ML20002B011

Natural Circulation Test.

ML20002B011
Person / Time
Site:	Sequoyah
Issue date:	12/05/1980
From:	TENNESSEE VALLEY AUTHORITY
To:
Shared Package
ML20002B010	List: ML20002B009 ML20002B011
References
NUDOCS 8012090312
Download: ML20002B011 (3)

Text

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ENCLOSURE SEQUOYAH NUCLEAR PLANT NATURAL CIRCULATION TEST Demonstrate the Ability to Cooldown, Depressurize, and Boron Mix Special Test 9B, Boron Mixing and Cooldown, was performed on unit 1 of Sequoyah Nuclear plant after zero power physics testing.

1.0 objective A. To borate and verify uniform boron mixing while in natural circulation.

B. To demanstrate the capability to cool down and depressurize while in untural circulation.

C. To provide operator training in the natural circulation mode during a primary system boron addition and cooldown.

2.0 Results With the reactor at approximately 2.5-percent power an6 natural circulation established, a slow boration of the reactor coolan' system was started (2.7 gal / min) and allowed to run for a 2-h or period. Core exit thermocouple maps were run periodically to deter-mine if a nonuniform boron distribution would develop in the core.

Along with the T/C maps, the incore movable detectors (6) worm positioned in the core at varying radial and axial positions.

The time delay from the initiation of the boron addition until the negative reactivity effects were observed in the core was approxi-mately the same as in forc;4 circul;; ion (4-5 minutes). The traces from the incore detectors shewed occasional indications of a slightly nonuniform distribution, but f'r the most part the flux levels recorded by the detectors trended consistently. The core exit thermocouple maps showed no indica' ion of nonuniform distribution as again the exit temperature distribution was slightly better than in the full flow case.

After sampling the system to ensure adequate mixing, a slow cool-down was started and again thermocouple maps were taken periodically to detect, in this case, nonuniform temperature distributions during l the cooldown. The overall temperature distribution remained very l uniforn throughout the cooldown with some indications of a slightly increased radial tilt as measured by the thermocouples. The thermo-couple calculated tilt is not considered extremely accurate, but the trend of the tilt from 550"F to 450"F should be a relatively reliable indication of the direction of changes.

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.. In the first performance of the test, the cooldown was accomplished using steam dump to the condenser. In the second performance, the .

main steam isolation valves were closed and the cooldown was accomplished using atmospheric relief valves. In this case, main-taining constant natural circulation in each loop proved to be difficult due to steaming off of individual steam. generators el different times. When one steam generator was steamed off, the natural circulation flow would quickly increase in that loop and reduce in other loops. The best way to cool down proved to be positioning the relief and feedwater valves to give a continuous steam and feed flow and establishing consistent natural circulation flow in each loop rather than letting the automatic system control feedwater and steam flow.

During the cooldown, the indicated temperatures in the upper head, as indicated by the upper head thermocouples, were monitored closely to determine if the upper head temperatures would drop with system temperature under natural circulation. As seen in Figure 1, the upper head temperatures lagged the core exit and hot leg temperatures but followed the cooldown very well indicating that some natural circulation flow was reaching the upper head region.

3.0 Conclusions i

Special Test 9B, Boron Mixing and Cooldown, demonstrated the ability to cool down and depressurize the plant and to demonstrate that boron mixing is sufficient under natural circulation circumstances. Approxi-mately 300 gallons of boric acid were added during the first portion of the test. The results of the test showed that the boron mixing was as expected and better than adequ..te. The results also show that boron addition can be made satis.factorily to bring the unit to the cold shutdown xenon-free condition.

The second portion of the test demonstrated the ability to cool 'down and depressurize under such circumstances. Approximately 100 F of cooldown and 400 psig of depressurization were performed during the test. The results of this portion of the test demonstrated satisfactorily that the unit could be cooled down and depressurized from hot standby to hot shutdown under such circumstances. The results also showed that the upper head temperature followed the cooldown very well indicating that some natural circulation flow was reaching the upper head region.

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