Understanding the risk coming from tritium exposure

Anca Melintescu

“Horia Hulubei” National Institute for Physics and Nuclear Engineering (IFIN-HH)

mercredi 28 novembre 2018

Tritium (3H) is a radioactive isotope of hydrogen that is ubiquitous in environmental and biological systems. Following the debate regarding the human health risk from exposure to tritium, there have been claims that the former bio kinetic model recommended by the International Commission on Radiological Protection (ICRP) may underestimate tritium doses (ICRP, 1989). Recently, ICRP published its new bio kinetic model for tritium (ICRP, 2016), but, there are still some uncertainties associated with OBT metabolism as described by the novel ICRP OBT model, particularly the retention of OBT in children and the effect of organ growth regarding the OBT retention (CNSC, 2010). In a country that develops Canada Deuterium Uranium (CANDU) reactors for production of nuclear energy, like Romania, tritium is the ‘national radionuclide’. Tritium can enter into the human body in many forms, but only tritiated water (HTO) and organically bound tritium (OBT) generate exposures of concern for members of the public.
Four decades ago, tritium was considered having the lowest radiological effectiveness (RBE), the RBE of 1 and the lowest dose conversion coefficients. At the beginning of 1990’s, claims of higher radiological impact started a long dispute between antinuclear groups and the majority of scientists. The high OBT concentrations measured in fish at Cardiff Bay (UK) (Hunt et al., 2009) amplified the confusion. Nowadays, there is a debate regarding the Linear No Threshold (LNT) assumption of the ICRP, affecting the perception regarding the risk coming from tritium exposure. It is demonstrated that the intake of HTO is much less harmful than that of OBT and the present contribution refers mostly to OBT. OBT can be classified as buried, exchangeable, non-exchangeable, biotic, abiotic, soluble or not soluble, volatile or not volatile. A quite recent human bio kinetic model (Galeriu and Melintescu, 2010) considers the age and gender dependence, as well as the OBT deposition in various organs. The model was favourably tested with both HTO and OBT experimental data in urine for humans following the HTO intakes. The model was also favourably tested with OBT data in urine for humans following the OBT intake. The HTO and OBT data in urine were provided by five volunteers who ate food containing fish from Cardiff Bay, and HTO and OBT in their urine were analysed for up to 150 days (Hunt et al., 2009). After a short presentation of the model, the model tests with HTO and OBT experimental data following the OBT intake is discussed in relation with the experimental errors due to the measurement techniques. The model also predicts the dynamics of HTO and OBT in urine and blood (red blood cells and plasma) and potentially offers a new approach for bioassay. The need for a standardised bioassay technique is discussed.
For health effects, it is important to understand the metabolism of each organic form in animal body coming from intakes to organs and cells. These differ between media (aquatic (salt/fresh water) or terrestrial) and depend on animal type (clade). The case of Cardiff Bay is presented. The adaptation of animals to environmental stress involves DNA repair after DNA damage induced by OBT. The arguments that LNT is not useful are discussed. OBT
measurements are needed for monitoring or research and their importance depend on the goal of assessment (aquatic or terrestrial release, decommissioning). The presentation shortly revises the above aspects, in order to open a discussion on analytical quality requirements, analyst - modeller interaction and the need of further work for well designed experiments to clarify open problems.

References
CNSC 2010 Tritium Studies. Health effects, dosimetry and radiological protection of tritium. INFO-0799. Canadian Nuclear Safety Commission (2010), available at http://www.nuclearsafety.gc.ca//pubs_catalogue/uploads/CNSC_Health_Effects_Eng-web.pdf
Galeriu, D., Melintescu, A. (2010) Retention of tritium in reference persons: a metabolic model. Derivation of parameters and application of the model to the general public and to workers. J. Radiol. Protect. 30: 445-468
Hunt, J., Bailey, T., Reese, A. , J. (2009) The human body retention time of environmental organically bound tritium. J. Radiol. Prot. 29: 23-36
ICRP 1989. Age-dependent doses to members of the public from intakes of radionuclides: Part 1. International Commission on Radiological Protection ICRP Publication 56. Ann. ICRP 20 (2) (1989) ICRP 2016. Occupational Intakes of Radionuclides: Part 2. ICRP Publication 134. Ann. ICRP 45(3/4) (2016).