20.2 Contamination Limits and Decontamination Goals
249
We have already discussed the tension between risk reduction and remediation
costs; another tension in developing remediation limits is that of scientifically justifiable risk-based limits versus the perception of those who do not have a strong
understanding of the science and calculations underlying the risk calculations.
Thus, a citizen with a radiation detector might be alarmed at the count rate they
are measuring, even if the dose rate is too low to pose a great risk. Alarmed by
seeing high count rates, it would be natural to demand cleaning up until the count
rate drops to normal background levels measured elsewhere in the city. At the same
time, radiation safety experts would note the high cost of remediation to those levels
compared to the marginal reduction in risk.
Comparison risk-based and technology-based remediation standards
Using the same sort of calculations described in the earlier text box we
can calculate that a Co-60 contamination level of 1000 dpm per 100 cm
2
(0.17 Bq cm
2 ) will produce a radiation dose rate of about 150 nSv hr
−1 for
an annual radiation exposure of about 1.3 mSv yr
−1 . A person living on this
surface for 80 years would receive an integrated exposure of about 10 mSv;
using the linear no-threshold hypothesis and a slope factor of 5% risk of developing a fatal cancer for 1 Sv of exposure this person would have a lifetime
cancer fatality risk of 0.005%, or about five for every 100,000 residents. This
risk can be adjusted to account for spending part of one’s time in areas with
more or less contamination.
Assume that the city’s Radiological Advisory Committee (RAC) agrees
that an acceptable risk level is one possible cancer fatality for every 100,000
residents (which would be stated as a cancer fatality risk of 1 × 10
−5 ) then the
acceptable average contamination level would be 0.035 Bq cm
−1 .
While this level of contamination produces very little risk, it will produce an
elevated count rate, although the actual count rate will depend on the instrument
being used. A small scintillation detector with a cesium iodide detector would
have a count rate on the order of several thousand counts per minute while a
GM tube might register only a few hundred counts per minute (although even
this is much higher than the normal background count rate of 50–100 CPM).
Thus, as with the previous case, an area that poses very little risk might still
produce a sufficiently high count rate as to alarm the public.
20.3 Decontamination Standards for Homes, Businesses,
Public Spaces, etc.
Another question that must be answered is whether or not to apply the same decontamination standards universally or to adjust them to account for the fact that different
249
We have already discussed the tension between risk reduction and remediation
costs; another tension in developing remediation limits is that of scientifically justifiable risk-based limits versus the perception of those who do not have a strong
understanding of the science and calculations underlying the risk calculations.
Thus, a citizen with a radiation detector might be alarmed at the count rate they
are measuring, even if the dose rate is too low to pose a great risk. Alarmed by
seeing high count rates, it would be natural to demand cleaning up until the count
rate drops to normal background levels measured elsewhere in the city. At the same
time, radiation safety experts would note the high cost of remediation to those levels
compared to the marginal reduction in risk.
Comparison risk-based and technology-based remediation standards
Using the same sort of calculations described in the earlier text box we
can calculate that a Co-60 contamination level of 1000 dpm per 100 cm
2
(0.17 Bq cm
2 ) will produce a radiation dose rate of about 150 nSv hr
−1 for
an annual radiation exposure of about 1.3 mSv yr
−1 . A person living on this
surface for 80 years would receive an integrated exposure of about 10 mSv;
using the linear no-threshold hypothesis and a slope factor of 5% risk of developing a fatal cancer for 1 Sv of exposure this person would have a lifetime
cancer fatality risk of 0.005%, or about five for every 100,000 residents. This
risk can be adjusted to account for spending part of one’s time in areas with
more or less contamination.
Assume that the city’s Radiological Advisory Committee (RAC) agrees
that an acceptable risk level is one possible cancer fatality for every 100,000
residents (which would be stated as a cancer fatality risk of 1 × 10
−5 ) then the
acceptable average contamination level would be 0.035 Bq cm
−1 .
While this level of contamination produces very little risk, it will produce an
elevated count rate, although the actual count rate will depend on the instrument
being used. A small scintillation detector with a cesium iodide detector would
have a count rate on the order of several thousand counts per minute while a
GM tube might register only a few hundred counts per minute (although even
this is much higher than the normal background count rate of 50–100 CPM).
Thus, as with the previous case, an area that poses very little risk might still
produce a sufficiently high count rate as to alarm the public.
20.3 Decontamination Standards for Homes, Businesses,
Public Spaces, etc.
Another question that must be answered is whether or not to apply the same decontamination standards universally or to adjust them to account for the fact that different