February 12, 1997
Mr. Oscar A. Paulson, Facility Supervisor
Sweetwater Uranium Facility
Kennecott Uranium Company
PO Box 1500
Rawlins, Wyoming 82301
Dear Mr. Paulson:
The U.S. Nuclear Regulatory Commission staff has completed its review of the "Revised Addendum to the Revised Environmental Report - Regional Seismicity" submitted in response to a staff comment on the "Revised Environmental Report" (ER) for the Kennecott Sweetwater Uranium site, Source Material License SUA 1350. The NRC staff's comment (Comment 9 to the ER), transmitted to KSU by a letter of May 3, 1995, dealt with two concerns related to the assessment of the seismic hazard at the Sweetwater site. Based on the information provided by KSU, the staff concludes that both concerns related to Comment 9 are resolved.
The staff's evaluation of KSU's response is enclosed. If you have any questions regarding this letter or its enclosure, please contact the NRC Project Manager, Ms. Charlotte Abrams, at (301) 415-5808.
Sincerely,
Joseph J. Holonich, Chief Uranium Recovery Branch
Division of Waste Management
Office of Nuclear Material Safety and Safeguards
cc:
R.Fransen, WYDEQ
J.Case, WYGS
Enclosure: As stated
Docket No. 40-8584
License No. SUA-1350
In August 1994, Kennecott Sweetwater Uranium (KSU) submitted a revised Environmental Report (ER) for U.S. Nuclear Regulatory Commission staff review. The ER stated that a random seismic event of magnitude 6.5 has a 105 probability of occurrence with a 24 km radius of the Sweetwater site, and the peak acceleration from this event would be 0.189. The NRC staff commented (Comment 9, NRC letter of May 3, 1995) that 1) the estimated peak ground acceleration (PGA) from a random earthquake may be too low, and 2) a deterministic evaluation of the Chicken Springs fault system (about 15 km from the site) should be included in the seismic analysis, based on evidence that it may be a capable fault.
KSU, in November 1995, submitted an addendum to the ER which specifically addressed Comment 9 of the staff's ER review. In March 1996, NRC staff received a report from the Wyoming State Geological Survey (WYGS; Case, 1996) under cover letter from Governor Geringer, that recommended Wyoming tailings sites (including the Sweetwater site) be designed to withstand a peak horizontal acceleration (PHA) with a recurrence time of 2000 years, which equates to a 200 year design life.
Following discussions between KSU, WYGS, and NRC staff, which included a visit to the Chicken Springs fault system, KSU submitted a revision to the ER addendum in June 1996. In the revised addendum, the Chicken Springs fault system, located 19 km from the Sweetwater site, is considered to be active based on work by the WYGS (Case, 1991, and personal communication, 1996). The addendum estimates that the maximum credible earthquake (MCE) magnitude for the Chicken Springs fault system is 6.5, which results in a PGA of 0.229, based on mean plus one standard deviation attenuation curves presented in Campbell (1981). KSU considers the PGA value of 0.229 to be conservative for the Sweetwater site.
Comment 9 addressed two concerns: 1) the estimated PGA from a random earthquake may be too low, and a 15 km epicentral distance should be used for the floating earthquake; and 2) the Chicken Springs fault system is a fault system with an assigned MCE which should be considered in the seismic hazard analysis. Therefore, the staff's analysis of the licensee's response is presented in two parts.
The estimated PGA from a random earthquake may be too low, and a 15 km epicentral distance should be used for the floating earthquake.
The KSU ER employs a Wyoming Basin floating earthquake analysis done by Geomatrix (1988) for the U.S. Bureau of Reclamation. This analysis assumes a magnitude 6.5 earthquake with an epicentral distance of 24 km, resulting in a PGA of 0.18 9 at the site. As stated in the original basis for Comment 9, NRC staff considers 15 km to be an appropriate epicentral distance to calculate PGA for a floating earthquake (NRC, 1993).
In the addendum to the ER, KSU has revised its methodology to assume a magnitude 6.25 earthquake occurs 15 km from the site, and takes the median ground motion from the Campbell (1981) attenuation model, yielding a value of 0.15 9. This value does not exceed the 0.22 9 PGA proposed by KSU for the Sweetwater site in the ER addendum of June 1996. Therefore, the staff's concern related to the floating earthquake PGA is resolved based on information contained in the November 1995 and June 1996 addenda to the ER.
The Chicken Springs fault system is a fault system with an assigned MCE which should be considered in the seismic hazard analysis.
A reconnaissance investigation in 1987 by WYGS identified probable Holocene movement along portions of the Chicken Springs fault system; therefore, the Sweetwater site seismic analysis should consider the Chicken Springs fault system to be capable. Case (1991 and 1996) and Case and others (1995) further examined faults in Wyoming, including those in the area of the Sweetwater site. In 1996, the WYGS (Case, personal communication) postulated a range of possible fault slip magnitudes between 6.3 and 6.9 on the Chicken Springs fault system. These scenarios consider various degrees of surface rupture and the attendant earthquake magnitude which might occur along the Chicken Springs fault.
The WYGS findings were summarized for the NRC staff during a site visit to the Sweetwater site and Chicken Springs fault on May 21, 1996. At that visit, four fault rupture scenarios for the Chicken Springs fault system, based on the 1987 and 1996 WYGS investigations, were presented (Case, personal communication).
KSU's June 1996 revised addendum to the ER outlined the four rupture scenarios for the Chicken Springs fault used by Case to determine a postulated MCE magnitude. Those scenarios are based on fault length and assume that fault rupture is- associated with: 1) the length of the entire fault system as one unit; 2) the sum of the northern, active fault segments; 3) the longest continuous observable trace, including inactive segments; or 4) the sum of all active fault segments. The geologic evidence from Case's ground investigations and aerial photo examination indicates that a M 6.5 event placed 19 km from the Sweetwater site is the most tenable of the 4. Such an event would cause a PGA of 0.22 9 at the Sweetwater site, using the attenuation curves of Campbell (1981).
The NRC staff reviewed the information on the Chicken Springs fault system provided by Case during the May 21, 1996, site visit and presented by KSU in the June 1996 revised addendum to the ER. The staff finds the analyses conservatively estimate the seismic hazard for the Sweetwater site. Although two of the scenarios presented by Case would lead to larger earthquakes on the Chicken Springs fault, resulting in larger horizontal accelerations at the site, the staff concludes that the geologic evidence supports the alternative interpretation presented by KSU (Scenario 2). The single, long rupture of scenario 3 relies on highly equivocal evidence of Quaternary slip over this length of the fault. Scenario 4 requires a great number of fault swarm segments to slip simultaneously, whereas portions of the swarm appear to have slipped independently in the past. Moreover, the spatial pattern of such a simultaneous slip would be very unusual.
Therefore, the staff finds that a PGA of 0.229 for the Sweetwater site, from a M 6.5 event placed 19 km away on the Chicken Springs fault system, conservatively estimates the seismic hazard. Comment 9 is resolved based on information provided in the KSU's June 1996 ER addendum.
Bernreuter, D., McDermott, E., and Wagoner, J., 1994, Seismic hazard analysis of Title II reclamation plans: Lawrence Livermore National Laboratory, prepared for the U.S. Nuclear Regulatory Commission, 145 p.
Campbell, K.W., 1981, Near-source attenuation of peak horizontal acceleration: Bulletin of the Seismological Society of America, vol. 71, p. 2039-2070.
Case, J.C., 1991, Earthquakes and active faults in Wyoming: Wyoming State Geological Survey, Laramie, Wyoming.
Case, J.C., Larsen, L.L., Boyd, C.S., and Cannia, J.C., XXXX, Earthquake epicenters and suspected active faults with surficial expression in Wyoming: Wyoming State Geological Survey, Laramie, Wyoming.
Geomatrix Consultants, Inc., 1988, Seismotectonic evaluation of the Wyoming Basin geomorphic province: Report prepared for the U.S. Bureau of Reclamation, Contract No. 6-CS-81-07310, 167 p.
NRC, 1993, Final standard review plan for the review and remedial action of inactive mill tailings sites under Title I of the Uranium Mill Tailings Radiation Control Act, Revision 1: U.S. Nuclear Regulatory Commission, June 1993, 66 p.