Como funcionam as bombas nucleares

Craig C. Freudenrich, Ph.D.

Surely you've read history books informing the nuclear bombs used in World War II. And also must have seen science fiction movies where nuclear bombs were dropped or detonated ("Fail Safe," "Dr. Strangelove," "The Next Day", "The Testament," "Shadows in the Future" and "The Peacemaker , "just to name a few). In the news, while many countries have negotiated disarmament of their nuclear weapons arsenals, others have sought to develop nuclear weapons programs.

It is known that these devices have an immense destructive power, but how do they work? In this article, we will discuss the physics that makes the nuclear bomb so powerful, as it is designed and what happens after the explosion.

Photo courtesy NARA
Test Atomic Cannon, 1953

Nuclear bombs utilizes the forces, strong and weak, that hold the nucleus of the atom together, especially atoms with unstable nuclei (see How Nuclear Radiation Works for details). There are two basic modes of nuclear energy to be released from an atom:

  • nuclear fission: the nucleus of an atom can fissioning into two smaller fragments containing neutrons. This method usually involves isotopes of uranium (uranium-235, uranium-233) or plutonium-239;
  • fusion: from two smaller atoms, usually hydrogen or hydrogen isotopes (deuterium, tritium), it is possible to form a larger atom (helium or helium isotopes); similarly, the sun produces energy.

In both processes, fusion or fission, a large amount of heat energy and radiation is emitted.

To build an atomic bomb is accurate:

  • a source fuse or fissile fuel;
  • a trigger;
  • a method that makes the most of fuel from fission or fusion before the bomb (or trigger pump will fail).

Danger in Iran

Iran informed the UN nuclear agency that is building a second uranium enrichment plant, which should exacerbate Western fears that the country wishes to develop a nuclear bomb.

Early pumps used for nuclear fission device, and the latest melt pumps require activation by fission pump. The following types of projects will be addressed devices:

  • fission bombs (in general);
  • fission bomb activation from pistol (Little Boy) that was detonated over Hiroshima, Japan, in 1945;
  • fission activation through implosion bomb (Fat Man), which was detonated over Nagasaki, Japan, in 1945;
  • Fusion bombs (in general);
  • the design of the hydrogen fusion bomb Teller-Ulam , which was detonated as a test on the Island Elugelap in 1952.

The fission bomb using an element such as uranium-235 to cause a nuclear explosion. If you have read How Nuclear Radiation Works , then you know what the underlying degeneration and the radioactive fission basic process. Uranium-235 has an extra property that enables both nuclear power generation and for the generation of a nuclear bomb. The U-235 is one of the few materials that support the induced fission . If a free neutron step inside a nucleus of U-235, it will be absorbed immediately, making the nucleus unstable and causing it to crack.

The figure at right shows the nucleus of the element uranium-235 with close proximity to a neutron. As soon as the nucleus captures the neutron, it will be cracked into two smaller atoms and expel two or three new neutrons (the number of ejected neutrons depends on how the U-235 atom was cracked). The two new atoms emit one gamma radiation as they adjust to their new states (see How Nuclear Radiation Works ). There are three aspects of the fission process that make it interesting:

  • the probability of U-235 as a neutron capture atom moves this is too large. In a pump operating properly ejected fission neutrons may cause other fissions. This condition is known as supercriticality;
  • the capture process and a neutron fission occurs very quickly, on the order of picoseconds (one second trillionth);
  • an incredible amount of energy is released in the form of heat and gamma radiation during the explosion of an atom. The energy released by a single occurs due to fission neutrons and fission products, together weigh less than the original U-235 atom.

The difference in weight is converted to energy at a rate governed by the equation e = mc 2 . In the case of 450 g (1 lb) of highly enriched uranium, as used in a nuclear bomb, will be equal to 1 million gallons or 3,785,412 liters of gasoline. When considering that 450 g of uranium occupy less volume than a baseball and that one million gallons of gasoline fill a cube edge of 15.24 meters (15.24 meters is the height of a five-story building), can If have an idea of the amount of energy available in just a little U-235.

To enable these properties of U-235, a sample of uranium must be enriched. Uranium for use in weapons is composed of at least 90% U-235.

Critical mass
of a pump fission, the fuel must be separated masses subcritical , that do not support fission, to prevent premature detonation. Critical mass is the minimum fissurável material required to ensure support for a nuclear fission reaction. This separation makes possible the occurrence of several problems in the fission bomb, which project should be solved:

  • submassas two or more critical should be grouped together to give a supercritical mass , will provide more than enough neutrons to provide a fission reaction at the time of detonation;
  • free neutrons will be introduced to the supercritical mass to start fission;
  • fissurável most of the material should prevent blow prior to failure.

To join the subcritical masses with supercritical mass, two techniques are used:

  • activation by spray
  • implosion

neutron generator . This generator is a small sphere of polonium-beryllium, separated by a blade inside the fuel fissurável. This generator:

  • The blade will be broken when the subcritical masses to group together and polonium emit alpha particles.
  • These alpha particles collide with beryllium-9 to produce beryllium-8 and release neutrons.
  • The neutrons will begin to fission.

Finally, the fission reaction is confined within a dense material, known as reflector nuclear reactor , which is usually composed of uranium-238. The reflector nuclear reactor heats up and expands through the central zone of fission. This expansion exerts a pressure back reflector and decelerates the expansion of the central zone. The nuclear reactor reflector will also reflect neutrons back into the central area of fission, increasing the efficiency of the reaction.

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