Uranium-235
General
Name, symbol	Uranium-235, 235U
Neutrons	143
Protons	92
Nuclide Data
Natural abundance	0.72%
Half-life	7.038·108 years
Decay products	Thorium-231
Isotope mass	235.0439299(20) u
Spin	7/2-
Excess energy	40914.062 ± 1.970 keV
Binding energy	1783870.285 ± 1.996 keV
Decay mode	Decay energy
SF	n/a MeV
Alpha decay	4.679 MeV

Uranium-235 is an isotope of uranium that differs from the element's other common isotope, uranium-238, by its ability to cause a rapidly expanding fission chain reaction, i.e., it is fissile. In fact, U-235 is the only fissile isotope found in any quantity in nature. It was discovered in 1935 by Arthur Jeffrey Dempster.

If at least one neutron from U-235 fission strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction will sustain itself, it is said to be critical, and the mass of U-235 required to produce the critical condition is said to be a critical mass. A critical chain reaction can be achieved at low concentrations of U-235 if the neutrons from fission are moderated to lower their speed, since the probability for fission with slow neutrons is greater. A fission chain reaction produces intermediate mass fragments which are highly radioactive and produce further energy by their radioactive decay. Some of them produce neutrons, called delayed neutrons, which contribute to the fission chain reaction. In nuclear reactors, the reaction is slowed down by the addition of control rods which are made of elements such as boron, cadmium, and hafnium which can absorb a large number of neutrons. In nuclear bombs, the reaction is uncontrolled and the large amount of energy released creates a nuclear explosion.

The fission of one atom of U-235 generates 200 MeV = J, i.e. 18 TJ/mol = 77 TJ/kg.

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The combustion of 1000 kg of coal releases 50 MJ of heat while the nuclear fission of 1 mol of uranium-235 (238 grams) releases 20000000 MJ.
malfas1 (Mafalda Torres) Tue, 29 Dec 2009 21:36:14 -0000
The combustion of 1000 kg of coal releases 50 MJ of heat while the nuclear fission of 1 mol of uranium-235 (238 grams) releases 20000000 MJ.

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