Re: Measurement of Neutron Exposure In NPP?

[bflood@SLAC.Stanford.EDU (Bob Flood)]

>I am surveying how to measure or evaluate neutron exposure to radiation
>workers in nuclear power plants  around the world.
>Would  you please  help me with the following?
>
I worked in the nuclear power field almost 20 years before coming to
Stanford. I think I can help a little.

The Nuclear Regulatory Commission (NRC) financed and published several
neutron spectrum measurements in light water reactors in the 1970's and
1980's. Ultimately they concluded that the variation in neutron spectra
within a plant and from one plant to another was small, and that the typical
nuclear plant spectrum was approximated to an acceptable degree by the
D2O-moderated Cf-252 spectrum used in proficiency testing for dosimetry
accreditation. Thus, the NRC accepted dosimetry and survey instruments
calibrated to moderated Cf-252 for use in American nuclear plants.

At TVA (where I worked), we handled this in 2 steps. First, the dose
algorithm for our Panasonic UD-802 dosimetry system employed the moderated
Cf-252 dose factors when calculating personnel doses. Second, our instrument
calibration facility determined by experiment the difference in readings on
a rem-ball (Eberline PNR-4) when irradiated by the moderated Cf-252 spectrum
and the spectrum from a PuBe source TVA used in the calibration facility.
Thus, instruments placed in the PuBe field could be adjusted to read as if
calcibrated in the same field used to calibrate the dosimetry system.


>  1. How to expect the dose( or dose rate) before any work is done in a
>neutron field.

Routine surveys using the calibrated rem-balls could be used with expected
stay-times for the work.

>  2. How to evaluate neutron dose after it is done.

The dose calculation algorithm reports the components of dose to the dose
tracking system (i.e., beta, photon, and neutron for shallow, lens, amd deep
doses), so the dose tracking database that can report total dose for a job
can also report neutron dose for a job. This can be compared to the neutron
dose estimate used when planning the work.

>  3. Its name and type if any instrument is used 

Survey meter: Eberline PNR-4
Dosimeter: Panasonic UD-802
Reader: Panasonic UD-710

>  4. How often neutron spectrum in an area where radiation workers may
>acces  or   enter is measured or evaluated?

No spectrum measurements were made at Browns Ferry (BWR) or Watts Bar
(Westinghouse PWR). One set of measurements were made by TVA staff at
Sequoyah (also a Westinghouse PWR) during startup in 1980.

>      What  is the measured spectrum  used for? or how? 

Bonner spheres were use for the measurements. Much arguing ensued over how
to analyze the data. In typical government fashion, eventually several
techniques were used with none selected as the most appropriate. (Anything
worth doing is worth overdoing.) All of the calculations fit the NRC's
position that the differences are small and the spectrum is similar to
moderated Cf-252.

>  5. If you have several methods , is  there any deviation between them?  

Sorry, I can't answer this one from memory. I recall the differences in the
calcuated results as being insignificant to the dosimetry program (my
specialty).

>  6. What has been done or is planned  as for the change of quality factors
>in        new 10 CFR  20 (effective in 1994) or radiation weighting factors
>in              ICRP   60.
>      There may be revision of a  TLD Algorithm,  or use of new remball or
>whatever.
>
Since the NRC hasn't changed it's position about in-plant spectra and
moderated Cf-252, the only TLD system changes needed because of changes in
the quality factors would be to the dose conversion factors relating element
response to neutron dose equivalent.

Hope this is useful.

Bob Flood
Unless otherwise noted, all opinions are mine alone.
(415) 926-3793
bflood@slac.stanford.edu