ML19331B591
| ML19331B591 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 08/08/1980 |
| From: | Mills L TENNESSEE VALLEY AUTHORITY |
| To: | Schwencer A Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8008120474 | |
| Download: ML19331B591 (3) | |
Text
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TENNESSEE VALLEY AUTHORITY CH ATTANOOGA. TENNESSEE 374ot 400 Chestnut Street Tower II August 8, 1980 Director of Nuclear Reactor Regulation Attention:
Mr. A. Schwencer, Chief Licensing Branch No. 2 Division of Licensing U.S. Nuclear Regulat.'r'r Commission Washington, DC 20555
Dear Mr. Schwencer:
In the Matter of the Application of
)
Docket Nos. 50-327 Tennessee Valley Authority
)
50-328 Enclosed is the information requested by M. Mendonca, Reactor Systems Branch, concerning the justitication of the Sequoyah Nuclear Plant head vent design.
Very truly yours, TENNESSEE VALLEY AUTHORITY Y
. M. Mills,. nager Nuclear Regulation and Safety Enclosure 36>l s
J/
8008120474 An Ecual Opportunity Employer
ENCLOSURE Because of the flowpath configurations which govern the hydraulic communication between the reactor vessel upper head and the rest of the reactor coolant system in plants of Westinghouse design, accumulation of gases in the vessel upper head region could only occur during periods of natural circulation.
During forced circulation periods, there is substantial flow through the upper head from the downcomer region to the upper plenum region of the reactor vessel which would tend to uniformly distribute gases throughout the reactor coolant system.
During natural circulatiog, the proposed Sequoyah reactor vessel head vent design could leave 160 ft of vessel upper head volume unvented since the design utilizes the existing UHI penetrations for head access. Since the reactor vessel head vent system is not intended for use during the initial mitigation phase of a transient, the head vent system will not interact with UHI operation. Procedures for use of the head vent system will not permit head vent operations during conditions where UHI delivery is required for RCS inventory addition.
It must also be noted that procedures for use of the head vent system will utilize appropriate system instrumentation, including vessel level indications, to establish appropriate conditions under which to vent the vessel head and at what point any previously unvented volume may have enlarged to a condition where subsequent vent operations would be effective in further reducing the mass of gas present in the vessel head.
In this manner, the existing vent may be used to maintain an essentially fixed unvented volume while continually reducing the amount (mass) of unvented gases.
An examination of the geometry of the Sequoyah reactor vessel upper head region shows that the location of this unvented volume is entirely above all flowpaths from the upper head to the active portions of the reactor coolant system. Since any flows through the upper head region are extremely low during natural 3 circulation, the upper head region is essentially stagnant and the 160 ft of unvented volume cannot interact with the rest of the system. Therefore, this unvented region represents a " dead-ended" volume which would not influence pressurizer control, heat transfer, or pump performance.
During f reed circulation following a period of natural circulation when a 3
160 ft unvented volume could exist, any gas in this space-would be swept l
out and mixgd uniformly in the reactor coolant system. The magnitude of this 160 ft volume as cgmpared to the volume of the reactor coolant system (approximately 13,000 ft ) is so small it is insignificant during forced circulation.
If a period of natural circulatiog is assumed to occur after forced circulation which had distributed this 160 ft of unvented volume through the system, it is highly unlikely that all of this potential unvented gas could collect in the high points of the RCS so as to interrupt natural circulation. However, even under the extremely conservative assumptions that the entire unvented volume consisted of noncondensible gas and that this volume became uniforgly dis-cirbuted in the U-bend regions of the four steam generators (70 ft U-bend e
d
region volume per steam generator), there is insufficient unvented volume to interrupt natural circulation heat transfer.
Based on these considerations, it is shown that the proposed reactor vessel head vent design provides an unvented volume sufficiently small to preclude adverse impacts on decay heat removal capability. Further, this " dead-ended" volume would not be expected to preclude pressurizer control and is sufficiently small to be insignificant with respect to the total effect in the RCS on pump performance.
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