Tritium in Drinking Water: Science, Regulation and Society

Transcription

1 Tritium in Drinking Water: Science, Regulation and Society TAM-E: Environmental Section Special Session: Tritium in the Environment Presented To: 57th Annual Meeting of the Health Physics Society Sacramento - July 22-26, 2012 Presented By: Dr. Douglas Chambers 1

2 Purpose Ontario s Drinking Water Advisory Council (May 2009) proposed a drinking water standard for tritium of 20 Bq/L as a running average over 52 weeks because you can This paper examines the science and politics surrounding this proposal 2

3 Tritium An isotope of hydrogen (H-3) Naturally occurring via interaction of cosmic radiation with upper atmosphere Thermal neutron capture by a deuterium atom in a nuclear reactor Low energy beta emitter mean energy of 5.7 kev (maximum of 18.6 kev) range in tissue of about 6µm Only results in radiation dose if taken inside the body 3

4 Tritium cont d Tritium is an isotope of hydrogen, it reacts chemically to form compounds in the same manner as hydrogen does and is seen as water and in organic compounds Most commonly in environment as water (HTO) Tritium can be incorporated into organic molecules (OBT) When bonded to O,S,N or P atoms considered exchangeable with biokinetics similar to HTO When bonded to C, considered non-exchangeable with longer residence in the body Can also be incorporated in DNA (tritiated thymidine) 4

5 Tritium cont d Radioactive half-life years Biological half-life (t 1/2 ) : 10 days HTO [99%] 40 days OBT [0.98%] ICRP recently introduced new OBT component with a larger t 1/2 of 350 days [0.02%] 5

6 Canadian Regulatory Considerations Current Health Canada and Ontario Drinking Water Guidelines based on WHO [2006] Recommend reference dose level (RDL) equal to 0.1 msv from 1 year s consumption of drinking water; Comprises 10% of the Canada s annual dose limit for members of the public; Dose coefficients for adults provided by the ICRP 6

7 Health Canada Guidelines Preparation Process Recommendations for drinking water quality guidelines are made by the Committee on Drinking Water, which is led by Health Canada and has representation from the provinces and territories. The recommendations go to the Committee on Health and the Environment for final approval. The Guidelines can then be adopted as regulations by the provincial and territorial authorities. 7

8 Formula for Calculating Maximum Acceptable Concentration (MAC) MAC (Bq/L) = 0.1 (msv/y) 730 (L/y) DC (msv/bq) Where: 0.1 msv/y = dose constraint for drinking water, 730 L/y = adult drinking water consumption (2L/day), DC (msv/bq) = dose coefficient for HTO of Sv/Bq from ICRP-72 [updated ICRP model Sv/Bq ( after Taylor 2003)]. 8

9 Tritium Drinking Water Standards* Nuclear Reactors Jurisdiction CANDU Total Tritium Standard (Bq/L) Canada msv rounded ,000 EU indicator parameter [TID = 7,600] Finland 0.5 msv ,000 Australia 1.0 msv ,103 Russia 0.1 msv ,700 Switzerland WHO value ,000 United States 0.04 msv *After Canadian Nuclear Safety Commission tritium Synthesis Report

10 Tritium Concentration (Bq/L) Background Tritium Concentrations 8.0 in the Great Lakes Thunder Bay (Lake Superior) Niagara Falls (Niagara River - Lake Huron) Kingston (Lake Ontario) Below Detection Limit Year 10

11 Tritium Concentrations in Water Supply Plant (Bq/L) Tritium in Water Supply Plants Near NGS* Year 2 0 * After CNSC Tritium Synthesis Report 2010 Water Supply Plants 11

12 Health Risks Once radionuclides have entered the body, they are absorbed, metabolized, distributed throughout the tissues and organs, and eliminated in urine and feces The effects on the organs and tissues depend on the type and energy of the radiation and the residence time within the body. Exposure can result in cancers to various tissues and organs. 12

13 Health Risks cont d Probability of a health effect is estimated as the product of dose and a risk factor that converts dose to risk ICRP 103 provides current risk conversion factors 13

14 Dose Coefficients ICRP Age-dependent factors for inhalation and ingestion For tritium, increase contribution of HTO via inhalation by 50% to account for absorption through skin Hamby* Used Monte Carlo sampling and calculated that the dose coefficient for intakes of HTO by adults varies by a factor of 15, with a median value of Sv/Bq and a geometric standard deviation of 1.6. Harrison** reports ranges of DECF from an uncertainty analysis Form 5%tile 50%tile 95%tile Sv/Bq Sv/Bq Sv/Bq HTO OBT * Hamby, D.M. Uncertainty of the tritium dose conversion factor. Health Phys. 77(3): (1999). ** Harrison et.al. Radiat. Prot. Dosim. 98(3): (2002). 14

15 Uncertainties in Dose Coefficients Physical Dose Absorption to blood Biological half life for HTO and OBT Effect of chemical speciation Heterogeneity of distribution in cells and tissues Effective Dose RBE Tissue weighting factor 15

16 Ranges of RBE from Literature RBE Range Endpoints # Reported Values In vivo In vitro Test Models Mortality; Reproductive capacity (fertility and fecundity); Cell survival; Chromosome aberrations; Vertebral abnormality; Tumour incidence Embryo survival; Chromosome aberrations; Cell survival; Mutations Forty two (42) Fish; Plant; Marine invertebrate; Mouse; Rat; Hamster; Human primary cells; Cell lines Reproductive capacity (germ cells survival and anomalies in total implants); Cell survival; Tumour development; Dominant lethal and sex-linked recessive lethal mutations in germ cells Chromosomal aberrations; Dicentrics; Mutation frequency; Survival of haematopoietic progenitors Twenty five (25) Fish; Insect; Mouse; Rat; Human primary cells; Cell lines >3.0 Reproductive capacity; Chromosome aberrations and mutations Chromosome aberrations; Mutations Seven (7) Fish; Mouse; Human primary cells; Cell lines Most of the tritium RBE studies have been done in mammals (80% of the data), either in vivo, with laboratory animals (mainly mice) or in vitro (human cells or established cell lines). 16

17 ICRP Risk Coefficients Latest cancer risk coefficients (ICRP 103) essentially unchanged from ICRP 60 values (about 5% per Sv) Risk of heritable effects reduced 6 to 8 fold (risk now summed over two generations only extending over a very large number of generations not now considered reasonable) No epidemiological evidence of harm below about 100 msv Linear No Threshold (LNT) model assumed for radiation protection purposes 17

18 ICRP Risk Coefficients* (% per Sv) Exposed Population Cancer Heritable Effects Total ICRP 103 ICRP 60 ICRP 103 ICRP 60 ICRP 103 ICRP 60 Whole Adult * ICRP 103 at para A115, risk estimates are based principally on incidence data, nominal risk adjusted for lethality and quality of life 18

19 Example Dose and Risk Calculation Tritium in drinking water at 20 Bq/L Annual dose from ingestion of tritium: Bq L -11 Sv Sv L y = Bq Corresponding risk of cancer: msv y = Sv/y 5 10 Sv 75y -6 ~ 1 10 lifetime risk of cancer 19

20 Example Doses to Most Exposed Receptors Dose (usv) PNGS DNGS Total (all radionuclides and pathways)* HTO Dose from Water Ingestion* Total % of Total Dose from HTO in Water 0.34% 2.0% * 2008 Doses from OPG REMP report,

21 Dose to the Public* * After CNSC Tritium Synthesis Report

22 22

23 Source: 23

24 Public Perception or Why are we concerned? Monitoring data demonstrates that water supplies can meet 20 Bq/L (52 week average) but Some measurements will exceed With new nuclear, some exceedances may occur and Public perceives reduction from a standard of 7000 Bq/L to 20 Bq/L as Proof that risks from tritium have been underestimated 24

25 Health Effects of Tritium: Epidemiology Studies Main Findings of CNSC Review* Existing information does not provide enough detail to estimate the health risks of tritium exposure, specifically Tritium exposures represent a small fraction of total radiation exposures, thus tritium risk is only a fraction of total risk, which is low The risk to workers, offspring and the general public is low No human health studies have demonstrated any tritium induced cancer or other health effect * CNSC Tritium Synthesis Study

26 Health Effects of Tritium: Laboratory Studies* Minimum dose or dose rate To induce effect Effect (Laboratory animals) 5 Gy Death ~ 0.5 Gy 3.7 MBq/l drinking water 0.17 MBq/g/day -cancer promotion -embryo/foetus development -genetic/reproductive * After CNSC Tritium Synthesis Study

27 Its all about the dose!!! 27

28 Comparative Doses (msv) 28

29 Doug s Conclusions The current drinking water guideline of 7,000Bq/L is consistent with WHO reference dose of 0.1 msv from 1 year of intake 0.1 msv is 10% of public dose limit Doses are a small fraction of dose (and variability in dose) from natural background radiation Doses are de minimis 29

30 Doug s Conclusions cont d The current radiological approach has doses from drinking water at de minimis levels Dose limit + ALARA has achieved very low HTO concentration in drinking water supplies (<20 Bq/L) lifetime risk in the order of 1 in a million current practices for tritium management are effective! 30