What happens during the fission of uranium?

Answer 1

Fission is a process of the splitting the atom, typically into two daughter atoms, plus a few neutrons. It can be caused when a fissionable atom is struck by a neutron, or, in the case of uranium and some synthetic elements, spontaneously. The actual products of fission are somewhat unpredictable. We can know a few things about what always, or nearly always, happens, however and among these are the following:

• The sum of the numbers of protons in the daughters is equal to the number in the parent.
• The sum of the numbers of neutrons in the daughters plus the neutrons emitted is equal to the number of neutrons in the parent.
• Typically two or three neutrons are emitted.
• The daughters are each very roughly half the size of the parent.

So, a typical equation for a fission reaction of 235U might look like this (the lower number preceding the atomic symbol is the atomic number or number of protons in the atom; the upper number is the Atomic Mass, which equals the number or protons plus the number of neutrons; and 01n represents a neutron): 92235U --> 3799Rb + 55134Cs + 2 01n

Note the upper numbers on both sides of the equations add up (235 = 99 + 134 + 2x1), as do the lower numbers on both sides of the equation (92 = 37 + 55 + 2x0).

The equation could just as easily have been slightly different: 92235U --> 3798Rb + 55134Cs + 3 01n

or very different: 92235U --> 40105Zr + 52127Te + 3 01n

Various isotopes of uranium can undergo fission, and what is true for one is true for the others. The uranium isotopes have long half lives, 703,800,000 years for 235U, and 4,469,000,000 years for 238U. Since heavier atoms can have a greater proportion of neutrons to protons, the daughter atoms nearly always have too many to be stable, and have very much shorter half lives. They handle this by converting neutrons to protons, ejecting beta particles (high speed electrons) as they do. This, for example, is the decay chain of the 99Rb in the first example equation, with half lives: 3799Rb - 0.050 seconds

3899Sr - 0.269 seconds

3999Y -- 1.47 seconds

4099Zr - 2.10 seconds

4199Nb - 15.0 seconds

4299Mo - 69.14 hours

4399Tc - 211,100 years

4499Ru - stable

Please note, however, that different decay chains do not always move from shorter to longer half lives; it is they usual way things go, but it is not inevitable.

When spent fuel is removed from the reactor, it needs to cool off in a special storage area for a few years because the shorter lived isotopes are decaying rapidly. Then it can be moved from short term storage to longer term storage.

The most abundant medium term decay products of uranium fission, with yield as a percentage of the daughter atoms produced, and half life are as follows: 137Cs .. 6.34% .. 32.23 years

90Sr .... 4.51% ... 28.9 years

141Sm . 0.53% ... 90 years

85Kr .... 0.22% ... 10.78 years

And the most abundant long term decay products are as follows: 135Cs . 6.91% .. 2,300,000 years

99Tc .... 6.14% ..... 211,000 years

93Zr .... 5.46% .. 1,530,000 years

107Pd . 1.25% .. 1,250,000 years

129I .... 0.84% ... 1,570,000 years

126Sn . 0.11% ..... 230,000 years

There are no fission products with half lives between 90 and 211,000 years.

Answer 2

When the nuclear energy is bigger than the electric energy that holds the protons and neutrons together, the fission is initiated .. that can be done by bombarding the nucleus with a neutron ..

Answer 3

Fission happens when an unstable nucleus breaks apart, creating two nuclei, each with very roughly half of the mass of the original.

Fission can result from the atom having a neutron collide with it, or it can happen spontaneously. When it happens, there is no way to predict exactly what the daughter nuclei will be, except that they will have the same number of protons between them as the parent had. Fission produces free neutrons along with the daughter nuclei, with the number being generally one to five. Also produced is a lot of energy, usually represented as kinetic energy measured in millions of electron volts (MeV). The energy can as easily be thought of as heat, since we are talking about atoms.

Though it might be possible for the daughter nuclei to be radiologically inert, in practice, they will be radioactive, and will mostly have short half lives, many going through a rapid succession of decays to produce atoms that have medium to long half lives. Because of this, the spent fuel is much more radioactive than the original fuel was.

Answer 4

The isotope 235U is easily fissionable with thermal neutrons; also the artifficial isotope 233U.

Answer 5

Nuclear fission with thermal neutrons; thermal neutron is a neutron with an energy of 0,025 eV (or a speed of 2,2 km/s).

Answer 6

Uranium-235 (natural) and uranium-233 (artificial).