ML19257A198
| ML19257A198 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 12/19/1979 |
| From: | Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| To: | Harold Denton, Reid R Office of Nuclear Reactor Regulation |
| References | |
| TASK-2.K.2.16, TASK-TM TAC-48129, NUDOCS 8001020451 | |
| Download: ML19257A198 (3) | |
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- SACRAMENTO MUNICIPAL UTILITY DISTRICT O 6201 S street, Box 15830, Sacramento, california 95813; (916) 452-3211 December 19, 1979 Director of Nuclear Reactor Regulation Attention: Mr. Robert W. Reid, Chief Operating Reactor, Branch No. 4 U.S. Nuclear Regulatory Commission Washington, D.C.
20555 Docket No. 50-312 Rancho Seco Nuclear Generating Station, Unit N7. 1
Dear Mr. Reid:
Your letter of November 21, 1979 requested additional infomation concerning small break loss of coolant accidents.
In particular, it requested infomation concerning slug flow in hot-leg piping and reactor coolant pump seal response to loss of seal cooling. The District's letter of December 6, 1979 provided a response schedule for your request. This letter provides the information concerning reactor coolant pump seal response without seal cooling. The specific question asked was, " Evaluate the impact of RCP seal damage and leakage due to loss of seal cocling on loss of offsite power. How long can the RCP seals sustain loss of cooling without damage?" The District's response to this question is:
During normal pump operation, both seal injection water and component cooling water are supplied to each pump. Seal injection water is provided by a makeup or high pressure injection pump which is powered from a safety grade bus which receives its power from either offsite or an associated diesel generator.
The component cooling water pump power supply is off site power.
At Rancho Seco the reactor coolant pump seal system consists of 3 seals.
For discussion purposes, the space between the upper and middle seals is known as the upper seal chamber.
The space between the lower and middle seals in known as the intermediate seal chamber and the space below the lower seal is known as the injection cavity.
During nomal operation seal injection flow provides about 8 gpm of water to eacn pump seal. About / gpm is injected into the external portion of the recirculation system which taps off of the injection cavity. When the reactor coolant pump is running there is a recirculation flow of approximately 70 gpm gener;ted by the recirculation impeller.
The recirculation flow passes through heat exchangers which are cooled by component cooling water.
This recirculation system provides cooling for
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the RCP seals in case of loss of seal injection flow.
1663 319 W V
p soonu qw AN ELECTRIC SYSTEM SERV!NG MORE THAN 600,000 IN THE HEART OF CA LIFO R NI A
s Mr. Reid December 19, 1979 About 7 gpm of the seal injection water flows down into the reactor coolant system (RCP bowl). The remainder of the 8 gpm injection flow (about 1 gpm) flows into the intermediate seal chamber, out through a pressure breakdown orifice and then into the upper seal chamber. From the upper seal chamber it passes through another pressure breakdown orifice and into the seal return line.
If offsite power is lost, component cooling water would be lost. Of course, reactor coolant pumps would be de-energized.
Though it is very unlikely, seal injection flow might also be lost.
Babcock & Wilcox recently perfonned an evaluation of this situation.
The evaluation shows that leakage would not increase appreciably for approximately ten minutes and would not be severe for up to sixty minutes.
In this evaluation it was assumed that at time zero the RCP's were stopped when both seal injection and cooling were lost, that the seal return valves would remain open and that the initial leakage was at a high level of two gallons per minute for mechanical f ace type seals.
In the situation described above, the Capability exists to manually reinitiate seal injection flow. At Rancho Seco procedures exist to cover such reinitiation.
In addition, the seal return valve will automatically isolate under such a condition.
This minimizes seal heat up rate and makes the analysis done to cover this situation very conservative.
The details of this analysis continue.
Seal cavity temperatures and seal leak rates for the first four to five minutes af ter time zero will remain essentially stable due to the mass of the heat sink at the shaf t seal cartridge and pump heat exchanger.
This time period could be extended Ly about two tc three minutes if the seal return valve is closed witni.. ninety seconds. Remember, the Rancho Seco seal return valve would close at time zero.
With the seal return open, when the seal cavity temperature starts to rise it will increase at a rapid rate. The seals will begin to pass steam within four or five minutes.
The temperature ramp will be turned around if seal injection can be gradually reinitiated or if the component cooling water flow is startL, within about ten minutes. Although, some internal damage may occur, the seal system will gradually stabilize and return to approximately the initial leakage rate.
Closure of the seal return valve within this time frame is most effective in slowing down the rate of temperature rise on those pumps that had low seal leakage at time zero.
Closure of the seal return valve within this time frame is most effective in slowing down the rate of temperature rise on those pumps that had low seal leakage at time zero.
Closure of the seal return valve shortly af ter time zero reduces the heat up rate by as much as 75 percent for seals which nonnally operated at low seal leak rates and by as much as 50 percent for pumps operating with hign seal leak rates.
If cooling continues to be unavailable, the seal cavity temperature will continue to increase.
It is predicted to reach at least 3500F within twenty minutes. At this time, the shaft directly above the seals will be at about 3000F and the recirculation heat exchanger will be at full system temperature (about 5400F).
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Mr. Reid December 19, 1979 Af ter steaming conditions are reached. significant seal degradation would not be expected for up to one hour af ter t '.?a zero.
The elastomers which make up a part of the seal assembly will start to sof ten at approximately 3000F and can start to extrude before 5000F. The amount of extrusion is based upon time, temperature and annular clearances. Experience shows that leakage frorr seals because of elastomer extrusion does not increase appreciably within the first 30 minutes.
It is estimated that under the worst condition, leakage from a static pump may reach 5 gpm in 30 minutes and 10 gpm in 60 minutes.
In summary, our analysis showns that some increase in RCP seal leakage will occur within the first hour after loss of component cooling water and seal injection. Extensive analysis on varying break sizes in varing locations with varying scenarios has shown Rancho Seco's ability to safely handle a broad spectrum of break sizes including any leakage caused by RCP seal damage.
Sincerely, hn J. Mattimoe Assistant General Manager and Chief Engineer SI A:nm:j ra 1663 321
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