ML20141E895

Forwards NRC Review of Us Army Corps of Engineers Study Re Possible Flood Levels at Facility.Study Showed Higher Flood Levels than Design Basis Flood Level.Permanent Flood Protection Unnecessary.Alternative Measures Suggested

ML20141E895
Person / Time
Site:	Trojan File:Portland General Electric icon.png
Issue date:	04/10/1986
From:	Chan T Office of Nuclear Reactor Regulation
To:	Withers B PORTLAND GENERAL ELECTRIC CO.
References
TAC-57971, NUDOCS 8604220416
Download: ML20141E895 (9)

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April 10, 1986 j Docket No. 50-344 ' DISTRIBUTION Vocket File NRC PDR L PDR Gray File PAD-3 Rdg OELD Mr. Bart D. Withers H. Thompson T. Chan Vice President Nuclear K. Johnston C. Vogan Portland General Electric Company E. Jordan B. Grimes 121 S.W. Salmon Street J. Partlow ACRS 10 Portland, Oregon 97204

Dear Mr. Withers:

Enclosed for your information is the staff's review of the U.S. Army Corps of Engineers' (COE) study regarding possible flood levels at the Trojan Nuclear Plant. Results of the COE study showed higher flood levels, in some instances, than the Trojan design basis floud level of 45 feet, mean sea level.

We have determined that the condition for which the COE assumed would result in flood levels greater than the plant grade of 45 feet exceeded the staff's assumptions regarding the design basis flood. However, for the condition which.

is consistent with our design basis flood, water levels would reach plant grade. This level is 4 feet higher than the maximum water level assumed in the Final Safety Analysis Report (FSAR). This difference is attributed to the actual determination of the Columbia River's discharge profile and is considered more accurate than the projected discharge information used in the FSAR.

We' conclude that permanent flood protection in this case is not necessary;.

alternative measures have been suggested in the enclosure.

/s/TChan Terence L. Chan, Project Manager PWR Project Directorate No. 3 Division of PWR Licensing-A

Enclosure:

As stated l

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.i Mr. Bart D. Withers Portland General Eletric Company Trojan Nuclear Plant cc:

Senior Resident Inspector U.S. Nuclear Regulatory Commission --

Trojan Nuclear Plant _

Post Office Box 0 Rainier, Oregon 97048 Robert M. Hunt, Chairman Board of County Commissioners

, Columbia County St. Helens, Oregon 97501 William T. Dixon Oregon Department of Energy i Labor and Industries Building t

Room 111 Salem, Oregon 97310 Regional Administrator, Region V U.S. Nuclear Regulatory Commission c

Office of Executive Director for Operations i

1450 Maria Lane Suite 210 l Walnut Creek, California 94596 i

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REVIEW 0F CORPS OF ENGINEERS REPORT CONCERNING FLOODING AT THE TROJAN NUCLEAR PLANT i

Background

The design basis flood considered by the staff in licensing the Trojan Nuclear Plant is the flood that would result from a catastr phic failure of Grand Coulee Dam, which is located on the Columbia River about 524 miles l upstream of Trojan. This flood would result in a maximum stillwater level

. of 41 feet at the Trojan Plant. To allow for higher water levels which could

[ result from wind generated water waves, the Trojan plant grade elevation was F

f set at 45 feet.

In 1982, the U.S. Army Corps of Engineers (COE) conducted inundation mapping studies for the lower Columbia River. The portion of the river li l studied included the reach of the river on which the Trojan Plant is s

located. The COE studies showed higher flood levels, in some instances, than the Trojan Plant design basis flood level of 45 feet msl.

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This report discusses the result of the staff's review of the COE report i

and the staff's conclusions regarding the safety of the Trojan Plant

with res.pect to flooding.

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Purpose and Results of the COE Studies . . .

[ The COE studies were performed under the COE's Dam Safety Program to determine the extent of fleoding associated with hypothetical dam failures on the Columbia River. The studies reflect events of an extremely remote nature and as such are not intended in any way to I reflect upon the integrity of the dams included in the study.

. The purpose of the studies was to prepare inundation maps showing the i

extent of flooding associated with various postulated conditions at each i

of the lower Columbia River dam projects. In downstream order these projects are McNary, John Day, the Dalles, and Bonneville Dams. The Trojan Plant is located about 73 miles downstream of Bonneville Dam.

l For each of the dams considered by the COE, three postulated conditions j were analyzed as follows:

i CONDITION 1 Probable Maximum Flood (PMF) Without Dam Failure i For. this condition it was assumed that a PMF would occur on the lower Columbia River. With the exception of Bonneville Dam, all r

other dams were found capable of discharging the PMF without overtopping. The COE study concluded that because of its outdated N

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s hydrologic design, Bonneville Dam would be overtopped under PMF Conditions. Under conditions of overtopping, Bonneville dam was assumed to fail, f CONDITION 2 Probable Maximum Flood (PMF) With Dam Failure Although none of the dams except Bonneville would be overtopped by

( the PMF, the COE in this scenario assumed that all dams would fail

. during the PMF. The dam failures were modeled using the National Weather Service (NWS) DAMBRK computer model. The outflow

! hydrographs were routed downstream through the Bonneville i

! tailwater. The Bonneville hydrograph was then routed down the Columbia River past the Trojan Plant to the Pacific Ocean using the NWS DWOPER unsteady flow model.

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I CONDITION 3 Dam Failure at Normal High Pool Level b

3 This condition assumed that a dam failure would occur during the i

spring normal high pool. The flood hydrograph from each failed dam i

was routed downstream to the Pacific Ocean using the same NWS

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? computer models as noted above.

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1 For the three conditions described above, the COE computed water surface elevations using cross-sections along the lower Columbia River.,_These elevations were then used to develop floodplain maps for the reach of the Columbia River extending from McNary dam to the Pacific Ocean.

1 k Staff Review of COE Studies i

s General Design Criteria (GDC-2), " Design Basis for Protection Against 4

Natural Phenomena," requires that structures, systems and components important to safety be designed to withstand the effects of natural phenomena such as floods. Regulatory Guide (RG) 1.59, " Design Basis i Floods for Nuclear Power Plants," describes combinations of flood-causing events which provide adequate design flood bases to meet the requirements

!- of GDC-2. With the exception of Bonneville Dam, all of the dams studied t

by the COE can safely pass their respective PMF's without be(ng over-0 topped. Therefore, hydrologic events would not cause any of the dams, j except Bonneville, to fail. Dam failures then, would have to be caused i .

! by seismic or other non-hydrologic events.

I I i RG 1.59 suggests that a 25 year flood be considered coincident with a non-hydrologic dam failure. Therefore, considering a PMF coincident with a dam failure, as was done by the COE in condition 2, is a sequence of events more severe than required to be considered for a design basis flood because a PMF'is a much more severe event than a 25 year flood.

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i The COE study determined flood levels at cross-sections located both ,

upstream and downstream of the Trojan Plant. The cross-sectioni closest to the Trojan Plant are at river miles 66.1 and 75. Water surface elevations at the Trojan Plant, which is located at river mile 72.4 were

determined by interpolation by the NRC staff. The tabulation below shows interpolated flood elevations at the Trojan site for the three conditions considered by the COE.

Columbia River Water Surface Elevations in ft msl

- at the Trojan Plant Condition 2 Condition 3

. Condition 1 .PMF with PMF Dam Failure PMF without multiple at normal high Dam dam failure dam failure pool level P McNary 45 48 16 s John Day 45 47 19

The Dalles 45 45 12

' - Bonneville 45 45 12 j This tabulation shows that for Condition 2, flood levels would equal or exceed the Trojan Plant grade elevation of 45 feet. However, as discussed above, Condition 2 floods are of greater magnitude than floods which have to be considered for design of nuclear power plants.

i Condition 1 floods, which are consistent with design bases floods, would result in water levels of 45 feet for all scenarios considereLby the COE.

Although this elevation is the same as the plant grade elevation, wind generated waves coul'd cause water levels to exceed plant grade elevation and potentially affect the safe operation of the plant.  ;

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The Columbia River drainage basin is very large, comprised of an area of

' approximately 260,000 square miles. The upper reaches of the drainage area are bounded by the Rocky Mountains on the east and the Cascade Range I on the west. It is in these mountainous areas that snow can accumulate in the winter months and contribute to high river flows when the snow

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melts in the spring. A PMF on the Columbia River would result from rapid melting of an extraordinarily large and extensive snow pack. Such a snow

pack would be evident several months before actual melting occurred.

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Flood forecasts for the Columbia River are made through the cooperative effort of the Columbia River Water Management Group (CRWMG). This group consists of representatives of eleven federal agencies and from the states that border the Columbia River. As part of the flood forecasting I

- program, the Soil Conservation Service (SCS) regularly conducts airplane flights over the Columbia River basin during the winter months to determine depth and extent of snow pack. Using tFe information obtained during these snow flights, the COE is able to forecast potential river I .

floods, weeks or even months before they actually occur.

i Staff Conclusions Since the Trojan Plant grade elevation of 45 feet is the same as the COE's predicted PMF level, only the effects of water waves running up on safety-related structures are of concern. However, since potential floods can be forecast well in advance of their occurrence, there would l

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be sufficient time for the licensee to implement some temporary actions to protect any safety-related structures or equipment which coul.d be adversely affected by flooding. Thus the staff concludes that permanent flood protection measures are not necessary. However, it would be prudent for the licensee to determine if wind wave runup could adversely affect the safe operation and shutdown of the plant. If the conclusion is that there would be no adverse effects, the basis for the conclusion should be provided. Otherwise, an operating procedure to define the f

j. actions to be taken in the event of an impending PMF on the Columbia River should be developed. These procedures should as a minimum identify the actions to be.taken and the event that will initiate implementation of those actions. An example of such an event could be a flood level in

! Bonneville or other upstream impoundments or a discharge from the dams or l any other measurable activity that could be identified with sufficient warning time to permit implementation of protective plans at the Trojan Plant before the actual flood occurred. In addition, the procedures l should describe the level to which ficod protection would be provided at the plant and identify the safety-related structures, systems or components that would have to be protected.

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