Tritium in the Environment

Radiobiological effects of tritiated water short-term exposure on V79 clonogenic cell survival

PURPOSE

We set out to improve the accuracy of absorbed dose calculations for in vitro measurements of the relative biological effectiveness (RBE) of tritiated water (HTO) for the clonogenic cell survival assay, also considering the influence of the end-of-track linear energy transfer (LET) of low-energy electrons.

MATERIALS AND METHODS

The COmputation Of Local Electron Release (COOLER) program was adopted to investigate the cell geometry and the tritium full beta-decay spectrum impact on the S-values and subsequently on the RBE of HTO for clonogenic cell survival at similar high dose rates (HDR).

RESULTS

S-values for cells growing in suspension are usually comparable to those for adherent cells. RBEs calculated at the 10% survival fraction through the use of the average energy are almost similar to those obtained with the beta-spectrum. For adherent cells, an RBE of 1.6 was found when HTO cell survival curves were compared to acute γ-ray exposures. Irrespective of the geometrical configuration, the RBE was 2.0 when the comparison was made with similar dose rates.

CONCLUSIONS

These results underline the importance of irradiating at equal dose rates and cell culture conditions when measuring in vitro RBE-values.

Estimation of Biological Effects of Tritium

Nuclear fusion technology is expected to create new energy in the future. However, nuclear fusion requires a large amount of tritium as a fuel, leading to concern about the exposure of radiation workers to tritium beta radiation. Furthermore, countermeasures for tritium-polluted water produced in decommissioning of the reactor at Fukushima Daiichi Nuclear Power Station may potentially cause health problems in radiation workers. Although, internal exposure to tritium at a low dose/low dose rate can be assumed, biological effect of tritium exposure is not negligible, because tritiated water (HTO) intake to the body via the mouth/inhalation/skin would lead to homogeneous distribution throughout the whole body. Furthermore, organically-bound tritium (OBT) stays in the body as parts of the molecules that comprise living organisms resulting in long-term exposure, and the chemical form of tritium should be considered. To evaluate the biological effect of tritium, the effect should be compared with that of other radiation types. Many studies have examined the relative biological effectiveness (RBE) of tritium. Hence, we report the RBE, which was obtained with radiation carcinogenesis classified as a stochastic effect, and serves as a reference for cancer risk. We also introduce the outline of the tritium experiment and the principle of a recently developed animal experimental system using transgenic mouse to detect the biological influence of radiation exposure at a low dose/low dose rate.

Quantification of the activity of tritium produced during the routine synthesis of (18)F fluorodeoxyglucose for positron emission tomography

Gamma emitting radioactive by-products generated during the cyclotron irradiation of (18)O labelled water by protons to produce (18)FDG (fluorodeoxyglucose) for positron emission tomography are well characterised. However, the production of tritium ((3)H) through the (18)O(p,t)(16)O nuclear reaction has not been investigated in detail. The aim of this study was to measure tritium activity produced during a large number of (18)FDG production runs in order to obtain a better perspective on its impact on radioactive waste management, particularly as regards storage and disposal. Tritium was assayed by liquid scintillation counting in recovered (18)O water from 24 separate production runs. The mean (SD) values of activity and activity concentration were 170 (20) kBq and 81 (8) kBq ml(-1) respectively. Both quantities were positively correlated with the activity of (18)F. Tritium was detected in much lower concentration in water used to rinse the target vessel. The activity of tritium is such that it is exempt from regulatory control and may be combined with bulk non-active waste for disposal as Very Low Level Waste. However, variations in the irradiation conditions or the procedures for the collection of recovered water might result in its classification as Low Level Waste, necessitating a more complex disposal regime.

Systematic review of epidemiological studies of exposure to tritium

Tritium (3H) is a radioactive isotope of hydrogen. A number of factors combine to create a good deal of interest in the risks arising from exposure to tritium of both workers and members of the public. Tritium is ubiquitous in environmental and biological systems and is very mobile due to its occurrence as tritiated water. In this paper we systematically review epidemiological data relating to tritium exposure with a view to assessing the risk of such exposure using those studies that are potentially informative. The usefulness of the available studies of cancer and other adverse health effects in workforces and members of the general public is often impaired by a lack of tritium-specific dose data, low doses and small numbers of cases. A number of workforce studies have been identified in which tritium-specific individual doses have been estimated, although none of them, as presently reported, enable reliable inferences to be made on risks associated with exposure to tritium. In general, the available epidemiological studies on the offspring of radiation workers or on pregnancy outcome in areas subject to releases of tritium do not contain enough detail to estimate risks from tritium exposure. Although the studies presently reported are uninformative on risks from tritium, a number of the occupationally exposed cohorts would be potentially informative, particularly if data were suitably combined.

Tumorigenesis in infant C3H/HeN mice exposed to tritiated water (HTO)

The purpose of this study was to determine the carcinogenicity and retention of tritiated water (HTO) in mice. A two-part study was undertaken. In an HTO-incorporation study, both sexes of 12-day old C3H/HeN mice were i.p. injected with 3.70 MBq/pup of HTO and sacrificed 3 hr and 1, 3, 7, 14 days after HTO administration; in a carcinogenicity study, pups were given a single i.p. injection of HTO at doses of 0, 0.23, 0.92 and 3.70 MBq/mouse, and then observed for 14 months. The survival rates of both sexes slightly decreased upon increasing the HTO administered doses. The results indicated that the administration of HTO to infants led to a significant increase of liver tumors in male mice, but not in females. In female mice, ovarian tumors were observed for the high-dose group of injected HTO.

The tritium toxicity program in the Medical Department at Brookhaven National Laboratory

A summary of all the findings to date is given in Table 2. It appears from this information that it is possible to detect somatic, cytogenetic, and genetic effects resulting from exposures at 33 to 100 times the mpc's for HTO. Similar effects also result from exposure to external gamma rays at an equivalent dose. The reduction in bone marrow cells in animals maintaining normal total cellularity is of interest since it demonstrates both the presence of an effect at the primitive cell level and the animal's ability to compensate for this effect by recruiting stem cells from the G0 resting state. This evidence of damage together with the observed cytogenetic changes leads one to contemplate the possible importance of radiation exposures at these levels for the induction of leukemia or other blood dyscrasias. Studies to investigate this question are now under way. As predicted on the basis of established principles of radiobiology, exposure to tritium beta rays from HTO ingestion results in measurable effects on several animal systems. The importance of position of incorporation of H into molecules of biological importance has not been well defined, nor have the low-dose portions of the dose-response curve for several effects of interest. Experiments designed to address these questions and measure H turnover as a means for analysis of cell kinetics in several systems are now under way.

Histopathological changes in the ovary following acute and chronic low-level tritium exposure to mice in vivo

Swiss albino mice were exposed to various doses of tritiated water, either through intraperitoneal injection (acute) or orally (chronic) for various lengths of time. The ovaries were examined for qualitative histopathological and quantitative changes in the number of oocytes. All experimental groups displayed varying degrees of dose dependent radiolesions. It was found that chronic irradiation was more damaging than multiple weekly injection, which in turn was more damaging than a single injection. Also, radiosensitivity varied with the age of the mice.

Effects of tritiated water ingestion on mice: III. Hexokinase isozymes in brain, liver and spleen up to five generations

The radiobiological consequences of chronic exposure to 3H at a dose level of 37 kBq/ml or 1 muCi/ml is reported. An inbred strain of Swiss albino mice was exposed up to 5 generations. Metabolic disorders were recorded by monitoring quantitative and qualitative changes in hexokinase and its isozymes from brain, liver and spleen from both sexes. The delivered dose ranged from approximately 41 mGy to 98 mGy. Nonetheless, the changes in enzyme activities as well as electrophoretic mobilities were significant. Deviations from normal levels were recorded in both sexes. The evidence indicates that direct effects of beta-irradiation can cause conformational changes in enzyme molecules. However, damage to DNA and membranes as being causal factors for variations in the enzyme levels cannot be ruled out. These disorders culminated in a gradual reduction in litter size. The implications of exposure to this dose of tritiated water is discussed in the light of NCRP recommendations and relevant literature.