----- 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