Re: Nd-144

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

Ted:

        In addition to Nd-144, there are a number of radionuclides that, while admittedly radioactive, have half-lives longer than the estimated age of the Earth (4.5 to 4.6 billion years old).  As a result, these nuclides are of such low SPECIFIC activity as to be of negligible regulatory concern, from the standpoint of radiation safety.    In other words, owing to their low rate of radiation emission, they pose little hazard owing to their radioactivity.  A partial listing of these radionuclides follows.  The listed specific activity is for 100% pure, elemental nuclide, free of any non-radioactive constituents, such as oxide, nitrate, etc. "RQ" = EPA-specified reportable quantity (40 CFR 302).  U-238 is included in the list below for purposes of comparison, since its half-life approximates that of the age of the Earth:

NUCLIDE HALF-LIFE (years)       Specific Activity (Ci/gram)     Weight of 10 mCi (lbs)  RQ (Ci)

Rb-87   4.75 E+10               8.651E--8                       255                     10
        
Cd-113  9.3E+15         3.402E--13                      64,820,000              0.1     

In-115  4.41E+14                7.049E--12                      3,128,000               0.1

Te-123  1E+13                   2.907E--10                      75,860                  10

La-138  1.05E+11                2.467E--8                       894                     1

Nd-144  2.29E+15                1.084E--12                      20,340,000              None Listed

Sm-147  1.06E+11                2.294E--8                       961                     0.01

Gd-152  1.08E+14                2.178E--11                      1,013,000               0.001

Lu-176  3.78E+10                5.374E--8                       410                     1       

Ta-180  1.2E+15         1.655E--12                      13,320,000              100

Re-187  4.35E+10                4.395E--8                       502                     1000

Th-232  1.405E+10               1.097E--7                       201                     0.001


U-238   4.468E+9                3.362E--7                       66                      0.1     


        I presume you based your question regarding Nd-144 being unregulated based on the fact that no numerical RQ value was ever specified by the EPA for this nuclide.  My personal conjecture is that the only reason the EPA did not specify an RQ value for Gd-144 is, at the time the RQ values were compiled (1989), some Table of Isotope books and some nuclide charts did not list a half-life for Nd-144; hence, the EPA staff might have concluded that this isotope was stable, no need to compile an RQ value for a non-radioactive substance, etc.

        It is my personal opinion also that us HP types are so schooled/used to thinking about nuclides in terms of their ACTIVITY, that we mentally tend to ignore the MASS required to produce a given amount of activity.  For 99.9% of the radioactive nuclides, the mass associated with a given activity is indeed negligible.  But for the above listing of very low specific activity nuclides (minus U-238), ignoring the tremendous mass that must be assembled in order to produce even a small amount of activity, is inappropriate.  Indeed, the EPA appears to have failed to consider the tremendous mass associated with a single RQ for the above nuclides (minus U-238).  For example, from the above table, one can compute that 1 RQ of Te-123 will weigh at least 75,860,000 lbs!  As a matter of practicality, it is inconceiveable that a single RQ quantity will ever be assembled in one place anywhere in the world--the biggest steam locomotive would have difficulty pulling that amount of weight.  Therefore, since it is practically physically impossible ever to assemble a single RQ of Te-123, why ever specify an RQ value to begin with?  There is simply no regulatory utility or "value added" in doing so.     

        Just my two cents worth.  Best regards  David



At 02:52 PM 12/1/1998 -0600, you wrote:
>Is there any rational for exempting Neodymium-144 (half-life 2.4E15 yr)
>from rad safety concern?
>
>
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DAVID W. LEE
Los Alamos National Laboratory
Radiation Protection Services, ESH-12
PO Box 1663, MS K483
Los Alamos, NM  87545
PH:   (505) 667-8085
FAX:  (505) 667-9726
lee_david_w@lanl.gov