Doses from single particles of Uranium Octoxide

Hazards from alpha irradiation of tracheobronchial lymphatic tissue from inhaled and translocated Depleted Uranium

The Uranium in armour piercing weapons burns on impact, forming insoluble ceramic particles of Uranium Oxide. This provides a radiation exposure route which can be expected to have serious effects on the lymph system (known to be a critical organ for leukaemogenesis1) without Uranium having to be ingested in quantities sufficient to cause heavy metal poisoning.

Particles in the size range 0.1 microns to [say] 5 microns are easily transported in air and can be resuspended. They are small enough to cross the lung wall when inhaled, and are scavenged to the lymph nodes.

We are aware of no measurements of Uranium dose to lymphatic tissue specifically but post mortem analyses2,3 of Plutonium show high concentrations in the tracheo-bronchial lymph nodes (in one subject2, the TBLNs had concentrated the Plutonium to values 40 times higher than the lungs; these tiny pieces of tissue, weighing 16.5 grammes, contained 17.5% of the Plutonium 239 in the entire body).

Chemical and physical similarities between Plutonium and Uranium make this relevant to discussion of DU weapons. We pointed this out at the Mariam Appeal seminar on DU in London 30th July, and in letters to the Guardian (17 Aug.) and the Independent (5th Oct.) There has been no rebuttal.

Our inquiries suggest that the MoD’s monitoring programme for DU in Gulf War veterans will not determine lymph node burdens.

A poorly understood area

The 1996 COMARE report1 on the Seascale leukaemia cluster and studies of Uranium in rats4,5,6 show how poorly understood this area of dosimetry is.

COMARE 4th Report cites NRPB paper R276 as showing that the dose from Sellafield is too low to account for the excess leukaemia. However, R276 models the human lymph system as the sum of virtually every internal organ - a large volume of tissue into which to dilute the radiation dose which is actually being delivered within the lymph nodes. COMARE has recently told us that they were misinformed about this reference; the correct reference is COMARE 95/40: Thoracic Lymph Doses due to Sellafield Discharges and Natural Background Radiation by J. R. Simmonds et al. This has not been published nor even internally reviewed, but we have obtained a copy. It is not based on measurement but on assumptions which include extrapolating doses to people aged 2 - 25 from the lung capacity and air intake of a baby from birth to 1 year of age, although the capacity and intake of the older people are far greater.

We are discussing this and other anomalies with NRPB’s Director of Dose Assessments and will publish a further report. It seems that COMARE may have to revisit the relationship between Sellafield discharges and the Seascale leukaemia cluster.

Similarly, health effects in people exposed to ceramic forms of Uranium cannot be dismissed without adequate consideration of local doses.

Studies of Uranium in rat lungs4,5,6 conducted to inform radiation protection agencies about risks to occupationally exposed workers show that insoluble forms (especially U3O8) are poorly transportable to blood and are retained in the lung to a far greater extent than soluble forms. They acknowledge that Uranium accumulates in the lymph nodes to an extent sufficient to complicate external monitoring of the chest but fail to distinguish between retention in lung and retention in the associated lymph nodes.

Table of doses to sphere of tissue 30 micron radius
by one particle of U3O8 of various diameters 7
Particle diameter microns Particle volume cm3 Mass U308(g) Mass U238(g) Activity of particle (Bq) Hits /day(dose mSv) Hits/year(annual dose mSv)
0.2 4.2 x 10-15 3.6 x 10-14 3.06 x 10-14 3.8 x 10-10 3.3 x 10-5
(3.96 x 10-3mSv) 		0.012
(1.44mSv)
0.5 6.5 x 10-14 5.6 x 10-13 4.8 x 10-13 5.9 x 10-9 5.1 x 10-4
(0.06mSv) 		0.186
(21.9mSv)
1 5.2 x 10-13 4.3 x 10-12 3.7 x 10-12 8.8 x 10-8 7.6 x 10-3
(0.91mSv) 		2.77
(332mSv)
2 4 x 10-12 3.5 x 10-11 2.9 x 10-11 3.6 x 10-7 0.031
(3.72mSv)		 11.32
(1358mSv)
5 6.5 x 10-11 5.6 x 10-10 4.75 x 10-10 5.9 x 10-6 0.51
(60mSv) 		186
(21900mSv)

The table shows that for particles as small as 0.2 microns diameter, average annual alpha dose to the lymphatic tissue surrounding the particles is about the same as the total average natural background dose of 2mSv. (It is, of course, additional to NBR) For larger particles the local dose rapidly increases.

Particle sizes from 0.1 to 5 microns are frequent in the environment. The dangerous size range for genetic mutation is between 0.5 and 5 microns since "Second Event" processes will occur for particles of this size. Hit density per day is such that cells damaged but not killed by radiation have an enhanced probability of being hit again within the ten hour period during which they undergo the repair and replication process. This hazard from inhaled particles is suggested in a letter from Dr Chris Busby which has been accepted by the International Journal of Radiation Biology.

Status of the "Second Event" theory

The National Radiological Protection Board originally denied that the "Second Event" effect exists, but Drs Cox and Edwards of NRPB have now submitted their own version of Busby’s calculations to IJRB, showing that for the example of Strontium-90 the Second Event hazard is greater than from Natural Background Radiation. Publication of their Commentary on the Second Event Theory of Busby8 is expected in the January 2000 edition, together with Busby’s reply. While not accepting the detail of their calculations, Busby points out that there are other sources of local dose which present even greater hazard than Sr-90 - e.g. Tellurium-132 and hot particles. Thus the debate has shifted from the existence of additional hazard from man-made isotopes and anthropogenic Uranium to its degree.

Recommendations

NRPB, MoD, and DoH should be pressed to take account of local doses from ceramic Uranium;

MoD should set up a lifetime study of Gulf War veterans and their children, including measurement of the Uranium content of veterans’ tracheo bronchial lymph nodes.

Notes and references

1 COMARE 4th Report
2 McKinroy, J. F. Kathren, R.L. Voelz, G.L. Swint, M. J. 1991 U. S Transuranium Registry Report on the 239 Pu distribution in a human body Health Physics, Vol. 60 No. 3 307-333 March 1991 [data is on www.llrc.org/lymphnodes.htm]
3 Popplewell DS Ham GJ et al.1985 Plutonium in autopsy tissue in Great Britain Health Physics 49 304 [data is on www.llrc.org/rat319.htm] NOTE: later published versions of this research had the TBLN data removed - e.g. Popplewell DS Ham GJ Dodd NJ Shuttler SD Plutonium and Cs-137 in Autopsy tissues in Great Britain Science of the Total Environment 70 (1988) 321 - 334.
4Stradling G N, Stather J W, Ellender M, Sumner S A, Moody J C, Towndrow C G, Hodgson A, Sedgwick D, Cooke N; Metabolism of an Industrial Uranium Trioxide Dust after Deposition in the Rat Lung Human Toxicol. (1985) 4, 563-572
5 Stradling G N, Stather J W, Gray S A, Moody J C, Ellender M et al; Metabolism of Uranium in the Rat after Inhalation of Two Industrial Forms of Ore Concentrate: the Implications for Occupational Exposure Human Toxicol. (1987) 6, 385-393
6 Stradling G N, Stather J W, Gray S A, Moody J C, Ellender M, Hodgson A, Cooke N; The Metabolism of Ceramic and Non-ceramic Forms of Uranium Dioxide after Deposition in the Rat Lung Human Toxicol. (1988) 7, 133-139
7 Assumptions: Uranium (U-238) Oxide is in the U3O8 form (density = 8.6); Specific Activity of U238 = 12.43 MBq/kg Alpha Decay Energy = 4.45MeV; Alpha Range = 30 microns; Relative Biological Effectiveness of alphas of 20 (ICRP) have been used to convert dose in Grays to effective dose in Sieverts; 1 micron or micrometre is 10-6 metre.
8 This is a reworking of Busby's calculations which can be seen at www.llrc.org/secevnew.htm

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