ML19319B354
| ML19319B354 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 12/16/1977 |
| From: | Roe L TOLEDO EDISON CO. |
| To: | Stolz J Office of Nuclear Reactor Regulation |
| References | |
| 408, TAC-8378, NUDOCS 8001150939 | |
| Download: ML19319B354 (5) | |
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[4191 259-5242 Serial No. 408 gr
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Mr. John F. Stolz, Chief
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Light Water Reactors Branch No. 1 Division of Project Management United States Nuclear Regulatory Ccmmission Washington, D.C.
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Dear Mr. Stolz:
Enclosed is our response to your September 16, 1977 letter addressed to my attention regarding possible boron dilution accidents at Davis-Besse Nuclear Power Station Unit 1.
Yours very truly, dh e/9 Enclosure THE TOLECO EDISON COMPANY EDISCN PLAZA 3C0 MAC!SCN AVENUE TOLECO, CHIO 43S52 l
i ENCLOSURE 1 FOR SERIAL NO. 408, DECEMBER 16, 1977 TOLEDO EDISON COMPANY ANALYSIS OF THE POTENTIAL FOR A BORON DILUTION ACCIDENT l
All systems with the potential of creating a moderator dilution incident have been examined to assure that there are adequate safeguards designed in the system to assure against the moderator dilution incident.
l The systems considered as dilution sources for the reactor coolant 4
system are as follows:
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Spent Fuel Pool in the refueling mode (mode 6)
Decay Heat Removal High Pressure Injection j
Core Flood Containment Spray j
Chemical Addition i
Make-up and Purification System The Fire Water System The protection provided in each system is described briefly below:
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The Spent Fuel Pool System in the Refueling Mode l
All sources of non borated water are isolated from the spent fuel pool l
by a minimum of two normally closed isolation valves.
I Decay Heat Removal, High Pressure Injection, and Containment Sprav Svstems l
j All lines which can dilute the fluid in the decay heat removal, high pressure injection systems or the containment spray system are isolated l
by a minimum of two normally closed isolation valves with the exception of the BWST heat up system which recirculates the BWST water. All i
potential dilution flow paths into the heat up system have been analyzed.
l All sources of non borated water into the BWST heat up system are isolated by a minimum of two valves with the exception of the lines from the primary water system and the demineralized water system. The primary q
j and demineralized water system flow paths into the BWST heat up system are isolated by one normally closed manual valve. Although the proba-bility of the isolation valve in the above lines being inadvertently i
left open or leaking excessively are very remote, an additional valve will be installed in the common supply line to provide added assurance against the possibility of dilution through this path.
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Core Flood System The core flood system has one potential flow path but it is isolated l
from the tanks by two normally closed valves.
In addition, the Davis-Besse Unit 1, Technical Specification r quires that the tank level and pressure be checked every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and the tank boren concentration be checked every 31 days or within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> from a change in tank level. A change in level or pressure will alert the operator in the control room of the possibility of baron dilution by alarms. There is also tank j
level and pressure indication in the Control Room which continuously 1
monitor the condition of the tanks.
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The Chemical Addition System All potential dilution flow paths into the boric acid tanks are isolated by two normally closed valves.
In addition there is a Davis-Besse Unit 1, Technical Specification requirement to check the tank concentra-tion every seven days. Tank level is also continuously monitored by level instrumentation in the Control Room.
The Make Up and Purification System The make up and purification system is the normal mode by which the boron concentration within the RCS is adjusted.
Since it is the normal method of boron dilution, several safeguards have been designed into the system to protect against excessive dilution of the reactor coolant.
The safeguards are as follows:
1.
The process of normal deboration of the reactor coolant cannot start unless specific control rod groups are withdrawn to a certain point which allows for deboration.
This control rod group position interlock through the integrated control system either permits or prohibits continuous dilution depending upon the control rod group position.
Because of this interlock, the demineralized water makeup valve (HV-MU40) and the three-way valve (HV-Mull) can be operated simultaneously only when the control rod group is withdrawn to a preset position. The demineralized water makeup valve is automatica11v closed, and the three-way valve position is automati-cally changed when the rods have been inserted to a preset position.
2.
The dilution valves are interlocked so that the operator must preset the desired dilution batch size before initiating the dilution cycle. The dilution cycle will automatically terminate when the dilution flow has been integrated to the preset batch size.
3.
The operator is able to manually terminate the dilution cycle at any time.
Most interfacing connections into the system, which could cause dilution are isolated by a minimum of two normally closed valves.
There is one line which is isolated by a single normally closed valve. This line goes to the lithium hydroxide and hydrazine mix tank.
This tank has a capacity of 50 gallons which is not a sufficient volume to dilute the RCS to such a degree that violation of the required shutdown margin would occur.
An analysis was conducted to determine the consequence of a failare of the make up system and is presented in Section 15.2.4 of the FSAR.
j In addition to the above the B&W chemistry manual requires that the RCS boron concentration be checked daily while operating or shutdown.
Reactor power is continuously monitored by the protective system and the operators which will sense and correct any unwarranted power escalation in the unlikely event it were to happen.
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l Fire k'ater System The only borated system which can be potentially affected by the fire water system is the spent fuel pool system while in t.e refueling mode l
i (mode 6).
A review of the system indicates that in the area of the spent fuel pool and refueling canal, there are no overhead sprinklers and no hose stations in the close praximity.
For the above reasons the fire water system is not considered a potential problem.
In conclusion, it is apparent that each system which has the potential to cause a moderator dilution incident has the necessary safeguards designed into it to assure with a high degree of confidence that, even assuming the most limiting single failure, the incident cannot occur.
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