A contribution to the development of sensitive and isotope-selective analytical methods based on sector-field ICP-mass spectrometry for supporting the development of Gen IV nuclear reactors (PhD)

The interaction of heavy liquid metals (HLMs) and structural materials can lead to corrosion.  The corrosion products can potentially undergo activation.  Furthermore, the chemical impurities present in HLMs and steels can be the starting material for the formation of activation products, or can influence the extent of liquid metal embrittlement.  Analytical methods which can quantify the corrosion in terms of the concentrations of the elements released during corrosion, or can quantify the impurities present in these materials are needed to support the development of HLM use in reactor designs.  Such analytical methods could be used as a routine part of corrosion testing/research programmes, but also to analyze "unexpected" samples.  These analytical methods must be sensitive and selective to allow identification of, for example, corrosion products at low concentrations.  The combination of high sensitivity with its low, medium or high resolution modes makes the sector-field ICP‑MS an ideal instrument upon which to develop such analytical methods.

ICP‑MS is well embedded within nuclear chemical laboratories for the analysis of radionuclides and impurities (stable isotopes).  A quadrupole ICP-MS was first made commercially available in 1983 and by 1988 a quadrupole ICP-MS had been modified and coupled to a glove-box.  A nuclearized quadrupole ICP-MS (PQ3, VG, Winsford, UK) has been operated at SCK•CEN since 1999.  In late 2008 a double-focussing sector-field inductively coupled plasma mass spectrometer (SF-ICP-MS) adapted for coupling to a nuclear glove-box was installed.  The glove-box is currently being equipped and has to undergo testing and acceptance before being closed definitively in time for the AWM project.

The higher resolution of the SF-ICP‑MS provides a simple means to physically resolve the inconvenient polyatomic interferences encountered in earlier quadrupole ICP‑MS instruments, which otherwise require either some kind of cell (reaction¹ or collision²) to remove the interferences or the use of cool plasma ^{3, 4, 5} conditions, as recently studied at SCK•CEN.⁶  These interferences limit the applicability of ICP‑MS, i.e. measurement is not selective for the analyte nuclide and/or detection limits are poor due to the background signal from the interference.  Not all interferences can be solved by the higher resolution of the SF-ICP‑MS.  For isobaric interferences the resolution of the SF-ICP‑MS is usually insufficient.^7,8  For this reason the mass spectrometric analyses must be allied with appropriate radiochemical separation in order to remove isobaric interferences prior to analysis.  This could be offline separation or could be based upon matrix separation and/or selective analyte preconcentration.  Whilst there are many issues (and interesting analytical challenges) which need to be addressed in order to arrive at useful analysis methods, the SF-ICP‑MS opens up a new range of potential analysis methods for challenging nuclear materials.

   2.   McCurdy, E.; Woods, G. J.Anal.At.Spectrom 2004, 19, 607-15.

   3.   Jiang, S. J.; Houk, R. S.; Stevens, M. A. Anal.Chem. 1988, 60, 1217-21.

   4.   Tanner, S. D.; Paul, M.; Beres, S. A.; Denoyer, E. R. Atomic Spectroscopy 1995, 16, 16-18.

   5.   Tanner, S. D. J.Anal.At.Spectrom. 1995, 10, 905-21.

   6.   Wijnen, M, vanderauwera, P, and Dobney, A. Voorbereidende studie van de koud plasma modus van een ICP-QMS voor de bepaling van ⁵⁵Fe, ⁵⁹Ni en ⁶³Ni.  2009.  Artesis; SCK-CEN. 

   7.   Becker, J. S.; Dietze, H. J. J.Anal.At.Spectrom 1997, 12, 881-89.

   8.   de Prins, M, vanderauwera, P, and Dobney, A. Studie van de interferenties bij de analyse van kritisch radionucliden in radioactief afval met SF-ICP-MS.  2009.  Artesis, SCK-CEN.

9.  Kellner, R,  Mermet, J-M, Otto, M,  Valcarcel, M, Widner, H. M.  "Analytical Chemistry – a modern approach to Analytical science" 2^nd edition, p3, 2004, Wiley-VCH, Weinheim

10.   Gysemans, M., Dobney, A., Adriaensen, L., and Sannen, L. DESTRUCTIVE RADIOCHEMICAL BURNUP DETERMINATION AT SCK.CEN USING ISOTOPES OF CS, CE AND ND AS FISSION PRODUCT MONITORS. Jülich, Germany, September 19-21, 2006. 2006.

The first objective is to investigate and characterize numerically how the SF‑ICP‑MS instrument performs in terms of parameters such as limits of detection, selectivity, sensitivity, linear range and robustness for the determination of critical nuclides (e.g. Fe-55, Ni-63) and chemical impurities (e.g. Fe, Ni, Nb) in relevant nuclear materials (including heavy liquid metals and alloys).  These parameters are commonly referred to as analytical figures of merit.  It is by determining these figures of merit that we will see where the limitations of the instrument in its standard configuration lie for samples relevant to SCK•CEN.  For example, analysis of uranium matrix samples for tin can be difficult due to formation in the plasma of doubly charged uranium ions which overlap the tin signals.  It is only once the actual figures of merit are available that one can assess, for this hypothetical case, whether a higher RF power or a sample desolvating unit is necessary in order to bring the doubly charged uranium ion formation down to a level at which the tin signals are no longer interfered.  It is already clear that HLM is not an easy matrix for radionuclide and chemical measurement.  Determining the analytical figures of merit provides the baseline against which we can decide where to concentrate our efforts to optimize the measurement procedure so that the results are more reliable.

The second objective, following on from objective one, is to make the optimized analytical method(s) for the determination of critical nuclides and impurities in heavy liquid metals and alloys available for routine use.  This implies a certain degree of validation (which will no doubt be hampered to some extent by a lack of suitable reference materials), certainly in terms of method robustness such that different operators generate metrologically comparable results, and this irrespective of the measurement day.  Previous work within RCA ⁶ indicates that of the critical nuclides Fe‑55 would be the most amenable to mass spectrometric determination.  Part of this second objective should be to validate the determination of Fe-55 in at least one relevant to SCK-CEN sample matrix, most probably an alloy matrix rather than a HLM.  These methods should be applicable to both un-irradiated and irradiated samples of these materials (always respecting the limits imposed when working within the glove-box).  Furthermore these methods must also take into account the presence in the samples of other radionuclides (especially decay products in irradiated lead bismuth) which will influence how the sample can be analysed (e.g. dilution might be required in order to comply with dose rate requirements but will the dilution bring the analyte concentration below the detection limit of the SF-ICP‑MS).  

In addition to transferring the existing operational analytical methods from the PQ3 quadrupole ICP‑MS to the SF-ICP-MS, the methods developed as part of objective II and the knowledge gained from objective I will form the basis of expanding our portfolio of ICP‑MS  analysis methods to other analytes and other sample types.

A third objective will be to identify samples relevant to SCK•CEN where isotope ratios can be measured using the SF-ICP‑MS, and to optimize measurement of the isotope ratios (in terms of lowest achievable measurement uncertainty calculated following the GUM approach).  These samples could be ones for which isotope ratios are not presently measured because they are incompatible with the TIMS instrument (i.e. we expand the measurement capabilities we are able to offer to SCK•CEN), or they could be existing samples.  Isotope ratio measurements are currently performed on our TIMS instrument.  Isotope ratio measurements are a valuable part of PIE and thus a crucial part of spent fuel characterization and burnup programmes, ¹⁰ as well as being required by groups from EHS (e.g. RDW, BIS).  The necessary sample preparation steps and the TIMS measurements are very time consuming, however, and results typically take more than a week to obtain.  The advantage of TIMS is its high precision which means overall relative uncertainties for Pu and U assay (based upon isotope dilution) are typically 0.25 – 0.35 % (k = 2).  The precision of SF‑ICP‑MS isotope ratios is inferior, perhaps 10-20x poorer precision than our TIMS when applied to U and Pu measurements, but it is much quicker in terms of sample preparation and analysis time.  In a private communication the analytical services manager at ITU informs me that ITU obtains uncertainties of 0.8% (k = 2) for single element IDMS assay using its SF‑ICP‑MS, and this when applied to impurity analysis, not just to major components.  The gap between SF‑ICP‑MS and TIMS therefore  appears to be not so large.  Of course the difference between the techniques is irrelevant if the SF‑ICP‑MS measurement uncertainty exceeds that required by the customer (e.g. a fuel producer requires a particular maximum uncertainty in order to qualify its product, verification of a code requires data with uncertainties below a particular value).  But the gap is not necessarily the same for all analytes.  Over recent years our TIMS analyses of Nd (an important burnup monitor) have yielded uncertainties in the range 0.3 – 1.5 % depending on the particular sample and measurement conditions.  Uncertainties for other commonly measured lanthanides (Sm, Eu, Gd) have also been typically in the range 0.5 ‑ 0.75% (k = 2).  It is quite clear to us that measurement uncertainties on lanthanide determinations by SF‑ICP‑MS could sometimes be competitive with our TIMS measurements.  Given the shorter analysis times of SF‑ICP‑MS such measurements could speed up the rate at which we can process burnup samples and spent fuel characterizations.  And even in cases where the SF‑ICP‑MS is not competitive with TIMS, TIMS can still benefit from having a good estimate of the concentration in order to select a good spiking ratio for the isotope dilution step.  Until now we have performed spiking largely on the basis of assumptions derived from previous experience with similar fuel types and similar burnups rather than on actual measurements.  A comparison of the uncertainties of IDMS measurements on nuclear materials achievable with our SF‑ICP‑MS and TIMS instruments is therefore valuable in deciding if we can switch certain analyses away from TIMS.