Nuclear Fission?

Question

In this process, a Uranium atom is split into 2 smaller fragments releasing a lot of heat. This process is done by adding a neutron to the atom making it unstable. I've always wondered, where does this neutron come from? Where is it taken from and how do they manage to get it into a uranium atom considering its size?

Answer 1

There are many forms of fission possible with many different isotopes and neutron velocities.

The neutrons that fission U235 or Pu239 may be derived from a neutron source, which could be a mixture of radium and beryllium. The alpha particles from the radium strike the beryllium and liberate a fast neutron and 5.71 Mev of energy, in the reaction: -

... 9 .... 4 ..... 12....1

Be + He ----> C + n + 5.71 Mev

Other neutron sources are (among others), Li7 under proton bombardment, Mn55 plus beryllium, In116 with beryllium, and La140 with beryllium, to name but a few.

The neutrons emitted by these sources are fast and must be slowed by scattering or moderating. Neutrons loose energy through inelastic scattering with medium or heavy nuclei. Graphite, beryllium and heavy water (D2O) are used as efficient moderators, within nuclear reactors. When the neutrons have been slowed to 'thermal neutrons' (v = 2200 m/s for a temperature of 293 K (room temperature), from Maxwell's v² = 2kT/m) they can be used for fissioning U233, U235, or Pu239. U238 may be fissioned by fast, or energetic neutrons. However, U235 can also be fissioned by fast neutrons! Furthermore, in 1939 it was discovered that uranium and (later discovered) plutonium undergo spontaneous fission without the initial absorption of an incident neutron.

The neutrons are emitted in a large ‘flux’ so that many, after moderation, encounter the target uranium or plutonium. The probability that a neutron will fission a U233, U235 or Pu239 nuclei is referred to as the cross-section and its unit is the barn (10^-28 m²). For U233 the fissioning cross-section is 525 +/- 4 b and for U235 it is 577 +/- 5 b plus for Pu239 it is 742 +/- 4 b. Finally, americium 242 has a fission cross-section of 6400 +/- 500 b but it only has a half-life of 100 years.

The neutron as it approaches the actinide nuclei does not have to overcome the electrostatic repulsion of the protons, in its core, as an approaching proton would have to do. If the neutron gets close enough to the nuclei, it is 'grabbed' by the strong nuclear force and absorbed by the uranium or plutonium nuclei. Once inside the nuclei the neutron triggers an energy imbalance, which is inhibited from radio-active decay and so must decay via fission.

The fission products can vary and have a range of atomic numbers from A = 72 to A = 158. About 97 % of U235 fission yield products fall into two groups with a = 85 - 104 for the light group and A = 130 - 149 for the heavy group. The most probable fission products occur in the mass range A = 95 to 139. Typically, at least two neutrons are emitted from the original target nuclei as it undergoes fission and this number increases with increasing initial neutron energy. Finally, a typical fission decay emits around 200 Mev of energy.

The liquid drop model (1939   Bohr) may be used to describe the fission process mathematically. This model was further improved upon by Bohr's son and J. A. Wheeler to account for the inhibition of radioactive decay processes in favour of fission decay. This later model has a feature known as double-humped-di-isomorphism.

I hope this is of some use.

Answer 2

From other atoms of uranium.

A radiation can be a stray neutron among other subatomic particles.  Uranium is unstable it has too many neutrons in the nuclease to hold itself together.  It decays at a known rate.  Uranium also gives off alpha particles (two neutrons and two protons) and those are easily absorbed or block so they can add more neutrons to the uranium increasing the number of possible fission collisions.

Fission happens when you get enough uranium together to reach critical mass.  This is one of the processes that keeps the earth's core molten.  But critical mass is not necessarily explosive mass.  To create a nuclear fission explosion you need a denser radiation and fission reaction so we use explosives to implode the uranium and squash it together in an implosion; only then will the critical mass be enough to cause a nuclear explosion.  This is why nuclear power plants don't blow up like atomic bombs.  They have their own dangers, but they are not nuclear bombs.

Another way to start the fission reaction would be to get a strong radiating material (like an x-ray machine:  http://en.wikipedia.org/wiki/X-ray) and aim it at the uranium pile starting the reaction.  If you have enough then the reaction becomes stable for a long time.  Normally the pile collects enough of the unstable isotope of uranium that all they need to start the fission reaction is to put the pile together.

There is a famous case of a Los Alamos scientist who had two blocks of uranium on his desk separated by a screwdriver.  The screwdriver was accidentally removed and the scientist shielded his colleges with his own body from the fission reaction.  He split the uranium pieces and the intense exposure killed him soon after.  All he did was let two pieces of uranium touch each other.

According to Wikipedia:  http://en.wikipedia.org/wiki/Uranium

"Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth (see uranium-thorium dating, uranium-lead dating and uranium-uranium dating). Many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 has the distinction of being the only naturally occurring fissile isotope. Uranium-238 is both fissionable by fast neutrons, and fertile (capable of being transmuted to fissile plutonium-239 in a nuclear reactor). An artificial fissile isotope, uranium-233, can be produced from natural thorium and is also important in nuclear technology. While uranium-238 has a small probability to fission spontaneously or when bombarded with fast neutrons, the much higher probability of uranium-235 and to a lesser degree uranium-233 to fission when bombarded with slow neutrons generates the heat in nuclear reactors used as a source of power, and provides the fissile material for nuclear weapons. Both uses rely on the ability of uranium to produce a sustained nuclear chain reaction."

According to Wikipedia:  http://en.wikipedia.org/wiki/Alpha_parti...

"Alpha particles (named after and denoted by the first letter in the Greek alphabet, α) consist of two protons and two neutrons bound together into a particle identical to a helium nucleus; hence, it can be written as He2+ or 42He2+. They are a highly ionizing form of particle radiation, and have low penetration. The alpha particle mass is 6.644656×10-27 kg, which is equivalent to the energy of 3.72738 GeV. The charge of an alpha particle is equal to +2e, where e is the magnitude of charge on an electron, e=1.602176462x10-19C.

Alpha particles are emitted by radioactive nuclei such as uranium or radium in a process known as alpha decay. This sometimes leaves the nucleus in an excited state, with the emission of a gamma ray removing the excess energy."

Half-life is the time it takes an unstable or radioactive material to decay down to other stable elements.  It can be a few fractions of a second for the elements that are high on the periodic table and it can differ by the isotope (number of neutrons in the nucleus).

According to Wikipedia:  http://en.wikipedia.org/wiki/Half-life

"The half-life of a quantity whose value decreases with time is the interval required for the quantity to decay to half of its initial value. The concept originated in the study of radioactive decay which is subject to exponential decay but applies to all phenomena including those which are described by non-exponential decays.

The term half-life was coined in 1907, but it was always referred to as half-life period. It was not until the early 1950s that the word period was dropped from the name."

A Polonium initiator can be used; it is an unstable radioactive material that can be found in uranium mines.  It to is an alpha particle emitter.

But according to Wikipedia polonium is not needed, instead the uranium is enriched so it can start its own fission reaction:  http://en.wikipedia.org/wiki/Nuclear_pil...

"Enriched uranium is uranium in which the percent composition of uranium-235 has been increased from that of uranium found in nature. Natural uranium is only 0.72% uranium-235; the rest is mostly uranium-238 (99.2745%) and a tiny fraction is uranium-234 (0.0055%).:

According to Wikipedia:  http://en.wikipedia.org/wiki/Polonium

"Polonium (pronounced /pəˈloʊniəm/) is a chemical element that has the symbol Po and atomic number 84. A rare and highly radioactive metalloid, polonium is chemically similar to bismuth and tellurium , and it occurs in uranium ores. Polonium has been studied for possible use in heating spacecraft. It is unstable; all isotopes of polonium are radioactive."

Normally graphite and boron are used to slow down and control the fission reaction.  The first nuclear reactor used graphite.

According to Wikipedia:  http://en.wikipedia.org/wiki/Control_rod

"Materials Used

Chemical elements with a sufficiently high capture cross section for neutrons include silver, indium and cadmium. Other elements that can be used include boron, cobalt, hafnium, dysprosium, gadolinium, samarium, erbium, and europium, or their alloys and compounds, e.g. high-boron steel, silver-indium-cadmium alloy, boron carbide, zirconium diboride, titanium diboride, hafnium diboride, gadolinium titanate, and dysprosium titanate. "

Answer 3

Fission   Uhmmm?

Answer 4

The neutron comes from the natural and spontaneous radioactive decay of a Uranium atom.  This neutron is usually travelling too fast to cause any subsequent radioactive decay in any other Uranium atoms.  

However, by placing some Boron in the vicinity of the Uranium, the emitted neutron collides with the Boron and loses some of its kinetic energy.  The slower moving neutron now has the right amount of kinetic energy to cause subsequent radioactive decay.  Thus, a rate of radioactive decay higher than normal can be achieved.  This can lead to an accelerating chain reaction.

http://en.wikipedia.org/wiki/Nuclear_cha...

http://www.chemistrydaily.com/chemistry/...

OK I thought it was Boron, but now I'm not so sure!!!!

Answer 5

When the Uranium splits it releases 3 neutrons and a bunch of energy.

The U is always decaying and releasing neutrons, that hit other U.  If the U is in a sufficient concentration you get a sustained nuclear reaction.  

If the Uranium is mostly U235 and compressed either with a shaped explosion or a slug of critical mas then more of the neutrons from the decaying U235 will hit other U235, and will start a chain reaction (kablam)

Answer 6

Certain metals (polonium for instance) decay releasing "slow" neutrons that can be captured by a uranium atom.

Answer 7

At the centre of the bomb or reactor they have a radioactive material called an "initiator" that spews out lots of neutrons. Normally made of polonium.

But in theory at least there is no need for this in a bomb because all you need is one neutron to start the chain reaction: 1-2-4-8-16-32-64 etc in nanoseconds progression, and you'll get a big big bang.

Answer 8

the neutron comes from the uranium atom. the atom splits into 2 small atoms and releases 3 neutrons. they get it into the atom pretty much by brute force. they just pack uranium pretty much as close together as they can get it and fire a small "bullet like" piece of matter into the uranium. think of it like a baseball being thrown into a glove, only that glove is going to explode when it catches the ball, and the ball is going to be thrown thousands of times faster.

Answer 9

Neutrons can come from many different sources - the slowly decaying Uranium atoms themselves, or from other radioactive sources.