Friends,
I promised to let you have a detailed evaluation of the toxicity of depleted uranium (DU) along with contributions toward toxicity from other contaminants like U-236, Pu-239 and Np-237. For comparison purposes, the Radiological toxicity has also been evaluated. Here it is.
I hope to finish my report on DU in the near future. It will contain a detailed rationale for the evaluation of dose based on realistic figures for biological half life. If the biological half life is too long -- about a hundred years, we will not detect DU in urine specimens. This is a bad news for those who wish to use it for evaluating quantitatively DU in one's body.
Re:- The Chemical and Radiological Toxicity of DU.
Depleted uranium has the same chemical behaviour as the natural uranium or the enriched uranium. The chemical toxicity is identical for the three types. In fact, chemical properties of an element or its chemical toxicity do not alter with changes in isotopic composition of the element. Chemical toxicity of U-238 is no different from U-236 or U-235 or U-234. However, each isotope differs in its radiological hazards.
Re:- Radiological Toxicity of DU.
The radiological hazard is quantified by the amount of energy dissipated during the residence of the radioisotope in a certain quantity of tissues and the type of radiation emitted by the radioisotope. The amount of energy dissipated in the tissues depends on the quantity of the radioisotope and its specific activity (number of particles or rays emitted per second by a gram of the radioisotope). This type of calculation may appear to be very complex to you but I believe, it is not if you can follow the example of each isotope of uranium. You will be able to judge for yourself the radiological toxicity of each isotope of uranium.
4.18 Million-electron-volt (MeV) alpha particles emitted by 1 gram of U-238 = number of nuclei or atoms of U-238 in 1 gram of U-238*0.693/half life of U-238.
Number of atoms in 1 gram of U-238 = 1gram*Avogadro's number/atomic weight of U-238 = 1*6.023E23/238 = 2.53E21.
* sign for multiplication, / sign for division.
Half life of U-238 = 4.46E09years*365.25days/year*24hours/day*3600sec/hour = 1.41E17 seconds.
Rate of emission of alpha particles from 1 gram of U-238 = 2.53E21*0.693/1.41E17 = 12,435 particles per second. 1 MeV = 1.602E-13 Joule.
Rate of energy deposition from alpha particles from 1 gram of U-238 = 4.18MeV/particle*1.602E-13Joule/MeV*12,435particles/sec = 8.33E-09 Joule per second.
or
= 8.33E-09Joule/sec.*3600sec/hour*24hours/day = 7.20E-04 Joule per day.
If 1 gram of U-238 is dipersed in the lungs that have one kilogram of tissues
(average weight of the human lungs), the dose rate deposited in the lungs
= 7.20E-04 Gray per day [Gray is a unit of radiation absorbed dose (RAD).
It is the deposition of 1 Joule of radiation energy in one kilogram of
tissues]. The quality factor of alpha particle = 20 (It is a relative damage
by the type of radiation with beta or gamma radiation = 1).
One
needs to multiply by the Quality Factor with radiation dose in Gray to
deduce radiation insult in Sievert (a unit for assessing harm to humans).
Radiation equivalent man (REM) or radiation insult from 1 gram of U-238 = 7.20E-04 Gy/day*20 = 0.0144 Sv./day.
If the clearance rate is in accordance with the biological half life, total integrated dose deposited in the lungs = Dose rate*the biological half life in days/0.693. The International Commission on Radiological Protection specifies for insoluble compounds of uranium as 500 days and for soluble compounds about a day. The biological half life of ceramic type of uranium dioxide is much longer than 500 days. However, for the present we shall evaluate cumulatve dose by taking the biological half life as 500 days.
For insoluble compounds of uranium (example uranium tetrafluoride or uranium dioxide), the integrated dose = 500*0.0144/0.693 = 10.39 Sv.
For soluble compounds, the integrated dose = 1day*0.014/0.693 = 0.02 Sv. It may be recalled that the biological half life for soluble compounds is about 1 day.
We can convert organ dose (in the present case the lungs) into whole body dose = 10.39 Sv.*0.12 = 1.2 Sv.
There is a lung model that takes into account the tissue sensitivities. According to that model the organ dose (lungs) for insoluble U-238 compounds is 3 Sv.
By following this procedure, we can evaluate the radiation dose for i gram of each of other radioisotopes of uranium. We need to know the physical half life of the radioisotopes and the energy of alpha particles or beta particles emitted by each radioisotope. This information is readily available, for example, in the 'Handbook of Chemistry and Physics' and is presented below:-
Isotope Half life Average energy of alpha or beta particles#
U-234
2.45E05 years 4.76-MeV
alpha particle
U-235
7.04E08 years ~4.4-MeV
alpha particle
U-236
2.34E07 years ~4.48-MeV
alpha particle
U-239
23 minutes
E(max)1.265-MeV beta particle, E(av)
0.43
MeV
#Several
groups of alpha particles are emitted to different energy levels.
The
average energy of alpha particles is computed from the decay data.
Specific activity and dose rate of uranium isotopes.
Isotope
Specific activity Dose rate, Gy per
day Total dose, Sv.
U-234
2.307E08 particles/sec 15.2 Gy/day
212,800 Sv.
U-235
79,950 part./sec. 4.87E-03 Gy/day
68 Sv.
U-236
2.395E06 part./sec. 0.15 Gy/day
2,079 Sv.
U-239
1.27E18 part./sec. 85,000Gy/sec
1.65E08 Sv.
In the lung model, tissue sensitivities are taken into consideration which I believe reduces the dose for the above energy of particles by a factor of almost 3. Epithelial tissues in the aveoli are not as sensitive as the other tissues. Part of the energy of alpha particles is deposited in the epithelial tissues.
For the evaluation of radiation dose for the soluble compounds, the biological half life is about a day. However, it is necessary to consider the pathways of uranium inside the body. The biological half life for the clearance of uranium from bones is 300 days. Only a very small fraction of ingested uranium is retained in the vertebral mass. However, one can evaluate the total dose deposited by the soluble compounds of uranium by multiplying with the biological half life of one day.
Radiation dose from soluble compounds having 1 gram of uranium isotope.
Isotope
Dose, Gy/day
Dose, Sv
U-234
15.2
420 Sv.
U-235
0.00487
0.14 Sv.
U-236
0.15
4.2 Sv.
U-238
7.2E-04
0.02 Sv.
It will be seen that neither the natural uranium nor the depleted uranium soluble compounds present radiological hazard.
Radiation Dose from soluble compounds of various types of uranium.
Basis:- 1 gram
Isotope Abundance %
Dose Sv./gram*abundance
Natural
uranium U-234 0.0055
0.000055*420 = 0.02 Sv
U-235 0.725
0.00725*0.14 = 0.001 Sv.
U-236 0.0
0.00
U-238 99.27
0.99275*0.02 = 0.02
Total dose from 1 gram = 0.041 Sv.
(Ingestion annual limit on intake (ALI) of the soluble type is 30 grams as deduced from the Radiological hazard only. Intake of this amount is far in excess of the amount set for uranium from chemical toxicity considerations.)
Depleted
uranium$ U-234 0.0008
0.000008*420 = 3.36E-03
U-235 0.20
0.002*0.14 = 2.8E-04
U-236 0.003
0.00003*4.2 = 1.26E-04
U-238 99.8
0.998*0.02 = 0.02
Total dose from 1 gram of DU = 0.0238 Sv.
$Depleted uranium is a generic term for elemental uranium that contains less than 0.7205 per cent U-235 and U-238 more than 99.275 per cent. DU with the above isotopic composition was used in the Gulf conflict.
Enriched
Uranium& U-234 0.90
0.009*420 = 3.78
U-235 91.00
0.91*0.14 = 0.13
u-238 8.1
0.081*0.02 = 1.62E-0
Total dose from soluble compounds that contain 1 gram of enriched uranium of the above isotopic composition = 3.91 Sv.
&Enriched
uranium is also a generic term. Any elemental uranium that contains more
than 0.7205 per cent U-235 or less than 99.275 per cent U-238 is called
enriched uranium. IT IS CONSIDERED AS A RADIOLOGIC HAZARD.
Radiation dose (organ) from insoluble inhaled compounds
that contain 1 gram of various types of uranium.
Basis:-
Biological half life = 500 days.
Type
Isotope Abundance %
Integrated dose, Sievert
Natural
uranium U-234
0.0055
212,800*0.000055 = 11.7
U-235 0.725
68*0.00725 = 0.49
U-236 0.000
= 0.00
U-238 99.275
10.4*0.99725 =
10.32
Total integrated organ (lungs) dose with clearance rate for inhaled insoluble
compounds compounds having 1 gram of natural uranium, given by the biological
half life = 500 days = 22.51 Sv. Multiply by 0.12 to get whole body dose.
Depleted
uranium U-234
0.0008
212,800*0.000008 = 1.70
(deployed
during U-235
0.200
68*0.002 =
0.14
the
Gulf conflict. U-236
0.003
2079*0.00003 = 0.06
U-238 99.8
10.4*0.998 =10.37
Total integrated organ dose from insoluble DU compounds with 1 gram of DU =12.27 Sv. It can be seen that the presence of U-236 hardly alters the total integrated radiation dose. Presence of a small amount of U-236 (30 parts per million) does not contribute towards either the radiological or the chemical toxicity of DU. It is interesting to note that the presence of trans-uranics [plutonium-239 with 100 parts per billion (ppb) in DU and 100 ppb of neptunium-237 in DU] does not significantly alters the total integrated dose either.
Physical half life of Pu-239 = 24,110 years.
Average
energy of alpha particles = `5.14 MeV (sum of abundance*energy of
particle
decaying to a discrete level).
Physical
half life of Np-237 = 2.14E06 years
Average
energy of alpha particles = 4.8 MeV.
Integrated dose deposited by 100 ppb of Pu-239 = 0.19 Sv. (Activity, A~229 Bq.). An Army report indicates that Pu-239 found in DU is less than 3 pCi or 0.1 Bq/gram of DU.
Integrated dose deposited by 100 ppb of Np-237 = 0.0024 Sv.(A = 2.59 Bq.). An army report indicates that the conc. of Np-237 is less than 4 pCi or 0.14 Bq/gram of DU.
Addition of 0.2 Sv to 12.27 Sv (calculated earlier by excluding Pu and Np) does increase the radiological toxicity of DU (about 2 per cent) in a significant way. Drawing attention to undue hazard attributed to U-236 and to the transuranics simply diverts attention from the main issue, i.e., the toxicity of DU itself. A typical set of data on transuranics in DU is presented for your perusal.
TABLE 1
Highest Concentration by Nuclide
Nuclide
Lowest value (pCi/g of DU armour) Highest Value (pCi/g
of DU)
Activity +/- 1 sigma
Activity +/- 1 sigma
Am-241
-0.80 +/- 1.3
4.4+/- 5.5
Np-237
<1.3 +/- NA
3.7+/- 0.92
Pu-239/240
-1.2 +/- 1.9
2.7 +/- 0.88
You can see that 100 ppb is much higher amount for Pu-239 and Np-237 than the amount stated above for the nuclides in the report.
Enriched
uranium U-234
0.9
212,800*0.009 = 1915.2
Bomb
grade U-235
91.0
68*0.91 = 61.9
U-236 0.0
00 = 0.0
U-238 8.1
10.4*0.081= 0.84
Total integrated organ dose from insoluble compounds of one gram of enriched uranium = 1977.94 Sv. It can be seen that the presence of U-234 enhances the radiation dose by more than thirty times.
If the biological half life is longer than 500 days, the radiation dose will increase accordingly. For example, if we find the biological half life of ceramic type of DU dioxide is 10 years, the integrated radiation dose will be about 80 Sv from a gram of DU in the lungs. If we scale down by a factor of three, it will still be 27 Sv. or about 3 Sv. as the whole body dose. Presence of DU dioxide has long-range health consequences. I am still investigating it in some more detail. It is known that it translocates itself into lymph nodes in alomost ten times the concentration found in the lungs. I understand that the presence of DU in lymph nodes may have some effect on the immune system.
There are large numbers of animal studies on radiation effects from ingestion of plutonium dioxide. Ceramic uranium dioxide is not very different from plutonium dioxide. The energy of alpha particles is about 1 MeV higher from plutonium dioxide. It appears that DU dioxide is likely to be much more toxic than it was previously believed. In my next e-mail, I shall deal with this aspect. I feel that you have had a good dose of this material in this e-mail.
As usual, I should like to have scientific comments from you. I beseech you that one must not attribute health effects without looking into it deeply. Such statements without presenting scientific backgound to suppert them, may not help the cause of veterans.
Hari Sharma