi) greater range than the M51.1.

France also has about 60 air-launched missiles with TN 80/TN 81 warheads, each with a yield of about 300 kt (1,300 TJ). France's nuclear program is designed to ensure these weapons remain usable for decades. Currently, France is no longer intentionally producing materials like plutonium and enriched uranium for weapons. However, it still uses nuclear energy for electricity, which produces Template:OptionalLink as a byproduct.

India's Nuclear Tests

Shakti-1

On May 11, 1998, India announced that it had exploded a thermonuclear bomb in its Operation Shakti tests (specifically, "Shakti-I"). In Hindi, 'Shakti' means power. Samar Mubarakmand, a Pakistani nuclear physicist, said that if Shakti-I was a thermonuclear test, the device had failed. However, Harold M. Agnew, former director of the Los Alamos National Laboratory, said that India's claim of detonating a staged thermonuclear bomb was believable. India states that their thermonuclear device was tested at a controlled yield of 45 kt (190 TJ). This was done because the Khetolai village was only about 5 kilometres (3.1 mi) away, and they wanted to make sure the houses there were not badly damaged. Another reason given was that radioactivity from yields much higher than 45 Kilotons might not have been fully contained. After the Pokhran-II tests, Rajagopala Chidambaram, former chairman of the Atomic Energy Commission of India, said that India can build thermonuclear bombs of any yield they want.

The actual yield of India's hydrogen bomb test is still debated among Indian scientists and international experts. The question of politics and disagreements between Indian scientists has made the matter more complicated.

In an interview in August 2009, K. Santhanam, the director for the 1998 test site preparations, claimed that the thermonuclear explosion's yield was lower than expected. He argued that India should not rush to sign the CTBT. Other Indian scientists involved in the test disagreed with Santhanam's claim, saying it was not scientific. British seismologist Roger Clarke suggested that the earthquake magnitudes indicated a combined yield of up to 60 kilotonnes of TNT (250 TJ), which matches India's announced total yield of 56 kilotonnes of TNT (230 TJ). U.S. seismologist Jack Evernden argued that to correctly estimate yields, one must "properly account for geological and seismological differences between test sites."

India officially states that it can build thermonuclear weapons of various yields, up to about 200 kt (840 TJ), based on the Shakti-1 thermonuclear test.

Israel's Alleged Weapons

Israel is believed to have thermonuclear weapons based on the Teller–Ulam design. However, it is not known to have tested any nuclear devices. It is widely thought that the Vela incident of 1979 might have been a joint Israeli–South African nuclear test.

It is well known that Edward Teller advised and guided Israel on general nuclear matters for about twenty years. Between 1964 and 1967, Teller visited Israel six times, giving lectures on theoretical physics at Tel Aviv University. It took him a year to convince the CIA about Israel's capability. Finally, in 1976, Carl E. Duckett of the CIA told the U.S. Congress, after getting reliable information from an "American scientist" (Teller), about Israel's nuclear capability. In the 1990s, Teller eventually confirmed media speculation that during his visits in the 1960s, he concluded that Israel had nuclear weapons. After he told the higher levels of the U.S. government, Teller reportedly said: "They [Israel] have it, and they were clever enough to trust their research and not to test, they know that to test would get them into trouble."

North Korea's Claims

North Korea claimed to have tested its miniaturized thermonuclear bomb on January 6, 2016. North Korea's first three nuclear tests (2006, 2009, and 2013) had relatively low yields and do not seem to have been thermonuclear weapon designs. In 2013, the South Korean Defense Ministry thought that North Korea might be trying to develop a "hydrogen bomb" and that such a device might be North Korea's next weapon test. In January 2016, North Korea claimed to have successfully tested a hydrogen bomb. However, only an earthquake of magnitude 5.1 was detected at the time of the test. This was similar to the magnitude of the 2013 test of a 6–9 kt (25–38 TJ) atomic bomb. These seismic readings made people doubt North Korea's claim that a hydrogen bomb was tested, suggesting it was a non-fusion nuclear test.

On September 3, 2017, North Korea's state media reported that a hydrogen bomb test was conducted and was a "perfect success." According to the U.S. Geological Survey (USGS), the explosion released energy equal to an earthquake with a seismic magnitude of 6.3. This was 10 times more powerful than North Korea's previous nuclear tests. U.S. Intelligence released an early estimate that the yield was 140 kt (590 TJ), with a possible range of 70 to 280 kt (290 to 1,170 TJ).

On September 12, NORSAR updated its estimate of the explosion magnitude to 6.1, matching the CTBTO's estimate. However, this was less powerful than the USGS estimate of 6.3. Its yield estimate was updated to 250 kt (1,000 TJ), noting that the estimate had some uncertainty.

On September 13, an analysis of satellite images of the test site suggested the test happened under 900 metres (3,000 ft) of rock. The yield "could have been more than 300 kilotons."

Public Understanding of Nuclear Weapon Design

Photographs of warhead casings, like this one of the W80 nuclear warhead, help people guess the relative size and shapes of the primaries and secondaries in U.S. thermonuclear weapons.

For many years, the Teller–Ulam design was considered one of the top nuclear secrets. Even today, official government publications do not discuss it in detail. The United States Department of Energy (DOE) policy is not to confirm when "leaks" happen, because doing so would confirm if the leaked information is accurate. Besides images of the warhead casing, most public information about this design comes from a few short statements by the DOE and the work of a few individual researchers.

DOE Statements

In 1972, the U.S. government declassified a document stating: "In thermonuclear (TN) weapons, a fission 'primary' is used to trigger a TN reaction in thermonuclear fuel referred to as a 'secondary'." In 1979, they added: "In thermonuclear weapons, radiation from a fission explosive can be contained and used to transfer energy to compress and ignite a physically separate component containing thermonuclear fuel." The U.S. government specified that "Any elaboration of this statement will be classified." The only information that might relate to the spark plug was declassified in 1991: "Fact that fissile or fissionable materials are present in some secondaries, material unidentified, location unspecified, use unspecified, and weapons undesignated." In 1998, the DOE declassified the statement that "The fact that materials may be present in channels and the term 'channel filler', with no elaboration," which might refer to the polystyrene foam (or a similar substance).

Whether these statements confirm some or all of the models discussed above is open to interpretation. Official U.S. government releases about nuclear weapons have sometimes been intentionally vague in the past. Other information, such as the types of fuel used in some early weapons, has been declassified, but precise technical details have not.

The Progressive Case

Most of the current ideas about how the Teller–Ulam design works became public after the Department of Energy (DOE) tried to stop a magazine article by U.S. anti-weapons activist Howard Morland in 1979. Morland's article was about the "secret of the hydrogen bomb." In 1978, Morland decided that revealing this "last remaining secret" would draw attention to the arms race and help citizens feel able to question official statements about the importance of nuclear weapons and secrecy. Most of Morland's ideas came from easily available sources, including drawings from the Encyclopedia Americana. Morland also interviewed many former Los Alamos scientists (though none gave him direct useful information). He used clever ways to get information from them, like asking questions such as "Do they still use spark plugs?" even if he didn't know exactly what that term meant.

Morland eventually concluded that the "secret" was that the primary and secondary parts were kept separate, and that radiation pressure from the primary squeezed the secondary before igniting it. When an early draft of his article, meant for The Progressive magazine, was sent to the DOE, the DOE asked that the article not be published and sought a temporary court order to stop it. The DOE argued that Morland's information was likely from classified sources, or if not, it was "secret" under the "born secret" rule of the 1954 Atomic Energy Act. They also said it was dangerous and would encourage nuclear proliferation.

Morland and his lawyers disagreed, but the court order was granted. The judge felt it was safer to stop the publication and allow Morland to appeal, which he did in United States v. The Progressive (1979).

Due to other complex events, the DOE's case weakened when it became clear that some of the information they claimed was "secret" had already been published in a students' encyclopedia years earlier. After another person, Chuck Hansen, published his own ideas about the "secret" (which were different from Morland's) in a Wisconsin newspaper, the DOE said The Progressive case was no longer relevant. They dropped their lawsuit, allowing the magazine to publish its article in November 1979. By then, Morland had changed his mind about how the bomb worked. He suggested that a foam material (polystyrene) rather than just radiation pressure was used to compress the secondary, and that the secondary also had a "spark plug" of fissile material. He published these changes as a short correction in The Progressive a month later. In 1981, Morland wrote a book about his experience, explaining how he reached his conclusions about the "secret."

Morland's work is thought to be at least partly correct because the DOE tried to censor it. This was one of the few times they went against their usual policy of not acknowledging "secret" material that had been released. However, it's not known for sure how much information is missing or incorrect. The difficulty that several countries had in developing the Teller–Ulam design (even when they seemed to understand the basic idea, like the United Kingdom) makes it unlikely that this simple information alone is enough to build thermonuclear weapons. Nevertheless, the ideas put forward by Morland in 1979 have been the basis for all current public speculation about the Teller–Ulam design.

Reducing Nuclear Weapons

In January 1986, Soviet leader Mikhail Gorbachev publicly proposed a three-step plan to get rid of all the world's nuclear weapons by the end of the 20th century. Two years before his death in 1989, Andrei Sakharov's comments at a scientists' meeting helped start the process of removing thousands of nuclear ballistic missiles from the U.S. and Soviet arsenals. Sakharov (1921–1989) joined the Soviet Union's nuclear weapons program in 1948. In 1949, the U.S. detected the first Soviet test of a fission bomb, and both countries began a desperate race to design a thermonuclear hydrogen bomb that was a thousand times more powerful. Like his U.S. counterparts, Sakharov justified his work on the H-bomb by pointing to the danger of the other country having a monopoly. But also like some U.S. scientists who worked on the Manhattan Project, he felt responsible for informing his country's political leaders and then the wider world about the dangers of nuclear weapons.

Sakharov's first attempt to influence policy came from his worry about possible genetic damage from long-lasting radioactive carbon-14. This carbon-14 was created in the atmosphere from nitrogen-14 by the huge amounts of neutrons released in H-bomb tests. In 1968, a friend suggested that Sakharov write an essay about the role of educated people in world affairs. Self-publishing was the way to spread unapproved writings in the Soviet Union at the time. Many readers would make multiple copies by typing with several sheets of paper and carbon paper. One copy of Sakharov's essay, "Reflections on Progress, Peaceful Coexistence, and Intellectual Freedom," was secretly taken out of the Soviet Union and published by the New York Times. More than 18 million copies were made in 1968–69. After the essay was published, Sakharov was not allowed to return to work in the nuclear weapons program and took a research job in Moscow. In 1980, after an interview with the New York Times where he spoke against the Soviet invasion of Afghanistan, the government sent him and his wife to Gorky, away from Western media. In March 1985, Gorbachev became the leader of the Soviet Communist Party. More than a year and a half later, he convinced the Politburo (the party's executive committee) to allow Sakharov and Bonner to return to Moscow. Sakharov was elected as an opposition member to the Soviet Congress of People's Deputies in 1989. Later that year, he had a cardiac arrhythmia and died in his apartment. He left behind a draft of a new Soviet constitution that focused on democracy and human rights.

Accidents Involving Thermonuclear Weapons

• On February 5, 1958, during a training flight by a B-47, a Mark 15 nuclear bomb, also known as the Tybee Bomb, was lost off the coast of Tybee Island near Savannah, Georgia. The U.S. Air Force says the bomb was not armed and did not contain the live plutonium core needed for a nuclear explosion. The Department of Energy believes the bomb is buried under several feet of silt at the bottom of Wassaw Sound.

• On January 17, 1966, a deadly collision happened between a B-52G bomber and a KC-135 Stratotanker over Palomares, Spain. The conventional explosives in two of the Mk28-type hydrogen bombs exploded when they hit the ground, spreading plutonium over nearby farms. A third bomb landed safely near Palomares, while the fourth fell 12 miles (19 km) off the coast into the Mediterranean Sea and was found a few months later.

• On January 21, 1968, a B-52G, carrying four B28FI thermonuclear bombs as part of Operation Chrome Dome, crashed on the ice of the North Star Bay while trying to make an emergency landing at Thule Air Base in Greenland. The fire caused a lot of radioactive contamination. Workers involved in the cleanup could not find all the pieces from three of the bombs, and one bomb was never found.

Different Designs and Variations

Ivy Mike's Design

In his 1995 book Dark Sun: The Making of the Hydrogen Bomb, author Richard Rhodes describes the inside parts of the "Ivy Mike" Sausage device in detail. He based this information on many interviews with the scientists and engineers who built it. According to Rhodes, the secondary was compressed by a combination of radiation pressure, foam plasma pressure, and tamper-pusher ablation. The radiation from the primary heated the polyethylene foam lining of the casing into a plasma. This plasma then re-radiated radiation into the secondary's pusher, causing its surface to ablate and push inwards. This compressed the secondary, ignited the sparkplug, and started the fusion reaction. It's not clear if this exact combination of principles applies to all thermonuclear bombs.

The W88 Warhead

In 1999, a reporter for the San Jose Mercury News reported that the U.S. W88 nuclear warhead had a prolate (egg or watermelon shaped) primary (code-named Komodo) and a spherical secondary (code-named Cursa). These were inside a specially shaped radiation case (known as the "peanut" because of its shape). The benefit of an egg-shaped primary is that a MIRV warhead's size is limited by the diameter of the primary. If an egg-shaped primary can work correctly, the MIRV warhead can be made much smaller while still delivering a high-yield explosion. A W88 warhead can produce up to 475 kilotonnes of TNT (1,990 TJ). Its main part is 68.9 inches (1,750 mm) long, with a maximum diameter of 21.8 inches (550 mm), and weighs between 175 to 360 kilograms (386 to 794 lb). A smaller warhead means more of them can fit onto a single missile, and it also improves flight characteristics like speed and range.

Most bombs do not have tertiary "stages" (additional fusion stages compressed by a previous fusion stage). The fissioning of the final uranium blanket, which provides about half the yield in large bombs, is not counted as a "stage" in this way.

The U.S. tested three-stage bombs in several explosions (see Operation Redwing) but is thought to have only used one such model: the heavy but very efficient 25 Mt (100 PJ) B41 nuclear bomb. The Soviet Union is believed to have used multiple stages (including more than one tertiary fusion stage) in their 50 Mt (210 PJ) (or 100 Mt (420 PJ) if fully used) Tsar Bomba. The fissionable jacket could be replaced with lead, as was done with the Tsar Bomba. If any hydrogen bombs have been made using designs other than the Teller–Ulam design, this is not publicly known. An exception might be the Soviet Union's early Sloika design.

In short, the Teller–Ulam design relies on at least two steps of compression:

1. First, conventional (chemical) explosives in the primary squeeze the fissile core, causing a fission explosion much more powerful than chemical explosives alone could achieve.
2. Second, the radiation from the primary's fission is used to squeeze and ignite the secondary fusion stage, resulting in a fusion explosion much more powerful than the fission explosion alone.

This chain of compression could theoretically continue with any number of tertiary fusion stages, each igniting more fusion fuel in the next stage. Finally, efficient bombs (but not "neutron bombs") end with the fissioning of the final natural uranium tamper. This fission could not normally happen without the neutron flux provided by the fusion reactions in the secondary or tertiary stages. Such designs are thought to be able to be scaled up to an extremely large yield, potentially to the level of a "doomsday device." However, most such weapons were not more than a dozen megatons, which was generally considered enough to destroy even the strongest targets. Even these large bombs have been replaced by smaller "nuclear bunker buster" type nuclear bombs.

As mentioned, for destroying cities and less protected targets, breaking the mass of a single missile payload into smaller MIRV bombs is much more efficient. This spreads the energy of the explosions over a wider area. This also applies to single bombs delivered by cruise missiles or bombers. As a result, most operational warheads in the U.S. program have yields of less than 500 kt (2,100 TJ).

Images for kids

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  A basic diagram of a thermonuclear weapon.
  Some designs use spherical secondaries.

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  Edward Teller in 1958