Nuclear Weapon Design

Specific designs

While every nuclear weapon design falls into one of the above categories, specific designs have occasionally become the subject of news accounts and public discussion, often with incorrect descriptions about how they work and what they do. Examples:

Hydrogen bombs

All modern nuclear weapons make some use of D-T fusion. Even pure fission weapons include neutron generators which are high-voltage vacuum tubes containing trace amounts of tritium and deuterium.

However, in the public perception, hydrogen bombs, or H-bombs, are multi-megaton devices a thousand times more powerful than Hiroshima's Little Boy. Such high-yield bombs are actually two-stage thermonuclears, scaled up to the desired yield, with uranium fission, as usual, providing most of their destructive energy.

The idea of the hydrogen bomb first came to public attention in 1949, when prominent scientists openly recommended against building nuclear bombs more powerful than the standard pure-fission model, on both moral and practical grounds. Their assumption was that critical mass considerations would limit the potential size of fission explosions, but that a fusion explosion could be as large as its supply of fuel, which has no critical mass limit. In 1949, the Soviets exploded their first fission bomb, and in 1950 President Truman ended the H-bomb debate by ordering the Los Alamos designers to build one.

In 1952, the 10.4-megaton Ivy Mike explosion was announced as the first hydrogen bomb test, reinforcing the idea that hydrogen bombs are a thousand times more powerful than fission bombs.

In 1954, J. Robert Oppenheimer was labeled a hydrogen bomb opponent. The public did not know there were two kinds of hydrogen bomb (neither of which is accurately described as a hydrogen bomb). On May 23, when his security clearance was revoked, item three of the four public findings against him was "his conduct in the hydrogen bomb program." In 1949, Oppenheimer had supported single-stage fusion-boosted fission bombs, to maximize the explosive power of the arsenal given the trade-off between plutonium and tritium production. He opposed two-stage thermonuclear bombs until 1951, when radiation implosion, which he called "technically sweet", first made them practical. He no longer objected. The complexity of his position was not revealed to the public until 1976, thirteen years after his death.^[23]

When ballistic missiles replaced bombers in the 1960s, most multi-megaton bombs were replaced by missile warheads (also two-stage thermonuclears) scaled down to one megaton or less.

Alarm Clock/Sloika

The first effort to exploit the symbiotic relationship between fission and fusion was a 1940s design that mixed fission and fusion fuel in alternating thin layers. As a single-stage device, it would have been a cumbersome application of boosted fission. It first became practical when incorporated into the secondary of a two-stage thermonuclear weapon.^[24]

The U.S. name, Alarm Clock, was a nonsense code name. The Russian name for the same design was more descriptive: Sloika, a layered pastry cake. A single-stage Russian Sloika was tested on August 12, 1953. No single-stage U.S. version was tested, but the Union shot of Operation Castle, April 26, 1954, was a two-stage thermonuclear code-named Alarm Clock. Its yield, at Bikini, was 6.9 megatons.

Because the Russian Sloika test used dry lithium-6 deuteride eight months before the first U.S. test to use it (Castle Bravo, March 1, 1954), it was sometimes claimed that Russia won the H-bomb race. (The 1952 U.S. Ivy Mike test used cryogenically-cooled liquid deuterium as the fusion fuel in the secondary, and employed the D-D fusion reaction.) However, the first Russian test to use a radiation-imploded secondary, the essential feature of a true H-bomb, was on November 23, 1955, three years after Ivy Mike.

Clean bombs

Bassoon, the prototype for a 3.5-megaton clean bomb or a 25-megaton dirty bomb. Dirty version shown here, before its 1956 test.

On March 1, 1954, America's largest-ever nuclear test explosion, the 15-megaton Bravo shot of Operation Castle at Bikini, delivered a promptly lethal dose of fission-product fallout to more than 6,000 square miles (16,000 km²) of Pacific Ocean surface.^[25] Radiation injuries to Marshall Islanders and Japanese fishermen made that fact public and revealed the role of fission in hydrogen bombs.

In response to the public alarm over fallout, an effort was made to design a clean multi-megaton weapon, relying almost entirely on fusion. Since the energy produced by fission is essentially free, using the vital tamper as a source of extra energy the clean bomb needed to be much larger for the same yield. For the only time, a third stage, called the tertiary, was added, using the secondary as its primary. The device was called Bassoon. It was tested as the Zuni shot of Operation Redwing, at Bikini on May 28, 1956. With all the uranium in Bassoon replaced with a substitute material such as lead, its yield was 3.5 megatons, 85% fusion and only 15% fission.

On July 19, AEC Chairman Lewis Strauss said the clean bomb test "produced much of importance . . . from a humanitarian aspect." However, two days later the dirty version of Bassoon, with the uranium parts restored, was tested as the Tewa shot of Redwing. Its 5-megaton yield, 87% fission, was deliberately suppressed to keep fallout within a smaller area. This dirty version was later deployed as the three-stage, 25-megaton Mark-41 bomb, which was carried by U.S. Air Force bombers, but never tested at full yield.

As such, high-yield clean bombs were a public relations exercise. The actual deployed weapons were the dirty version, which maximized yield for the same size device.

Cobalt bombs

Main article: Cobalt bomb

A fictional doomsday bomb, made popular by Neville Shute's 1957 novel, and subsequent 1959 movie, On the Beach, the cobalt bomb was a hydrogen bomb with a jacket of cobalt metal. The neutron-activated cobalt would supposedly have maximized the environmental damage from radioactive fallout. This bomb was popularized as the 'Doomsday Device' in the 1964 film Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb in the film the bomb brings about the end of mankind by covering the planet in a radioactive shroud for 93 years. The element added to the bombs is referred to in the film as 'cobalt-thorium G'

Such "salted" weapons were requested by the U.S. Air Force and seriously investigated, possibly built and tested, but not deployed. In the 1964 edition of the DOD/AEC book The Effects of Nuclear Weapons, a new section titled Radiological Warfare clarified the issue.^[26] Fission products are as deadly as neutron-activated cobalt. The standard high-fission thermonuclear weapon is automatically a weapon of radiological warfare, as dirty as a cobalt bomb.

Initially, gamma radiation from the fission products from an equivalent size fission-fusion-fission bomb are much more intense than Co-60: 15,000 times more intense at 1 hour; 35 times more intense at 1 week; 5 times more intense at 1 month; and about equal at 6 months. Thereafter fission drops off rapidly so that Co-60 fallout is 8 times more intense than fission at 1 year and 150 times more intense at 5 years. The very long lived isotopes produced by fission would overtake the ⁶⁰Co again after about 75 years. ^[27]

Fission-fusion-fission bombs

In 1954, to explain the surprising amount of fission-product fallout produced by hydrogen bombs, Ralph Lapp coined the term fission-fusion-fission to describe a process inside what he called a three-stage thermonuclear weapon. His process explanation was correct, but his choice of terms caused confusion in the open literature. The stages of a nuclear weapon are not fission, fusion, and fission. They are the primary, the secondary, and, in one exceptionally powerful weapon, the tertiary. Each of these stages employs fission, fusion, and fission.

Neutron bombs

Main article: Neutron bomb

A neutron bomb, technically referred to as an enhanced radiation weapon (ERW), is a type of tactical nuclear weapon designed specifically to release a large portion of its energy as energetic neutron radiation. This contrasts with standard thermonuclear weapons, which are designed to capture this intense neutron radiation to increase its overall explosive yield. In terms of yield, ERWs typically produce about one-tenth that of a fission-type atomic weapon. Even with their significantly lower explosive power, ERWs are still capable of much greater destruction than any conventional bomb. Meanwhile, relative to other nuclear weapons, damage is more focused on biological material than on material infrastructure (though extreme blast and heat effects are not eliminated).

Officially known as enhanced radiation weapons, ERWs, they are more accurately described as suppressed yield weapons. When the yield of a nuclear weapon is less than one kiloton, its lethal radius from blast, 700 m (2300 ft), is less than that from its neutron radiation. However, the blast is more than potent enough to destroy most structures, which are less resistant to blast effects than even unprotected human beings. Blast pressures of upwards of 20 PSI are survivable, whereas most buildings will collapse with a pressure of only 5 PSI.

ERWs were two-stage thermonuclears with all non-essential uranium removed to minimize fission yield. Fusion provided the neutrons. Developed in the 1950s, they were first deployed in the 1970s, by U.S. forces in Europe. The last ones were retired in the 1990s.

Energy distribution of weapon

	Standard	Enhanced
Blast	50%	40%
Thermal energy	35%	25%
Instant radiation	5%	30%
Residual radiation	10%	5%

A neutron bomb is only feasible if the yield is sufficiently high that efficient fusion stage ignition is possible, and if the yield is low enough that the case thickness will not absorb too many neutrons. This means that neutron bombs have a yield range of 1–10 kilotons, with fission proportion varying from 50% at 1-kiloton to 25% at 10-kilotons (all of which comes from the primary stage). The neutron output per kiloton is then 10–15 times greater than for a pure fission implosion weapon or for a strategic warhead like a W87 or W88. ^[28]

Oralloy thermonuclear warheads

In 1999, nuclear weapon design was in the news again, for the first time in decades. In January, the U.S. House of Representatives released the Cox Report (Christopher Cox R-CA) which alleged that China had somehow acquired classified information about the U.S. W88 warhead. Nine months later, Wen Ho Lee, a Taiwanese immigrant working at Los Alamos, was publicly accused of spying, arrested, and served nine months in pre-trial detention, before the case against him was dismissed. It is not clear that there was, in fact, any espionage.

In the course of eighteen months of news coverage, the W88 warhead was described in unusual detail. The New York Times printed a schematic diagram on its front page.^[29] The most detailed drawing appeared in A Convenient Spy, the 2001 book on the Wen Ho Lee case by Dan Stober and Ian Hoffman, adapted and shown here with permission.

Designed for use on Trident II (D-5) submarine-launched ballistic missiles, the W88 entered service in 1990 and was the last warhead designed for the U.S. arsenal. It has been described as the most advanced, although open literature accounts do not indicate any major design features that were not available to U.S. designers in 1958.

The above diagram shows all the standard features of ballistic missile warheads since the 1960s, with two exceptions that give it a higher yield for its size.

• The outer layer of the secondary, called the "pusher", which serves three functions: heat shield, tamper, and fission fuel, is made of U-235 instead of U-238, hence the name Oralloy (U-235) Thermonuclear. Being fissile, rather than merely fissionable, allows the pusher to fission faster and more completely, increasing yield. This feature is available only to nations with a great wealth of fissile uranium. The U.S. is estimated to have 500 tons.

• The secondary is located in the wide end of the re-entry cone, where it can be larger, and thus more powerful. The usual arrangement is to put the heavier, denser secondary in the narrow end for greater aerodynamic stability during re-entry from outer space, and to allow more room for a bulky primary in the wider part of the cone. (The W87 warhead drawing in the previous section shows the usual arrangement.) Because of this new geometry, the W88 primary uses compact conventional high explosives (CHE) to save space,^[30] rather than the more usual, and bulky but safer, insensitive high explosives (IHE). The re-entry cone probably has ballast in the nose for aerodynamic stability.^[31]

Notice that the alternating layers of fission and fusion material in the secondary are an application of the Alarm Clock/Sloika principle.

Reliable replacement warhead

Main article: Reliable Replacement Warhead

The United States has not produced any nuclear warheads since 1989, when the Rocky Flats pit production plant, near Boulder, Colorado, was shut down for environmental reasons. With the end of the Cold War two years later, the production line was idled except for inspection and maintenance functions.

The National Nuclear Security Administration, the latest successor for nuclear weapons to the Atomic Energy Commission and the Department of Energy, has proposed building a new pit facility and starting the production line for a new warhead called the Reliable Replacement Warhead (RRW).^[32] Two advertised safety improvements of the RRW would be a return to the use of "insensitive high explosives which are far less susceptible to accidental detonation", and the elimination of "certain hazardous materials, such as beryllium, that are harmful to people and the environment."^[33] Since the new warhead would not require any nuclear testing, it could not use a new design with untested concepts.

References:

1. ^ Nuclear Weapons FAQ: 4.1.6.3 Hybrid Assembly Techniques, accessed December 1, 2007. Drawing adapted from the same source.
2. ^ Nuclear Weapons FAQ: 4.1.6.2.2.4 Cylindrical and Planar Shock Techniques, accessed December 1, 2007.
3. ^ "Restricted Data Declassification Decisions from 1946 until Present", Section V.B.2.k "The fact of use in high explosive assembled (HEA) weapons of spherical shells of fissile materials, sealed pits; air and ring HE lenses," declassified November 1972.
4. ^ Until a reliable design was worked out in the early 1950s, the hydrogen bomb (public name) was called the superbomb by insiders. After that, insiders used a more descriptive name: two-stage thermonuclear. Two examples. From Herb York, The Advisors, 1976, "This book is about . . . the development of the H-bomb, or the superbomb as it was then called." p. ix, and "The rapid and successful development of the superbomb (or super as it came to be called) . . ." p. 5. From National Public Radio Talk of the Nation, November 8, 2005, Siegfried Hecker of Los Alamos, "the hydrogen bomb – that is, a two-stage thermonuclear device, as we referred to it – is indeed the principal part of the US arsenal, as it is of the Russian arsenal."
5. ^ ^a b Howard Morland, "Born Secret," Cardozo Law Review, March 2005, pp. 1401-1408.
6. ^ "Improved Security, Safety & Manufacturability of the Reliable Replacement Warhead," NNSA March 2007.
7. ^ A 1976 drawing which depicts an interstage that absorbs and re-radiates x-rays. From Howard Morland, "The Article," Cardozo Law Review, March 2005, p 1374.
8. ^ "SAND8.8 - 1151 Nuclear Weapon Data -- Sigma I," Sandia Laboratories, September 1988.
9. ^ The Greenpeace drawing. From Morland, Cardozo Law Review, March 2005, p 1378.
10. ^ Herbert York, The Advisors: Oppenheimer, Teller and the Superbomb (1976).
11. ^ "The ‘Alarm Clock' . . . became practical only by the inclusion of Li6 (in 1950) and its combination with the radiation implosion." Hans A. Bethe, Memorandum on the History of Thermonuclear Program, May 28, 1952.
12. ^ See map.
13. ^ Samuel Glasstone, The Effects of Nuclear Weapons, 1962, Revised 1964, U.S. Dept of Defense and U.S. Dept of Energy, pp.464-5. This section was removed from later editions, but, according to Glasstone in 1978, not because it was inaccurate or because the weapons had changed.
14. ^ "Nuclear Weapons FAQ: 1.6".
15. ^ "Neutron bomb: Why 'clean' is deadly".
16. ^ Broad, William J. (7 September 1999), "Spies versus sweat, the debate over China's nuclear advance," The New York Times, p 1. The front page drawing was similar to one that appeared four months earlier in the San Jose Mercury News.
17. ^ Jonathan Medalia, "The Reliable Replacement Warhead Program: Background and Current Developments," CRS Report RL32929, Dec 18, 2007, p CRS-11.
18. ^ Richard Garwin, "Why China Won't Build U.S. Warheads", Arms Control Today, April-May 1999.
19. ^ Home - NNSA