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Risk from tritium exposure

VIEWS: 22 PAGES: 10

									                            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: galdan@ifin.nipne.ro, ancameli@ifin.nipne.ro
             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.
                                              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. The
uncertainty in the dose coefficients at low dose, including all above factors, is still less
than overall uncertainty in our radiological assessments for routine or accidental tritium
releases (Galeriu, 2007c).


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