ML20132C141

Forwards Hydraulic Engineering Rept & Recommendations Re Postulated Breakout of Spirit Lake & Impacts on Plant. Recommendations Based on Preliminary Info & Subj to Change

ML20132C141
Person / Time
Site:	Trojan File:Portland General Electric icon.png
Issue date:	05/11/1984
From:	Johnston W Office of Nuclear Reactor Regulation
To:	Lainas G Office of Nuclear Reactor Regulation
Shared Package
ML19264D658	List: ML20132B237 ML20132B532 ML20132B567 ML20132B600 ML20132B606 ML20132B636 ML20132B645 ML20132B648 ML20132B753 ML20132B785 ML20132B792 ML20132C141 ML20132C361 ML20132C780 ML20132C836 ML20133F110 ML20133F531 ML20133F542 ML20133F569 ML20133F596 ML20133F656 ML20133F674 ML20133F730 ML20133F743 ML20133F773 ML20133F786 ML20133F847
References
FOIA-85-353 NUDOCS 8405220470
Download: ML20132C141 (12)

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UNITED STATES f.1 f* [ 'g NUCLEAR REGULATORY COMMtssION q :e C0%

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William V. Johnston, Assistant Director flaterials, Chemical & Environmental Technology, DE

SUBJECT:

HYDROLOGIC ENGINEERING REPORT AND REC 0f;MENDATIONS CONCERNING SPIRIT LAKE AND THE TROJAN PLANT Attached is a Hydrologic Engineering report that addresses potential flooding

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of the Trojan Plant due to a postulated breakout of Spirit Lake. In this report, we are recommending several conditions to assure the continued safe operation of the Trojan Plant should there be a breakout of the Lake. These

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recommendations are made on the basis of preliminary informAtion and are subject to change should additional infornation become available.

The U.S. Geological Survey, in response to a request from the Federal Emergency Management Agency, is currently conducting a detailed study of the impacts of a breakout of Spirit Lake on the Columbia River. A y preliminary report on this study is scheduled for completion later this

month. We will review the USGS report at that time and notify you if there j is any new infonnation that would affect the recommendations of this report.

, We recently received the final version of the Trojan Plant / Spirit Lake study that USGS did for NRC together with a comparison of the assumptions and t

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results of this study with the independent study done by the licensee.

Copies of these are attached.

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William V. Johnston, Assistant Director Materials, Chemical & Environmental Technology 3 Division of Engineering

Attachment:

1. Hydrologic Engineering Report 1

' 2. USGS Study done for NRC-WRI Report 83-4197

3. Study comparisons by USGS, Tacoma, Washington i

cc: w/ attachment no.1 H. Denton R. Vollmer D. Eisenhut w/ attachment nos.1, 2, A 3 W. Johnston ,

R. Ballard -

J. Miller C. Trammell M. Fliegel R. Gonzales s

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.I d' HYDROLOGIC ENGINEERING REPORT AND RECOMMENDATIONS CONCERNING A POSTULATED BREAK 0UT OF SPIRIT LAKE AND ITS IMPACTS ON THE TROJAN PLANT Purpose ,

' On May 18, 1980, Mount St. Helens erupted and caused a massive mudflow flood that deposited a large amount of sediment in the Columbia River near the mouth of the Cowlitz River. The Trojan Nuclear Plant which is located on the west bank of the Columbia River about 4.5 niles upstream of the mouth of the Cowlitz River (See figure 1) was not directly affected by the sediment deposited in the Columbia River, although large deposits were measured near the intake structure. However, there now exists a potential for a more severe mudflow than the one that followed the May 1980 eruption because the outlet channel of Spirit Lake is blocked with debris deposited by the 1980 eruption. The debris-blockage which is a massive and unstable deposit, has caused a dramatic increase in the volume of water stored in Spirit Lake. Because of the unstable nature of the debris blockage, Spirit Lake could breach the blockage and cause a mudflow flood that could possibly affect the safe operation of the Trojan Plant. This report addresses the potential effects of how such a mud-flow flood at the Trojan Plant and reconmends several conditions to assure the safe operation of the plant.

Backaround The 1980 eruption of Mount St. Helens caused a failure of the north #1ank of the volcano. The resultant avalanche of rock, nud and ice swept down -

the slopes of Mount St. Helens, noving at high velocity into Spirit Lake and the Upper Toutle River Valley. As the debris avalanche slid into Spirit Lake, it displaced the lake level upward by more than 200 feet.

The avalanche also deposited an estimated 3.9 billion cubic yards (bcy) of sedinent in the upper 17 miles of the North Fork Toutle River Valley and buried the outlet channel of Spirit Lake with debris ranging in depth to 500 feet. Before the eruption, the volume of water in Spirit Lake was about 123,000 acre-feet. By December 1982, it had increased to 275,000 acre-feet due to the blockage of the outlet channel.

Mudflows associated with the May 1980 eruption moved down the Toutle River and carried more than 50 million cubic yards (mcy) of material into the Cowlitz River and its overbank areas. An additional 45 mcy were deposited in the Columbia River, mostly in a nine mile reach of the river extending fra, about 5 miles downstream of the mouth of the Cowlitz River to a miles upstream.

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j a In the fall of 1981, a task force organized by the U.S. National Forest c,

Service determined that the effective crest of the debris blocking the Spirit Lake outlet would deteriorate fran elevation 3490 ft msl to 3475 ft msl during the winter of 1982-83 because of subsidence and erosion. If was estimated

, that Spirit Lake would fill to elevation 3475 ft msl (314,000 acre-feet) sometime in March 1983 assuming that no action was taken to reduce the level and volume of the lake and that the annual inflow was average. Were this to happen, there would be a very high potential for failure of the blockage and catastrophic flooding downstream. To reduce the potential for failure, the Corps of Engineers, as an interim measure, constructed a pumping facil-ity at Spirit Lake. Pumping began on November 5,1982, and is expected to continue until a permanent solution to the Spirit Lake flooding potential is in place. With normal inflow, the goal of the pumping operation is to stabilize the lake level at an elevation of about 3,462 ft.

l msl which corresponds to a lake volume of about 275,000 acre-feet. However, j greater-than-normal rainfall, failure or disruption of the pumping systen and/or addition of debris into Spirit Lake from a subsequent eruption could cause the lake level to rise excessively. In an extreme case the debris blockage could fail.

Because of the potential public safety hazard associated with a breakout of Spirit Lake, the U.S. Geological Survey (USGS) in cooperation with the Federal Emergency Management Agency (FBiA) conducted a study (L'ater Resources Investigations 82-4125) to determine the extent of inundation that night result downstream in the Toutle and Cowlitz Rivers should a breakout generate a catastrophic mudflow flood. The results of the USGS study were published -

in early 1983 and the NRC received a copy of the USGS report in March 1983.

In its study, the USGS concluded that the level of nudflow flooding at the confluence of the Cowlitz and Columbia Rivers due to a postulated Spirit Lake breakout would be of significantly greater nagnitude than the nudflow that followed the May 1980 eruption of Mount St. Helens. The USGS did not assess the effects of the nudflow on the Columbia River. However, in review-ing the report, it appeared to the NRC . staff that the predicted mudflow level at the mouth of the Cowlitz, estimated to reach an elevation of about 40-50 feet msl, could potentially result in some flooding of the Tro,ian plant which is located on the west bank of the Columbia River about 4.5 miles upstream of tne mouth of the Cowlitz River. FEMA also expressed concern about the inpact on the Columbia River so they requested that USGS conduct additional nudflow studies to include effects on the Columbia River.

One of the problems encountered by the USGS is the lack of a verified computer model to handle transport of a mudflow. The models that are avail-able do not handle transport in an upstream direction at river confluences.

The mudflow that followed the May J980 eruption noved mud and debris upstream '

in several tributary streams. Tnis was significant in the Cowlitz River 4

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' above the mouth 'of the Toutle River and in the Columbia River above the mouth of the Cowlitz River. It is expected that a mudflow resulting from a failure of the Spirit Lake debris-blockage would also result in signiff-cant sediment movement at river confluences. The USGS tnus ' decided to develop a new computer model before assessing the effects on the Columbia River. This effort was expected to take about a year to complete.

NRC Concerns Because of immediate safety concerns, the NRC contracted with the USGS to furnish in the shortest time possible, a conservative estimate of the flows and elevations in the Columbia River at the Trojan Plant that would result frcn a Spirit Lake breakout. To expedite this analysis, the NRC instructed

.the USGS to use the mudflow hydrograph which had been generated in the USGS-FEMA Report (Water Resources Investigation 82-4125) as the inflow into the Columbia River. Using this mudficw hydrograph, the USGS considered two scenarios: (1) the coincident occurrence of a mudflow and a Columbia River r

flood and (2) the occurrence of a mudflow depositing sediment in the Columbia River during a low flow period and then being followed by a Columbia River flood flow. .

In the firsg of these scenarios, a hypothetical mudflow with a peak discharoe of 1.1 x 10 cfs at the mouth of the Cowlitz River was postulated to occur coincident with a high flow in the Columbia River. The hydraulic properties of the combined Cowlitz River mudflow/ Columbia River flood flow, could be those for either a clear-water flow or a mudflow. Therefore, the USGS -

computed flood elevations in the Columbia River at the Trojan Plant for both clear-water and mudflow assumptions. It was also assumed that very little, if any, sediment would be deposited in the Columbia River upstream of the Cowlitz River during this assumed high flow. For a coincident flow of about 690,000 cfs in the Columbia River, the water level at the Trojan plant would rise as high as the plant grade elevation of 45 ft msl assuming combined mudflow conditions. With clear water assumptions, a coincident Columbia River flow as high as the 850,000 cfs resulted in a flood elevation of only 32 ft. msl at the Trojan Plant.

In the second scenario, it was assumed that the mudflow would occur coincident with a low flow in the Columbia River. The low flow would not transport the sedinent downstream so it would deposit and aggrade the. Columbia River channel.

This event by itself would not impact on the safe operation of the Trojan Plant unless it was followed by a high flow in the Columbia River. It was detennined that if the Columbia became blocked to the extent estimated by the USGS during low flow conditions, a subsequent flow of about 430,000 cfs or greater would result in a flood level at Trojan at or above plant grade '

"S elevation of 45 ft msl.

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Flood elevations in the Columbia River at the Trojan Nuclear Plant i

e llSGS Analysis {c Licensee's Scenario No. I Scenario No. 2 l Columbia River ~ Analyses Mudflow Clea r-lfa ter Discharge i Conditions Conditions b l-125,000 cfs 31 ft msl l

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250,000 cfs t 39 ft msl  !-

400,000 cfs 35 ft msl 430,000 cfs 38 ft msl 25 ft msl 45 ft msl 640,000 cfs 34 f t msl 28 ft msl 49 ft msl '

• 690,000 cfs *45 ft msl '

790,000 cfs 47 ft msl 30 ft msl 52 ft msl ',

800,000 cfs 39 ft msl '

850,000 cfs 48 ft i.nsl 32 ft msl 696,000 cfs flow in the Columbia River was not considered by either the IJSGS or the Licensee. This flow was extrapolated by the NRC staff to correspond to a water level at the Trojan Plant of 45 ft .7sl which is the plant grade elevation. i k

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USGS ANALYSIS i' Scenario No.1 Scenario No. 2 l-Licensee's Mud flow Clea r-lla ter Analyses Conditions Conditions  !.

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[I Spirit Lake volume / elevation 310,000 ac-ft/ 314,000 ac-ft/ 314,000 ac-ft/ 314,000 ac-ft/

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percent by volume entrained from debris blockage l Sediment concentration in 45% 65% 65% 65%

percent by volume in mudflow dt the mouth of the Cowlitz i-

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5 0 Peak of mudflow into 3.86X10 cfs 1.1X10 cfs 0 1.1X10 cfs Columbia River 1.1X10 cfs Sediment volume deposited 0.28X109yd 0 0 0.50X109yd3 in Columbia River Slope of sediment deposited -7.5 f t/mi in Columbia River upstream NA NA -2.5 ft/mi of Cowlitz River -

Assumed tide level in 7.Sft 9.0 ft 9.0 ft 9.0 ft Columbia River at down-stream end of reach Assumed hydraulic properties clear water mud flow clear water clear water of flow in Columbia River downstream of Cowlitz River ,

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The licensee, in an independent study, also addressed the potential impact of a Spirit Lake breakout on the Trojan Plant. The licensee, however, did not use the mudflow hydrograph from the USGS-FDIA report. Instead they developed their own inflow hydrograph to the Columbia River based on their analysis of the effects of a Spirit Lake breakout in the Toutle-Cowlitz system. Their resulting inflow hydrograph to the Columbia River was less severe than that used by the USGS. In analyzing the effect of a nudflow on the Trojan Plant, the licensee conservatively assumed that during a coinci-dent high flow in the Columbia River, there would be sediment deposition of the mudflow in the Colunbia River.

The worse case considered by the licensee was based on a flow of 800,000 cfs in the Columbia River occurring coincident with a mudflow in the Cowlitz River. For this case, a flood elevation of 39 ft nsl was estimated for the Columbia River at the Trojan Plant. This elevation is lower than the Trojan Plant grade elevation which is 45 f t msl. This analysis is comparable to the USGS scenario no.1 except that the licensee conservatively assuned that sediment would deposit in the Columbia River upstream of the mouth of the Cowlitz River while the USGS assumed no deposition.

Initially, the licensee did not consider a case similar to the USGS's second scenario which analyzed a mudflow occurring during a period of low flows in the Columbia River with consequent high sedinent deposition to be followed by a high flow in the Columbia River. However, in response to a staff ,

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question concerning this scenario, the applicant stated that it considered I this scenario to be of negligible probability. Additionally their analysis T included the ~ assumption of sediment deposition in the Columbia during high flows so there was no need to assume a low Columbia flow during the nudflow followed by a high Columbia flow. They reported that if a flow of 1,000,000 cfs was to occur instead of the 800,000 cfs previously considered, the flood elevation in the Columbia River at the Trojan Plant would be 42.1 ft msl which is lower than the grade level of the plant.

In analyzing how a breakout of Spirit Lake would affect flood levels in the Columbia River at the Trojan Plant, both the USGS and the licensee had to make many assumptions. Table 1 compares some of the assunptions made.

Table 2 is a tabulation of the flows assumed in the Columbia River coincident with a mudflow from the Cowlitz River and the resultant flood elevations at Trojan.

The staff has reviewed both the USGS and the applicant's analys'es. All of the cases considered contain both conservative assunptions and assumptions which may be too optimistic. For example, the licensee's assumptions with respect to sediment concentration, peak discharge of the mudflow into the 'r Columbia, hydraulic properties of flow in the Columbia, and tide level are .

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% O 4 more optinistic than USGS's. However, their assumption of sediment deposition in the Columbia during an assumed concurrent flood is more con-servative than the USGS's analysis which assumed no deposition. Because of the uncertainty of the applicability of the computer models used and of the many assumptions that necessarily have to be made, the staff concludes that the results of the more conservative USGS analyses should be used as a basis for establishing limiting conditions for the operation of the Trojan Plant.

Both the licensee and USGS studies indicate that the elevation to which the Columbia River water level would rise in the vicinity of the Trojan Plant following a failure of the debris-blockage and a breakout of Spirit Lake, is directly dependent on the stage of the Columbia River at the time the nudflow flood would discharge into the Columbia River. In the USGS's first scenario, a discharge in the Columbia River of about 690,000 cfs would result in a flood level at the Trojan Plant equal to the plant grade elevation of 45 ft msl, assuming that a catastrophic mudflow flood moves into the Columbia River at the same time. In their second scenario, a Columbia River flow of 430,000 cfs, following a nudflow during a low Columbia River flow, result in water at plant grade.

The possibility of sediment fra, a mudflow blocking the intake structure and affecting the service water systen which provides safety-related cooling water, has been addressed by the licensee. In the event that the intake to the service water system is lost, the licensee is required by existing Technical Specifications to shut down the plant. In lieu of the service water system, adeanate cooling can be provided for a minimum ,

of 165 hours0.00191 days <br />0.0458 hours <br />2.728175e-4 weeks <br />6.27825e-5 months <br /> 'by the circulating water system and the cooling tower basin, assuming no makeup to the tjstem. This cooling capacity can be maintained in the event of concurrent loss of off-site power by use of the cooling tower make-up pumps which can be connected to an emergency electrical bus supplied by the diesel generators by closing an electrical breaker.

If the water in the cooling tower should be exausted by evaporative losses before the intake to the service water is restored, additional water can be punped into the cooling tower by tenporary pumping systems or fire pumpers. Water could probably be taken from the Columbia Piver.

An alternative source of water could be from the on-site Reflecting Lake or Recreation Lake. The conbined water volume of these lakes is approxi-mately 100 million gallons. Assuming that only 70 percent of this volume is available and no inflow to the lakes, make-up water for the cooling tower basin is available for about 97 days.

Recommendations Based on the above, we reconmend that during the estinated 2-3 year time '

interval until the Corps of Engineers installs permanent facilities to control the level of Spirit Lake, the following measures be taken while the Trojan Plant is operating.

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1) Monitor and record the discharge of the Columbia River in the plant vicinity once every 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.

2) Whenever the Columbia River discharge is in excess of 500,000 cfs, the following will be implemented:

$ (a) On a daily basis, confinn with the appropriate authorities that the early warning system installed to signal an uncontrolled release from Spirit Lake is operational.

i' (b) Procedures designed to implement emergency measures in the event of a Spirit Lake breakout will be reviewed to confinn that the necessary actions can be successfully implemented.

3) In the event of a failure of the Spirit Lake debris-blockage, the Trojan Plant will be shut down immediately regardless of the flow in the Columbia River.

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(1) U.S. Army Corps of Engineers, Portland District, " Mount St. Helens Eruption, The Challenge to Restore and Protect", October 1981.

l- (2) U.S. Geological Survey, "Mudflow Hazards Along the Cowiitz River fron a Hypothetical Failure of Spirit Lake Blockage", Water Resources Investigation 82-4125, Tacoma, Washington,1983.

, (3) U.S. Geological Survey, " Preliminary Estimate of Possible Flood Elevations in the Columbia River at Trojan Nuclear Power Plant Due to Failure of Debris Dam Blocking Spirit Lake, Washington". Water Resources Investigation 83-4197, Tacoma, Washington, 1984.

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