radioactivity of uranium
Richard J. Fox
>I've got a question that hopefully someone can provide an answer.
>Uranium is mined and processed into fuel rods to be used in reactors.
>What is the change in the radioactivity between the raw uranium ore and
>the spent fuel rod?
>
>Yuchou
The Uranium is never very hot (radioacitve), either before or after being
burned in a reactor. The intense radioactivity associated with spent fuel
is a result of the fission products and the transmutation of U238 into
Pu239. As for the what the radioctivity of a fuel rod after vs. before use in a
reactor, in a word; HUGE! Sorry I don't have the numbers but this is
what the whole waste issue is about.
Richard J. Fox
Steinn Sigurdsson
The issue is not that simple. While U is not very "active" several of
its daughter isotopes are, and a large quantity of uranium can produce
a large net exposure to people. In particular, natural uranium decay to Radon and from
Radon to Polonium is a very large contribution to the background rate
and to the radiogenic cancer rate.
In fact if you look at time averaged exposure, the integrated activity
of U/Th plus daughters is about the same as the integrated acticvity
of U+fission products - the fission products may be more active but
then necessarily have a shorter half-life. If we were looking at very
long term exposure only the integrated exposure is not significantly
increased by fissioning the uranium, the waste problem is more a
question of how the activity is distributed and shielded.
* Steinn Sigurdsson Lick Observatory *
* ste...@lick.ucsc.edu "standard disclaimer" *
* If you ever have to go to Shoeburyness *
* Take the A-road, the ok road, that's the best! *
* Go motoring on The A13! - BB 1983 *
Hu Yuchou
Daniel K. Jarrell
Most of the radioactivity comes from the isotopes created by the fission
process. These daughter products are generally unstable and highly
radioactive. (I seem to remember most of them being beta and gamma
emitters.)
Dan Jarrell
It's only me.
Daniel Burstein
We're all familiar with the standard geiger counter. question: I realize
it does a good job at picking up Beta, but does it register Gamma in any
sort of reliable way?
thanks
danny
dan...@panix.com
(all usual disclaimers apply, whatever they may be...)
John De Armond
>A quick (and no doubt embarrassing) question:
>We're all familiar with the standard geiger counter. question: I realize
>it does a good job at picking up Beta, but does it register Gamma in any
>sort of reliable way?
>thanks
>danny
Not at all embarrasing. Lots of misunderstanding regarding GM tubes.
The GM detector will detect gamma rays, though the efficiency varies
widely depending on the structure. Very few gamma actually interact
with the fill gas. More common is the gamma interacting with the
housing in a manner that kicks out secondary electrons or soft X-rays.
Thus, the design of the housing heavily influences the efficiency.
Unfortunately, and despite the fact that most GM counters also have a
dose scale, nothing regarding dose can be inferred from the count rate.
This is because the GM tube registeres the same count in response to a
very weak gamma as to a very energetic one. A GM counter's scale CAN be
calibrated in terms of mR/hr for a single type of gamma spectrum. Civil
Defence-type detectors are typically calibrated against Cs-137
radiation, a gamma at 0.667 MEV. This is convenient because the
radiation environment post bomb with old fallout tends to have its
average energy around 0.650 MEV. Same for the degraded background
radiation typical in normal workspaces of a nuclear plant. This does
NOT hold, however, for measuring a point source - even a Cs-137 one -
and for radioactive sources with energies widely different from the
calibration point. It also does not hold in a mixed beta/gamma
environment.
John
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Frank - Hardware Hacker - Borger
>A quick (and no doubt embarrassing) question:
>
>We're all familiar with the standard geiger counter. question: I realize
>it does a good job at picking up Beta, but does it register Gamma in any
>sort of reliable way?
>
Geiger-Mueller tubes are best at picking up X-ray and Gamma radiation,
(ie electromagnetic radiation, but of a different wavelength than
radio or light.)
When you try to measure electrons, (Beta particles,) you have a problem.
They generally will not penetrate the metal wall of the GM tube. You
have to make special "windows" on the side or end of the tube, out of
some very thin material, often a thin foil, that will let the radiation
in and not let the special gas in the tube mix with the atmosphere.
In extreme cases, your tube "window" will be just be a long slot in
the side of the tube, and you keep a constant flow of gas from a small
tank of compressed gas and a pressure regulator flowing thru the tube.
Frank R. Borger - Physicist ___ George Halas had two heirs; one carried
Michael Reese - U of Chicago |___ his seed and one carried his soul. ...
Center for Radiation Therapy | |_) _ One can imagine the whisper of George
net: Fr...@rover.uchicago.edu | \|_) Halas, rooting for his soul. -
ph: 312-791-8075 fa: 567-7455 |_) Jeannie Morris
Jeremy Whitlock
>We're all familiar with the standard geiger counter. question: I realize
>it does a good job at picking up Beta, but does it register Gamma in any
>sort of reliable way?
The same way they register betas, by ionization of the gas. The only
difference here is the method of free electron creation: with photons it's
by mainly by Compton scattering, photoelectric absorption, or pair production.
These processes are reliable, in that they are characterized by energy-
dependent cross-sections that are well known for the particular gas used.
--
Jeremy Whitlock "My thoughts are mine, not Mac's"
Dept. Engineering Physics
McMaster University
Hamilton, Ontario, Canada
John De Armond
>>We're all familiar with the standard geiger counter. question: I realize
>>it does a good job at picking up Beta, but does it register Gamma in any
>>sort of reliable way?
>>
> Bzzzt---wrong
> Geiger-Mueller tubes are best at picking up X-ray and Gamma radiation,
> (ie electromagnetic radiation, but of a different wavelength than
> radio or light.)
Well while we're being rude,
BZZZT- Wrong.
GM tubes will demonstrate efficiencies on beta approaching theoretical
for the geometry given a suitable window. Since it is rare to see a
survey instrument these days that does NOT use a mylar window tube,
a generalized statememt like the original fellow made is quite
accurate.
> In extreme cases, your tube "window" will be just be a long slot in
> the side of the tube, and you keep a constant flow of gas from a small
> tank of compressed gas and a pressure regulator flowing thru the tube.
Not for GM tubes. Proportional counters are commonly operated with
a flow gas and no window. Any reasonably energetic beta will penetrate
the common 10 mg/sq meter mylar windows. The reason windowless chambers
are used for proportional counting is so the scatter caused by the
window will not broaden the energy spectra and so the unit may be
used for alpha counting where the alpha energy spectrum is important.
Even for proportional counters, except for the most sensitive detectors,
gas flow chambers have fallen out of favor. I have a nice Eberline
alpha proportional counter here in my lab that uses only dry air as
the fill gas.
John
(Note that I'm not here to debate whether a gas flow GM tube is possible.
Lord knows someone is likely to have made one somewhere, sometime.)
Frank - Hardware Hacker - Borger
>>fr...@rover.uchicago.edu (Frank - Hardware Hacker - Borger) writes:
>Well while we're being rude,
>
> BZZZT- Wrong.
>
>GM tubes will demonstrate efficiencies on beta approaching theoretical
>>for the geometry given a suitable window. Since it is rare to see a
>survey instrument these days that does NOT use a mylar window tube,
>a generalized statememt like the original fellow made is quite
>accurate.
I was attempting to do 2 things.
1/ Make a generalized statement that the average person could
comprehend, without getting into specialties and
2/ describe what a window was for, because, although typical
instruments these days are useable, much junk is available on
the surplus market, often picked up by naive users.
>> In extreme cases, your tube "window" will be just be a long slot in
>> the side of the tube, and you keep a constant flow of gas from a small
>> tank of compressed gas and a pressure regulator flowing thru the tube.
>
>Not for GM tubes. Proportional counters are commonly operated with
>a flow gas and no window. Any reasonably energetic beta will penetrate
>the common 10 mg/sq meter mylar windows. The reason windowless chambers
>are used for proportional counting is so the scatter caused by the
>window will not broaden the energy spectra and so the unit may be
>used for alpha counting where the alpha energy spectrum is important.
Again, I fall prey to trying to oversimplify. Should I have gone
into a long discourse on operation as ionization chamber, proportional
counter, or GM, thereby totally wasting bandwidth?
Let's try to give a better explanation for the general user.
1/ When I look at a GM counter in operation, all I get is a meter
reading in counts/minute. I have to convert that number to some
meaningful radiation measurement, by exposing the counter to
varying levels of known radiation and noting how many counts/min
I get.
2/ If I am using the instrument to measure Alphas or Betas, I have
two additional problems.
1/ The wall of the tube (or window,) will significantly
absorb and/or degrade the energy of the particles.
2/ Although GM tubes can be built to give reasonably flat
response over the range of photon energies normally
encountered, its tricky to do this for Alphas and Betas.
3/ I really have to have a good idea of the energy of the
particle before I can accurately measure it. Operating
the tube at reduced voltage, it becomes a proportional
counter, where the size of the current pulse is proportional
to the energy of the particle, and with fancier electronics,
I can get some handle on how energetic the particle is.
Again, some corner cutting above to try to simplify, the bottom line
I was trying to say is. If you see some surplus GM counter out there
in A & B's surplus electronics and close out center catalog, remember
TANSTAFFL. A tool is only as good as the person using it.