Understanding the Mechanism of Uranium Splitting in Nuclear Reactions

[saschouch]

Hi, in my environmental chem class at school, we vaguely touched on the subject of nuclear power and such. my question has to do with the splitting of the U235 or 238 atoms in reactors, as well as those in, say, nuclear weapons. my teacher (and diagrams) said that the splitting occurs when the uranium atoms are hit with neutrons, splitting them apart. but where does this initial neutron come from to split the first one, or does it happen some other way? thanks.

 

[Tzemach]

There are two types of fission creating free neutrons, the first is spontaneous emmission where natural radioactive decay releases neutrons the second is induced fission where neutron emmissions are modified to induce additional releases and create a chain reaction. The first part of a reaction begins with a spontaneously released neutron.

 

[LURCH]

Just to clarify; the reaction supplies its own neutrons. When a U atom splits, it usually splits almost perfectly in half, forming two new atoms of nearly equal mass. But, it also realeases two or three neutrons at fairly high speed. Since these neutrons are released inside a lump of Uranium, there exists a significant probaility that one of them will strike the nucleas of another Uranium atom, causing it to split and release nore neutrons to strike more nuclei, etc.

In fact, the purpose of the "controll rods" you're allways hearing about, is to give the neutrons something else to run into, so the chain reaction can be slowed down or stopped when necessary. It was the removal of these controll rods that set off Chernoble. Without them in place, all the neutrons freed by the reaction are allowed to collide with other Uranium nuclei, and the self-perpetuating reaction gets quickly out of hand.

 

[saschouch]

ahhh...i see now. thanks.

 

[Astronuc]

Commercial nuclear reactors require 'startup neutron sources' until they build a sufficient inventory of transuranics to allow for spontaneous fission.

In the past alpha-emitters were combined with Be. The Be nuclei which undergoes an (alpha,n) reaction (decomposition) when a high energy alpha strikes them.

More modern plants use a photo-neutron source (Sb-124 (1.69 MeV gamma) - Be), in which gamma rays from an isomeric transition causes Be9 to eject a neutron.

The startup neutron source allows sufficient fissions from which neutrons are detected and so the reactor power can be monitored at low powers. The rate at which power is allowed to increase are strictly controlled.

 

[saschouch]

wow. that's an even better answer. you never cease to amaze me, astronuc. thanks a lot.