ML20028G622

Forwards Addl Info Re Instrumentation for Detection of Inadequate Core Cooling.Info Closes Out SER Outstanding Item 9

ML20028G622
Person / Time
Site:	Byron, Braidwood, 05000000
Issue date:	02/08/1983
From:	Tramm T COMMONWEALTH EDISON CO.
To:	Harold Denton Office of Nuclear Reactor Regulation
References
5946N, NUDOCS 8302170149
Download: ML20028G622 (21)

Text

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[ N Commonwealth Edison

) one First Nationil Plaza. Chictgo. Illinois

[p.

7 Addr:ss R: ply to: Post Office Box 767

/ Chicago. Illinois 60690

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February 8, 1983 Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555

Subject:

Byron Station Units 1 and 2 Braidwood Station Units 1 and 2 Instrumentation for the Detection of Inadequate Core Cooling NRC Docket Nos. 50-454, 50-455, 50-456, and 50-457-References (a):

October 27, 1982 letter from B. J.

Youngblood to L. O. De1 George.

(b):

June 7, 1982 letter from T. R. Tramm to H. R. Denton.

(c):

August 13, 1982 letter from T. R. Tramm to H. R. Denton.

Dear Mr. Denton:

This is to provide additional information regarding instrumenta-tion to be installed at our Byron and Braidwood stations for the detection of inadequate core cooling.

Review of this information should close Outstanding Item 9 of the Byron SER.

Attachment A to this letter contains responses to eight questions transmitted in reference (a) relative to information provided in references (b) and (c).

A summary of all this information will be included in the Byron /Braidwood FSAR in Appendix E.

Please address further questions to this of fice.

One signed original and fifteen copies of this letter and the attachment are provided for your use.

Very truly yours, TI N

oc><>l l

8302170149 830208 V

PDR ADOCK 05000454 T. R.

Tramm E

PDR Nuclear Licensing Administrator 1m Attachment 5946N

ATTACHMENT-A ADDITIONAL INFORMATION REGARDING INSTRUMENTATION'FOR-THE-DETECTION OF INADEQUATE CORE-COOLING BYRON /BRAIDWOOD UNITS 1 & 2 Question 1 Describe the planned modifications to the HJTC probe and/or to the reactor vessel and components to accommodate the C-E design in a Westinghouse plant.

Response

I.

Pressure Boundary Modifications Modification of the reactor head pressure boundary at two spare control rod drive mechanism (CRDM) locations is required to provide a penetration for each of the two heated junction thermocouple (HJTC) probe assemblies.

At the two locations selected for the HJTC probe assemblies (shown in Figure 1), the pressure boundary modifications consist of:

1.

Removal of the existing blank flange at the upper end of the spare CRDM.

2.

Replacement with a new pressure housing assembly (See Figure 2).

The pressure housing assembly consists of a standard quick disconnect flange, a pressure tube and a CRDM adaptor plug.

The assembly is manufactured in the shop and field installed.

The pressure housing assembly is threaded on to the spare CRDM nozzle and the Omega seal weld is completed.

The elevation chosen for the new flange was chosen to provide ease of accessibility for installation and electrical and mechanical disconnects during a refueling outage.

Since the HJTC probe remains with the upper internals during a refueling operation, a bellmouth sleeve is installed in the new pressure housing assembly (see Figure 3) to provide a lead-in and smooth path for the HJTC probe when the reactor vessel head is installed.

The bellmouth sleeve is virtually identical to those that are currently utilized for the same

purpose, i.e., the installation of the reactor vessel head over the rod cluster control assembly (RCCA) drive shafts.

" II.

Reactor Internals Modifications To protect the HJTC probe during operation and postulated

~

accidents requiring operability of the probe, a protective guide path for the probe in the upper internals is needed.

The protective guide path is provided by a probe holder assembly.

The probe holder assembly consists of a~RCCA guide thimble that is conceptually identical to the existing plant equipment, modified to provide a cylindrical guide path for the probe assembly and perforated at discrete locations to provide hydraulic communication between the interior of the RCCA guide thimble and the upper plenum.

The probe holder assembly is installed in the upper internals by removing the existing cover and orifice plates at the two designated spare RCCA locations and replacing them with the probe holder assembly.

The installed probe holder assembly is held in place utilizing the bolting and locking arrangement employed in the original design of the upper internals.

Figure 3 shows the reactor vessel internal arrangement of the probe holder assembly.

There are no modifications other than overall length adjustments of the Byron /Braidwood HJTC probes from the System 80 HJTC probes.

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. Question 2 Provide analysis, evaluation and test data to describe the performance of the HJTC system in the Braidwood plant.

Include the relationship of the shroud geometry to the hydraulic forces in the specific vessel locations for operation with main coolant pumps on or off and with or without voiding in the primary coolant system.

In the later designed (System 80)

C-E plants is was necessary to split the shroud to avoid the effects of differential pressure across the upper support plate which could yield incorrect indication of collapsed water level.

The potential for similar eifects when installed in a Westinghouse designed reactor and vessel should be analyzed and evaluated.

Expected indications during all phases of small break LOCA transients should be described.

RESPONSE

A.

Introduction The HJTC Systein is designed to measure the water inventory in the reactor vessel above the fuel alignment plate, even when a steam / water two-phase mixture exists in the reactor vessel.

This is accomplished by use of a separator tube which acts like an internal sight glass and creates a single phase water level inside it when surrounded by a two-phase mixture.

This effect is shown schematically in Figure 4.

The height of the water level inside the separator tube is equal to the height of the collapsed water level (water inventory) in the two-phase mixture surrounding the separator tube.

Tests performed at C-E during the Phase II portion of the HJTC test program have demonstrated that a single phase water level is created inside the separator tube when immersed in a two-phase mixture and that the HJTC sensors, which are placed inside the separator tube, measure that level.

The reactor vessel configuration of the Byron /Braidwood units PWRs consist of two major regions above the fuel alignment plate:

the upper plenum and the upper head, (see. Figure 3).

The upper plenum is the region between the fuel alignment plate and the upper internals support plate.

The upper head is the region between the upper internals support plate and the top of the reactor vessel head.

These two regions have only limited hydraulic communication through leakage paths around the control rod drive shafts at the top of the RCCA guide thimbles.

Thus, during loss-of-inventory or volume reduction accidents, the loss of water inventory proceeds essentially independently in the two regions.

Based on analyses presented in Reference (1), the upper plenum is expected to drain faster than the upper head in the Byron /Braidwood units.

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B.

Description of HJTC Probe Design i

Two. identical;HJTC probe assemblies are.. installed in each of the Byron /Braidwood units.

.These probe assemblies are identical to System 80 probe assemblies.

There.are eight. heated / unheated-thermocouple pairs (sensors) tin each. probe assembly.

The Byron /Braidwood HJTC probe assemblies are designed.to measure j

the collapsed water-level in the upper head independently from the

?

' collapsed water level in the upper ~ plenum.

This'is accomplished by-use of a " split" probe assembly which creates two functionally separate probe sections, one in the upper head and the other in the i

upper plenum.

A divider disk is located inside the separator tube of the probe at the upper. internals support plate elevation to divide the two probe segments hydraulically.

Flow holes at the top and bottom of each separator tube section allow the collapsed water level in each region to be formed and measured inside the two spearator tube sections.

1 The axial locations of the eight HJTC sensors are shown in Figure 3.

The HJTC sensor arrangement for the Byron /Braidwood units has-two sensors located in the upper head and six sensors in the upper i

plenum.

Only two sensors are placed-in the upper head because once the water level falls below the top of the RCCA guide thimbles,- the upper head inventory no longer communicates with the upper plenum i

and reactor core.

For the upper head to drain, water-must exit through orifice holes located around the periphery of the upper-internals support plate and drain into the downcomer annulus (reactor inlet region).

H One of the upper head sensors is located as close as practical to the top of the reactor-vessel head.

This provides the reactor operator with information about the loss of reactor coolant inventory from the upper head region as early as possible.

4 l

The second upper head sensor is located just above the upper internals support plate which separates the upper head region from the upper plenum region.

This sensor indicates to the operator when the coolant inventory in the upper head has been fully I

depleted.

The remaining six sensors are utilized to provide more detail on coolant inventory in the upper plenum which is in direct communication with the reactor core.

One sensor is located as c

close as practical to the upper internals support plate.

This sensor indicates the formation of a void space as early as possible and that loss of coolant inventory in the upper plenum has begun.

i t

i

- The second sensor is=1ocated halfway between the upper internals support plate and the top of the hot leg.

a minimum distance between senr.3rs in the upper portion of theThis location upper plenum of 39 inches, thus maximizing the continuity of indication of the reactor coolant inventory change in the region The third sensor is located at the top of the hot leg elevation This location is chosen to provide information about the natural circulation capability of the plant.

If the water inventory falls below this elevation, the loss of natural circulation becomes imminent.

A fourth sensor is located at the midpoint of the reactor vessel hot leg elevation and a fifth sensor is located at the bottom of the_ hot leg.

information in this region.These sensors provide the operator with more detailed important to the operator because when the water level drops below the bottom of the hot leg, communication between the liquid inventory in the reactor coolant system piping and the reactor drop more rapidly than before. vessel ceases and the water inventory in t The sixth sensor is located as.close as practical to the fuel alignment plate.

This sensor tells the operator that loss of coolant inventory has proceeded to the point where core uncovery may become imminent.

using the core exit thermocouple indications. Subsequent ICC monitoring sh For recovery from an inadequate core cooling incident, actions or the automatic safety systems are working.at th the sensors C.

_HJTC Performance With Reactor Coolant Pumps Off A sample HJTC response for the Byron /Braidwood units plants I

l is shown in Figure 5.following a representative 4 inch diameter (0.09 ft ) pi 2

the upper head and upper plenum is shown.In the figure the collapsed liquid leve shown when a particular HJTC sensor becomes uncovered.In addition, the time is collapsed level data used in the analysis presented in Figure 5 was The i

obtained from mixture level data from Reference (1) and void i

fraction information for the upper plenum provided for this case by Westinghouse.

The 0.09 ft.2 break transient presented shows that the liquid inventory in the upper plenum drains completely several minutes before the upper head drains.

upper head drains quickly (in less than 100 sec)The liquid inventory in the of the top of the RCCA guide thimbles and then drains slowlyto tne elevation through the orifice holes around the periphery of the upper internals support plate.

. D.

HJTC-Performance With Reactor Coolant-Pumps On j

The.overall effect of Reactor Coolant Pump (RCP) operation is to circulate'more vigorously and homogenize more uniformly.(relative to no RCP's in operation) the two-phase mixture which is produced by flashing and/or boiling of the coolant.

The capability of the separator tube _to create a single-phase. water level inside it.is L

not affected by the RCP operation.

Also, due to the very small flow in the upper head region of the reactor vessel (as will be discussed in more detail in the followingfparagraph), RCP operation will have no significant effect on the two-phase response in this-region.

Therefore, the HJTC response in the upper head region is 4

expected to be the same with or without the RCP's in operation.

The expected normal operating, single-phase liquid, flow pattern with RCP's running in the reactor vessel is shown in Figure 6.

The bulk of the coolant flows down in the downcomer, flows through the core, enters the upper plenum through the fuel-alignment plate and 4

leaves the upper plenum through the hot leg pipes.

A small bypass flow from the inlet annulus enters the upper head through the orifice holes around the periphery of the upper internals support plate.

The bypass flow travels down through the RCCA guide thimbles and exits into the upper plenum above the fuel alignment plate through the open section of the RCCA guide thimbles.

The split of the total coolant flow into a main portion flowing through the core and a small bypass flowing through the upper head results in a difference in pressure in the upper head (P ) relative to 1

1 I

the pressure in the upper plenum (P )-

2 In addition, it is expected that there is an axial variation (beyond the one due to changing elevation heads) in the static pressure within the upper plenum region with the RCP's running.

High velocity flow jet: through the holes in the fuel alignment plate (FAP) result in.a reduced static pressure, P3, immediately l

downstream of the FAP, relative to the pressure, P, in the bulk 2

1 of the upper plenum.

Recovery of the static pressure occurs as the flow velocity decreases farther away from the FAP.

Thus, there is i

I a greater static pressure at the top of the upper plenum (just l

below the upper internals support plate) than at the bottom (just above the FAP).

In general, the pressure difference between upper head and upper j

plenum as well as the axial pressure variation in the upper plenum

[

will decrease as the void fraction in the primary coolant system I

increases.

This is due to degradation of the pump performance that l

occurs as the void fraction increases and results in a decrease in

(

mass flow rate.

Thus, as water inventory is lost during a small break LOCA with the RCP's running and system voiding increases, the i

effect of the RCP's on the indicated collapsed level is expected to decrease.

The effect of the RCP's is also smaller if not all RCP's are running.

. The HJTC indication in the upper plenum is expected to be affected by the axial pressure variation when the RCP's are running.

lower indication of the collapsed level than actually exists The the pumps are running.

This is because the static pressure at the top of the upper plenum is greater than the static pressure at the bottom of the upper plenum due to the pressure recovery downstream of the holes in the FAP.

Thus, the greater static pressure at the top holes of the upper plenum separator tube depresses the collapsed water level inside the separator tube relative to the collapsed level in the upper plenum.

than actual indicated water level.

This would result in a lower Therefore, based on present information, it will be recommended that the reactor operator disregard the indicated level from the lower portion of the probe (upper plenum) when the RCP's are running.

Because of the split probe design, the HJTC measurement in the' upper head is not affected by the operation of the RCPs.

Sensors in this region will correctly indicate voiding in the upper head even with the RCP's running.

displayed separately for these two regions. Display information is processed and i

REFERENCE 1.

Volume III, WCAP 9601, Section 4.1, Plots for Case B, Mixture Level In-Core and Mixture Level-in Upper Head.

5946N l

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QUESTION-3

-Describe any additional testing programs for.evalution, qualification,.

~and calibration for application;to the Braidwood Station. -This'should include discussion of the validity and applicability of the C-E Phase III testing to the Braidwood Station.

i

RESPONSE

An_ extensive test program has been performed to demonstrate that.the HJTC System will operate.as intended.

Three separate _ test-phases were undertaken in the program, which are:

Phase I - Proof of Principle-Testing l

Phase II - Design Development Testing l

Phase ~III - Prototype Testing i

The Phase I testing consisted of a series of,five_ tests performed at.C-E and ORNL test facilities.

These tests demonstrated the feasibility of using the HJTC as a level sensing device and provided information.

l necessary to the development of a preliminary RVLM probe assembly.

Phase II was~a design verification test series for the probe assembly.

i The objective of these tests was to simulate the thermal-hydraulic conditions surrounding the HJTC probe assembly that might exist in a PWR and verify the HJTC probe assembly performance under these conditions.

The Phase II HJTC probe assembly consisted of three HJTC sensors with splash shields installed inside a 12 foot long separator tube.

Single l

phase, two phase and depressurization transient tests were run.

The tests covered a pressure range from_ atmospheric to 1450 psig, with blowdown tests initiated at 1875 psig.

The two phase mixture void fraction varied from 0 to 0.52.

The collapsed level change rate tested varied from 0.5 to 3.0 in/sec for drain tests and 0.2 to 1.0 in/sec for

. refill tests.

For comparison, the representative liquid level plot for Byron /Braidwood presented in Figure 5 shows a peak drain rate _of about l.3 in/sec for a 0.09 ft2 break.

-The results of the Phase II test series show-that the separator. tube is-ccpable of creating a collapsed water level that can be detected by the HJTC_ sensor when the probe is immersed in a two-phase mixture.

The

. separator' tube produces a region of all liquid below a region of nearly dry steam.

The HJTC sensor responds to the movement of this steam / water interface past the sensor elevation.

Good agreement is obtained between the water level indicated by the HJTC sensors and-the collapsed water level measured independently by a DP cell.

In conclusion, the Phase II tests demonstrate that the HJTC probe assembly functions correctly to measure the collapsed water level under thermal-hydraulic conditions which the probe might be exposed to in a PWR.-

These tests, therefore, verify the performance of the HJTC probe assembly as an instrument to measure the-water inventory in the upper head.or upper plenum of a reactor.

i

. The Phase III test series was developed -to verify the final design of the HJTC probe assembly and associated electronics.'

A complete prototype HJTC system was tested'under normal _.and accident thermal-hydraulic conditions that the probe may_be exposed to in a.PWR.

Since the thermal-hydraulic performance of the probe _ assembly was verified by the Phase II tests, these tests concentrated on the performance of the integrated HJTC system, consisting of the_ probe assembly, signal processor and sensor heater power control.

The Phase III HJTC probe assembly consisted of eight HJTC-sensors inside a 12 foot long separator tube.

Single phase, two-phase, blowdown, and repressurization transients were run.

The tests covered a pressure range from about 50 to 2000 psig and two-phase void fractions from 0 to 0.62.

The probe assembly thermal-hydraulic performance, which had been verified in the Phase II tests, was reconfirmed in these tests.

That is,_the collapsed water level is formed and measured inside the separator tube while a two-phase mixtureLexists outside.

The Phase III tests also show that the signal processor generates an-uncovered-or covered signal when the sensor. T (which is the temperature difference between heated and unheated thermocouples) reaches a setpoint_

value.

When this occurs for each sensor, the percent level display changes to show the new collapsed water level.

The sensor heater power control system successfully limits the maximum temperature and T by reducing the power supplied to the heaters.

The heater power control system maximizes the power supplied to the sensor heaters (to minimize the response time) while-preventing damage to the sensor.due to high heated junction temperatures.

Even at low pressure where the heater power is reduced, the HJTC system still provides a good indication of the collapsed water level.

The integrated HJTC system was tested in the Phase III tests under simulated PWR thermal-hydraulic conditions and performed very well.

Based on the analyses presented in Reference (1), the representative thermal-hydraulic parameter ranges for Byron /Braidwood were adequately covered by the Phase III as well as by the previous Phase II tests.

Thus, the test results are applicable to Byron /Braidwood.

Furthermore, it can be concluded that the integrated HJTC system will indicate to the reactor operator the status and trend of the water inventory in the reactor vessel during an accident.

REFERENCE l.

Volume III, WCAP 9601, Section 4.1, Plots for Case B, Mixture Level In-Core and Mixture Level in Upper Head.

5946N

10 -

Question 4 Describe the guidelines and precautions for use of the additional 1ICC instrumentalon and analyses-used to develop procedures.

Response

./

General guidelines for use of the additional instrumentation are being prepared by the Westinghouse Owners Group as part-of the effort undertaken to resolve NUREG-0737 Item I.C.l.

Plant specific procedures will be prepared after the guidelines have been approved by the NRC.

Minor departures from these guidelines will be necessary because the Byron /Braidwood heated junction _ thermocouple instrumentation covers a level range different from the standard differential pressure instruments.

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. 5946N 4

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o Question-5 Describe the operator instructions in emergency operating procedures for ICC and how these procedures will be modified when the final system is implemented.

Response

Refer to Item 8 in the attachment to the August 13, 1982 letter from T.R..

Tramm to H.R. Denton.

i I

e

. Question 6

Describe the spacing of the sensors in relation to.the core alignment

- plate and reactor vessel head.

How would the decrease in resolution due

. to loss of a single sensor affect the ability of the system to detect approach to Inadequate Core Cooling.

RESPONSE

See the response to Question 2 for information regarding spacing of sensors.

The loss of a single sensor would not affect the ability to detect the approach to inadequate core cooling since the system contains independent, redundant sensors at each location.

5946N 4

,+

4--

Question 7 Describe the display measurement units in the SPDS.

Response

j The display measurement units on the SPDS for reactor vessel level will be " percent level" above the fuel alignment plate.

5946N L

J l

. Question 8 Provide tables covering the evaluation of conformance of core exit thermocouples, the subcooling margin monitor, and the HJTC system with NUREG-0737:

II.F.2, Attachment I and Appendix B, with suitable reference to conformance documentation.

Response

Refer to the attachment to the August 13, 1982 letter from T.R. Tramm to H.R. Denton.

5946N l

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