[ RadSafe ] Skyshine measurements

[Victor Anderson]

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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