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DISPERSION OF RESIDUAL RADIONUCLIDES FROM MURUROA AND THROUGH THE OCEAN FROM THE TO THE SEAS BY PAVEL PETER POVINEC AND EKKEHARD MITTELSTAEDT

odelling the Working Group. A model of The outputs from these transport of the mixing of water was used models are estimates of Mradionuclides to estimate radionuclide radionuclide concentrations in through the marine concentrations in the lagoons the ocean as a function of time environment is a complex for given releases from and location. These exercise. As part of the Study underground into the lagoons, concentrations were used to on the Radiological Situation and the flow rates of estimate doses to real and at the Atolls of Mururoa and radionuclides from the lagoons hypothetical people at various Fangataufa, scientists into the surrounding ocean. locations and times in the investigated the dispersion of A sediment model was future. radionuclides after their release developed to predict the from Mururoa and Fangataufa movement of sediment MODELLING OF lagoons and from underground between the lagoons and the THE LAGOONS sources. The investigation ocean. Estimates were made of Radionuclides in the lagoon covered mixing processes the amount of sediment, and the waters arise from the leaching within the lagoons, discharge corresponding quantity of of radioactive material from into the ocean, and transport plutonium, leaving the lagoons the lagoon sediments and from to the shores of neighbouring annually under average weather migration of material from the 34 islands and distant continents. conditions, or with a severe underground sources. Their This article highlights work storm. concentrations in the lagoon carried out through the Study The rates of release of waters will be determined by to assess the dispersion of radionuclides from the lagoon their rate of release into the residual radionuclides through sediments were assessed, leading lagoon and the rate at which the ocean, and summarizes the to estimates of the release rates the lagoon waters are major findings. of plutonium, caesium-137, exchanged with the ocean. If strontium-90 and tritium from radionuclides are released into METHODOLOGY & the lagoons to the oceans as a the lagoon, the first two stages APPROACHES function of time. in the dispersion process are For modelling purposes, the Three compartment models mixing in the lagoon and the area around the source of were used to model dispersion exchange of lagoon water with radioactivity (i.e. the point at of radionuclides in the regional the ocean. A hydrodynamic which radionuclides are field. These models cover model of circulation in the released into the marine envi- different areas with different ronment) was divided into resolutions, and each has Mr. Povinec is Head of the three major zones : the near particular strengths and Radiometrics Section of the field (the lagoons); the regional weaknesses. Taken together, IAEA Marine Environment field (broadly, the area of they give an indication of the Laboratory in Monaco. Mr. ); and the far likely uncertainty in the Mittelstaedt, of the Federal field (the South Pacific beyond dispersion estimates, and Maritime and Hydrographic the regional field). improve the robustness of the Agency in Germany, served as To understand the water and final conclusions. Chairman of the Working sediment dynamics in the near Transport and dispersion in Group on Marine Modelling of field, two models were the far field was assessed using the Study on the Radiological developed by the Mururoa a predictive model of world Situation at the Atolls of IAEA BULLETIN, 40/4/1998 Study’s Marine Modelling ocean circulation. Mururoa and Fangataufa. Doc-09.qxd 1/21/99 10:19 AM Page 35

lagoons was developed and the ments and from underground the lagoons were estimated and average turnover time for sources. predictions for the future were exchange of water with the For tritium (a half-life of made. No attempt was made ocean was estimated. The 12.3 years), which is observed to distinguish Mururoa and turnover time, averaged over all in the form of water, there is Fangataufa as separate sources seasons, was estimated to be no contribution from the since, on an oceanic scale, they 98+ 37 days, and 33+ 12 days sediments. The observed are so close together as to be for Mururoa and Fangataufa concentrations (about 200 indistinguishable. The releases lagoons, respectively. Bq/m3, a factor of two above via the lagoons are considered The ultimate fate of the the oceanic background) are to be surface releases; releases unconsolidated sediments on entirely due to underground direct to the ocean are assumed the bottom of the lagoons (and sources. Tritium to occur at a depth of 400 the plutonium in them) is concentrations in the lagoons meters. important in assessing the may remain fairly constant for The total release rates of long-term implications of the the next few decades before tritium, strontium-90, and radiological conditions at the declining slowly. caesium-137 will decrease with atolls. For strontium-90, time, with the dominant A sediment transport model expectations are that both contribution from was developed for Mururoa leaching and underground underground sources coming lagoon; it led to a tentative sources are contributing to from releases directly into the estimate for the average annual concentrations. Although a ocean. (See graphs.) On the removal rate of bottom considerable scatter in the past other hand, in the case of sediments through the pass to data on strontium-90 was plutonium, which will migrate the ocean of 80,000 tonnes per observed, it appears that the very slowly, the underground year. A cyclonic storm is, radionuclide’s concentrations contribution will peak after however, capable of removing could rise above current levels about 6000 years at a much very large quantities of for a few decades, though not lower peak value than at 35 sediment from a lagoon — one more than by a factor of two. present. such storm could remove about In the case of caesium-137, In the case of the assumed 4 x 106 tonnes of sediment, the current release from disruptive event, i.e. a mostly over the rim of the underground sources is slightly carbonate rock slide with an . related to leaching of instantaneous release of The annual equivalent sediments. Its concentrations radionuclides to the marine release rate of plutonium from in the lagoons have been environment from this source was estimated to be decreasing with an apparent underground sources, the of the order of 0.1 TBq. The half-life of about seven years estimated releases were 1PBq source term is expected to and it is unlikely that they will for tritium, 30 TBq for decrease with an effective half- exceed present levels at any caesium-137, 10 TBq for life of about ten years because time in the future. strontium-90 and 10 TBq for of the progressive burial of For plutonium, leaching of plutonium-239,240. plutonium-bearing sediments lagoon sediments is currently by fresh sediment. the only significant source. REGIONAL MODELS OF On the basis of observations it DISPERSION RELEASE RATES OF is estimated that plutonium RADIONUCLIDES concentrations in the lagoons Three compartment models Four radionuclides (tritium, will decrease with an effective were developed to assess dis- strontium-90, caesium-137 half-life of about ten years. persion of radionuclides on the and plutonium-239,240) were On the basis of regional scale. This covered considered as the most impor- measurements of these such neighbouring atolls as tant for modelling exercises. radionuclides in the lagoons Atoll and those as far The analysts took into account and the open ocean provided away as , and considered the contributions from both by the Marine Working Group, dispersion over the long term. leaching of the lagoon sedi- their present release rates from The three models differ in IAEA BULLETIN, 40/4/1998 Doc-09.qxd 1/21/99 10:19 AM Page 36

RELEASE RATES OF RADIONUCLIDES AT MURUROA & FANGATAUFA

The graphs show the predicted time-dependent release rates of tritium, strontium-90, caesium-137, and 36 plutonium-239,240 to the biosphere.

their spatial resolution, the material in the environment estimates of the surface water degree to which vertical struc- (e.g. to the plutonium in the concentrations at various ture in the ocean is taken into sediments). Events considered Pacific islands over different account and the time over included glaciation, leading to time periods for predicted which it is convenient to fol- falling sea levels and the releases from Mururoa and low the dispersion. The mod- exposure of the lagoon bed; Fangataufa and for the els were applied to “continuous” and a slide of carbonate rock instantaneous releases. The sources arising from geosphere leading to an “instantaneous” predicted concentrations at the migration and varying in source release of the radionuclides atolls of French Polynesia were strength over time, and to exposed by the rock slide. found to be lower than the “instantaneous” releases, when a The only disruptive event present concentrations due to major fraction of the source is that was found to warrant a global fallout observed in the released within a relatively short thorough assessment was the open ocean. time of a year or so.) Both cases hypothetical major breakaway Only the plutonium release could conceivably arise as a con- and slide of the carbonate for a disruptive event (a rock sequence of disruptive events. formations in the northern slide) gives a higher A number of hypothetical zone of Mururoa Atoll; this is concentration (by about two disruptive events were in the area where underground orders of magnitude) at the considered. They included safety trials and some nuclear closest atoll (Tureia) than the extreme events and events due tests that produced cavity- present levels due to global to climatic changes that could chimneys penetrating into the fallout. However, after ten lead to enhanced release rates carbonate formations were years the predicted of radioactive material now in carried out. concentrations will be below the cavity-chimneys or to The compartment modelling the present background IAEA BULLETIN, 40/4/1998 enhanced rates of exposure to of regional dispersion provides concentrations. Doc-09.qxd 1/21/99 10:19 AM Page 37

FAR FIELD nuclear testing. Plutonium event — a slide of carbonate MODELLING concentrations in the ocean at rock, releasing activity from A far field general circulation Tureia would return to the the carbonate zone correspond- model was used to estimate the background concentrations ing to the inventory from one concentrations in the ocean of within ten years. safety trial and one cavity- radionuclides released from the In the case of a more realistic chimney test; and (2) a time- atolls and transported and dis- time-dependent release of dependent source arising from persed as far as the east coast of radionuclides, the predicted migration of material from the Australia and the west coast of long-term concentrations of underground cavities and the South America. The model was radionuclides will be below leaching of sediments in the applied to continuous and background oceanic levels lagoon. instantaneous sources and to beyond 100 kilometers from Most of the simulations have cases in which the release Mururoa and Fangataufa Atolls. assumed release to the surface occurs into the surface layer For example, for plutonium, layers. However, some and in which it takes place at in the case of a surface release, calculations assumed the source 400 meters depth, below the the predicted maximum would be located at depth (400 thermocline. In the latter case, surface concentrations, after meters), simulating release from the released material is trapped ten years of release, would be the karstic layers. below the thermocline and dis- about 0.2 mBq/m3 in the For these scenarios, persion is inhibited; higher vicinity of Mururoa. This is maximum elevations in concentrations will be found at about an order of magnitude concentrations were estimated farther distances from the below present levels. in the lagoons and at islands points of release, but at depths and atolls in the South Pacific. of about 400 meters. OVERALL Predicted concentrations at the Only in the case of a CONCLUSIONS closest inhabited atoll (Tureia) hypothetical disruptive event — In summary, the Study’s marine were compared with a rock slide, release at 400 meters modelling work reached the fol- background concentrations in 37 depth —could sea level lowing conclusions: the open ocean due to global concentrations of tritium, ■ The estimated turnover time fallout. Only the plutonium strontium-90 and caesium-137 of the lagoon waters is 98+ 37 release for a disruptive event at Tureia Atoll reach the present days for Mururoa and 33+ 12 would give a higher levels observed in the South days for Fangataufa. concentration than the Pacific Ocean due to global ■ The transfer of particulate background level and then fallout. (sediment-bound) plutonium- only for a few years. For example, for tritium the 239,240 from the Mururoa For releases at depth, the maximum predicted elevation in lagoon into the Pacific Ocean is concentrations at Hao and concentration is about 10 estimated to be 8 GBq per year Tahiti are predicted to be Bq/m3 after five years halfway to with mean winds and tides, and higher than at Tureia. Australia, and about 1 Bq/m3 0.7 TBq in the event of an However, when the data for off the coast of Australia five extreme storm (per storm). The these locations are averaged years later. These compare with frequency of an extreme storm is over the total depth of 450 the background tritium level of estimated to be one in ten years. meters, they are all below the 50-100 Bq/m3 at 400 meters The source term due to storms is maximum values. depth. The release of the expected to decrease with time, Overall, the modelling of the plutonium would result in a due to depletion of plutonium- dispersion of radionuclides predicted ocean concentration at bearing sediments and gradual from Mururoa and Fangataufa Tureia Atoll of 100 mBq/m3. burial of these sediments. The atolls to the open ocean This is about two orders of source term due to plutonium recorded clear results. The magnitude higher than the from a storm is an order of mag- work has shown that expected present very low ocean nitude below that released by the concentrations in nearby concentration there (about 1 hypothetical disruptive event. inhabited islands would not be mBq/m3) due to global fallout ■ Two main release scenarios high enough to be of any associated with all atmospheric were modelled: (1) a disruptive radiological interest. ❐ IAEA BULLETIN, 40/4/1998