Why does a chain reaction occur when uranium 235 undergoes fission?

Answer 1

Neutrons do this.

When 235U undergoes fission, the result is the production of two daughter atoms, each very roughly half the mass of the original atom, a number of neutrons, and heat. The neutrons then can collide with other fissile or fissionable atoms causing them to undergo fission much more quickly than they otherwise would.

Answer 2

A reaction in which enough neutrons are released for each fissioned nucleus to enable at least one to be captured by a further nucleus causing another fission, and so on. This can happen with U-235 and Pu-239, provided the geometry of the fuel arrangement and moderator is optimised.

Answer 3

Fission of the nucleus of U-235 occurs when a neutron is captured. To form and maintain a chain reaction at a steady rate, the neutron population in the active reactor must be held constant. U-235 nuclei both capture neutrons and emit neutrons, and in fact produce 2.5 neutrons per fission on average (the fission can occur in various different ways so it is not a whole number on average though for any one fission it must be a whole number). This gives the possibility of a chain reaction, but if uncontrolled the neutron population would rise quickly and you would have a bomb. Therefore in a reactor you have to design it so that every neutron born from fission has to produce just one more free neutron, and the population remains steady. Some excess neutrons are lost at the core boundaries, some are absorbed in the reactor structure and moderator, and some in the control rods. The control rod position can be varied so that things are just balanced, and the chain reaction just continues.

Answer 4

Please note that this doesn't work for ANY uranium; only for uranium 235, which makes up about 0.7% of naturally occuring uranium.The chain reaction works because:

* A single neutron, striking a U-235 atom, will make it split.

* Not only will the U-235 atom split into two or three smaller atoms; but critically, it will also produce two or three extra neutrons. These can continue the reaction, if they happen to crash into another U-235 atom.

Answer 5

The release of neutrons by the fissioned nuclei, which enter and cause the decay of the next nuclei in the chain.

Answer 6

The nuclear fission of uranium is not a decay chain reaction.

Answer 7

The cause is the bombardment with thermal neutrons.

Answer 8

The nuclear fission of uranium is not a chain decay.

The radioactive disintegration of uranium is a decay chain.