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UN1001: REACTOR CHEMISTRY AND

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UN1001: REACTOR CHEMISTRY AND

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    1. 1 UN1001: REACTOR CHEMISTRY AND CORROSION Section 16: Material of Construction and Chemistry of Reactors By D.H. Lister & W.G. Cook Department of Chemical Engineering University of New Brunswick

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    11. 11 Principal Materials of Construction – BWRs:- - reactor vessel: fuel cladding; Zircaloy 2 (Sn 1.2-1.7%, Fe 0.2%, *Ni 0.08%, Cr 0.15%, balanced with Zr) * Ni-free less susceptible to hydriding fuel assembly accessories; Alloy-600 (Inconel) (Ni 72%, Cr 14-17%, Fe 6- 10%, C 0.15%) Alloy-X750 (Inconel) (Ni 70%, Cr 14-17%, Fe 5- 9%, C 0.08%, Nb+Ta 0.7-1.2%, Ti 2.25-2.75%, Al 0.4-1.0%) neutron-absorbing control rods; B4C powder in * SS or Inconel sheath *Note: Most of the stainless steel we consider in the reactor circuits is of the “300-series”, e.g., type 304 (18-8 for 18%Cr & 8%Ni). This material is austenitic – it has a FCC structure and is non-magnetic. The nickel alloys like Inconel are also austenitic.

    12. 12 jet-pump supports; Alloy-X750 most vessel components; Type 304 SS (Cr 18-20%, Ni 8-10.5%, *C 0.08%, Si 1.0%, Mn 2.0%) * important for cracking propensity – less-susceptible Type 304L has < 0.03%C. pressure vessel; *high-strength, low-alloy steel (C ~0.15%, Mn ~1%, Si ~0.2%, Cr ~0.6%, Ni ~1%, Mo ~0.5%, B ~0.005%, Cu ~0.4%, V ~0.06%, balanced with Fe) *note: inside surface overlaid with SS. recirculating water piping: Type 304 SS (replacement material generally low-carbon, type 304L). steam piping, feedwater lines: Carbon steel (e.g., C 0.17%, Mn 0.90%, balanced with Fe) Stainless steel.

    13. 13 - condenser: Stainless Steel (fresh water); Admiralty brass (e.g., Cu 70-73%, Sn ~1%, As 0.02-0.10%, balanced with Zn); Al-bronze (e.g., Cu ~90%, Al ~7%, Fe ~3%); Titanium F.W. heater: Type 304 SS; C.S.; 70/30 Cu-Ni (Ni ~30%, Fe~0.7%, Mn ~1%, Zn 1%, balanced with Cu)

    14. 14 BWR Chemistry

    15. 15 Radiolysis In-Core: Radiation tends to radiolyse water:- H2O H2 + 1/2O2 In the process, many transient species formed … Note H2O2 decomposes at high T … The decomposition is fast at high pH; but, at BWR temp. and chemistry, the lifetime of H2O2 is a few seconds, so H2O2 can be seen by sampling at core outlet. H2O2 is more aggressive than dissolved oxygen: it is an important species.

    16. 16 In a closed system, back reactions occur, and an equilibrium is established; H2O H2 + 1/2O2 is the overall effect. If H2 is added to the system, back reactions are promoted, and radiolysis is effectively suppressed.

    17. 17 BWR Normal Water Chemistry Since the coolant boils in the core, dissolved gases are stripped to the vapour phase – the radiolysis equilibrium is disturbed, and O2+H2 levels are high (for a reactor system) Typical [O2] in RW = 200 ppb typical [H2] in RW = 25 ppb High [O2] ? IGSCC (InterGranular Stress Corrosion Cracking) of sensitized stainless steel

    18. 18 Sensitization Rapid cooling of very hot S.S. (etc.) metal (e.g., after welding) will precipitate Cr (that is normally distributed evenly in solution across grain boundaries) as microscopic particles of carbide; the Cr cannot then protect the boundary from corrosion with a protective oxide film (based on Cr2O3) and attack occurs – exacerbated by stress and Cl-. [O2] is particularly aggressive; Low-carbon alloy (e.g. 304L SS) helps; Mo additions (e.g. 316 & 316L SS) helps.

    19. 19 Several BWRs have had to replace cracked recirc. water lines. One or two plants trying HWC.

    20. 20 BWR Hydrogen Water Chemistry (HWC) ~1ppm [H2] in FW reduces [O2] in RW to 2-3 ppb, despite boiling and stripping in-core. High [O2] in NWC increases Zircaloy corrosion. If poorly heat-treated, Zirc-2 fuel cladding in BWRs shows nodular corrosion – local oxide nodules which can lead to clad perforation. HWC helps. Also – high O2 in NWC exacerbates IASCC (Irradiation-Assisted Stress Corrosion Cracking). This is cracking of SS, Ni alloy components caused by irradiation and metallurgical factors. HWC should help.

    21. 21 Feedwater Purity Control All BRWs have condensate demineralizers (ion exchange). Powder resin systems (e.g., POWDEX R ) also act as filters (very efficient) … Slurried IX powder deposited on candles (cylindrical screens with wound fibre and an inert, porous precoat) in layer ~7mm thick. Reduce particulate impurities effectively: generally cannot cope with large condenser leaks – particularly at seawater-cooled plants:- provision for rapid addition of resin powder important. Powdered resin – usually mixed cation/anion.

    22. 22 Schematic …

    23. 23 Deep Bed Systems Very high capacity for demineralization, not very effective filters. Usually employed at seawater-cooled plants; sometimes in conjunction with powdered-resin systems. Mix-bed types (cation/anion) usual; capability to slurry resin into separate beds to regenerate important. Regeneration … anion … NaOH cation … H2SO4 Important to separate anion & cation resin in regen. cycle – otherwise Na+ will contaminate cation, SO42- will contaminate anion – will “throw” into FW in subsequent operation.

    24. 24 Schematic … Condensate demineralizers can reduce Na, Cl, etc. to sub-ppb levels when operating smoothly.

    25. 25 Comparison Deep Bed and Powdered Resin Deep Bed Powder Corrosion Products (plant activation, fuel) startup 90% Fe, 50-90% Cu 90% steady operation 40% 90% Silica (turbine performance) soluble very good poor – good particulate 85% medium Na+ Throw can get up to 5 ppb only with poor resin Anion Throw occasionally only with poor resin Resin Release not usually problem Condenser Leak Capacity 140 days 7 days (1 USGPM, seawater)

    26. 26 Note: In reactor circuits, ppbs of circulating crud (corrosion products in suspension) are haematite (Fe2O3) with some magnetite (Fe3O4). On corroding steel surfaces, mixture of Fe chromite (FeCr2O4) with some magnetite overlaid with Fe2O3. Any Co impurity in corrosion products is activated in-core; this subsequently contaminates oxides on out-core surfaces with Co-60. Deposits on fuel can become thick: if Cu present, can severely limit boiling heat transfer and lead to fuel failure through overheating.

    27. 27 PWRs

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    33. 33 Principal Materials of Construction – PWRs:- - reactor vessel: fuel cladding; Zircaloy 4 (Sn 1.2-1.7%, Fe 0.2%, Ni 0.007%, Cr 0.1%, balanced with Zr) fuel assembly accessories; Ni alloy (e.g., Alloy-X750) fuel grids; formerly Ni alloy, changing to Zircaloy vessel; low-alloy steel, – SS clad (austenite) control rods; SS-clad alloy (Ag 80%, In 15%, Cd 5%) - out-core piping: austenitic SS (e.g., Type 304) - steam generators: channel heads; Carbon steel, 304 SS weld overlay tubing; *Alloy 600; some replacement S.G.s have *Alloy 690. German PWRs have Alloy 800 (Incoloy) *Inconel

    34. 34 steam piping, feedwater lines: Carbon steel; low-Cr steel F.W. heater: SS; CS; 70/30 Cu-Ni (becoming obsolete); condenser: SS; Admiralty brass; Titanium; Al-bronze

    35. 35 PWR Chemistry Control (after EPRI Guidelines … Electric Power Research Institute, Palo Alto, CA, USA) Primary Coolant H2O should be kept alkaline to reduce crud – material transport (mainly corrosion products in suspension and dissolved) and minimize corrosion. For example, assume materials are steel, so that corrosion products are magnetite (Fe3O4) … 3Fe + 4H2O ? Fe3O4 + 4H2

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    37. 37 At pH > minimum, corrosion products will tend to dissolve off hot surfaces and precipitate on cold surfaces: keeps core clean, minimizes contamination.

    38. 38 Difficulty is, PWR PHTS uses chemical shim – soluble neutron poison for reactor control … burnable … 2000 ppm B at shutdown; 1100 ppm B at start of fuelling cycle; 0 ppm B at end of fuelling cycle. B added as boric acid obtained from boric oxide … B2O3 + 3H2O = 2B(OH)3 Boric acid in water accepts OH- … B(OH)3 + H2O B(OH)4- + H+ ? weak acid. Strong base, LiOH, added to keep pHT above minimum (i.e., above ~7.0). NOTE: neutral pH300 = 5.7.

    39. 39 Li added because it does not readily activate, although: 63Li + 10n ? 31T + 42a High concentrations of Li will attack Zr alloys (damage fuel) and crack Alloy 600 (damage S.G.s). Therefore, at start of operating cycle (when high [B(OH)3] promotes acid), [Li] usually limited to less than ideal for control of activity transport (i.e., crud deposition in-core).

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    41. 41 General recommendations (from EPRI) for primary coolant chemistry control …

    42. 42 Secondary Coolant Steam generators – large, complex, mixture of materials, vulnerable to attack … Note: SG tubing presents thinnest barrier to release of primary coolant. SG or boiler “blowdown” S.G. is a boiling “pot” on the end of the FW line – concentrates impurities …

    43. 43 Input rate of impurity = F . CF g/s Rate change of impurity in bulk = g/s Output rate = B . CM g/s or so, final concentration ( as t ? 8) If B = 0.1% F,

    44. 44 Blowdown is used to limit impurity concentration in recirculating SGs – cannot be too big because of losses (e.g., if B = F, no S, since S = F – B). Blowdown very ineffective for sludge (secondary side crud – suspended corrosion products etc.) – tends to deposit before it can be blown down. Secondary coolant controlled:- to minimize general corrosion; to minimize corrosion product transport into S.Gs; to provide a chemical buffer against impurity excursions from condenser. Note: in specifying a chemical control regime, one must consider all the feed- train materials – condenser, LP feedwater heater, de-aerator (if there is one; unusual in PWRs), HP feedwater heaters, S.G. tubing; also – presence or absence of a corrosion product (CP).

    45. 45 To illustrate complexity of FW systems:

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    47. 47 pH: High pH throughout feedtrain desirable – add volatile amine (ammonia, morphine, cyclohexylamine). If feedtrain all ferrous – pH25 in SG water = 9.0-9.5. If feedtrain contains Cu – pH25 in SG water = 8.5-9.0 N.B. A few PWRs add Na phosphates to SG – good buffering agent, non- volatile, but Na3PO4 + H2O Na2HPO4 + NaOH Na2HPO4 + H2O NaH2PO4 + NaOH 2NaH2PO4 Na2HPO4 + H3PO4 Must control [Na]:[PO4] to avoid high caustic (NaOH ? caustic SCC of Inconel). Also must control [Na]:[PO4] to avoid acid condition (acid ? wastage)

    48. 48 chloride: aggressive to steels and ferrous alloys generally:- particularly if oxygen present (can dent tubes at support plates in SGs by forming acid chlorides in crevice: local concentrations > 4000 ppm measured …

    49. 49 sulphate: promotes IGA (intergranular attack) of Alloy 600. silica: deposits on turbines; hardens sludge in S.G. oxygen: promotes CP transport in feedtrain (though, it will reduce attack at some levels on low-alloy steels); forms aggressive conditions in S.G. if Cl- and/or Cu2+ present). controlled with hydrazine: N2H4 + O2 ? N2 + 2H2O hydrazine decomposes in S.G. ? ammonia. iron: total Fe monitored to quantify sludge buildup, monitor feedtrain corrosion. Recommended .. = 20 ppb in final feedwater. copper: as Fe except .. reducible Cu promotes S.G. denting. Recommended … = 2 ppb in final feedwater.

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    51. 51 CANDUs

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    55. 55 Principals Materials of Construction – PHTS:- fuel: Natural U as UO2 fuel sheath: Zircaloy-4 pressure tube: Zr – 2.5 wt.% Nb end fitting: Type 403 SS (Cr 12%, Ni < 0.5%, Mn < 0.5%, balanced with Fe) feeders, headers, S.G. heads, piping: Carbon steel steam generator tubing: Alloy-600 (Inconel) at Bruce NGS (Cr 15%, Ni 72%, balanced with Fe) Alloy-800 (Incoloy) at Darlington NGS and CANDU-6s (Cr 21%, Ni 32%, balanced with Fe) Alloy-400 (Monel) at Pickering NGS (Cr 0%, Ni 70%, Fe 2%, Cu 28%)

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    57. 57 Purification System Capacity … Assume PHTS as a stirred tank containing M kg D2O …

    58. 58 If purification half-life =

    59. 59 PHTS Chemistry Control Coolant D2O: > 97.5% w/w = 98.6% w/w Alkalinity: pH*25 (LiOH) = 10.2 – 10.4 N.B. pH25* is apparent pH of LiOD in D2O … i.e., indication of a standard pH meter in the D2O solution. true pD25 = pH*25 + 0.4 Constant high alkalinity, (= pH300 ˜ 7.6 – 7.8) - no chemical shim. - C.S. corrosion low. - Activity transport low.

    60. 60 Dissolved D2: 3 – 10 cm3/kg (STP) minimizes radiolysis (keeps [O2] low); should not contribute to the hydriding (deuteriding) of Zr – 2.5 Nb pressure tubes. Note: H2 (not D2) usually added; exchanges rapidly with D in D2O; not enough to downgrade isotopic content significantly. Cl-: < 200 ppb F-: < 100 ppb crud: < 10 ppb I-131: < 10 µCi/kg

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    63. 63 Principal Material of Construction – Moderator:- Calandria vessel: Type 304 SS Calandria tubes: Zircaloy-2 Piping: Type 304 SS Cover gas: He Annulus gas: CO2 Moderator HXs: 70/30 Cu-Ni … changed to SS ? Alloy-800

    64. 64 Moderator Chemistry Cover gas: – nominally pure He:- D2: < 2% (> 6% means immediate shut down of reactor). O2: < ½ D2 (to ensure complete recombination of D2: D2 + ½O2 ? D2O) Moderator D2O (~60oC): > 99.75% isotopic pH*25: < 7.0 (alkalinity will promote precipitation of Gd) Cl-: < 200 ppb F-: < 100 ppb

    65. 65 Poison Control: Boron (as boric oxide, B2O3) added to a new core at start of reactor life to compensate for excess reactivity (c.f. PWR). Initial [B]: 2 ppm (as B) N.B. 10B … 20% abundance.

    66. 66 - Gadolinium (as nitrate, Gd(NO3)3) added after long shutdown to compensate for FP Xe-135 which attains equilibrium levels in fuel during operation, then decays during S/D with = 9.2 h. Xe-135 has highest know st-n. st-n = 2.6.106 b [Gd] for Xe compensation ~ 40 – 50 ppb (at level difficult to measure – amount gauged by reactivity control). Some B (~0.2 ppm) may also be present for Xe comp. Gd required for gauranteed S/D: ~13 ppm Gd also injected rapidly for emergency S/D … SDS-2 system

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    69. 69 Principal Materials of Construction – Secondary Coolant:- S.G. tube: Alloy-600 (Inconel) – Bruce Alloy-400 (Monel) – Pickering Alloy-800 (Incoloy) – Darlington, CANDU-6s. FW heaters (HP): 90/10 Cu-Ni; Type 304 SS FW heaters (LP): Admiralty brass; Type 304 SS Condenser: *Admiralty brass (being replaced); Type 304 SS; Titanium. *sometimes with SS in vulnerable areas - bundle periphery, air extraction zone Piping and vessel shells: Carbon steel.

    70. 70 CANDU Secondary Coolant Chemistry Similar to control at PWRs. pH (all ferrous) in condensate: 9.5 – 10.0 pH (Cu-bearing) in condensate: 8.8 – 9.3 Compare with specifications for S.G. water … pH (all ferrous) in S.G. water: 9.3 – 9.8 pH (Cu-bearing) in S.G. water: 8.8 – 9.3

    71. 71 pH additive: NH3, morpholine. Note: Point Lepreau is only CANDU with condensate polishing (CP) unit. It operates with morpholine (previously phosphate) and has an all-ferrous feedtrain (Ti condenser). Typical phosphate chemistry: pH25 (SG water): 9.4 phosphate conc: 6 ppm [Na+]/[PO43-] mole ratio: 2.3 [O2] in condensate: < 10 ppb (controlled with N2H4 down feedtrain at 40 – 60 ppm all ferrous, 10 – 30 ppm Cu-bearing).

    72. 72 Make-up: To compensate for leaks, blowdown, samples removed, etc., make-up water is required. This is a local fresh-water supply (well, etc.) which is purified in the water treatment plant: - filtration; - clarifier; - ion-exchange (note, possible release of acid, caustic from regen.); - storage. Other treatment methods: - reverse osmosis.

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