[ RadSafe ] Skyshine measurements

Thu Mar 7 12:33:39 CST 2013

Skyshine can be pretty tricky.  The geometry of the source is often
complex, and doesn't simplify into a point or a line very well.  If the
source is not well shielded on the sides direct radiation swamps
skyshine out to a fair distance, if it is well shielded you may well
have a region where the closer you get to the source, the weaker the rad
field.  

I agree with Victor that taking the measurement is better than modeling.
If you rely too heavily on modeling you are likely to fall into the
"Assume the elephant is a sphere..." trap.  Measuring to show compliance
shouldn't be too hard.  Measuring to characterize under a variety of
conditions isn't too hard conceptually; it just requires a lot more
money than most managements are going to want to spend.

-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Victor
Anderson
Sent: Wednesday, March 06, 2013 8:58 PM
To: 'The International Radiation Protection (Health Physics)
MailingList'
Cc: JPreisig at aol.com
Subject: Re: [ RadSafe ] Skyshine measurements

Good Evening,

This is indeed interesting. Perhaps Ted could share some of his results.
With a BWR, skyshine is a more straight forward issue.  The major
radiation source is N-14 in the steam as it travels through the turbine.
While working at Peach Bottom I had the opportunity to measure dose
rates in the turbine building alongside the turbines at full power.  As
I recall dose rates were about 1 rem/hr at roughly 2 to 3 meters from
the turbine surface.
Skyshine is a function of what lies directly over the turbines.  Most
plants will have a thin steel roof over the turbine deck.  This has more
to do with the weather than anything else.  After the N-16 photons
radiate into the air above the turbine roof, they are subject to
scatter.  As someone has pointed out moisture in the air, air density,
etc will determine scattering of the
N-16 photons.  If the turbine building is subject to low lying fog, then
the fog could act as thin shield.  A low cloud bank will increase the
scattering.  How much is an interesting question.  For those of you with
a masochistic streak, the calculation can be done using a good monte
carlo code such as MCNP 2.7.0.  In my opinion, the best method is to
make measurements.  The good news is that you only have to measure the
scattered
N-16 gammas.  The bad news is that dose rates are very low.  The trick
of using 1 inch (2.54 cm) by 12 inch (30.48 cm) compensated GM tubes is
intriguing.  One big question is how sensitive these detectors are.  At
the site boundaries, the best method is to take integrated doses over
long periods of time (e.g., months).  One problem that our Swedish
friend has is this requirement to take measurements at 120% of power and
with low laying clouds.  The low laying cloud issue is a problem simply
because clouds will not stay low, maintain the same composition, or even
keep the same altitude.
What will be required is a good weather data base. Even then, it will be
hard to determine what part of the measured rates are due to low laying
clouds.  One technique could be to measure closer to the turbine
building where measurable readings can be made.  With some luck, the
influence of low laying clouds may be seen.  In my opinion, ultimately
what counts is the annual rates at the site boundary.  As long as
measurements show that annual dose rates are well within regulatory
limits, the plant has met standards.

Victor

-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Ted de Castro
Sent: Wednesday, March 06, 2013 6:39 PM
To: The International Radiation Protection (Health Physics) Mailing List
Cc: JPreisig at aol.com
Subject: Re: [ RadSafe ] Skyshine measurements

Sounds like lots of very expensive instrumentation!

I set up an active area monitoring network at Lawrence Berkeley
Laboratory many years ago - maybe you were at the San Jose mid year some
time ago and heard my paper on it.  My work was basically an improvement
on a system started by Lloyd Stephens there in 1963.

In any event - it uses cheaper, simpler and very reliable
instrumentation to good effect:  A 1 inch dia by 12 inch long energy
compensated GM and a Hankins Modified, high energy modified moderated
He3 detector.  The system runs reliably for years on end and is
sensitive enough to clearly show variations in background radiation - be
it seasonal or even when its raining!  The system integrates data in 10
minute intervals - this allows tracking changes with changes in
accelerator operations - either by comparison to accelerator running
logs and/or by comparing readings from local instruments with those from
the perimeter instruments on the network.

Using this I've been able to characterize things like perimeter
contributions from the ALS accelerator during fill as opposed to normal
running conditions.  Never got around to writing that up as a paper -
just an internal note.

These instruments would certainly be able to characterize the fields
discussed in this thread under the various conditions mentioned as a
temporary set-up and probably even be of use as a permanently installed
monitoring system (not requiring the high energy neutron configuration -
of course).

On 3/6/2013 10:40 AM, JPreisig at aol.com wrote:
> Dear Radsafe,
>   
>       Skyshine is discussed in Patterson and Thomas's  Accelerator 
> Health Physics and/or Cossairt's Course notes on Accelerator Health 
> Physics.
>   
>       Guess other Radsafers have commented on the  meteorology of 
> skyshine measurements.
>   
>       For field measurements of Skyshine, perhaps use a  portable MCA 
> counting system with NaI or Ge detector, or whatever you have 
> available.  If there is a neutron component, you can make flux density

> spectra measurements with a set of Bonner Spheres and one or two 
> LiI(Eu) detectors.  Hopefully reactor neutron and/or photon/gamma 
> signals are not highly variable in time, so you don't need
> 7 LiI(Eu) detectors, just one or two.  Bonner Spheres (polyethylene)  
> are fairly expensive.  You can analyze Bonner data with BON4/BON5,  
> LOUHI???, MAXED??? or other unfolding codes available from RSICC 
> (Radiation Shielding Information Center at ORNL).  If you end up using

> an Anderson-Braun detector only, maybe you would want to make a set of

> measurements as a function of angle (measured from horizontal) pointed

> up towards the sky.
>   
>      Like the other guy said, you can mathematically model  the 
> problem using MCNP (Monte Carlo Neutral Particle Program).  MCNPX can 
> be used for high energy neutron/hadron transport, but that may not be 
> necessary in this case.
>   
>      Have fun.    Regards,  Joseph R.  (Joe) Preisig
>   

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