ML20044G135

Evaluation of Effect on Plant Operation of MSIV Low Turbine Inlet Pressure Isolation Setpoint Change at Pilgrim Nuclear Power Plant.

ML20044G135
Person / Time
Site:	Pilgrim
Issue date:	07/31/1986
From:	Alvarez E, Cornwell K, Sozzi G GENERAL ELECTRIC CO.
To:
Shared Package
ML20044G126	List: ML20044G125 ML20044G133 ML20044G135 ML20044G140
References
MDE-70-0586, MDE-70-0586-R01, MDE-70-586, MDE-70-586-R1, NUDOCS 9306020068
Download: ML20044G135 (15)

Text

4 8 MDE-70-0586 REVISION .1,

DPJ B21-00254 July 1986 EVALUATION OF THE EFFECT ON -

4 PLANT OPERATION OF MSIV LOW TURBINE INLET PRESSURE ISOLATION SETPOINT CHANGE AT PILGRIM NUCLEAR POWER STATION t

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P Prepared by:

F E. M. Alvarez K. F. Cornwell i

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r$ q { x Approved by: 8 ;/ h m-G. L. Sozzi'," Manager Application Engineering Senrices i

f GENER AL h ELECTRIC 9306020068 930520 PDR ADOCK 05000293

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IMPORTANT EDTICE RECARDING CONTENTS OF THIS REPORT  ;

4 Please read carefully The only undertakings of General Electric Company respecting information in this document are contained in the contract between the customer and General Electric Company, as identified in the purchase order for this report and nothing contained in this document shall be construed as changing the contract. The use of this intonnation by anyone other than the customer or for any purpose other than that for which it is ,

intended, is not authorized; and with respect to any unauthorized use, General Electric Company makes no representation or warranty, and.

assumes no liability as to the completeness, accuracy, or usefulness of the information contained in this document.

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.- .i TABLE OF CONTENTS

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1. INTRODUCTION 1-1 .!

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2. BACKGROUND 2-1  !

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SUMMARY

AND CONCLUSIONS 3-1 f i

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4. ANALYSIS 4-1  !

4.1 Simulation of the Pressure Regulator 4-1 {

Tailure (Open) Event 4.2 Resu3ts of the Pressure Regulator 4-2  ;

Tailure (Open) Event i i

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5. REFERENCES 5-1 l

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ILLUSTRATIONS j i

Figura Title Page

, 4-1 Pressure Regulator Failure (0 pen) Response 4-4 4

with MSIV Isolation at 750 psig for Pilgrim -

Nuclear Power Station t

4-2 Water Level Swell for Pressure Regulator 4-5 Tailure (0 pen) a 9

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1. INTRODUCTION  !

t The purpose of this analysis is to document the evaluation of the effect on plant operation associated with reducing the main steam ,

isolation valve (MSIV) low pressure isolation analytical limit

• from the 4

present setting of 880 psig to 750 psig at the Pilgrim Nuclear Power L Station (PNPS).  ;

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The MSIV low pressure isolation setpoint is a part of the reactor j vessel isolation control system, and its main purpose is to prevent j excessive vessel depressurfration. Lovering the setpoint introduces the }

possibility that the vessel will depressurire for a longer time period prior to isolation. Two items warrant consideration due to the extended depressurization time. First, the thermal gradient and resulting thermal stress imposed upon the vessel and internals will be larger due to the i lowered isolation pressure setpoint. The results of the thermal stress

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evaluatien and other safety issues are documented in Reference 1.

Secondly, the extended vessel depressurization results in more coolant .

I voiding and consequently the potential for a higher bulk water level i swell. This document describes the analysis performed to evaluate the  !

irpact of the water Icvel swell on plant operation for PNPS.

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• The term " analytical limit" is defined as the value of the sersed

.. process variable established as pert of the safety analysis, prior to )

1 which a desired action is to be initdeted to prevent the process variable i from reaching the associated design safety limit.

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2. BACKGR0l'ND  ;

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The single failure which produces the highest depressurization rate l and consequently the maximum level swell is the Pressure Regulator j Failure (open) transient. This failure results in the highest level swell .l vhen initiated from low power conditions. Under these conditions the lov l pressure MSIV isolation may occur when the water level in the vessel is  !

above the steamline nozzle. Thus, the MSIV closure may result in some liquid being trapped in the steamlines between the vessel and the inboard  :

r MSIV. If this occurs, there is a potential that the safety relief valves  !

I (SRVs) may be required to open, discharging high pressure liquid or two phase flow. l i

Previous analyses (Reference 2) have been performed which conservatively estimate the quantity of trapped liquid in the steamlines  !

due to the initiation of the Pressure Regulator Failure (open) transient }

from a low reactor power. These analyses were perforraed for a BWR plant l l vith both full and partial bypass capacity, and at an MSIV low pressure  !

isolation setpoint of 825 psig. The results of these analyses l demonstrated that the quantity of trapped liquid in the steamlines was not -sufficient to fill the portion of the steamline where the relief

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s valves are mounted, thereby demenstrating that the SRVs vould not be

! required to discharge high pressure liquid or two phase flow.

The probability of this transient occurring at low power levels. l

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3 below 40%, is extremely low (4x10 /yr), mainly due to the infrequent l operation at these low power conditions. For reactor powers of greater f

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9 than 40% the probability increases to less than 6x10 '/yr (Reference 2). t

- As the reactor pcwer increases, howevct, the consequences of level swell

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!, are reduced because the depressurization rate caused by the Pressure ,

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! Regulator Failure (open) transient is slower. This results in lower peak water level swells and a decrease in the predicted quantity of liquid l trapped in the steamlines. l

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In conclusion, if it can be shown that the quantity of Ifquid .

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trapped in the steamlines does not impact SRV performance, then no  !

operational concerns vill arise due to the reduction of the MSIV low  ;

i pressure isolation analytical limit to 750 psig. j k

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3.

SUMMARY

AND CONCLUSIONS 4

A Pressure Regulator Failure (open) transient was simulated for the PNPS f ro:n low power conditions with a MSIV low pressure isolation analytical limit of 750 psig. The intent of the aralysis was to quantify

, the amount of liquid predicted to be trapped in the steamlines. From this analysis it was determined that the water level did not increase to the bottora of the steamline elevation. Hence, no liquid would be trapped in the steamIfnes as a result of the transient and consequently SRV perforcance is not impacted.

Therefore, it is concluded that reducing the MSIV pressure isolation analytical limit to 750 psig will not introduce any operational concerns at PNPS.

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4. ANALYS7S

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l In order to determine if plant operation is impacted it is necessary to evaluate the quantity of trapped liquid in the steamlines with the {

v reduction of the MSIV low pressure isolation limit from 880 psig to 750  :

psig. This is done by examining the results of the Pressure Regulator  !

Failure (open) transient. The maximum amount of liquid predicted to be

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trapped in the steamline is obtained by initiating the transient from a i low power condition. Thus, the analysis is bounding for all possible '

I operating power / flow conditions, i l

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Simulation of the Pressure Regulator Failure (Open) Event e

The turbire at PNPS is provided with two pressure regulators. These  !

two regulators have slightly different pressure setpoints such that one  ;

functions as a controlling regulator and the other as a backup pressure  !

regulator. The controlling pressure regulator is used to control both the turbine control valves and the turbine bypass valves to maintain

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constant turbine inler pressure. If either the controlling pressure l regulator or the backup pressure regulator fails in an open direction, j it will~ cause the main turbine control valves to respond by opening I

further, thus increasing steam flow and dropping turbine inlet pressure.

In a short time, the pressure drops to the MSIV closure (MSIVC) setpoint which initiates isolation followed by a reactor scram. The ;

t 4 depressurization of the reactor may also cause an increase of the bulk  !

3 fluid void volume which can produce a level swell. If the resulting depressurization is rapid, the vessel water level may reach the high trip level (Level B) before the turbine inlet pressure drops to the MSIV low pressure isolatien setpoint. In this case, the high level trip initiates

, a main turbine stop valve closure (MTSVC) and possibly a feedwater pump trip. The MTSVC in turn initiates reactor scram. With MSIVC or MTSVC (whichever occurs first) the pressure decrease will ultimately be terminated by the MSIV low pressure isolation.

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t The severity of the water level swell is directly related to both L

the vessel depressurization rate and the initial power level assumed for the 'recsure Regulator Failure (open) transient. The most severe water level swell is achieved when the Pressure Regulator Failure (open) transient is assurred to occur at initial conditions of 2% power /30% flow. l The low power level is more severe because at a lower initial power there ,

is proportionally less steam generation which produces a more rapid blevdown. This results in note flashing and a higher water level swell. ,

The GE thermal-hydraulic and nuclear kinetics coupled transient

- code, REDY (Reference 3), is used to evaluate the dynamic system response to the Pressure Regulator Failure (open) event previously discussed. The j following basic assumptions and initial conditions are used:

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. 1. The initial reactor power is at 39.96 MWt (2% rated).

2. Initial dome pressure is 963 psia. '

3. Initial core flow is 30% of rated core flow.

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4. Conservative end-of-cycle scram, void and doppler reactivity ,

- are assumed, based on Cycle 7 fuel loading conditions.

5. The scram is initiated by the MSIV closure which actuates the

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position scram signal. l

6. The MSIV closure time is 5 seconds. l

7. The turbine bypass valves, with 25% capacity, are open for i faster depressurization.  !

8. The pressure regulator upper limit is set at 125% of steam flov ii demand.

i 4.2 Results of the Pressure Regulator Failure (0 pen) Event

• The results of the Pressure Regulator Failure (open) transient calculation are presented in Figure 4-1 for the MSlv low pressure-isolation setpoint of 750 psig. In both figures the vessel steam flov (curve a.5) initially increases rapidly as the turbine control valves

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4 open due to the pressure regulator failing in the open direction. With-4 4-2 l i

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the high steam outflow, the ve.ssel depressurizes (curve b.1) which causes

the water lerel to swell as the bulk fluid void volume is increased ,

(curve c.1 represents the reactor water level inside the dryer skirt).

At approximately 2.4 seconds, the water level reaches the high water r

level (Level 8) setpoint which initiates a MTSVC. A feedwater pump trip occurs shortly thereaf ter when the water level rises an additional foot l I t to the trip setpoint. The vessel steamflow is reduced to that of the bypass flow due to the MTSVC and the vessel continues to depressurize .

until the low turbine inlet pressure is reached at 28 seconds. At this  !

time the MSIV closure is initiated. This terminates the vessel  !

4 i depressurization and the level increase, in addition to initiating a  ;

reactor scram. After the MS1V closure the vessel begins to slowly.

repressurize and the reactor water level recedes.

1 FiFure 4-2 shows the reactor water level response outside the dryer l skirt relative to the location of the bottom of the steamline. The water j level does not re::ch the steamline elevation during the course of the i transient and hence no liquid will be trapped in the steamlines as a  !

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result of the MSIV closure.  :

Therefore, from the results discussed above it is concluded that the j plant operation at PNPS will not be impacted as a result of reducing the  ;

,e analytical limit of the MSIV le" pressure isolation setpoint to 750 psig.  ;

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TIGURE 4-1 i P/ISSURE REGULATOR FAILURE (OPEN) RESPONSE WITH MSIV .

ISOLATION AT 750 PSIG FOR PILGRIM NUCLEAR PO'w'ER STATION  !

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FIGURE 4-2 4

WATER LEVEL SWELL FOR PRESSURE REGULATOR FAILURE (OPEN) 2% INITIAL POWER, 30% CORE FLOW, PILGRIM NUCLEAR POWER STATION 11 10 -

ROTTOM OF STEAM I,INE ELEVATION 9-C 8-4 d

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5. REFERDiCES I i

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1. " Safety Evaluation of MSIV Low Turbine Inlet Pressure Isolation Setpoint Change for Pilgrim Nuclear Power Station, General Electric  !

t Company, May 1986 (NEDO-31296). [

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2. D. B. Waters, BVR Owners Group, Chairman; letter to R. H. Vollner, Nuclear Regulatory Commission, Director, dated September 17, 1980.  :

"NUREG-0758 Requirement 2.1.2 - Performance Testing of BWR and PWR [

Relfef and Safety Valves."

3. " Analytical Methods of Plant Transient Evaluations for the General j Electric Boiling Water Reactor," General Electric Company, February 1973 (NEDO-10802).

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Attachment E to BEco Letter #93-062 P

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