Radioactivity in the Underground Environment of the Cambric Nuclear Explosion at the Nevada Test Site

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Radioactivity in the Underground Environment of the Cambric Nuclear Explosion at the Nevada Test Site ‘Lawrence Livermore Laboratory, Livermore, CA 94550. “US Geological Survey, Denver Federal Center, Denver, CO 80225. of the University of California LOS ALAMOS. NEW MEXICO 87545 An Affirmative Action/Equal Opportunity Employer ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION

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II. PHYSICAL SETTING ...................................................................................................... 5 III. DRILLING AND INITIAL CONSTRUCTIONOF RNM-1 ...................................................19 IV. GROUND-WATER SAMPLING FROM THEVICINITY OF THE CAMBRIC UNDERGROUND NUCLEAR EXPLOSION ..................................................................... 25 C. Zone II - Within Lower Cavity Region.................................................... 28 D. Zone III - Within Upper Cavity Region............................................... 29 G. Comments on Hydraulic Characteristics of Material in Zones II and III....................................................................................... 31 V. CHEMICAL AND RADIOCHEMICAL ANALYSES ........................................................... 31 C. Radiochemical Analyses and Results ................................................. 34 2. Pumped Water Samples ....................................................................... 42 3. Pressurized Water Samples .......................................................... 49 VI. RESULTS AND DISCUSSION.............................................................................................50

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APPENDIX: A. CHRONOLOGICAL HISTORY OF RNM-1 (WELL COMPLETED IN CAMBRIC B. SELECTED CHEMICAL ANALYSES OF WATER SAMPLES FROM RNM-1 ... 66 I. LASL Procedure for Removal of Kr, HT, and HTO from Sidewall Cores and Pressurized Water Samples ...................... 67 II. Gamma-Spectral Analyses of Sidewall-Core and Water Samples III. Decay Data Used iny-SpectralAnalyses at LLL and LASL. . 70 I. Activity Ratios for Filtered, Unfiltered, and Acidified II. Results of y-Spectral and Sr Analyses of B-Series Water III. Results of y-Spectral and Sr Analyses of C-Series Water IV. Results of y-Spectral and Sr Analyses of D-Series Water VI. Results of y-Spectral Analysisof Sidewall Core Samples. 81 VII. Tritium Analyses of PumpedWater Samples ................................. 82 A. Comparison of LASL and LLL Results of T and Li Analyses of Filtered and Unfiltered B-Series Water

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The site of the Cambric underground nuclear test was re-entered nearly ten years after its detonation at the USERDA Nevada Test Site. The experimental results obtained from radiochemical analysis of sidewall cores and water taken during the re-entry drillback and sampling operations are summarized and dis cussed in this report. The investigation was part of the Radionuclide Migration Project sponsored by the Nevada Operations Office of ERDA. The entire report was reviewed and modified by all three of the authors, but primary responsibility for the compilation and interpretation of the data and for writing given sections was divided among the authors as follows: Darleane C. Hoffman, Los Alamos Scientific Laboratory: Summary; Section I, Introduction and Background; Section V, Radiochemical and Chemical Analyses; Section VI, Discussion; Appendix, C, Procedures; Appendix D, Results of ySpectral and Radiochemical Analyses. Randolph Stone, Lawrence Livermore Laboratory: Section III, Drilling and Construction of RNM-1; Section IV, Water Sampling Operations; Appendix A, Chrono logical History of RNM-1. William W. Dudley, Jr., U.S. Geological Survey, Denver: Section II, Physical Setting; Appendix B, Selected Chemical Analyses of Water Samples from RNM-1. Although the recommendations, Section VII, were written by D. C. Hoffman, they were the result of suggestions and input from many people, in particular,

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RADIOACTIVITY IN THE UNDERGROUND ENVIRONMENT OF THE CAMBRIC NUCLEAR EXPLOSION AT THE NEVADA TEST SITE The experimental results obtained from investigation of the radionuclide distribution in the environment around the detonation point of the 0.75-kt nuclear test. Cambric, fired 300 m underground in alluvium at the Nevada Test Site in 1965, are presented and discussed. Analyses of sidewall cores obtained ten years later from near ground surface to below the explosion cavity showed that most of the radio activity is still contained within solid material in the lower cavity region. Water pumped from the region of high est activity at the bottom of the cavity showed only T and 90sr at levels higher than the recommended concentration guides for drinking water in uncontrolled areas. Recommen dations for future studies are given. The investigation is part of the Radionuclide Migration Project sponsored by the Nevada Operations Office of ERDA. Although nuclear tests have been conducted underground at the Nevada Test Site for more than IS years, the Radionuclide Migration (ENM) Project is the first investigation designed specifically to characterize the radionuclide dis tribution in both the water and rubble around the detonation point after a nuclear test. Prior to this study, water had been punped from the chimney of only one underground test. In Phase 1 of the ENM study, the site of the 0. 75-kt Cambric event, conducted in Frenchman Flat some ZOO m underground in alluvium

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below the water tablet was successfully re-entered ten years after the detonation. An extensive suite of sidewall core samples was obtained from near ground level to below the explosion cavity. A technique involving casing the drillback holey appropriate placement of internal and external packers, and perforation of the casing at selected depths below the water table was successfully used to isolate five different zones from below the cavity to outside the chimney. Representa tive water samples were pumped from each zone in succession with apparently little cross-contamination, thus allowing a well-defined determination of the vertical distribution of the radioactivity in the ground water. No activity above background was observed in water pumped from the region below the cavity. Water from the region of highest radioactivity at the bottom 144 239 90minimal amounts of Ce and Pu. Of these, only T and Sr were present in levels higher than the recommended concentration guides for drinking water in un After ten years, most of the radioactivity is still retained in vitrified or fused debris within the region of the original cavity created by the explosion. Although some Kr and T were observed in water and core samples from above the cavity, there was no evidence that they had moved above the water table, and more than 99.9% of the T was present as HTO. By comparing the amounts of various radionuclides produced in the Cambric device with the amounts detected in the pumped water, using T as the reference nuclide, effective "retention" factors for >10 for Pm and Pu were estimated. The very high values for Pm and 239Pu may be related to the high pH of the water of 8 to 8.5 as well as to a low 90removal rate from the glassy material. Higher retention values for Sr and 137Cs were found in the puddle than in the chimney region, indicating that in corporation of the activity in the fused debris in the puddle region may retard dissolution of the radionuclides and result in higher retention factors. Water is now being pumped from a satellite well 91 m south of the Cambric detonation point in an attempt to draw HTO from the cavity and determine whether any of the other radionuclides accompany it. Based on the RNM studies to date, the following recommendations have been made for future work:

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1) Complete the sampling at the satellite well and re-sample the original well in the Cambric cavity. 2) Initiate Phase 2 of the projected RNM program by selecting a suitable nuclear test for study of the early as well as the late-time radionuclide source term and migration. Consider the possibility of a study designed specifically to investigate migration of radionuclides above the water 2) Conduct studies similar to the Cambric investigation for nuclear devices tested in other media at the NTS, such as tuff and rhyolite. 4) Carry out laboratory studies to determine the effects of different variables on the interactions between water and various geologic media. 5) Perform modeling studies to predict migration rates of radionuclides of interest for various media at the NTS and determine the relative importance of the various parameters.

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I. INTRODUCTION AND BACKGROUND The Radionuclide Migration (RNM) Project was initiated at the Nevada Test Site (NTS) in January, 1974 under the sponsorship of the Nevada Operations Office (NV) of ERDA with the participation of the Lawrence Livermore Laboratory (LLL), was designated Project Manager, and 0. E. Sattizahn, LASL, and R. H. Ide, LLL, were named Technical Directors. The stated intent of the project was "to provide data and interpretation thereof which will quantify the radionuclides which migrate away from underground nuclear explosion cavities and chimneys and will determine the time-distance relationships of such migration under the conditions of the test. This information may then be applicable to the hydrologic program studies which have been and are in progress relating to long-term safety with regard to contamination of water supplied both on- and off-site. It may also provide a base for studies relating to underground disposal of high-level radio The project was divided into two experiments. The first, called Experiment No. 1, was started at once and is the subject of this report. In Experiment No. 1, the site of the 0.75-kt nuclear test. Cambric, which was fired beneath the water table in alluvium was chosen for examination. The choice was based on several considerations. The Cambric cavity is within the Area 5 water-supply aquifer. It was predicted that sufficient time had elapsed so that the cavity and chimney had filled with ground water to the preshot static water level, 73 m above the detonation point. If so, radionuclides might be present in the water and constitute a potential source for migration. The Cambric detonation point is only 294 m below ground surface, and thus the re-entry drilling and sampling operations would be less difficult and expensive than for some of the more deeply buried tests. Although the Cambric chimney was believed not to have progressed to the surface, a small subsidence was detectable at the surface. The existence of some uncertainty concerning the stability of the area precluded drilling vertically through the chimney, and therefore, it was planned to drill a slant hole into the chimney-cavity region. The site was far enough from the areas of active nuclear testing so that damage or interruption of the re-entry and sam pling operations from those activities would be unlikely. Sufficient tritium was present to provide an easily measurable tracer for water from the cavity region.

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The postshot debris also contained plutonium and fission products whose concen trations 1n the rubble and ground water from the cavity and chimney regions could be measured and compared. The small yield was expected to have had little effect on the local hydrology. Further, it was judged that the alluvium consituted a good medium for hydrologic studies because it was more permeable than tuff and did not have large fissures or cracks through which the water might selectively flow. A satellite well was to be drilled some distance from the Cambric re entry hole; pumping of the satellite well was expected to induce a sufficient artificial gradient to draw water from the Cambric cavity and provide an oppor tunity for study of radionuclide migration under field conditions. Detection of tritium would signal the breakthrough of water from the Cambric cavity region. The initial phases of Experiment No. 1 have now been successfully completed. The cavity region was re-entered and sidewall sampling was performed in order to measure the vertical distribution of radionuclides. The hole was cased and the casing perforated to permit pumping of water in different regions. In this way, variations in the radionuclide content of water from the various Intervals were determined. The satellite well has been drilled and pumping is in progress. Much useful and interesting information has been obtained concerning the source term, i.e., the distribution and concentration of radionuclides around the Cambric detonation point ten years after the test. This report gives a detailed descrip tion of the physical setting, construction, and sampling of RNM-1, of the analyses performed on the samples, a discussion of the results, and recommendations for The data from Experiment No. 1 will be used to plan Experiment No. 2. It is envisioned as a "cradle-to-grave" study in which preshot chemical, geologic, and hydrologic conditions, the radionuclide distribution in soil and water immedi ately after the test, the rate of infill of water, and early as well as late-time radionuclide migration will be studied. II. PHYSICAL SETTING The Cambric device was detonated on May 14, 1965 with a yield of about 0.75 kt. The explosion point was at a depth of 294.3 m in alluvium in western Frenchman Flat, located in the southeastern part of NTS, about 100 km northwest of Las Vegas (Fig. 1). Frenchman Flat ranks fourth--behind Yucca Flat, Rainier

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Rock Volley Mercury Ridge 1 Index map of Nevada Test Site and vicinity western Frenchman Flat in southeastern NTS Cambric site is in

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duadrangla map*. 1060-1901. dlnataa. In matara. Fig. 2. West-central Frenchman Flat, showing location of the RNM Experiment

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Mesa, and Pahute Mesa--as a testing locality; of 338 announced underground nuclear tests at NTS through June 30, 1975, only 10 were conducted In Frenchman Flat^. Three water-supply wells and one former well, shown in Fig. 2, are in west ern Frenchman Flat. Two of the supply wells, 5B and 5C, produce water from the alluvial materials in which Cambric was detonated. Wells 5B and 5C (and formerly Well 5A) are used for multiple purposes including the general water supply for Mercury, the housing and operational center of NTS. Distances from Cambric to 1.8 km northwest of the Cambric site, is used as a source of construction water. Semi-annual monitoring of these wells by the Environmental Protection Agency has revealed no radioactivity above background. on land aurfaca Approximate Cambric cavity radlua NEVADA CENTRAL ZONE COORDINATES. EAST (m) Fig. 3. Principal features of the RNM Experiment No. 1 site and nearby vicinity. Section A-A' is shown in Fig. 5. Figure 3 shows the Cambric site and the facilities constructed for RNM Experiment No. 1. Locations and depths of the drill holes and wells and their approximate land-surface altitudes are as follows:

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Cambric emplacement hole Satellite well for pumping and sampling Nearby exploratory hole providing additional information RNM-1 was drilled in May 1974 through and beneath the Cambric cavity for the purpose of obtaining sidewall cores and for subsequent water sampling. RNM-2 was an unsuccessful attempt to complete a satellite pumping well; after severe hole enlargement and caving, 6-cm tubing was installed to provide access for water-level measurements. The present satellite well for pumping and sampling, RNM-2S, was completed in April 1974. It was completed before drilling RNM-1 in order to avoid the possibility of contamination with radioactivity from Cambric. Frenchman Flat is a closed basin, having interior surface drainage to Frenchman Lake, a normally dry playa about 20 km in area and about 938 m in altitude (Fig. 2). Gently sloping (10 to 20 m/km) desert of sparsely vegetated alluvial soil surrounds the central playa up to an average altitude of about 1030 m. Mountainous terrain that reaches about 1900 m in altitude surrounds most of the basin.

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Well-defined …