Risk from tritium exposure D. Galeriu1, A. Melintescu1, H. Takeda2 1 Horia Hulubei National Institute of R&D for Physics and Nuclear Engineering, Department of Life and Environmental, 407 Atomistilor St., PO BOX MG-6, RO-077125, Bucharest - Magurele, Romania Contact author email: email@example.com, firstname.lastname@example.org Phone: +40 21 404 23 59, Fax: +40 21 457 44 40 2 National Institute of Radiological Sciences, Environmental and Toxicological Sciences Research Group, Chiba, 4-9-1, Anagawa, Inage-ku, Chiba-shi, 263-8555, Japan 'IRPA Regional Congress for Central and Eastern Europe' www.irpa2007romania.com Brasov Romanina Sept. 24-28 2007 In a country which develops nuclear energy with CANDU6 reactors, tritium is the “national radionuclide”. In the early „90s a literature search mentions some claims for increased tritium risk (Rudran 1988, Fairlie 1992). Tritium can enter into human body in many forms but for public only tritiated water (HTO) and organically bound tritium (OBT) is of concern. Atomic workers in a CANDU environment have mostly intake of HTO, while in a tritium removal facility they are subject to many other intake types. Using the opportunity of a common work at AECL, we contributed to the role of OBT in the dose after HTO intake. At those times, an accidental intake of a large HTO activity occured in a CANDU facility in Canada and the assessment of workers dose was on debate. The official recommendation of ICRP was an increase of about 10 % due to metabolised OBT in human body after HTO intake (ICRP 1993), while Indians claimed a much more increase. We proved, by objective analyze of urine tritium concentration that the dose increase was between 3.5% and 8.9% of the total body water dose (Trivedi, 1997) and the low values reflects increased water uptake after the incident but also metabolic variability between people. Subsequently, workers chronic intakes of HTO was analysed and it was shown that the dose increase was between 4.7% and 9.9% (Trivedi, 2000). These findings are supporting ICRP recommendation but a more carefully analyses of experimental data revealed some problems concerning our understanding of OBT metabolism in humans and limitation on human tritium dosimetry (Galeriu and Trivedi, 1999). As human experiments are not allowed we need to use animal experiments and find an objective, scientifically base, method to expand for humans. First of all, we must understand animal metabolism of hydrogen and its isotope, tritium, and test our understanding with available experimental data. A carefully analysis of experimental data, mostly on rat and sheep reveals some generic rules that can be used in the development of a dynamic model for 3H and 14 C in mammals (Galeriu, 2003); but most important was testing ICRP assumptions. ICRP model for human intake of OBT assumes that 50 % of ingested OBT is immediately transformed in HTO and eliminated from the body with the water turnover time (10 days for adult). The remaining 50 % is taken by body OBT and further eliminated with the “carbon half time”, estimated from the organic carbon intake and body carbon content. Applying the same approach to rat or sheep, it was observed that the ICRP assumptions do not correspond with experimental data (Galeriu, 2003). This finding opens the question on validity of human ICRP tritium dosimetry. A study on parameter uncertainty of ICRP model for dose coefficients for public after tritium intake was published (Harrison, 2002) with very interesting results. Allowing ranges of values for the parameters (mostly the intake fraction and retention half times) the adult dose coefficient after OBT intake has a median values higher with 35 % from ICRP recommendation but 4 fold variability (between 5 and 95 % of the probability distribution). It was also analysed the influence of tritium relative biological effectiveness (RBE), being in a controversial aspect in literature. ICRP was using a value of 1 for tritium but experimental values shows a bit wide range and a dependence of the reference radiation (Gamma or X rays). As a compromise, a uniform probability distribution between 1 and 2.5 was adopted and the final uncertainty in the dose coefficients has been obtained. The median value for adult, after OBT intake was higher than ICRP by a factor of about 2 and the 95 % of the probability distribution by a factor of 4.7. Due to RBE effect the dose after HTO intake was also higher than ICRP values but in a slightly lesser extend. In an independent committee for assessing radiation risk from internal emitters (CERRIE 2004) a well known scientist and ecologist, Dr. Ian Fairlie, was claiming a much more increase of dose coefficients, a factor of 12-15 for the HTO values comparing with ICRP and 5 times more for OBT case. For an outsider, the arguments seem valid and, if accepted, nuclear industry would have sufferings. We concentrate on an independent approach with more scientific bases. To be able to use animal experimental data in order to get to a human model, we need a common trend, a basic fact. It was known that mammal basal metabolic rate has a mass dependence, with a power exponent between 2/3 and ¾ for adults and also a metabolic theory of ecology was published (Brown, 2004). We considered in more detail the energy metabolism and make the hypothesis that energy turnover rate is a measure of organic matter turnover rate (14C and OBT included). We also considered net energy demand and define energy turnover as a ratio between specific metabolic rate (MJ/kg fw) of whole body or any organ and the energy content (combustion energy) of fresh mass unit. Full justification on the validity of our hypothesis and tests has been presented recently (Galeriu, 2007a) and a full model description with parameters for key laboratory and farm animals will be soon available (Galeriu, 2007b). In an absence of any calibration, model predictions are very close with experimental observations when appropriate input is taken from animal physiology, nutrition, and metabolism. A first application for human dosimetry was published (Galeriu, 2005) and now we will refer to recent results (Melintescu, 2007) and discuss the limits and merits of the model for human dosimetry. A flowchart of the model is given in Figure 1 and shows similar structure as other bio kinetic model of ICRP. It has a compartment for free T (hydrogen) - Whole Body_water, more compartments for organically bound T (hydrogen) and a simple intake pathway for dry matter (stomach content, small intestine content). Intake of HTO is directly distributed in whole body water compartment. As 25-35 % of body bound hydrogen, at equilibrium, comes from metabolised free hydrogen, we have a transfer from body water (free hydrogen) to blood plasma (organic hydrogen). From the dry matter in the diet, the metabolisable fraction is transferred to the body but exchangeable fraction is passed directly to body water. The non-exchangeable OBT enters into the blood plasma compartment and later fraction dis partly transferred to body water as a respiration flux. Few model parameters are distributed (probabilistic approach) as given in the Table 1. WB_water F11 C1 viscera RBC stomach F12 F17 F1 F2F18 F14 F10 F19 F16 F3 smint bloodpl muscle F4 F5 F15 F13 F7 F8 F6 milk_OBT F9 rem C4 urineOBT adipose Figure 1. Flowchart of tritium model We considered 3 types of committed dose: 1) The committed equivalent doses H (RBE = 1, uniform tritium distribution) obtained with the model considering whole body integrated tritium concentration. This can be directly compared with the ICRP values in Table 2. 2) The committed effective doses E (RBE=1), take into account the non uniform distribution of tritium (OBT) in the body. Adipose tissue has the largest integrated concentration but is less radiological sensitive. We consider the tissue radiation weighting factor (ICRP 2007) and our compartment composition. The results in Table 2 demonstrate that although the retention of tritium is longer than the ICRP approach and the non-uniform distribution compensates a lot. 3) The committed effective doses E (RBE>1), give the overall probability distribution of the dose coefficients, including the influence of RBE. It must be compared with the parameter uncertainty of ICRP dose coefficient (Harrison, 2002) Table 1. Model distributed parameters Variable Distribution Range Comment RBE HTO Lognormal 1-3.3 (mean All values for X and γ ray 1.8) RBE OBT Lognormal 1- 4 (mean 2) All values for X and γ ray Metabolisable Uniform 0.9-1 Equivalent with fraction transferred to fraction of the diet blood in other models Non exchangeable Uniform 0.4-0.75 From organic tritium in the diet, fraction transferred to body OBT after digestion Bound H from free Uniform 0.25-0.35 Fraction of OBH in the body, at H equilibrium, coming from free H in body water Diet composition Uniform 0.00325- OH in diet per MJ of diet energy (kg OH/MJ) 0.00354 Water intake Lognormal Depends on Adapted from USA and Canada water case intake distributions. In order to be consistent to ICRP, we considered reference humans of age 1, 5, 10, 15 and adults but investigated gender effect at age 15 and adults. First results were for Caucasians and a briefing is done in Table 2. The reference adult has low to moderate activity and a mass of 73 kg. We investigated also the variability of the dose coefficient for an obese adult (mass 104 kg) and an athlete (mass 67 kg) and found that change in the dose coefficient were less than ± 30% with lower values for the athlete (Melintescu, 2007). We expanded the study to Japanese reference humans, taking appropriate data for body and organ mass, as well as diet and energy needs. The variability of dose coefficients was moderate (Galeriu, 2006) and values were 5%-25 % lower than in Table 2, except Japanese adult male with same dose coefficient as Caucasians. Considering the above results, values in Table 2 can be used in general risk assessment with average values over gender as it is the current practice. Keeping RBE at unit value, as in current ICRP dosimetry, the increase in dose coefficients is less than 50 %, while allowing increased RBE, at 95 % of probability distribution the increase is about a factor of 3-4. Table 2. Dose coefficients for tritium (unit 10-11 Sv/Bq) ICRP H_RBE=1 E;RBE=1 E(RBE>1) HTO 1y 4.8 4.6 (2.9-6.6) 4.9 (3-7) 8.6 (4.9-13) 10 y 2.3 3.1 (1.8-4.8) 3.2 (2-5) 5.8 93-9.5) Adm 1.8 1.8 (1.-2.9) 1.7 (1-3) 3. (1.5-5) Adf 1.8 2.4 (1.3-3.9) 2.4 (1.3-4) 3.2 (1.6-5.4) OBT 1y 12 16 (14-18) 12.2 (10-14) 25 (16-36) 10 y 5.7 9.5 (8-11) 6 (5-7.5) 15.2 (10-22) Adm 4.2 5 (4-6) 4.7 (3-5) 7.8 (5-11) Adf 4.2 10.2 (9-11) 7 (6-8) 11.8 (7.5-17) Discussion The results presented are based on a new approach describing the transfer of tritium in mammals on a general scientific basis and model tests confirm the reliability of predictions. Complains against ICRP tritium dosimetry can now be analyzed in deeper details. First of all, there is indeed higher tritium retention of OBT in human body, up to a factor of 2 but most of retained tritium is in adipose tissue, with low radio sensitivity. For case of male adult the integrated concentration in adipose tissue is 3 times higher than for other tissues but when the tissue weighting factors (wT) are taken into account, the contribution of adipose tissue is minor (Table 3). In practice, considering the ICRP advise, we are using gender averaged dose coefficients and in this case our values (E, RBE=1) for the median are only 40 % higher than ICRP, much less than claimed (a factor of more than 3). Even at the 95 % of probability distribution there is only 55 % increase in the dose coefficients, as biokinetic halftimes are regulated by body metabolism in a restricted range (42-105 days for adult male, longer for obese). It remains now to include the effect of RBE, and we are faced with many end points which have to be considered and to choose the reference radiation as both X ray and gamma rays. As the quality factor (an old unit linked directly with RBE and radiation 60 weighting factor) of X ray is about 2 times higher than for Co gamma ray, RBE for tritium can differ by a factor 2 depending on the reference radiation. In an absence of advice from ICRP (ICRP 2007) we select gamma ray as reference radiation, not to be suspected of diminishing voluntary the tritium risk. Based on all available data for HTO we use a lognormal distribution with 99 % upper end at 3.3, a value higher than those considered in the previous uncertainty analysis (Harrison, 2002). For OBT, we limit the upper range of RBE at 4 and this can be an underestimate if considering recent microdosimetric estimations (Chen, 2006), indicating a 1.7 ratio to HTO. Table 3. Integrated activity and concentration and weighted values for adult male (OBT intake) Integrated activity Total T int. WT Weighted Bq*d conc. int. conc. Percentile Bq/kgfw Compartment 5 50 95 HTO 6.5 12.6 20.7 OBT 36.9 39.6 42.5 OBT_adipose 22.2 23.8 25.5 1.6496 0.008 0.013197 OBT_muscle 7 7.5 8.1 0.4954 0.008 0.003963 OBT_viscera 1 1.1 1.15 0.4867 0.45 0.219047 OBT_remainder 5.7 6.1 6.5 0.4877 0.42 0.204837 OBT_RBC 0.8 0.85 0.91 0.8406 0.12 0.100876 Including RBE effect, the committed effective dose increase by a factor between 1.7 and 2.8 for the median values comparing with ICRP (see last column in Table 2). For the 95 % of the probability distribution, the increase is less than a factor of 4.3. For both HTO and OBT dose coefficients, the RBE is the major contributor to the uncertainty, and for HTO also the water intake contributes significantly. It is useful to compare our result (adult, gender averaged) with the analysis of ICRP parameter uncertainty (Harrison, 2002) as shown in Table 4. Table 4. Probability distribution of dose coefficients (unit 10-11 Sv/Bq) 5% 50% 95% HTO, old 2.1 3.9 6.6 HTO,new 1.6 3.1 5.2 OBT,old 3.9 8.7 20 OBT,new 6.2 9.8 14 We used an expanded range for RBE and our range of biokinetic rates are given by metabolism and not by a presumed range (Harrison, 2002). It is interesting to observe the close mean values in the two approaches and few differences at range extremity. We have used an improved model for tritium retention, tested with animal data, so our results are robust in giving a limit to tritium risk, disregarding the ICRP retention model. This is definitely lower than claims of prominent activist, as Dr Ian Fairlie who recently publishes under Greenpeace (Greenpeace 2007) a document claiming 12 fold increases for HTO dosimetry, comparing with ICRP. As we included reference humans at various ages, genders and races in our study we also give support to abandoning the ”Reference Man” approach, as recently claimed by activists (IEER 2006). We note also that our result for adult reference male is very close with earlier work (Richardson, 2003) when metabolism of carbohydrates, proteins and lipids has been considered in more detail. Concerning the communication with the public, we want to point a present difficulty. For prospective and compliance dosimetry, ICRP is using RBE=1, but allow higher values for retrospective dosimetry or risk estimates (ICRP 2007). This is difficult to be explained to the public, in a credible and transparent way, when we must expose the effect of tritium releases from nuclear industry. The dose limits itself have been established considering risks, so dose coefficients must be also risk related and the distinction between risk estimate and compliance or prospective dosimetry is difficult to understand. We encounter also a difficult problem in applying compliance dosimetry to atomic workers. Their urine is monitored and dose is assessed from bioassay - this is in fact a retrospective dosimetry. It will be much easier to communicate with the public or to argue on the debate on tritium risks if ICRP and IRPA will agree to use average values of RBE in all cases and TO DISTINGUISH BETWEEN HIGH AND LOW DOSE CASES. Risk factors as well as RBE were established at high dose or dose rate. In practice we must assess public or occupational dose at low dose (< 100 mGy/y). It is agreed by all International Committees that at low dose the radiation effects are lower and Linear No Threshold (LNT) extrapolation must be accompanied by a dose rate effectiveness factor (DDREF) with a value close to 2. If we consider an average value of RBE near 2 for HTO intake, for compliance or prospective dose, there will be no dramatic changes due to influence of DDREF, using our values (column 4 in Table 2). For public, when OBT must be considered and average RBE of 3 suffices and again for routine release there will be a limited change from actual dosimetry. Even for most of design basis accident the proposed approach will change the actual assessment in lesser extend than the overall uncertainty. Only when we expect dose near or higher than 100 mGy, a full uncertainty assessment will be needed - including all RBE distribution, debate on LNT assumption and DDREF best estimate. Considering the debate on tritium risk and RBE (see for example Eliot, 2005; Fairlie, 2007) our proposal will make radiological assessment more transparent for the public, without loosing scientific basis. There is also a debate on various effects - as bystander, adaptive response and genomic instability which contribute to the uncertainty. There are also counterarguments at low dose, as a hormetic response due to adaptation (Feinendegen and Neumann, 2005) is effective at doses less than 0.1Gy. 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