ML20010G202

Description of Heated Junction Thermocouple (Hjtc) Sys Under Consideration for Use at Prairie Island Nuclear Generating Plant to Aid in Detection of Inadequate Core Cooling.

ML20010G202
Person / Time
Site:	Prairie Island
Issue date:	09/01/1981
From:	NORTHERN STATES POWER CO.
To:
Shared Package
ML20010G196	List: ML20010G195 ML20010G202
References
TASK-2.F.2, TASK-TM NUDOCS 8109150434
Download: ML20010G202 (10)

Text

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Attachment Director of NRR September 1, 1981 DESCRIPTION OF HEATED JUNCTION THERMOCOUPLE (HJTC) SYSTEM UNDER CONSIDERATION FOR USE AT THE PRAIRIE ISLAND NUCLEAR GENERATING PLANT TO AID IN DETECTION OF INADQUATE CORE COOLING l

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1.0 Description The HJTC system measures reactor coolant liquid inventory with dis-crete sensors located at different levels within a separator tube ranging ,

from the top of the core to the reactor vessel head. The basic principle of system operation is the detection of a temperature difference between adjacent heated and unheated thermocouples.

1.1 Sensor Design As pictured in Figure 1-1, the HJTC sensor consists of a C.iromel-Alumel thermoccuple near a heater (or heated junction) and anot her Chromel-Alumel thermocouple positicned away from the heater (or unheated junction). In a fluid with relatively good heat trans fer properties, the temperature dif ference between the adjacent thermocouples is very small. In a fluid with relatively poor heat transfer properties, the temperature difference between the thermocouples is large.

Two design features ensure proper opration under all thermal-hydraulic conditions.

First, each HJTC is shielded to avoid overcooling due to direct water contact during two phase fluid conditions. The HJTC with the splash shield is referred to as the RJTC sensor. Second, a string of HJTC sensors is enclosed in a tube that separates the liquid and gas phases that surround it.

The separator tube creates a collapsed liquid level that the HJTC sensors measure. This collapsed liquid liquid level is directly related to the average liquid fraction of the fluid in the reactor head volume above the fuel. This mode of direct in-vessel sensing reduces spurious effects i due to pressure, fluid properties, and non-homogeneities of the fluid medium. The string of HJTC sensors and the separator tube is referred to as the HJTC instrument.

The HJTC System is composed of two channels of HJTC instruments. Each HJTC ins t rument is manufactured into a probe assembly. The probe assembly includes eight HJTC sensors, a seal plug, and electrical connectors (Figure 1-2). The eight HJTC sensors are electrically independent and located at eight levels from the reactor vessel head to the fuel.

The probe assembly is housed in a stainless steel structure that protects the sensors from flow loads and serves is a guide path for the sensors. Installa-tion arrangements are being developed in conjunction with Westinghouse.

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Installation details will be provided in future documentation if NSP decides to install the HJTC System at Prairie Island.

1.2 Processing Equipment The processing equipment for the HJTC System wilt perform the following funct ions :

1. Determine if liquid inventcry exists at the RJTC positions.

The heated and unheated thermocouples in the HJTC are connected in such a way that absolute and differential temperature signals are available. This is shown in Figure 1-3. When water surrounds the thermocouples, their temperature and voltage output are approximately equal. V( A-C) on Figure 1-3 is, therefore , approximately

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zero. In the absence of liquid, the thermocouple temperature and to rise.

output When V voltages become of the unequal, individual HJTCcausing rises V($bhhe a predetermined setpoint, liquidkkvhntoryisassumednot to exist at this HJTC position.

2. Determine the maximum upper plenum / head fluid temperature from the unheated thermocounles for use as an input to the subcooling monitors (optional).

3. Process all inputs anc calculated outputs for display.

4. Provide an alarm output to the plant annunciator system shen any of the HJTC's detect the absence of liquid level.

5. Provide control of heater power for proper HJTC output signal level.

Figure 1-4 shows a single channel conceptual design which includes the heater power controller.

1.3 Displav Design The HJTC outputs will probably be displayed through a human engineered cathode ray tube (CRT) based primary display with separate backup displays.

As shown in Figure 1-5, each channel of the HJTC instrument system will also have safety grade backup displays. Both primary and backup displays are intended to be designed consistent with the criteria in NUREG-0737, Action Item II.F.2, Attachment 1, and Appendix A.

The following information is planned for the primary display:

1. Two charnels of 8 discrete HJTC positions for indicated liquid inventory above tha fuel.

2. Maximum unheated junction temperature of each of the two channels.

The following information is anticipated to be displayed on the backup displays:

1. Liquid inventory level above the fuel derived from the 8 discrete

' HJTC positions

2. Unheated junction temperature at the 8 positions

3. Heated junction temperature at the 8 positions

[ 2.0 System verification The HJTC System is a new system developed to indicate liquid inventory

above the core. Since it is a new system, extensive testing has been performed and further tests are planned to assure that the HJTC System j

will operate to unabiguously indicate liquid inventory above the core.

l The testing is divided into three phases:

Phase 1 - Proof of Principle Testing l

l Phase 2 - Design Development Testing Phase 3 - Prototype Testing

, The first phase consisted of a series of five tests, which have been completed.

! The testing demonstrated the capability of the HJTC instrument design to l

measure liquid level under simulated reactor vessel thermal-hydraulic conditions (including accident conditions).

Test 1 Autoclave test to show HJTC (thermocouples only) response to water or steam.

In April 1980, a conceptual test was performed with two thermocouples in one sheath with one thermocouple as a heater and the other thermocouple as the inventory sensor. This configuration was placed in an autoclave (pressure vessel with the capabilities to adjust tamperature and pressure). The thermocouples were exposed to water and then steam envirorments. The results demonstrated a significant outp te difference between steam and water conditions for a given heater power level.

Test 2 Two phase flow test to show bare hJ1 Sensitivity to voids.

In June 1980, a RJTC (of the present differential thermocouple design) was placed into the Advanced Instrumentation for Reflood Studies (AIRS) test facility, a low pressure two phase flow test facility at Oak Ridge National Laboratory (ORNL).

The HJTC was exposed to void fractions at various heater power levels. The results demonstrated that the base RJTC output was virtually the same in two phase liquid as in subcooled li .uid. The HJTC did generate a significant output in 100% quality steam.

Test 3 Atmospheric air-water cest to show the ef fect of a splash shield A splash shield was designed to increase the sensitivity to voids. The splash shield prevents direct contact with the liquid in the two phase fluid. The HJTC output changed at intermediate void fraction two phase fluid. The results demonstrated that the HJTC sensor (heated junction thermocouple with the splash shield) sensed intermediate void fraction fluid conditions.

Test 4 High pressure boil-off test to show HJTC sensor response to reactor thermal-hydraulic conditions.

In September 1980, a CE HJTC sensor (with splash shield) was tested at the ORNL Thermal-Hydraulics Test Facility (THTF). The device is still installed and available for further tests at ORNL. The HJTC sensor was subjected to various two phase fluid conditions at reactor temperatures and presures. The results verified that the HJTC sensor is a device that can sense liquid inventory under

! normal and accident reactor vessel high pressure and temperature two phase conditions.

Test 5 Atmospheric air-water test to show the effect of a separator tube A separator tube was added to the HJTC design to form a collapsed liquid level so that the HJTC sensor directly measures liquid inventory under all simulated l two phase conditions. In October, 1980, atmospheric air-water tests were l

performed with HJTC sensors and the separator tube. The results demonstrated I that the separator tube did form a collapsed liquid level and the HJTC output

! did accurately indicate liquid inventory. This test verified that the HJTC l

instrument, which includes the HJTC, the splash shield, and the separator tube, is a viable measuring device for liquid inventory.

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The Phase 3 test program will consist of high pressure and temperature tests on the HJTC instrument. These tests will provide input for the HJTC instrument design and manufacturing ef fort. The Phase 3 test program is expected to be completed this year.

The final processing and display design for the HJTC System has not been completed.

As the design ef fort proceeds, design evaluations will be performed prior to installation. Correct implementation of the sof tware and hardware will be included and documetted as part of the desing effort.

3.0 System Qualification The qualification progrtm for the HJTC System has not been completely defined.

However, plans are being developed based on the following three categories of instrumentation:

1. Sensor instrumentation within the pressure vessel.

6 2. Instrumentation components and systems which extend from the primary pressure boundary up to and including the primary display isolator and including the backup displays.

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3. Instrumentation systems which comprise the primary display equipment.

A preliminary outline of a qualification program for each classification ic given below.

The in-vessel sensors will meet the NUREG-0737, Appendix A, requirement to install the best equipment available consistent with qualification and scheinlar requirements. Design of the equipment will be consistent with the guidelines of Appendix A as well as the clarification and Attachment 1 to Item II.F.2 in NUREG-0737. Specifically, components will be designed such that they meet appropriate stress criteria when subjected to normal and design basis accident loadings. Verification testing will be conducted to confirm operation at DBA pressure and temperature conditions. Seismic testing to safe shutdown conditions will verify function after being subjected to the seismic loadings.

The out-of-vessel instrumentation system, up to and including the primary display isolator, and the sackup displays will be environmentally qualified in accordance with IEEE-323-1974 as interpreted by Combustion Engineering Document, CENPD-255, " Qualification of Combustion Engineering Class IE Instruments." This document describes the metnod which will be used to qualify out-of-vessel Class IE equipment.

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