ML20126K682

Submits Hazard Evaluation of Airborne Ash at Facility in Response to 810429 Memo Re 810408 Meeting on Assessment of Mount St Helens Activity.Evaluation of Likely Failure Modes Due to Ash Fall Initiated.Comments Invited

ML20126K682
Person / Time
Site:	Trojan File:Portland General Electric icon.png
Issue date:	05/12/1981
From:	Buske N SEARCH TECHNICAL SERVICES
To:	Godard D OREGON, STATE OF
References
1363, NUDOCS 8105200230
Download: ML20126K682 (2)

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12 May 1981 _ /, w 00 goy 14209 M PORTLE 10 OR;GOn 97214 u (503)235-o059 Mr. Don Godard Cregon Department of Energy J m(./ V o"j Tf /

Labor & industries Sullding Sa l em, Oregon 97310

REFERENCE:

HAZARD EVALUATION OF AIPSCRNE ASH AT TROJAN PLANT SITE Proj ect : 1363

Dear Con:

In your Memo of 29 Aprii 1981, concerning the 3 ApriI meeting with

?SP, you noted tha t an assessment of Mt. St. Wetens he:ards should be made ulTh ref erence to Mt. St. Helens, herself.

This can be done f or events with suf f iciently high prcbability of occurrence so that The short historical record of Mt . St. Helens activity will suffice.

USGS Bu lletin 1383-C (1978, c. C6) maps areas covered by Mt. St.

Helens' ash deposits greater than 7.9" + hick. Layer Wn , whicn is about 500 years old, is shown w!+h tnis 7.9" depth as f ar as 31 miles f rcm the mou n a i n, sugges+ing at least 5" depth 34 miles f rom the mountain. La y er Yn , whicn is about 40C0 years old, has greater than 7.9" of ash tere than 50 m i l es f rom the summ i t . Frcm this it appears that at leas? two events in the last 4000 years have produced ash deposits greater than 5" in thickness, 34 mi les f rom *he vo f cano. Based on this record, the mean probability per year of such an ash fall 34 miles from Mt. St. Helens is a? least 2/4C00 yr = 0.5x10-3/yr This probability is for a 5" ash fall sce'ewhere around Mt. St. Helens, at a 34-mi !e rad ius. . .not necessar i ly at the Trojan Plant site. In our repcrt of 5 October 1980, it was estNated that there was en;y a 20,5 chance of a g iven, heavy ash f all being direc?ed toward Trojan. Thus, the mean probability per year of such ar asn fati at -ne Trojan site is abou*

0.1x10-3/yr Final ly, one must correct for the times in which we live. Cbviously, ash f a!!s occur only when Mt. St. Welens is active. The record ( f or examp l e, Fire and !c e, 1980, pp . 174-175) suggests that ac+1ve periods cccur less than 10% of the t ime. Since the present is an active period, the probability for the present time is increased at least ten-fold.

On this basis, I es+imate that the present probability per year 00/

of an ash fall at Trojan, equivaient to 5" of compac+sd ash, is 3 1 x10- 3/yr

/o 8105200 30 W

i 12 May 1981 PAGE TWO Of course, that which will end up as 5" of compacted ash in the geologic record may start out as considerably more, fresh, wet ash, in glancing back over these considerations, I note that the estimates are quite cautious in the sense of not overestimating the severity of the problem. That is, one might argue that the situation is actually worse.

As you know, Trojan has a set of building design criteria which have been reported as an ability to witnstand 4i" of wet ash. The above calculation suggests that there is about a 4% chance *nat this condition will be exceedet during a 40-year ;! ant life.

Un'ortunately, there is no sol id basis f or eva luating this probl em.

The consequences of exceeding t9e building design criteria are apparently unknown. An NRC official has no+ed that this is not a proper area of NRC concern. He noted that bridge desigrers are rot concerned with the consecuences of a po*ential bridge collapse. A more cauticus approach woulc be to assume that, in the absence of contrary 'nformation, the abcve probability represents The probability of severe core damage. . .f rcm an ash fall event.

Furthermore, the overall probability of severe core damage at Trojan must be greater than the probabi' :'y of severe core damage cue to one event type--ash fall. But how much grez 'er ?

R. Eernero, Directcr of '.RC's Syster.s and Rel iab i l i ty Pesearch has, suggested that with severe core damage probabilities in the range of 10 '

'o 10-3/yr, corrective action should be taken within months. In 'his case, corrective action could include:

Installation of sloping roofs on critical structures provision of secure sources of emergency cooling nater and power development of realistic energency procedures investigation of o+her failure medes associated with high ash fall SEARCH has begun an evaluation of !ike y failure modes due +o ash fall at Trojan. A scenario-developmen* approach i s be i ng tr i ed . A +irst pass suggests interest in air quali+y systems f ai!ures with secondary failures.

Also of interest is a scenario involving a dry ash f all . As the ash accumulates toward dangerous ' evel s, emergency rencva l procedures would apply nater o wash away ash. This wou!d render the asn electrically conductive and more dense, potentially precipitating electrical and mechanical failures.

Your comments are welcome.

Very *r' / yours, xc: USNRC, Washington Q

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PSR, Pcrtland 1

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