Reactor Coolant System Operational Chemistry Presented To: KEPRI & KHNP Presented By: Dr. Robert Litman

Objectives Identify the metallurgical composition of: Reactor vessel internal cladding RCS piping Letdown system Reactor internals Fuel cladding Steam Generator Tubes List the 5 significant radionuclides produced from corrosion product activation.

Objectives Identify the chemicals used in the RCS during power operation Describe the effect of these chemicals on pH Describe the effect temperature changes (and power changes) have on pH

Objectives Explain the relationship of RCS chemistry to Stress Corrosion Cracking (SCC) of Inconel, stainless steel and Zircalloy. Explain the relationship of RCS chemistry to fuel clad deposits and fuel integrity.

Reactor Coolant System (RCS) Chemicals During power operation only three chemicals are used in the RCS: Boric Acid Lithium Hydroxide Hydrogen Gas

Boric Acid Purchased as H 3 BO 3 It should have certifications for the following contaminants: Chloride Fluoride Iron Aluminum Calcium + magnesium So that the boric acid storage tank samples will have low levels as shown on the next page.

(a) < 100 or 300 (a) Iimpurity limits may require accurate determination of contaminants present in the “as delivered” boric acid, or ion exchange purification of the storage tank contents.

Boric Acid Nuclear Reactions of boron: 10 B(n,  ) 7 Li 11 B(n, 3 H) 9 Be Boric acid exists as the following univalent anions: H 2 BO 3 - H 4 B 3 O 7 -

Lithium Hydroxide Purchased as LiOHH 2 O Enriched in the 7 Li isotope to > 99.9 atom %. It should have certification for low levels of contaminants: Chloride Fluoride sodium

Lithium Hydroxide Nuclear reactions of lithium isotopes: 6 Li(n,  ) 3 H 7 Li(n,  )2  Chemical reaction to avoid: 2Li + aq + H 2 O + CO2  Li 2 CO 3 + 2H +

pH isopleths are at 25 0 C

Hydrogen Purchased as H 2 at >99.999% purity Major contaminants that should be certified are HCl HF N 2

Hydrogen The principal function of hydrogen is to suppress the formation of oxygen through radiolysis of water: H 2 + O 2  H 2 O Hydrogen will also help to suppress metal corrosion reactions

RCS Components Stainless Steel RCS piping, internal vessel cladding, steam generator inlet and outlet cladding, reactor coolant pump internals. Accounts for 5% of surface area in RCS Contains ~18% chromium, ~8% nickel, ~76% iron Has the highest corrosion rate of all internal RCS surface materials Forms two different layers of corrosion after initial power cycle.

RCS Components (stainless steel) RCS Fluid Flow Several Hundred microns

RCS Components (Inconel) Inconel SG tubes (alloy 600 MA, 600 TT, and 800) Reactor vessel upper and lower internals Inconel Composition ~74-77% nickel, 14-17% chromium, 6-10% iron Minor constituents (0.01-1%): Al, Cu, Ti, Si, Mn, C, Co, and S.

RCS Components – Inconel RCS Flow 20-50% of the SS thickness

to 120 microns, Depending on fuel Duty RCS Components – Zirconium Alloy RCS Flow 4

Corrosion Reactions in Water The principal equations involving oxygen (molecular and from water): Fe + H 2 O ↔ FeO + H 2 2Fe + H 2 O + O 2 ↔ Fe 2 O 3 + H 2 Fe + O 2  FeO 2 [ Note that there is no water here ] 2Ni + O 2 ↔ 2NiO Ni x Fe 3-x O 4 + 4(x/3) H 2 ↔ xNi 0 + (1- x/3)Fe 3 O 4 4(x/3)H 2 O Zr + 2H 2 O ↔ ZrO 2 + 2H 2

Effects of Temperature on pH

Elevated pH of 7.4 at C (572 0 F)

Coordinated pH of 7.1 with Maximum Li at 3.5 ppm (300 0 C)

Design of Chemical Volume and Control System Provides a means of adding chemicals B, Li, H 2, H 2 O 2, N 2 H 4, Zn Provides a means to maintain low contaminant levels by use of demineralizers and filters Used to monitor radiation levels “real time”. (Bolded chemicals added during power operation)

AAAA To MB effluent From MB Effluent HC M A Boron Thermal Regeneration System (BTRS) A = Anion resin H = Reheat Heat Exchanger C = Chiller Heat Exchanger M = Moderating Heat exchanger From regenerative HX To Letdown Hx

Boron Thermal Regeneration System Most significant application in recent years has been in de-boration of RCS at end of cycle. Calculate bed capacity in equivalents to estimate total boron removal possible. Almost all plants have stopped using BTRS to borate due to contaminant introduction (especially, silica and fluoride)

Boric Acid Storage Tank System (BAST) Storage Tanks (2) - 24,000 gallons ppm boron Transfer pumps supply blended boron to the VCT tank effluent (low pressure point of the system). Diluted with RMW through the boric acid “blender” RMW and BAST are sources of low level oxygen intrusion during the power cycle

Guidance for Monitoring RCS Chemistry Parameters Technical Specifications for Power plants in the US are stipulated by the NRC: Oxygen <100 ppb Chlorides/Fluorides < 150 ppb DEI < 1.0 μCi/g Gross Activity < 100/E-bar These limits are established conservatively to protect from SCC and Fuel clad corrosion, and to protect the public from radiation releases.

What Other Non-radioactive Parameters Should Be Monitored?? Sodium Sulfur (as sulfate) Silica Al, Ca, Mg H 2 Li Specific Conductivity pH (measured vs. theoretical) Boron (Total and 10 B)

How is the Data Evaluated? The theoretical RCS specific conductivity should closely parallel the measured specific conductivity The theoretical RCS H 2 concentration should closely parallel the measured hydrogen. The theoretical pH at room temperature should be plotted with the measured room temperature pH Related parameters plotted with power level. Important plant evolutions are denoted on trend graphs to identify points where changes may have occurred.

What Else Should Be Monitored?? Specific Radionuclides (by gamma spectrometry) Activation products: Co, Mn, Fe, Cr, Zn, Np Radioactive gasses: Ar, Xe and Kr Wear Products: Ag, Cd Fission Products Sb, Zr, Nb, Cs, I, La, Ba, Ru, Rb Hard to Detects (by Alpha or liquid scintillation spectrometry) 3 H, 14 C, 55 Fe, 59/63 Ni, 99 Tc, 90 Sr 241 Am, 242 Cm, 238 Pu, 239 Pu,

Nuclear Reactions There are four significant nuclear reaction types that take place in the core: (n,  ) (n,  ) (n, p) (n, f) (p, n)

Radionuclide Formation Analysis within two hours of sampling Radionuclide Reaction Source 85m, 87, 88 Kr FP Tramp U/Pu, Fuel 133m, 133, 135m, 135, 138 Xe FP Tramp U/Pu, Fuel 131, 132, 133, 134, 135 I FP Tramp U/Pu, Fuel 56 Mn 55 Mn(n,  ) Corrosion 24 Na 23 Na(n,  ) Make-up Water, SAT leakage 88 Rb FP Tramp U/Pu, Fuel 105 Ru- 105 Rh FP (indicative of fuel melt) Tramp/Pu, Fuel 41 Ar 40 Ar(n,  ) Air 69m Zn 68 Zn(n,  ) CA 71m Zn 70 Zn(n,  ) CA 138 Cs FP Tramp U/Pu, Fuel

Radionuclide Formation

Radionuclide Formation

Activation to Form Transuranics

Reactor Coolant Expected Activities No Defects (1-5)x10 -5 Slight increase during the cycle to ND to 5x10 -5 ND to 5x10 -5 Tight Defects (1-10)x10 -4 Activity spike at time of defect- becomes less significant with successive defects to Open Defects >1x10 -3 Increases over the cycle >1x to to Residual Fuel Fragments (1-10)x10 -4 Drops slowly over the first cycle <10 -3 <1x to I 92 Sr 134 Cs 137 Cs