5. TRANSPORT OF RADIONUCLIDES THROUGH
5.2. Aquatic food chain transport
Radionuclides discharged into the aquatic environment are also assimilated by living organisms. Some of the assimilated radionuclides are passed along the aquatic
food chains and may eventually reach humans. Models are used for dose assessments to simulate the transport of radionuclides in aquatic environments.
5.2.1. Basic model
Models that describe the transport of radionuclides from liquid discharges to aquatic foods generally take the form
Caf,i= Cw,iBp/1000 (38)
where
Caf,i is the concentration of radionuclide i in aquatic food p (Bq/kg);
Cw,i is the concentration of dissolved radionuclide i in water (Bq/m3);
Bp is the equilibrium ratio of the concentration of radionuclide i in aquatic food p to its dissolved concentration in water (Bq·kg–1/Bq·L–1, or L/kg), known as the bioaccumulation factor;
1000 is the conversion factor from m3to L.
Methods for the generic estimation of Cw,iwere considered in Section 4.
5.2.2. Bioaccumulation factor Bp
The transfer of radionuclides from water, through various trophic levels of aquatic life, to those organisms consumed by humans is condensed into one parameter — the bioaccumulation factor Bp. This parameter is quite variable, with values ranging in some cases over several orders of magnitude for a given radionuclide and organism [6].
The most important factor governing the value of Bpis the trophic level of the organism. Other factors are
(a) Suspended sediment concentration, (c) Chemical composition of the water body, (d) Chemical state of the released radionuclide, (e) Characteristics specific to the aquatic organism.
For the purpose of these generic calculations default values of Bp(Table XIII) have been selected to ensure that the transfer of dissolved radionuclides from water to aquatic organisms is conservatively estimated, and to avoid the possibility of substantial underestimation occurring for any specific application. As a result, the values given in Table XIII may differ from listings of Bpgiven in other reports.
TABLE XIII. ELEMENT SPECIFIC BIOACCUMULATION FACTOR Bpa Element Freshwater fish Marine fish Marine shellfish
(Bq·kg–1/Bq·L–1) (Bq·kg–1/Bq·L–1) (Bq·kg–1/Bq·L–1)
Ac 15 50 1 000
Ag 5 500 10 000
Am 30 50 20 000
As 500 1 000 2 000
At 15 10 50
Au 35 100 1 000
Ba 4 10 1
Bi 10 20 1 000
Br 400 3 10
Cb
Cd 200 1 000 20 000
Ce 30 50 5 000
Cm 30 50 30 000
Co 300 1 000 5 000
Cr 200 200 800
Cs 2 000–10 000c 100 30
Cu 200 700 2 000
Eu 50 300 7 000
Fe 200 3 000 30 000
Ga 400 700 700
Hb
Hg 1 000 20 000 20 000
I 40 10 10
In 10 000 1 000 10 000
Mn 400 400 5 000
Mo 10 10 100
Na 20 0.1 0.3
Nb 300 30 1 000
Ni 100 1 000 2 000
Np 30 10 400
P 50 000 30 000 20 000
Pa 10 50 500
Pb 300 200 1 000
Pd 10 300 300
Pm 30 500 5 000
Po 50 2 000 50 000
Pu 30 40 3 000
Ra 50 500 1 000
Rb 2 000 100 20
5.2.3. Adjustment of Bpfor the effect of suspended sediment
For generic assessment purposes it is often simpler to use the total concentrations of radionuclides (Cw, tot) estimated by the equations in Sections 4.3 to 4.6 rather than the dissolved radionuclide concentrations estimated in Section 4.7.
This introduces conservatism for particle reactive radionuclides because some fraction of these will be adsorbed on to suspended particulate matter and thus will be unavailable for biological uptake. If this pathway is important and better estimates are needed, then the dissolved concentrations may be estimated (see Section 4.7).
5.2.4. Adjustment of Bpfor caesium and strontium in freshwater fish
In Table XIII ranges are given for the values of Bpfor strontium and caesium in freshwater fish. For regions with sedimentary bedrock, clay rich soil and hard water the lower values should be selected. For regions with igneous bedrock, sandy or organic soils, and soft water the higher values are appropriate. If dissolved potassium and suspended sediment concentrations are known, the site specific Bp value for TABLE XIII. (cont.)
Element Freshwater fish Marine fish Marine shellfish (Bq·kg–1/Bq·L–1) (Bq·kg–1/Bq·L–1) (Bq·kg–1/Bq·L–1)
Rh 10 100 1 000
Ru 10 2 2 000
S 800 2 4
Sb 100 400 400
Se 200 6 000 6 000
Sr 15–75c 2 2
Tc 20 30 1 000
Te 400 1 000 1 000
Th 100 600 1 000
Tl 1 000 5 000 5 000
U 10 1 30
Y 30 20 1 000
Zn 1 000 1 000 50 000
Zr 300 20 5 000
a Values derived from Refs [6, 55, 68–74].
b Models for tritium and carbon are covered separately in Annex III.
c See Section 5.2.4 for explanation.
caesium can be estimated as described in Ref. [6]. Similarly, if dissolved calcium concentrations are known, the site specific Bpfor strontium can be estimated [6, 68].
5.2.5. Biota not included in this Safety Report
Although the categories of aquatic biota presented in Table XIII encompass those most frequently consumed, it is recognized that in some regions freshwater crustaceans and marine macroalgae are also consumed. Bp values for freshwater crustaceans are generally unavailable, but may be assumed to be ten times greater than those for freshwater fish, with the exception of caesium, whose Bpvalue is three times lower. For marine macroalgae Bpvalues are presented in Ref. [55].
5.3. UNCERTAINTY ASSOCIATED WITH THESE PROCEDURES
As discussed earlier, the models and default parameter values presented here are intended for use in calculating doses for screening purposes, such that they are generally likely to overestimate doses received and are unlikely to underestimate real doses by more than a factor of ten under any circumstances. They have been based partly on recommendations for similar models presented elsewhere (see Ref. [9]), where further discussion on uncertainties is given. Particular points are also made here.
(a) The models for terrestrial food chain transfer are thought to be generally conservative. In particular, they do not include any allowance for the reduction in radionuclide concentrations owing to food preparation and processing, which can be significant.
(b) The models for transfer of radionuclides to milk and meat are based on information for cattle. However, it is expected that their use for other animals should not lead to substantial underestimation. In particular, the predicted concentrations in milk should not be more than a factor of three less than the actual concentration, even for milk from other species.
(c) For predicting the transfer of radionuclides to aquatic foods, a simple concentration factor approach is adopted through the use of bioaccumulation factors. These factors have been chosen specifically for screening procedures and thus are thought to be conservative.