Tamper (nuclear weapon)
It is used in nuclear weapon design to reduce the critical mass and to delay the expansion of the reacting material through its inertia, which delays the thermal expansion of the fissioning fuel mass, keeping it supercritical longer.
The first nuclear weapons used heavy natural uranium or tungsten carbide tampers, but a heavy tamper necessitates a larger high-explosive implosion system and makes the entire device larger and heavier.
The primary stage of a modern thermonuclear weapon may instead use a lightweight beryllium reflector, which is also transparent to X-rays when ionized, allowing the primary's energy output to escape quickly to be used in compressing the secondary stage.
In Atomic Energy for Military Purposes (1945), physicist Henry DeWolf Smyth describes the function of a tamper in nuclear weapon design as similar to the neutron reflector used in a nuclear reactor: A similar envelope can be used to reduce the critical size of the bomb, but here the envelope has an additional role: its very inertia delays the expansion of the reacting material.
[1] The concept of surrounding the core of a nuclear weapon with a tamper was introduced by Robert Serber in his Los Alamos Primer, a series of lectures given in April 1943 as part of the Manhattan Project, which built the first nuclear weapons.
He noted that since inertia was the key, the densest materials were preferable, and he identified gold, rhenium, tungsten and uranium as the best candidates.
He believed they also had good neutron-reflecting properties, although he cautioned that a great deal more work needed to be done in this area.
It followed that 15% more fissile material was required to get the same energy release with a gold tamper compared to a uranium one, despite the fact that the critical masses differed by 50%.
This was important not just for neutron reflection but also for its strength in preventing the projectile from blowing through the target.
Tungsten carbide has a high density and a low neutron absorbency cross section.
Despite being available in adequate quantity during the Manhattan Project, depleted uranium was not used because it has a relatively high rate of spontaneous fission of about 675 per kg per second; a 300 kg depleted uranium tamper would therefore have an unacceptable chance of initiating a predetonation.
[6] Tungsten carbide was commonly used in uranium-233 gun-type nuclear weapons used with artillery pieces for the same reason.
[9] In the Fat Man type used in the Trinity test and at Nagasaki, the tamper consisted of 7.0-centimetre (2.75 in) shells of natural uranium and aluminium.
[13] In a boosted fission weapon or a thermonuclear weapon, the 14.1-megaelectronvolt (2.26 pJ) neutrons produced by a deuterium-tritium reaction can remain sufficiently energetic to fission uranium-238 even after three collisions with deuterium, but the 2.45-megaelectronvolt (0.393 pJ) ones produced by deuterium-deuterium fusion no longer have sufficient energy after even a single collision.
An important development after World War II was the lightweight beryllium tamper.
In a boosted device the thermonuclear reactions greatly increase the production of neutrons, which makes the inertial property of tampers less important.
[14] The secondary's tamper (or "pusher") functions to reflect neutrons, confine the fusion fuel with its inertial mass, and enhance the yield with its fissions produced by neutrons emitted from the thermonuclear reactions.
It also helps drive the radiation implosion and prevent the loss of thermal energy.
It has an atomic weight nearly as high as uranium and a lower propensity to fission, which means that the tamper has to be much thicker.
[15] It is possible that a state seeking to develop nuclear weapons capability might add reactor-grade plutonium to a natural uranium tamper.
It was used in thermonuclear weapons to protect the plutonium spark plug from stray neutrons emitted by the uranium-238 tamper.
[16] In the Fat Man type the natural uranium tamper was coated with boron.
The use of lead and bismuth reduces nuclear fallout, as neither produces isotopes that emit significant amounts of gamma radiation when irradiated with neutrons.
Natural tantalum is almost entirely tantalum-181, which when irradiated with neutrons become tantalum-182, a beta and gamma ray emitter with a half-life of 115 days.
[15] The British Tadje nuclear test at Maralinga used cobalt pellets as a "tracer" for determining yield.
doesn't depend on the direction, so we can use this form of the Laplace operator in spherical coordinates: Solving the separable partial differential equation gives us:[23] where and For the tamper, the first term in the first equation relating to the production of neutrons can be disregarded, leaving: Set the separation constant as
this can be approximated as: If the tamper (unrealistically) is a vacuum, then the neutron scattering cross section would be zero and
Since the volume is proportional to the cube of the radius, we reach Serber's conclusion that an eightfold reduction in the critical mass is theoretically possible.
