What are the sources of neutrons in starting up a nuclear reactor?

[tiepngh]

Thank for all if you understand my ideal.
My problem is i don't know what's neutron which cause of fission in critical accident?
seem it must be have the first neutrons, and then reaction fission occurred ?
eg The critical model in some nuclear accident, what is neutron source?
The nuclear bomb? ...
thanks...

 

[mathman]

There are always some neutrons just floating around. Spontaneous fission and cosmic rays are examples.

Also what kind of accident are you concerned about. Reactors are going all the time, so there are plenty of neutrons. Accident means it got out of control somehow.

 

[Drakkith]

The fuel itself is decaying naturally, releasing neutrons. In a reactor the fuel is arranged in such a way that the neutrons cause MORE decays, but even a small amount by itself is releasing neutrons at a fairly steady rate.

 

[googlefans]

forward：Sorry i Can't include a link
There are several sources of neutrons for starting up a reactor:

a) Cf-252 spontaneously fissions and thereby releases neutrons.
Small wires of Cf-252 are inserted into selected fuel assemblies
(the spontaneous fission rate is so high that only gram amounts
of Cf-252 are needed).

b) Antimony-berylium or Pu-Be sources are also used. In these sources,
the Antimony or the Plutonium emits a high-energy alpha which
is absorbed by the Berylium which then emits a neutron, among
other things.
c) As the fuel is irradiated, Pu-240 builds up. It also spontaneously
fissions, giving rise to neutrons. There are some spontaneous
fissions in U-238 as well. I seem to recall that there may be some
(alpha, n) reactions directly in the fuel, although I am uncertain
as to the magnitude of this contribution.
It is likely that after a core has been irradiated it has enough background
neutrons that the Cf-252 and berylium sources are no longer needed. So
far as I know, however, no one has put this theory to the test in a power
reactor.

 

[Thermalne]

forward：Sorry i Can't include a link
There are several sources of neutrons for starting up a reactor:

a) Cf-252 spontaneously fissions and thereby releases neutrons.
Small wires of Cf-252 are inserted into selected fuel assemblies
(the spontaneous fission rate is so high that only gram amounts
of Cf-252 are needed).

b) Antimony-berylium or Pu-Be sources are also used. In these sources,
the Antimony or the Plutonium emits a high-energy alpha which
is absorbed by the Berylium which then emits a neutron, among
other things.
c) As the fuel is irradiated, Pu-240 builds up. It also spontaneously
fissions, giving rise to neutrons. There are some spontaneous
fissions in U-238 as well. I seem to recall that there may be some
(alpha, n) reactions directly in the fuel, although I am uncertain
as to the magnitude of this contribution.
It is likely that after a core has been irradiated it has enough background
neutrons that the Cf-252 and berylium sources are no longer needed. So
far as I know, however, no one has put this theory to the test in a power
reactor.

Cores do not need a new neutron source after being through one cycle unless they are starting with completely fresh fuel. The once and second burned fuel provides enough neutrons to not require a new neutron source. This isn't a theory, this is actual practice.