ML20148E749
| ML20148E749 | |
| Person / Time | |
|---|---|
| Site: | Fermi  |
| Issue date: | 10/30/1978 |
| From: | Jens W DETROIT EDISON CO. |
| To: | Stolz J Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 7811070096 | |
| Download: ML20148E749 (4) | |
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- s Wayn3 H. Jins Assistant %ce President Eng neering and Constructton (313) 237-8860
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i October 30, 1978 EF2 - 41684 Revision 1 Mr. John F. Stolz, Chief Light Water Reactors Branch No. 1 Of fice of Nuc1 car Reactor Regulation United States Nuclear Regulatory Commission Washington, D. C.
20555
Subject:
Tornado Missile Proection for Residual Heat Removal Cooling Towers
Reference:
Enrico Fermi Atomic Power Plant - Unit 2 NRC Docket 50-341
Dear Mr. Stolz:
Edison's initial response to your February 1,1978 letter on this subject was provided in our letter (EF2 - 41684) to you on July 14, 1978.
NRC Staff comments, subsequent to these letters, indicated that additional information would be helpful. Edison, therefore, provides this revision to our July 14, 1978 letter, including all information contained in the previous letter.
Edison feels that sufficient justification does exist for making assumptions to show the probability of tornado borne missiles is acceptably low and that justification is provided in this letter.
However, Edison has considered the consequences of postulated damage to all fans in the cooling towers and has concluded that the reactor can safely be shut down. Edison is convinced that the blades on one fan can be replaced before it is needed in the shutdown sequence.
The fan motors and other components of the towers can withstand or are protected from the tornado missiles defined in Standard Review Plan 3.5.1.4, Revision 1.
Edison has not identified a need for the protection of the fan blades against postulated tornado borne missiles and, therefore, does not intend to provide a structure or grate to protect the towers from postulated tornado missiles.
PROBABILITY OF TORNADO MISSILES The Enrico Fermi 2 site is located in an area with the highest ground elevation within one mile of the site 10 feet above grade.
Using the guidelines in the Standard Review Plan 3.5.1.4, the only design missile that can be elevated to the top of the towers (54 feet above grade) is the 1 inch diameter by 3 foot, 8 pound rebar.
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Mr.' John F. Stolz EF2 - 41684 Revision 1 October 30~, 1978 Page 2 The cooling tower structure is designed to withstand horizontal and vertical tornado missiles. The cooling fan motor is enclosed in a concrete cubicle designed to repel both types of missiles. The cooling tower gear hub and shaft are protected by missile shields.
The tower vendor has stated that the fan blades could be damaged by the 8 pound rebar if the velocity is sufficiently high.
It would be expected that some damage would occur to the mist elimi-nators and/or spray nozzles.
Damage to the mist eliminators and nozzles is. inconsequential compared to the overall area of the mist eliminator and-number of spray nozzles.
Miscellaneous debris could fall into the tower from the tornado.
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However, sufficient time is available to clean up this debris.
(The time available is described later in this letter.)
The cooling tower vendor has confirmed that as long as the drive shaf t is protected against missiles, the only item that can be damaged is the fan blade (s).
Any force caused by a missile break-ing a moving fan blade is not. sufficient to damage other drive components such as the shaf t, gear hub, or motor.
In addition, i
the starting force produced by the motor even if the fan assembly is jammed by debris is not ~ sufficient to cause damage to any com-ponent.
The probability for damage to the fan blades was developed using the following assumptions:
1.
The fans will not be operating.
The fans would be shut down (by adm1nistrative control) whenever a tornado warning is issued.
2.
If the 8 pound missile strikes the fan blade, it will damage it.
3.
Because rebars are small relative to the space between the blades, many would pass between the fan blades.
The damage prone area is 25% of the fan discharge area.
4.
The rebars could land anywhere within 250 feet of r
the point of pickup. Therefore, a rebar must be within 250 feet of the cooling tower to reach the fan discharge.
5.
Ricochetting missiles retain 50 percent of their energy af ter bouncing off the fan discharge inner wall.
The probability calculations show that the_ probability of 20 rebars damaging two RHR cooling tower fans is 2.5 x 10-10 per year,of three fans 8 x 10-14 per year, and of all four fans 5 x 10-18 per year.
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Revision 1 Mr. John F. Stolz EF2 - 41684 October 30, 1978 Page 3 The calculations also show that we could allow 395 missiles within 250 feet of the RHR Cooling Towers and yet only begin to approach a 10-7 per year probability of damage to two fans.
REACTOR SHUTDOWN ANALYSIS Notwithstanding the extremely low probability of damage to the f an blades of all four cooling towers, Edison has reviewed the ability to shut down the plant, given this event. For this analysis, the following assumptions are made:
1.
The tornado would also cause a loss of offsite power.
2.
The joint occurrence of low probability events (a LOCA and an earthquake) is sufficiently small and, therefore,
not considered (in accordance with Regulatory Guide 1.117, Tornado Design Classifications).
3.
An independent failure of an EDG to start or run and the subsequent loss of ESF equipment on that bus is assumed.
The reactor shutdown would be an isolation shutdown because of the loss of offsite power. The shutdown cooling system of the RHR would be initiated using the RHR and RHRSW pumps available while accounting for the loss of an EDG, The large quantities of water in the sup-pression chamber and the RHR reservoir would heat up as the decay heat is transferred to the suppression pool and the RHR service water system. The decay and sensible heat values are taken from FSAR -
Table 9.2.8.
The shutdown can then proceed until torus water tem-perature reaches an arbitrary limit of 1700F or RHR cooling water reservoir temperature reaches 125 F.
Calculations show that if no steps are taken to reject the heat from the RHR reservoir, it would take about 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> to reach the above limiting temperature of the torus and reservoir.
l Many options are available to the plant operators to supplement the plant cooling during this period, any of these changes would extend the time before the cooling tower fans are needed.
The 120 KV system can be reenergized by starting one of the onsite combustion turbine generators. Repowering the 120 KV system would allow use of many non-class 1E systems to assist in the reactor shutdown.
One of the systems available would be the circulating water system and the main unit condenser. These systems would allow blowdown of the reactor steam to the main condenser as in a normal shutdown.
Another cooling method would be to add as much makeup water to the RHR reser-voir as possible and overboard to the circulating water reservoir (this method is cooling by displacement with cooler water).
The make-up water system for the RHR reservoir is supplied from the general service water system.
In addition, the electric and diesel fire pump j
could provide cooling water through fire hoses placed in the reservoir.
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Mr. John F. Stolz EF2 - 41684 Revision 1 October 30,~1978 Page 4 The diesel fire pump is self-starting without onsite or offsite AC power. The RHR service water would be run through the cooling towers i
even if the fans were damaged and achieve cooling by natural draf t.
Some or all of these techniques would be used to extend the avail-
' able time before the RHR cooling tower fans would be-needed.. In fact,. it is possible that cooling tower fans would never be required r
for-the plant shutdown.
f As an additional back-up, a spare set of RHR cooling tower fan blades I
and the necessary tools will be stored on site so that one cooling tower can be restored to operation. The tools and materials required
- are readily available in power plants (such as ladders, timber logs, manual pulleys, and planks). The tools that are necessary will be identified.and dedicated for use as cooling tower fan blade replace-ment tools.
The cooling. tower vendor has stated that only four hours are required to replace blades and restore the fan to an operative condition.
Thus, restoring the operation of one tower within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> is feasible even if none of the previous steps were taken. As will be explained next, one cooling tower is sufficient to reject the plant decay heat and thus maintain shutdown.
As the hot water (cooling tower inlet) temperature increases, the heat rejection rate - of the tower increases. The cooling tower was very conservatively designed for the design basis LOCA to' discharge 0
the rated heat with a 116 F hot water temperature.
Based on the cooling tower curves, if the hot water temperature increases to' 0
140 F, the resulting 70% increase in heat rejection capacity is suf-ficient to stabilize the reservoir temperature. The 140 F hot water temperature is the approximate temperature calculated when the RHR reservoir temperature is at 125 F.
i In conclusion, Edison's position is that it is extremely unlikely that all RHR cooling tower fans would be damaged by tornado missiles.
i l-Even if this did occur, alternate methods exist to supplement or replace the need for the cooling tower fans. One tower fan can be repaired and made operable within the time period necessary without taking credit for the alternate methods of supplementing the cooling heat rejection.
Finally, Edison will purchase a spare set of fan blades, tools for removal and replacement of the blades, and the equipment necessary for replacement of the fan blades. The blades, tools and equipment i
will be dedicated for fan blade replacement and will be maintained on site as an appropriate alternative to providing increased pro-
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i tection. for the fans as requested by the NRC.
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Sincerel l
-WHJ/RCA/LES/jr G
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