Plutonium Isotopes Concentration in Seawater along the Algerian Coast

The International Atomic Energy Agency has organised in the framework of the regional project RAF/7/004, in collaboration with “Commissariat à l’Energie Atomique” (COMENA) and “Institut des Sciences de la Mer et de l’Aménagement du Littoral” (ISMAL), during August 2001, a scientific campaign along the Algerian coast, on board of the research vessel M.S. Benyahia of ISMAL is. Three stations, at the centre, east and west, were selected to collect five seawater samples for each water column reaching a maximum depth of 2000 m, using a stainless-steel water sampler of a volume of 250 litres. After recording the marine environment parameters (temperature and conductivity), seawater samples were conditioned and preconcentrated to precipitate plutonium isotopes using MnCl2 in the form of MnO2 in order to proceed to plutonium extraction by radiochemical separation and prepare the source by coprecipitation using neodymium fluoride (NdF3) by vacuum filtration and an evaluation of the activity by alpha spectrometry. Concentration results in units of μBq/l of plutonium isotopes were obtained in the range of 6.7±1.00 to 25.5±3.70 for 239+240Pu and 0.21±0.04 to 0.77±0.15 for 238Pu. Distribution of Pu through the plot of its profile was studied and the concentration was estimated. The obtained results were compared to137Cs and those found by other authors in the same Mediterranean area.


INTRODUCTION
The Mediterranean sea is a semienclosed sea located between Europe, Asia, and Africa, subject to properties and seawater exchanges, coming particularly from the Atlantic Ocean.
This sea is characterized by an important evaporation and a weak rate of dilution.It becomes thus vulnerable; and therefore studying the radioecological marine phenomena is required.This medium is thus exposed to the introduction of natural radioactivity since the creation of the earth, and artificial radioactivity since the advent of nuclear industry.An increase of this radioactivity in the different marine medium compartments has occurred according to physicochemical properties of the receiving medium and including the radioelements of interest.
The evaluation of the concentration of some artificial radioelements of interest becomes necessary to study their behavior in the marine medium (seawater, sediments, marine organisms, etc.).For this purpose, and within the framework of a radiological monitoring project of the marine medium along the Algerian coast, several sampling campaigns were carried out from 1997 to 2004.
The results presented in this work are those obtained in the oceanographic campaign carried out in August 2001 in the framework of the regional project RAF/7/004 organized by the IAEA, in collaboration with CEA and ISMAL, on board of the research vessel M.S. Benyahia of ISMAL, along the Algerian coast, at the centre, east, and west.Samples of water columns (surface/depth, maximum depth of 2000 m) were collected, treated, and preconcentrated on board, maximal 2000 m), to be subject to a fine radiochemical separation analysis at the laboratory in order to extract Pu isotopes, with the aim of determining Pu profiles against depth (m).

Sampling and preconcentration
Three stations (Figure 1) were considered for the measurement campaign, Algiers (N 36 • 54.059, E 003 • 20.452) at the centre, Jijel at the east, and Ténes (N 36 • 44.707, E 01 • 38.421) at the west.Seawater samples were collected at several layers, from surface seawater to a maximum depth of 2000 m, using a stainless-steel bottle of a volume of 250 liters (Figure 2) in each station.After recording different parameters (temperature in • C and conductivity in mS) for each layer, the seawater sample was filtered and transferred through a pump towards a polyethylene tank of a volume  of 200 liters to be treated and to preconcentrate radioelements of interest, namely, 137 Cs, 90 Sr, and, in the case of this work, plutonium isotopes.In addition to the radioactive spikes ( 134 Cs, 85 Sr, and 242 Pu) and the stable carriers introduced in each sample, an appropriate amount of Mncl 2 was added in order to precipitate plutonium isotopes in the form of MnO 2 .After this step of preconcentration, samples were reduced from a volume of 150 liters to a volume of around ten liters, for an analysis by fine radiochemical separation at the laboratory in order to extract, in our case, the isotopes of plutonium and to determine the concentration in μBq/l.

Radiochemical analysis
Presence in seawater of some alpha-emitting radioelements particularly plutonium isotopes is known to be at the state of traces.Their concentration, according to the degree of contamination of the medium, can fluctuate between a few μBqls −1 until some mBqls −1 Consequently and in order to be able to extract by radiochemical separation the radioelement of interest and to quantify by alpha spectrometry the specific activity, significant quantities reaching hundreds of liters (up to 500 l) must be taken and treated on board with the aim of preconcentration of the required radioelements.
Samples of seawater collected on board of the research vessel M.S. Benyahia of ISMAL were acidified to a pH (1-2), the radioactive tracers corresponding to the radioelements of interest (isotopes of Pu, 137 Cs, and 90 Sr) were added in order to determine at the end of the analyses the chemical yield.With the aim of preconcentration of plutonium isotopes, KMnO 4 was added in order to oxidize the organic matter as well as plutonium at the state VI.
The solution is maintained in a basic medium (pH ∼ 9) by MnCl 2 , after addition of NaOH.Plutonium precipitates in the form of MnO 2 are stored in a plastic container reaching a volume of around ten liters for its analysis at the laboratory.
The precipitate was then dissolved by adding a quantity of HCl 2 M and N 2 HOH•HCl while carrying out an agitation so that the solution becomes clear.While heating the last solution, Fe (III) as FeCl 3 dissolved in 0.1 M HCl and NH 2 OH•HCl were added in order to reduce Pu to the state of oxidation III.Fe (II) and Pu (III) were oxidized with the state Fe(III) and Pu(IV), respectively, by adding NaNO 2 , Pu(IV) was precipitated by the ammonium hydroxide to a pH between 8-9.The solution was left overnight for decantation until a minimal pH between 6-7 was reached.The solution was centrifuged; the supernatant recovered and then passed through a paper filter.The latter was washed with concentrated HCl; the filtrate was recovered in the tube of centrifugation with the precipitate.The whole was transferred to a 250 cc glass beaker; evaporated nearly to dryness.The residue was then dissolved with concentrated nitric acid and centrifuged.The supernatant was recovered in a glass beaker after filtration.The obtained solution was transferred in a glass beaker, and iron was tested with ammonium cyanide (NH 4 SCN-5 M).Some drops of basic reducing agent (hydrazinium Hydroxide-N 2 H 5 OH-5M) were added to initiate a reduction-oxidation reaction of ferric-iron (Fe(III)) which is reduced to ferrous-iron (Fe(II)), this will involve a reduction of plutonium to state III.The solution is cooled and 5 grams of sodium nitrite (NaNO 2 ) are added to convert plutonium to the state of oxidation IV, that is, (Pu(III) to Pu(IV)).
An anion exchange column (resin AG1X8 100 to 200 meshes) is conditioned with 100 mL of HNO 3 -2 M and 50 mL of HNO 3 -8 M. The solution passed through the column, Pu(IV) is retained by the resin.Plutonium is then eluted by pouring through the resin 100 mL of HCl 9 M + 1.45NH 4 I Once the elution is carried out, plutonium is trapped in the form of complexes while using NdF 3 and deposited by vacuum filtration on membrane filters of 0.1 μm porosity.The evaluation of the spectrum is made by alpha spectrometry [1, 2].The whole procedure is described in Figure 3.

Evaluation by alpha spectrometry
For the determination of plutonium concentration, 242 Pu was used as radioactive tracer for the determination of the chemical yield of the analysis.The energy position of the radioactive tracer in a spectrum of energy and its interaction with other radioelements of close energy can appear after extraction of the plutonium isotopes, and can be the object of  tailing phenomena contribution.Once the source is prepared and the counting is carried out using a surface barrier detector, the evaluation by alpha spectrometry can be made reliably if there is no alpha-particle attenuation related to the thickness of the source.In the case of this work, the source prepared was counted with a PIPS detector.Surfaces of the peaks of interest were delimited while taking into account the background noise and the contribution of the neighbouring peaks to determine the chemical output as well as the specific activity.

RESULTS AND DISCUSSION
The means of collecting seawater column samples as well as locality of the considered stations in this work are shown in Figures 1 and 2. Temperature ( • C) and conductivity (mS) parameters recorded along the water column using a Conductivity Temperature and Depth (CTD) probe are given in Table 1 and they are plotted and shown in Figures 4(a) and 4(b).The concentrations of plutonium isotopes, particularly 239+240 Pu and 238 Pu, with the ratios 238 Pu/ 239+240 Pu and 239+240 Pu/ 137 Cs, are given in Table 2.However, their profiles against depth (m) are shown in Figures 5(a), 5(b), and 5(c) In the Ténes station, the concentration of 239+240 Pu and that of 238 Pu varies from 8.2 μBq/l to 25.5 μBq/l, and from 0.47 μBq/l to 0.61 μBq/l; the average values are 18.0 μBq/l and 0.55 μBq/l, respectively.In the Algiers station, the values vary from 6.70 μBq/l to 21.70 μBq/l for 239+240 Pu and from 0.21 μBq/l to 0.77 μBq/l for 238 Pu.The results obtained within the framework of this work were compared with other works carried out in the same area in the north of the Mediterranean sea, where concentrations of 239+240 Pu vary between 14 and 26 μBq/l, in the west, the variation is between 8 and 33 μBq/l [3-5].
The surface 239+240 Pu concentrations found in this study seem to be in good agreement with values reported by Leon Vintro (their average value is 7.8 ± 1.4 μBq/l).Data reported by other authors indicate that surface Pu concentrations have been exponentially decreasing since 1976 from 33 μBq/l to 17 μBq/l [6] down to 7.5 ± 1.0 μBq/l (mentioned in this paper).The 239+240 Pu subsurface concentration peak shown in Figures 5(a), 5(b), and 5(c) at about 250-meter water depth can be explained by Pu transport from surface to deeper water layers through its association with suspended matter.However, the second peak at about 1000 m is not only associated with suspended matter but also with water masses movement at deep layers from the other parts of the Mediterranean sea and the north of the Atlantic ocean.The profile of 239+240 Pu and 137 Cs along the water column as shown in Figures 4(a) and 4(b) are similar, but 137 Cs is most affected by water movement.If we estimate the ratios of 239+240 Pu to 137 Cs as presented in Table 2, we will confirm again that Pu isotopes is more effectively removed from surface layers than 137 Cs due to adsorption to suspended particles.

CONCLUSION
This study enabled us for the first time to obtain results of concentration of 239+240 Pu in a water column in the centre and the west of the Mediterranean sea, along the Algerian coast.The values seem to be in good agreement compared with those found in other works, but reflect a displacement of the 239+240 Pu along the water column profiles which seem to be similar to those given by other works in the same area, along with the 137 Cs.Concentrations of 239+240 Pu increase along the water column at the station of Ténes and a peak is observed at 1000 meters depth.

MnO 2 2 2 2 -Figure 3 :
Figure 3: Scheme of the whole radiochemical procedure of the Pu in seawater.

Table 2 :
Concentration of 239+240 Pu and 238 Pu along the water column in the stations of Ténes and Algiers.