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MPR ASSOCIATES. INC.

June 1, 1987

Mr. Robert C. Jones, Jr. |U.S. Nuc1 ear Reguletory Commission 1

Office of Nuclear Reactor Regulation |

7920 Norfolk AvenueBethesda, MD 20814

Subject: Effects of Modeling Assumptions on the Computation of ReactorPressure Response, B&W Owners Group Sensitivity Study

Dear Mr. Jones:

Following up the telephone conversation among you, Jack Ramsey (NRC),Stuart Rose (DPC), and me on March 20, 1987, I am enclosing herewith adiscussion of our analyses of the errors and uncertainties in thecalculation of pressurizer level and pressure response to loadrejections (Enclosure 1).

Volume I of the Sensitivity Study compared model predictions againstactual plant responses. This comparison led to the conclusion that themodel is conservative in its pressure predictions for load rejections --actual pressure peaks are lower than model calculations. We consideredit prudent, however, to investigate potential sources of the differencesin model and plant response; hence, the analyses described inEnc 1osure 1.

As discussed in the enclosure, the differences between the sensitivityStudy model and actual plant response to load rejections are, ingeneral, identifiable and, to a degree, quantifiable. The model isindeed conservative. Thus, the conclusions drawn by the Study that reston pressurizer response to insurge -- most notably that relating to theinherently limited response of B&W units to load rejections and turbinetri ps -- remain valid.

The comparative data provided in the Sensitivity Study for outsurges(e.g. , Fi gures 1, 5, 7, and 8 of Volume I) indicate good agreementbetween calculated and actual pressurizer level and pressure responsesto outsurges. Given an outsurge, the pressurizer model itself isproviding good predictions of pressure, for the range of transientsconsidered by the Sensitivity Study. There is no reason to reconsiderthe conclusions of the study related to outsurges -- for example, those yrelating to post-reactor trip pressurizer response and overcooling.

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1050 CONNECTICUT AVENUE. N.W. WASHINGTON. D.C. 20036 202 659 2320

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Mr. ' Robert Jones : - 2L- June 1, 1987-;

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' As you req:iested during our conversation, the enclosure-provides the.

volumetric surge and pressure data for the transient'on which theinsurge: comparisons are based - . the full load rejection (turbine trip)without reactor.-tri p at TMI-1.(Figure 2'of the Volume.1 of theSensitivity Study).- The figures of.the enclosure show

~(1) steam pressure,- steam gen'erator power transfer, surge and' reactor pressure data as calculated by the Sensitivity Study

model,.

(2) steam pressure,. steam generator' power transfer, surge' andreactor pressure data as. calculated by a more sophisticated

.model, and-|

(3) actual plant steam pressure, surge, and reactor pressure data.~

:If you have.any questions on the enclosure or would like to explore theanalyses . described in it in greater depth, please do not hesitate tocall us.

Sincerel y,

he.H. Estrada

)!cc: S. T. Rose

G. R. SkillmanG. Swindlehurst !

C. TurkJ. DunneL. Lanese ;

.R. Schoma ker

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. M P R ASSOCIATES. INC. PR et r tedJune 1, 1987

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| BOUNDS OF THE MODELING ERRORS IN THE REACTOR

PRESSURE RFSPONSE TO PRESSURIZER INSURGES

INTRODUCTION

The ' conclusions of the Sensitivity Study relative to the responses of|

B&W and other PWRs to sudden reductions in steam demand rely in part on

the accuracy of the predictions of pressurizer level and pressureres pons e. To ensure that the approximations of the models used by the

'

Sensitivity Study have not misied us, we have explored alternative and,in most cases, more detailed analytical descriptions of the two aspectsof the physical process that dominate this response:

o the heat transfer from the reactor coolant to the secondary

side of the steam generator, and

o the pressurizer itself.

More specifically, we have evaluated the uncertainties in modelpredictions due to

(1) Reactor Coolant to Secondary Side Heat Transfer |)

o The use of a single node to characterize primary to secondaryheat transfer in the boiling region (separate nodes are usedfor the superheater region of the OTSG and for the preheaterfor the Westinghouse UTSG).

||

o The effect, on primary to secondary heat transfer of theoscillatory nature of the steam pressure and flow responsefollowing a sudden load rejection or turbine trip. The sudden

closure of the turbine control and stop valves following aturbine trip causes a pressure wave to traverse the steamsupply lines. This wave alternates from compression to

rarefaction and, during the compression portion of the cycle,

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could.cause the heat transfer in.the' steam generator to depart i

from nucleate boiling-(that is, the heat transfer could be ;

momentarily impaired because the difference between metalsurface temperature 'and bulk fluid' temperature is insufficient.to sustain nucleate boiling).

I

o the effect, on primary to secondary heat transfer, of the heatcapacity of the steam generator tubes.

(2) Pressurizer Modeling -

o' The effect.of heat and mass transfer to the pressurizerwalls -- condensation during an insurge will remove both

. energy and mass from the steam bubble (the study modeli

neglects these losses).

o The effect of heat and mass transfer from the steam bubble to r

the pressurizer liquid surface (during an insurge the model !

treats this surface as adiabatic).

o The effect of the time response of the pressurizer spray valve(for B&W and CE plants, the study assumes that this valveresponds instantaneously when the pressure reaches itssetpoint).

'

i

RESULTS

j

The results of the analyses are as follows:: |

1. Reactor Coolant to Steam Generator Heat TransferThe most demanding transients with respect to surge and pressurizer i

modeling are turbine trips and load rejections, wherein reactor fpower persists at a high level after the steam demand is reduced. ;

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Figure. 2, Volume I, of the Sensitivity Study is such a transient --a turbine trip from full load at TMI-1. Following the trip, thereactor remained on-line (the turbine trip was performed as a testof the load rejection capability of TMI-1 in 1976). The predic-tions of the heat transfer and pressurizer models used in theSensitivity Study were compared in ' detail with the actual plantresponse, and also with the. predictions using a multinode dynamicmodel of the steam generator.:

The computer code used for the latter model is conceptually similarto RELAP. The steam generator is comprised of 45 nodes -- fourboundary nodes,16 primary nodes, and 25 secondary nodes (thelatter including the downcomer and outlet annuli). In addition,

the main steam piping down to the turbine stop valves are modelled

with 30 nodes. Dynamic heat, mass, and momentum balances arewri tten ~around each node. The multinode model explicitly. considers-

the heat capacity of the steam generator tubes in the " connectors"between primary and secondary fluid nodes.

,

.

The purposes of the multinode model were two-fold:

o To generate the compression and rarefaction wave produced by a

sudden closure of the turbine steam admission valve and toassess the effect of this wave on primary to secondary heattransfer, and

o To calculate more accurately the effective driving tempera-tures for steam generator heat transfer (the simplified modelemploys only two primary nodes. One of these is assumed to

operate at coolant outlet temperature (for heat transfer inthe wetted region); the other is assumed at coolant inlettemperature (and governs heat transfer in the superheatregi on ) .

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The heat transfer calculations of the multinode model werecoupled to the primary system and pressurizer models used inthe Sensitivity Study. It can be demonstrated that the !

pressurizer surge can be calculated with excellent precision j

from (i) a knowledge of the net difference between the power i

generated by the reactor core and the power transferred by thesteam generators and (2) a knowledge of the incremental ;

properties of the reactor coolant -- the partial differential ;

of specific volume with respect to specific enthalpy is themost important of these. A detailed nodalized model of thereactor coolant system is not required for the surgecal cul ation.

|

The predictions of the Sensitivity Study model and themul tinode steam generator model as regards steam generator

'

pressure, heat transfer, and pressurizer insurge and level are i

shown in Fi gures 1, 2, and 3, respectively. The actual plantsteam generator pressure and pressurizer level responses are

also shown. From these comparisons, the following conclusions

can be drawn: q

I

a. The multinode model prediction of steam pressure1

(Figure 1) agrees extremely well with actual plant -- ]even the small " blips" due to the compression wave appear |

to correl ate well. The pressure rise of the two-node(Sensitivity Study model) is somewhat more rapid than the l

other two in the 1 to 2 second time frame, but for the

balance of the transient, its agreement with the actual ||

response is reasonable. i

b. The forms of the power transfer responses calculated bytwo-node and multinode models (Figure 2) are in general

a greement . The large oscillatory component of the,

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multinode response is due to the oscillations in steamflow resul ting from the compression wave. The two-nodemodel does not consider momentum storage in the steam

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line and is therefore not capable of generating such a -

wave. The somewhat more rapid restoration of power

transfer in the single node model is consistent with itsmore rapid steam pressure response.

c. The actual response of the pressurizer level (Figure 3)contains an oscillatory component of about 16 inches at i

about 1 Hz. This oscillation is believed due tovariations in the magnitude of the insurge flow. Theoscillatory power transfer response calculated by the ;

multinode model (Figure 2) produces variations in+ surge4

flow, but as long as the instantaneous di/ference betweenreactor power and the power transferred through the steamgenerator remains positive, the net surge flow mustalways be into 'the pressuri zer. A drop in pressurizer

level below its initial value (as occurs in the measuredresponse at 2.6 seconds) wouia mean steam generator poweris greater than reactor power at this point and is notconsidered likely. It is believed that the oscillationsin measured level are due to impingement of insurgingfluid on the lower tap of the level instrument. The |

design of the B&W surge line nozzle is directedhorizontally and would impinge on the pressurizer lowerhead just below the tap at TMI-1 (Figure 4 is a sectionaldrawing of the TMI-1 pressurizer.) Surge velocitiesduring the insurge are about 20 ft/second. Thestagnation head at this velocity is about 6 feet, thusonly a small fraction of the insurging fluid " playing" onthe tap could produce the observed effect.

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The level predictions of both the two-node and multinodemodel agree well' with the general trend of the actual |

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l evel . As would be expected from the somewhat more rapid j

restoration of power transfer of the two-node (Sensitivity |'

Study) model, its level rises slightly more slowly thanthe multinode level over the first 5 seconds. Over the

~

10-12 second time frame, the two-node calculation of the-

level change is within two inches of the actual --considered excellent overall agreement. The simplifi-cations implicit in the use of a single boiling regionnode operating, on the primary side, at steam generatoroutlet temperature do not appear to lead to large errorsin the coolant volume calculaion. !

l

From the standpoint of the reacter pressure computation, it is thelevel (i .e. , surge ) calcul ati on that is important. We concluaa

from Figure 3 that the Sensitivity Study's model is doing a goodjob of this.

2. Pressurizer Response

Figure 5 is a plot of the actual plant pressure response for theTHI-1 turbine tri p. Calculated pressure responses are also plotted

in Figure 5. Note that the actual plant pressure is that at thereactor vessel outl et. Pressurizer pressure will differ by (1) theelevation difference between the tot leg tap and the pressurizer '

liquid surface, (2) the head produced by the fluid velocity in the ;

coolant pipe (this approximately offsets the elevation head), and(3) the pressure drop in the surge line, which is calculated to beas much as 35 to 40 psi during portions of the transient (thoughnot when the pressure peak occurs). For the mutlinode model, both

coolant loop and pressurizer res'ponses are given; for the two-nodemodel only pressurizer pressure is calculated.

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iThere is pneral ' agreement between both calce?ded, pressureresponse and tith rc'tual response over the firstjfbur seconds of the j

transiy,$t. Calcubt,ted res ponses are one t(~?-1/2, Ndconds auf cker

but part of this -- perhaps 1/2 second -- is duc to the delay of ;{a

/>the pressere instrument which is, of course, included in the actual

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About iour seconds into the transient, however, the achal response

turre down abruptly, from a peak pressure of 2336 psia. The calcu-lated re ponses, on the other hand, continue upward. The response

!produced; by the two-node steam generator model turns down at

9 secor.ds froh, a pressure peak of. 2396 psi a (the mul tinode modelresponse was, not calculated ~ beyond 5 seconds).

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Several possibilities were investigated to explain t'ie differencesbetween the actual and'ealculaty. pressure responses in the 4 to 9 s

I (second time' frame. / .; '

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(a) Heat and mass transfer between the steam bubble and the a

pressurizcf walls, neglected by the Sensitivity Study modelwill ten'd t/ reduce the p; assure peak.

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(b) Delays in the operation of the pressurizer spray valve willtend to increase the pressure peak. To ensure the Study'sassumption of negligible' delay was not offsetting another

I

error, valve delay was investigated.)

(c) A PORV capacity larger than design would tend to reduce thepressure peak (for the M1-1 test, the PORV opened at2270 psia).

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(d ) A pressuri zer spray 'l ow more effective than calculated wauld

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k ,; (e) Significant energy and mass removal from the steam bubble dueto turbulent mixing of insurging fluid and pressurizer fluidwould tend to reduce the peak.

The effects of heat and mass transfer to the pressurizer wallswere evaluated by incorporating an eight-node metal slab into thepressurizer portion of the Sensitivity Study model. The slab wasdimensioned to represent that portion of the pressurizer vess'elabove the liquid interface. Condensing heat transfer was assumed .]to govern the flow of energy from the steam bubble to the inner |wall of the slab. The outer wall.was assumed to be adiabatic. The 1

.1

energy removed from the steam leads to condensation; fluid mass ]

removal from the steam bubble due to condensation was also |accounted. I

The analysis indicates that heat and mass transfer from the steambubble owing to condensation on the pressurizer walls may reduce

. the peak pressure by about 8 or 10 psi -- not negligible, but alsonot a substantial error.

On most B&W units the spray valve is operated by a fast acting

motor. Reactimeter data from the TMI-1 turbine trip test indicate

an opening delay of about two seconds. Based on an analysis with 3

the Sensitivity Study model, a six-second time constant on spray1

valve opening produces an increase in the peak pressure of about7 psi. It is concluded that the error due to ignoring the sprayvalve delay in the OTSG model is small -- probably no more than5 psi -- in the nonconservative direction. The error will tend to {be offset by the also small error due to neglecting heat transferto the walls.

For the PORV to arrest the pressure increase of Figure 5 (which is3produced by a 10 ft /second insurge), a capacity more than twice

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- design would| 6e required.' F'or the PORV to reverse the pressure;.

h trise-'(as occurs at about 5.6 s9conds');a capacity close to thrpe,

. times' design would be required. Though variations. of PORV' capacity.,

from design values |have been noted in service none has been thislarge.

' For the ; spray flow to arrest the pressure rise of Figure 5,.an.' effectiveness .f ar. greater than.. assumed - by the 'model would be -

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required. - An increase .'in .effe'ctiveness (or flow rate) by an orderof magnitude would be required to explain.the observed response.

. This|is not considered plausible. '

|- During an .insurge some' steam from the steam bubble will condense'on

the surf ace 'of tne pressurizer liquid. However, if the surface is

placid, the newly condensed liquid will serve as an insulator to'further heat and mass transfer. A calculation of the effect on

- pressure of such condensation indicates that the peak. pressure- prediction for the TMI-1 turbine trip transient would be reduced byabout 1 psi -- a' negligible error.

As noted in the study report, however, a more substantial reductioncould occur if there is significant " roiling" of the surface --mixing of' condensed liquid on the surface with cooler liquidbelow. Such " roiling" could be produced by the insurging fluidfrom the surge line. All pressurizer designs analyzed in the Studytake steps to dissipate the kinetic energy of the insurge either bydirecting it horizontally (B&W) or by diffusing it through ascreen-like device (Westinghouse and CE). There is evidence,however, that following the turbine trip transient at TMI-1, theinsurging fluid is creating turbulence in the pressurizer liquid asa whole. The oscillations in the actual pressurizer level response(Figure 3) described above are indicative of this turbulence --

oscillating fluid velocities of five to six feet /second amplitude

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in the vicinity of the tap would produce the oscillations.!

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recorded. The surge line at TMI-1 is about 45 feet long. Thevelocity produced by the insurge averages over 20 feet /second overthe first 5 or.10 seconds of the transient. Water from the coolanthot ieg will- make its way into the . pressurizer about two secondsafter the turbine tri p. If some of this water makes its way

unmixed to the liquid surface of the pressurizer, the condensation.

of steam produced thereby could mitigate the pressure rise producedby the i ns urge .

The arrival of relatively cool water from the coolant 1 cop at thesurface of pressurizer liquid at about 4 seconds is plausible -- itis consistent with a two-second transport delay in the surge line,and an upward velocity component of about 10 feet /second (half thesurge line velocity) in the pressurizer itself. Si gni ficant I

condensation of steam brought about by the presence of cool liquidat the steam / water interface is considered the most plausible

explanation for the early turnaround of the coolant pressureresponse in Figure 5.

* * * * * * * * * * * * *

!

Summing up, the Sensitivity Study's calculation of pressurizerlevel response appears to match actual plant response well; anincrease in the model sophistication by a more detailed descriptionof the steam generator is not required to obtain adequateaccuracy. The pressure increase predicted by the model for therapid insurge produced by a full load rejection is greater thanactual. Calculational means to reduce this discrepancy are notobvious, however, since it appears to be due to turbulent mixing of ]pressurizer liquid with insurging fluid, and the consequent removalof heat and mass from the steam bubble by condensation. Further- 4

l more, the model is clearly conservative, in that it overstates theIpressure increase produced by an insurge.

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k< - ) The' Sensitivity. Study concluded that the pressure response of a B&W-. unit to a turbine trip .is inherently limited and therefore does not -require an . automatic reactor trip. A conservative'model is a-

!satisfactory basis for this conclusion.1

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