ML19291C516
| ML19291C516 | |
| Person / Time | |
|---|---|
| Issue date: | 12/31/1979 |
| From: | Scarano R NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Kerr G NRC OFFICE OF STATE PROGRAMS (OSP) |
| References | |
| FOIA-81-8, REF-WM-28 NUDOCS 8001240533 | |
| Download: ML19291C516 (4) | |
Text
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{C UNITED STATES 4
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g NUCLEAR REGULATORY COMMISSION 3
-E W ASHINGTON, D. C. 20555
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O DEC 311979 MEMORANDUM FOR:
G. Wayne Kerr, Assistant Director for State Agreement Program Office of State Programs FROM:
Ross A. Scarano, Chief Uranium Recovery Licensing Brar.ch Division of Waste Management
SUBJECT:
INFORMATION NOTICE TO AGREEMENT STATES REGARDING CHURCH ROCK FAILURE Enclosed is a sumary of the probable cause of failure of the Church Rock uranium mill tailings impoundment and the Uranium Recovery Licensing Branch's recomendations regarding embankment inspections and evaluations to be perfonned by Agreement States. We recommend that this enclosure be fomarded to all appropriate Agreement States in the fonn of a State Program: Infonnation Notice.
Gr/' "
Ross A. Scarano, Chief Uranium Recovery Licensing Branch Division of Waste Management
Enclosure:
As stated g
1793 235 8001240
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ENCLOSURE EVALUATION OF URANIUM MILL TAILINGS RETENTION SYSTEMS FOLLOWING THE EMBANKMENT
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FAILURE AT CHURCH ROCK The Church Rock Embankment Failure:
The United Nuclear Corporation dam impounding tailings and raffinate from the Church Rock mill was observed to have been breached early on July 16, 1979.
We have reviewed the company's evaluations of the probable cause of the accident and generally concur with the report ~ d findings. The dam e
was located on a site containing alluvial soils overlying bedrock having an irregular surface.
Depths of alluvium ranged from less than 20 feet up to a maximum of about 100 feet.
Consolidation tests were conducted on samples of the alluvial soils during the preconstruction design phase and after construction of the starter embanknent. These tests indicated that settlement of about 5 percent would result from the loading of the embankment under unsaturated conditions. After saturation, additional settlement ranging from 1-1/2 percent to 13 percent was experienced due to collapse of the soil structure.
As a result of the potential for large compression of the allu)ium and because of the irregular bedrock surface, large differential settlenent of the dam occurred.
Settlement in excess of 3 feet was measured in Janur y 1979 by a -.nsultant to United Nuclear Corporation.
As a result of differential settlenent, c 'acks developed in the enbankment.
Longitudinal cracking along 1250 feet of the embankment has been well documented by observations made by the operator's consultant in December 1977 and photographs of the cracks in July 1978.
Transverse cracks, perpendicular 1793 236
_2 to the dam axis and entending nearly to the downstream shell, have been observed in the breach area after the failure.
For some time period immediately prior to the failure, tailings water was maintained in direct contact with the enbanbnent near the breach area. The high pore water pressure in the embankment resulted in a decrease in the strength of the embankment and caused instability to develop. After the instability was created, the large flow of water through the cracks resulted in internal erosion which accelerated the breach.
One factor which may have helped to accelerate the internal erosion is the fact that the high acidity of the tailings water causes the embankment soil to be highly dispersive.
Recommended Evaluation Procedures:
It is the recommendation of the Uranium Recovery Licensing Branch of the NRC that the appropriate Agreement States perform inspections and evaluations of all uranium mill tailings dams under their respective jurisdictions to insure that an incident similar to the Church Rock s
failure will not occur. To accomplish this goal it is recommended that in addition to regular state inspection procedures, the following specific evaluations should be performed by a licensed professional engineer:
a.
Embankments should be visually inspected for signs of slumping, cracking, movement or co? entration of seepage.
Records should be viewed to determi..' whether slumping, cracking, movement or concentration of set' age has been evident in the past and if so, what remedial measures have been performed.
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.. b.
If the embankment inspection performed under item "a" reveals evidence of slumping,. cracking, movement or concentration of seepage, the following detailed analyses should be performed.
1.
The embankment and foundation soils should be checked for their susceptibility to collapse when saturated.
2.
The embankment and foundation soils should be checked for their dispersive characteristics when exposed to the impounded tailings solution.
3.
The foundation profile should be examined to determine the existence of any factors which could. lead to significant differential settlenent.
Regardless of the results of items "a" and "b", the operators c.
should be required by license condition to initiate a continuing program that meets the criteria presented in NRC Regulatory Guide 3.11.1, " Operational Inspection and Surveillance of Embankment Retention Systems for Uranium Mill Tailings."
If additional information regarding this matter is desired, contact Mr. Ross A. Scarano, Chief, Uranium Recovery Licensing Branch, U. S.
Nuclear Regulatory Commission at 301-427-4103.
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