ML20148Q196
| ML20148Q196 | |
| Person / Time | |
|---|---|
| Site: | Seabrook  |
| Issue date: | 10/28/1978 |
| From: | YANKEE ATOMIC ELECTRIC CO. |
| To: | |
| Shared Package | |
| ML20148Q180 | List: |
| References | |
| NUDOCS 7811290344 | |
| Download: ML20148Q196 (5) | |
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Report on the Seabrook Station Ref ueling Water Storage Tank Design Deficiency Prepared by: Yankee Atomic Electric Company for Public Service Company of New Hampshire Octobe r 28. 1978 0N 31129
l Int roduct ion On September 28, 1978 Mr. Seth Folsom of the Region 1 of fice of the NRC was informed by Mr. John Haseltine of Yankee Atomic Electric Company, Seabrook Station Project Manager, of a design deficiency associated with the refueling water storage tanks (RWST) at Seabrook Station. The deficiency was reported under 10CFR50.55a(e) and was reported within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of confirmation of the deficiency.
The deficiency was reported to Mr. Folsom as an undersizing of the RWST.
The undersizing was discovered in a routine design review of the Net Positive Suction Head (NPSH) for the containment spray and residual heat removal p ump s.
The design review verified proper NPSH but it also showed that there did not appear to be sufficient water inthe RWST to complete the transfer of pump suctions from the tank to tae containment sump before it was emptied.
This report serves as a written report under the 30 day requirements in 10CFR50.55a(e)(3) and is arranged to provide the inf ormation reques ted in the regulation.
Description of Deficiency The water storage capacity of the RWST serves dual purposes in the s tation design. One is to flood the cavity and canal area above the reactor prior to refuelings and the other is to supply borated water to the containment spray and injection pumps during a loss of coolant accident.
As reported to the IE Inspector on September 28, the deficiency concerns the undersizing of the RWST's useable volume for a loss of coolant accident.
The tank was adequately sized for the flooding of the reactor cavity and canal.
The tank capacity is dependent upon a number of parameters. The major parameter is the flooding of the containment during a loss of coolant accident to a height which will provide sufficient NPSH for the containment spray and residual heat removal pumps and proper containment sump screen s ubme rge nce. The 375,000 gallon design submitted in the Preliminary Saf ety Analysis Report (PS AR) met this requirement.
Approximately 350,000 gallons of. the tank was assumed to be injected and this volume did allow suf ficient flooding of the containment to meet pump NPSH requirements and screen cove rage. The deficiency arose in the ability of the remaining 25,000 gallons in the tank to meet the other design parameters.
The other design parameters are instrument error, a working allowance, transfer allowance, single f ailure, and unuseable volume. The following is an explanation of what these parameters are and how they were satisfied or not satisfied in the original design of the RWST.
1.
Ins trument error - Increased volume is necessary to account for the accuracy of the instrumentation reading the RWST level.
Initially a nominal 1% f ull scale instrument accuracy error was assumed which translates into 3500 gallons. This error has to be taken twice - once - -.
to assure that the tank does initially contain 350,000 gallons and a second time to assure that 350,000 gallons has-been injected. Thus, 7,000 gallons of capacity was necessary for. ins trument error.
Recentip. we.were informed by Wes tinghouse, who is responsible for the RWST. level ins trumentation, that they pref er a 3% f ull scale
-instrument error which causes an increase in the allowance for ins trument error to 21,000 gallons.
2.-
Working allowance - Some allowance above the required capacity of 350,000 gallons is necessary to prevent alarms with only minor water losses stom the tank.
It would not be practical te maintain a level at the exact setpoint and thus, a nominal 3,000 gallons over and above j
the design capacity is desired for this margin.
3.
Transfer allowance - Additional RWST capacity must be provided to accommodate a reasonable delay time associated with the transfer of certain pump suctions from the RWST to the containment sump.
As stated in RESAR-3, when ~the proper quantU r of water has' been transferred to the containment, the containmer u mp valves open. This automatically shif ts RHR and cont
.m e nt spray pump suctiona to the containment sump. However, the sut 9ty injection and charging pumps continue to draw from the RWST.If you assumed 'a 10 minute realignment time, the allowance would be 24,000 gallons since the flow rate of.
the pumps remaining on the RWST is -2,400 gallons per minute (gpm).
4.
Single f ailure - Becau.c of the automatic. transfer of RER and containment pumps to the sump, some allowance' must be included for a single f ailure. The mos t limiting single failure for tank. capacity results if ?.he containment sump valve for one of the trains failed to open upon a low level siga C. f rom the RWST.
Should this worst single failure occur, the associated 12R and containment spray pumps in that trait will continue to draw irom the RWST at a rate of 7800 GPM. If you assume it takes 5 minutes to recognize and correct the condition, i
then an additional 39,000 2311ons would have to be added to the tank to corpensate for the single f ailure.
5.
Unuseable volume - Once the invert of the tank pump suction pipes are reached the p mps lose auction and any remaining water in' the tank is unusable. This unusable volume must be included in the des tyn calculations and for a 375,000 gallon tank it is approximstaV/ 14,000 gallons.
The design parameters. for instrument error, transfer allowance and l
single f ailure have chany.ed since the original sizing of the tank. The
.f ollowing table ~1s a'sudfary of required capacities under the' old and' revised design parameters:.
4 Required R"ST Volume Design Parameter Old New
- 1. Injection capacity 350,000 gal 350,000 gal
- 2. Ins trumect error 7,000 gal 21,000 gal
- 3. Working allowance 3,000 gal 3,000 gal
- 4. Transfer allowance 24,000 gal
- 5. Single failure 39,000 gal
- 6. Unusable volume 14,000 gal 14,000 gal Total 3
,000 gal 451,000 gal As shown by the table, the original 375,000 gallon tank would have met all the old design parameters but is subs tantially unde rsized f or the new or revised parameters.
Analysis of Safety Implications If the tank size had remained at 375,000 gallons, it is possible t' hat taa pumps taking suction f rom it would have run out of water and lost s uct ion. The safety injection and charging pumps are multistage pumps and upon loss of suction may cease and become inoperable. Thus, the path from the discharge of the residual heat removai pumps through these pumps may have Seen lost. However, even with worst single failure for tank sizing, the containment spray and res1@tal heat removal paths into the reactor
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coola nt system and containmeqt respectively would have remained operable g
and water could have been supplied to the core and containment atmosphere.
$f No accident analysis was made of a loss of coolant accident under
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these conditions since the design has been changed and these conditions no longer credible.
.. rect ive Act ion t
3 The RWST storage capacity has been increased f rom 375,000 gallons
~o 475,000 gallons. This larger size is more than adequate for all the asign considerations and leaves some margin for possible future changes.
No work on the tank foundation, piping or ercetion has started and thus, no field modification needs to be taken to enlarge the tank.
- However, design drawings.for the tank, piping and foundations were complete and certified for construction and will have to be modified. The spray chemical addition tank will also have to increase in size proportional to the RWST or the concentration of chemicals in it will have to increase to compensate -
for the RWST' enlargement.
A decision on what will be done to the spray chemical addition tank will be made in' the 'near f uture and documented in the Final Saf ety Analysis Report.
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