Formation and deposition of platinum nanoparticles under boiling water reactor conditions

Journal of Nuclear Materials 494 (2017) 200e210

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Formation and deposition of platinum nanoparticles under boiling water reactor conditions Pascal V. Grundler*, Lyubomira Veleva, Stefan Ritter Paul Scherrer Institut (PSI), Nuclear Energy and Safety Research Division, 5232 Villigen PSI, Switzerland

h i g h l i g h t s
Water chemistry plays an important role in the in-situ formation of Pt particles.
Pt injection rate exerts some control on size distribution of the Pt nanoparticles.
Deposition of the particles is significantly influenced by the flow conditions.
Morphology and nature of the oxide film influence the retention of the Pt particles.
Power plant Pt applications can be reproduced reasonably well in the lab.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 March 2017 Received in revised form 26 June 2017 Accepted 8 July 2017 Available online 10 July 2017

Stress corrosion cracking (SCC) is a well-known degradation mechanism for components of boiling water reactors (BWRs). Therefore the mitigation of SCC is important for ensuring the integrity of the reactor system. Noble metal chemical application (NMCA) has been developed by General Electric to mitigate SCC and reduce the negative side-effects of hydrogen water chemistry used initially for SCC mitigation. NMCA is now widely applied as an online process (OLNC) during power operation. However, the understanding of the parameters that control the formation and deposition of the noble metal (Pt) particles in a BWR was still incomplete. To fill this knowledge gap, systematic studies on the formation and deposition behaviour of Pt particles in simulated and real BWR environment were performed in the framework of a research project at PSI. The present paper summarizes the most important findings. Experiments in a sophisticated high-temperature water loop revealed that the flow conditions, water chemistry, the Pt injection rate, and the pre-conditioning of the stainless steel surfaces have an impact on the Pt deposition behaviour. Slower Pt injection rates and stoichiometric excess of H2 over O2 produce smaller particles, which may increase the efficiency of the OLNC technique in mitigating SCC. Surfaces with a well-developed oxide layer retain more Pt particles. Furthermore, the pre- and post-OLNC exposure times play an important role for the Pt deposition on specimens exposed at the KKL power plant. Redistribution of Pt in the plant takes place, but most of the Pt apparently does not redeposit on the steel surfaces in the reactor system. Comparison of lab and plant results also demonstrated that plant OLNC applications can be simulated reasonably well on the lab scale. © 2017 Elsevier B.V. All rights reserved.

Keywords: Nanoparticles Platinum Boiling water reactor Stress corrosion cracking Water chemistry Electrocatalysis

1. Introduction Components of boiling water reactors (BWR) made of stainless steel, a material otherwise resistant to uniform corrosion, are sensitive to stress corrosion cracking (SCC) under the combination of mechanical loads and an aggressive environment, the latter

* Corresponding author. E-mail address: [email protected] (P.V. Grundler). http://dx.doi.org/10.1016/j.jnucmat.2017.07.018 0022-3115/© 2017 Elsevier B.V. All rights reserved.

being the result of the radiolysis of water (the less volatile H2O2 accumulates in the liquid phase) [1]. A consequence of the excess of oxidising species is an increased electrochemical corrosion potential (ECP) ranging from þ100 to þ250 mVSHE. The injection of H2 into the reactor feed water was found to be effective to reduce the content of oxidising species and bring the ECP to low values, thus helping to mitigate SCC (hydrogen water chemistry, HWC) [2]. However, a side effect of the relatively high H2 concentrations e required to achieve the desired ECP level e was the speciation of 16 N into the steam and as a corollary, often a significant increase in

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the steam line dose rates. To minimise this issue, General Electric has developed a technology [3] that uses noble metals to catalyse the reaction between H2 and the oxidising species O2 and H2O2, thus allowing the desired decrease in ECP with significantly lower H2 concentrations (stoichiometric concentration is sufficient) and a much smaller effect on the steam line dose rate. This technology, marketed as NobleChem™ by General Electric [4], is based on the injection of soluble noble metal compounds into the hot water of the reactor feed stream. Under the effect of the increased temperature, the soluble noble metal compounds form metallic nanoparticles. It is claimed [4] that the nanoparticles deposit on all water wetted surfaces where they develop their catalytic activity and therefore help to mitigate SCC by decreasing the ECP when sufficient H2 is present. Despite the fact that the technology is already in use in numerous BWRs [5e7], there are still many open questions on the efficiency of the technology and its potential for improvement. Therefore, a joint project between PSI, the Swiss Federal Nuclear Safety Inspectorate (ENSI) and the nuclear power plants of Leibstadt (KKL) and Mühleberg (KKM) in Switzerland was started to obtain phenomenological insights and a better basic understanding of the Pt distribution and deposition behaviour in BWRs. Beside the work in the laboratory at PSI, experiments were also performed at the KKL plant to collect data from full-scale on-line noble metal chemical applications (OLNCs). The present paper summarizes the major findings of this project, which has lasted 3.5 years. 2. Material and methods 2.1. Specimens and Pt solutions Coupon specimens 13  10  4 mm made of AISI 304L stainless steel (DIN no. 1.4306) were used either in “as-received” (AR) condition or pre-oxidized (PO) for two weeks under BWR/HWC conditions (T ¼ 275  C, 0 ppb O2, 170 ppb H2, high-purity water with k < 0.06 mS cm1) prior to the Pt application tests. Surface roughness for all coupons was N5 (Ra ¼ 0.4 mm). The chemical composition of the stainless steel is given in Table 1. Pt was provided by KKM as an aqueous solution of Na2Pt(OH)6 (1% Pt, total of other Pt group elements < 350 ppm), the actual solution used at the plant. This stock solution was diluted at PSI with high-purity water (k < 0.06 mS cm1) to a 100 ppb Pt solution prior to injection into the loop with a high-pressure dosing pump. 2.2. Experimental procedure of the loop tests A sophisticated loop with autoclave, made of stainless steel (Fig. 1), is in use at PSI to reproduce the water chemistry of a BWR, however without the radioactive component. In the lowtemperature part, the water chemistry can be precisely adjusted (H2, O2) and closely monitored (dissolved H2 and O2 concentrations, conductivity and flow). Due to the tendency of H2O2 to decompose, its concentration is difficult to control. For this reason, the deliberate choice to add only O2 to the water, and not H2O2 as well, was made to achieve well defined conditions. Specimens are exposed in the high-temperature part in an externally heated autoclave (volume z 0.9 L, flow rate ¼ 10 kg h1) and two specimen holders (SHs) connected in series to the exit of the autoclave. In the SHs, the

201

hot water flows over the two large sides of the specimens in ducts of rectangular section (for details see Ref. [10]). Pt is injected ahead of the autoclave into the hot water stream (T ¼ 280  C) coming from the pre-heater. The ECPs of stainless steel coupons and Pt sheets (redox potential) connected to wires are recorded before the Pt injection point (control measurement) and inside the autoclave. Potentials are measured with respect to a Cu/Cu2O ZrO2-membrane electrode [11] and referenced to the standard hydrogen electrode (SHE) scale. In a typical test run, pre-conditioning of the specimens (AR and PO) in the autoclave and SHs under BWR/HWC conditions lasts eight days. Note that the PO specimens were pre-oxidized under a different water chemistry (H2, O2 contents) than the Pt application tests, thus possibly changing the initial nature of the oxide film [8]. However, the pre-conditioning time before the start of the Pt application is considered to be sufficient for the re-equilibration of the oxide film [9]. Then Pt is injected as a solution of Na2Pt(OH)6 for the required duration. After Pt injection has ended, the system is kept under the same BWR/HWC conditions for three more days before shut down and removal of the specimens from the loop. 2.3. Experimental procedure of plant tests To study the Pt deposition behaviour and to assess the effectiveness of the OLNC technology under real plant conditions, specimens were also exposed at two locations in KKL (Fig. 2): (i) Mitigation Monitoring System (MMS) in the reactor water clean-up system (T ¼ 275  C, flow rate ¼ 318 L h1, hemicyclic channel on each side of the specimens, flow velocity across specimens z 0.5 m s1), (ii) Backup Deposition Monitoring System (BDMS) in the reactor water sample line (T ¼ 277  C, flow rate ¼ 71 L h1, flow velocity across specimens z 0.6 and 1.1 m s1). Individual SHs in the MMS were inserted and removed at different times during the plant cycles to vary pre- and post-application exposure times. 2.4. Analytical procedures Microstructural investigations were performed using a Zeiss ULTRA 55 scanning electron microscope (SEM) equipped with a field emission gun and X-ray energy dispersive spectrometer (EDS) for chemical analyses. A secondary electron detector was used to investigate the oxide layer topography, whereas a back scattered electron detector was used to identify Pt particles (chemical contrast) across the oxide layers. For particle size measurements, contrast and threshold of back scattered SEM micrographs were adjusted to clearly delineate the particles prior to converting the images into binary mode. Feret diameters of the individual particles were then obtained from the binary images using ImageJ software [12]. Active specimens from KKL were not examined by SEM. Instead, a film of cellulose acetate dissolved in acetone was applied to the surfaces of interest. After drying the film was peeled off, thus providing an extractive replica of the surface. The particles (oxides and Pt) attached to the film were then transferred to copper grids for investigation by transmission electron microscopy (TEM) with a JEOL 2010 instrument. Replicas and TEM were also used for investigation of inactive specimens when the study of particles below

Table 1 Chemical composition of the AISI 304L (DIN no. 1.4306) stainless steel in weight-%. C

Si

Mn

P

S

Cr

Mo

Ni

Co

Cu

N

Nb

Ti

0.024

0.35

1.49

0.026

0.005

17.9

0.247

10.00

0.088

0.305

0.059

0.001

0.001

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Fig. 1. Schematic of the PSI high-temperature water loop as used for the Pt deposition experiments. All parts in contact with the water are made of stainless steel or an inert polymer (usually PTFE).

Fig. 2. Schematic of the set up at the KKL plant and locations of the MMS and BDMS.

the resolution limit of SEM (approx. 3e4 nm) was required [13]. Quantitative Pt loadings on specimen surfaces were determined by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) [14]. Spots ranging from 60 to 120 mm in diameter were ablated with a laser beam and the aerosol transported by a helium stream to the ICP-MS. For calibration of the instrument, stainless steel specimens coated with well-defined surface loadings of Pt were used as references. Platinum loadings ranged from 0.03 to 30 mg cm2 corresponding to a nominal Pt layer thickness of 0.014e14 nm. For each specimen, 20 to 60 ablation points were measured. The detection limit was down to 1 ng cm2 in most cases.

3. Results and discussion 3.1. Effect of environmental conditions Most tests were performed under a set of water chemistry conditions which were defined for the purpose of the present study as standard conditions (see column “10” in Table 2). Two tests were performed with markedly different water chemistry conditions to allow studying the effect of different dissolved gas levels on formation and deposition of Pt particles. In Test 12 a stoichiometric excess of O2 over H2 was chosen whereas in Test 11 H2 was in large excess, much more than used in standard tests. Table 2 lists the

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203

Table 2 Summary of experimental parameters for the tests discussed in the paper. Test number

7

8

10

11

12

13

Experiment duration [h] Pressure [bar] T autoclave [ C] Water mass flow [kg h1]

477 92 280 10

305 91 280 10

433 91 280 10

526 90 280 10

503 92 280 10

351 91 280 10

Water chemistry

Inlet

Outlet

Inlet

Outlet

Inlet

Outlet

Inlet

Outlet

Inlet

Outlet

Inlet

Outlet

Molar ratio (H2/O2) H2 content [ppb] O2 content [ppb] Conductivity [mS cm1] Pt conc. of solution [ppb] Pt injection period [h] Pt injection rate [mg h1] Pt conc. in water [ppt]a Total Pt injected [mg]

4.8 75 250 0.055 9.7 207 0.2 23 49

e 46 0 0.070

4.8 76 250 0.055 9.6 40 1.2 115 48

e 44 0 0.060

3.6 76 338 0.055 94.2 168 3.9 392 660

e 33 0 0.078

32 80 40 0.055 91.3 168 3.8 380 639

e 75 0 0.104

1.3 40 500 0.055 84.1 168 3.5 350 589

e 0 200 0.172

4.0 77 305 0.055 96.6 57 12.1 1208 692

e 0 0 0.104

Test number

16

17

19

22

24

26

Experiment duration [h] Pressure [bar] T autoclave [ C] Water mass flow [kg h1]

527 91 281 10

432 90 220 10

441 90 281 10

524 90 220 10

400 90 281 10

524 91 280 10

Water chemistry

Inlet

Outlet

Inlet

Outlet

Inlet

Outlet

Inlet

Outlet

Inlet

Outlet

Inlet

Outlet

Molar ratio (H2/O2) H2 content [ppb] O2 content [ppb] Conductivity [mS cm1] Pt conc. of solution [ppb] Pt injection period [h] Pt injection rate [mg h1] Pt conc. in water [ppt]a Total Pt injected [mg]

3.7 75 320 0.055 95.0 256 2.1 207 529

e 33 0 0.084

4.0 75 296 0.056 96.8 168 4.0 400 672

e 33 0 0.066

3.7 75 322 0.055 101.0 165 4.2 427 699

e 34 0 0.096

3.5 71 325 0.055 97.4 258 2.2 217 560

e 30 0 0.076

3.5 65 292 0.055 93.5 94 6.9 690 650

e 30 0 0.079

4.3 82 306 0.055 90.5 452 1.6 160 721

e 33 0 0.077

a Estimated on the basis of the concentration of the injection solution and the respective mass flows. This concentration decreases gradually from the vicinity of the injection point onwards due to Pt deposition.

major experimental parameters. Pt injection rate, temperature, flow rate and duration were kept as close as possible to the standard conditions (Test 10). The ratio of dissolved H2 to O2 has a strong effect on the redox conditions prevailing in the system as can be seen from the plot of the ECPs (Fig. 3). Before the start of the Pt injection, the ECP values were reflecting the prevailing redox conditions. In Test 10 the potentials of the three materials are distinct, the AR specimen having the highest potential. The autoclave is at a lower potential than the specimen because it has experienced previous Pt applications (a routine cleaning is not sufficient to remove all traces of Pt). As soon as the Pt injection starts the potential of the specimen drops rapidly until eventually reaching the minimum potential and staying there, even after the end of the Pt injection. In the case of Test 11, the Pt injection did not produce any change in ECP (the small fluctuations of the potential curves are due to short interruptions, when replacing an empty bottle of the Pt injection solution, and to water sampling), because the overwhelming excess of H2 is sufficient on its own to essentially neutralise all O2 present without the need for a catalyst. In the case of Test 12 (excess O2) the picture is less clear. The ECP of the specimen is slightly but regularly increasing during the Pt injection and stabilises at its new level after the end of the Pt injection. In case of the Pt sheet and autoclave, the start of the Pt injection coincides with a slight drop in ECP before an upward trend is taking over. This trend does not end with the termination of the Pt injection. The small increase in O2 feed rate (500e520 ppb) during the injection period can only account for a part of this ECP increase. This test essentially demonstrates that without at least a stoichiometric amount of H2 the presence of Pt alone does not lower the ECP.

The water chemistry has also a strong effect on the size distribution of the Pt particles (Fig. 4). The average Pt particle size from both, AR and PO specimens was found to be around 9 nm in Test 11 and 30 nm in Test 12. The fully reducing environment leads to finer Pt particles and homogenous dispersion whereas the oxidising environment leads to much larger Pt particles (Fig. 5) and inhomogeneous dispersion. Test 10, with intermediate conditions, fits well between these two tests. Due to the limited throwing power in high purity water, for a given Pt loading, finer Pt particles eevenly distributed across the specimen surfacee are regarded as more advantageous for the mitigation of SCC compared to larger, inhomogeneously dispersed ones [15]. Although an above average Pt loading is observed for the autoclave specimens (Fig. 6) this trend is not found at the two other locations (see Table 3 for numerical values). There is an inherent variation in Pt loadings as is exemplified by the differences observed between Tests 10 and 19 where the latter is basically a control of the former. Overall, the water chemistry (molar H2/O2 ratio) seems to have only a marginal influence on the amount of Pt deposited compared to the other parameters investigated. 3.2. Effect of water temperature Most tests were performed at 280  C (270  C in SHs 1 and 2, which are not actively heated), which is close to the average temperature of the water inside the reactor and recirculation loops. However, in nuclear power plants the Pt injection is made into the feed water line at a lower temperature; e.g., approx. 220  C in KKL and 195  C in KKM. Therefore the formation and deposition of Pt particles at the lower temperature of 220  C was investigated

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Fig. 3. Course of the specimen in AR condition ( ), the autoclave ( ) and the redox ( ) potentials during Tests 10, 11, 12, 16, 17 and 22. Shaded area represents the time during which Pt was injected. Start of Pt injection is t ¼ 0. Note the positive scale on the ordinate for Test 12.

during Tests 17 and 22. Compared to Test 10, performed at 280  C with a very similar injection rate, the ECP of the stainless steel coupon in test 17 decreases at a much slower rate (Fig. 3). The same observation holds

for Test 22 when compared to Test 16 (Fig. 3). In Test 22 the minimum potential (¼ Pt potential) is not reached; possibly an indication of an insufficient Pt deposition under these conditions. SEM investigation of coupons exposed during Test 22 showed a poor

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205

Fig. 4. Particle size distribution under moderate (Test 10) and large (Test 11) H2 excess, and under O2 excess (Test 12) for AR and PO specimens.

Fig. 5. Back scattered electron images of PO specimens from Tests 11 (left) and 12 (right). The white dots are Pt particles resting on the crystals of the oxide film.

coverage with Pt particles which made the evaluation of the particle size distribution difficult due to low counts. But the average size is nevertheless very similar to the one found in Test 16 (9.8 ± 1.2 vs 10.5 ± 4 nm). This could explain the limited decrease of the ECP. However, it does not correlate with the Pt loading determined by LA-ICP-MS (Table 4) which is in the usual range when compared to tests with similar total amounts of injected Pt. Overall, the quantity (according to LA-ICP-MS) and quality (size) of the particles is not affected by the lower temperature. Yet, the dispersion on the surface seems to be suboptimal with respect to the efficient lowering of the ECP. With the present experimental set-up it was not possible to have a deposition zone at a higher temperature than the injection point to actually simulate the thermal path found in a nuclear power plant. Therefore the hypothesis that the higher temperature is important for the optimal dispersion of the Pt particle on the surface could not be tested. During the tests at 280  C, the temperature in the SHs is usually about 10  C lower (e.g. 270  C) and no negative effect on the Pt behaviour correlated to this smaller temperature difference could be observed. 3.3. Effect of specimen pre-treatment Fig. 6 shows a clear trend in Pt loading between AR and PO specimens. Therefore the nature of the oxide has an influence on the Pt deposition. Stainless steel exposed to water under BWR conditions develops an oxide film on its surface. The nature of the oxide film reflects the composition of the stainless steel and the water chemistry; common phases are the mixed oxides NiFe2O4, FeCr2O4, FeFe2O4, and

NiCr2O4 [16]. The oxide film usually consists of two layers. The inner layer, which forms first, is chromium rich (chromite). Upon further exposure an outer oxide layer starts to growth which consists of ferrite (iron rich) [17]. The outer layer often consists of larger euhedral crystals which contrast with the fine grained and compact structure of the inner layer. Test 13 was selected to illustrate the present discussion, because it is the test with the shortest run time (fastest Pt injection rate) of the tests with standard environmental conditions (DH, DO, temperature). Therefore, this is the test where the difference in exposure time to the hot water between AR and PO specimens was the greatest. Pre-oxidation time was 310 h for PO specimens and the duration of Test 13 was 351 h. Fig. 7 nicely illustrates the difference in oxide layer morphology between AR and PO specimens. In the case of the PO specimen the edges and faces of the large oxide crystals tend to accumulate Pt particles whereas the inner layer is only lightly scattered with Pt particles. With the absence of a well-developed outer oxide layer the amount of Pt particles that covers the AR specimen is also reduced, although the particles seem to be quite evenly spread. These qualitative visual observations correlate well with the results from the LA-ICP-MS analyses which regularly found higher Pt loadings on PO specimens compared to AR ones within the same test and at the same location Fig. 6. The fact that Pt particles preferentially deposit on the PO specimens may be explained by the increased micro-roughness (increased specific area) due to the presence of a well-developed outer oxide layer. The fact that corners and edges of the crystal protrude from the surface also reduces the distance the particles have to diffuse through the stagnant water layer before reaching

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This can be explained by the fact that at a lower injection rate the Pt concentration in the high-temperature water is lower (down to 24 ppt in case of the test with the lowest injection rate) thus limiting the ability of the particles to grow by accretion after the nucleation phase [18]. A higher injection rate leads to a faster decrease of the ECP of a stainless steel coupon exposed to the fluid carrying the Pt. Increasing the injection rate from about 4 to 12 mg h1 reduces the time to go from 0.27 to 0.49 VSHE by a factor of two. 3.5. Effect of flow conditions on Pt deposition

Fig. 6. Pt surface loadings on as-received (AR) and pre-oxidized (PO) coupons exposed to water with Pt nanoparticles at the three different locations of the high-temperature water loop.

the surface. In some limiting cases the increased roughness could also contribute to the formation of turbulences which seem to be beneficial to the deposition of particles (cf. Section 3.5). Another factor may be the chemical nature of the oxide layer. Perhaps the Pt particles have a higher affinity for ferrite type compounds than for the chromite ones. However, more specific experiments would be needed to test this hypothesis. From the discussion above it is concluded that the pre-oxidation time of the steel plays a vital role in the Pt deposition behaviour and the nature of the oxide may be an important factor as well. 3.4. Effect of Pt injection rate Seven experiments were performed focussing on the effect of the Pt injection rate on the Pt deposition behaviour. Results from LA-ICP-MS showed no major variation in the amount of Pt deposited as a function of the injection rate, when corrected for the total amount injected. On the other hand particle size distribution analysis showed a relation between the Pt injection rate and the particle size (Fig. 8). From the data displayed in Fig. 8, one easily observes that the average Pt particle size increases with increasing injection rate reaching a maximum above about 5 mg h1 (corresponding to a maximal Pt concentration in the fluid of 500 ppt).

During all experimental runs, specimens were exposed to water at three different locations of the loop: (i) suspended in the autoclave with quasi-stagnant flow conditions (few mm s1), (ii) in a specimen holder (SH1) with a large rectangular duct on each side of the specimens (0.10 m s1), (iii) in a specimen holder similar to SH1 but with smaller rectangular ducts (SH2) (0.52 m s1). The dimensions of the SHs are shown in Fig. 9. When comparing the Pt loadings obtained by LA-ICP-MS for a series of eleven tests (Fig. 6), that have been performed under varying conditions (Pt injection rate, total Pt amount injected, temperature, and dissolved oxygen and hydrogen levels), one recurrent observation is that specimens in SH1 always have a lower Pt loading than specimens in SH2 and in the autoclave. However, the particle size distribution within a test seems not be significantly affect by flow conditions [10]. The high Pt loadings on autoclave specimens can be explained by the proximity to the Pt injection point and therefore the higher Pt concentration compared to locations further downstream and also because of the relatively long residence time of the fluid in the autoclave (several minutes). The SHs are located in series after the autoclave and SH2 has a systematically higher Pt loading despite its location after SH1. Therefore the argument of being up stream and thus closer to the Pt source does not apply to SH2. The major parameter that distinguishes the two SHs is the fluid flow. Computation of the Reynolds numbers, Re, for both SHs shows that they are above the critical value for a turbulent flow regime in both cases (Table 5). However a developed turbulent flow regime is only present over a significant length in SH2. This is an important factor when considering the deposition behaviour at these two locations [10]. Under turbulent conditions, the stagnant layer (in which transport takes place by slow diffusion) is very thin and there is throughout mixing of the fluid. These are favourable conditions for the deposition of Pt particles because they have only to travel a reduced distance by diffusion and the turbulences allow a fast transport over the whole cross section of the duct (Fig. 10a). On the other hand under quieter flow conditions, with the limiting case of laminar flow, the velocity component perpendicular to the duct walls is likely to be much smaller than the parallel component (Fig. 10b) and transport towards the wall will significantly rely on a diffusion contribution. This is consistent with results from simulation of the deposition of nanoparticles in laminar flows [19]. Consequently more particles reach the surface per unit of time under turbulent conditions producing higher Pt surface loadings. This is well illustrated in the case of the SHs. 3.6. Exposure of specimens at the KKL plant SHs have been inserted and removed from the MMS at KKL at different times during one plant cycle. During this cycle two OLNC applications took place, separated by 5.3 weeks. The Pt surface loading of the specimens exposed in the MMS is given in Fig. 11. It shows that specimens exposed to the reactor water for 12.3 weeks before and 10 weeks after the OLNC

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Table 3 Pt surface loadings in mg cm2 as determined by LA-ICP-MS for specimens exposed to different H2/O2 molar ratios during Pt application.

Table 4 Average Pt surface loadings on the autoclave specimens from Tests 16 and 22 as determined by LA-ICP-MS. Test Nr Total Pt injected [mg]

16 529

Specimen condition Pt surface loading [mg/cm2]

AR 0.172 ± 0.039

22 560 PO 0.234 ± 0.032

AR 0.167 ± 0.043

PO 0.376 ± 0.091

Fig. 7. SEM images in BSE mode from Test 13; AR (left) and PO specimens (right) which were located in SH2. The white dots are Pt particles resting on the oxide film.

applications (MMS-B) have the highest Pt surface loadings, whereas specimens with the same pre-OLNC exposure time but longer postexposure time (26.9 weeks, MMS-C) “lost” more than half of their Pt. In the case of specimens inserted just before the OLNC application (MMS-E), the Pt loading is even lower than on MMS-C specimens. The Pt surface loading on specimens from SHs B, C

and E therefore strongly depends on the pre- and post-OLNC exposure time, as this is the only difference between those specimens. The lab tests have shown that PO specimens tend to have higher loadings than AR specimens (see section 3.3). The difference between MMS-E and MMS-B is certainly also related to the development of an oxide film beneficial to Pt deposition and retention

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Fig. 8. Effect of the Pt injection rate (and water chemistry, Tests 11 & 12) on the average Pt particle size.

Fig. 11. Pt loadings on specimens from the MMS at KKL, inserted and removed at different times during a plant cycle with two OLNC applications with a 5.3 weeks gap between them.

Fig. 9. Cross sections of specimen holders SH1 and SH2. The hydrodynamic characteristics are listed in Table 5.

Table 5 Bulk flow velocity, Reynolds numbers and entrance lengths for the SHs at 270  C.

Hydraulic diameter DH [m] Aspect ratio (width to height) a [-] Duct length [m] Bulk flow velocity UB [m s1] Reynolds number Re [-] Critical Reynolds number Recrit [-] Entry length turbulent flow le [m], le =DH z4:4Re1=6 Entry length laminar flow le [m], le =DH z0:06Re

SH1

SH2

0.00400 2.0 0.100 0.0999 3131 ~1900 0.067 0.75

0.001057 10.3 0.100 0.517 4282 ~2700 0.019 e

during the additional exposure time. The amplitude of this difference is however larger than expected from the loop tests. An additional factor to consider is that the Pt application during this plant cycle took place in two stages. During the first one, 10 g of Pt were injected. The second stage took place six weeks later when 354 g of Pt were injected. MMS-E was inserted just before the first application and only a minimal oxide would have developed before it received Pt. Although only a small quantity, this Pt may have influenced in some way the further development of the oxide film, thus leaving it less receptive for Pt during the main application, which eventually led to the lower Pt loadings observed compared to MMS-B. Two SHs did not experience the OLNC applications. MMS-A was removed before the first and MMS-F was inserted after the last application for about 17 weeks. Only traces of Pt could be detected on the MMS-A specimens and in case of MMS-F the specimens showed an extremely low Pt loading. Therefore it seems that the Pt, once “eroded” from the surface, is not able to sufficiently re-deposit on other surfaces. The Pt loss is likely to happen by erosion of oxide particles carrying the Pt on their surface. The Pt is lost for protection as these oxide particles become CRUD on fuel components or are removed by the water clean-up system. Another OLNC (re-) application would be necessary to deposit a relevant amount of Pt on plant components which were not in service during OLNC applications. 3.7. Comparison of lab and plant results

Fig. 10. Qualitative view of particle transport in a pipe under: (a) turbulent flow and (b) laminar flow conditions (small arrows on particle represent the random diffusion).

Lab tests allow a greater flexibility in experiment design than would be possible in a nuclear power plant and thus are essential to test new ideas and gain new insights into the Pt deposition behaviour. However, to verify if such lab data can be transferred to the plant scale, results from the exposure of specimens in an actual BWR plant need to be compared to the lab results. In terms of Pt surface loading, AR and PO specimens exposed at KKL follow exactly the same trend as observed on lab specimens (i.e., more Pt on PO specimens); compare Figs. 6e11 and 12.

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environment, usually achieve higher Pt loadings than surfaces, in the same setting, with a thin oxide layer. The comparison of the results from the lab to BWR plant tests shows that OLNC can be reproduced reasonably well in the lab and therefore the more detailed lab results can be used to understand the behaviour in the power plant. In this context, and from a scientific point of view, some plant recommendations for OLNC may be formulated: The Pt injection should be performed at a slow rate under reducing conditions for extended periods of time. Applications should be repeated on a regular basis to compensate for Pt erosion and to protect new surfaces. Pre-oxidation of new components may contribute to achieve higher Pt loadings early on. Acknowledgements

Fig. 12. Comparison of Pt surface loadings on specimens from the MMS exposed to reactor water (KKL) during an ONLC application and from a PSI lab test simulating such an OLNC application at KKL.

Some lab tests were performed which simulated an actual OLNC application at KKL in terms of total amount of Pt injected and the injection rate. However, the pre- and post-OLNC exposure periods were about ten times longer at KKL. Nevertheless, Pt surface loadings on plant and lab coupons are fairly comparable (Fig. 12). The higher Pt concentrations on the lab specimens can be explained by the much shorter post-OLNC exposure period of only three days, leaving less time for Pt losses by erosion (see Section 3.6) and possibly by the difference in flow conditions (in SH2 turbulent flow is established, whereas the MMS specimen holders are still in a transitional flow regime). Another reason for the lower Pt loadings on the MMS specimens is that in a nuclear power plant quite some Pt may directly accumulate on the boiling surface of the fuel cladding or be removed by the water clean-up system. Quantifying the amount of Pt lost in that way is difficult, but it is likely to be larger than in a high-temperature water loop system with a proportionally much smaller surface area and without such “Pt traps”. 4. Conclusions The NORA project has been successful at obtaining new insights into the behaviour of Pt particles in BWR environments in the context of the OLNC SCC mitigation technology. It showed that the water chemistry plays an important role in the in-situ formation of Pt nanoparticles; oxidising conditions produce larger particles than reducing ones. A lower temperature, 220 vs 280  C, was found to produce a possibly suboptimal dispersion on the surface with respect to the efficient lowering of the ECP. However, the quantity (according to LA-ICP-MS) and quality (size) of the particles is not affected by the lower temperature. The Pt injection rate also exerts some control on the particle size distribution. A slower injection rate yields smaller particles due to the lower Pt concentration in the high-temperature water, which limits the amounts of growth of the particles beyond the nucleation phase. Furthermore, the deposition of the particles is significantly influenced by the flow conditions prevailing at specific locations. Turbulent flow conditions seem to provide the best deposition rate. However, higher nanoparticle concentrations or longer residence times of the carrier fluid can also produce high surface loadings. Finally the morphology, and possibly the nature of the oxide layer, have a significant effect on the retention of the Pt particles. Stainless steel surfaces with a welldeveloped oxide layer, the result of a longer exposition to BWR

The financial support by the Swiss Federal Nuclear Safety Inspectorate (ENSI) is gratefully acknowledged. We are also indebted to the Swiss nuclear power plants KKL and KKM for in kind contributions to the project. Special thanks are expressed to A. Ramar, I. Guenther-Leopold, B. Baumgartner, L. Nue, P. Reichel, S. Abolhassani, J. Kobler-Waldis, N. Kivel, M. Streit, B. Niceno, S. Koechli (all PSI), Ch. Weber (KKM), G. Ledergerber (KKL), M. Lichtinghagen (KKL), and H. Glasbrenner (ENSI) for their support of this project. Parts of this paper were presented at the Nuclear Plant Chemistry conference (NPC 2014) held in Sapporo [20]. References [1] R. Kilian, A. Roth, Corrosion behaviour of reactor coolant system materials in nuclear power plants, Mater. Corros. 53 (10) (2002) 727e739. [2] R.L. Cowan, C.C. Lin, W.J. Marble, C.P. Ruiz, Hydrogen water chemistry in BWRs, in: 5th Int. Symposium on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, NACE, Monterey, CA, USA, 1991, pp. 50e58. [3] P.L. Andresen, T.M. Angeliu, Y.J. Kim, T.P. Diaz, S. Hettiarachchi, in: U.p. office (Ed.), Application of Catalytic Nanoparticles to High Temperature Water Systems to Reduce Stress Corrosion Cracking, General Electric Company, USA, 2004. [4] S. Hettiarachchi, R.L. Cowan, T.P. Diaz, R.J. Law, S.E. Garcia, Noble metal chemical addition… from development to commercial application, in: 7th Int. Conference on Nuclear Engineering, JSME, Tokyo, Japan, 1999. [5] L. Oliver, B. Helmersson, E. Fredriksson, G. Ledergerber, W. Kaufmann, G. Wikmark, K. Lundgren, Change in CRUD deposition, water and ECP response after the transition to HWC/OLNC at KKL - an update, in: Nuclear Plant Chemistry (NPC) Conference, SFEN, Paris, 2012 p. Paper 124 O38. [6] S. Hettiarachchi, C. Weber, Water chemistry improvements in an operating boiling water reactor (BWR) and associated benefits, in: Nuclear Plant Chemistry (NPC) Conference, CNS, Quebec City, Canada, 2010 p. Paper No. 2.04. [7] S.E. Garcia, J.F. Giannelli, M.L. Jarvis, Advances in BWR water chemistry, in: Nuclear Plant Chemistry (NPC) Conference, SFEN, Paris, France, 2012 pp. Paper No. 80eO01. [8] Y.-J. Kim, Analysis of oxide film formed on type 304 stainless steel in 288 C water containing oxygen, hydrogen, and hydrogen peroxide, Corrosion 55 (1) (1999) 81e88. [9] T. Otake, D.J. Wesolowski, L.M. Anovitz, L.F. Allard, H. Ohmoto, Mechanisms of iron oxide transformations in hydrothermal systems, Geochim. Cosmochim. Acta 74 (21) (2010) 6141e6156. [10] P.V. Grundler, A. Ramar, L. Veleva, S. Ritter, Effect of flow conditions on the deposition of platinum nanoparticles on stainless steel surfaces, Corrosion 71 (1) (2015) 101e113. [11] L.W. Niedrach, Electrodes for potential measurements in aqueous systems at high temperatures and pressures, Angewandte Chemie Int. Ed. Engl. 26 (3) (1987) 161e169. [12] T.J. Collins, ImageJ for microscopy, BioTechniques 43 (1) (2007) 25e30. [13] A. Ramar, P.V. Grundler, V. Karastoyanov, I. Günther-Leopold, S. AbolhassaniDadras, N. Kivel, S. Ritter, Effect of Pt injection rate on corrosion potential and Pt distribution on stainless steel under simulated boiling water reactor conditions, Corros. Eng. Sci. Technol. 47 (7) (2012) 489e497. [14] M. Guillong, P. Heimgartner, Z. Kopajtic, D. Günther, I. Günther-Leopold, A laser ablation system for the analysis of radioactive samples using inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 22 (2007) 399e402. [15] P.L. Andresen, Y.-J. Kim, T.P. Diaz, S. Hettiarachchi, Mitigation of SCC by on-line NobleChem, in: 13th Int. Conference on Environmental Degradation of

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P.V. Grundler et al. / Journal of Nuclear Materials 494 (2017) 200e210

Materials in Nuclear Power Systems - Water Reactors, NACE/TMS/ANS, Whistler, B.C., Canada, 2007. [16] B. Beverskog, I. Puigdomenech, Pourbaix diagrams for the ternary system of iron-chromium-nickel, Corrosion 55 (11) (1999) 1077e1087. [17] R.A. Castelli, Chapter 1-The Corrosion Source, Nuclear Corrosion Modelling, Butterworth-Heinemann, Boston, 2009, pp. 1e31. chignac, [18] J. Livage, D. Roux, Specific features of nanoscale growth, in: C. Bre P. Houdy, M. Lahmani (Eds.), Nanomaterials and Nanochemistry, Springer,

Berlin, 2007, pp. 383e394. [19] A.A. Brin, S.P. Fisenko, A.I. Shnip, Brownian deposition of nanoparticles from a laminar gas flow through a channel, Tech. Phys. 53 (9) (2008) 1141e1145. [20] P.V. Grundler, L. Veleva, A. Ramar, S. Ritter, A comprehensive investigation of the platinum application to BWRs to mitigate stress corrosion cracking, in: Nuclear Plant Chemistry (NPC) Conference, Atomic Energy Society of Japan, Sapporo, Japan, 2014 p. Paper No. 10128.