Contact Dose Rates (RE: Drill Scenario)

["Gerry W. Kindred" <gwkindred@centerior.com>]

I agree totally with Ted's counselling on the proper use
of instrumentation for measuring dose rates.  However, during
my career, I have only been able to review surveys of 
irradiated fuel that were taken underwater. That is where the 
initial values I used came from.  Therefore, a 30 cm or 100 cm
dose rate (thru the water) would not lend itself very well for
ascertaining the dose rates (in air) of an irradiated fuel
bundle.  I just wanted to provide the RADSAFE community 
with a rational of the time required to receive a lethal dose
in the scenario Bruce described. My point, is even if I was low
by an order of magnitude, most prople on the floor would have 
time to exit the area (with less than a lethal dose).  As soon as
the fuel bundle neared the surface of the pool, the people in 
the area would know something was happening (i.e. radiation
monitors would be alarming).  If I were to provide an official
calcualtion, I would not use measured dose rates by a survey
meter.  I would use other empirical data including the irradiated
fuel's EFPY, the associated neutron fluences (which would vary,
because in a commerical plant, a fuel bundle is burned typically 
for three cycles, and is in a different core location each cycle),
fuel bundle composition, enrichment, and other factors.  

    

----------
> From: tdc@ehssun.lbl.gov
> To: radsafe@romulus.ehs.uiuc.edu
> Subject: Re[2]: Drill Scenario
> Date: Monday, January 12, 1998 6:25 PM
> 
> 
> 
> 
> >The dose rate on the lateral surface of an irradiated fuel bundle that
has
> >decayed for >30 days is in the neighborhood of 5E5 to 1E6 R/hr contact.
> 
> >1/4 inch    = 1E6 R/hr
> >8 inches   =  33,750 R/hr 48 sec
> >~5 feet     4200 R/hr  6.4 minutes
> >~ 21 feet  1000 R/hr  27 minutes
> You know - this is a very good example of the ridiculous use of the
> meaningless quantity "contact dose"!
> It really is not very useful yet its use persists in the industry.
> For the "contact dose" to represent the reading at 1/4 inch the detector
> assembly would have to be only 1/2 inch thick.  Not too many dose
> responding detectors are that small that are not also influenced by local
> scatter factors too!
> It is really hard to know WHAT distance so called "contact" measurements
> represent and they certainly don't estimate REAL contact risks like skin
> dose.
> This is instrument misuse at best.
> When you use such a measurement at an assumed and inaccurate small
geometry
> and then try to extrapolate that out to very large distances - the errors
> are enormous.
> The 30 cm - or better yet 1M readings are so much more meaningful and
> accurate.  Such measurements lend themselves well to far field
> extrapolations.
> Ted de Castro
> tdc@ehssun.lbl.gov
> University of California Lawrence Berkeley Laboratory
> Bldg 90 Rm 0026B
> Berkeley, CA 94720
> (510) 486-5256
> (510) 486-6939 - FAX
>