Fission Resonance

1. Apr 14, 2005

Clausius2

When one looks at the diagram neutron energy vs. probability of fission $$\sigma_f$$ (measured in barns) there is a zone of resonance, characterized by peaks and valleys on the function $$\sigma_f$$. Why doesn't conventional nuclear power plants works in this zone, instead of using a moderator to slow down fast neutrons?. I know maybe it is more instable, but also sometimes $$\sigma_f$$ would be greater.

2. Apr 14, 2005

Morbius

Clausius2,

Although the fission cross-section has peaks - so does the parasitic
absorption - and the peaks of the parasitic absorption are bigger.

You may see peaks in the fission cross-section of U-235; but 96+% of the
material in the fuel is U-238; which also has peaks but not fission peaks,
but absorption peaks. [ U-238 won't fission unless the neutron has an
energy above a threshold of about 1 MeV.] So the peaks of the U-235
fission cross-section in the resonance region; are totally swamped by
the parasitic absorption peaks in U-238.

In fact, this is one of the main negative feedbacks for a nuclear reactor.

First, you have to realize that those peaks are fairly narrow. Also, when
neutrons slow down - they don't slow down continuously like you do when
you put your foot on the brake in your car. Neutrons slow down in a
series of collisions - so the neutron's energy is a series of discrete jumps.
The neutrons will most likely "jump over" a resonance when in a single
collision it goes from having energy greater than the resonance to an
energy below the resonance. So the effective fission cross-section is
higher at thermal energies - because only a small number of neutrons
will ever see those fission resonances - most jump over them.

The resonances however can be Doppler broadened - that is, the neutron
can still be absorbed parasitically in the resonance if the thermal
motion of the target nucleus compensates for the fact that the neutron
doesn't quite have the resonance energy. If a neutron has an energy
below the resonance energy - it may encounter the target nucleus while
the target nucleus is moving toward the neutron - and hence the
relative energy of the neutron as seen by the target nucleus is equal to
the resonance energy.

Or if the neutron has an energy that is greater than the resonance
energy - it may encounter the target nucleus while the target is moving
away. Again, it looks like the neutron has the resonance energy to the
target nucleus - and the neutron is absorbed.

The hotter the reactor's fuel - the greater the mismatch between the
neutron's actual energy and the energy of the resonance that can be
accomodated and have the neutron absorbed in the resonance. Therefore,
as the reactor's fuel gets hotter - the parasitic resonance absorption
increases - and that lowers the reactor's reactivity.

This feedback mechanism is called "Doppler broadening". It is the chief
feedback mechanism that made the Integral Fast Reactor [ IFR ] design
passively safe. That is, the IFR didn't depend on control rods or other
engineered safeguards for its safety - it was "inherently safe" aka
"passively safe". Courtesy of PBS's Frontline, an interview with my
former boss at Argonne National Laboratory, Dr. Charles Till:

http://www.pbs.org/wgbh/pages/frontline/shows/reaction/interviews/till.html

By the way - you actually can have a reactor whose neutron density or
flux peaks in the resonance region - it is called a "fast reactor" - like the
IFR.

Although the idea of a fast reactor is to have the reactor operate at
the energies of the fast neutrons released by fission - so you avoid
putting any light material like a moderator in it - there's really no
escaping moderation. The heavy materials in the reactor will still do
a limited degree of moderation - and so the peak neutron flux is found
at an energy of a few hundred keV - which is right in the midst of the
resolved resonance region.

A fast reactor has a much lower neutron lifetime - so it can increase
its power a lot faster. Therefore, it was for SAFETY reasons too that
commercial power plants were built with thermal reactors.

However, in the 1980s, Argonne National Laboratory developed the
technology to make fast reactors like the IFR actually SAFER than
the light water reactors that we have today.

However, the USA hasn't built any new nuclear power plants since
Argonne developed the IFR technology - so the USA hasn't been able to
take advantage of this safer technology.

Additionally, as Dr. Till explains in the above interview - in 1994, then
President Clinton cancelled the IFR program.

Dr. Gregory Greenman
Physicist

Last edited: Apr 14, 2005
3. Apr 15, 2005

Clausius2

Great answer. You have answered widely to my question. Thanks and sorry for not having any additional comment.

Regards.

Javier.

4. Apr 17, 2005