Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

INTERNAL RADIATION SAFETY 625
In the case under consideration, the respiration rate is 0.5 L per inspiration × 20
inspirations per minute = 10 L min−1 . Substituting into Eq. (11.19) to solve for C0,
we have
1.47 × 10 9 Bq = 10 L
min × C0 Bq/L
0.26 min −1
C0 = 9.43 × 10 7 Bq/L (2.55 mCi/L) .
1 − e −0.26 min−1 ×2min
Since the room volume is 27 m 3 , or 27,000 L, and since only 1.275 kg of the 3 kg
of water will be in the vapor state, the maximum amount of tritium that may be
contained in the tritiated water is
3
Bq
× 9.43 × 107
1.275 L vapor × 2.7 × 104 L vapor = 6.0 × 10 12 Bq (162 Ci) .
According to these calculations the use of no more than 6.0 × 10 12 Bq (162 Ci)
tritium in the 3 L of tritiated water would ensure against overexposure in the event
of the maximum credible accident. If this is not enough activity for the purpose of
the experiment, then additional precautions, such as enclosure of the process or
increased ventilation, would have to be employed.
Radon
Radon gas is a naturally occurring radioactive gas that occurs in the uranium (4n+2
series) and the thorium (4n series). Since both these series are ubiquitous, radon
gas is produced, to a lesser or greater degree in all soils, depending on the concentration
of uranium or thorium. The gas diffuses out of the ground and into the air.
Because of the relatively slow diffusion rate and the short half-lives of the two Rn isotopes,
most of the radon that is exhaled from the ground originates in the top 30 cm
(1 foot) of soil. Radon exhalation from the soil is on the order of several picocuries
per square meter per second, the exact rate depends on the concentration of the
uranium and thorium series precursors in the soil. The concrete foundation or
slab on which a building rests acts as a barrier against the radon. However, radon
gas can gain entry into the building by diffusing through the concrete or through
cracks that may occur in the concrete. Entry rates through such concrete barriers
are usually less than one-half the exhalation rate from the soil. Once in the
house, air exchange by general ventilation and radioactive decay will lead to a radon
and radon daughter concentration that depends on the rate of radon influx into
the house, on the ventilation rate, and on the radon concentration in the outside
air with which the house is ventilated. In the United States, the average outdoor
Rn concentration is 0.4 pCi/L (15 Bq/m 3 ) and the mean indoor concentration is
1.5 pCi/L (54 Bq/m 3 ).
In general ventilation, the concentration C to which the airborne contaminant
is reduced is determined by the rate of generation G of the contaminant and the
ventilation rate Q. The rate of change of the quantity of contaminant in the air is
simply the difference between the rate of generation and the rate of removal of the