CA9400063 —

(*l=CL.107S91COG-92-352V

ATOMIC ENERGY OF CANADA LIMITED

ENERGIE ATOMIQUE DU CANADA LIMITEE

FACTORS AFFECTING THE CONSOLIDATION OF STEAM GENERATOR SLUDGE

FACTEURS AGISSANT SUR LA CONSOLIDATION DES BOUES DANS LES GENERATEURS DE VAPEUR

C.W. TURNER, K. SHAMSUZZAMAN and R.L. TAPPING

Presented at "NACE 93' (National Association of Corrosion Engineers) New Orleans, Louisiana 1993 March 7-12

Chalk River Laboratories Laboratoires de Chalk River

Chalk River, Ontario KOJ 1J0

February 1993 fdvrier

ABCL Research

FACTORS AFFECTING THE COHSOLIDATIOR OF STEAM GEHERATOR SLUDGE

by

C.V. Turner, K. Shamsuzzaean, and R.L. Tapping

Presented at "HACK 93"(National Association of Corrosion Engineers)

New Orleans, Louisiana 1993 March 7-12

System Chemistry & Corrosion BranchChalk River LaboratoriesChalk River, Ontario

KOJ 1J0

1993 FebruaryAECL-10759COG-92-352

EACL Recherche

FACTBURS AGISSANT SUR LA CONSOLIDATION DBS BOUES DANS LBS g6n6RATEURS DE VAPEUR

par

C.U. Turner, K. Shamsuzzaman et R.L. Tapping

RESUMfi

On formula 1'hypothese que la consolidation des boues est favorisee par des reactions chimiques lnteressant les divers composants de ces boues, bien que la durete du produit final dependra en outre de la poroslte totale.Les conditions d’oxydation et les temperatures superieures produisent des boues plus dures. La precipitation du Zn2Si04 -- un liant possible — peut aussi favoriser la consolidation des boues. On suggere plusieurs solutions pour empecher la consolidation des boues.

Chimie et Corrosion des Systemes Laboratoires de Chalk River

Chalk River (Ontario) KOJ 1J0 1993 fevrier

AECL-10759COG-92-352

AECL Research

FACTORS AFFECTING THE CONSOLIDATION OF STEAM GENERATOR SLUDGE

by

C.W. Turner, K. Shamsuzzaman, and R.L. Tapping

ABSTRACT

It is hypothesized that sludge consolidation is promoted by chemical reactions involving the various sludge constituents, although the hardness of the final product will also depend on the total porosity. Oxidizing conditions and higher temperatures produce a harder sludge. The precipitation of Zn2Si04—a potential binding agent—may also promote sludge consolidation. Several solutions to prevent sludge consolidation are suggested.

System Chemistry & Corrosion BranchChalk River LaboratoriesChalk River, Ontario

KOJ 1J0

1993 February

AECL-10759COG-92-352

VALUE AI.J IMPLICATIONS

The value of this work is that it has identified some of the possible consolidation mechanisms that lead to hard sludge. One of these is the chemical reaction between oxidizing species (Cu2+, Cu*) and iron; the other is the formation of villemite, Zn2Si04.

The implications are that oxidizing conditions in the feedtrain and the steam generator should be avoided, and Zn-containing materials (as veil as silicate contamination) should be removed from the system.

Branch Manager's Signature

System Chemistry & Corrosion BranchChalk River LaboratoriesChalk River, Ontario

KOJ 1J0

1993 February AECL-10759COG-92-352

comnsINTRODUCTION .......................................................... 1

EXPERIMENTAL METHODS.................................................. 1

RESULTS ............................................................... 2

DISCUSSION ............................................................ 2

SUMMARY....... 4

ACKNOWLEDGEMENTS ...................................................... 4

REFERENCES ...... 4

LIST OF TABLES AID FIGURES

Table 1 ................................................................ 5Table 2 .......................................... 5Table 3 ............ 6Table 4 ............................................................... 6

Figure 1 .............................................................. 7Figure 2 .............................................................. 8Figure 3 .............................................................. 9

INTRODUCTIONSome CANDU1 nuclear generating stations have considerable amounts of hard sludge deposits in the steam generators. The increasing amounts of deposit that are accumulating in the steam generators have begun to affect the generators' performance, directly by interfering with both the steam/water flow and with heat transfer, and indirectly by providing sites for cracking, intergranular attack, crevice corrosion, and/or pitting of tube and support structures under the adherent deposits.

Small samples of deposit extracted from the steam generators consisted of mixtures of magnetite and metallic copper, and some oxides of copper, zinc, silicon, calcium, and magnesium. The deposits were hard, tenacious, and of extremely low (<5%) porosity.

It is our contention that typical operating temperatures of steam generators are sufficiently low that sintering by a solid-state diffusion process cannot contribute significantly to sludge consolidation, and that consolidation proceeds as a result of chemical reactions between the various sludge components. This report presents results of a study at Chalk River Laboratories on the effect of chemical reactions between sludge components on the consolidation of sludge.

EXPERIMENTAL METHODS

The various sludge components were mixed together as either oxides or metallic powders in water to form a slurry. For static autoclave tests, the slurries were poured into Teflon cups and autoclaved at 265°C or 300°C for one or two weeks in deoxygenated water adjusted to pH25 9.0 with morpholine. In some cases, the slurries were dried in the Teflon cups at slightly elevated temperature and re-wetted before placing the cups into the autoclave. Other sludges were prepared under heat transfer conditions by packing the sludge mixture around the tube bundle of a model boiler, or by inserting a bayonet heater into the sludge mixture and autoclaving as described above.

The sludges were examined by scanning electron microscopy and elemental X-ray analysis (SEM/EDX) to determine their morphology and elemental composition. Sludges were also examined by X-ray diffraction (XRD) and Mossbauer spectroscopy to determine the crystalline phases present. These are complementary techniques for the determination of some phases of iron oxide. Where it was warranted, the sludges were also measured for hardness, on parallel-sided discs cut from the sludge, using an MTS load cell at 100 kN load. This is actually a crush test, commonly used to evaluate crushability of brittle materials, but the results are commonly referred to as "hardness".

Some sludge mixtures were mixed with a 40 wt% solution of sodium silicate and air-dried for seven days. Since they were not submitted for autoclaving, no chemical reactions occurred between the various sludge components. Pore size distributions of these sludges were measured by mercury porosimetry (Micromoretics model 9310) and the hardness was measured as described above. A

1. CANDU: CANada Deuterium Uranium. Registered trademark.

range of porosities and hardnesses was achieved by varying the chemical constituents of the sludge to examine the effect of porosity on sludge hardness independent of chemical compound formation.

RESULTS

Table 1 shows an average composition for sludge deposits removed from steam generators at the Pickering Nuclear Generating Station and Bruce Nuclear Generating Station--both plants having mixed iron/copper feedtrains. The effect of porosity on the hardness of air-dried mixtures of iron, copper, oxides of these metals, and silica is shown in Figure 1. Table 2 shows the hardness and XRD analysis of sludges prepared by the reaction between copper oxide and metallic iron for selected times and temperatures; Table 3 shows the phase analysis by Mdssbauer spectroscopy for the same sludges. Figures 2 and 3 show SEM micrographs and elemental maps for sludges prepared at 265°C by heating mixtures of CuO:Fe and Cu20:Fe, respectively. Table 4 shows hardness and composition for mixtures of zinc oxide and silica autoclaved for selected times and temperatures.

DISCUSSION

A mixture of corrosion products plus other impurities from the condenser cooling water and the water treatment plant are continuously being transported into the steam generator with the boiler feedwater. The corrosion products for plants with brass heat exchanger tubes (such as the condenser or low-pressure feedwater heaters) will include copper and zinc, and these are expected to be transported with the feedwater primarily as soluble species. Iron is transported in both soluble and particulate form, where the latter is defined as the component that is removed by a 0.45 /im filter. The oxidation state of the copper and iron depends on the concentration of dissolved oxygen in the low-temperature part of the feedwater circuit. Colloidal silica is likely introduced with the make-up water from the water treatment plant. Small amounts of calcium, magnesium, sodium, chloride, and sulphate can be introduced from either condenser leaks or from the water treatment plant. This mixture of corrosion products plus impurities from condenser leaks and make-up water accumulates in deposits in the boiler, where further chemical reactions can occur between the various constituents. All of the above elements were found in steam generator sludge deposits removed from stations with mixed feedtrains, as shown in Table 1 (1,2).

The hardness or strength of sludge will have both a mechanical component- -related to the porosity—and a chemical component. The latter is related to how the sludge particles are "cemented" together. The data in Figure 1 show how porosity affects the apparent hardness of sludge where no chemical reactions have occurred between the various sludge constituents. In this case, binding between the sludge particles was likely provided by sodium silicate. Note that since no chemical reaction has taken place between the constituents, the relationship shown is strictly the effect of porosity on sludge hardness.Despite the scatter in the data, there is a clear trend of increasing hardness for decreasing porosity, as might have been expected.

To explore the effect of chemical reactions between sludge components on sludge consolidation, different mixtures of metallic iron and either cuprous oxide or cupric oxide were heated at 265°C or 300°C for one and two weeks. Table 2 shows that in each case the iron was oxidized to magnetite and the copper oxide was reduced to copper metal during the exposure to high-temperature water. With the exception of the equimolar CuO:Fe mixture, there was not sufficient copper present to oxidize all of the iron in the mixture, so part of the iron must have been oxidized to magnetite by the water. Table 3 shows the relative proportions of metallic iron and magnetite in the sludge, as determined by Mossbauer spectroscopy, after exposure of the mixtures to high-temperature water. Table 2 shows that the sludges prepared at 300°C were consistently harder than those prepared at 265°C. Another trend shown in Table 2 is that sludge prepared by the reduction of cupric oxide is harder than sludge prepared by the reduction of cuprous oxide, and that the hardness of sludge prepared from a CuO:Fe mixture increases with increasing proportion of iron. These results suggest that oxidizing conditions in the feedtrain, which would lead to higher concentrations of cupric ions relative to cuprous ions, may produce harder sludge in the boiler. In a further test, a mixture of magnetite and metallic copper was autoclaved at 300°C for two weeks, and this mixture showed no tendency to consolidate. The fact that magnetite/copper sludges formed by the oxidation of iron by copper oxide were hard, whereas a mixture of magnetite and copper did not harden under identical conditions of temperature and pH, suggests that a chemical reaction between sludge constituents is an important part of the consolidation process.

SEM microscopy and elemental mapping of iron and copper showed that the sludge formed from the CuO:Fe mixture is heterogeneous, with Cu-rich regions extending for over 50 ym, as shown in Figure 2. This mixture also produced the hardest sludge, as shown in Table 2. In contrast, the sludge prepared from the Cu20:Fe mixture was more homogenous, as shown in Figure 3, with Cu-rich regions extending for only 10 ym at most. These sludges were also softer.

Several synthetic sludges having a composition approximately equal to that shown in Table 1 were heated under heat transfer conditions using either a model boiler or a bayonet heater in an autoclave. Despite attempts to remove dissolved oxygen from the water, all of the iron and some of the copper components of the sludge were oxidized during the exposure to high-temperature water. Mossbauer analysis of the sludge after the exposure showed that the iron was entirely in the ferric form (Fe3+). The XRD pattern could be indexed as either magnetite or maghemite (7-Fe203). All of the iron in maghemite is in the ferric form, whereas magnetite contains a mixture of ferric and ferrous; hence Mossbauer and XRD together establish that the magnetite had been oxidized to maghemite. Although no qualitative measurements were made, the sludge was observed to be quite hard.

This is in contrast to the magnetite/copper sludge that was autoclaved without oxidation and showed no tendency to consolidate. Again, we see consolidation associated with chemical reactions of the sludge constituents—this time oxidation of the magnetite to maghemite.

Sludge from stations with mixed feedtrains has often shown the presence of villemite Zn2Si04 in the XRD pattern. Willemite is not a major component of natural water, and hence is likely formed in-situ by a precipitation reaction

- 4 -

between zinc and silica that has entered with the feedwater. To check this hypothesis, mixtures of zinc oxide and silica were autoclaved for s' lected times and temperatures under steam generator chemistry conditions. Willemite did precipitate under these conditions (see Table 4). The reaction took one to two weeks to reach completion and proceeded faster at 300°C than at 265°C. The mixture increased in hardness as the reaction reached completion. Hence, the precipitation of willemite in steam generator sludge deposits has the potential to promote sludge consolidation.

SUMMARY

Chemical reactions between sludge constituents promote sludge consolidation. Oxidizing conditions and higher temperature appear to promote the formation of harder sludge than do lower temperatures or less oxidizing or reducing conditions. Barkatt et al. (3) reached a similar conclusion regarding the effect of oxidizing conditions on the consolidation of sludge. Our study suggests two possible remedial solutions to prevent the formation of consolidated sludge. The first is the avoidance of oxidizing conditions in both the feedtrain (to minimize the Cu2+/Cu+ ratio in plants with mixed feedtrains) and the boiler (to prevent oxidation of magnetite to maghemite). The second remedial solution is to avoid the formation of willemite, a potential binding agent in the sludge. This can be accomplished by eliminating brass heat exchanger tubes in the balance-of-plant, or by enforcing stricter limits on the silica entering the steam generator. Another potential solution, yet to be evaluated, is to add a chelating agent to complex the zinc ions and prevent the precipitation of willemite in the steam generator sludge deposits.

ACKNOWLEDGEMENTS

We wish to acknowledge the valuable contributions made to this program by the following people: P.A. Lavoie and M.E. Blimkie for preparing the sludge compositions; M.E. Blimkie for the SEM microscopy; J.E. Vinegar for the XRD analysis; and Dr. J.A. Sawicki for the Mossbauer analysis. Financial support for this work was provided by the CANDU Owners Group (COG) under Working Party 19, VPIR 1923.

REFERENCES

(1) F. Gonzalez, "Analysis of Bruce NGS Unit 3 Steam G^ner:tor 3 Deposits", Ontario Hydro Research Report 86-82-K (1986).

(2) F. Gonzalez, "Analysis of Pickering NGS A Unit 2 Boiler 10 Sludge Pile Deposits", Ontario Hydro Research Report 86-65-K (1986).

(3) A. Barkatt, L. May, E. Labuda, M. Wozniak, and G. Cherepakhov, "Characterization and Simulation of Hematite-Rich Deposits in PVR Steam Generators", in 'Steam Generator Sludge Deposition in Recirculating and Once Through Steam Generator Upper Tube Bundle and Support Plates', ed. R.L. Baker and E.A. Harvego, NE-Vol. 8, Nuclear Engineering Division, ASHE. Presented at the 1992 International Joint Power Generation Conference, Atlanta, Georgia, 1992 October 18-22.

- 5 -

Table 1: Elemental Composition of Steam Generator Sludge*

ELEMENT CONCENTRATION6

Iron 30.2Copper 12.1Zinc 10.2Silicon 11.9Magnesium 2.8Calcium 13.3Chromium 1.8Aluminum 1.8Sulphur 1.1Manganese 4.3Nickel 8.8

* Averaged from reported elemental compositions of sludges from Bruce NGS and Pickering NGS.

b Expressed as g per 100 g.

Table 2: Characteristics of Sludges Prepared from Iron and Copper Oxides and Heated for Selected Times and Temperatures.

STARTINGMIXTURES

HOLERATIO(Cu:Fe)

HEATINGTIME(week)

TEMP.(°C)

HARDNESS*(PSI)

PHASE COMPOSITION6

CuO + Fe 1:1.42 1 265 669 Fe304(M), Cu(M), Fe(m), Cu20(t)

1 300 1049 Fe304(M), Cu(M), Fe(m), Cu20(t)

1:1 2 265 496 Fe30,(M), Cu(M), Fe(t), Cu20(t)

2 300 712 Fe304(M), Cu(M), Fe(t)

Cu20 + Fe 1:1.28 1 265 Soft": Fe304(M), Cu(M), Fe(m), Cu20(t)

1 300 262 Fe304(M), Cu(M), Fe(t), Cu20(t)

1:1 2 265 Sof tc Fe304(M), Cu(M), Cu20(m)

2 300 305 Fe304(M), Cu(M), Cu20(m)

* By crush test described in Section 2.1.b By X-ray diffraction; (M) = major, (m) = minor, and (t) = trace.

Qualitative assessment.

- 6 -

Table 3: Mossbauer Analysis of Sludges Prepared with Metallic Iron and Copper Oxides.

Sample Hole Ratio (Cu:Fe)

Temperature(°C)

Fe Phases (Z)

Fe30€ Fe Metal

CuO + Fe 1:1.42 265 75.7 ± 0.3 24.3 ± 0.5300 83.7 ± 0.6 16.3 ± 0.6

1:1 265 78.7 ± 2.1 21.3 ± 2.1300 87.1 ± 0.4 12.9 ± 0.4

Cu20 + Fe 1:1.28 265 70.4 ± 0.5 25.5 ± 0.5300 98.7 ± 1.3 1.3 ± 1.3

1:1 265 91.1 ± 0.5 2.7 ± 0.2300 100

Table 4: Characteristics of Silica/Zinc Oxide Mixtures Heated for Selected Times and Temperatures.

STARTINGMIXTURES

HOLERATIO(Zn:Si)

HEATINGTIME(week)

TEMP.(°C)

HARDNESS*(PSI)

PHASE COMPOSITION1’

ZnO + Si02 2:1 1 265 Soft Zn2Si04(m), Si02(M), Zn0(M)

1 300 Soft Zn2 Si04(M), Si02(m), Zn0(M)

2:1 2 265 Hard Zn2Si04(H), Si02(t), Zn0(m)

2 300 Hard Zn2Si04(M), Zn0(m)

* Qualitative assessment.b By X-ray diffraction; (M) = major, (m) = minor, and (t) = trace.

HA

RD

NES

S (PSI

)

- 7 -

12000

8000

6000 --

4000

2000

POROSITY (*>)

Figure 1: Influence of the porosity of simulated sludges on hardness. Oxides of Fe, Cu and Si were mixed In various proportions, slurried with sodium silicate solution and set at ambient temperature.

8

Figure 2: SEM micrograph of sludge prepared from the reaction between Fe and CuO in 265°C water at pH25 = 9 (a), plus elemental maps showing the distribution of Cu (b) and Fe (c) in the sludge.

9

Figure 3: SEM micrograph of sludge prepared from the reaction between Fe andCu20 in 265°C water at pH25 = 9 (a), plus elemental maps showing the distribution of Cu (b) and Fe (c) in the sludge.

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