Lithium and boron analysis by LA-ICP-MS results from a bowed PWR rod with contact

A previously published investigation of an irradiated fuel rod from the Ringhals 2 PWR, which was bowed to contact with an adjacent rod, identified a significant but highly localised thinning of the clad wall and increased corrosion. Rod fretting was deemed unlikely due to the adhering oxide covering the surfaces. Local overheating in itself was also deemed insufficient to account for the accelerated corrosion. Instead, an enhanced concentration of lithium due to conditions of local boiling was hypothesised to explain the accelerated corrosion. Studsvik has developed a hot cell coupled LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometer) equipment that enables a flexible means of isotopic analysis of irradiated fuel and other highly active surfaces. In this work, the equipment was used to investigate the distribution of lithium (Li) and boron (B) in the outer oxide at the bow contact area. Depth profiling in the clad oxide at the opposite side of the rod to the point of contact, which is considered to have experienced normal operating conditions and which has a typical oxide thickness, evidenced levels of ∼10–20 ppm Li and a B content reaching hundreds of ppm in the outer parts of the oxide, largely in agreement with the expected range of Li and B clad oxide concentrations from previous studies. In the contact area, the B content was similar to the reference condition at the opposite side. The Li content in the outermost oxide closest to the contact was, however, found to be strongly elevated, reaching several hundred ppm. The considerable and highly localised increase in lithium content at the area of enhanced corrosion thus offers strong evidence for a case of lithium induced breakaway corrosion during power operation, when rod-to-rod contact and high enough surface heat flux results in a very local increase in lithium


Introduction 1.Background
Results presented at the 2014 WRFPM [1] concerned a bowed fuel rod with rod-to-rod contact from the Ringhals 2 PWR in Sweden.The contact was identified in the peripheral row of an assembly during routine inspection at end of cycle unloading.Because poolside camera inspection indicated possible increased local corrosion at the contact area, it was decided to transport the rod to Studsvik for hot cell post-irradiation examinations (PIE).
The previously presented PIE [1] identified a significant but highly localised thinning of the clad wall and increased corrosion at the contact area.Rod fretting was deemed unlikely due to the adhering oxide covering the surfaces.Local overheating in itself was also deemed insufficient to account for the accelerated corrosion.The increased clad oxidation rate was, however, explainable by proposed Li induced corrosion enhancement under local boiling [2,3].Enhanced concentrations of Li and B due to conditions of local boiling in the crevice-like rod-to-rod contact area was thus hypothesised to explain the accelerated corrosion.The potential role of B might, however, also be of a beneficial nature [4].
In this work, additional examinations to investigate the distribution of lithium ( 7 Li) and boron ( 11 B) in the outer oxide at the bow contact elevation are presented.

Fuel and operating history data
Key fuel and operating data are summarised below.Additional data can be found in [1]. Figure 1 shows an overview of the contact region at 0°at 1142 mm elevation (from previous PIE of the rod).Figure 2 shows views of the outer oxide at 0°, 90°, 180°and 270°.

Additional post-irradiation examinations 2.1 Scope
The scope of investigation consisted of cutting of the sample, ∼10 mm long near the rod-to-rod contact point followed by LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) investigation of the 7 Li and 11 B content in the outer oxide.

LA-ICP-MS method
The LA-ICP-MS technique consists of a pulsed laser that ablates the material to be studied.A carrier gas transports the created aerosol for analysis to an Inductively Coupled Plasma Mass Spectrometer (ICP-MS).
The ablation equipment is a New-Wave UP-213 Nd:YAG laser mounted on a motorized X-Y-Z stage, in connection to an ablation chamber that is housed in a hot cell.The transport gas from the ablation cell is injected into a Perkin-Elmer Elan 6100 DRC II ICP-MS, coupled to a glove box.The laser operates at a wavelength of 213 nm with a pulse length of <4 ns.The ablated spot size can be varied between ∼5 and 160 mm with an ablation frequency of 1-20 Hz.The equipment can be used for spot analysis (drilling) or for line scans (typical traversing speed 10-140 mm/s).
For the results presented in this paper, a laser spot size of either 160 mm diameter (example in Fig. 3) or a square 95 mm line scanning beam was used (example in Fig. 4).An ablation frequency of 5 Hz and 100% intensity was employed.The carrier gas through the ablation chamber was He (∼800 ml/min).A makeup flow of ∼700 ml/min of Ar was added to the carrier gas prior to the ICP-MS.The ICP-MS was optimised for the low mass range.
The laser is of the flat beam type as can be seen in Figures 3 and 4, which are examples from ablation on inactive autoclave oxidised claddings during the calibration of the instrument.Figure 3 shows example SEM images of the ablated craters after laser ablation on zirconium oxide for 1, 3, 6 and 12 seconds (5 Hz, 160 mm spot).
Figure 4 shows a SEM image of the ablated track after multiple passages with a square beam (95 mm side) traversing between two pre-ablated spots, creating a rectangular track.
An ablation depth rate of ∼0.5 mm/s was achieved for the spot-wise analysis, alternatively a lateral depth resolution of ∼300 nm per passage was obtained when the beam was traversing the surface.Calibration was performed by ablation on a set of inactive standards obtained via PSI (Paul Scherrer Institute, Switzerland).The standards consisted of pieces of cladding with an outer oxide, grown by autoclave exposure.The standards were implanted with 7 Li ions at ETH (Swiss Federal Institute of Technology), and were of the same kind as those used for 7 Li calibration of the SIMS equipment at PSI. SIMS measurements and SRIM calculations (Stopping and Range of Ions in Matter) of the implanted depth profile showed a peak 7 Li content ∼2.1 mm inside the oxide.Reference [5] provides additional information on the SIMS analysis and on the use of the same type of implanted reference materials.With the above information and the implanted dose, the calculated peak 7 Li oxide content was used for calibration.The ablation depth rate was obtained by transforming the ablation time from the first rise in 91 Zr signal to the time to reach the 7 Li peak in the implanted standards, resulting in a depth rate of 0.5 mm/s (in good agreement with the SEM results).The same laser and ICP-MS settings were used for the standards and the samples within the analysis campaign.Figure 5 shows the Zr-normalised 7 Li calibration plot.
The uncertainty of the 7 Li calibration is estimated at ±10%, or ±0.5 ppm for the lower concentrations.
As a secondary objective, non-matrix matched 11 B intensity calibration was estimated from ablation on NIST 610 and 612 (National Institute of Standards and Technology, USA), standard reference material glasses, using the averaged B concentrations reported by Jochum et al. [6].Non Zr-normalized 11 B calibration was performed since the NIST glasses only contain minor amounts of Zr.The different matrixes, glass vs. Zr/ZrO 2 in the samples as well as variations in the ablated mass rate (geometry effects, sample density, etc.) could significantly affect the validity of the comparison.
The 11 B calibration uncertainty is thus larger and is estimated at ±100%.

7 Li and 11 B results
Cladding analysis of the irradiated fuel rod was performed at a sample cut out in the lower area of the rod bow contact (∼1131 to 1141 mm, marked by solid lines in Fig. 6), directly below the transversal metallography cross section at 1142 mm from the rod bottom end (marked by the dashed line in Fig. 6).The sample was transported to the laser ablation hot cell without any further preparation (no defueling required).
Figure 7 shows spot wise laser ablation performed at a rod elevation of ca.1140 mm at four different rotations angles, using the same zero angle as in the original PIE work [1].The 0°7Li depth profile shows the enormously elevated 7 Li content in the direction of the contact (near the area of maximum oxide thickness).
Figure 8 shows a contour plot with 7 Li results based on multiple line scans at an axial position of ∼1141 mm in the circumferential direction near 0°.
The line scans cover approximately ±18°of the circumference around the 0°position.Each line pass corresponds to a step of ∼300 nm into the oxide from the oxide/coolant interface.Although it may appear that the outer surfaces are flat in the Li and B plots, it should be pointed out that this is an effect of defining the x-axis as the ablated depth from the outer oxide surface (from the rise in Zr-signal during ablation).In reality, both the outer and  inner boundaries of the outer oxide (as well as the thickness) are actually quite irregular (Figs. 1 and 2).Since the sample is not rotated as the laser traverses the sample in the circumferential direction, there is also a small geometrical bias to overestimate the oxide thickness when the beam is the furthest from the normal plane (∼0.5 mm bias at the ±18°endpoints in Fig. 8).
Figure 9 shows a contour plot with 11 B results based on multiple line scans at the axial position of ∼1141 mm in the circumferential direction near 0°(same scan as the 7 Li results in Fig. 8).
Figure 10 shows a contour plot with 7 Li results from multiple line scans at the axial position of ∼1132 mm (a few millimetres below the contact) in the circumferential direction near 0°.
Figure 11 shows a contour plot with 11 B results based on multiple line scans at the axial position of ∼1132 mm in the circumferential direction near 0°(same scan as the 7 Li results in Fig. 10).
Figure 12 shows a contour plot with 7 Li results based on multiple line scans at the axial position of ∼1140 mm in the circumferential direction near 180°.
Figure 13 shows a contour plot with 11 B results based on multiple line scans at the axial position of ∼1140 mm in the circumferential direction near 180°(same scan as in Fig. 12).

Discussion & summary
The results show that the 7 Li content in the oxide with a normal thickness (∼10 mm) are in agreement with previous results from irradiated M5 TM claddings [7], showing a maximum of ∼10 to 20 ppm 7 Li about ∼1 to 2 mm inside the oxide.This 7 Li oxide distribution is illustrated in Figure 14 (left), which is an alternative plot showing the same data as in Figure 12.
Figure 14 (right), which is a 7 Li plot from the contact area with the maximum oxide thickness near 0°, illustrates the highly localised and strongly enhanced 7 Li content at that   location.The circumferential 7 Li concentration profile appears to follow the oxide thickness profile with a maximum 7 Li concentration of almost 600 ppm on the surface of the oxide close to the point of maximum oxide thickness (∼50 mm).This peak Li value equates to ∼0.35 atom% Li, if the bulk of the oxide is assumed to be ZrO 2 .One should point out that the axial elevation of the sample (∼1140 mm from the bottom end plug) is from a location with no or very limited conditions of local boiling during normal operating conditions.The maximum 7 Li gradient inside the oxide, in the contact zone, is considerable, with 7 Li concentrations going from >500 ppm at the surface of the contact to <10 ppm approximately 10 mm inside the oxide (local oxide thickness 30-50 mm).The enhanced Li content thus seems to be related to the outer surface of the oxide, and not to the deeper parts of the oxide or the clad-oxide interface.These results contrast with the flatter Li oxide depth profiles from experiments on Li enhanced rapid corrosion of Zry-4 at high local voids [2,4].Keeping in mind that the cladding in this study is the Nb containing M5 TM alloy, the results do however have similarities with Li and B profiles from other Li and B corrosion tests [8].Interestingly, [8] tentatively identifies a beneficial effect of a more compact (impermeable to Li) oxide close to the metal interface of Nb containing Zr-alloys.A recent out-of-pile autoclave study points in the same direction [9].Analysis of the 11 B content (Figs. 9, 11 and 13) was largely in agreement with previous investigations [7], although it should be noted that the 11 B results are more uncertain, being a secondary objective of this study.At the location of the rod-to-rod contact, the 11 B content could possibly be slightly enhanced, reaching ∼1500 ppm in a very thin outer layer, in comparison with values of ∼500 to 1000 ppm at the outer surfaces of the other locations.The thin outermost 11 B layer is probably due to drying in of spent fuel pool water (2000+ ppm B, no added Li).The 11 B profiles also typically displayed a second peak ∼2 mm into the oxide, and sometimes a third more diffuse peak at ∼3 to  4 mm depth.These 11 B peaks do not appear to be correlated with the layering of the oxide (Fig. 2).In the areas with normal oxide thickness away from the contact zone, the peak 7 Li and 11 B concentrations appear to occur at different depths in the oxide.
In summary, the considerable and highly localised increase in Li content at the area of enhanced corrosion offers strong evidence for a case of Li induced breakaway corrosion during power operation.One could also argue that the exact Li bulk concentration in the reactor water (± a few ppm Li) may probably be of minor importance, compared with the likely key factor, the local boiling introduced by the high enough heat flux and poor flow conditions around the rod-to-rod contact, causing a very local increase in Li concentration.
Vattenfall Nuclear Fuel AB is acknowledged for commissioning of the work.

Fig. 6 .
Fig. 6.Overview of the laser ablation sample relative to the contact area.

Fig. 14. 7
Fig.14.7 Li plot with same data as in Figures12 (left) and 8 (right).The left plot shows the7 Li distribution in the oxide with normal thickness (8-10 mm) at 180°.The right plot shows the strongly elevated7 Li at the area of rod-to-rod contact (∼35-50 mm oxide thickness).

8 A.
Puranen et al.: EPJ Nuclear Sci.Technol.3, 2 (2017) The contact occurred during the last cycle, as evidenced by video from the previous end of the cycle inspection.-Lastcyclelinearheat rate at contact elevation decreasing from ∼19 to 13 kW/m during the cycle.No boiling is expected at this elevation during normal operation.-Lastcyclereactor7Li concentration 5 ppm, decreasing to ∼2 ppm.