corrosion and basic chemistry in primary circuits in ... and basic chemistry in primary circuits in...

Corrosion and basic chemistry in primary circuits in Pressurized Water Reactors

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Dolores Gómez-Briceño Structural Materials Division

CIEMAT

IAEA Training Workshop TR 47184, Madrid, September 2014

Assessment of Degradation Mechanisms of Primary Components in Water Cooled Nuclear Reactors: Current Issues and Future Challenges

CORROSION OF PRIMARY CIRCUITS MATERIALS

General corrosion

Influence of Zn additions

Boric acid corrosion

Flow accelerated corrosion ( FAC)

Stress corrosion cracking ( next talk)

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PWR WWER

PHWR

TYPES OF PRESSURISED WATER REACTORS

Main materials in the primary system of pressurized water reactors PWR

(W / EDF)

PWR (Siemens-KWU)

WWER 440/1000

PHWR

Carbon steel

Austenitic steels

Stabilised austenitic steels

Alloy 600/ 690

SG tubes

A600/690

A 800 NG

Stabilised SS

All the materials show low corrosion rates the in primary water conditions of the different types of pressurized water reactors

MAIN MATERIALS IN THE PRIMARY SYSTEM OF PRESSURISED WATER REACTORS

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Control materials degradation

Avoid fuel performance issues Minimize radiation effect

The optimization of primary water chemistry is aimed at :

Primary water chemistry has to be reducing and alkaline

H2 injection to suppress oxygen produced by radiolysis Li or K to raise the acidic pH produces by boric addition

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The optimum pH chemistry is consequence of a balance among the three goals

Radiation field control has lower priority than pressure boundary integrity and fuel performance

Materials corrosion of primary systems

Release of corrosion products (CP)

Activation of corrosion products in the core

Deposition of activated corrosion products out of core

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Pressure High enough to prevent boiling Local boiling may occur and cause formation of deposits that lead to Axial Offset Anomaly (AOA) Boric Acid Controls nuclear reactions Decreases throughout fuel cycle LiOH Controls pH; from 2.1-3.5 ppm to reduce activity in the circuit Hydrogen To prevent radiolysis( oxygen>0.1ppm)

Typical

Pressure(MPa) 14.2

Temperature(ºC) 286-323

Oxygen(ppm) <0.1

Conductivity(µS/cm) 1-40

H2 (ml/kg) at STP 20-50

Li(ppm) as LiOH 0.1-3.5

B(ppm) as H3BO3 0-2300

Chloride(ppm) <0.15

Fluoride(ppm) <0.15

SiO2 (ppm) <0.20

pHT 6.8-7.4

PWR primary water chemistry

Natural Li is not used to avoid the tritium formation from 6Li. Li enriched in 7Li (99%) is used Natural B only contains about 20% of 10B, the useful isotope for neutron absorption.

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Constant chemistry: Li, B coordination

300ºC 300ºC

Modified chemistry: Li, B coordination

Elevated chemistry: The coolant pH is maintained at or above pHT =7.2 for the entire cycle This strategy can be carried out following a “ Constant chemistry” or a “Modified Chemistry”

Options for PWR primary chemistry

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Sweeton, 1968

In the early times, it was thought that the main CP was magnetite, at pH300ºC= 6.9 magnetite solubility is at a minimum. Later, it was identified that nickel ferrite is the major constituent of CP,at pH300ºC= 7.4 nickel ferrite solubility is at a minimum

300ºC

Magnetite solubility an Fe corrosion rate

Options for WWER primary chemistry

KOH as pH control agent pH260ºC = 7.1-7.3 Reducing environment H2 produced by NH3 injection

direct gas H2 injection

After Zmitko, 2004

2.2-4.5ppm H2

3 ppm ammonium

VVER 440 VVER 1000

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Primary water chemistry for Pressurized Heavy Water Reactors (PHWR)

Heavy water (D2O) is used both as coolant in the primary heat transport system (PHTS) and moderator of the nuclear reaction.

PHTS Chemistry Specification in Normal Operation

specified desired

Dissolved deuterium 3-10 ml/kg 3-7 ml/kg

Dissolved oxygen < 10mg/kg

pHRT 10.2-10.8 10.2-10.4

Lithium 0.4-1.1 mg/kg 0.5-0.7 mg/kg IAEA TECDOC 1666

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GENERAL CORROSION OF PRIMARY LOOP MATERIALS

Corrosion metals in aqueous environment involves various electrochemical and chemical reactions at the material /environmental interface. Primary loop materials undergo general corrosion in high temperature, neutral or alkaline water to form thin oxides, with low corrosion rates.

M + H2O Maq z+ + ze-

M + H2O MOnn- + 2nH+ + ne-

M + H2O MO + 2 H2O + 2e- (1)

Maq z+ + H2O MO + H+ (2)

(1)Oxidation

(2)Dissolution/precipitation

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288ºC

The composition and characteristics of the oxide layers formed in high temperature water depend on the type of materials.

Tomlinson, 1981

Carbon steels Double oxide layer formed by Fe3O4

Austenitic stainless steels Outer layer: NiFe2O4

Inner layer: FeCr2O4

Nickel base alloys Outer layer: Fe-Ni spinel ( Ni Fe2O4)

Inner layer: Ni (Cr,Fe)3O4 spinel

OXIDE LAYERS OF MATERIALS IN PRIMARY LOOP COMPONENTS

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Terachi, 2005


Double oxide layers is a consequence of different diffusion coefficients and different transport rates of alloy elements. Fe and NI exhibit much higher transport rates than Cr.

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Outer layer

Inner layer

Anions

Fe2+ Ni2+

Fe2+

Ni2+ Fe2+

Austenitic Alloy

NiFe2O4 + Fe3O4

FeCr2O4

Outer layer is porous and loosely adherent, formed by cristallites (from hundres nm-few µm). Inner layer is non porous and thightly adherent It is the protective layer ( 200-500 nm)

316L in PWR primary water, 320ºC

Terachi, 2005 Terachi, 2005


Temperature and water chemistry ( B, Li, pH) have influence on thickness an properties of oxide layer, but the influence of surface state is more accused

Polished surface

Ground surface

Polished surface: Internal layer Cr rich, 50 nm Ground surface : Internal layer Cr rich, 10 nm

T and Cr content effect

Cisse, 2012

Influence of Zn additions on general corrosion of primary loop materials

• Zn additions inhibit the cobalt incorporation into the oxide layer of ex-core regions (mainly iron and nickel chromites FeCr2O4, NiCr2O4and nickel ferrites NiFe2O4) reduce activation

• Substitute the cobalt ions already incorporated in the oxide structures. • Diminish the corrosion release from primary circuit material.

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Zn2+ + CoCr2O4 Co2+ + ZnCr2O4

Zn2+ + FeCr2O4 Co2+ + ZnCr2O4

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Stability regions of spinel oxides in primary water

NI- base alloys Fe- base alloys

Miyajima, CORROSION 2001

Alloy

Corrosion

(mg/dm2/month)

Corrosion release

(mg/dm2/month)

With Zinc Without

Zinc

With Zinc Without

Zinc

304 SS 1.1 3.5 0.1 1.3

316SS 1.3 3.5 0.1 1.4

600MA 1.5 2.6 0.3 0.8

600TT 0.5 2.1 0.2 0.9

690TT 0.2 1.3 0.1 0.6

X-750 0.6 2.6 0.2 1.2

Stellite 0.4 14.7 0.1 12.0

Reduction of corrosion rates and corrosion products release by Zn additions

Zinc is added to the primary loop as Zinc acetate ( Zn(CH3COO)2 2H2O by Its high solubility ( 430 g/l at 25ºC) and its high purity.

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Corrosion product generation and transport to the core is reduced In addition to this, PWSCC susceptibility of nickel alloys decreases

B0RIC ACID CORROSION

Corrosion of carbon and low-alloy steel (C&LAS) components by leaking borated water has posed significant maintenance problems for PWR plants: gasketed joints, valve packing, mechanical seals, CRDMs …… The water can become oxygenated and the boric acid can concentrate as the water boils off or evaporates.

Oconee 1

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David Besse RPV

BORIC ACID CORROSION

Corrosion rates are a function of oxygen concentration, pH, boron concentration temperature and flow velocity.

The mode of corrosion of greatest concern due to leakage of borated water is uniform corrosion, often called "wastage,” of C&LAS

pH and oxygen influence

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In the absence of moisture , no corrosion was observed for any of the materials in H-B-O environments at 150, 260 and 300ºC In aerated saturated solution of boric acid at 97,5ºC and ambient pressure, a corrosion rate of 40mm/y was measured in A533 Gr.-B In deaerated solutions, corrosion rates were 40% lower than in aerated one. Corrosion rates up to 150 mm/y were measured at 150ºC

Typical values of Corrosion rates of C&LAS in primary water are 0.025 mm/y.

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Corrosion rates of C&LAS in environmental conditions postulated in the CRDM nozzle/head crevice

FLOW ACCELERATED CORROSION (FAC) IN THE PRIMARY LOOP OF PHWR

PHWR’s use carbon steels for the primary heat transport system (PHTS) piping that connects the fuel channels to the rest of the PHTS circuits. At the conditions of primary water at CANDU-6 reactor, pH ~ 10, magnetite is stable and the corrosion rates of carbon steels are low. But in particular locations as the outlet feeders with Turbulent flow of Fe-unsaturated water High mass transfer at tight radius bend Corrosion rates of 175 µm/y are been measured

Slade, 2005

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The low Cr content of the outlet feeder materials promote this high corrosion rates

FAC is a consequence of a flow-accelerated increase in mass transfer of dissolving and reacting species at a high flow or highly turbulent location.

Flow accelerated corrosion (FAC) is a degradation mechanism affecting metallic materials (carbon steels) that not form tightly adherent passive surface films when the materials are exposed to fluid flow environments in power reactor systems.

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FLOW ACCELERATED CORROSION (FAC) OF C & LAS

Water Single phase FAC

Steam + water Two-phases FAC

FAC does not occur in dry steam

The surfaces experiencing single-phase FAC often look like "orange-peeled" and have small cavities in them. Areas with very high FAC rate can have a polished appearance with no cavities. In certain areas, in which the rate of FAC is slow, pit-like features are encountered on the surface.

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Factors influencing rate of flow accelerated corrosion

Flow accelerated corrosion ( FAC) is an electrochemical corrosion process dependent:

Hydrodynamic factors : fluid velocity, pipe configuration, roughness pipe,.

FAC rate is controlled by mass transfer ( low fluid velocity) or by chemical reaction water/ oxide ( high fluid velocity)

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Corrosion rate increases when flow increase. A maximum is found around 130ºC The roughness of the surface increase the dependence of mass transfer of the fluid velocity. Hydrodynamic disturbances, such as elbows, tees, reductions in the pipes or downstream of valves or control orifices increase the FAC rate

Metallurgical factors: Chromium content

Cr concentration is critical for FAC resistance.

For concentrations < 0.04-0.05% there is not protection

Cr contents >0.2% are recommended

Alternative materials to the carbon steelsP11(1.25Cr-0.5Mo)

P22 (2.25Cr- 1Mo)

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Single- phase FAC

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Higher temperatures lower ferrous ion concentrations Higher temperatures faster mass transfer


Environmental factors: Temperature

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Environmental factors: pH and ECP

E-JAM Vol1, Nº4 Sturla, 1973

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