H-bomb

The H-bomb (also called hydrogen bomb, fusion bomb, or thermonuclear bomb) is a nuclear bomb whose main energy comes from the fusion of light nuclei.

More powerful and complex than a nuclear fission bomb, called an “A-bomb”, an H-bomb is divided into two stages:

• The operation of the first stage is that of a “conventional” plutonium fission atomic bomb;
• The second stage consists of fusion fuels, the isotopes of hydrogen-deuterium and tritium; it is its operation that constitutes the thermonuclear explosion itself.

H-bomb historical

As early as 1940, the Hungarian-American nuclear physicist Edward Teller saw the possibility of using the enormous thermal power (to reach a temperature of 10 K, or one hundred million kelvins, or degrees Celsius) produced by the explosion of an A-bomb to trigger the nuclear fusion process. In 1941, Teller joined the Manhattan Project, which aimed to develop the fission bomb.

After preliminary work in Chicago with Enrico Fermi and in Berkeley with Robert Oppenheimer, Teller went to Los Alamos National Laboratory to work on the atomic bomb under Oppenheimer’s direction. But given the difficulty of making a fusion bomb, the H-bomb trail is not followed, much to Teller’s disappointment.

In 1949, after the Soviets detonated their own fission bomb on August 29, U.S. intelligence analysis showed it was a plutonium-using bomb. The monopoly of the United States no longer existed and the new one caused a considerable psychological shock. Indeed, the Americans believed they could maintain the monopoly of nuclear weapons for a decade. They then embark on a new epic, that of the search for a bomb even more powerful than the fission bomb: the fusion bomb.

U.S. President Harry S. Truman asked Los Alamos National Laboratory to develop a bomb that would work by melting nuclei. Oppenheimer is against this decision, considering it just another instrument of genocide. Teller was then appointed program manager. However, its model, although reasonable, does not achieve the intended goal.

The Polish-American mathematician Stanislaw Marcin Ulam, in collaboration with C. J. Everett, performed detailed calculations that showed that Teller’s model was inefficient. Ulam then suggests a method that will be retained. By placing a fission bomb at one end and thermonuclear material at the other end of an enclosure, it is possible to direct the shock waves produced by the fission bomb. These waves compress and “ignite” the thermonuclear fuel.

At first, Teller dismisses the idea and then understands the merits, but suggests the use of radiation rather than shock waves to compress thermonuclear material. The first H-bomb, Ivy Mike, exploded on Eniwetok Atoll (near Bikini Atoll in the Pacific Ocean) on November 1, 1952, to Teller’s satisfaction, despite the disagreement of much of the scientific community. This bomb had a power of 10.4 Mt.

“Radiation implosion” is now the standard method for creating fusion bombs. The two creators, Ulam and Teller, have patented their H-bomb.

“Teller-Ulam” H-bomb

Structure

• The upper part or primary part: it is the fission bomb (A-bomb type) which, by exploding, causes a very strong increase in temperature and thus the triggering of fusion. The United States will use the Tsetse primary in particular.
• The lower part or secondary part: this is the material that will fuse, here lithium deuteride, accompanied by a plutonium core and a uranium 238 envelope. This part is surrounded by a polystyrene foam that will allow a very high rise in temperature.
• Finally, it is possible to use a third stage, of the same type as the second, to produce a much more powerful hydrogen bomb. This additional stage is much larger (on average ten times larger) and its fusion is initiated by the energy released by the fusion of the second stage. It is therefore possible to make H-bombs of very high powers by adding several stages.

The power of the primary stage, thus its ability to cause the explosion of the secondary, is increased (doped) by a mixture of tritium, which undergoes a nuclear fusion reaction with deuterium. Fusion emits a large number of neutrons, which substantially increase the fission of plutonium or highly enriched uranium present in the stages. This approach is used in modern weapons to ensure sufficient power despite a significant decrease in size and weight. Such a fission-fusion-fission bomb is called Teller-Ulam architecture.

The bomb itself is surrounded by a structure that makes it possible to retain the massive supply of X-rays produced by the explosion of the fission bomb. These waves are then redirected in order to compress the melting material and the total explosion of the bomb can then begin.

The explosion

Reactions involving fusion can be as follows (2
1D being a deuterium nucleus ²H, 3
1T a tritium nucleus ³H, n a neutron and p a proton, 3
2He and 4
2He indicating helium 3 and helium 4 nuclei respectively):

1. 2
   1D + 3
   1T ⟶ 4
   2He + 1
   0n + 17,6 MeV ;
2. 2
   1D + 2
   1D ⟶ 3
   2He + 1
   0n + 3,3 MeV ;
3. 2
   1D + 2
   1D ⟶ 3
   1T + 1
   1p + 4,0 MeV ;
4. 3
   1T + 3
   1T ⟶ 4
   2He + 2 1
   0n ;
5. 3
   2He + 2
   1D ⟶ 4
   2He + 1
   1p ;
6. 6
   3Li + 1
   0n ⟶ 3
   1T + 4
   2He ;
7. 7
   3Li + 1
   0n ⟶ 3
   1T + 4
   2He + 1
   0n.

The first of these reactions (deuterium-tritium fusion) is relatively easy to start, the temperature and compression conditions are within the reach of high-performance chemical explosives. It is by itself insufficient to start a thermonuclear explosion, but can be used to boost the reaction: a few grams of deuterium and tritium in the center of the fissionable core will produce a large flux of neutrons, which will significantly increase the rate of combustion of the fissionable material. The neutrons produced have an energy of 14.1 MeV, which is enough to cause the fission of U-238, leading to a fission-fusion-fission reaction. Other reactions can only take place when a primary nuclear explosion has produced the necessary conditions of temperature and compression.

The explosion of an H-bomb takes place over a very short time interval: 6 × 10 s, or 600 ns. The fission reaction requires 550 ns and the fusion reaction 50 ns.

1. After the chemical explosive is ignited, the fission bomb goes off.
2. The explosion causes the appearance of X-rays, which reflect on the envelope and ionize the polystyrene, which passes into the state of plasma.
3. X-rays irradiate the buffer that compresses the fusion fuel (LiD) and primer it into plutonium, which, under the effect of this compression and neutrons, begins to fission.
4. Compressed and heated to very high temperatures, lithium(LiD) deuteride starts the fusion reaction. The following type of fusion reaction is usually observed. When the melting material fuses at more than a hundred million degrees, it releases a tremendous amount of energy. At a given temperature, the number of reactions increases as a function of the square of the density: thus, a compression a thousand times higher leads to the production of a million times more reactions.
5. The fusion reaction produces a large neutron flux that irradiates the buffer, and if it is composed of fissile materials (such as U), a fission reaction will occur, causing a new release of energy, of the same order of magnitude as the fusion reaction.

Power and effect of the explosion

Thermonuclear bombs have qualitatively similar effects to other nuclear weapons. However, they are generally more powerful than A-bombs, so the effects can be quantitatively much greater.

A “classic” value of the energy released by the explosion of a fission bomb is about 14 kt of TNT (or 14,000 t), one ton of TNT developing 10 cal, or 4.184 × 10 J. By design, the maximum value hardly exceeds 700 kt.

In comparison, H-bombs would theoretically be at least 1,000 times more powerful than Little Boy, the fission bomb dropped on Hiroshima in 1945. For example, Ivy Mike, the first American fusion bomb, released an energy of about 10,400 kt (10.4 Mt). The most powerful explosion in history was that of the Soviet Tsar Bomba of 57 Mt of power, which was to serve as a test for 100 Mt bombs. It was a bomb of type “FFF” (fission-fusion-fission) but “bridled”, the 3rd stage being inert. Khrushchev explained that it was a matter of not “breaking all the mirrors of Moscow.”

The maximum energy released by a fusion bomb can be increased indefinitely (at least on paper). The Tsar Bomba cleared 2.84 × 10 J.

Other H-bombs

Russian bombs

The structure of some Soviet and Russian H-bombs uses a different approach, in layers instead of separate components, which allowed the USSR to develop the first transportable H-bombs, therefore suitable for use in bombing. The first Soviet H-bomb explosion occurred on August 12, 1953, during the RDS-6s test (named Joe 4 by the Americans), which was more of a “doped” A-bomb. The USSR later used the Teller-Ulam concept, (re)discovered by Andrei Sakharov. Russia was also responsible for the most powerful nuclear explosion ever carried out, that of the TSAR, reaching 57 megatons.

Bombs from other countries

The British did not have access to American technology to design their fusion bomb and fumbled until 1957 to succeed in producing a multi-megaton bomb.

The People’s Republic of China (1967) and France (1968) built and tested megatonnic “H” bombs. Because of the secrecy surrounding nuclear weapons, the Teller-Ulam structure was “reinvented” (in France by Michel Carayol).

India claims to have done the same, but several experts, referring to seismographic recordings, dispute this result.

North Korea claimed to have successfully designed and tested an H-bomb on January 6, 2016. The U.S. Geological Survey (USGS) and South Korea’s meteorological agency have detected an earthquake with a magnitude between 4.2 and 5.1: too weak according to experts to authenticate a thermonuclear bomb. The country also claims to have tested an H-bomb on September 3, 2017, apparently successfully, with various government agencies detecting large artificial earthquakes. The estimated magnitude of this earthquake was 6.3.

“Clean” H-bomb

The military calls it a “clean” H-bomb when less than 50% of its total energy comes from the fission reaction. Indeed, fusion alone does not directly produce any radioactive compounds. The radioactive fallout of a “clean” H-bomb would therefore be a priori less important than that of a conventional A-bomb of the same power, while the other effects remain just as devastating. The difference comes from the design of the fusion stage. If the buffer is uranium, then it will fission, releasing half the power of the bomb, but causing 90% of the radioactive fallout. By replacing it with a pad made of another heavy, but not fissionable metal, such as lead, the bomb will lose half of its power, but with much lower fallout. However, the large amount of neutrons released will be absorbed by the surrounding matter, which will become radioactive.

“Famous” fusion bombs

• Ivy Mike, an American bomb, is the first H-bomb to be tested. It exploded on Eniwetok Atoll (Marshall Islands) on November 1, 1952. It had a power of 10.4 Mt.
• Castle Bravo is the most powerful H-bomb tested by the United States. With a power of 15 Mt, the explosion took place on Bikini Atoll, the Marshall Islands in Oceania, on March 1, 1954.
• Tsar Bombais the most powerful H-bomb in history, a three-stage H-bomb developed by the Soviet Union. With an estimated power of more than 50 Mt, 57 Mt according to several sources, it exploded on October 30, 1961, over Novaya Zemlya, Russian archipelago in the Arctic Ocean (on the “Site C” of Sukhoi Nos. 73° 51′ N, 54° 30′ E), during a show of force. It was the most powerful man-made nuclear explosion in history.
• Canopus was the first H-bomb tested by France on August 24, 1968, over Fangataufa Atoll in French Polynesia. It reaches a power of 2.6 Mt.

Among the accidents involving operational H-bombs, two were particularly famous:

• The Palomares nuclear accident took place near the village of Palomares near Almería in southern Spain on January 17, 1966. A U.S. Air Force Boeing B-52, carrying four H-bombs, exploded after a mid-air collision with a tanker plane. Three bombs fell on the coast, two of which were destroyed on impact, spreading radioactive material (their emergency parachute did not open), one was recovered intact and the last found intact in the Mediterranean, at a depth of 869 meters, after several weeks of search;
• The Thule accident took place near the namesake U.S. air base in northwest Greenland on January 21, 1968. A B-52 carrying four H-bombs crashed after a fire on board. All four bombs were destroyed in the explosion and their radioactive contents escaped.

However, the thermonuclear nature of these bombs did not intervene in these accidents, the correct ignition of the secondary stage being impossible in accidental circumstances.

References (sources)

Richard E. Rowberg, Clifford Lau, The Department of Energy’s Tritium Production Program, 1997; The Soviet Nuclear Weapons Program, section « The Soviet Thermonuclear Weapons Program (Part 2): 1954-1955 »; Bomb H, Wikipedia; « La Corée du Nord annonce avoir testé une bombe H », Le Monde, 2017.