Boric Acid Corrosion

Ryan Jones and Glenn White

Dominion Engineering, Inc.

Industry-NRC 2012 Meeting BAC Testing Program

February 29, 2012

Boric Acid Corrosion: Implications Assessment of BAC Test Programs

2 © 2012 Electric Power Research Institute, Inc. All rights reserved.

Contents

• Objective of Implications Assessment

• Summary of Modeling for Full-Scale Mockup Test Program (Task 4)

(Basis for Implication Assessment)

• Implications of Full-Scale Mockup Test Program on the Current Safety

Assessments and Inspection Requirements


Objectives of Implications Assessment

• The objective is to address the implications of the BAC test program

(culminating in full-scale mockup testing) on the assumptions and

technical bases that were used to develop the MRP safety

assessments (i.e., MRP-110, MRP-117, MRP-167, and MRP-206)

supporting the current inspection requirements for the RPV top and

bottom heads (ASME Code Cases N-729-1 and N-722-1, respectively).

• Technical Concern Addressed by Safety Assessments – If a leaking

nozzle is not detected during an inspection, can wastage conditions

develop that result in structurally significant loss of material before the

next inspection opportunity and successful leak detection.

• The full-scale mockup results provide the best experimental simulations

of conditions expected to exist in an actual leaking nozzle.

– Confirm that the wastage rates observed in the full-scale mockup tests

support the assumptions used in the safety assessments

– Confirm the effectiveness of visual inspection to detect the existence of a

leak


Summary of Modeling for Full-Scale

Mockup Test Program


• Application to Support Design of Full-Scale Mockups – For an assumed leak rate,

use PICEP (or alternative) to evaluate initial effluent conditions.

– Use Lockhart-Martinelli (LM) approach to quantify two-phase flow pressure profile (Tsat) along annulus.

– Iterate with thermal FEA models to determine effective dry-out area.

– Calculate concentration of boric acid.

– Calculate chemistry and resulting pHT.

– Estimate wastage.

Primary

Conditions

PICEP

x0

Lockhart-

Martinelli

Analysis

Tsat

P

Heat Transfer

Analysis

A

ANSYS

Twall

Dryout

x(z)

Chemistry

Calculation

Li(z)

B(z)

pHT(Li,B,T)

CR(pHT)

pHT(z)

CR(z)

Part 1

Part 2

Hydra

ulic

T

herm

al

BMN Model

Mockup Model

Chem

ical

Corr

osio

n

Implications Assessment of BAC Testing Modeling – Thermal-Hydraulic Analyses


• RVH Model Description: – Simulated a nozzle leak between

outer diameter of CRDM nozzle (above weld) and head bore using 1/8th symmetry model based on typical reactor vessel head geometry.

– Representative materials of construction and thermal boundary conditions.

RVH Thermal Model Mockup Thermal Model

Implications Assessment of BAC Testing Modeling – Thermal Analyses


Implications Assessment of BAC Testing Modeling – Dry-Out Calculation

• Coupled heat transfer and two-phase hydraulic models are used to determine area vs. temperature curves.

• ANSYS independently generates data points for dry-out area vs. temperature.

• Intersection of thermal-hydraulic modeling and FEA analyses defines the dry-out area and temperature (two equations and two unknowns).

0.01

0.1

1

10

100

200 250 300 350 400 450 500 550 600

He

at T

ran

sfe

r A

rea

[in

2]

Average Wall Temperature on Dryout Region [°F]

10 mil gap (CR Model)

1 mil gap (CR Model)

10 mil gap (RVH ANSYS Model)

1 mil gap (RVH ANSYS Model)

10 mil gap (Mockup ANSYS Model)

1 mil gap (Mockup ANSYS Model)

Heat Transfer Area for Dryout0.01 gpm

4" diameter 5.5" length annulus ~ 69 in 2


Implications Assessment of BAC Testing Modeling – Wastage Rate Estimates

• Calculate Concentration Factors in Annulus

– Slug of fluid entering the annulus through the orifice travels to the

top of the annulus without mixing with the fluid preceding or

following it.

– All chemical species are conserved in a particular slug.

– Differences in chemistry along the annulus result from differential

partitioning and concentration of species as the quality changes.

• Calculate pHT as a function of [Li], [B], and Temperature

– Compute the pHT of lithium/boric acid solutions per the method

described in the EPRI Primary Water Chemistry Guidelines.

– Model predictions expected to be more aggressive than conditions

which will actually develop during testing (e.g., does not include

precipitates and effect of corrosion products).

– Assumes only significant species in the annulus are Li, B, and

Water.


0.001

0.01

0.1

1

10

0 0.5 1 1.5 2 2.5 3

Co

rro

sio

n R

ate

[in

/ye

ar]

Distance from Injection Site [in]

1 mil Annulus

10 mil Annulus

Dryout

0.05 in3/year

0.07 in3/year

0.01 gpm

Implications Assessment of BAC Testing Modeling – Wastage Rate Estimates


Implications Assessment of BAC Testing Modeling – CFD Simulations

• CFD Model used to simulate flow within mockup annulus (two-phase flow with heat

transfer from LAS).

• Confirmed correlation between cooled region and dominant attack which corresponds to

concentrating boric acid at edge of two-phase regions.

• Pre-machined flow channel

• Pre-machined cavity

• Attack is predominantly within the narrow annulus region just outside the pre-machined regions.


Implications Assessment of BAC Testing Full-Scale BMN Mockups – Wastage Correlation with Cooling

• Interpolation of TC data confirms correlation between regions of annulus cooled by liquid and regions of attack within the annulus.


Implications of Full-Scale Mockup Test

Program on the Current Safety

Assessments and Inspection

Requirements


Implications Assessment of BAC Testing Wastage Rate Modeling in Safety Assessment – MRP 110

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.001 0.01 0.1 1. 10. 100.

Size of Wastage Cavity at Time of Detection (in3)

Cu

mu

lati

ve

Dis

trib

uti

on

Fu

nct

ion

, F

1.5 EFPY Fuel Cycles

2.0 EFPY Fuel Cycles

Bare metal visual (BMV) examinations

performed every refueling outage for a

plant with a head temperature of 605°F

The probability of the wastage cavity size exceeding

the allowable wastage volume of ~150 in3 is less than

1 10-4

for both 1.5 and 2.0 EFPY cycles.

Results of Probabilistic Wastage

Calculation used in MRP-110

• The MRP-110 model showed

high confidence that Code

allowable stresses (i.e.,

stress associated with a

wastage volume of ~150in3

as documented in Appendix

D of MRP-110) would not be

exceeded given a leaking

nozzle and periodic bare

metal visual examinations

performed at an appropriate

interval.


Implications Assessment of BAC Testing Wastage Rate Modeling in Safety Assessment – MRP 110

0.00

0.50

1.00

1.50

2.00

2.50

3.00

1E-05 1E-04 1E-03 1E-02 1E-01 1E+00

Corr

oso

in R

ate

(in

/yr)

Leak Rate (gpm)

Exponential behavior assumed for

leak rates between LRlow and LRcrit:

CR = 0.069e35.8 LR

(LRcrit,CRcrit)

(LRlow,CRlow)

Upper shelf behavior assumed for leak

rates greater than LRcrit: CR = 2.5 in/yr

Linear behavior assumed for

leak rates less than LRlow:

CR = (LR/LRlow)CRlow

LRlow and LRcrit chosen

based on calculation of

extent of local cooling

as a function of leak rateLRlow = 0.001 gpm

LRcrit = 0.1 gpm

Linear Corrosion Rate used in MRP-110

• Linear corrosion rates (in/yr)

presented in MRP-110 were

intended to provide a

conservatively high estimate for

application to a large cooled

region of the RVH for an

extended period of time.

– LRlow and LRcrit were leak

rates required to produce

<10°F temperature drop and

produce Tavg = 212°F,

respectively, on the model

heat sink area.

– CRlow = 0.072in/yr and

CRcrit = 2.5in/yr were limiting

values for linear corrosion

rates documented in the

Boric Acid Corrosion

Guidebook, Rev. 1.


Implications Assessment of BAC Testing Wastage Rate Modeling in Safety Assessment – MRP-167

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1E-05 1E-04 1E-03 1E-02 1E-01 1E+00

Corr

oso

in R

ate

(in

/yr)

Leak Rate (gpm)

Exponential behavior assumed for

leak rates between LRlow and LRcrit:

CR = 0.069e71.7 LR

(LRcrit,CRcrit)

(LRlow,CRlow)

Upper shelf behavior assumed for leak

rates greater than LRcrit: CR = 1.25 in/yr

Linear behavior assumed for

leak rates less than LRlow:

CR = (LR/LRlow)CRlow

LRlow and LRcrit chosen

based on calculation of

extent of local cooling

as a function of leak rateLRlow = 0.0005 gpm

LRcrit = 0.05 gpm

Linear Corrosion Rate used in MRP-167

• Linear corrosion rates (in/yr)

presented in MRP-167 were intended

to provide a conservatively high

estimate for the area of uncovered

cladding.

– Leak Rate Factor: LRlow and LRcrit were

reduced by a factor of two since target

surface temperatures were achieved in

the BMN with half the flow rate

required in the CRDM configuration.

– Linear Corrosion Rate Factor:

Accounts for the inverted BMN

geometry and also the spatially

averaged corrosion rate being less

than the local maximum corrosion rate,

considering that the potential for

pooling of concentrated boric acid

solution does not exist to the same

extent as for the top head.

– Linear corrosion rate curve was

multiplied by a reducing factor sampled

as triangular distribution with

Lower=0.3, Mode=0.5, and Upper=1.0.

(plot is shown with mode factor

applied).


Implications Assessment of BAC Testing Comparison of Safety Assessment and Full-Scale Testing

• The following describes how wastage volumes were determined

from the safety assessment models and compared to the mockup

tests.

• Volume of Material used in Safety Assessment Models

– At the time of detection of a simulated leak during a regular

inspection, the modeled wastage volume is then compared to

structurally unacceptable volume (i.e., ~150 in3 for the top head

or equivalent volume required to uncover (3in)2 of cladding for

the bottom head).

– The values for CRSA and ASA are known as function of the time-

dependent leak rate which increases with time.

• Volumetric Wastage Rates Measured in Full-Scale Mockup Tests

– Total volume of material is measured at the end of the test and

divided by the test duration.

– In contrast to safety assessments, the values for CRFSM and

AFSM are unknown, but these values are expected to vary with

time despite the leak rate being a controlled constant for the

full-scale mockup tests.

0

inspectT

SA SA SAVol CR A dt

0

where test

FSMFSM

Test

T

FSM FSM FSM

VolWR

T

Vol CR A dt


Implications Assessment of BAC Testing Comparison of Safety Assessment and Full-Scale Testing

• Comparison of safety assessments and full-scale mockup wastage rates

– Wastage rates provide most applicable comparison, but still not one-

to-one since the corrosion rates and area terms are not the same

between the mockup and model. Moreover, the full-scale mockup

quantities represent conservative early stages of the wastage

process whereas the safety assessments are intended to be applied

for all stages of the wastage process.

• Comparison of volumetric corrosion rates is most meaningful because:

– Maximum linear corrosion rates are rather localized and not

representative of structurally significant material loss

– Average linear corrosion rates are sensitive to estimates of the

affected areas, which are in practice uncertain

where ( ) and

both & ( )

SA SA SA

SA

SA

WR CR A

CR fn LR

LR A fn t


Implications Assessment of BAC Testing Comparison of Wastage Rates – MRP-110

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0001 0.001 0.01 0.1 1. 10. 100. 1000.

Cu

mu

lati

ve

Dis

trib

uti

on

Fu

nct

ion

, F

Volumetric Wastage Rate Applied in MRP-110 App. E Model (in3/yr)

MRP-110 App E at LR = 0.001 gpm



Mockup Data at LR = 0.001 gpm



Full-Scale Mockup Wastage Rate Compared to

Probabilistic Model used in MRP-110

• Used MRP-110 model to determine the

instantaneous wastage rate at the instant

the leak rate matched the nominal leak

rate of the full-scale CRDM tests.

• Cumulative distribution of instantaneous

volumetric wastage rates (WRSA)

compared to the time-averaged values

(WRFSM) from the full-scale mockup tests.

– The wastage rates observed during

the full-scale tests support the

wastage rates used in the safety

assessment.

– The wastage rates observed during

the full-scale tests are representative

of early stages with a restrictive

annulus. These wastage rates are

expected to be bounding and

decrease with time as: (1) the annulus

opens, (2) velocities are lower, and (3)

there is reduced back pressure to

promote moisture retention.



0.0001

0.001

0.01

0.1

1

10

100

1000

Vo

lum

etr

ic W

asta

ge

Ra

te [

in3/y

r]

0.0012 3 4 5 6 7 8

0.012 3 4 5 6 7 8

0.12 3 4 5 6 7 8

1

Volumetric Leak Rate [gpm]

CRDM Nozzle Mockups MRP-110 MC Code MRP-110 (Best Fit) MRP-110 (1%) MRP-110 (99%)

Comparison of CRDM Full-Scale Nozzle Mockups and MRP-110 (Top Head)

Volumetric Wastage Rate per MRP-110

Probabilistic Model Compared to Full-Scale

Mockup Results (Top Head)

• The MRP-110 model was

used to generate

instantaneous volumetric

wastage rates covering the

range of flow rates tested

with the full-scale mockups.

• The instantaneous wastage

rate vs. leak rate data was

then evaluated to determine

the mean and bounding

percentile (1st and 99th)

curves (non-parametric fit).

• The full-scale mockup results

are bounded by the statistical

variations of the model used

in the MRP-110 safety

analysis for top heads.



0.0001

0.001

0.01

0.1

1

10

100

1000

Vo

lum

etr

ic W

asta

ge

Ra

te [

in3/y

r]

0.0012 3 4 5 6 7 8

0.012 3 4 5 6 7 8

0.12 3 4 5 6 7 8

1

Volumetric Leak Rate [gpm]

Comparision of BMN Full-Scale Mockupsand MRP-167 (Bottom Head)

BMN Mockups MRP-167 MC Code MRP-167 (Best Fit) MRP-167 (1%) MRP-167 (99%)

Volumetric Wastage Rate implied by MRP-167

Probabilistic Model Compared to Full-Scale

Mockup Results (Bottom Head)

• The MRP-167 model was used to

generate instantaneous volumetric

wastage rates covering the full

range of flow rates tested with the

full-scale mockups.

– MRP-167 used corrosion length (ℓ) to

determine the area of unsupported

head cladding. BMN Mockup data

used to estimate a proportionality

function C(ℓ) in order to estimate

volume; V=[C(ℓ)][ℓ3].

• The wastage rate vs. leak rate data

was then evaluated to determine the

mean and bounding percentile (1st

and 99th) curves (non-parametric fit).

• The full-scale mockup results are

bounded by the statistical variations

of the model used in the MRP-167

safety analysis for bottom heads.


Implications Assessment of BAC Testing Effectiveness of Visual Examinations – Modeled in Safety Assessments

0.1

1.

10.

100.

1,000.

10,000.

100,000.

1,000,000.

1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00

Leak Rate (gpm)

Vo

lum

e o

f B

ori

c A

cid

Dep

osi

ts

Rel

ease

d D

uri

ng

Fu

el C

ycl

e (i

n3)

18-month Fuel Cycle

24-month Fuel Cycle

Cycle average boron concentration of 750 ppm and

zero deposit porosity assumed. The density of boric

acid crystals is 1.44 g/cm3 (0.052 lb/in

3).

Assumed sensitivity of bare metal

visual (BMV) leak inspections:

10 in3 (½ lb) of boric acid deposits

5 in3 (¼ lb) lower bound

20 in3 (1 lb) upper bound

Volume of Boric Acid Deposits

for a Fuel Cycle versus Leak Rate

• The effectiveness of the visual

examination modeled in the safety

assessments is represented by a

probability of detection (POD) curve

that estimated the probability of

detecting a given volume of boric acid

deposits.

• The POD curve used in MRP-110

was represented by a triangular

distribution with lower, mode, and

upper values set to 5in3, 10in3, 20in3,

respectively.

• Models used in safety assessments

account for the possibility that a

substantial fraction of the boric acid

might be advected away from the

location of the leak with the liquid

phase of the effluent that has not

been vaporized.


Implications Assessment of BAC Testing Effectiveness of Visual Examinations – Observations in Full-Scale Tests

• All full scale mockup tests (i.e., CRDM nozzle, Inverted CRDM nozzle, and BMN) had visual evidence

local to the exit of the annulus for all conditions tested including: (a) leak rates ranging from 0.001 gpm

to 0.1 gpm, (b) conforming and offset insulation configurations, and (c) varied ventilation conditions.

• Volume of Evidence – There was a positive correlation between the amount (both local to the annulus

exit and remote from the mockup) of evidence and the leak rate of the test.

• Type of Evidence – The Type of Evidence local to the annulus exit was dependent on ventilation and

insulation configuration, but the Volume of Evidence was relatively independent of testing parameters

other than leak rate. As leak rate increased, the Volume of Evidence local to the annulus exit shifted

from white boric acid deposits at low leak rates to tenacious corrosion products at intermediate to high

leak rates.

• Wastage Progression – The full-scale mockups were representative of early stages of the wastage

process and variations in test results suggested the possibility of wastage being initially concentrated

deep in the annulus. Regardless of the wastage progression, there was always evidence of a leak.

There was no evidence that a structurally significant subsurface cavity could develop in the absence of

evidence readily detectable by visual inspection.

• Based on the results of the full-scale mockup tests, the assumptions regarding effectiveness of visual

examinations used in the technical analyses of the MRP safety assessments are consistent with

detection for an actual leaking penetration.


Implications Assessment of BAC Testing Conclusions Regarding Current Inspection Requirements

• Current inspection requirements for reactor vessel top and bottom head

nozzles are defined in ASME Code Cases N-729-1 (top head) and

N-722-1 (bottom head), which are made mandatory by NRC regulation

subject to specific conditions (10 CFR 50.55a(g)(6)(ii)(D) and (E))

• The technical assessments of these inspection requirements (MRP-

110, MRP-117, MRP-167, MRP-206) apply two key elements: (1)

wastage rate dependence on leak rate, and (2) ability to detect a leak

by visual examination.

– Based on the full-scale mockup tests, both the volumetric wastage rate and

effectiveness of visual examination to detect a leak have been shown to support the

modeling elements used in the technical analyses of the safety assessments.

• The current inspection requirements are conservative and adequate

such that a leaking nozzle would be detected and not result in

structurally significant wastage.


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