[ RadSafe ] Field determination of radon progeny

Tue Apr 13 13:08:08 CDT 2010

In terms of the effective half-life of radon daughters on clothing, I believe the good working value is just about 30 minutes. If clothing measured contamination drops by a factor of 2 in 30 min, and 4 in 1 hour, it is almost certainly Rn daughter contamination. 

Bi-214 has a half-life of about 20 min, and Pb-214 about 27 min, with a rough overall effective half life for the Rn-daughter mix of 30 min.

Another interesting source of Rn-daughter contamination  that might show up at a nuclear plant is that due to rainfall.  I was involved with gathering and reviewing High Pressure Ionization Chamber measurements of exposure rate  near an operating nuke plant 30 years ago. Measurements were being made with a custom built instrument [like the Reuter Stokes
 RSS-111, which had a 10" diameter sealed sphere, pressurized to 25 atm with Ar, so it was quite sensitive to ionization from x rays
 and gamma rays]. I had a custom unit built because I needed to record data to a tape unit for long-term exposure rate measurements at the site boundary a BWR nuclear plant [only 700' from the turbine]. The State of VT had set an annual limit at the site boundary of only 5 mR/year  [0.05 milli-Sv/hr] from plant operations, with turbine shine being the major factor at only 700'.

I had plotted hourly average radiation exposure rates. I also had gathered [for another purpose] concurrent data of hourly rainfall rates in mm/hr.  Putting the exposure rate data together with the rainfall rate in mm/hr some very interesting things became evident.

 During the start of initial hours of high rainfall, the exposure rate increased from 8 uR/hr [0.08 uGy/hr] to
 about 14 micro-R/hr [0.14 microGy/hr], an increase in background exposure rate of about 75%, due to ground-plane deposition of Rn-daughters around the HPIC system located at the site boundary. 

Subsequently, after a few hours the rainfall rate went to almost zero. The exposure increment began to drop with about a half hour half life, returning toward 8 uR/hr. Then a few hours of increase and drop of rainfall rate,  and the total exposure rate went up and down concurrent with the rainfal rate affecting the deposition of Rn daughters. After about 10 hours the rainfall ended, and the 6 uR/hr [0.06 uSv/hr]  increase in background due to Rn-daughters decayed away, and the total exposure rate returned [with about a 30 minute half-life] to the background of about 8 uR/hr [0.08 uGy/hr] for that location. Of note, the plant was not operating at the time, so there was no contribution from N-16 turbine shine, and no plant releases occurring.

This 75% increase in background measured at the time was due to rain washing ["scrubbing"] Rn-daughter particulates from the air column and depositing them to the ground where they caused an increase in exposure rate measured
 by the fixed HPIC.

So, it is worth being aware that background exposure rates at or near a nuclear facility [which might be monitored by HPIC or detectors able to measure slight increases in background] can increase by 75% [or who knows how much more depending on rainfall rate and average Rn-daughter concentration in the air mass at the location being measured at that time]. Then the increase exposure rate above background, not due to plant releases, will decay away with about a half hour half-life.

Stewart Farber, MSPH

Farber Medical Solutions, LLC

Bridgeport, CT 06604

[203] 441-8433 [office]

website: http://www.farber-medical.com
farber at farber.info
=======================

--- On Tue, 4/13/10, Eric.Goldin at sce.com <Eric.Goldin at sce.com> wrote:

From: Eric.Goldin at sce.com <Eric.Goldin at sce.com>
Subject: [ RadSafe ] Field determination of radon progeny
To: radsafe at agni.phys.iit.edu
Date: Tuesday, April 13, 2010, 10:46 AM

I didn't ever see too much on the actual answer to Rick Hansen's question 
on field determination of radon progeny:

I have a question for radsafe:

What are some methods to use in the field to determine if low levels of
radiation detected on a person or clothing is due to radon daughters
rather than radioactive contamination from other sources?

Nuclear plants have a duty to not release people who are contaminated with

licensed radioactive material
 (plant-related).  So to distinguish between 
naturally occurring rad material (i.e. radon progeny) and plant-related 
noble gases (typically Xe-133), the worker is typically asked to simply 
wait for decay.  With most radon progeny having short half-lives (less 
than 20 minutes), a short decay period will usually drop the 
electrostatically bound radon daughters to a sufficiently low level such 
that they will pass the whole body contamination monitors.  Anything 
longer-lived is likely plant-related and therefore requires documentation, 
decontamination, investigation, . . . .     Of course an option is to take 
the clothes and place them on a HpGe detector for a qualitative evaluation 
- if the count room has the time.

Eric M. Goldin, CHP
<Eric.Goldin at sce.com>