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AN EVALUATION OF BORON PRECIPITATION-

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DURING LONG-TERM COOLING:

FOLLOWING A POSTULATED LOSS OF COOLANT ACCIDENT;_

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I. Introduction

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Reactor system circulation and boron concentration in the post-LOCA,long-term cooling environment have been analyzed for the potential ofunacceptably high boric acid concentrations in the core region. Bothlarge and small breaks of the reactor inlet and outlet pipes are con-sidered and the large break of a reactor vessel inlet pipe is shownto be the most limiting case.

The analysis indicates that natural circulation within the reactorvessel, where the flow path is downcomer-core-upper head-vent valvesdowncomer , provides adequate circulation to prevent rapid increases insolute concentrations, including the limiting case. The resulting slowconcentration buildup indicates that a time period in excess of 30 daysis available for alignment and operation of alternate flow paths. Ifboth Low Pressure Injection (LPI) strings are operable, then the pro-cedure, one day af ter the postulated LOCA, is to establish suctionon the reactor vessel outlet pipe (hot leg) through the decay heat linewith one LPI string. This will force flow from the LPI string throughthe core. This procedure, (Mode 1), will limit concentration buildupto a factor of two or less.

In the event the decay heat line is not available, the procedure (Mode 3)is to open the auxil,iary spray to the pressurizer. This will act ashot leg injection by routing dilute injection to the area above thecore. The flow path will be through the auxiliary spray line into thepressurizer, out of the pressurizer through the surge line, into thehot leg and then into the reactor vessel. This flow path will act firstas a dilutant for care water and then, reversing the direction of coreflow, providing long-term sensible heat removal. The concentrationbuildup rate decreases as auxiliary pressurizer spray becomes effectiveand the concentration gradually decreases as sensible heat removalproceeds. The minimum auxiliary spray capacity is 40 gpm. Thisauxiliary spray rate, if required, will limit concentrat on buildup toC/Co=ll.

The decay heat line may not permit circulation of saturated water because- the line routing takes it above the hot leg pipe. This decay heat lineis, however, capable of being aligned (Mode 2) such that a connectionfrom the LPI pump discharge to the decay heat line will route flow back-wards through this line into the hot leg. This will route diluteinjection into the area above the core. The analysis description thatfollows describes hot leg injection by way of the auxiliary pressurizerspray. However, the hot leg injection results presented.are independentof source, e.g., auxiliary pressurizer spray or reverse flow through thedecay heat line.

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II. Discussion of Analysis

1

A. Reactor Inlet - Large Break]

The reactor vessel internal natural circulatior. flow paths are shownin Figure 1. In this circulation mode, the driving head for thecore flow is provided by downcomer fluid. The downcomer head issufficient to promote significant core flow and to provide anadequate pressure differential to open the vent valves.

The core flow rate as a function of time is shown in Figure 2. Thisflow rate from the core and through the vent valves is assumed tomix with the injection flow (3000 gpm) and exit from the vesselthrough the inlet break. For concentration calculations, however,the concentration entering the core is conservatively assumed to beequal to that leaving the core at a given time step. (In effect,no boron is assumed to leave the vessel through the break.) Theresulting concentration ratio of boron as a function of time isshown in Figure 3. The resultant natural circulation flow is suchthat very low quality mixture is produced which produces relativelylow concentrating rates. The rate of concentration is such thatalignment of an alternate flow path is not required for a time periodin excess of one month.

However, in order to provide measurable assurance that boron con-centration is minimized, alignment of the decay heat line isperformed within one day af ter the postulated LOCA. The flow pathin the reactor vessel in this flow mode called " forced circulation,"is shown in Figure 4. The minimum core flow in this " forced circu-lation" mode, assuming no density gradient in the core is approxi-mately 118 lb/sec, whichis more than adequate to provide decay heatremoval without evaporation within five days. Figure 5 shows theboron concentration change and peak value following alignment of thedecay heat line after one day of natural circuation operation.

In the event that the decay heat line is not operational (failureof isolation valve to open), the alternate system to promote lowboron concentration and to dilute the boron concentration is align-ment of the auxiliary pressurizer spray. The flow paths for thismode of operation (defined as " hot leg injection") are shown inFigure 6.

Hot leg injection has little effect on circulation and concentrationuntil the steaming rate, assuming evaporation only from the core, isequal to or less than the bot leg injection rate. As the injectionrate approaches the steaming rate the hot leg injection acts toreduce the rate of core boron concentration by adding non-concentratedwater. As the decay heat rate decreases with time., the hot leginjection becomes adequate to remove decay heat by sensible h::tremoval. The core flow direction reverses, and the concentration ofboron in the core becomes dilute with time. The concentration withtime for " hot leg injectica" is shown in Figure 5, for an injectionrate of 40 gpm.

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Figure 7 shows the model used for calculating the concentration )after the decay heat letdown is opened or after the pressurizer l

spray injection is initiated. The concentration, C,' is given by )

C = Mass SoluteMass Solvent

C= cCi+f in in "tft out

c

!o"# "+ t1 dt - 1 dt (1)o e o e o

where Ci is the concentration at t1tl is time of vent valve opening or spray initiationCo is same reference concentration

Equation 1 is used to calculate the concentration ratio after thenatural circulation phase of the transient is terminated.

The important assumptions used for this part of the transient are:

(1) Constant mixing r. ass, Mc = 30000 LBM

(2) 40 gpm flow rate from pressurizer sprays

(3) Constant LPI injection rate and concentration.

(4) No' density gradients in calculation of forced circulation

.

(5) 1000 BTU /LBM needed to produce steam

(6) Maximum flow out break for letdown line case

(7) K-factors based on full flow values -

Summary: Reactor Inlet - Large Break

The results of the calculations are shown in Figure 5. They showthat the maximum concentration ratio for the letdown line case is1.19 at 1 day; the maximum concentration ratio for the hot leginjection case is 11 at 60 days. The solubility curves show that aconcentration ratio of 35:1 is needed before precipitation willoccur. Thus, it has been shown that the proposed method for con-trolling concentration buildup in the long-term cooling phase of aLOCA is credible. This is particularly true when the conservatismsinherent in the analysis are considered. These conservatisms include:

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_ ___ _. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - _ _ _ _

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(1) No dilution of injection E 0 is considered2

(2) Absorption of boric acid by concrete is neglected(3) Volatility of boric acid was neglected

B. Pump Suction - Large Break

For large breaks postulated at or in the pump suction line, the flow l1paths are as shown on Figure 8. In this case the leak fluid must pass

through the RC pump which is some 3 feet above the centerline of coldleg nozzle. This condition provides increased downcomer driving headfor core flow which results in more core flow and a lower outletquality and boron concentration when operating with LPI inj ection," Natural Circulation mode." In this case, the core flow versus timewill be equal to or greater than that shown in Figure 3.

Operations in the forced circulation mode will produce flow patterns |

equivalent to that shown in Figure 9, with a near constant minimumflow rate of approximately 118 lb/sec, which is more than adequateto provide decay heat removal without evaporation within five days.Switch over to this mode will provide a concentration ratio equalto or less than that shown in Figure 5.

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In the event the decay heat line is not operational, the alternate |system, pressurizer spray, dilutes the boron concentration as !described iE"Section II.A. above. The hot leg injection flow paths ,

are shown in Figure 10, and the concentration ratio versus time is Iequal to or less than that shown in Figure 5. !

C. Reactor Outlet - Large Break

in the case of a postulated large break, the flow paths are as shownin Figure 11. Any parallel alignment such as opening the decay heatline, or hot leg injection, will produce little or no effect. Thiscase corresponds to the forced circulation cold leg break with thedecay heat line open, except that all injection water flows throughthe core. The minimum core flow with one LPI pump operating isexpected to be 3000 gpm (416 lb/sec) which will prevent boiling andconcentration within approximately 18 hours. The resulting concen-tration ratio, then will be significantly less than that shown inFigure 5.

D. Reactor Outlet - Top of 180 Bend - Large Break

In the case of a postulated large break at the top of the 180 bend,reactor outlet, the overall flow paths are as shown in Figure 12.With one LPI injection pump assumed operating, the injection flowwill split approximately 50/50 between the ' core and steam generatorsuntil the steam generator stored heat is removed (approximately oneday). During this period the flow paths within the vessel areapproximately those of forced circulation. The corresponding coreflow rates are equal to or greater than (approximately 200 lb/sec)those shown in Figure 2, because of increased drit ing head.

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the decay heat line with one LPI string (see Figure 13). The flowis E cough existing valves LP-1, LP-2, LP-3, LP-4 (for Units 1 and 2),LP-9, LP-12, LP-17 and into the reactor vessel through the core flood ,

nozzle. I

B. Mode 2 - Hot Leg Injection Through the Decay Heat Line i

|This flow path uses a connection from the LPI pump discharge to the |

decay heat line to route flow backwards through the decay heat lineand into the hot leg.

The flow path will be different for Oconee Units 1 and 2 and Unit 3.Oconee Units 1 and 2 have a cross connect (8" line) from LPI String B(upstream of the LPI cooler) to the decay heat line. Flow path isthrough the cross connect (valve LP-68) into the decay heat line andbackwards in the decay heat line through valves IP-3,12-2 and LP-1to reach the hot leg. Oconee Unit 3 does not have this cross connectto the decay heat line. For Unit 3, cross connects to the letdownline of the HPI System will be used. The cross connect from LPI StringB cooler outlet is a 2" line connecting the HPI System letdown linedownstream of the letdown control valve and upstream of the demineralizers.A return cross connect leaves the HPI System letdown line downstreamof the purification demineralizers but upstream of the letdown storagetank and connects into the LPI System downstream of the valve IP-4 inthe decay heat line. The normal functiorof these cross connects tothe HPI System is to obtain purification flow during cold shutdownaf ter the HPI System is shut down. When using this path during long-term cooling, the filters and demineralizers in the HPI System wouldbe bypassed. Units 1 and 2 will be able to achieve, as a minimum,several hundred GPM into the hot leg with only one LPI pump operationaland a large majority of the pump flow split between the two LPI lines.Unit 3 will be able to achieve a flow rate greater than the 40 GPMminimum into the hot leg.

C. Mode 3 - Open Auxiliary Spray Line to Pressurizer

This flow path uses an HPI pump taking suction from an operating LPIstring through cross connect valves LP-15 or LP-16. The flow pathto the hot leg will be through the auxiliary pressurizer spray linewhich is through valves HP-355, HP-340 and LP-45. Changes are requiredto make this a workable flow path as there are manually operated valves

[ inside the reactor building which must be opened and closed duringl the long-term cooling period to establish this flow path. Changes

required are shown on Figure 3 and are: (1) change valve LP-45 tolocked open, (2) add on an electric motor operator (EMO) to valveHP-340, and (3) add on EHO to valve HP-356. Valve HP-356 mustremain open during normal operation to provide a continuous minimummakeup line flow to minimize thermal transients on the RC pipe nozzle.This valve (HP-356) must be closed for Mode 3 to prevent the flowfrom diverting to the HPI line. Valve HP-340 must remain closed

| during normal operation to prevent the continuous minimum makeup line| flow from diverting to'the pressurizer. The flow rate for this path

| to the hot leg is controlled and limited to 40 GPM by the block orificedownstream of valve HP-340.

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IV. Single Failure Analysis

The three modes listed above are necessary to satisfy the single failurecriteria. Operations must be designed for a single failure in eitherthe short-term or long-term cooling period but not a single failure inthe short term and then another single failure in the Ivag-term period.Since the operator may not be able to determine the location of thebreak, Mode 1 is attempted first (if both LPI strings are operable) todetermine if the normal decay heat removal path can be established. It

can be established if the break location is high enough in elevation sothat the decay heat suction nozzle on the hot leg is sufficiently floodedto prevent gas or steam entrainment at the LPI pump flow rate. Even ifthere is no single failure during the attempt, success is not assuredbecause of the possibility of gas or steam entrainment in the decay heat

, suction nozzle. If Mode 1 fails because LPI pump flow is erratic orloses prime, then Mode 2 is attempted. Success or failure of Mode 2 willbe indicated by flow indicators. On Oconee Units 1 and 2, opening of thelast valve to establish reverse flow in the decay heat line will causethe LPI string flow to decrease. On Oconee Unit 3, a flow indicator inthe letdown line of the HPI System will indicate the reverse flow ratein the decay heat line. If Mode 2 fails due to single failure, thenMode 3 is performed. The above was for a single failure in the long term.The single failure could occur in the short term with the single failurebeing such that only one LPI string is operable. Mode 1 then cannot beattempted because prime could be lost on the only LPI pump operating. So,Mode 2 would then be performed.

All valves located within the reactor building, that must be operated inany of the three modes will be electric motor operated and locatedoutside the secondary shield wall. All of the existing valve EM3'sare qualified for the LOCA environment and all EM0's to be added will

i

be qualified EM0's. If power is not available to any of the EM0'srequired to implement any of the three modes, electrical jumper cableswill be used to connect power to the EMO controller.

V. Operating Procedures for Long-Term Cooling

The three operating modes which have been described for long-term coolingcannot presently be fully effected. The existing equipment is adequateto provide the necessary flow paths for Modes 1 and 2; however, dosecalculations are not completed for areas of the Auxiliary Building inwhich manual valves must be operated. Operating Mode 3 cannot beimplemented until electric motor operators are added to valves HP-340and HP-356 which are located in the Reactor Building and valve LP-45 ismodified to be locked open.

The ECCS systems will be placed in one of the following three modes ofoperation (Mode 3 will only be possible after modifications) within oneday of the accident. Injection flow to the RC System'should be maintainedthrough two paths while attempting to place the system in one of thethree operating modes. The two injection flow paths can be either thetwo LP injection lines or one LP injection line combined with one HPIstring (LPI pump acting as a booster pump for the EPI pump).

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Mode 2 - Route Flow Backwards Through the Decay Heat Line Into theHot Leg

Oconee Units 1 and 2

a. If Mode 1 is not successful, maintain injection flow to reactor vesselthrough two injection flow paths by any of the following:

(1) Two LPI strings

(2) One LPI pump operating with LPI pump discharge header open(valves LP-9 and LP-10 open) and the flow split between thetwo injection lines by the control valves.

(3) One LPI string in series with one HPI string.

b. Decay heat line EMO valve LP-1, LP-2, and LP-3 are open. Close manualvalve LP-4.

c. Open manual valve LP-68 in cross connect from LPI String B to decayheat line.

,1) If two LPI pumps are operable, use String A to inject through(LPI line to reactor vessel. Use String B's full flow throughthe cross connect (valve LP-68) to decay heat line and into thehot leg. Close String B LPI line EMO valves LP-18 and LP-14.

(2) If only one LPI pump is operable, split the flow between oneLPI line to the reactor vessel and the cross connect path to thehot leg. The flow split is accomplished by throttling the LPI

line EMO control valve (LP-18) and manual valve LP-68 in thecross connect. Flow split indication is confirmed by LPI pumpdischarge pressure indication remaining the same (same as for3000 GPM LPI line injection flow rate) and the LPI line flow

| rate indicator at approximately 1500 GPM.

d. If Mode 2 is successful, injection flow to the reactor vessel is-

through two injection flow paths and dilute injection is reachingthe area above the core (if the break is a cold leg break).

Oconee Unit 3

a. Step a, for Oconee Units 1, 2 above,.is applicable.

b. Decay heat line EMO valves LP-1 and LP-2 are open. -EMO valve LP-3in decay heat line is closed.

c. Line up cross connect (valve IP-96) flow path from LPI String B toHPI System letdown line. Complete the flow path up to the returncross connect (valve BP-363) to the LPI System. In the HPI System: letdown line, isolate the filters and demineralizers and open thebypasses around them. Close the inlet valve to the letdown storagetank.

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d. LPI pump P1A must not be operating and its suction EMO valvesLP-5 and LP-6 nnst be closed. The building spray pump assSciatedwith LPI String A must be shut off and the sump EMO outlet valveLP-19 must-be closed. Open manual valve HP-363 in return crossconnect-to LPI System. Open EMO valve LP-3 in decay heat dropline.

; e. The flow path is now complete. The flow rate indicator in the HPISystem letdown line indicates the flow rate in this path from LPIString B to EPI System letdown line to LPI String A suction to decayheat line and backwards through this line to the hot leg.

1

Mode 3 - Open Auxiliary Spray Line to Pressurizer

i a. This operating mode will be used if Mode 2 is not successful.

b. .Close main pressurizer spray line EMO valve RC-1 or RC-3 or both.2

,

* c. Open auxiliary spray line EMO valve HP-340. Open manual valve HP-355; in Auxiliary Building. C1cse EMO valve HP-356 to force flow to the

auxiliary spray line,

d. Start any one of the three HPI pumps taking suction from an operatingLPI string through the cross connect.

e. HP injection can be stopped by closing injection valves HP-26, HP-27,and HP-120 or HP injection flow can be maintained in parallel withthe auxiliary spray flow by controlling valve HP-26 or HP-120.Injection flow to the reactor vessel should be maintained through

| two paths per Item a of Mode 2. Auxiliary spray line flow rate isindicated by the flow measurement upstream of manual valve HP-355(normal makeup line flow indication).

VI. Summary of Results.

The reconsnended operating procedures for Oc'onee 1, 2, and 3 to minimizeboron concentration following a LOCA are, in order of preference:

i A. Always maintain a minimum of 3000 GPM LPI injection into the down-comer. This provides for a natural circulation flow path within,

the reactor vessel and the maximum concentration buildup is C/Co =1.19 for the first 24 hours after a LOCA. (Co is initial concentration- 2200 ppm boron solution.)

| B. Align decay beat line in a low flow mode within 24 hours after-the LOCA (see Step e of Mode 1 in Section V) which provides a,

j minimum core flow of 500 gpm. This is called a force circulationmode and if' successful will provide a maximum concentration ratioof 1.3 as shown in Figure-5.

| In addition, if this operating mode is successful and operation proceedsI to the normal decay heat system flow rate, the forced core flow is

3000 gpm =in4== resulting in even lower concentration ratios (< 1.19)_

as shown in Figure 5.

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C. Should Step B not be successful, the decay heat line is alignedfor a reverse flow of 40 gpm minimum within 24 hours af ter the LOCA.This is called hot leg injection and limits the concentration ratioto 11.0 at 60 days (Figure 5). If larger reverse flow rates areavailable through the decay heat line, the concentration ratio will

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be held to a smaller value 1.9 at 7 days (140 gpm hot leg injectionFigure 5).

,

D. In the event that the decay heat line is not operational(single failure), the auxiliary pressurizer spray ficw is alignedto provide 40 gpm minimum flow within 24 hours after the LOCA.This mode is also called hot leg injection and produces the sameresults as discussed in C above. (C/Co=11.0 60 days)

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