The determination of source terms from radioactive particles in a marine environment using SIMS and IPCMS

Radioactive particles are determining the diffusion and transport mechanisms of radionuclides in the environment as well as their bioavailability. Therefore, their characterisation as for elemental and isotopic composition is of great relevance. Considerable advances in secondary ion mass spectrometry (SIMS) have been made over the last decade to develop precise and accurate methods for the determination of the isotopic composition of individual radioactive particles. These parameters are fundamental for the characterisation of such particles in safeguards as well as in environmental and forensic applications. Particularly, in nuclear forensics, the U, Pu and U/Pu isotopic ratios give important information to trace the origin of nuclear fuel material. Following an accidental discharge, highly radioactive particles about few millimetres in size have been found for more than ten years on the seabed and occasionally on beaches in the neighbourhood of a nuclear establishment in Scotland. The particles were identified as an aluminium-uranium matrix fuel used in the material test reactors. To search for U particles in the sample, the secondary ion mass spectrometer was set to mass 238U and alternatively to mass 235U to obtain a mapped distribution of the U particles. The sample was scanned across the surface with a field of view of 500x500 μm and the analysis performed is shown. This SIMS image of the 235U secondary ion signal distribution in the sample reproduces exactly the same uranium spots found by the scanning electron microscope laboratory, as shown. To check whether the uranium spots found in the particle were located at the same place with some other elements, a series of images was recorded during the same analysis scanning consecutively the analysed area for the masses 23Na, 24Mg, 27Al, 31P, 40Ca, 56Fe and 235U. The SIMS analysis confirmed the nature of the matrix to be Al and showed also that the other major elements present in the sample are not directly located with the uranium spots. In fact, no evident contribution of any uranium in the same area of analysis of the detected iron particles, for instance, was found. For all the U particles analysed, homogeneous results concerning the ratio 235U/238U and the minor isotope ratios were obtained. The results for the minor isotopes are much higher than 'normal' values, especially for the 236U. The hydride contribution of 235UH+ to the mass 236 was checked by applying a correction from the 238UH+/238U ratio measurements of certified standard reference U3O8 uranium oxide powders, namely CRM U020, CRM U100, CRM U500 and CRM U900 with 2%, 10%, 50% and 90% 235U, obtained from NIST. In these cases, the ratio was always of the order of 10-4 or even less. For this reason, if any hydride contribution in the sample is present, this one is negligible because the 236U is very high in intensity. It was not possible to use the value of the 238UH+/238U ratio in this sample for correction because the sample is a spent fuel particle and the mass 239 contains also 239Pu and not only 238UH+. This was shown also clearly from the measured 239/238 ratio, i.e. 3.06x10-2. To confirm the SIMS results, few micrograms of the sample were dissolved and measured by inductively coupled plasma mass spectrometry (ICPMS). The ICP-MS results are consistent with the values obtained by SIMS. The plutonium isotope ratios (239Pu/240Pu, 239Pu/238U) were calculated from the measured count rates of the different Pu and U isotopes. Pu has generally a considerably higher ionisation efficiency than U. Therefore the measured Pu/U ratios have to be corrected with the 'relative sensitivity factor' (RSF) for Pu/U. The RSF takes into account the differences in the ionisation efficiencies of the elements at the fixed conditions of the measurement (e.g., primary ion beam energy and current). In a previous work, the ionisation efficiency difference of Pu and U was measured analysing standard Pu materials with known Pu/U ratios. The RSF of 2.41 obtained in that study is applied in this work for the correction of the 239Pu/238U ratio. For the ICP-MS measurements, due to the negligible amount of 238Pu, no U/Pu separation has been performed. The results of the two techniques agree quite well demonstrating a good reliability of the results. The SIMS and ICP-MS U/Pu isotopic ratio results were used for the identification of the reactor and fuel type. The experimental composition is consistent with the theoretically calculated composition of a HEU fuel (enrichment >90%) irradiated in an MTR reactor to a burnup of 25 to 30%. (author)