From: email@example.com (B. Alan Guthrie)
Subject: Re: nuclear fission questions
Date: 13 Oct 1997 13:35:34 GMT
Keywords: nuclear fission
In article <firstname.lastname@example.org>,
Marshall Carroll <email@example.com> wrote:
>Hi. I am a high school teacher with some questions about nuclear fission
>1) What is the source of the initial impinging high energy neutrons?
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
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
>2) How are these neutrons "aimed" at the U-235 fuel?
They are not aimed. The neutrons are released isotropically by fission.
The fuel rods in a water reactor are surrounded by water where the
neutrons slow down (are thermalized).
>3) The U-235 can break into Ba-141 and Kr-92 but it can also break into
>Te-137 and Zr-97 (and, of course, other fission products are possible).
>Does the U-235 favour splitting into Ba-141 and Kr-92 over any of the
>other fission products? Is there any statistical evidence proving that
>the U-235 favours one set of fission products over another?
The fission product spectrum is a function of the fissioning species
and the energy of the incident neutron. The mass numbers of the
fission products are centered around the mass numbers of about 90
to about 100 and from about 134 to about 144, although mass numbers
of fission products with other mass numbers are possible. It is
not possible to predict the fission products of a single particular
fission. For fission induced by a thermal neutron, a fission product
with a mass number of say 95 is about 500 times more likely than one
with a mass number of 115.
>4) Roughly 2 x 10^13 J/mol are released when U-235 undergoes fission to
>Ba-141 and Kr-96. Is it gamma rays that carry off this energy, does the
>energy go directly into heat or ...?
First, when U-235 fissions, it probably will not fission into Ba-141
and Kr-96 because you have too much mass. The starting components
have a mass of 236 (235 from the uranium and 1 from the neutron), and
the resulting products need to include two or three neutrons; thus,
the fission products need to have a total mass of 233 or 234.
I will use MeV for my energy partition, because I am too lazy to
convert to J/mole.
U-233 U-235 Pu-239
Light fission fragment 99.2 99.8 101.8
Heavy fission fragment 67.9 68.4 73.2
Prompt neutrons 5.0 4.8 5.8
Prompt gammas 7.0 7.5 7.0
Beta-decay 8.0 7.8 8.0
Delayed neutrons 4.2 6.8 6.2
Neutrinos 10.0 10.0 10.0
Total 202 205 212
The values in the tables above show the energy in MeV for
The fission fragments' energy is manifested as kinetic energy. They
quickly slow down in the fuel matrix and the kinetic energy is
converted to heat. The same argument holds with the beta particles
and the gammas. The neutrons tend to lose their energy in the
moderator (as do some of the gammas). The neutrinos are
lost from the system, since the probability of interaction between
a neutrino and anything else on Earth is so small.
>5) Regarding nuclear waste, the spent U-235 still has some radioactivity
>left in it because gamma radiation can still occur. Is that correct?
U-235 can decay by alpha-decay but the half-life is pretty long -
about 700 million years, so the activity (decay/second) is pretty
low. Of more concern are the fission products which tend to have
more neutrons than are needed for nuclear stability. (At low
atomic masses, the number of neutrons in a stable nucleus is about
the same as the number of protons (a/k/a, the atomic number); as
the mass increases, the number of neutrons required for stability
increases. For instance, lead-208, as stable as they come, has
82 protons and 126 neutrons.) The fission products are radioactive
with relative short half-lifes.
>6) When balancing nuclear reactions, the sum of the mass numbers on the
>left hand side must equal the sum of the mass numbers on the righ hand
>product side. (same goes for atomic numbers). Well, if these are
>conserved, how is any energy released at all?
It has to do with a quantity known as binding energy. You see, the
mass of a U-235 nucleus is not 92 * proton mass plus 143 * neutron
mass but is actually somewhat less. The energy released by fission,
then, comes from the difference in the atomic mass of the U-235
and the atomic masses of the fission products (and neutrons).
>You can see I'm not an expert at all this and have not found answers in
>the high school/freshman college texts I have looked at. That's why I
>appeal to you experts. Thanks very much. Marshall Carroll from snowy
>Winnipeg, Manitoba, Canada
Hope that the above helps.
B. Alan Guthrie, III | Quis Custodiet Ipsos Custodes?
firstname.lastname@example.org | My opinions only