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An impact assessment for the French nuclear test sites in French Polynesia
Volume 2 6 / N u m b e r 9/September 1993
Table 3. According to this Table, the most important associations were found between Fe and Cd (r 2= 0.98, p=0.0001) and Mo and Pb (r2--0.98, p -- 0.0001). These associations are depicted in Figs 2 and 3. The sediment in the Antarctic region is mainly derived from the weathering of local rocks, and the inter-element interactions suggested in Table 3 are likely to reflect the composition of the parent rocks. As shown in Table 1, certain of the sediment samples from Horse Shoe Island and Marsh Martin were collected from obviously contaminated areas. Relatively high concentrations of Cd, Cr, and V were found in the samples collected from Horse Shoe Island, whereas concentrations of Cd, Co, Cu, Mo, Ni, Pb, V, and Zn were higher in the samples collected from Marsh
Martin. These data suggest that the nearby anthropogenie activities have started to impact sediment quality. Some contamination was found due to leaking fuel barrels and this is likely to constitute a source of several trace metals, especially Ni and V.
Ah. rine Pollution Bulletin,
presented as acceptable as they are in good agreement with the known features of the general circulation of the South Pacific. This point, which is secondary to the following reasoning, is not discussed here either. It should simply be pointed out that although the general circulation of the South Pacific may indeed be dominated by a major tropical anticycline cell, much recent data, particularly that obtained with floating buoys released from Mururoa, show a far more complex situation. In reality, a zone of mobile convergence, reaching as far as the Samoa-Cook-SocieteTuamotu-Gambier line, in the cold season, separates two circulation cells. To the East, the warm Easter Island cell, which is the larger one, is virtually permanent, whereas to the West, the cold cell of Melanesia, is more mobile and less stable. The result of this situation is normally a highly-contorted pattern in the superficial oceanic trajectories leaving Mururoa. Many of them turn back on themselves and do not manage to escape from the highly self-contained zone constituted by French Polynesia. Those which escape are captured, depending on the season, either by the Westward South Equatorial current in the North, or by the Eastward Subantarctic circumpolar drift in the South. Though we have no particular criticism of the model itself, or more specifically the advection-diffusion equation, used for the impact evaluation, we have our reservations concerning the numerical values attributed to the diffusion coefficients which, according to the authors, represent the parameters of the vertical and horizontal turbulent exchanges.
Voume 26, No. 9, pp. 527-530, 1993, Pri-lted in Great Britain.
0025 326X/93 $6.00+0.00 © 1993PergamonPressLtd
An Impact Assessment for the French Nuclear Test Sites in French Polynesia In an article published in 1990 in the Marine Pollution Bulletin (21(11), 536-542), J. Ribbe and M. Tomczak of the Ocean Sciences Institute of the University of Sydney, Australia, describe, for two different scenarios, the results of an assessment of the impact on the South Pacific of underground nuclear tests in French Polynesia. The results were obtained for hypothetical releases of cesium-137, considered an indicator, dispersed only in the oceanic environment. The model used was a conventional three-dimensional numerical transport/diffusion model. The transport, or advection, is covered by a coupled ocean/atmosphere model. The diffusion is modelled by a Monte Carlo technique applied to Lagrangian monitoring of particles. The following aspects of the work call for criticisms or comments: • the release hypotheses, • the numeric parameters and data necessary for calculating advection, • the numeric parameters and data necessary for calculating diffusion, • the discussion and the conclusion. The release hypotheses are not discussed here. It must however be pointed out that 3.2× 1017 Bq of cesium-137, corresponding to 60 Mt, released instantaneously, or 3.2 X 1016 Bq of cesium-137, corresponding to 6 Mt, released annually for 10 years, are equally unrealistic, but the fact that they are clearly overestimated only adds further weight to the comments following the conclusions. The data concerning the processing of advection are
The authors are indebted to the Saudi Government for bestowing an opportunity to participate in the Trans-Antarctic Expedition. The authors gratefully acknowledge the support of the Research Institute of King Fahd University of Petroleum and Minerals in carrying out this study. Appreciation is also extended to the laboratory staff who participated in the analytical work.
Sadiq, M. & Zaidi, T. H. (1985). Metal concentrations in the sediments from the Arabian Gulf coast of Saudi Arabia. Bull. Environ. Contain. Toxicol. 34,565-571.
H o r i z o n t a l D i f f u s i o n Coefficient K h The horizontal diffusion is correctly assumed to be isotropic, but it is also, and in this instance wrongly, assumed to be constant over the time of transfer. The frequency range of the spectrum of the turbulence 527
M a r i n e Pollution Bulletin
responsible for horizontal diffusion, both in air and in water, as well as the inevitable coupling between these frequencies and the dimensions of the radioactive cloud, indeed, suggests that the intensity of the turbulent exchanges depends directly on the size reached by such a cloud and consequently the age, or the duration of transfer, of the cloud. Full-scale studies (Okubo, 1969) have substantiated this point and a French oceanic model (Doury & Badie, 1973; Doury, 1989) has been created, corresponding to the results of these tests, with horizontal diffusion coefficients which are functions of the duration of transport and which allow for frequency/dimension coupling. From the comparison between his model and that of Ribbe & Tomczak, it becomes apparent that the single value of 1 0 3 m 2 s -~, used by the authors for the horizontal turbulent diffusion coefficient, actually corresponds to a transfer duration of 107 s, i.e. 4 months, and that the diffusion, slower before this period (10 -4 m 2 s -~ at 100 s), becomes in reality considerably greater beyond it (105 m 2 s-~ at 10 yr). If the time scale proposed was restricted to a few months, the consequences of the disagreement would be negligible. After 10 years, the resulting overestimation of concentration reaches a factor of 40.
Q = 3,2.10 17 Bq
100 000 Bq.m -3 10 000
,,,. t
ooo
~,.~
\
100
4,
"<.
~
1°t
" ~ ' ~ ' 0 . . . . . -0 . . . . .
"
....
Previous Level.
,, ~oo,''' [
•
20
,~8o,o---~....~. ?'lP,, 40
60
80
100
,Days, Months
4 Months
Fig. I M a x i m u m concentration of cesium-137 at the surface (Bq m ~) as a function of the duration of transfer for the 'worst case' scenario (unrealistic). R.T.: Ribbe & Tomczak's calculation. D.B.: D o u r y & Badie's calculation.
T A B L E 1.
Vertical Diffusion Coefficient K z Due to the narrower range of the vertical turbulence frequency spectra, the vertical diffusion coefficients depend much less on the duration of transfer than their horizontal counterpart, with the result that this dependence can even be disregarded. On the other hand, the same does not apply to the relationship with depth of the layers considered. It is thus, in agreement with Okubo, that the French model utilizes the following median values for the vertical diffusion coefficient Kz: • • • •
in superficial layer: 3 x 10 -3 m 2 s -~ near the bottom and in estuaries: 3 × 10 -4 m 2 s -~ in deep layers: 1 X 10 .4 m 2 s-] in the thermocline layer: 1 X 10 -5 m 2 s -~
It would appear that in the superficial layer, the one in question in this instance, Ribbe & Tomczak have adopted a value for Kz which, in our opinion, is 30 times too low, i.e. 10 -4 m 2 s -], and which would be more suitable for deep layers. The result is an underestimation of vertical diffusion resulting in a further overestimation of concentrations by a factor of 30 °5, i.e. 5.5.
Discussion of the Two Scenarios Proposed The article proposes two scenarios based on different hypotheses, one, somewhat unjustifiably, designated as 'realistic' and the other, justifiably, as 'severe'. Here we shall simply point out the highly unrealistic and conservative nature of the two hypotheses.
'Realistic'hypothesis It is assumed that 10% of the total amount of cesium137 arising in the 60 Mt of fission explosions regularly 528
C o m p a r i s o n of French and Australian calculations for a c o n t i n u o u s oceanic release of 1.0X 10 ~ Bq s ~.
Distance (km) 4 90 250 500 700 1000
Maximum concentration D o u r y & Badie Ribbe & Tomczak 0.1 m s ' ( I m s -~ 0 . 0 2 m s -~ Bq m 3 Bq m ~ Bq m ~ 45 000 270 45 15 9 4.5
10 000 60 10 3.3 2 1
Illegible 60 50 3O 2O 10
escapes from the Mururoa atoll every year for 10 years. This would involve a permanent continuous release of 3.2 x 10 j6 Bq yr -], i.e. 1.0 x 10 s Bq s -1 for 10 years. If a reasonable mean advection velocity of 10 cm s -] represents covering a distance of 3000 km yr -1, it can be assumed, without knowing the true trajectories, that steady-state conditions, i.e. at constant concentration, are fully established after 10 years. Thus, by considering an instant beyond the initial transient conditions, and disregarding the directions, it is possible, using the French model, to immediately calculate maximum possible concentrations located at points on circles of different radii centred on the source point, i.e. Mururoa. The following table gives six values of such maximum concentrations for six distances, as well as six corresponding approximate values deduced from Fig. 3 (Ribbe & Tomczak, p. 540). As the Australian calculations were made for a very low mean transport velocity (0.02 m s-t), in Table 2, alongside a French calculation for a more probable mean velocity of 0.1 m s -1, a second French calculation for a velocity of virtually zero is given. It can be seen that, as could be expected in view of the values taken
Volume 2 6 / N u m b e r 9 / S e p t e m b e r 1993 TABLE 2 Comparison of French and Australian calculations for instantaneous oceanic release of 3.2 X 10 ~vBq s -L. Distance Duration of transfer (days) 3(I 1211 7_(1tt 600 I b 00 3('00
0.02 m s ~ km
0.10 m s ~ km
60 200 34(I 100(I 3200 5200
300 1000 1700 5000 16 000 26 000
for the diffusion coefficients, the evaluations of the two models coincide precisely between 100 and 200 km, i.e. the very distance compatible with the diffusion velocity of 1 km day -~, implied by the French model, or also with 3-6 months of transfer. It is thus naturally seen that the French assessments, which are higher before the time of coincidence, become rapidly lower beyond it. up to a factor of 10. ' 1Vorst case' hypothesis
There has been no compunction in imagining that all the cesium-137 produced in 60 Mt of fission explosions, i.e. 3.2 X 1017 Bq, escapes instantaneously from the Mururoa atoll at once. This gives rise to a single 'cloud' for which it is possible to immediately calculate the maximum concentration at its centre of mass, using the French model. In the following table, six values are given for the maximum concentration corresponding to six transfer durations and six corresponding values deduced from Fig. 4 (Ribbe & Tomczak, p. 541). In this table, and even more clearly in the following figure, it is seen how the two series of estimates of maximum concentration as a function of the duration of transfer compare to each other. It will, in particular, be noticed that there is an intersection at 120 days, i.e. 4 months, at which the initially higher French values deep below the Australian values, after which the French values become increasingly lower than the Australian values, by up to a factor of around 200 towards the end of the 10-year period adopted by the authors of the article. If it is wished to express the duration of transfer in terms of distance covered, the mean advection velocity hypothesis becomes primordial. Experience shows that the value of this velocity must reasonably be between 0.01 and 0.10 m s- l and that the real trajectories, with their complicated meanderings, are unknown and will remain so for a long time. An argument based on the mean velocity and the shortest distance has at least the advantage of supplying a first concentration value which is certainly overestimated. It is thus that with a velocity close to the lower limit, i.e. precisely 0.02 m s-I adopted by the Australian authors, it is found, according to Table 2, that the countries bordering the South-West Pacific, at a minimum distance of 300051100 kin, would be affected by concentrations which would certainly be less than 160 Bq m -3 proposed by the Australian calculations and the 2 Bq m -3 proposed bv the French calculations. With the higher value at 0.10 m s-I, the distances reached, all other conditions being
Maximum concentration Doury & Badie Ribbe & Tomczak Bq m -3 Bq m 3 1.95 × 105 3.20 )<' 1 0 4 0.96 × 103 32 1.9 0.5
1.0 X 105 4.0 )< 104 1.8 X 104 2150 160 100
equal, by the same concentration levels would of course be five times higher, but in view of the meanders and the exact length of the real trajectories, the situation would probably be equivalent to that of the preceding case.
Conclusions In all cases, and extremely luckily, the models confirm that the highest concentrations of radioactivity would be found in the vicinity of the potential source. In the case of the 'realistic' case, whatever the value adopted for the current velocity, the French evaluations remain lower than the Australian evaluations up to 250 km, and reach the previous levels of 3-4 Bq m -3 at around 1000 km, where the Australian estimations are still around 10 Bq m -3. In the highly unrealistic 'worst case', the French estimate falls below the Australian estimate 4 months after the release, and thereafter decreases far more rapidly to below the previous levels during the first year of transfer and below 100th of the Australian estimate during the 10th year of transfer. It is true, furthermore, that the Australian hypothesis for the average current velocity, whose value (0.02 m s-1) is a little small, tends to compensate, particularly in the 'worst case', for overestimation of concentration by underestimation of distance. But it is nonetheless true that, the true trajectories being unknown, a concentration calculation over the shortest distance with a more realistic average velocity of 0.1 m s-j would represent a considerable overestimation as the true trajectories between two points, particularly in the superficial layer of the ocean, are always longer than the shortest distance between the two points. A more comprehensive calculation, including, if possible, constructions of average trajectories constituting a better representation of the real trajectories would be helpful. The advantage of the calculation for the shortest distances nevertheless resides in the fact that, in view of the uncertainty concerning the true trajectories, the observer is assured that there will never be greater exposure than that deduced from the calculation.
Note The conclusion of the article by Ribbe & Tomczak is preceded by a curious paragraph intended to prove that both the 'worst case' and the 'realistic' hypotheses should be taken seriously because, although nuclear industry generally accepts that there would be an 529
Marine Pollution Bulletin
which produced good agreement with observations. We tested our model of the South Pacific Ocean with tritium data and again found good agreement with the observed distribution of tritium. We concur with A. Dury that a relationship exists between time and the size of the dispersing cloud and consequently the horizontal diffusion coefficient. This has been derived from field experiments with, for example, rhodamine B and is well established knowledge. However, it is important to note that such experiments and the derived relationships describe the spreading of the diffusive material close to the source (the near-field behaviour). Our study was concerned with the far-field distribution when horizontal dispersion of the tracer is dominated by the advective field and turbulent diffusion becomes relatively unimportant (a number of authors therefore occasionally propose to A. DOURY omit horizontal diffusion altogether). Using a constant French Atomic Energy Commission, 2 rue de l'Eglise, diffusion coefficient is certainly acceptable for such a F-92420 Vaucresson, France situation. The cross-over point between near-field behaviour and far-field behaviour is found where the Ribbe, J. & Tomczak, M. (199(I). An Impact Assessment for the French two approaches give identical concentrations. According Nuclear Weapon Test Sites in French Polynesia. Mar Polha. Bull. 21, to Doury (his Table 1) this occurs somewhere near 536-542. 100-200 km from the source. Rather than disproving Okubu, A. & Pritchard, D. (1969). Summary of our present knowledge of the physical processes of mixing in the ocean and coastal waters, our results, Doury's calculations complement ours by and a set of practical guidelines for the application of existing giving more accurate estimates for the near-field, where diffusion equations in the preparation of nuclear safety evaluations of the use of nuclear sources in the sea. Report NYO 3109-40, the concentrations turn out to be much higher than Chesapeake Bay Institute, The John Hopkins University. estimated by our model. Doury, A. & Badie, Ch. (1973). Une mdthode pratique pour la Finally, we agree with A. Doury that the current provision numdrique des pollutions ocdaniques. Rapport CEA-Rvelocities used in our model are on the small side and 4512, Saclay. Doury, A. (1989). Une mdthode simple et pratique d'6valuation de la not necessarily representative for currents that might be pollution ocdanique. Rapport CEA-R-5507, Saclay. found at a particular place and time. Again, Doury's comments on current features in the vicinity of Polynesia are of importance when the near-field distribution is considered but do not influence the farfield situation, where eddies and contortions are represented through long-term means. Our decision to base the circulation data on a numerical model was motivated by a desire not to assume possible but unWe appreciate the opportunity to respond to some verifiable current patterns. We considered a circulation points raised by A. Doury in relation to our own study. derived from physical principles and based on the We do not intend to discuss how realistic our release equations of motion the next best option, even if it has hypotheses are. They were obviously designed to relate its intrinsic limitations. A better approach would to the French underground testing of nuclear bombs, a require an eddy-resolving model, in which all conpractice fortunately halted some time ago that has very tortions are resolved and all velocities are therefore little chance of being revived again. In that sense the realistically large. Experience with such models shows present debate is now only of academic interest. that they change the circulation in some regions, We agree with A. Doury that horizontal and vertical particularly in the equatorial zone, in wester boundary mixing processes are far more complex than expressed currents and in the Circumpolar Current, but not in the and parameterized in our study, which used constant centres of the subtropical gyres. diffusion coefficients. We disagree with the approach In summary, Doury's study is a valid addition to our taken by A. Doury for a number of reasons. results. It gives more accurate (and significantly higher) With the exception of upwelling and downwelling estimates of cesium concentrations in the vicinity of regions, vertical current velocities in the ocean are Polynesia but does not invalidate our findings for the generally quite small. The most important factor for remainder of the South Pacific. vertical transport of conservative tracers is vertical mixing. Its parameterization is therefore crucial to the J O A C H I M R I B B E and M A T T H I A S T O M C Z A K Flinders Institute for Atmospheric and Marine Sciences, success of any model. Our parameterization has been Flinders University of South Australia, GPO Box 2100, used successfully in a number of studies by others, Adelaide 5001, Australia accident every 100 000 years, during the 50 years following the first nuclear fission, major uncontrolled releases of artificial radioactivity have occurred, particularly at Windscale in 1957, near Harrisburg in 1976 and near Chernobyl in 1986. Here it obviously must be pointed out that although the releases from Chernobyl may have been considerable (natural level at 1000 km), those of Windscale were relatively small (natural level at 1 km), and those at Harrisburg (1979 and not 1976) were virtually null (natural level at 100 m). It can also be added that it would be hard to imagine a fire similar to that at Chernobyl causing all the vitrified products in the Polynesian basalt to be released into the environment. One is left wondering about the nature of the probability calculations which could give rise to such declarations.
Reply
530
Table 3. According to this Table, the most important associations were found between Fe and Cd (r 2= 0.98, p=0.0001) and Mo and Pb (r2--0.98, p -- 0.0001). These associations are depicted in Figs 2 and 3. The sediment in the Antarctic region is mainly derived from the weathering of local rocks, and the inter-element interactions suggested in Table 3 are likely to reflect the composition of the parent rocks. As shown in Table 1, certain of the sediment samples from Horse Shoe Island and Marsh Martin were collected from obviously contaminated areas. Relatively high concentrations of Cd, Cr, and V were found in the samples collected from Horse Shoe Island, whereas concentrations of Cd, Co, Cu, Mo, Ni, Pb, V, and Zn were higher in the samples collected from Marsh
Martin. These data suggest that the nearby anthropogenie activities have started to impact sediment quality. Some contamination was found due to leaking fuel barrels and this is likely to constitute a source of several trace metals, especially Ni and V.
Ah. rine Pollution Bulletin,
presented as acceptable as they are in good agreement with the known features of the general circulation of the South Pacific. This point, which is secondary to the following reasoning, is not discussed here either. It should simply be pointed out that although the general circulation of the South Pacific may indeed be dominated by a major tropical anticycline cell, much recent data, particularly that obtained with floating buoys released from Mururoa, show a far more complex situation. In reality, a zone of mobile convergence, reaching as far as the Samoa-Cook-SocieteTuamotu-Gambier line, in the cold season, separates two circulation cells. To the East, the warm Easter Island cell, which is the larger one, is virtually permanent, whereas to the West, the cold cell of Melanesia, is more mobile and less stable. The result of this situation is normally a highly-contorted pattern in the superficial oceanic trajectories leaving Mururoa. Many of them turn back on themselves and do not manage to escape from the highly self-contained zone constituted by French Polynesia. Those which escape are captured, depending on the season, either by the Westward South Equatorial current in the North, or by the Eastward Subantarctic circumpolar drift in the South. Though we have no particular criticism of the model itself, or more specifically the advection-diffusion equation, used for the impact evaluation, we have our reservations concerning the numerical values attributed to the diffusion coefficients which, according to the authors, represent the parameters of the vertical and horizontal turbulent exchanges.
Voume 26, No. 9, pp. 527-530, 1993, Pri-lted in Great Britain.
0025 326X/93 $6.00+0.00 © 1993PergamonPressLtd
An Impact Assessment for the French Nuclear Test Sites in French Polynesia In an article published in 1990 in the Marine Pollution Bulletin (21(11), 536-542), J. Ribbe and M. Tomczak of the Ocean Sciences Institute of the University of Sydney, Australia, describe, for two different scenarios, the results of an assessment of the impact on the South Pacific of underground nuclear tests in French Polynesia. The results were obtained for hypothetical releases of cesium-137, considered an indicator, dispersed only in the oceanic environment. The model used was a conventional three-dimensional numerical transport/diffusion model. The transport, or advection, is covered by a coupled ocean/atmosphere model. The diffusion is modelled by a Monte Carlo technique applied to Lagrangian monitoring of particles. The following aspects of the work call for criticisms or comments: • the release hypotheses, • the numeric parameters and data necessary for calculating advection, • the numeric parameters and data necessary for calculating diffusion, • the discussion and the conclusion. The release hypotheses are not discussed here. It must however be pointed out that 3.2× 1017 Bq of cesium-137, corresponding to 60 Mt, released instantaneously, or 3.2 X 1016 Bq of cesium-137, corresponding to 6 Mt, released annually for 10 years, are equally unrealistic, but the fact that they are clearly overestimated only adds further weight to the comments following the conclusions. The data concerning the processing of advection are
The authors are indebted to the Saudi Government for bestowing an opportunity to participate in the Trans-Antarctic Expedition. The authors gratefully acknowledge the support of the Research Institute of King Fahd University of Petroleum and Minerals in carrying out this study. Appreciation is also extended to the laboratory staff who participated in the analytical work.
Sadiq, M. & Zaidi, T. H. (1985). Metal concentrations in the sediments from the Arabian Gulf coast of Saudi Arabia. Bull. Environ. Contain. Toxicol. 34,565-571.
H o r i z o n t a l D i f f u s i o n Coefficient K h The horizontal diffusion is correctly assumed to be isotropic, but it is also, and in this instance wrongly, assumed to be constant over the time of transfer. The frequency range of the spectrum of the turbulence 527
M a r i n e Pollution Bulletin
responsible for horizontal diffusion, both in air and in water, as well as the inevitable coupling between these frequencies and the dimensions of the radioactive cloud, indeed, suggests that the intensity of the turbulent exchanges depends directly on the size reached by such a cloud and consequently the age, or the duration of transfer, of the cloud. Full-scale studies (Okubo, 1969) have substantiated this point and a French oceanic model (Doury & Badie, 1973; Doury, 1989) has been created, corresponding to the results of these tests, with horizontal diffusion coefficients which are functions of the duration of transport and which allow for frequency/dimension coupling. From the comparison between his model and that of Ribbe & Tomczak, it becomes apparent that the single value of 1 0 3 m 2 s -~, used by the authors for the horizontal turbulent diffusion coefficient, actually corresponds to a transfer duration of 107 s, i.e. 4 months, and that the diffusion, slower before this period (10 -4 m 2 s -~ at 100 s), becomes in reality considerably greater beyond it (105 m 2 s-~ at 10 yr). If the time scale proposed was restricted to a few months, the consequences of the disagreement would be negligible. After 10 years, the resulting overestimation of concentration reaches a factor of 40.
Q = 3,2.10 17 Bq
100 000 Bq.m -3 10 000
,,,. t
ooo
~,.~
\
100
4,
"<.
~
1°t
" ~ ' ~ ' 0 . . . . . -0 . . . . .
"
....
Previous Level.
,, ~oo,''' [
•
20
,~8o,o---~....~. ?'lP,, 40
60
80
100
,Days, Months
4 Months
Fig. I M a x i m u m concentration of cesium-137 at the surface (Bq m ~) as a function of the duration of transfer for the 'worst case' scenario (unrealistic). R.T.: Ribbe & Tomczak's calculation. D.B.: D o u r y & Badie's calculation.
T A B L E 1.
Vertical Diffusion Coefficient K z Due to the narrower range of the vertical turbulence frequency spectra, the vertical diffusion coefficients depend much less on the duration of transfer than their horizontal counterpart, with the result that this dependence can even be disregarded. On the other hand, the same does not apply to the relationship with depth of the layers considered. It is thus, in agreement with Okubo, that the French model utilizes the following median values for the vertical diffusion coefficient Kz: • • • •
in superficial layer: 3 x 10 -3 m 2 s -~ near the bottom and in estuaries: 3 × 10 -4 m 2 s -~ in deep layers: 1 X 10 .4 m 2 s-] in the thermocline layer: 1 X 10 -5 m 2 s -~
It would appear that in the superficial layer, the one in question in this instance, Ribbe & Tomczak have adopted a value for Kz which, in our opinion, is 30 times too low, i.e. 10 -4 m 2 s -], and which would be more suitable for deep layers. The result is an underestimation of vertical diffusion resulting in a further overestimation of concentrations by a factor of 30 °5, i.e. 5.5.
Discussion of the Two Scenarios Proposed The article proposes two scenarios based on different hypotheses, one, somewhat unjustifiably, designated as 'realistic' and the other, justifiably, as 'severe'. Here we shall simply point out the highly unrealistic and conservative nature of the two hypotheses.
'Realistic'hypothesis It is assumed that 10% of the total amount of cesium137 arising in the 60 Mt of fission explosions regularly 528
C o m p a r i s o n of French and Australian calculations for a c o n t i n u o u s oceanic release of 1.0X 10 ~ Bq s ~.
Distance (km) 4 90 250 500 700 1000
Maximum concentration D o u r y & Badie Ribbe & Tomczak 0.1 m s ' ( I m s -~ 0 . 0 2 m s -~ Bq m 3 Bq m ~ Bq m ~ 45 000 270 45 15 9 4.5
10 000 60 10 3.3 2 1
Illegible 60 50 3O 2O 10
escapes from the Mururoa atoll every year for 10 years. This would involve a permanent continuous release of 3.2 x 10 j6 Bq yr -], i.e. 1.0 x 10 s Bq s -1 for 10 years. If a reasonable mean advection velocity of 10 cm s -] represents covering a distance of 3000 km yr -1, it can be assumed, without knowing the true trajectories, that steady-state conditions, i.e. at constant concentration, are fully established after 10 years. Thus, by considering an instant beyond the initial transient conditions, and disregarding the directions, it is possible, using the French model, to immediately calculate maximum possible concentrations located at points on circles of different radii centred on the source point, i.e. Mururoa. The following table gives six values of such maximum concentrations for six distances, as well as six corresponding approximate values deduced from Fig. 3 (Ribbe & Tomczak, p. 540). As the Australian calculations were made for a very low mean transport velocity (0.02 m s-t), in Table 2, alongside a French calculation for a more probable mean velocity of 0.1 m s -1, a second French calculation for a velocity of virtually zero is given. It can be seen that, as could be expected in view of the values taken
Volume 2 6 / N u m b e r 9 / S e p t e m b e r 1993 TABLE 2 Comparison of French and Australian calculations for instantaneous oceanic release of 3.2 X 10 ~vBq s -L. Distance Duration of transfer (days) 3(I 1211 7_(1tt 600 I b 00 3('00
0.02 m s ~ km
0.10 m s ~ km
60 200 34(I 100(I 3200 5200
300 1000 1700 5000 16 000 26 000
for the diffusion coefficients, the evaluations of the two models coincide precisely between 100 and 200 km, i.e. the very distance compatible with the diffusion velocity of 1 km day -~, implied by the French model, or also with 3-6 months of transfer. It is thus naturally seen that the French assessments, which are higher before the time of coincidence, become rapidly lower beyond it. up to a factor of 10. ' 1Vorst case' hypothesis
There has been no compunction in imagining that all the cesium-137 produced in 60 Mt of fission explosions, i.e. 3.2 X 1017 Bq, escapes instantaneously from the Mururoa atoll at once. This gives rise to a single 'cloud' for which it is possible to immediately calculate the maximum concentration at its centre of mass, using the French model. In the following table, six values are given for the maximum concentration corresponding to six transfer durations and six corresponding values deduced from Fig. 4 (Ribbe & Tomczak, p. 541). In this table, and even more clearly in the following figure, it is seen how the two series of estimates of maximum concentration as a function of the duration of transfer compare to each other. It will, in particular, be noticed that there is an intersection at 120 days, i.e. 4 months, at which the initially higher French values deep below the Australian values, after which the French values become increasingly lower than the Australian values, by up to a factor of around 200 towards the end of the 10-year period adopted by the authors of the article. If it is wished to express the duration of transfer in terms of distance covered, the mean advection velocity hypothesis becomes primordial. Experience shows that the value of this velocity must reasonably be between 0.01 and 0.10 m s- l and that the real trajectories, with their complicated meanderings, are unknown and will remain so for a long time. An argument based on the mean velocity and the shortest distance has at least the advantage of supplying a first concentration value which is certainly overestimated. It is thus that with a velocity close to the lower limit, i.e. precisely 0.02 m s-I adopted by the Australian authors, it is found, according to Table 2, that the countries bordering the South-West Pacific, at a minimum distance of 300051100 kin, would be affected by concentrations which would certainly be less than 160 Bq m -3 proposed by the Australian calculations and the 2 Bq m -3 proposed bv the French calculations. With the higher value at 0.10 m s-I, the distances reached, all other conditions being
Maximum concentration Doury & Badie Ribbe & Tomczak Bq m -3 Bq m 3 1.95 × 105 3.20 )<' 1 0 4 0.96 × 103 32 1.9 0.5
1.0 X 105 4.0 )< 104 1.8 X 104 2150 160 100
equal, by the same concentration levels would of course be five times higher, but in view of the meanders and the exact length of the real trajectories, the situation would probably be equivalent to that of the preceding case.
Conclusions In all cases, and extremely luckily, the models confirm that the highest concentrations of radioactivity would be found in the vicinity of the potential source. In the case of the 'realistic' case, whatever the value adopted for the current velocity, the French evaluations remain lower than the Australian evaluations up to 250 km, and reach the previous levels of 3-4 Bq m -3 at around 1000 km, where the Australian estimations are still around 10 Bq m -3. In the highly unrealistic 'worst case', the French estimate falls below the Australian estimate 4 months after the release, and thereafter decreases far more rapidly to below the previous levels during the first year of transfer and below 100th of the Australian estimate during the 10th year of transfer. It is true, furthermore, that the Australian hypothesis for the average current velocity, whose value (0.02 m s-1) is a little small, tends to compensate, particularly in the 'worst case', for overestimation of concentration by underestimation of distance. But it is nonetheless true that, the true trajectories being unknown, a concentration calculation over the shortest distance with a more realistic average velocity of 0.1 m s-j would represent a considerable overestimation as the true trajectories between two points, particularly in the superficial layer of the ocean, are always longer than the shortest distance between the two points. A more comprehensive calculation, including, if possible, constructions of average trajectories constituting a better representation of the real trajectories would be helpful. The advantage of the calculation for the shortest distances nevertheless resides in the fact that, in view of the uncertainty concerning the true trajectories, the observer is assured that there will never be greater exposure than that deduced from the calculation.
Note The conclusion of the article by Ribbe & Tomczak is preceded by a curious paragraph intended to prove that both the 'worst case' and the 'realistic' hypotheses should be taken seriously because, although nuclear industry generally accepts that there would be an 529
Marine Pollution Bulletin
which produced good agreement with observations. We tested our model of the South Pacific Ocean with tritium data and again found good agreement with the observed distribution of tritium. We concur with A. Dury that a relationship exists between time and the size of the dispersing cloud and consequently the horizontal diffusion coefficient. This has been derived from field experiments with, for example, rhodamine B and is well established knowledge. However, it is important to note that such experiments and the derived relationships describe the spreading of the diffusive material close to the source (the near-field behaviour). Our study was concerned with the far-field distribution when horizontal dispersion of the tracer is dominated by the advective field and turbulent diffusion becomes relatively unimportant (a number of authors therefore occasionally propose to A. DOURY omit horizontal diffusion altogether). Using a constant French Atomic Energy Commission, 2 rue de l'Eglise, diffusion coefficient is certainly acceptable for such a F-92420 Vaucresson, France situation. The cross-over point between near-field behaviour and far-field behaviour is found where the Ribbe, J. & Tomczak, M. (199(I). An Impact Assessment for the French two approaches give identical concentrations. According Nuclear Weapon Test Sites in French Polynesia. Mar Polha. Bull. 21, to Doury (his Table 1) this occurs somewhere near 536-542. 100-200 km from the source. Rather than disproving Okubu, A. & Pritchard, D. (1969). Summary of our present knowledge of the physical processes of mixing in the ocean and coastal waters, our results, Doury's calculations complement ours by and a set of practical guidelines for the application of existing giving more accurate estimates for the near-field, where diffusion equations in the preparation of nuclear safety evaluations of the use of nuclear sources in the sea. Report NYO 3109-40, the concentrations turn out to be much higher than Chesapeake Bay Institute, The John Hopkins University. estimated by our model. Doury, A. & Badie, Ch. (1973). Une mdthode pratique pour la Finally, we agree with A. Doury that the current provision numdrique des pollutions ocdaniques. Rapport CEA-Rvelocities used in our model are on the small side and 4512, Saclay. Doury, A. (1989). Une mdthode simple et pratique d'6valuation de la not necessarily representative for currents that might be pollution ocdanique. Rapport CEA-R-5507, Saclay. found at a particular place and time. Again, Doury's comments on current features in the vicinity of Polynesia are of importance when the near-field distribution is considered but do not influence the farfield situation, where eddies and contortions are represented through long-term means. Our decision to base the circulation data on a numerical model was motivated by a desire not to assume possible but unWe appreciate the opportunity to respond to some verifiable current patterns. We considered a circulation points raised by A. Doury in relation to our own study. derived from physical principles and based on the We do not intend to discuss how realistic our release equations of motion the next best option, even if it has hypotheses are. They were obviously designed to relate its intrinsic limitations. A better approach would to the French underground testing of nuclear bombs, a require an eddy-resolving model, in which all conpractice fortunately halted some time ago that has very tortions are resolved and all velocities are therefore little chance of being revived again. In that sense the realistically large. Experience with such models shows present debate is now only of academic interest. that they change the circulation in some regions, We agree with A. Doury that horizontal and vertical particularly in the equatorial zone, in wester boundary mixing processes are far more complex than expressed currents and in the Circumpolar Current, but not in the and parameterized in our study, which used constant centres of the subtropical gyres. diffusion coefficients. We disagree with the approach In summary, Doury's study is a valid addition to our taken by A. Doury for a number of reasons. results. It gives more accurate (and significantly higher) With the exception of upwelling and downwelling estimates of cesium concentrations in the vicinity of regions, vertical current velocities in the ocean are Polynesia but does not invalidate our findings for the generally quite small. The most important factor for remainder of the South Pacific. vertical transport of conservative tracers is vertical mixing. Its parameterization is therefore crucial to the J O A C H I M R I B B E and M A T T H I A S T O M C Z A K Flinders Institute for Atmospheric and Marine Sciences, success of any model. Our parameterization has been Flinders University of South Australia, GPO Box 2100, used successfully in a number of studies by others, Adelaide 5001, Australia accident every 100 000 years, during the 50 years following the first nuclear fission, major uncontrolled releases of artificial radioactivity have occurred, particularly at Windscale in 1957, near Harrisburg in 1976 and near Chernobyl in 1986. Here it obviously must be pointed out that although the releases from Chernobyl may have been considerable (natural level at 1000 km), those of Windscale were relatively small (natural level at 1 km), and those at Harrisburg (1979 and not 1976) were virtually null (natural level at 100 m). It can also be added that it would be hard to imagine a fire similar to that at Chernobyl causing all the vitrified products in the Polynesian basalt to be released into the environment. One is left wondering about the nature of the probability calculations which could give rise to such declarations.
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