Nuclear Engineering and Design 238 (2008) 2746–2753

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Nuclear Engineering and Design

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Predicted impact of power uprate on the water chemistry of Kuosheng boilingwater reactor

Mei-Ya Wanga, Tsung-Kuang Yeha,∗, Charles F. Chub

a Nuclear Science and Technology Development Center, National Tsing-Hua University, 101, Sec. 2, Kuangfu Road, Hsinchu, Taiwanb Nuclear Generation Department, Taiwan Power Company, 242, Sec. 3, Roosevelt Road, Taipei, Taiwan

a r t i c l e i n f o

Article history:Received 26 December 2007Received in revised form 9 May 2008Accepted 16 May 2008

a b s t r a c t

A theoretical model was adapted to evaluate the impact of power uprate on the water chemistry of acommercial boiling water reactor (BWR) in this work. In principle, the power density of a nuclear reactorupon a power uprate would change immediately, followed by water chemistry variations due to enhancedradiolysis of water in the core and near-core regions. It is currently a common practice for commercialBWRs to adopt hydrogen water chemistry (HWC) for corrosion mitigation. The optimal feedwater hydro-gen concentration may be different after a power uprate is implemented in a BWR. A computer codeDEMACE was used in the current study to investigate the impact of various power uprate levels on majorradiolytic species concentrations and electrochemical corrosion potential (ECP) behavior of componentsin the primary coolant circuit of a domestic BWR-6 type reactor operating under either normal waterchemistry or HWC. Our analyses indicated that under a constant core flow rate the chemical species

concentrations and the ECP did not vary monotonously with increases in reactor power level at a fixedfeedwater hydrogen concentration. In particular, the upper plenum and the upper downcomer regionsexhibited uniquely higher ECPs at 108% and 115% power levels than at the other evaluated power levels.

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. Introduction

Power uprates have become a common practice for the powertilities owning light water reactors (LWRs) to meet the increas-

ng electricity demand under the restrictions of constructing newuclear power plants. Since September of 1977, more than 100ower uprate applications in the United States alone have receivedpproval from the regulatory body (Hettiarachchi, 2005). Amonghe three types of power uprate, measurement uncertainty (<2%),tretch power uprate (2–7%), and extended power uprate (7–20%,urrently approved maximum percentage), there is no commonelection for general nuclear power plants and a thorough eval-ation is always required for every single LWR.

One practical way of increasing the reactor power is to delib-rately adjust the fuel loading pattern and the control rod patternnd thus to avoid replacing the primary coolant pump with a new

ne of larger capacity. In this case, the power density of the reac-or will increase with increasing power, and the mass flow rate inhe primary coolant circuit (PCC) will only increase slightly (usu-lly by less than 5%) or even remain unchanged. The higher power

∗ Corresponding author. Tel.: +886 3 5742864; fax: +886 3 5713849.E-mail address: tkyeh@mx.nthu.edu.tw (T.-K. Yeh).

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029-5493/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.nucengdes.2008.05.007

© 2008 Elsevier B.V. All rights reserved.

ensity in the reactor core due to a power uprate will lead to theroduction of more steam in the core fuel channels, and this will inurn result in higher linear liquid and steam velocities in the boilingore region and shorten the total residence time of the coolant inhe core region. The combined effect of higher neutron and gammahoton dose rates and a shortened coolant residence time in theeactor core complicates the water chemistry in the PCC of an LWRue to enhanced radiolysis but a less irradiated time of the primaryoolant.

In boiling water reactors (BWRs), intergranular stress corrosionracking (IGSCC) on stainless steel and nickel-base alloy compo-ents has been a major issue of material degradation for decades.he IGSCC susceptibility of structural materials in a BWR is pri-arily dominated by the physical and chemical properties of theaterials, tensile stresses, and the primary coolant chemistry. Foritigating IGSCC in an operating BWR, improvement on the pri-ary coolant chemistry by hydrogen water chemistry (HWC) has

een adopted worldwide (Hettiarachchi, 2005). The HWC tech-ology is deemed effective when the electrochemical corrosion

otential (ECP) in the PCC of a BWR at a specific feedwater hydrogenoncentration ([H2]FW) is reduced to below −0.23 VSHE that signi-es immunity of structural components to IGSCC. The U.S. Nuclearegulatory Commission has recognized an ECP value of −0.23SHE to be the critical corrosion potential (Ecrit) of sensitized Type

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04 stainless steels (electrochemical potentiodynamic reactivationEPR) = 15 C/cm2) in typical BWR environments (i.e., a coolant tem-erature >270 ◦C and coolant conductivity near 0.3 �S/cm).

The ECP in the PCC of a BWR is a mixed potential reflecting themounts of major redox species (i.e. hydrogen, oxygen, and hydro-en peroxide) present in the reactor coolant. The concentrationsf these redox species, however, are directly related to the degreef water radiolysis, which is dominated by the power density ofhe reactor and by the coolant residence time in the reactor core.

hen a reactor’s power is uprated, changes in power density (i.e.eutron and gamma photon dose rates) and coolant flow velocity inhe reactor core would lead to concentration variations of the majoredox species. Accordingly, the required [H2]FWs at different powerevels for the HWC technology to take effect on IGSCC mitigation

ay thus be different. However, it has been demonstrated throughheoretical analyses that the effectiveness of HWC actually variesrom region to region (e.g. from core to downcomer or to recircu-ation system) in the PCC of a BWR due to various degrees of wateradiolysis in these regions (Yeh et al., 1995; Yeh and Macdonald,996, 2006; Yeh and Chu, 2001; Yeh, 2002). Therefore, it is antici-ated that the impact of power uprate on the effectiveness of HWC

n the PCC of a BWR may also be different in different regions.The concentrations of radiolysis products and the ECP values dif-

er significantly in the PCC of a BWR. It is very difficult to actuallyeasure the water chemistry data directly at various locations of

n actual reactor. Accordingly, the impact of power uprate on theater chemistry of a BWR operating under HWC can only be eval-ated theoretically through computer modeling. A deterministicodel by the name of DEMACE was developed in the past for ana-

yzing water chemistry variation and corrosion behavior of metallicaterials in the PCC of a BWR (Yeh and Chu, 2001; Yeh, 2002; Yeh

nd Macdonald, 2006). In the current study, a local BWR, Kuoshengnit 1 (Kuosheng-1), was selected for demonstrating the impactf various power uprates on the major redox species concentra-ions and on the ECP behavior of structural components in the PCC.uosheng-1 is a BWR-6 type reactor with a rated thermal powerf 2894 MW, and its commercial operation started in December of981. It is currently operating under HWC with a 0.5 ppm (parts perillion) [H2]FW. Numerical simulations for Kuosheng-1 were car-

ied out for [H2]FW ranging from 0.0 to 2.0 ppm and for power levelsanging from 100% to 120%. Variations in oxygen (O2), hydrogenH2), and hydrogen peroxide (H2O2) concentrations and in ECP atour selected locations in the PCC of Kuosheng Unit 1 (Kuosheng-1)ere analyzed. The impact of power uprate on the corrosion miti-

ation effectiveness of HWC is discussed. Optimal HWC operatingonditions for Kuosheng-1 at various power uprate levels are thenroposed.

. Modeling approaches

For simplicity in modeling, the entire PCC of a BWR was dividednto 12 regions in the DEMACE computer model, as shown in Fig. 1.he DEMACE computer code consists of a radiolysis model for cal-ulating chemical species concentration, a mixed potential modelor calculating ECP, and a coupled environment fracture model foralculating the crack growth rate. More detailed descriptions abouthese three models can be found in the literature (Yeh and Chu,001; Yeh, 2002; Yeh and Macdonald, 2006; Macdonald, 1992;acdonald et al., 1996).

.1. Power uprate simulation

When considering a power uprate in an NPP, the utilities gener-lly have two options. One is to go for a higher mass flow rate with a

X

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Fig. 1. Conceptual configuration of a typical BWR primary coolant circuit.

ew recirculation pump of higher capacity (Yeh and Wang, in press),nd the other is to maintain the same mass flow rate but with newuel loading and control rod patterns (Wang and Yeh, in press). Theecond option would reduce the cost for power uprate to a signifi-ant extent since a major component replacement is not required.lmost all uprated NPPs in the United States have adopted the sec-nd approach. Our earlier study focused on the change in coolantass flow rate due to a change in power level on a specific loadline

Yeh and Wang, in press). We have also looked into the case of powerprates under a fixed core flow rate in a BWR-4 type reactor (Wangnd Yeh, in press). In the current study, we again took into accounthe condition of a fixed core flow rate, but the evaluation targetas switched to a BWR-6 type reactor (Kuosheng-1) to demon-

trate that the impact of power uprate on the water chemistry of aWR would actually vary from plant to plant.

The plant data used in the computer simulations are the same ashose operating in the HWC ramping test conducted in 2006 excepthat the neutron and gamma photon dose rates and the mass flowate of liquid coolant in the flow path between the core channelnd the mixing plenum were recalculated based on a specificallyelected operating power level. The dose rates of neutron andamma photon were assumed to vary linearly as the reactor powerevel increased. The linear flow velocities of the liquid coolant inhe upper plenum, stand pipe, and separator sideway regions wereecalculated based on the steam quality produced in and the liquideft the reactor core at different power levels. The steam quality (X)s defined as the mass fraction of vapor in the mixture of liquid andapor and can be expressed by

= H − Hf + Hi

Hfg, (1)

here H is the enthalpy of the mixture at the core outlet, Hf is thenthalpy of the saturated liquid at the system pressure, Hi is the

2 ring and Design 238 (2008) 2746–2753

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nthalpy of the liquid at the core inlet, and Hfg is the enthalpy ofaporization at the system pressure. H is a function of total pro-uced power and core flow rate and may be expressed as

= FcQc

m, (2)

here Fc is the hot channel factor in the core, Qc is the total pro-uced power in the core, and m is the core flow rate. As the reactorower level increases at a fixed pressure and a fixed core flow rate,would accordingly increase and lead to an increase in X.The increases in steam quality as a function of reactor power

evel were calculated using the ZEBRA computer code (Bladeslee,974), and the results are shown in Fig. 2. For the core channelegion, the variation in coolant flow velocity along the core channelas dominated by the steam quality (and hence the void fraction),

nd the liquid flow velocity became higher as the steam qualityncreased in this region at a fixed power level, as also shown in Fig. 2.he foregoing changes in radiation dose rates and in coolant flowelocity were expected to influence the steady-state water chem-stry of the reactor since the degree of radiolysis and the time forhe coolant to undergo radiolysis were altered at different powerevels.

.2. Modeling procedures

Before the actual modeling work on Kuosheng-1 under vari-us power uprate conditions commenced, the DEMACE computerodel was deliberately calibrated by fitting the predicted oxygen

oncentration at the recirculation system outlet of this reactor witheasured data, obtained from the results of the HWC ramping

est conducted in 2006. The operating conditions of Kuosheng-1n 2006 were used in the model calculation during the calibrationrocess. By adjusting the gas transfer coefficients in the core boil-

ng channels of Kuosheng-1, a routinely taken technique (Yeh andacdonald, 1996; Yeh and Chu, 2001; Yeh, 2002), and by applying

he least square fitting algorithm, we were able to obtain predictedesults that are in reasonably good agreement with the measuredlant data as shown in Fig. 3.

The power uprate conditions considered in this study rangedrom 0% to 20% with selected power levels of 100%, 102%, 104%,07%, 108%, 109%, 114%, 115%, 116%, and 120%. At the beginning of

his work, a 1% difference in power level was actually selected inhe modeling work for comparison purpose. In order to simplify theesultant figures for discussion, only power levels leading to rela-ively more oxidizing coolant environments are discussed in thisaper. At a specific power level, the coolant flow velocities in the

ig. 2. Steam quality and liquid coolant flow velocity as a function of power level athe core exit of Kuosheng-1 operating at a fixed mass flow rate.

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ig. 3. Comparison of the calculated and the measured oxygen concentrations inhe recirculation system of Kuosheng-1.

ore and other near-core regions were recalculated based upon theteam quality produced in the core fuel channels. The dose rates ofeutron and gamma photon in Kuosheng-1 were calculated usingn independent computer code by the name of DRONG (Yeh et al.,000). In the meantime, the new power level at a fixed coolant flowate would enable the calculation of steam quality and void frac-ion in the core region, and this part of the study was done using theEBRA computer code. Changes in void fraction in the core boilingegion of a BWR are expected to have an important impact on theffectiveness of HWC since gas transfer rates of oxygen and hydro-en between the steam and liquid phases in this region are alteredccordingly. After the foregoing parameters were derived, waterhemistry and ECP modeling was then carried out for the entire PCCf Kuosheng-1 with [H2]FWs ranging from 0.0 to 2.0 ppm. To avoidengthy discussion, only four particular locations of major IGSCConcerns in Kuosheng-1 were selected to demonstrate the impactf various power uprate levels on the effectiveness of HWC. The fourpecific locations of major IGSCC concerns selected were the outletf the upper plenum region, the outlet of the upper downcomeregion, the outlet of the recirculation system, and the outlet of theottom lower plenum region which approximately represent theop guide, the belt area of the core shroud and jet pump risers, theecirculation piping, and the core shroud base and support, respec-ively. These locations have a long history of IGSCC incidents aseported in the literature (USNRC, 1994a,b, 1995).

. Results and discussion

The simulation results are categorized into two portions show-ng variations in the concentrations of major redox species and inhe ECP as a function of [H2]FW at the four selected locations.

.1. Species concentrations

Although there are a total of 11 radiolysis species taken intoccount in the DEMACE model, only three major redox speciesith relatively significant concentrations are used in the calcula-

ion of ECP. Details on the full set of chemical reactions among thesepecies and their respective rate constants can be found elsewhere

− +

Yeh and Chu, 2001). Among the 11 radiolysis species (e , H, H , OH,H−, H2O2, HO2, HO2

−, O2, O2−, and H2), H2 and H2O2 are directly

roduced during the course of water radiolysis, and O2 is then pro-uced through the decomposition of H2O2, not a direct radiolysisroduct. These three species actually dominate the magnitude of

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CP because the concentrations of the other redox species (e.g. OHnd HO2) are comparatively small and can thus be neglected. Theoncentration variations of the three major species as a functionf [H2]FW at the designated locations of Kuosheng-1 are of greatmportance.

.1.1. Concentrations of H2The predicted H2 concentration ([H2]) in the reactor coolant

t each selected location of Kuosheng-1 is shown in Fig. 4. Itas observed that at a fixed power level the [H2] monotonously

ncreased with increasing [H2]FW up to 2.0 ppm at the outlets of thepper downcomer, the recirculation system, and the bottom lowerlenum. However, the [H2] was relatively low at the upper plenumutlet, which resulted from the gas stripping process in the coreoiling region, although it still increased with increasing [H2]FWp to a maximum of 45 ppb (parts per billion) at 2.0 ppm [H2]FW,hown in Fig. 4(a). Under the power uprate condition, it seemedhat a higher power uprate percentage would lead to a higher [H2]n the reactor coolant at all four locations, but the difference in [H2]mong various uprate percentages at low [H2]FWs was small.

.1.2. Concentrations of O2Variations in O2 concentration ([O2]) at the selected locations

re shown in Fig. 5. In general, a higher [H2]FW led to predictedower [O2]s at all four locations no matter what the power levelas. Among these locations, the [O2] at the recirculation systemutlet, shown in Fig. 5(c), was reduced most effectively under HWC,ith a 0.5 ppm [H2]FW being sufficient to scavenge all dissolved O2

lrtu[

Fig. 4. Variations in [H2] as a function of [H2]FW at four selected locations of Kuoshen

nd Design 238 (2008) 2746–2753 2749

n the coolant. In the meantime, the dissolved O2 in the coolantas not fully depleted until the [H2]FW at 120% power reached.0 and 1.5 ppm at the upper downcomer outlet and the bottom

ower plenum outlet, respectively. On the other hand, the [O2] athe upper plenum outlet (Fig. 5(a)) at all power levels never wentown to zero even with a 2.0 ppm [H2]FW. It was clearly demon-trated that the effectiveness of HWC on reducing the [O2] in a BWRight vary from region to region along the PCC due to different

egrees of radiolysis in these regions. In addition to the radiolysisffect, changes in power level would be expected to influence theWC effectiveness.

In view of the effect of power level, we found that at lowH2]FWs ranging from 0 to 0.2 ppm, increases in power level ledo decreases in [O2] at all selected locations. One exception washat the [O2] at the recirculation system outlet, shown in Fig. 5(c),ecreased with increasing power level at all [H2]FWs. For the case ofH2]FW being equal to 0, the difference in [O2] could be as much as0 ppb between the power levels of 100% and 120%. However, whenhe [H2]FW was greater than 0.2 ppm, certain power levels wouldnduce particularly higher [O2]s, such as those seen at 108% and15% power at the upper downcomer outlet. In fact, the [O2] at thepper downcomer outlet at 108% or 115% power never went belowppb even with a 2.0 ppm [H2]FW while the [O2]s at the other power

evels became much lower than 5 ppb, shown in Fig. 5(b). Similaresults were also observed at the upper plenum outlet and the bot-om lower plenum outlet. It was therefore foreseen that a powerprate of 8% or 15% causing undesirably higher [O2]s at higherH2]FWs would eventually influence the effectiveness of HWC on

g-1 with operating power levels ranging from 100% to 120% of the rated power.

2750 M.-Y. Wang et al. / Nuclear Engineering and Design 238 (2008) 2746–2753

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Fig. 5. Variations in [O2] as a function of [H2]FW at four selected locations of Ku

orrosion mitigation in Kuosheng-1, and it was indeed proven soased upon the predicted ECP results discussed later.

.1.3. Concentrations of H2O2The other important oxidizing species in the BWR coolant is

2O2, a direct radiolysis product. Fig. 6 exhibits variations in2O2 concentration ([H2O2]) as a function of [H2]FW at the four

elected locations. In contrast to the [O2], the [H2O2] did notecrease monotonously as the [H2]FW increased. Except for thepper plenum outlet, the [H2O2]s were predicted to increase firstnd then decrease later on with increasing [H2]FW. Four chemicaleactions were speculated responsible for this unique outcome, andhey are

+ OH ⇔ H2O, (3)

H + H2 ⇔ H + H2O, (4)

H + H2O2 ⇔ HO2 + H2O, and (5)

+ H2O2 ⇔ OH + H2O. (6)

q. (3) with the greatest rate constant of all dominated the con-entrations of species H and OH. When a mild amount of H2 wasdded into the reactor coolant, Eq. (4) started taking effect andore H was produced. More H then led to more consumption of

H via Eq. (3), which in turn led to less consumption of H2O2 viaq. (5). Although some H2O2 could have reacted with H via Eq. (6),he decrease in [H2O2] was comparatively less significant. Once theH2]FW increased to an effectively high level, the concentration of Henerated via Eq. (4) became vital and Eq. (6) started to dominate.

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g-1 with operating power levels ranging from 100% to 120% of the rated power.

he [H2O2] then promptly decreased via both Eqs. (5) and (6) aseen in Fig. 6. Since H2O2 is a highly oxidizing species and a few ppb2O2 could keep the ECP well above the Ecrit, it is essential to elimi-ate all H2O2 in the coolant to achieve considerably low ECPs in theCC of a BWR. At NWC, the [H2O2]s at some locations near to coreeduced with increasing power level due to the degree of radiolysis.s [H2]FW gradually increased, the competition between the con-umption and the production of [H2O2] would be obvious. However,ue to differences in degree of radiolysis, the [H2O2] at various loca-ions responded to the [H2]FW quite differently at selected powerevels.

For a fixed power level, the [H2O2] at the upper plenum outlet,hown in Fig. 6(a), steadily decreased as the [H2]FW increased fromto 0.2 ppm and then slowly decreased with increasing [H2]FW.

he [H2O2] at this location would be more than 20 ppb at 2.0 ppmH2]FW at most power levels, except at 100% and 107% power lev-ls. In view of the impact of power level, we found that the [H2O2]ecreased with increasing power level at 0 or 0.1 ppm [H2]FWs.owever, the [H2O2] did not show consistent changes with increas-

ng power level when the [H2]FW became higher. One distinctxample was that the [H2O2] remained at more than 40 ppb at 108%nd 115% power levels with 2.0 ppm [H2]FW while those at the otherower levels went down to below 30 ppb at the same [H2]FW. Thisnique phenomenon highlighted our argument that a combined

ffect of enhanced radiolysis and huge recombination upon a powerprate could arbitrarily alter the effectiveness of HWC.

At the upper downcomer outlet, the response of [H2O2] toH2]FW was quite different, shown in Fig. 6(b). Variations in power

M.-Y. Wang et al. / Nuclear Engineering and Design 238 (2008) 2746–2753 2751

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ig. 6. Variations in [H2O2] as a function of [H2]FW at four selected locations of Kuo

evel did not cause any significant change in [H2O2] at NWC. Irreg-larity in [H2O2]s of different power levels became discernable at0.1 ppm [H2]FWs. For the base case of 100% power level, a 2.0 ppmH2]FW was required to scavenge nearly all H2O2 in the coolant athis location. Power levels of 104%, 107%, 114% and 120% would bexpected to lead to the same H2O2 response. However, a [H2]FW ofhe same level was not enough to reduce the [H2O2] to below 20 ppbt 108% and 115% power levels. The outcome implicated that smallhanges in power and hence in degree of radiolysis and recombi-ation could result in significant changes in HWC effectiveness.

At the recirculation system outlet, the [H2O2], shown in Fig. 6(c),ecreased with the increasing power level. Changes in [H2O2] at dif-erent power levels became distinct when the [H2]FW was greaterhan 0.1 ppm. The HWC application exhibited tremendous effec-iveness at this location in terms of H2O2 reduction. The requiredH2]FW to effectively lower the [H2O2] at the recirculation sys-em outlet was 0.5 ppm at 100% rated power, but it would be only.3 ppm at 115%, 116%, or 120% power levels. In other words, theequired [H2]FW to effectively lower the [H2O2] was reduced as theower level increased at this location.

Similar to what happened at the upper downcomer outlet, varia-ions in power level did not cause any significant change in [H2O2] atow [H2]FWs at the bottom lower plenum outlet, shown in Fig. 6(d).

n the other hand, the required [H2]FW to effectively lower the

H2O2] at the this location was 1.7 ppm at the original rated power,ut this required [H2]FW level became 1.8 and 1.5 ppm at 108%nd 120% power levels, respectively, which again highlighted theensitivity of HWC to the reactor operating power.

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-1 with operating power levels ranging from 100% to 120% of the rated power.

Summarizing the concentration results from the two oxidizingpecies, we found that for the Kuosheng-1 BWR an 8% or 15% powerprate tended to promote a more oxidizing coolant environment forhe structural components located at or near the top guide, the beltrea of the core shroud and jet pump risers, the recirculation piping,nd the core shroud base and support than the others. Increases inhe concentrations of H2O2 and O2 at these particular uprate levelsould pose a significant impact on ECP according to the mixedotential model (Macdonald, 1992). Therefore, a similar outcome

s expected to appear in the ECP response to power uprate.

.2. ECP

In the current study, the well-recognized ECP of −0.23 VSHE waselected as the Ecrit of highly sensitized Type 304 stainless steelsn typical BWR environments. Based upon this criterion, a compo-ent is considered protected from IGSCC and the HWC technique

s considered effective if the predicted ECP is below the Ecrit. Fig. 7hows variations in ECP as a function of [H2]FW and power level athe four selected locations.

At the upper plenum outlet, one important phenomenon to noteas that the ECP at all simulated power levels never went below

he Ecrit even though the [H2]FW was as high as 2.0 ppm, shown in

ig. 7(a). Similar situation occurred at the upper downcomer outletFig. 7(b)), but the ECP would likely be reduced to below the Ecrithen the [H2]FW reached 2.0 ppm and the power level was main-

ained at 100%, 104%, 107%, 114%, or 120%, exactly consistent withhe [H2O2] response at this location. The ECP response to [H2]FW

2752 M.-Y. Wang et al. / Nuclear Engineering and Design 238 (2008) 2746–2753

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Fig. 7. Variations in ECP as a function of [H2]FW at four selected locations of Kuo

nd power level at the recirculation system outlet was compar-tively simple. The ECP monotonously decreased with increasingower level, shown in Fig. 7(c). Except for the power levels above14% at which an effective ECP reduction at 0.3 ppm [H2]FW wasbserved, a 0.4 ppm [H2]FW was required at all uprated power lev-ls. The HWC effectiveness on ECP reduction at this location waslightly improved at higher power uprate levels. ECP variations athe bottom lower plenum outlet, shown in Fig. 7(d), behaved quiteimilar to those at the upper downcomer outlet. A 1.7 ppm [H2]FWas required to reduce the ECP effectively at 100% power level. Aigher 1.8 ppm [H2]FW at 108% power level but a lower 1.5 ppmH2]FW at 120% power level both enabled effective ECP reductions.n general, the overall ECP responses agreed in trend with theH2O2] variations under different [H2]FWs and power levels at allour evaluated locations.

Summarizing the predicted ECP results at these four loca-ions, we noted that no significant ECP differences due to powerprate were observed when the [H2]FW was either much less orreater than the critical concentration at which the ECP markedlyecreased to below the Ecrit. A particular uprate percentage, how-ver, would be expected to induce a relatively more oxidizingnvironment and hence led to a poorer HWC efficiency that wasommonly seen at most of the evaluated locations of Kuosheng-

with a 108% or 115% power level. On the other hand, the HWC

fficiency would be slightly improved at the upper downcomer out-et and the bottom lower plenum outlet at certain higher powerevels of 107%, 114% and 120%. Finally, it is important to notehat the uniquely oxidizing environment at the uprated power

a

otr

-1 with operating power levels ranging from 100% to 120% of the rated power.

evel of 108% or 115% was applicable to Kuosheng-1 only. A BWRith a different power density and different physical dimensionsay experience a similar outcome at a different uprated power

evel. An individual analysis on the impact of power uprate on theorrosion mitigation effectiveness of HWC in a BWR is thereforeecessary.

. Conclusions

The responses of water chemistry and ECP to HWC at someelected locations in Kuosheng-1 BWR under different postulatedprated power levels were successfully evaluated. Upon a powerprate, the degree of radiolysis and coolant residence time in theore of a BWR may vary, causing changes in radiolytic speciesoncentrations and resulting in varied HWC efficiency at differentocations.

For the Kuosheng-1 BWR, an 8% or 15% power uprate would tendo promote a more oxidizing coolant environment for the structuralomponents and therefore lead to downgraded HWC effectivenessn terms of ECP reduction and corrosion mitigation. In contrast, theWC efficiency would likely be slightly improved at higher power

evels of 107%, 114% and 120% for the four selected locations. Noonsistent trend could be found for changes in the effective [H2]FW

s a function of uprated power level.

Based upon the predicted results, the impact of power upraten the HWC effectiveness in a BWR is expected to vary from loca-ion to location and eventually from plant to plant due to differentadiation dose rates and physical dimensions. The outcome derived

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M.-Y. Wang et al. / Nuclear Enginee

n this work for Kuosheng-1 may not be applicable to other BWRsf the same type, and a new, full-scale analysis is highly recom-ended.

cknowledgments

The authors wish to express sincere gratitude to National Sci-nce Council and Taiwan Power Company for the financial supportsnder contract numbers NSC 95-NU-7-007-004 and TPC-056-95-8014, respectively. We also acknowledge the technical informationupported from Ms. Ching Chang and Mr. Feng-Can Huang of Taiwanower Company.

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