Values for Exemption of Sealed Radioactive Sources

[David W Lee <lee_david_w@lanl.gov>]

QUESTION POSED by Kathleen McIntyre, Brookhaven National Laboratory:  Does
ANYONE know how the exemption values for sealed sources were derived for DOE
N 441.1, Attachment 1?  What is the technical basis for the values?!  We're
trying to construct a table to list isotopes not specified in the
attachment...unfortunately, our numbers don't jive when comparing our
calculated values to those of 441.1.

DETAILED RESPONSE FOLLOWS:

        a.  Circa the 1990 time frame, what is now called EH-52 at DOE HQ
contracted with Oak Ridge for ORNL to scientifically generate some "Derived
values for exemption of sealed sources from inventory."  Once generated,
EH-52 would attach this table to its sealed radioactive source
accountability guidance (DOE N 5400.9 (Dec 91)which has since evolved
basically unchanged into the present DOE N 441.1) which was then be pushed
out to the DOE complex for implementation.  The concept behind having a
radioactive sealed source (RSS) accountability table was that for RSSs ABOVE
the accountability activity threshold level, rigorous controls would be
applied to such RSSs primarily in the form of periodic physical (eyeball)
inventories (semiannually), periodic leak testing (semiannually), and
detailed RSS labeling.  Below the accountability threshold activity value,
far less rigorous controls could be locally developed and applied, such as
merely having the group owning the source sign off on a sheet of paper once
a year that they still had the RSS, etc., no leak testing required.

        b.  Much of the work associated with compiling this radioactive
sealed source (RSS) accountability table appears to have been done by George
Kerr and Keith Eckerman at ORNL.  A short summary of Kerr and Eckerman's
(K&E) work can be found in Health Physics, Vol 59, No. 6 (December 1990, pp
931-934.)  Quite unfortunately, however, K&E never explicitly specified the
equations they used to calculate the numbers in their original "derived
values for exemption of sealed sources from inventory" table, thus rendering
obscure the calculation of similar exemption values for isotopes that K&E
did not originally list.

        c.  K&E's HPJ paper implies that they calculated exemption
(accountability) values for all non-gaseous, non-liquid radionuclides of
half-lives greater than 30 days.  This is not the case, however, as the
following radionuclides are missing from their original table:

        Nb-91 (680 years); Nb-91m (60.86 days); Nb-92 (3.47 E+7 years);
        Cd-115m (44.6 days); Te-168 (93.1 days); Re-183 (70.0 days);
        Bi-208 (3.68 E+5 years); Po-208 (2.898 years); Po-209 (102 years);
        Es-252 (471.7 days); Es-255 (39.8 days)

        d.  Radsafers should contact George Kerr at ORNL to find out exactly
what equations he and Keith Eckerman used to calculate the exemption values
of K&E's original table (I don't know specifically what equations they used,
and like most radsafers lament the fact that K&E never explicitly specified
same in their 1990 HPJ paper).

        e.  For those radsafers who wish to locally calculate their own RSS
accountability threshold values using the general methodology suggested by
K&E, then it should be noted that K&E's general methodology involved the
calculation and comparison of TWO activities, the LESSER of which was chosen
as the actual accountable threshold value.  Specifically, the amount of
activity required to impart a specified EXTERNAL dose equivalent was
calculated over a chosen exposure time and exposure distance (external dose
scenario), and the amount of activity required to impart a specified
INTERNAL committed effective dose equivalent was calculated using a chosen
net intake factor and a dose conversion factor (internal dose scenario).
The most limiting of these two scenario activity values is the
"accountability" or "exemption from inventory" value.  In the case of K&E,
whenever the external and internal dose scenario activity values for a
particular radionuclide BOTH exceeded 1000 microcuries, K&E chose to impose,
arbitrarily, a maximum activity cap of 1000 microcuries.  Thus, in K&E's
original exemption table, all RSSs are "accountable" above 1000 microcuries
each.  COMMENT:  there is no dose-based reason for the imposition of any
arbitrary maximum activity cap; whatever the most limiting of the activity
values of the external and internal dose scenario calculation turn out to
be, these values should then be allowed to fall where they may and not be
arbitrarily capped.

        f.  The accountable/exemption from inventory activity value in the
external dose scenario can be calculated from the following equation:

        A = (numerator of K times(D) times (d-squared)divided by a
denominator of (gamma) times (T) and then this quotient should be multiplied
by the following quotient which takes into account the radioactive decay of
the RSS over the course of the chosen decay period.  This quotient is:
(numerator of (lambda) times t divided by a denominator of (1 - e to the
minus lambda (t))

In order for A to be in microcuries, then 

K = constant of proportionality to yield A in microcuries which then is
        K = 1 E+6 Bq/MBq divided by the product of 37,000 Bq/microcurie
        and 100 mrem per mSv)

D = External dose equivalent chosen for the scenario in mrem

d = Exposure distance chosen for the scenario in meters

Gamma = specific gamma ray constant (from Shleien - new Rad Hlth Handbook) in
        units of (mSv m-squared per hour per MBq)

T = Exposure time chosen for the scenario over which Reference Man is externally
        exposed to penetrating radiation from an RSS in units of hours

t = scenario-dependent radionuclide decay time in units of hours

lambda = RSS radionuclide decay constant in units of inverse hours.

        g.  The internal dose scenario accountable activity threshold may be
calculated by the following equation:

                A (microcuries) = D divided by the product of (NIF) and
(DCF) and (k) where:

        D = internal committed effective dose equivalent in mrem chosen for
                the scenario

        NIF = Net Intake Factor (explained below)

        DCF = Effective Inhalation Dose Conversion Factor extracted from
                EPA Report No. 11, compile by Eckerman et al 1988

        k = constant of proportionality to yield A in microcuries
                (100 rem/Sv) (1000 mrem/rem) (37,000 Bq/microcurie) = 3.7 E+9

        h.  Thus, one may locally calculate one's own RSS accountability
threshold values by calculating the A-external dose scenario activity and
the A-internal dose scenario activity, and then taking the most limiting of
these two activity values.

        i.  The NIF is the product of the fraction of activity assumed to
escape from the RSS capsule (Release Fraction) and the fraction of th
escaped amount that is assumed to be inhaled by Reference Man (Uptake
Factor).  IAEA Report No. 7 states that release fractions typically range
from 1 E-2 to 1 E-3 and uptake factors range from 1 E-3 to 1 E-4 which
results in NIFs in the range of 1 E-5 to 1E-7.

        (Release Fraction)     (Uptake Factor)  =  (Net Intake Factor)
          1 E-2 to 1 E-3        (1 E-3 to 1 E-4)        (1 E-5 to 1E-7)

Franke documented NIF's in the range of 1 E-6 to 1 E-10, and Brodsky has
found that NIFs are typically much less than 1 E-6 (Health Physics Vol 39,
pp 992--1000, 1980)

        j.  In some instances, Eckerman et al in EPA Report No. 11 compiled
effective inhalation DCFs for the various lung clearance classes (D, W, or
Y).  For those radionuclides where D, W, or Y values are listed, it is best
to select the LARGEST of these values, because since the DCF appears in the
denominator of the internal dose scenario equation, selecting the largest
DCF results in the smallest (most conservative) internal dose scenario
accountable threshold activity value.

        k.  In the case of K&E, they assumed that an RSS was misplaced in
the controlled work environment where perhaps the RSS ended up 1 meter away
from some radiation worker's gonads as the worker worked at his desk 8
hours/day.  Over the course of 1 calendar year, then this hypothetical
person (Reference Man) would have a total exposure time of 8 hr/day X 250
work days/year = 2000 hours exposure time = T.  The source would constantly
decay over the course of the year, i.e. decay time t = 8760 hours.  K&E
chose a scenario limiting dose (D) of 10 mrem for both the internal and
external dose scenarios.  It appears, however, that in the internal dose
scenario calculation, K&E appear to have used a NIF value of 1 E-2.  Such a
value is appropriate as a Release Fraction, but it is very far from any
known NIF reality, as discussed above, where NIFs basically are no higher
than 1 E-5.  Use of a NIF as large as 1 E-2 is not scientifically defensible
or technically justifiable.

        l.  At any rate, if you use K&E's scenario parameter values,
specified above, and remember that K&E arbitrarily imposed a maximum
activity cap of 1000 microcuries, you will be able to come close to the
values listed in their original exemption table.  One must note however,
than after K&E calculated the internal and external dose scenario activities
and selected the most limiting of these two values, that K&E reduced this
most limiting value further by rather crude decade down-rounding.  Thus, if
the most limiting of the two scenario values was say 99 microcuries, since
99 microcuries in NOT greater than 100 microcuries, K&E would have listed
this radionuclide's accountability activity threshold value in the "less
than 10 microcuries" category of their original table.

        m.  As Paul Harvey says, "now, for the rest of the story."  When K&E
provided their original exemption table to EH-52, EH-52 decided that K&E's
RSS accountability threshold activity values were not LOW enough.  So EH-52
chose to arbitrarily modify K&E's values in a manner never intended or
proposed by K&E.  The way that EH-52 did this was to in effect convert K&E's
original WORK PLACE-based internal and external dose scenarios involving a
trained radiation worker in a controlled WORK PLACE environment into an
ACCIDENT/INCIDENT scenario involving a member of the public where an RSS was
magically assumed to find its way out of a laboratory and into the home of
Reference Man where it then located itself under Reference Man's bed 1 meter
away.  Reference Man was then further presumed to spend 100% of his time
continuously in bed for an entire calendar year--continuously exposing
himself to the RSS.  EH-52 did this without any rational consideration of
the fact that the dose limit (D) in an accident/incident scenario logically
should be greater than in a non-accident/incident (WORK PLACE) scenario, and
the exposure time (T) in an accident/incident scenario should be LESS (since
accidents/incidents do not normally take place over an extended length of
time) than in a controlled WORK PLACE scenario.  Note that in the external
dose scenario equation, above, since D is in the numerator and T in the
denominator, assuming an accident/incident scenario where realistically D
would be greater and T would be less for an accident, compared to a WORK
PLACE scenario, the result would be a LARGER calculated value of A.  But
LARGER values of A were precisely what EH-52 did NOT want.  So, EH-52
ignored any consideration of the fact that in an RSS incident involving a
member of the public, D should be larger and T smaller, and instead chose to
LOWER K&E's original values by the ratio of 2000 hrs to 8760 hrs which is
approximately 0.3, thus lowering K&E's original values by another 70% and
lowering K&E's arbitrary activity cap from 1000 microcuries down to 300
microcuries.

        n.  Thus, while DOE's present DOE N 441.1 table values are "based"
on the work of K&E, EH-52 corrupted K&E's original values by arbitrarily
manipulating K&E's values in a non-scientifically defensible or technically
justifiable way.
       
        o.  Another error that EH-52 made was to arbitrarily impose
accountability threshold activity values upon radionuclides that had such
low SPECIFIC ACTIVITY as to be virtually useless as RSSs.  K&E wisely
recognized in their original work that certain nuclides were of such low
specific activity as to be impractical as RSSs; hence, they recommended that
these nuclides should be "unlimited" (have no accountability activity
threshold limit) as RSSs.  These nuclides were: Rb-87, Cd-113, In-115,
Te-123, Gd-152, Ta-180, and Re-187.  K&E overlooked a few others of
low-specific activity such as La-138, Sm-147, and Lu-176, and Th-232.  At
any rate, consideration of low-specific activity was of no concern to EH-52,
EH-52 went ahead and arbitrarily imposed accountability activity threshold
limits on these nuclides seemingly not choosing to recognize their excessive
bulk, weight, and consequent impracticality as RSSs.

        p.  In summary, K&E's original work appears to be technically flawed
in that they appear to have used an NIF value of 1 E-2 which is at least 3
orders of magnitude LARGER than any scientifically known reality and the
imposition of a maximum activity cap has no dose-based justifiable reason.
Added to this was EH-52's arbitrary, non-technically defensible further
diddling of the numbers.

        o.  Any opinions expressed by the author of this reply are not
necessarily those of the University of California or DOE.  This
notwithstanding, if any radsafer has any further questions on this issue, I
will be happy to answer them.