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Re: Double Checking Your Calculations



Kai and friends,
 
My mistake(s)!  Like any other calculations, back of the envelope calculations should also be checked ...
 
1 -  The gamma constant for Bi 214 is 2.25 E-4 Rem per h per MBq (not Bq). 
2 - I mixed up flowrate units - I used liter per second instead of metre cube per second (concentrations were in Bq per meter cube). 
 
For 37 Bq/m3 of Rn and 1 L/s the dose rate at 1 m would be about 150 pSv/h.   At 3 m3/s and Rn concentration = 37 Bq/m3, the dose rate would be about 0.5 microSv/h.  It would take a lot of radon for filters to emit a significant dose rate.
 
My facer is red, sorry for the confusion. 
 
The Oak Ridge table for gamma constants can be found at   http://www-rsicc.ornl.gov/documents/pdf/reports/RSIC45r1.pdf
 
Philippe Duport
----- Original Message -----
Sent: Thursday, December 13, 2001 1:23 AM
Subject: Double Checking Your Calculations

Hi Philippe,
 
My Email has been acting up the last few days, so I don't know if anyone on radsafe has responded to you and I'm not sending this to the list. Here are my thoughts:
 
A  gamma constant of 2.25 e-6 Sv/h.Bq seems a bit high, perhaps you are missing a prefix? At what distance is this dose rate?
 
6 e4 Bq of Bi 214 at equilibrium also sounds a bit high for your assumptions. In 5 hours a total of 666 Bq of radon passes through your filter (37 Bq/m3 x 0.001 m3/s x 3600 s/h x 5 h). You can not have more Bi 214 than Rn 222 activity. My guess is that there would be about 60 Bq Bi 214 on the filter. 
 
6 e4 Bq x 2.25 e-6 Sv/h.Bq = about 1.4 e-1 Sv/h not 1.4 e-7 Sv/h (prefix ??).
 
This is how I would do the calculation:
 
The situation was for dehumidifier filters in waterworks. The radon concentration in waterworks can be elevated, especially if well water is used. We could use 740 Bq/m3 instead of 37 Bq/m3. (20 times your assumption.) I don't know where the dehumidifiers would be. If you are trying to dehumidify the general air, you would need flow rates comparable to the building ventilation. Otherwise you are not really doing anything. This is typically a few air changes per hour or a few m3/s. (Say 1000 times your assumption of 1 L/s.) If you are trying to dehumidify secondary ventilation, you would have less air volume but correspondingly higher radon concentrations (at lower equilibrium factors).
 
In this example the source term is 20 000 times your assumption and I would estimate the amount of Bi 214 on the filter at 1.2 e6 Bq (60 Bq x 20 000). This is about the same amount of Bi 214 (and gamma radiation) as in 1 kg of 10% uranium ore. Not a huge gamma radiation hazard, but a measurable gamma field.
 
If the filters are changed every few months, the long lived activity of Pb-210 and Po-210 would be about 1% of the Bi-214 equilibrium activity or 1.2 e4 Bq. This would not decay away if you shut the fan off for a few hours. The gamma field from that is not much but, if you let it dry and tossed it around, the stuff could probably get airborne and inhaled. It might become an issue if there was a contractor that went around to all the waterworks and handled all the filters.
 
Best Regards,
 
Kai Kaletsch
 
 
----- Original Message -----
Sent: Wednesday, December 12, 2001 1:19 PM
Subject: Dose from Rn decay products collected on a filter

A few days (weeks?) ago someone was asking about dose rate from an air filter collecting radon progeny.  Perhaps someone else gave the answer directly the interested person. 
 
A back-of-an-envelope calculation gives the following for a Rn concentration of 37 Bq/m3 (1 pCi/L) at equilibrium 0.7, 0.5, 0.3 for Po 218, Pb 214, Bi 214 respectively, a sampling rate of 60 L/min (1 L/s) and a gamma constant of 2.25 e-6 Sv/h.Bq (ORNL/RSIC-45/R1 Report (1982) :
 
Bi 214 is the only significant gamma-emitting radon decay product.  The equilibrium activity for Bi 214 (sampling time more than 5 hours) on the filter is about 6 e4 Bq, giving a steady state dose rate at one meter of
 
 6 e4 Bq x 2.25 e-6 Sv/h.Bq = about 1.4 e-7 Sv/h or 0.14 microSv/h  for 37 Bq/L of radon at stated equilibrium factors.
 
IT IS WORTH DOUBLE CHECKING MY CALCULATIONS !!!
 
 
Philippe Duport
International Centre for Low Dose Radiation Research
University of Ottawa
555 King Edward Ave.
Ottawa, ON, Canada, K1N 6N5
Tel: (613) 562 5800, ext. 1270
pduport@uottawa.ca