Contribution of close-in fallout from the French atmospheric tests in inventories of 137Cs, 241Am and plutonium (238, 239, 240) in Gambier Islands (French Polynesia) - Signatures of stratospheric fallout in the Southern Hemisphere.

The inventories of 137Cs (503 ±â€¯34 Bq.m-2), 241Am (11.3 ±â€¯1.2 Bq.m-2), 241Pu (33.7 ±â€¯3.4 Bq.m-2), 238Pu (6.82 ±â€¯0.87 Bq.m-2) and 239+240Pu (113.0 ±â€¯5.9 Bq.m-2), sum of 239Pu (100 ±â€¯11 Bq.m-2) and 240Pu (14.5 ±â€¯1.7 Bq.m-2), in the Gambier archipelago (23°S) of the French Polynesia, are well higher the global fallout at this latitude, in unequal proportions for the different radionuclides. (240Pu/239Pu)AR (AR: atomic ratio) of 0.0394 ±â€¯0.0062, and (241Pu/239Pu)AR of (2.03 ±â€¯0.39)10-4, confirm that the overwhelmingly dominant source of these radionuclides comes from local fallout during the 1970s of the French atmospheric tests of Moruroa and Fangataufa located nearly 400 km from Gambier. The signatures of the local fallout were deduced from the excess of its inventory in 137Cs and from the mixing lines established from the signatures of the global fallout, some of the test sites and the isotopic ratios measured in Gambier. Signatures obtained are 2.0 ±â€¯0.4 for 137Cs/239+240Pu, 0.045 ±â€¯0.008 for 238Pu/239+240Pu, 0.031 ±â€¯0.009 for 241Am/239+240Pu, 0.092 ±â€¯0.027 for 241Pu/239+240Pu, 0.0163 ±â€¯0.0049 for (240Pu/239Pu)AR, (0.78 ±â€¯0.23)10-4 for (241Pu/239Pu)AR. The concordance of the mixing lines of the [(240Pu/239Pu)AR, (241Pu/239Pu)AR] and the linear regression of these ratios measured in the stratosphere (40°S) during the 1970s, indicates that the signatures of the close-in deposition are also those of the stratospheric injections of the French tests. The signatures of stratospheric fallout in the Southern Hemisphere were evaluated by considering that the fission energy of these injections represents 11% and that of the Northern Hemisphere represents 89% and that the isotopic ratios of stratospheric injections remained the same over the period 1970-1974. The activity ratios deducted are 21.9 ±â€¯0.1 in 137Cs/239+240Pu, 0.11 ±â€¯0.05 in 238Pu/239+240Pu, 1.03 ±â€¯0.12 in 241Pu/239+240Pu and 0.35 ±â€¯0.04 in 241Am/239+240Pu. The associated atom ratios are 0.157 ±â€¯0.011 for (240Pu/239Pu)AR and (8.33 ±â€¯0.48)10-4 for (241Pu/239Pu)AR. These signatures appear to be consistent with the results of the inventories at Hiva Oa, located more than 1,000 km north of both French test sites, and with those found in the Australian continent, in regions not impacted by UK-test debris. The proportions of close-in tropospheric fallout from the French tests are about 90% in Gambier. They represent a proportion in the inventories of 40% for the 137Cs, 60% for 241Am and in the range between 80 and 90% for Pu isotopes.

Inventory and vertical distribution of 137Cs, 239+240Pu and 238Pu in soil from Raivavae and Hiva Oa, two French Polynesian islands in the southern hemisphere.

Atmospheric nuclear weapons tests carried out by the United States, the former Soviet Union, the United Kingdom, France and China between 1945 and 1980 resulted in radioactive fallout over the earth's surface of long-lived radionuclides, such as 137Cs, 239+240Pu and 238Pu that could be detected more than 50 years after their production. In addition, the burnup in the upper atmosphere of a thermoelectric generator fueled by 238Pu, SNAP-9A, contributed to the inventory of 238Pu deposited on the ground. In order to estimate the deposition densities of 137Cs, 239+240Pu and 238Pu in French Polynesia, we collected undisturbed soil samples up to 30 cm deep at eight sites in two islands (Hiva Oa, 139°W - 10°S and Raivavae, 148°W - 24°S) in 2015-2016. The top 0-10 cm of the soil cores were sliced into five 2-cm layers and the bottom 10-30 cm into four 5-cm layers for gamma spectrometry and alpha spectrometry measurements. We found that more than 50% of the radioactive inventories are still contained within the first 10 cm and that the average vertical migration velocities of 137Cs and Pu are less than 0.2 cm y-1. The average accumulated depositions, deduced from the profile measurements, are 236 ±â€¯11 Bq.m-2 and 313 ±â€¯39 Bq.m-2 for 137Cs, 12.1 ±â€¯1.5 Bq.m-2 and 22.1 ±â€¯1.7 Bq.m-2 for 239+240Pu, and 1.23 ±â€¯0.46 Bq.m-2 and 1.58 ±â€¯0.60 Bq.m-2 for 238Pu, in Hiva Oa and Raivavae, respectively. The 238Pu/239+240Pu ratios are 0.102 ±â€¯0.050 at Hiva Oa and 0.072 ±â€¯0.033 at Raivavae. Both values are higher than the ratio in nuclear weapons tests fallout estimated to be 0.016 in 2016 (Hardy et al., 1973), because of the contribution of 238Pu fallout from SNAP-9A, which is latitude dependent. The 137Cs/239+240Pu ratios, 19.5 ±â€¯3.2 at Hiva Oa and 14.2 ±â€¯2.8 at Raivavae are in the lower part of the range of values observed in other regions of the world.

Specific activity to H*(10) conversion coefficients for in situ gamma spectrometry.

The typical situations found in in situ gamma spectrometry have been simulated by Monte Carlo techniques to obtain the energy spectra of the photon fluence rate existing at 1 m above ground. The main difficulty found with the model is its slow convergence. A method to speed up the calculation has been derived. The results of the model have been thoroughly tested against existing data. In a final stage, the specific activity to H*(10) conversion coefficients have been derived for the typical scenarios encountered in the in situ gamma spectrometry.

Mechanisms of enrichment of natural radioactivity along the beaches of the Camargue, France.

A field study was carried out along the Golfe du Lion, that focussed on the beaches of the Camargue, to locate the main areas where enriched U and Th are found, and to better understand the processes that concentrate radioactivity on beaches. Indeed enriched areas are observed on some Camargue beaches, where high-dose rates are recorded due to excess U and Th activity (>1000 Bq kg(-1)). The coastline was mapped by means of an aerial gamma survey and the results indicated that the main actinides deposits occurred in the Camargue area. This concentrating effect is possibly due to a greater sedimentary contribution from the River Rhone relative to other minor Mediterranean rivers. Across the along-shore profile, the variability in actinides observed at the eastern part of Beauduc spit is mainly explained by variations in heavy and light mineral contents. Such variability can be accounted for by redistribution of the sand caused by erosion/deposition processes occurring in the eastern part of the spit. Further parameters such as grain size and heavy minerals content were studied in connection with the distribution of U, Th and (40)K in the field at a more localised level (i.e. across-shore beach profile). The <200-micro m fraction contains more than 50% of the radioactivity and heavy minerals (especially zircon) are the main contributors to the high levels of external radiation. Therefore the enriched areas, where U and Th exceed 1000 Bq kg(-1), presumably result from the sorting of sand grains according to their size and density.

Characterization of the NIST seaweed Standard Reference Material.

The National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) for seaweed was developed through an interlaboratory comparison with 24 participants from 16 countries. After evaluating different techniques to calculate certified values for the radionuclides, the median method was found to be the most representative technique. The certified values were provided for 13 radionuclides and information values were given for 15 more radionuclides. Results for the natural decay series showed disequilibrium in both the uranium and thorium series.

In situ metrology of 85Kr plumes released by the COGEMA La Hague nuclear reprocessing plant.

The IRSN (Institut de Radioprotection et de Sûreté Nucléaire, France) has started an in situ study of the behaviour of atmospheric releases close-by the COGEMA La Hague nuclear reprocessing plant. The study is designed to improve information on the dispersion of radioactive pollutants very close to the emission point--a 100 m height chimney. In this situation, close to the emission and height of the emission, Gaussian models generally used to predict the behaviour of atmospheric releases are not well adapted. The study is based on the characterisation of the 85Kr emitted during normal operations of the reprocessing process. Temporal and spatial variations of the plume shape were investigated with intensive in situ measurements. Live in situ techniques to measure the electrons and the photons emitted by the 85Kr have been implemented and will be described. Preliminary results showing the interest of the techniques were presented. Variations of the dose rates created by the photon flux of a plume were directly measured and correlated to other quantities.

Sector field inductively coupled plasma mass spectrometry, another tool for plutonium isotopes and plutonium isotope ratios determination in environmental matrices.

The transfer of radio nuclides into the different compartments of the environment are widely studied and leads to the elaboration of transfer models in order to evaluate potential impact onto the environment and humans. Accurate experimental data are needed to validate these models for all types of matrices (air, water, sediments, soils, biota and food...). Among these radionuclides, 238Pu, 239Pu, 240Pu and 241Pu, are often mentioned. They have been released into the environment by nuclear weapon tests, nuclear facilities, reactors or satellite accidents. These different sources have different 240Pu/239Pu ratios and therefore this ratio is used to provide information on the source of contamination into the environment. The most conventional analytical tools used for plutonium isotope determination are liquid scintillation and alpha spectrometry, and thermal ionisation mass spectrometry (TIMS) is still considered as the primary method for determination of plutonium isotope ratios. During the last decade, mass spectrometers equipped with plasma ion sources and sector field analysers were developed and can offer now another alternative method for the accurate determination of isotope content and ratios of long-lived radionuclides in environmental samples. This paper presents and discusses the results obtained for 239Pu, 240Pu and 241Pu content and isotope ratios by sector field ICP-MS in different environmental matrices.

Assessing exposure to cosmic radiation on board aircraft.

The assessment of exposure to cosmic radiation on board aircraft is one of the preoccupations of organizations responsible for radiation protection. The cosmic radiation particle flux increases with altitude and latitude and depends on the solar activity. The radiation exposure has been estimated on several airlines using transatlantic, Siberian and transequatorial routes on board subsonic and supersonic aircraft, to illustrate the effect of these parameters. Measurements have been obtained with a tissue equivalent proportional counter using the microdosimetric technique. Data have been collected at maximum solar activity in 1991-92 and at minimum in 1996-98. The lowest mean dose rate measured was 3 microSv/h during a Paris-Buenos Aires flight in 1991; the highest was 6.6 microSv/h during a Paris-Tokyo flight using a Siberian route and 9.7 microSv/h on Concorde in 1996-97. The mean quality factor is around 1.8. The corresponding annual effective dose, based on 700 hours of flight for subsonic aircraft and 300 hours for Concorde, can be estimated between 2 mSv for least-exposed routes and 5 mSv for more exposed routes.

Determination of 241Am in sediments by isotope dilution high resolution inductively coupled plasma mass spectrometry (ID HR ICP-MS).

Trace levels (pg kg(-1)) of 241Am in sediments were determined by isotope dilution high resolution inductively coupled plasma mass spectrometry (ID HR ICP-MS) using a microconcentric nebulizer. 241Am was isolated from major elements like Ca and Fe by different selective precipitations. In further steps. Am was first separated from other transuranic elements and purified by anion exchange and extraction chromatography prior to the mass spectrometric measurements. The ID HR ICP-MS results are compared with isotope dilution alpha spectrometry.

Assessing exposure to cosmic radiation during long-haul flights.

The assessment of exposure to cosmic radiation on board aircraft is one of the concerns of organizations responsible for radiation protection. Cosmic-particle flux increases with altitude and latitude and depends on solar activity. To illustrate the effect of these parameters, exposure has been estimated on several airlines operating subsonic and supersonic aircraft on transatlantic, Siberian and transequatorial routes. Measurements have been made with a tissue-equivalent proportional counter using the microdosimetric technique. This type of system provides the absorbed dose, the ambient dose equivalent, the mean quality factor, and the dose distribution as a function of lineal energy. Data were collected at maximum solar activity in 1991-1992 and at minimum activity in 1996-1998. The lowest mean dose rate measured was 3 microSv h(-1) during a Paris-Buenos Aires flight in 1991. The highest rates were 6.6 microSv h(-1) during a Paris-Tokyo flight on a Siberian route and 9.7 microSv h(-1) on Concorde in 1996-1997. The mean quality factor is around 1.8. The corresponding annual effective dose, based on 700 h of flight for subsonic aircraft and 300 h for Concorde, can be estimated at between 2 mSv for the least-exposed routes and 5 mSv for the more-exposed routes.

Population dose near the Semipalatinsk test site.

To determine the consequences of atmospheric atomic bomb tests for the population in the surroundings of the former nuclear weapons test site near Semipalatinsk in Kazakhstan, a pilot study was performed by an international cooperation between Kazakh, French, Czech and German institutions at two villages, Mostik and Maisk. Together with Kazakh scientists, eight experts from Europe carried out a field mission in September 1995 to assess, within the framework of a NATO supported project, the radiological situation as far as external doses, environmental contamination and body burden of man were concerned. A summary of the results obtained is presented. The actual radiological situation near the test site is characterized by fallout contaminations. Cs was found in upper soil layers in concentrations similar to those of the global fallout. Also Sr, Am and Co were observed. The resulting present dose to the population is low. Mean external doses from soil contamination for Maisk and Mostik (0.60-0.63 mSv/year) presently correspond to mean external doses in normal environments. Mean values of the annual internal doses observed in these two villages are below 2 microSv/year for 90Sr. For other radionuclides the internal doses are also negligible.

Quality factor and dose equivalent investigations aboard the Soviet space station MIR.

Since Dec 1988, date of the French-Soviet joint space mission "ARAGATZ", the CIRCE device (Compteur Intégrateur de Rayonnement Complexe dans l'Espace) had recorded dose equivalent and quality factor inside the MIR station (380-410 km, 51.5 degrees). After the initial gas filling two years ago, the low pressure tissue equivalent proportional counter is still in good working conditions. Some results of three periods, viz Dec 1988, Mar-Apr 1989 and Jan-Feb 1990 are presented. The average dose equivalent rates measured are respectively 0.6, 0.8 and 0.6 mSv/day with a quality factor equal to 1.9. Some detailed measurements show the increasing of the dose equivalent rates through the SAA and near polar horns. The real time determination of the quality factors allows to point out high LET (Linear Energy Transfer) events with quality factors in the range 10-20.

Real time quality factor and dose equivalent meter "CIRCE" and its use on-board the Soviet orbital station "MIR".

During the French-Soviet space mission "Aragatz," the experiment CIRCE (Compteur Integrateur de Rayonnement Complexe dans l'Espace) recorded the dose rate and quality factor values inside the MIR station. This paper presents results obtained with a new active dose equivalent meter based on microdosimetric techniques and using a low pressure tissue equivalent proportional counter. In terms of lineal energy CIRCE device works in the 0.2-1200 keV micrometer-1 range in tissue. Preliminary studies were performed in photon, neutron and heavy ion beams, and in the real stratosphere cosmic radiation field. Long term measurements on-board MIR station from December 1988 to April 1989 gave an average quality factor value equal to 1.9 +/- 0.3. Through the South Atlantic Anomaly (SAA), the quality factor was equal to 1.4. The temporal orbital variations of the dose rates and quality factors have been established in space dosimetry for the first time.