Thursday, November 24, 2011

Accidental Fallout from LANL?

Although contrary to today’s festive spirit, let's think for a bit about possible future releases of radioactive material from LANL, as a result of earthquake and/or wildfire. How much radioactive fallout would be experienced by local residents? Could this radioactive fallout constitute any sort of health hazard?  Could a retired physicist like myself, with no particular knowledge of environmental science, have anything sensible to say about such questions? Well, let’s see!    

 A release of Pu-239 from PF-4, or the still to be built CMRR-NF, could occur sometime in the future. In the event of a major earthquake (>7 on the Richter scale) followed by fire, there might be a release from either of these facilities of Pu-239 in dust form, or in the form of fumes from burning Pu-metal. Since many tons of Pu-239 is expected to be stored at PF-4, and/or at the CMRR-NF, one might imagine that a release of >1 ton Of Pu-239 could be possible. [1 ton, or 1000 kg of Pu-239 corresponds to 73,000 PE Ci.]

 Such a possibility was said last week, at a meeting in Santa Fe, NM, by the Defense Nuclear Facility Safety Board Chairman, Dr. Peter Winokur, to be “not far-fetched.” (The DNFSB is a group of scientists and engineers tasked by the US Congress to provide advice on safety practices, at US nuclear weapons facilities, to the US President and to the DOE Secretary.)

 A wildfire burning over Area-G might have serious consequences. Currently, there are 10,000 barrels of TRU-waste stored above ground at Area-G, and these are only poorly protected against the effects of  wildfire. Each barrel contains ~10 PE Ci of actinide residue, mainly Pu. If these barrels were to burst during a wildfire, ~100,000 PE Ci could go up into the air. (At the DNFSB meeting in Santa Fe last week, DOE/NNSA manager Anderson testified that the las Conchas wildfire  "was very scary" and that "if the wind hadn't shifted when it did, LANL might have been consumed by fire.")

 Thus, there are at least two different scenarios that might result in the release of ~100,000 PE Ci into the air over LANL.

 If a bad accident at LANL, accompanied by a hot fire, were to release 100,000 Ci of Pu-239 into the atmosphere, and if the ensuing fallout exhibited circular symmetry with an activity decreasing exponentially with distance r from the source, over a characteristic distance R (that is, if r=R, then the concentration at r would be  1/e = 0.37 of its value at the source, according to the law C(r) = C(0) * exp( -r/R), where C(r) is the activity per unit area, at r, in units of Ci per square meter, or Ci/m**2) then the activity of Pu-239 deposited on the ground at a distance r, and for R = 1.0 mile, would be:

                        Table 1

         r (miles)              C(r)  (Ci/m**2)
         --------            --------------------

          0.01                     30.7
          0.10                     0.307
          1.00                     0.00113                     

          5.00                     8.28 * 10**(-7)

          10.0                     1.39 * 10**(-9)

          20.0                     1.59 * 10**(-14)

If, instead, R = 5.0 mile, then:

                       Table 2

         r (miles)             C(r) (Ci/m**2)
         --------            --------------------

          0.01                     30.7
          0.10                     0.307
          1.00                     0.00307                     

          5.00                     4.52 * 10**(-5)

          10.0                     4.16 * 10**(-6)

          20.0                     1.41 * 10**(-7)

 For a small fire, one would expect fallout to be concentrated over and around the facility which was burning, with the density of fallout and, therefore, of activity, both areal and volumetric, decreasing with distance from the facility; i.e., as described by the numbers in Table 1.  For a hotter and more extensive fire, Table 2 might instead pertain. But, if the fire were both very hot and very extensive, such that the hot gases being generated rose up into the stratosphere, then the fallout would be distributed over larger distances than those indicated in the Tables, and the fallout pattern would  elongate along the direction of the prevailing winds. In such a case, both Tables would need to be modified. (The Fukushima-Daiichi disaster exhibited a pattern of fallout that extended in the northwest direction over a range ~10 times that of its width. Since the damaged nuclear reactors were located on the eastern seacoast, strong prevailing sea breezes blew the fallout away from the coast and towards the northwest. In May, 2000, smoke from the Cerro Grande wildfire extended from its source in the Jemez Mts., through Los Alamos, and towards the northeast, over the town of Española, and over the pueblos of  Santa Clara and Ohkay Owingeh, again blown by the prevailing winds. The length of the smoke plume was ~3 times its width, and it extended all the way into Oklahoma.)

 But, to continue:

 Let's estimate the amount of Pu-239 that would be deposited in the lungs of a local resident over the ~1 year's time that it would take for all of the fallout to come to earth. Let's assume that the amount of Pu-239 suspended in the air, during that 1 year's time, remains at a constant value and is distributed evenly in a vertical air column up to an altitude of 1000 m. (One could object to this number since, as we will see, it plays a critical role in our calculation; however, as a rough value I think that it may not be so bad.) The volumetric activity of Pu-239, as a function of r, would then be given by the numerical values in the above Tables, reduced by a factor of 1000 (and in units of Ci/m**3). Then, since the volume of air exchanged per breath by the human lungs is ~0.5 liter, or 0.0005 m**3, and the number of breaths taken per year is ~2 x 10**6, the volume of air exchanged in 1 year would be ~1,000 m**3. If we assume that all of the inhaled Pu-239 is deposited in the lungs, and remains in the lungs, then the activity of the Pu-239 concentrated in the lungs in 1 year, as a function of r, is given by the numerical results appearing in the above Tables (but, now in units of Ci).

 A LANL compendium of radiation effects published in June, 2000 (Los Alamos Radiation Monitoring Notebook, by J. T. Voss) noted that Pu-239 deposited in the lungs of dogs was fatal, within a year, if its activity exceeded ~5 x 10**(-7) Ci/gm (Ci per gm-weight of lung tissue.) Therefore, for adult humans with an average lung mass of 900 gm, inhaled Pu-239 might be lethal for activities > 4 x 10**(-4) Ci (corresponding to a dose > 4000 Rem.) Since single doses < 10 Rem are usually considered to be marginally safe, one could say that for adult humans inhaled Pu-239 would be marginally safe if the activity of all the inhaled material was < 10**(-6) Ci.

 Returning to the Tables, we see that, for R = 1 mile (Table 1), and at a distance of 5 miles from a Pu-239 release of 100,000 Ci into the atmosphere, the amount of Pu-239 concentrated in the lungs of a local resident, within 1 year of the release, does not exceed a marginally safe value. However, at a distance of just 1 mile from the release point the amount accumulated would be lethal. Therefore, for this case, a zone of exclusion extending out to at ~5 miles from the release point would be necessary.

 Similarly, for R = 5 mile (Table 2), and at a distance of 10 miles from a Pu-239 release of 100,000 Ci into the atmosphere, the amount of Pu-239 concentrated in the lungs of each local resident, within 1 year of the release, is marginally safe; but, at a distance of 5 miles the amount accumulated would be unsafe and possibly lethal. In this case, the zone of exclusion would have to extend to ~10 miles.

 Shocking stuff? Well, maybe.

 Clearly, many assumptions have been made in these rough "calculations", some more critical than others. The form of the distribution assumed is very important, as is the value of R chosen, if the distribution were to be exponential. The height of the air column through which the fallout occurs is, obviously, very important; i.e., the actual volumetric density of Pu-239 at ground level is critical. Moreover, there is uncertainty in the amount of Pu-239 which would be inhaled by persons in the vicinity of the release point and the maximum dose of inhaled Pu-239 which can be tolerated by humans is also not very well known.

  But, in spite of these uncertainties, I claim that my rough "calculations" suggest the need for a careful study of these important questions by independent qualified experts. But, perhaps such studies have already been performed? Then I wonder what their results have shown and why they aren’t already available as public information?  Maybe they’re just too shocking?

No comments: