20.2 Contamination Limits and Decontamination Goals
247
from Federal Guidance Report #15 [14] we can easily calculate the radiation dose rate
resulting from any level of contamination from any of a large number of contaminating radionuclides. With Co-60, for example, a surface contamination level of
10 dpm cm
−2 (a common cleanup limit in the US) will yield a dose rate somewhat lower than 0.020 µSv hr
−1 and will produce an annual exposure of only about
175 µSv yr
−1 to a person spending 100% of their time on this surface. Living on such
a surface for 80 years would produce a lifetime dose of about 14 mSv, comparable
to the dose from a single whole-body CT scan. Clearly this is not a significant health
risk. Remediating to such low contamination levels adds very little risk reduction
at a great cost, especially when we consider that how unlikely it is that any single
person would spend 100% of their lives in so restricted an area.
Setting cleanup limits based on the lifetime risk of developing a fatal cancer is
another possibility. With a risk coefficient of 5% added fatal cancer risk Sv
−1 we can
calculate the dose rate and corresponding contamination levels to produce a given
level of risk, as shown in this text box.
When confronted with contamination it is not uncommon for the public to demand
that it be cleaned up to pre-accident background levels. This is understandable, but
it offers little actual risk reduction at a potentially high cost. From an “aesthetic”
standpoint, this standard would return the affected areas to more or less exactly as
they had been prior to the attack; from a health and safety standpoint it makes little
sense.
It is also possible to remediate to a level based on risk—the total radiation exposure—to the general population, as described in the following text box. This is the
most cost-conscious cleanup standard, but this would also leave behind the highest
levels of contamination and might prove to be unacceptable to members of the public
who are inordinately frightened of radiation and radioactivity [1].
Calculating a risk-based contamination limit
Assume that a government or advisory committee has determined that they
will remediate an area until the risk posed by that area is no more than 0.01%
(one fatal cancer for every 10,000 people) to a person spending 100% of their
time on this site over an 80-year lifetime. Having determined the acceptable
risk, it is necessary to determine the level of contamination that will generate
this level of risk.
Using a risk factor of 5% Sv
−1 we can easily calculate the lifetime dose
that will produce a risk of 10
−4 ; dividing 10
−4 by 0.05 Sv
−1 shows us that a
lifetime radiation dose of 0.0002 Sv (0.2 mSv) will produce this level of risk.
Over an 80-year lifetime, this is equivalent to 2.5 × 10
−5 Sv yr
−1 or slightly
less than 3 nSv hr −1 since a year has 8760 h.
According the Federal Guidance Report #15 [14] the radiation exposure
from Cs-137 contamination is 7.85 × 10
−18 Sv s
−1 (or 2.83 × 10
−14 Sv hr
−1 ) for
every Bq m
−2 of contamination. Using this conversion factor we can calculate
that, to produce a dose rate of 3 nSv hr
−1 requires a surface contamination
247
from Federal Guidance Report #15 [14] we can easily calculate the radiation dose rate
resulting from any level of contamination from any of a large number of contaminating radionuclides. With Co-60, for example, a surface contamination level of
10 dpm cm
−2 (a common cleanup limit in the US) will yield a dose rate somewhat lower than 0.020 µSv hr
−1 and will produce an annual exposure of only about
175 µSv yr
−1 to a person spending 100% of their time on this surface. Living on such
a surface for 80 years would produce a lifetime dose of about 14 mSv, comparable
to the dose from a single whole-body CT scan. Clearly this is not a significant health
risk. Remediating to such low contamination levels adds very little risk reduction
at a great cost, especially when we consider that how unlikely it is that any single
person would spend 100% of their lives in so restricted an area.
Setting cleanup limits based on the lifetime risk of developing a fatal cancer is
another possibility. With a risk coefficient of 5% added fatal cancer risk Sv
−1 we can
calculate the dose rate and corresponding contamination levels to produce a given
level of risk, as shown in this text box.
When confronted with contamination it is not uncommon for the public to demand
that it be cleaned up to pre-accident background levels. This is understandable, but
it offers little actual risk reduction at a potentially high cost. From an “aesthetic”
standpoint, this standard would return the affected areas to more or less exactly as
they had been prior to the attack; from a health and safety standpoint it makes little
sense.
It is also possible to remediate to a level based on risk—the total radiation exposure—to the general population, as described in the following text box. This is the
most cost-conscious cleanup standard, but this would also leave behind the highest
levels of contamination and might prove to be unacceptable to members of the public
who are inordinately frightened of radiation and radioactivity [1].
Calculating a risk-based contamination limit
Assume that a government or advisory committee has determined that they
will remediate an area until the risk posed by that area is no more than 0.01%
(one fatal cancer for every 10,000 people) to a person spending 100% of their
time on this site over an 80-year lifetime. Having determined the acceptable
risk, it is necessary to determine the level of contamination that will generate
this level of risk.
Using a risk factor of 5% Sv
−1 we can easily calculate the lifetime dose
that will produce a risk of 10
−4 ; dividing 10
−4 by 0.05 Sv
−1 shows us that a
lifetime radiation dose of 0.0002 Sv (0.2 mSv) will produce this level of risk.
Over an 80-year lifetime, this is equivalent to 2.5 × 10
−5 Sv yr
−1 or slightly
less than 3 nSv hr −1 since a year has 8760 h.
According the Federal Guidance Report #15 [14] the radiation exposure
from Cs-137 contamination is 7.85 × 10
−18 Sv s
−1 (or 2.83 × 10
−14 Sv hr
−1 ) for
every Bq m
−2 of contamination. Using this conversion factor we can calculate
that, to produce a dose rate of 3 nSv hr
−1 requires a surface contamination