Sources and Effects of Ionizing Radiation
1977 UNSCEAR Report
to the U.N. General Assembly
[ UNSCEAR = U.N. Scientific Committee
on the Effects of Atomic Radiation ]
Developmental effects of radiation
[that is : Harmful Effects on Infants Exposed in the Womb]
V. INTERNAL IRRADIATION
[this includes tritium]
The first reported observations on the effect of injected radioisotopes in the foetal mammal are those of Bagg. He injected radon solutions in amounts of tens of millicuries into pregnant rats at different gestation times and described post-implantation death and various types of malformations in the offspring.
More recently, some data on teratogenic effects of internal irradiation have been published, but information on any one nuclide is still very scanty. It does not seem possible to draw general conclusions, and the main value of the existing data is the indication of possible levels of toxicity for the various radioisotopes.
The available data will be reviewed according to the nuclide tested, but only for those nuclides for which there seems to be significant information. It was felt that the present state of knowledge in this field would not justify any attempt to express the data in terms of the radiation dose actually received by the conceptus or its organs and tissues. Exposures or dose data are therefore given according to the way they were reported in the original publication.
Two documents have been submitted to the Committee summarizing the effects on foetus and progeny of mothers exposed to radionuclides before conception and reviewing the various effects in animals treated with radioactive substances in the course of intra-uterine development; both of them dealt with work carried out in the USSR.
An indexed bibliography dealing specifically with the transfer through the placenta and into the foetus of radioactive substances injected to mothers is also of interest for reference to published data on this subject.
The emphasis of the presentation to follow, in accordance with the objects specified in the introduction, will however be centred on the teratological effects [malformations] themselves induced by different doses of the administered nuclides, rather than on the mechanisms and rate of transfer of such nuclides from the mother to the foetus. For these the reader is referred to the specialized literature included in the previously mentioned publications.
Concerning the passage of tritium administered under the form of tritiated water from the mother through the placenta and into the foetus,
Lyaginskaya investigated the relationships between the pregnancy stage at the time of treatment with HTO [tritiated water] in the rat and the ensuing effects in post-natal development. For a dose of 0.3 microcuries per gram, little relationship was found among these variables, although
LD-50 values ["lethal-dose 50 percent values"] for foetal mortality in rats after single HTO [tritiated water] treatments at various doses were
Cahill and Yuile evaluated the resulting foetal tissue doses [radiation doses to unborn babies] in rats, the body water of which was maintained throughout pregnancy at a constant tritium level in the range 1 to 100 microcuries per millilitre. This was achieved by adjusting the ingestion of tritiated water. Dose rates in embryos and foetuses were calculated to range from 0.3 to 30 rads per day. Most mothers were sacrificed before birth for observation of the conceptuses, but in some cases the observations were carried out on born offspring and followed to their adulthood.
Several statistically significant effects were found at various HTO [tritium] levels, in no apparent relationship with dose. These included
The incorporation of tritium into foetal organs was directly proportional to the maternal HTO [radio-]activity during gestation and amounted to 20 to 30 percent of this [radio-]activity . After 180 days of life, stunting only persisted in males with concentrations in the range 50 to 100 microcuries per millilitre.
A paper by Dobson dealt specifically with the RBE [relative biological effectiveness] of tritium relative to cobalt-60 gamma radiation in long-term exposures. To this end, tritiated water in various doses was administered in drinking water to pregnant mice throughout gestation and lactation for a total of 33 days, the specific [radio-]activity of the body water being checked by the radio-assay of urine samples. Other groups of mice were exposed to cobalt-60 gamma radiation at various dose rates for the same time lapse.
The number of oocytes in the progeny of these animals was counted at the end of treatment, and it was found that oocyte survival decreased exponentially with tritium concentration in body water with an LD-50 level ["lethal-dose 50 percent level"] of about 2 microcuries per millilitre body water, corresponding to an effective dose to the foetus and the new-born offspring of about 6.5 rads. The survival curve of oocytes with respect to cobalt-60 gamma rays was convex upward, indicating a decreased killing effectiveness of the gamma-ray treatment at low doses [but not for the tritium treatment -- see next paragraph].
By comparing individual gamma-ray and tritium experiments it was shown that tritium was more effective [than gamma rays from cobalt-60] , and limiting RBE [Relative Biological Effectiveness] values were found to vary between 2.5 and 4.2 with a likely maximum value of about 3 at doses approaching zero. Short-term and protracted exposures were also compared by the same end-point, and higher RBEs for protracted irradiations were obtained.
Recent attempts were made by Cahill et al. to assess by a number of different end-points (morphological, biochemical and functional) the effects on rats of a simultaneous long-term administration of lead and tritiated water.
The experiments allowed the important general conclusions that combination of the treatments resulted in less-than-additive effects and that a significant reduction of brain weight by tritium exposure was apparent at dose rates of the order of 300 millirads per day continuously from conception to 180 days of age.
Haemopoietic [blood] disturbances (anisocytosis, leukopenia, thrombopenia) in the progeny of rats given tritiated water at doses of 0.008 to 0.3 microcuries per gram were described by Zhukova. Animals from mothers treated at 0.3 microcuries per gram on day 4 p.c. [after conception] were the most severely affected.
Finally, Lyaginskaya reported a decrease in the life-span of three subsequent generations of rats irradiated in utero with tritiated water on day 4, 11 and 17 p.c. [after conception] , particularly upon treatment on day 4 p.c. [after conception] .
Concerning administration of tritium in the form of tritiated thymidine (3HTdR), it has been possible to study a number of effects on both the mother and the foetus -- induced by a continuous perfusion of tritiated thymidine to pregnant rats from day 9-22 p.c. [after conception] .
Activities of 1.6 microcuries per gram per day were in general well tolerated; but higher levels, from 3.2 to 6.4 microcuries per gram per day, produced a marked bone-marrow syndrome in the mother.
Although the litter size at birth did not appreciably change at these levels, the percentage of still-born offspring increased with the [radio-]activity injected, while the number of offspring surviving more than 12 hours after birth decreased in proportion to that [radio-]activity .
With 8 microcuries per gram per day, no rat was born alive. Retardation of growth and macroscopic and microscopic malformations of the head, brain, eyes, ears, mouth and extremities were observed at these high levels.
With 6.4 microcuries per gram per day, no gross external abnormalities were seen, but the general development and the haemopoietic system of the animals were severely impaired in proportion to the administered
With 8 microcuries per gram per day, no rat was born alive. Retardation of growth and macroscopic and microscopic malformations of the head, brain, eyes, ears, mouth and extremities were observed at these high levels.
With 6.4 microcuries per gram per day, no gross external abnormalities were seen, but the general development and the haemopoietic system of the animals were severely impaired in proportion to the administered[radio-]activity . The post-natal growth and weight of surviving animals also showed [radio-]activity-related pathological changes.
In another paper, the incorporation of tritium from tritiated thymidine in developing rats was studied by biochemical techniques. The total incorporated [radio-]activity and the DNA [molecule] specific [radio-]activity showed a direct relation to the [radio-]activity of the tritium injected in the mother.
The distribution of radioactivity between the DNA [molecules] and the low-molecular-weight fraction was independent of the administered [radio-]activity , and the time variation of the DNA [molecule] specific [radio-]activities was characteristic for each organ.
Because of all these facts, the system of continuous perfusion was recognized as a suitable procedure for studying tritiated thymidine toxicity in the embryo.
Mouse embryos were also grown in vitro from the 2-cell stage to the blastocyst stage in the presence of tritiated thymidine. Concentrations of tritium above 0.1 microcuries per millilitre were definitely lethal, and concentrations between 0.01 and 0.1 microcuries per millilitre caused a highly significant reduction of the number of cells in the blastocyst.
The latter effect could be largely accounted for by selective cell death occurring at the 16-cell stage. As the cells in the inner mass were most susceptible to killing, it was possible to obtain blastocysts composed entirely of trophoblast.
In 1978, the principal airborne releases of radioactivity from Pickering A were measured at 26 000 curies of tritium; 4 100 curies of noble gases and assumed to be 1 100 curies of carbon-14 . (A curie is a common unit for measuring the radioactivity from different elements.)
Although all releases are of concern and require emission standards, the noble gases are of somewhat less concern because their short half-life (a few hours to a few days) ensures that they cannot accumulate in the environment.
Carbon-14 and tritium are of comparable and special concern for similar reasons.
: 5 730 years for carbon-14 and 12.3 years for tritium. Long half-lives allow them to accumulate in the environment around a reactor and in the global biosphere.
Thus the radiological significance of both elements is not related to their inherent toxicity, as each is a very low energy form of radiation, but to their easy incorporation in the body.
The basis for regulatory emission standards is the 'safe limits' established by the International Council for Radiological Protection (ICRP). The ICRP is an international body drawn from experts in radiology from all over the world, and including many of the leading authorities in radiological protection. Some critics claim it is a self perpetuating body drawn from the nuclear establishment with no inclination to 'rock the boat' by imposing overly tight standards. Its supporters claim that it is made up of leading scientists and medical doctors whose reputation is beyond dispute and who safeguard the independence of the Commission by not allowing membership selection by political bodies.
The ICRP has a Chairman and permanent Secretariat who call the Commission together when, in their opinion, there is a need to review overall or specific standards. The ICRP have established a 'safe limit' of 500 millirem per year for the most exposed individual from a nuclear facility and 5 rem or 5 000 millirem per year for workers. (A "rem" is the unit used to measure the exposure of man to radiation; a "millirem' is 1/1 000 of a rem). The ICRP also emphasize the "ALARA" principle which means that despite the 'safe limit', radiation doses should be kept As Low As Reasonably Achievable , economic and social factors being taken into account.
The AECB, as a matter of explicit policy, accepts the ICRP approach and specific standards. In their view it provides the most authoritative basis for setting emissions standards for radioactive products. The AECB take responsibility for the next phase of regulatory control, reviewing and approving the specific allowable levels for the various categories of radioactive releases, including air and water borne tritium, iodine, noble gases, and particulates and establishing limits for normal operations and accident conditions. The AECB require the licensee to conduct a "pathways analysis", analyzing the various pathways by which the radioactivity can reach man and determining the resultant radiation dose to the most exposed individual. On the basis of this analysis, the AECB then sets the release limits that equate curies of radioactive releases to the ICRP limit of 500 millirem to the most exposed individual.
In the routine operation of nuclear reactors, emissions can be kept considerably lower than the derived limit. Designers and operators of Hydro stations set a release target of one percent from each category of radioactive release. In most cases the targets can be met. For example, Hydro's pathways analysis shows that they could release 10 400 000 curies of airborne tritium before the most exposed individual in the public would receive a dose of 500 millirem. The release target is set at one percent of that derived limit or 104 000 curies. In 1978 the Pickering release was 26 000 curies or 1/4 of one percent of the derived limit resulting in a dose to the theoretically calculated "most exposed individual" of about 1 1/4 millirem.
Dr. E.P. Radford, a Professor of Environmental Epidemiology at the University of Pittsburgh was critical of this approach when he appeared as a witness before the Committee. Dr. Radford is familiar with radiation protection as the Chairman of the National Academy of Science's Committee on the Biological Effects of Ionizing Radiation. Dr. Radford believes that regulatory authorities should set the regulatory limits at the low levels that can be achieved by careful design and operation -- the one percent level -- rather than accepting the potential risks implicit in the ICRP's 'safe limit'.
At this time there is no derived release limit for carbon-14 for two reasons.
Ontario Hydro's Health Physics Branch recently concluded some work on measuring releases and on tracing pathways to man that could result in a dose to the most exposed individual that is somewhere between a small fraction of a millirem and 2 1/2 millirem. At that potential dose level the AECB has become more interested in carbon-14 and is now awaiting a submission from Ontario Hydro on derived release limits and operating targets.
By focusing on carbon-14 at this preliminary stage of developing a derived release limit, the Committee was able to see clearly the range of assumptions that make up a pathways analysis and the consequent uncertainty in the very specific calculations that are made.
At this time the uncertainties are in:
In addition to these admitted uncertainties, the Committee itself was concerned that neither Ontario Hydro nor AECB know enough about the propensity of carbon-14 to accumulate in the food chain and biosphere around a plant, and the rate at which it is dispersed as it moves further from the source.
Carbon-14 is not just a problem of the 'most exposed individual'. Its long half-life makes it a problem for the global environment. In 1977 a group of experts of the OECD's Nuclear Energy Agency identified four nuclides that may be of greater global than of local concern. The four are krypton-85, iodine-129, tritium and carbon-14 . The half-lives of these nuclides vary from 10.8 years for krypton-85, to 12.3 years for tritium, to 5 730 years for carbon-14 and 17 million years for iodine-129. Each of these elements occurs naturally in the environment as a result of the interaction of cosmic rays from the sun with the atmosphere. They are of concern in nuclear power programs because their artificial production is a significant proportion of the natural production which will result in an increase in the global inventory of each nuclide. The large-scale atmospheric testing of nuclear weapons in the '50s did, in fact, result in large increases in the global inventory. Since atmospheric testing ended, global inventories in man's immediate environment have been declining as these elements sink into the deep oceans.
As long as Canada does not reprocess its used fuel, krypton-85 and iodine-129 are of lesser importance to Canadian regulators. These nuclides remain bound up in the used fuel and will only be released in reprocessing or in a severe accident involving fuel melting.
Tritium and carbon-14 are of special concern to Canada.
The Committee is concerned that Canada -- and Ontario in particular -- may have a special global responsibility for controlling carbon-14 and tritium that goes beyond consideration of local effects. Although Ontario Hydro and AECL have programs ongoing to consider ways of further reducing tritium and carbon-14 releases, there is no national or regulatory framework for guiding their implementation.
The Committee also noted that radiation protection and the appropriateness of radiation standards is a health topic of great concern to people in Ontario -- the workers in the industry and populations located near industry facilities. Legislative responsibility for nuclear safety rests with the Federal government and Ontario has deferred responsibility to the AECB.
The AECB, with its limited staff, accepts the recommendations of the ICRP. The major Canadian role in standard-setting has been to provide representatives -- and in one case the chairman -- to the ICRP from AECL. Thus, the basic Canadian regulatory standards come from a body reputed to be dominated by those the critics call the nuclear establishment -- manufacturers, utilities, regulators.
The AECB has recently established an advisory committee of experts in radiation including some people from outside the "nuclear establishment."
The Committee believes that there is an additional need to provide a body that can focus on Ontario problems, that can do so openly and with participation from the public, and that can relate directly to the federal body.
WITH GIVEN TERMS OF REFERENCE
AND REPRESENTATION FROM WITHIN AND OUTSIDE
THE NUCLEAR ESTABLISHMENT
TO PROVIDE AN INSTITUTIONAL FORUM
FOR PUBLIC PARTICIPATION
AND A FOCUS FOR CONCERNS
ABOUT RADIATION PROBLEMS IN ONTARIO
TO BUILD UP ONTARIO-BASED TECHNICAL KNOWLEDGE TO DECIDE WHAT THE STANDARDS SHOULD BE
AND TO OVERSEE
AS MUCH EPIDEMIOLOGICAL WORK AS IS NECESSARY
FOR THE HEALTH AND SAFETY
OF PEOPLE IN ONTARIO.
TO DECIDE WHAT THE STANDARDS SHOULD BE
THE COUNCIL SHOULD REVIEW
PARTICULAR PROBLEMS OF RADIATION
ASSOCIATED WITH OPERATING OR PLANNED REACTORS,
INDEPENDENT OF ONTARIO HYDRO AND THE GOVERNMENT.
THE COUNCIL SHOULD WORK TOWARD
THE ESTABLISHMENT OF A FEDERAL-PROVINCIAL WORKING GROUP
TO CO-ORDINATE THE NATIONAL STANDARDS
WITH THE WORK AND FINDINGS OF THE PROVINCIAL GROUP.
THE POWERS OF THE COUNCIL SHOULD BE THAT OF
MAKING INDEPENDENT AND PUBLIC RECOMMENDATIONS
TO AN APPROPRIATE MINISTER.
IS AN INDEPENDENT REVIEW
OF THE ADEQUACY OF CURRENT PROPOSED RELEASE LIMITS
FOR CARBON-14 AND TRITIUM,
TAKING INTO ACCOUNT BOTH LOCAL AND GLOBAL CONCERNS.
[ THESE RECOMMENDATIONS WERE NEVER ACTED ON . . . ]
TESTIMONY OF DR. EDWARD RADFORD
ON ONTARIO HYDRO AFFAIRS
The Safety of Ontario's Nuclear Reactors
Tuesday, July 10, 1979
MEMBERS OF THE SELECT COMMITTEE ON ONTARIO HYDRO AFFAIRS:
Counsel: Schwartz, Alan M. Consultant: Fisher, J.D., Canada Consulting Group, Toronto
Clerk: Richardson A. Assistant Clerk: Stenky, J.
Broem, Dr. I.D.J., Director of Biostatistics, Roswell Park Memorial Institute for Cancer Research, Buffalo, New York.
Radford, Dr. E.P., Professor of Environmental Epidemiology, Graduate School of Public Health, University of Pittsburgh, Pittsburgh, Pennsylvania.
Dr. Radford: The low energy of the tritium beta means that when tritium is taken into the body the radiation dose to the tissues per unit of radioactive decay expressed in millicuries or microcuries is low, compared to other radionuclides that are present in reactor waste. In addition, the fact that it will usually be ingested or inhaled in water molecules means that it will be excreted from the body mostly as water and with a turnover rate characteristic of body water. We excrete about half of our body water every 10 days, of course making up the loss with fresh intake, so this means that tritium stays in the body only a relatively short time compared, say, with radioactive radium atoms, which may remain in the body for years or even decades.
I would like to add to that that we have to consider two situations with regard to tritium uptake and release-state situation. One is what I will call a steady-state situation.
The kinds of arguments I just expressed here and which were expressed yesterday by Dr. Myers I believe means that we come into a new equilibrium with the intake. For example, take the water outside the Pickering reactors. It means the population that has been drinking that water has now come into equilibrium with it. The ratio of tritium atoms to hydrogen atoms in their bodies is the same as the ratio in the water supply. That is what that means. It doesn't mean there is any really serious problem associated with it, but that is the way that tritium operates.
In this regard tritium is somewhat different from other radionuclides in that there is no so-called concentration factor. That is, there exists no biochemical mechanism that means the body knows the difference between a tritium atom and anything else or a hydrogen atom. Therefore it is always diluted in proportion to the hydrogen.
This is not exactly true, but it is close enough to being true. Somehow the body does know the difference between tritium and hydrogen but it doesn't know that much that it makes any difference.
This is in contrast, for example, to zinc which is a common contaminant of coolant water. Where the body knows about zinc it grabs it up avidly. It takes it out of the intestinal tract contents and has enzyme mechanisms to do just that because zinc is an essential element and we have to have it in our bodies all the time. In that situation the concentration of zinc in the body of a person drinking the water or eating a food or whatever that contains the radionuclide may be much higher than would be present in the food or the water that they drink. So that is the difference with tritium which in effect works to reduce the hazard from it.
For all these reasons tritium is sometimes said to be one of the least harmful radionuclides among all those potentially reaching the worker or the public from the nuclear cycle. I believe, however, that current evidence indicates that the harmful effects of tritium ingestion are somewhat greater than the standard view indicates, especially for effects on the foetus in pregnant women who are exposed.
Mr. Schwartz: Could you tell us what evidence you are referring to and help define the phrase, "somewhat greater," for us?
Dr. Radford: First, with regard to the relative effectiveness of this very weak beta, there is now experimental evidence, both in terms of changes in the developmental effects on foetuses in utero in animals and also in studies of cancer induction, that suggest that tritium has somewhat more effect. How much more? Four or five times more effective than would be predicted just on the basis of its energy alone. It has what we call an RBE [a "relative biological effectiveness" factor] of about four or five. So that factor has to be borne in mind.
There is another more subtle factor which may or may not be very practically important -- and I want to emphasize that. It goes to the question of what I will call "pulse labelling" with tritium.
We have the case of a water supply. The people drink it and pretty soon their tritium level is the same as in the water. I have already explained that. It may be very low and unimportant. But let's take the case of sending a burst -- a high concentration -- of tritium out into a lake or river somewhere. That burst now goes into the water supply and is drunk by people.
On the classical model it wouldn't make any difference: it would just be averaged out over the time. But there is one special case that I think is important.
What if a woman is in the early stages of pregnancy and the child is a girl -- 50 per cent chance? That woman is going to be laying down her ova in the uterus at the time that slug of tritium comes in. Now the DNA of the ova will be labelled with the level of concentration of tritium that is appropriate at that time, within a day or two or three, rather than averaged over a longer time. As far as we know that tritium that is laid down in the DNA of the ova of that developing girl will remain for her whole reproductive lifespan. There is no exchange of that type of hydrogen. It is a very different kind of hydrogen as far as the body is concerned. Most of the hydrogen in our body exchanges readily with tritium.
Mr. Nixon: So those nuclei could (inaudible) with tritium and not with hydrogen?
Dr. Radford: They will have tritium in them as long as that girl is alive. After she is born the tritium is still there, it is hung on to. That hydrogen is held very tightly, in contrast to most hydrogen. Therefore, the probability is that that tritium will decay at some time during the reproductive period of that girl. Under those circumstances the concentration of tritium in that particular, very small compartment of the body of that woman is much higher than one would predict on the basis of the classical models.
Again I come back to the point that this is perhaps more a theoretical concern than a practical one because pulsing out of large volumes of highly tritiated water is not a common practice, I would guess.
Ms. Gigantes: It just happened in Pickering.
Dr. Radford: How about that? It is not common, but it can happen. At least on theoretical grounds, that might be a cause for concern; namely, you now have, you might say, a time bomb sitting in the DNA of that ovum.
Mr. Nixon: The pulse that produced the pollution level that Ms. Gigantes mentioned to you when she was talking about the specific level.
Dr. Radford: I'm sorry, what is the question?
Mr. Nixon: Referring to a pulse that we just had, it produced a level of tritium pollution, if I can call it that, that was referred to by Ms. Gigantes in her specific question.
Ms. Gigantes: Eighteen thousand curies.
Dr. Radford: What was the concentration in the water? That is the item that's important.
Ms. Gigantes: It certainly raised the drinking water readings. I don't have a drinking water reading associated with that specific pulse. There is an average for the first four months of the year and this happened in February.
Dr. Radford: This is one theoretically possible circumstance in which averaging over a year, or two, or ten, is not necessarily giving the full health impact of pulsing out a lot of tritium all at once. Whether this happens as a practical matter I leave to others to discuss.
Mr. Nixon: You may be sure that it will be discussed.
Dr. Radford: Have I answered your question, Mr. Schwartz?
Mr. Nixon: I will wait until the next sentence to ask the next questions.
Dr. Radford: I discussed the foetus. Moreover, because tritium is more difficult to contain at the source than other radionuclides, especially during nuclear fuel reprocessing, vigilance is warranted in keeping exposures adequately low to the general public.
Mr. Nixon: My question there is for you to define for us what "adequately low" means.
Dr. Radford: As I say, for now I am perfectly satisfied with 10 picocuries per millilitre [370 becquerels per liter] . In other words, if the body of water is big enough to dilute it and it is kept at that level, I see no real public health problem. If it goes above that by a substantial amount, then there may be a problem.
Mr. Nixon: I am sorry to keep coming back to numbers, but what is "a substantial amount" above that in your mind? When would we be concerned that we are getting into a public health problem?
Dr. Radford: Ten times that [3700 becquerels per liter] .
The Health Dangers of Uranium Mining
and Jurisdictional Questions
published in August 1980 by
The British Columbia Medical Association
LD-50 = "lethal dose 50 percent" = dose required to kill half of the individuals exposed within 48 hours.
In the case of LD-50 for oocyte exposure, it is the death of oocytes that is in question, not the death of animals.
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