ML20135E333

Criticality Sar

ML20135E333
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Issue date:	02/21/1997
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{{#Wiki_filter:O CRITICALITY SAFETY EVALUATION REPORT

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i February 21,1997 O P'em m8m, PDR

1 1 (7 TABLE OF CONTENTS 1 v Section Eage 1. EXECUTIVE

SUMMARY

.1 II. INTRODUCTION... A. Site Description........ .................................................................I B. Remediation Alternatives. ...................2 C. Purpose of the Study, ..2 D. Approach to the Study.................. ..,..............3 III. URANIUM INVENTORY....... ......................3 A. Gamma Logging Results............... .4 B. Laboratory Measured Gamma Activity in Boring Samples........... ..............6 C. Laboratory-Measured Gamma Activity in Solids Filtered From Leachate Samples............ 6 D. Conclusions Regarding Inventory of U-235. .7 IV.

SUMMARY

OF CRITICALITY CALCULATIONS.. ... 7 A. Calculational Methodologies, Computer Codes, and Validations......... ................8 B. Parameters, Parameter Ranges, and Calculational Models........................... 9 j C. Results.... ........ 1 1 D. Conclusions Regarding Criticality Calculations........ . 13 n\\ ( / V. SITE-SPECIFIC HYDROGEOLOGIC EVALUATION.............. ........ 14 x~./ A. Approach........ ...14 B. Hypothetical Conditions for Convergent Groundwater Flow and High-Hydraulic Conductivity Pathways.................. .15 C. Field Measurements. Site Observations of Flow Convergence, and High-HyAmisc Conductivity Features..... .19 D. SIP ' s, Remediated Site With Degraded Engineered Components...................... 21 d E. Potential for Preferential Flow Paths.... ............ 2 2 F. Conclusions.. .............. 2 3 VI. EVALUATION OF POTENTIAL FOR RECONCENTRATION OF URANIUM............ 23 A. Chemical Constraints on Uranium Reconcentration.. .... 24 B. Conclusion... .... 2 8 VII. TECHNICAL ANALYSIS AND CONCLUSIONS. .. 28 VIII. REFERENCES.............. ..................28 FIGURES AND TABLES 's 02 aim s 32 rw

mracomepocarext poc j

_____m-_.__ TABLE OF CONTENTS (Continued) O Appendix A MEMORANDUM TO FILE: Explanation for Revisions to Table 7, Volume 2 of 5 of the 1995 Field Work Report, Appendix A, Rev. 0 2/5/96 Appendix B Validation of Computer Codes and Cross Section Libraries Appendix C Tabular Results Supporting Chapter IV and Selected Computer Code input Files Appendix D Trench 6 Water Balance Calculations O 1 I O

-.... ~.. -.. ACRONYMS DOS Disposal-Off-Site NEPA National Environmental Policy Act NRC ' U.S. Nuclear Regulatory Agency ORP oxidation / reduction potential PVC polyvinyl chloride SCR Site Characterization Report SIP +B Stabilization in Place With Mine Backfill i SLDA Shallow Land Disposal Area SOS Stabilization-on-Site i TWSP temporary waste sampling point U uranium ( i I i l i l l 1 1 Ag i i i 02/2197 5 23 PM J PROMRCOf!N 41bOCfD(TE.XT DOC jjj L

l l l /^ I. EXECUTIVE

SUMMARY

l! l This report evaluates the potential for criticality in nine shallow trenches and one closed l sediment basin at the Parks Shallow Land Disposal Area (Parks SLDA) located in Parks Township, Armstrong County, Pennsylvania. In this report, criticality is defined as the l condition in which a compact mass of U-235 achieves a neutron multiplication factor of l.0 or higher. The conditions required for criticality are evaluated with bounding i calculations and detailed evaluations of site conditions. Criticality is evaluated for three conditions related to remedial action alternatives at the site: No-Action, Stabilization in Place With Mine Backfill (SIP +B), and SIP +B with engineered components in a degraded condition. The SIP +B alternative precludes criticality by causing all seepage from Trenches 1 to 9 to ) occur as diffuse vertical unsaturated flow. The results of criticality calculations and evaluations of site data lead to the conclusion that there is no credible combination of hydrogeologic and geochemical conditions that would allow the formation of a critical mass in the trenches under the No-Action and the SIP +B alternative with degraded engineered components. j l II. INTRODUCTION 3 This study evaluates the potential for criticality at the Parks SLDA. This section of the report includes a statement of the purpose of this study, and to provide context, also includes brief descriptions of the site, remedial actions being considered for the site, the approach used in the study, and conclusions. Tables and figures follow the text in this document. A. Site Description The Parks SLDA is a site where waste from an Atomic Energy Commission-licensed nuclear materials facility was disposed in accordance with the requirements of the Code of Federal Regulations in existence at the time of the disposals (10 CFR S20.304). A complete description of the site and wastes is included in the Site Characterization Report (SCR) and only a brief description is provided in this report (ARCO /B&W,1993). There are nine trenches and one closed sediment basin in two areas at the site (see Figure 1). Trenches 1,2, and 4 through 9 are in the upper terrace area. The sediment basin, sometimes designated as " Trench 3," is also in the upper area. Trench 10 is in the lower terrace area. The trenches are monitored by 58 temporary waste sampling points (TWSPs). These include 36 TWSPs installed in 1993 with 2-inch polyvinyl chloride (PVC) pipe and 22 installed in 1995 with 4-inch PVC pipe. G) l f l o2,2>,.7 3 22 ru m uucom umemmn mc

i l i f The upper trenches and the sediment basin were excavated into the soil. The soil overlies O bedrock consisting ofinterbedded sandstones and shales. At a depth of 75 to 80 feet l beneath the trenches there are abandoned coal mine workings that have remained stable ~ I since they were mined more than 70 years ago. Trench 10 is founded on the bedrock I stratum that was exposed when coal was surface mined at the outcrop of the coal seam. j \\ The wastes in the upper trenches are heterogeneous and include soils, sludges, metal objects, and fibers. Constituents of screened and selected interest include nuclides of uranium in low concentrations and,' in very small amounts, thorium and residues of chemicals used to process materials. The waste in Trench 10 is primarily metal debris. J Surface water has infiltrated the trenches since wastes were first emplaced over 30 years ago. Seepage of leachate from the trenches has caused some migration of radiological and nonradiological constituents from the trenches. Radiological constituents are confined to the soils immediately adjacent to the trenches. The very limited migration of radiological constituents is primarily a result of natural geochemical processes. B. Remediation Alternatives Remediation of the site is being proposed to control potential radiation doses to the public. As part of the U.S. Nuclear Regulatory Commission (NRC) regulatory process O-for determining future action at the site, remediation alternatives are being evaluated in an environmental review process through the National Environmental Policy Act (NEPA) that compares alternate courses of action. In addition to the No-Action alternative, three basic remediation alternatives are being evaluated: (1) SIP +B, (2) Stabilization-on-Site (SOS), and (3) Disposal-Off-Site (DOS). Both the SIP +B and SOS alternatives include isolation of the material on-site. The SIP +B alternative involves isolation in place and the SOS alternative involves excavation of trench materials and placement in a specially constructed cell. Based on its evaluation of alternatives, the ARCO Project Management Team considers the SIP +B alternative to be the preferred alternative. A comprehensive description of the SIP +B design and performance are included in the Stabilization-In-Place: Description & Performance Report (ARCO /B&W, 1995). Design and performance details are describ:d in this Criticality Safety Evaluation Report only to the extent necessary for the limited purposes of this study. C. Purpose of the Study The major radiological constituent in the trenches is uranium (U), and much of the uranium in the trenches is enriched in the U-235 isotope. It is generally agreed that the concentrations of U-235 in the trenches are far too low in the present condition to support I criticality, and will undoubtedly remain so far into the future. However, the question of possible reconcentration of U-235 in the distant future in such a way that criticality might occur has not been systematically explored. The purpose of this report is to provide such morwomumemmme 2

i an analysis. The scope of this work explicitly includes evaluation of the No-Action j alternative and the SIP +B attemative, performing both as designed and with degraded 4 engineered components. This analysis of the SIP +B alternative is considered to apply to i the SOS altemative, as well. The criticality issue does not apply to the DOS alternative at the Parks SLDA site. l l \\ f D. Approach to the Study Certain conditions are required to support criticality. The foremost is a minimum quantity of U-235 in a form sufficiently concentrated relative to present concentrations in the trenches. Further, the U-235 must be present within a limited range of concentrations . and geometric configurations. Water or other material that crn act effectively as a moderator must also be present within a limited concentration range. Because the uranium concentrations in the trenches are presently far too low to support criticality, mechanisms to reconcentrate the uranium would be necessary. These mechanisms ] generally require convergent flow of water in the trenches to a point of potential l reconcentration combined with appropriate geochemical processes that allow uranium to be mobilized and reconcentrated. Diffusion as a possible transport mechanism is i. expected to be far too slow to be effective and has not been analyzed explicitly in this study. The approach followed in this study assesses each of the factors listed above in sequence. The quantity or inventory of U-235 in the Parks SLDA trenches is discussed in Section III. The conditions required for criticality using site-specific data and NRC nominal soil data as input parameters are analyzed in Section IV. Convergent flow mechanisms are evaluated in Section V and geochemical and transport processes are examined in Section VI. Although some integration is included in the transition to subsequent chapters, integrated analysis is completed in Section VII where conclusions are also presented. ) l III. URANIUM INVENTORY Important questions in relation to the potential for a critical event are "Is there enough uranium available in the waste / soil / rock / water system to allow such an event to occur?" ) and "Is this uranium sufficiently enriched to allow such an event to occur?" The question j of uranium inventory in the trenches at the Parks SLDA has been addressed in two i previous reports including the SCR (ARCO /B&W,1993) and the 1995 Field Work 1 Report (ARCO /B&W,1996). These reports estimated the inventory of total uranium to be between 3 and 6 curies on the basis of the results of gamma logging of the 2-inch TWSPs and burial limits / records. In 1995, a series of 4-inch TWSPs were installed to increase the ecverage of sampling points for gamma logging and leachate sampling. The 4-inch diameter of the new TWSPs allowed the use of a more sensitive, larger diameter gamma detection system. ) '""'2'* I ?h0AARCOffNALDOCVINTEXT. DOC 3

4 i I l In addition to gamma logs of the 2-inch and 4-inch TWSPs, data on uranium activity concentrations were obtained on solid trench samples recovered from perimeter borings. i These borings were designed to detect migration of uranium in soils outside the trenches

but some borings inadvertently penetrated solid wastes in the trenches. Gamma activities I

1 measured in the laboratory on solid samples recovered from these borings were originally 1 reported in the SCR (ARCO /B&W,1993). These data provide additional information on uranium inventory. i i l J j Finally, measurements of gamma activity were made on solids filtered from leachate d samples obtained during the 1995 field program from each of the TWSPs. These l measurements were intended to serve several purposes, including (1) identification of radionuclides other than those known to have been buried in the trenches, and (2) i l determination of the uranium activity in the solids. The activities measured in the solids I can be compared with the uranium activities measured in the liquids to calculate sorption coefficients and provide information on enrichment and additional data regarding uranium inventarf. A. Gamma Logging Results ) i Values for the U-235 activity measured at one foot intervals in 2-inch TWSPs in 1993 and 1995 are listed in Table 7 of Appendix A of the 1995 Field Work Report (ARCO /B&W,1996). The 1995 4-inch TWSP data are contained in Tables 11 through j 32 of the same report. Revised Table 7 of Annendix A of1995 Field Work Reoort A detailed review of the gamma logging data and results reported in Appendix A of the 1995 Field Work Report revealed several inconsistencies in the data as reported (ARCO /B&W,1996). For example, the highest activity reported in Table 7 of Appendix A was for the Il-foot level in TWSP 02-4. Yet, the graph of activity with depth for this TWSP showed no unusual gamma activity at this depth. To clear up these inconsistencies, the original gamma logs were reviewed and net activities were recalculated. A revised Table 7 has been produced and is included in this report as Table

1. An explanation of the basis for the revisions of the original Table 7 of Appendix A of the 1995 Field Work Report is included as Appendix A of this report (ARCO /B&W, 1996).

Statistical Analysis of Gamma Lopping Data In order to evaluate the consistency of the various gamma log data sets (i.e.,2-inch vs. 4-inch TWSPs and 1993 vs.1995 data for 2-inch TWSPs), a statistical evaluation of the revised gamma logging data was carried out. As is evident in Figures 2 through 4, this evaluation shows that the data points for each TWSP size and measurement year form s 02/2iM7 $ 23 PM NgtOTAltCOTINALDO(VINTEXT DOC 4

4 1

A exponential distributions.

Averages and standard deviations calculated for these () distributions are listed in Table 2. I The averages for the 2-inch TWSPs are 4.85 picocuries per gram (pCi/g) and 5.07 pCi/g for the 1993 and 1995 measurements, respectively. The fact that these two averages are so close indicates consistency in measurement technique (e.g., calibration). The average for the 1995 measurements in the 4-inch TWSPs is slightly lower at 3.93 pCi/g, but the averages are well within the standard deviations reported in Table 2. This suggests the 2-3 l - inch and 4-inch TWSPs are sampling similar materials in terms of uranium activity. l Inventory Calculations Based on Gamma Logging Data 1 l The three gamma logging data sets (1993,2-inch; 1995,2-inch; 1995,4-inch TWSPs) are used in this section to derive estimates of uranium inventory. The procedure used in j these calculations is the same as that explained in Section 2.1.3 (page 2-7) of the 1995 j Field Work Report (ARCO /B&W,1996). For a given trench, the U-235 activity ~ l measured by ganuna logging at each 1-foot interval in a given TWSP was assumed to be representative of solids in a horizontal 1-foot thick with aerial dimensions derived from i the total trench area divided by the number of TWSPs in the trench. The U-235 activity in the slab was converted to U-235 mass using the U-235 specific activity. The U-235 masses were summed to obtain the U-235 inventory represented by a given TWSP. The inventory in each trench was obtained by summing the inventories calculated for each V TWSP in the trench. The estimated total (site) inventory is simply the sum of the estimated inventories calculated for each trench. The mass U-235 results are reported in Table 3 as two separate data sets. One set reflects the assumption of 0.0 pCi/g for U-235 measurements reported as below the limit of detection. In the other set, a value of one-half (0.5) the minimum detectable activity concentration (3.5 pCi/g) was used for these measurements. The results for individual TWSPs are reported in Table 3 with separate listings for results for 2-inch and 4-inch TWSPs and 1993 and 1995 data for the 2-inch TWSPs. Two separate inventory estimates are listed for each trench based on whether 1993 or 1995 2-inch TWSP data were combined with the 1995 4-inch TWSP data. Total U-235 inventory estimates summed over all the trenches are also listed. Data for the U-235 enrichment in individual TWSPs, originally. reported in the 1995 Field Work Report are also listed in Table 3 (ARCO /B&W,1996). 4 The separate estimates of the total buried U-235 inventory summed over all the trenches are generally within 10 percent of the average of the four estimates and range from 64 to 78 kilograms (kg) of U-235. These estimates are more consistent than those presented in the 1995 Field Work Report (ARCO /B&W,1996). Inventory estimates for individual trenches and individual TWSPs show greater variation on a percentage basis. The range of estimated trench inventories is from 0.2 to 24 kg U-235 with only Trenches 2,5, and 6 having estimated inventories greater than 10 kg U-235. Only 2 TWSPs (02-3 and 01-1) have estimated inventories significantly greater than 4 kg U-235 (see Table 3). e2/2nn 23 ru 5

. - ~ . -. ~ c - - - - -. ~ -.-. B. Laboratory-Measured Gamma Activity in Boring Samples 1 Drilling log descriptions of the solid materials that were recovered from the perimeter l borings completed in 1993 indicate that a number of these borings penetrated wastes buried in the trenches. A review of the logs presented in Appendix B of the SCR suggest that 25 to 30 of the borings in the upper trench area recovered materials indicative of buried wastes (ARCO /B&W,1993). The total uranium activities measured in the boring samples were reported in Table 5-6 of the SCR. The data for the borings that penetrated waste materials have been reproduced in Table 4. The total uranium activities measured in all the boring samples were plotted in Figure 5-8 of the SCR. The highest total uranium activity (1107 pCi/g) was measured in a sample i taken at 10 feet from boring OlUO6 near Trench 1. This total uranium activity corresponds to a U-235 activity of 51.5 pCi/g, which is within the range of U-235 activities measured by gamma logging and considerably less than the maximum measured gamma log activity of 345 pCi/g (see Table 1). A comparison of boring and gamma log data indicates that gamma log results provide a conservative estimate of inventory. ) C. Laboratory-Measured Gamma Activity in Solids Filtered From Leachate Samples As explained previously, measurements of the gamma activities of solid samples filtered from trench leachate were intended to serve several purposes. These include (1) identification of radionuclides other than those known to have been buried in the trenches, and (2) determination of the uranium activity in the solids to compare with the uranium activities measured in the liquids to allow the calculation of sorption coefficients, to provide information on enrichment and to provide additional data regarding uranium inventory. Conclusions with regard to unexpected radionuclides, calculation of sorption coefficients, and enrichment are detailed on pages 2-6 to 2-10 of the 1995 Field Work Report (ARCO /B&W,1996). The conclusions with regard to uranium inventory presented on page 2-9 of the 1995 Field Work Report were as follows: " Uranium detected in solids filtered from trench leachate samples averaged approximately 2300 pCi/g. Results ranged as high as 30,300 pCi/g, but only three results exceeded 10,000 pCi/g. The average is somewhat higher than averages based on other assessment methods, including the average based on down-hole monitoring, but the peak values are in the same range as observed in down-hole gamma monitoring. The higher average concentration in the solids filtered from leachate samples may result from a difference in solid particle size and O composition between solids filtered from leachate and typical soil in the trenches. These have not been analyzed, but it is likely that the particle size of material mowcomooemnmoc 6

j O filtered from leachate is smaller than bulk solids in the trenches and it is likely v that the solids filtered from leachate samples contain more clay and dissolved organics. If so, it is also likely that the solids filtered from leachate also have a greater affinity for dissolved uranium than the bulk solids m the trenches, and the greater affinity could explain the somewhat higher average uranium concentrations in this material." The activities referred to in this quote are for total uranium. In terms of U-235 activities, the average value is 94.2 pCi/g, which is higher than the averages of 3.9 to 5.2 pCi/g (see Table 2) obtained by down-hole gamma monitoring. However, nine of the TWSP samples had U-235 activities greater than 100 pCi/g and these have a major impact on the calculated average value. Explanations for the differences in the averages obtained from gamma logging and those obtained from leachate solids offered in the 1995 Field Work Report are still valid with some additions (ARCO /B&W,1996). In order for solids typical of the materials in the trenches to remain suspended in water for any length of time, they must be of a small particle size. Small particles have large surface areas on a per unit weight basis. Large surface areas equate to large numbers of surface sites for the adsorption of chemical species from solution. Because organic solids generally have higher affinities for uranium than most inorganic solids such as quartz, feldspar, and clays TWSP leachate samples with suspended solids dominated by organics would show higher U-235 activities than TWSP leachate samples where inorganic soil constituents dominate (d the suspended fraction (Zielinski and Meier,1988). This explanation implies that the N uranium activities measured in the solid materials filtered from trench leachate samples are not representative of the average uranium activities in the trench sections from which they were recovered. D. Conclusions Regarding Inventory of U-235 The main sources ofinformation available regarding the inventory of U-235 buried at the Parks SLDA include analytical data collected from (1) burial limits / records, (2) perimeter borings that penetrated trench wastes, (3) analytical data collected from solids filtered from trench leachate samples, and (4) gamma log data. An evaluation of data from these four sources leads to the conclusion that the gamma log data are the most consistent and representative. Therefore, the inventory estimates based on the gamma log results will be used in the evaluation of the potential for criticality at the site. IV.

SUMMARY

OF CRITICALITY CALCULATIONS In order to perform a site-specific criticality analysis, information must be available on the various parameters that impact the critical mass. These parameters include (1) the n uranium inventory, its distribution, and enrichment; (2) soil density and composition; and () (3) the water content of the soil or other media in which uranium could credibly accumulate. Values for some of these parameters can be reasonably well specified on the momeomooce, awe 7

basis of \\g existing data (e.g., uranium inventory, enrichtnent and soil density and composition). However, other parameters such as the spatial distribution of uranium and the water content of the host media are difficult to specify with certainty both in the present day and into the future. Water content plays a key role in determining the limits of criticality. Fission of an atom of U-235 yields several " fast" neutrons with relatively high energies. To sustain a chain reaction (i.e., criticality), at least one of these neutrons must collide with and induce fission in another atom of U-235. However, fast neutrons are not readily absorbed by the U-235 nucleus. Fast neutrons can be slowed, or " moderated," by collisions with atomic nuclei, eventually reaching thermal equilibrium with the surrounding medium, provided the collisions do not lead to capture. Such " thermal" neutron:, have a much higher probability of inducing fission in U-235. Water is a particulary effective moderator, being composed oflight atoms with small neutron capture cross sections. If the water content of the soil is low, fast neutrons will have a comparatively high probability of being absorbed by the soil matrix before being slowed enough to induce fission in U-235. Thus, the drier the soil, the larger the mass of U-235 required to sustain a chain reaction. As a corollary, for a given mass of U-235, criticality may be possible only above a threshold water content. It was decided to determine the system bounds for which criticality was theoretically possible. That is, neutronics evaluations were performed over ranges which spanned the ( ) practical values the parameters listed above could possess. The majority of the calculations assumed a spherical and homogeneous geometry. This is conservative, resulting in criticality over a wider range of conditions than would be encountered in the trenches. A. Calculational Methodologies, Computer Codes, and Validations The majority of the criticality calculations were performed with the one-dimensional, deterministic, discrete-ordinates code ONEDANT and the sixteen group Hansen-Roach cross section set in spherical geometry. This geometry is neutronically conservative compared to the actual, three-dimensional distributions of the uranium in the trenches; that is, calculations performed in a spherical geometry will indicate that criticality is attained when in fact a cylindrical, box-shaped, or any irregular geometry would lead to suberitical results. Another conservatism associated with the calculational model was to assume that the uranium concentration was constant over the volume enclosed by the chosen geometry. This isjudged to be significant in that uranium may reconcentrate as a layer or shell (see Section VI) which could preclude criticality even for inventories far greater than are estimated to be present. The ONEDANT code was also run with the 27-and 44-gr - hraries of the SCALE 4.3 package to provide additional confidence in the accurac). me results. p s 021197 5 21 PM J PROJ. ARC 0flNALDOCf!NTEXT DOC g

d t To investigate some three-dimensional geometries and to compare against the deterministic codes and their cross sections, several calculations were made with the KENO and MCNP Monte Carlo codes. KENO is also a part of the SCALE package. t Calculations were performed with the same three cross section libraries as with ONEDANT. The MCNP code was run with a continuous energy cross section library. Details of these codes, cross section sets and their benchmarking, validation. and verification are provided in Appendix B. Consistent with the philosophy of conservatively determining the approximate bounds of 1 parameter combinations for which criticality is possible, a subcritical limit of the neutron multiplication factor (Km) was determined to be about 0.97. This value was derived from the validation studies in Appendix B; and while a multiplication factor of 1.0 defines the critical condition, this lesser value in essence provides another margin of conservatism in the determination of the critical bounds of parameters. B. Parameters, Parameter Ranges, and Calculational Models It is axiomatic that the uranium was disposed of at very low concentrations and densities, and thus not of concern with regard to criticality as it was emplaced. While dissolution, flow, and reconcentration arguments may indicate that the conditions necessary for p criticality are not credible, it was nevertheless decided to investigate the ranges of A parameters for which criticality would be theoretically possible. Parameters and Parameter Ranges 4 Parameters which could have potentially significant impact on criticality include:

1. Uranium-235 mass,

2. Uranium enrichment,

3. Uranium concentration in the soil matrix,

4. Soil chemical composition, with particular reference to strong neutron absorbers,

5. Soil density,

6. Soil water content,

7. Geometry of the uranium bearing zone,

8. Reflection medium (what surrounds the uranium bearing zone), and

9. Possible neutronic interaction among discrete uranium bearing zones.

Some of these parameters, such as uranium masses, are expected to have different values for different trenches while others are largely independent of any particular trench. In developing the calculational models, a nominal or Base Case was derived and then most of the parameters listed above were varied to determine the effect of realistic variations O_ about the nominal values. 02/21H 5 23 PM NROFARCOfM ALIXCENTEXT DOC g

] Calculational Models N.s Based on nominal soil data for the Parks site and the uranium masses and enrichments presented in Table 3, the following model was selected as the Base Case (ARCO /B&W, 1993):

1. Uranium-235 mass = 16.0 kg; uranium enriclunent = 20 percent; uranium present as UO. This uranium inventory bounds that found in most of the 2

trenches but is most closely representative of Trench 6.

2. Soil composition as per Table 5 at a dry density of 1.5 grams per cubic 3

centimeter (g/cm ) (no UO2 Present). The soil has a porosity of 35 volume percent (v/%).

3. Reflecting medium = soil as in item 2 above at 35 v/% water such that the 3

3 total density of the soil plus water = 1.5 g/cm soil + 0.35 g/cm water. A saturated reflecting medium is conservative, promoting criticality. The major variables were the uranium concentration in the soil and the water content of the soil. The geometry was spherical for the large majority of the calculations in order to i provide neutronic conservatism in light of the unknown configuration of the uranium as it q migrates. V As will be shown in the discussion of the calculational results, the U-235 masses and concentrations necessary to achieve criticality are sufficiently high such that only very small zones could even theoretically reach the critical state. Thus, the likelihood of more than one near-critical zone being in close proximity to another is vanishingly remote. Therefore, it was judged to be unnecessary to perform calculations ofinteracting zones. The U-235 mass was varied from the minimum necessary to achieve criticality, shown later in this section to be approximately 4.0 kg, up to twice the Base Case value (i.e., up to 32.0 kg). Masses in excess of 16 kg were evaluated to test the sensitivity of the results to U-235 mass. Water content was varied from the minimum necessary (13 v/%) to reach criticality under otbrwise optimum conditions up to a value of 35 v/% of the soil plus 1 uranium oxide-bearing zone. The calculations show that the water content of the soil influences criticality more than the U-235 mass. While most calculations focused on the soil / water / uranium oxide medium, a model was also derived for an iron oxide / water / uranium oxide medium. This model was generated to investigate the criticality characteristics of theoretical, large accumulations of iron oxide (rust) which had adsorbed uranium on the surface. Such acumulations might result from the corrosion of machinery and metal debris buried in the trenches. The iron oxide O. 'Ile parameter variations for this medium were the was assumed to be present as Fe2 3 water and iron oxide contents; these then defined the volume for which criticality could be possible. "" 2m momcomuoocontxi me 10

Due to the multivariate nature of the calculations, an iterative approach to determining the subcritical limit of K,g = 0.97 was necessary. For example, given the uranium mass and enrichment, water density, and the properties of the reflecting medium, it.was still necessary to adjust the soil density for the volume occupied by the uranium oxide. This i required that several calculations be made to determine each combination of uranium j mass (and enrichment) and water density which yielded K,g = 0.97. C. Results 1 The individual results are not all reproduced in this report, but representative tables of j results and code input listings are provided in Appendix C. Figures and tables (distilled l from these calculations) that are the significant results supporting the arguments and i conclusions are provided in the repon. l Soil / Water / Uranium Oxide Systems. ) i For the Base Case, the volume boundaries demarcating the suberitical and possibly critical zones are shown in Figure 5 as a function of the water content of the soil plus uranium oxide mixture. This clearly illustrates the very high degree of uranium ] concentration required to even theoretically attain criticality. The other significant point ) O-to be noted is that if the water content of the soil is less than about 13 v/% then criticality i is not possible for U-235 masses of 16 kg or less. One final point which is not specifically highlighted by Figure 5 is that the calculational model assumes a constant uranium concentration throughout the zone. As has been mentioned, this is judged to be a significant modeling conservatism. Further, if the geometry departs s*gnificantly from spherical then criticality may be impossible for any realistic water content. Figures 6 and 7 illustrate this point. Figure 6 is taken from a compendium of measured critical mass data and shows that either squat or elongated cylinders can readily increase the critical mass over that required in a spherical geometry by a factor of 3 or 4. Since the minimum critical mass at 35 v/% water is about 4.0 kg, modest depanures from spherical geometry preclude criticality. To emphasize this point, the Base Case model was mod;fied to a slab from a sphere and . the 16 kg of U-235 was distributed initially over the entire trench volume of about 5000 cubic meters. Calculations were made at successively greater uranium concentrations as the height of the uranium bearing zone was reduced but the lateral extent was maintained. That is, the uranium was assumed to migrate downward, compacting uniformly into thinner and thinner pancakes. Figure 7 shows dramatically that this geometry is always subcritical. O re i vesti ate etser.1iii ee serva1ive. 8 t rerwaFs se-e-a t -ere re >isti 8ee-etrr. 8 a spherical shell model of the uranium-bearing zone was analyzed. This model includes a 1 i c2:wn s 3 ru monaco mu.txxvmnxTooc

l fN central sphere of the same composition as the reflector in the Base Case model, namely b 1.5 g/cm soil plus 35 v/% water. The shell betsveen the central core and the reflector was of Base Case soil composition and uranium mass and enrichment. These results are shown in Figure 8 from which it is clear that beyond about 0.37 meters inside radius the system cannot achieve criticality. Thus, it is apparent from Figures 6,7, and 8 that as one adds more realism to the calculational model, such as geometric shape or uranium i concentration, the volume over which criticality is theoretically possible rapidly shrinks. To investigate the possibility that less than 16 kg accumulations of U-235 could possibly attain criticality. the Base Case model was run for fixed U-235 masses of 12 and 8 kg. Finally, a search was made for the minimum critical U-235 mass at 35 v/% water, the most reactive water content. These results are shown in Figures 9 and 10. The volume over which criticality is even theoretically possible diminishes rapidly as the U-235 mass is reduced below 16 kg. In effect, unless nearly all of the enriched uranium in Trench 6 3 migrates and concentrates in a sufficiently small volume (< l.5 m ) with an appropriate geometry (e.g., spherical), then criticality is not possible. Additionally, based on the inventory data in Table 3, the same principles apply to Trenches 2 and 5. Trenches 1,7, and 10 are judged to be so close to minimum critical combinations of mass and enrichment that criticality is not a credible concern even under the most ideal conditions. Sensitivity Studies for Soil / Water / Uranium Oxide Systems (3 V In order to understand the influence of the parameters discussed in Section IV-B on the multiplication factor, calculations were made for parameter values both above and below the Base Case values. These were performed for 35 v/% water content and for 16 kg U-235 with the results given in Table 6. Several points are noteworthy. First, as the uranium enrichment increases, the system becomes only slightly more reactive; i.e., the multiplication factor increases by about 1 percent for a 50 percent increase in U-235 enrichment, for constant U-235 mass. Thus, even for large uncertainties in the enrichment there is essentially no impact on the critical conditions. Second, the nominal concentrations of iron and gadolinium in the soil have similar, modest effects on the system reactivity. Cadmium has a much smaller effect on the critical state due to its lesser thermal neutron absorption cross section compared to gadolinium at a similar atom concentration. Finally, the soil density itself has only a relatively small effect on the critical state. Therefore, it is primarily the water content of the soil, as already portrayed in Figures 5 and 9, that has a significant influence on the critical conditions for a given uranium mass. As indicated in Figures 9 and 10, the quantity of U-235 does have a substantial effect on the critical conditions. To investigate the impact of U-235 masses greater than 16 kg, calculations were made at 24 and 32 kg, both at 20 percent enrichment. As shown in Table 6, the neutron multiplication factor increases by a seemingly large amount. To put ( this increase in perspective, however, one can estimate the volume increases to which these multiplication-factor increases equate. The lower branches of the curves in Figure 9 12

_.m i will not change but a few percent; the upper branches will increr e to about 2.7 and 3.5 O 3 m, respectively. Considering the total volumes of the trenches, this would seem to be a relatively small increase in the likelihood of uranium achieving the concentrations necessary to possibly reach the critical state. Iron oxide / Water / Uranium oxide systems 1 This medium was investigated in the event that massive rus; t.umulated here as iron oxide and water) accumulations were present in the trenches from corroding machinery and other metal debris and that significant uranium would adsorb to the iron. Results for this system are shown in Figure 11; all parameters are the same as the Base Case except the soil is replaced by iron oxide, represented as Fe2O. The lower branches of the curves 3 terminate at end points representing the limit of zero iron in the mixture. ~ Neutron absorbing properties ofiron preclude criticality for iron-to-uranium atom ratios greater ) than about 27:1 (iron to U-235 ratios greater than about 135:1), as represented by the i terminations of the upper branches of the curves. Based on the adsorption behavior of uranium on iron oxide on page 2-18 in the 1995 Field Work Report, the minimum iron to j uranium atom ratio is expected to be no less than 400:1, indicating that criticality is precluded for these iron oxide systems (ARCO /B&W,1996). i D. Conclusions Regarding Criticality Calculations O For an average composition representative of soils found at the Parks SLDA, the ] theoretical minimum critical mass is about 4 kg of U-235. This theoretical minimum critical mass is for a spherical geometry, a constant uranium density throughout, 20 percent enrichment, and 35 v/% water. A nonuniform distribution of uranium in the reconcentration zone and/or a reconcentration zone with an irregular geometry would require larger masses of U-235 for criticality. An irregular geometry would preclude criticality even in those trenches with the highest estimated inventories of U-235. Further, should the water content of the trench materials drop below about 13 v/%, as in the SIP +B alternative, criticality is precluded in any of the trenches. Criticality in iron oxide / water / uranium oxide systems is theoretically possible at an inventory of 16 kg U-235, but only if the iron / uranium mass has an ideal spherical geometry, the water content exceeds 30 v/%, and the iron / uranium atom ratio is less than approximately 27:1. This iron / uranium ratio is 1 to 3 orders of magnitude smaller than the minimum iron / uranium atom ratio expected in the trenches calculated on the basis of the adsorption behavior of uranium on iron oxide. Therefore, criticality in a iron oxide / uranium mass within the trenches is precluded. O ,3

m V. SITE-SPECIFIC IIYDROGEOLOGIC EVALUATION ( ) v As explained in Section IV, for criticality to be a concern, the uranium in a sufficiently large volume of a trench having adequate U-235 inventory would need to be reconcentrated into a sufficiently small volume with an appropriate geometric shape (e.g., spherical). Figure 12 shows a cross section of a model trench drawn to scale with dimensions representative of the trenches at the Parks SLDA. The minimum and maximum critical masses / volumes calculated in Section IV for an inventory of 16 kg U-235 have been drawn to scale in the figure. The minimum volume comes directly from the criticality calculations and reflects a spherical geometry with 35 v/% water. The maximum volume comes from the maximum total estimated inventory for the trench and the calculated maximum critical volume given that inventory. Clearly, the uranium in the trench must be efficiently reconcentrated into very small portions of the trench. Any mechanism for accomplishing this would require strongly convergent water flow which sweeps the trench volume containing the necessary uranium. This flow would have to pass through a uranium reconcentration zone with a sufficiently small volume (<l.5 3 m )so that the required chemical reactions could operate to reconcentrate the uranium to a critical mass. This section evaluates the groundwater flow regime at the Parks SLDA and particularly the issue of convergent flow within the trenches. It concludes that the current presence or future creation of high-hydraulic conductivity pathways that could lead to sufficient reconcentration of uranium to create a critical mass is not credible. O An evaluation of potential scenarios for future conditions at the Parks SLDA suggests two hypothetical failure scenarios that could involve strongly convergent flow regimes. V It is important to emphasize that these hypothetical scenarios are considered to be very improbable but are included here for completeness. The two scenarios are (1) creation of a hydraulic sink within the trenches by a hypothetical mine subsidence fracture intersecting a trench in te No-Action alternative, and (2) creation of a hydraulic sink within the trenches for the SIP +B alternative with degraded engineering features. In the latter scenario, the trench cover has been degraded and the trenches have become resaturated. Water flows preferentially along the cut slope of the bottom of the trenches and collects in the lowest part of the trench in a limited volume. The hydrology of the SIP +B alternative does not need to be evaluated for criticality. In addition to optimizing geochemical attenuation in subsoils and bedrock, the alternative ensures that no convergent groundwater flow will occur in the remediated condition. Exfiltration from the trenches occurs as diffuse, vertical unsaturated flow. A. Approach The approach in this section is to first list and discuss hypothetical conditions that could lead to convergent groundwater flow and high-hydraulic conductivity pathways in O Subsection V.B. This is followed in Subsection V.C by an evaluation of whether or not b any of the hypothetical condinons are present at the site or expected at the site in the c::iv s 22 ru momeomoocumn noc 34

,2 i nV future. In essence, this is an evaluation of the existence of convergent flow and high-hydraulic conductivity pathways in the No-Action alternative. The TWSP with the highest estimated U-235 inventory is discussed separately. A water i balance model for the trench section represented by that TWSP is also presented to further define the hydrology of the trench section. I The potential for the creation of convergent flow and high-hydraulic conductivity pathways in the SIP +B alternative with degraded engineered components is discussed in Subsection V.D. i Finally, the potential for preferential flow paths through the trench contents are discussed in Subsection V.E. Whether or not preferential flow paths exist in the trench contents is important to the discussion in Section VI. B. Hypothetical Conditions for Convergent Groundwater Flow and High-Hydraulic Conductivity Pathways Under the No-Action alternative, any potential for sufficient reconcentration of uranium [g in one or more of the trenches to allow criticality requires sufficient convergent lQ groundwater flow within the trench. This requires the following conditions: The pathway must have a hydraulic conductivity several orders of magnitude greater e than the surrounding materials. For purpose of discussion, this is considered to be high-hydraulic conductivity pathway. The pathway must extend from a trench to a zone oflower hydraulic head. The pathway must produce convergent flow to a small area withm a trench so that the e geometry of the cross-sectional area of the pathway is consistent with that required for the formation of a critical mass, given the inventory in a trench. The downgradient end of the pathway must be able to dissipate the groundwater flow e without backing up. Enough of the groundwater passing through the trench must pass through the reconcentration zone to allow the tormation of a minimum critical mass with an appropriate geometry given appropriate uranium reconcentration processes. In this section, the likelihood of occurrence of these conditions is evaluated on the basis of hydraulic principles. In Subsection V.C, the evaluation is performed in terms of field measurements and observations of site conditions. naouncomanoemun me 15

At the Parks SLDA site, only a limited number of hydrogeologic conditions or features C could give rise to a high-hydruulic conductivity pathway with the characteristics listed above. They could occur as a result of(l) the dissolution of carbonate formations to form fissures and channels, (2) the presence of porous and permeable sandstone stringers in bedrock, (3) sand stringers within soil, (4) a topographic feature intersecting a trench, (5) fracture zones in bedrock, (6)a mine shaft or adit in bedrock, or (7) a hypothetical subsidence fracture. Dissolution of Carbonate Bedrock Dissolution features in carbonate rocks may cause large apertures or solution cavities that channelize groundwater flow. However, bedrock in the stratigraphic section between the trenches and the mine does not contain appreciable carbonate layers and no dissolution features were observed in the site cores. None of the hydraulic conductivities measured by slug tests in the bedrock monitoring wells approach those that would be expected if there were dissolution cavities or channels. Even the carbonate units that were observed below the coal mine did not exhibit evidence of dissolution. Therefore, it is concluded that this condition does not exist beneath the trenches. Porous and Permeable Sandstone Stringers in Bedrock A sandstone stringer is a zone of sandstone that is much longer than it is high or wide. In (p terms of form, sandstone stringers typically represent stream channels which occurred in the sediments when they were originally deposited. Sandstone stringers occur in the regional formations and may exist beneath the Parks SLDA. In some cases, bedrock core samples from the Parks SLDA contain altemating layers of sandstone and shale. However, testing of rock core samples and hydrogeologic assessment of the site dau indicate that all the bedrock units have very low-primary porosity and hydraulic conductivity (ARCO /B&W,1996). Therefore, any sandstone stringers that may be present would not be expected to have a sufficiently high-hydraulic conductivity to act as a preferential pathway. Because sandstone stringers would follow the bedding of the bedrock, they would tend to be horizontal and would not form a vertical connection from a trench to an area of significantly lower hydraulic head. Porous and Permeable Sand Stringers in Soil Sand stringers in soil would represent a buried stream channel or relict bedrock structure in residual soil. The extensive boring program conducted at the Parks SLDA shows that the site soil is composed primarily of silty clay. No sand stringers were identified in soil horizons during the site investigation efforts. The only sand encountered without significant percentages of clay occurred in fill materials such as the Trench 10 area mine spoil and the sand used in the placement of a " swimming pool" in the " Trench 3" area. As noted elsewhere, these areas do not have the charweistics required for the formation of a minimum critical mass of U-235. an 1m nnmncomewmm

5

r3 Tonographic Feature b A topographic ravine or notch which intersects a trench could result in a zone of convergent groundwater flow. For example, the drainage ditch close to Trench 2 appears to produce convergent flow lines in this area. Similarly, a topographic notch near Trenches 4 and 5 might be conducive to the formation of a convergent flow regime in these trenches. However, it is unlikely any topographic feature would create groundwater flow that could result in suflicient reconcentration of uranium to allow criticality. For this feature to contribute to the creation of a critical mass, the location of a point of discharge associated with a topographic feature must be static for a sufficiently long period of time to allow the reconcentration of a significant portion of the uranium in a trench. If the point of discharge were the result of gully erosion, it would migrate as the gully was enlarged and the mass of reconcentrated sediment would be eroded. Ifit were a steep-sloped feature due to human activities (such as the drainage ditch adjacent to Trench 2 or the notch at Trenches 4 and 5), erosion would occur at the upper lip of the slope, and initially, sediment deposition might occur at a point of ground water discharge. With time, erosion would predominate and move the point of discharge toward and into the trench, causing the erosion of any mass of uranium that might have reconcentrated at the former point of discharge. i Fracture Zones in Bedrock 73b Potentially, a fracture zone in the bedrock that intercepts a trench could create a high-hydraulic conductivity pathway. Because bedrock below the trenches has effectively no primary porosity and permeability, groundwater flow in the bedrock is controlled by a system of secondary permeability features or fractures. This system of fractures at the site is potentially a high-hydraulic conductivity pathway for groundwater flow. Fracture systems at the Parks SLDA consist predominantly of horizontal partings and bedding plane joints in shale layers (ARCO /B&W,1996). Vertical fractures are limited to discrete joints within individual sandstone layers. Because of the shale layers separating individual sandstone layers, continuously connected vertical fractures and fracture zones are not common. This is consistent with the regional geology; the nearly horizontal layers of sedimentary rocks have not been sipificantly deformed and faults are uncommon. Although regional lineaments have been inferred to be present at the site, field data do not support the presence of vertical fractures under the trenches associated with regional lineaments. The groundwater elevation map also does not indicate the presence of vertical conduits beneath the trenches as saturated conditions and relatively uniform groundwater flow is prevalent at the site. The weathemd bedrock unit below the trenches consists of a shale in which the joints and fractures have been plugged with clay minerals as a result of percolation ofinfiltration waters. Field hydraulic conductivity test results for this unit fall within a narrower k statistical distribution than for the other bedrock or soil units. Generally, no zones of 02'2iM S 21 PM J PROMRCOflN ALDOCVINTEXT DOC g =j

( high-hydraulic conductivity were identified in the weathered bedrock. The hydraulic ( conductivities measured in weathered bedrock had a small standard deviation. A high-hydraulic conductivity pathway would have a greater volume of flow and receive a greater amount of percolating water than other zones and thereby would experience a greater degree of weathering. Eventually, the weathering products would clog this pathway and reduce its hydraulic conductivity. This process could have caused the observed uniformity in hydraulic conductivity values measured in the weathered bedrock and implies a lack of high hydraulic conductivity pathways. Fractures in bedrock are unlikely to produce a high-hydraulic conductivity pathway conducive to the creation of a suitable reconcentration zone. If a fracture zone were a preferential pathway beneath a trench, it would give rise to a narrow planar or at best linear reconcentration zone. As noted in Section IV, planar geometries generally do not lead to the formation of a critical mass given the estimated inventories of U-235 in the trenches. Although the intersection of two fracture zones could produce a small central zone of high-hydraulic conductivity, the four arms radiating from the intersection would receive the bulk of the groundwater flow. The resulting accumulation zone would be in the shape of a cross, which would again not be an apnropriate geometry for the formation of a critical mass. This applies to both existing fracture zones in the bedrock and to fractures that might form in the future. O Mine Adit As described in the SCR (ARCO /B&W,1993), Trench 10 is located within materials used to backfill the former strip mine and is adjacent to the Upper Freeport Coal mine workings. The mine, in this case, is investigated as a potential high-hydraulic conductivity pathway. Water enters Trench 10 as infiltration at the trench surface and laterally as groundwater underflow from nearby Dry Run. Much of the water in Trench 10 appears to flow into one or two mine adits and subsequently enters the mine. The saturated thickness ranges from zero in the southern end of Trench 10 to approximately 1 meter (m) at the northern end. Because most of Trench 10 is unsaturated and the mine is open to atmospheric oxygen, reconcentration of uranium by precipitation of tetravalent uranium oxide minerals is precluded. The possibility for reconcentration by adsorption of uranium onto iron oxides and carbonaceous materials is discussed in Section VI. Hvnothetical Subsidence Fracture Probably, the worst-case high-hydraulic conductivity feature that could cause convergent flow within the trenches is a hypothetical fracture developing as the result of differential subsidence of the underlying coal mine. This will not occur in any condition except the No-Action alternative because the other alternatives and site conditions involve stabilizing the mine. Under this failure scenario, it is hypothesized that a fracture system develops as the coal mine subsides and that these fractures intercept a trench, causing Q concentrated flow in the vicinity of the trench. It is also hypothesized that the trench fluids are concentrated in the bottom area of a trench to create a hydraulic sink by flow anm mowcomumvamoc

g

M 4 ( - down the fracture. For the reasons explained previously above (" Fracture Zones in the ( Bedrock") and the additional analysis presented in this report, this worst-case scenario is implausible. The volume of flow transported through the fracture would be small relative to the horizontal and vertical movement of fluids in other directions from the trench. Although the hydraulic conductivity of the fracture is higher than the surrounding trench materials, its cross-section area is very small relative to the large surface area of the unfractured trench bottom and moderate hydraulic conductivity. Any high-flow fracture would be filled by sloughing of trench material or soil as is the case with all neu-surface fractures. Some subsidence may already have occurrea at the site and no related high-hydraulic conductivity features are evident. C. Field Measuren:ents, Site Observations of Flow Convergence, and High-Hydraulic Conductivity Features l Based on the arguments presented above, the present or future existence of a high-hydraulic conductivity pathway that would allow the creation of a critical mass of p) uranium is considered highly unlikely. To further assess this issue, site data were ( evaluated in detail to look for any evidence for such a pathway in the trenches at the Parks SLDA. In addition, the hydrology in the portion of Trench 6 represented by TWSP j 06-1 was investigated in some detail because this TWSP has the highest estimated inventory of U-235. Groundwater flow through a reconcentration zone with the (small) dimensions required for the formation of a critical mass would produce effects on a water-level map similar to a pumping well. If a sufficiently convergent groundwater flow were present in a trench section, it would result in a characteristic groundwater elevation pattern. On a water-level map, this would appear as a cone of depression and a resultant capture zone. However, it should be mentioned that an apparent capture zone and lower water levels could also be created by several other possible site conditions. The watcr level in an individual trench segment depends on the relative hydraulic conductivity of the sides and bottom of the trench and the infiltration rates through the top. Any likely combination of the following conditions could result in a depressed water level: The bottom of a trench segment could have a slightly greater than average hydraulic conductivity. The infiltration rate above the trench segment could be lower than average.

The upgradient side of the trench could have a lower hydraulic conductivity than the n () downgradient side. These conditions can be distinguished from c.,nvergent flow associated with a high-hydraulic conductivity pathway based on tb shape of the groundwater depression. A small, compact pathway would produce c cone of depression having steep sides near a small central depression, which would become shallower with distance. A general lowering of the water level due to the conditions described above would produce a bowl-shaped depression that would be relatively flat near the center and steeper at the upgradient edge. To distinguish between these two conditions would require a closely spaced network of monitoring wells (on the order of 1 to 2 m apart). Although the existing monitoring network cannot distinguish between a small or a large pathway, it is sufficient to assess the maximum possible capture zone for a given trench segment. The capture zone would be defined by the area of the trench included in convergent groundwater flow lines. A groundwater elevation map of the soil zone was produced from the site monitoring data (see Figure 13). To utilize the greatest number of monitoring points, the water levels subsequent to installation of the last group of TWSPs were used. These water level measurements were collected from November 1995 through September 1996 and are contained in Table 7. Average water levels in each monitoring well for the period were used in contouring the map. The use of an average water level is more appropriate than a mQ single reading because it allows interpretation of the long-term net flow direction and gradient. Areas of Flow Convergenss Only three locations in or immediately adjacent to a trench show apparent lower groundwater elevations and indicate possible convergent groundwater flow. In all other areas, evidence for convergent groundwater flow is absent, with the implication that flow in these areas is effectively parallel or divergent. The three locations for which the field data may reflect convergent flow are listed and evaluated below. Area 1: Near the south east end of Trench 1 (TWSP 01-10) Area 2: Near the middle of Trench 2 (TWSP 02-6, TPZ 02U13) Area 3: Within " Trench" 3 (TWSP 03-1) Area 1. The contours in this area are due to a slightly higher water level in TWSP 01-4 than the general trend within Trench I and a slightly lower level in TWSP 01-10. If a significant pathway were in the vicinity of TWSP 01-10, then TWSP 01-4 would also be lower than the trend (not higher, as is observed). In several areas upgradient from Trench 1, standing water frequently accumulates on the surface. It is believed that one of these is near TWSP 01-4 explaining the observed higher water-level. The low near TWSP 01-10 O(D is believed to primarily represent the normal variations in water levels. If there is a momeomoocmxmc 20

p pathway in the vicinity of TWSP 01-10 that causes the lower water level, it could be v capturing water only from a small portion of Trench I as shown by the contour map (see Figure 13). The estimated inventory within the potential capture zone of TWJP 01-10 (1 ' kg U-235) is below that required for criticality even under ideal conditions. Area 2. The converg' at groundwater flow regir suggested by the contours in Area 2 is the result of the dr dnage ditch which abuts the trench. This ditch presumably was excavated at the sa ne time a pipe was installed through Trench 2 to drain surface water from the Trench 8 area. The pipe currently is plugged but the excavation may be acting as a preferential pathway for water from the middle of Trench 2 into the drainage ditch. Relative to t1 - time frame required for reconcentration of uranium, the drainage ditch is a temporary feature and therefore will not be a preferential pathway in the future. Area 3. Area 3 is the suspected former location of a " swimming pool" used to collect water during the exhumation of Trenches 4 and 5. The sand identified in borings at this location is believed to be the bedding sand for the " swimming pool" that was covered during subsequent grading operations. Therefore, the lower groundwater elevations and I convergent flow in this area may be the result of a buried drainage channel resulting from the exhumation effort. The estimated inventory of U-235 in " Trench 3" is less than 0.2 kg, far below the minimum U-235 mass needed to achieve criticality. TWSP 06-1 Because TWSP 06-1 has the highest estimated U-235 inventory (11-15 kg), the hydrology of the trench section represented by this TWSP was considered in more detail. A water baDnce calculation was performed -to evaluate whether high-hydraulic conductivity pathways were evident in this trench section. The groundwater contour map (see Figure 13) does not indicate convergent flow in the vicinity of TWSP 06-1. To evaluate the possibility of a convergent zone large enough to accumulate sufficient uranium for a minimum critical mass but too small to cause a detectable capture zone in the contour map, a water balance calculation was performed. This is included as Appendix D of this report. Water balance calculations suggest there is no convergent flow zone large enough to produce criticality. D. SIP +B, Remediated Site With Degraded Engineered Components This section evaluates the possibility of creating convergent flow and hydraulic sinks in trenches under the SIP +B alternative with degraded engineered components. In this hypothetical scenario trenches are proposed to saturate over unsaturated soils or weathered bedrock. Perched groundwater could hypothetically move along the cut slope of the bottom of the trenches and to collect and infiltrate preferentially in the lowest part of the trench. 2,

l 4 O - This worst-case scenario will not occur because the bottom of the trench is sufficiently i V - permeable to allow water drainage from the trench along the cut slope. Therefore, most = 4 of the water that flows along the trench bottom will not be concentrated in a sn. ail area. i The trenches are not analogous to " bathtubs." The hydraulic conductivities of the top, i bottom, and sides vary only by approximately.1 order of magnitude. Currently most of i the water entering a trench percolates through the sides and bottom. Only a small fraction I could possibly pass through any single location of convergent groundwater flow. If the. l cover degrades to the point where infiltration is approximately equal to the current rate, conditions will be similar to the unremediated current conditions, which are addressed above. However, a significant difference between No-Action conditions and the SIP +B [ remediated site with degraded engineering components is that topographic features } capable of causing convergent groundwater flow will be absent in the long-term degraded condition due to the surface grading conducted as part of the remediation effort. i For the resaturation of the trenches to result in a larger relative contribution of. j groundwater flow into a single convergent flow location would require that the hydraulic l conductivity of the trench bottom decrease by several orders of magnitude and the hydraulic conductivity of a pathway causing convergent flow increase significantly. The j hydraulic conductivity of the unsaturated soil and weathered bedrock is expected to be lower than the current saturated hydraulic conductivity. However, as these materials resaturate, the hydraulic conductivities will retum to their current values. The time required for the weathered bedrock to return to its current hydraulic conductivity would , O, be very short relative to any plausible time required for the degradation of the cover i j function. Even if the surface infiltration rate were to immediately retum to the current 4 rate, the volume of water that could preferentially migrate through a location at the downslope end of a trench would be limited. Therefore, the total mass of dissolved j uranium at maximum saturation would be far less than the minimum needed to achieve critical conditions. The wetting front would be proceeding vertically downward into the entire bottom of the trench, allowing the weathered bedrock to resatmate. The clay i components in unsaturated weathered bedrock would be expected to absorb water quickly i due to capillary action. It is also likely that the unsaturated weathered bedrock will have i numerous small desiccation cracks, which will enhance the absorption of water entering l the trenches. Therefore, the resaturation of the trenches, due to degradation of engineered l components does not present a reasonable mechanism to increase the relative contribution to a convergent flow location. 1 i' E. Potential for Preferential Flow Paths The tsenches were filled with assorted process wastes and construction debris. Burials j - were systematically separated by and covered with the natural site soil. The soil in the i upper trenches area is primarily clayey and has a low-hydraulic conductivity. The waste } and debris would be expected to have a wide range of hydraulic conductivities but would i likely have a higher average hydraulic conductivity than the soil backfill. On a small scale (several meters), there is undoubtedly great variation in the placement of wastes. i { 22 ,--~m 1 1 -e m ,x r,

p Zones of waste and debris could form preferential pathways at least for short distances, as V observed in municipal solid waste by Zeiss and Major (1992-1993). Even relatively homogeneous soils are known to possess preferential pathways for water flow and transport (Economy and Bawman,1992). Both the volume and velocity of groundwater flow would be greater within these pathways. The range of expected hydraulic conductivity within the trenches is several orders of magnitude. Observations during the perimeter boring program support this view. Some of the borings that encountered trench debris, as evidenced by the presence of gloves, boots, and other waste materials, filled immediately with water while o'hers accumulated water slowly. F. Conclusions Evaluation of site hydrologic conditions at the Parks SLDA leads to the conclusion that the current presence or future creation of high-hydraulic condedivity pathways that could result in sufficient reconcentration of uranium to create a critical mass is not credible. VI. EVALUATION OF POTENTIAL FOR RECONCENTRATION OF URANIUM As concluded in Section IV, if a trench had sufficient U-235 inventory, and if the buried U-235 in a given trench were to be reconcentrated into a sufficiently small volume with homogeneously distributed uranium and if this volume had an appropriate geometry and ifit had a water content of greater than 13 to 35 v/% criticality would become possible. In those trenches with estimated U-235 inventories that exceed the amount needed to form more than one minimum (spherical) critical mass (i.e.,4 kg), several hypothetical conditions could be envisioned, including the following: More than one minimum critical mass of 4 kg might form, Only a portion of the U-235 inventory in these trenches might reconcentrate to form a e 4 kg spherical mass, A spherical critical mass oflarger volume might form, and e A critical mass with a less ideal geometry (i.e., non-spherical, annular, etc.) might form. The likelihood of any of these hypothetical conditions occurring would be a function of the processes that transport and reconcentrate uranium in the trenches. The flow processes were addressed in the previous section. In this section, the issue of transport and reconcentration of uranium is addressed. As discussed in Section V, the reconcentration of U-235 buried in a given trench into a small (e.g., <l.5 m') volume mommunocumxTwc 23

p would require that water percolate into the trench and leave the trench through a 2 V hydraulic sink with dimensions on the order of 1.5 m or less. As noted in Section V, the present or future existence of such a sink is unlikely. However, even if the hydrologic requirements for a sink were met at some time in the future, there are chemical requirements that must be met in order for uranium to reconcentrate. Specifically, uranium must be mobilized from the bulk wastes in the trenches and efficiently reconcentrated in a sufficiently small volume with an appropriate geometry. This section discusses chemical constraints on uranium reconcentration in the trenches. The potential for reconcentration of uranium in the bedrock is not considered because the geometries of pathways through the bedrock would be inappropriate for the formation of a critical mass as discussed in Section V. A. Chemical Constraints on Uranium Reconcentration The U-235 buried in the trenches at the Parks SLDA was originally dispersed through the trenches in various waste forms. In order for U-235 to become reconcentrated into a sufficiently small mass with an appropriate geometry for criticality, two chemical processes must occur in addition to the hydrologic processes already discussed. The uranium in the bulk wastes must first be transferred to the mobile fluid (i.e., water) and then transferred back to the solid phase before the m6lle fluid leaves the trench through A the sink. This overall process could take place .atedly as long as there is a net () migration of uranium towards the sink without its passing through the sink. Uranium exists in nature in several oxidation states. The two states of greatest interest to this discussion are the tetravalent (U") and the hexavalent (U") states. In the hexavalent state, uranium is much more soluble in water than it is in the tetravalent state (Langmuir, I 1978). Under the high oxidation / reduction potentials found in waters in contact with the atmosphere, uranium is present predominantly in the hexavalent state and is therefore relatively soluble. Conversely, under the low oxidation / reduction potentials commonly associated with decomposing organic debris, uranium is present predominantly in the tetravalent state and is therefore relatively insoluble (Killops and Killops,1993). This suggests two basic mechanisms for transferring uranium into and out of the fluid phase. One way is to oxidize it in one place and adsorb the oxidized form onto solid materials (i.e., substrates) in another place. Another is to oxidize it in one place and reduce it in the other. Adsorotion Mechanisms Adsorption of the oxidized-solubilized form of uranium onto solid substrates does not require reduction of the uranium from the hexavalent state back to the tetravalent state and therefore does not require reducing conditions. It does require the existence of a solid phase that has a high affinity for adsorption of hexavalent uranium. In the trenches, ,-3 (j one phase likely to meet this requirement in the long-term is iron oxide or oxyhydroxide (Hsi and Langmuir,1985). However, as noted in Section IV, iron oxide-uranium systems o2mn 2m momeomoocumn we 24

i l (q are not likely to produce critical masses in the trenches because reconcentration of j enough uranium to produce a critical mass also results in a high concentration ofiron, which is a neutron absorber. Systems with iron-to-uranium ratios greater than approximately 30:1 cannot achieve criticality in the trenches (see Figure 11). Because the iron-to-uranium ratios of potential iron oxide-uranium systems in the trenches would exceed this value by 1 to 3 orders of magnitude, this reconcentration mechanism cannot produce a critical mass. Organic materials in the trenches could also act as substrates for the adsorption of hexavalent uranium from solution. Hexavalent uranium is known to be readily adsorbed to organic substrates such as peat (Zielinski and Meier,1988). It may be partially reduced to tetravalent uranium on the surfaces of these substrates even if the oxidation / reduction potential of the surrounding solution is not sufficiently reducing to precipitate tetravalent uranium (Zielinski and Meier,1988). Unlike the iron-uranium system, criticality in the organic-uranium system is not inhibited by neutron absorption in the substrate. However, this reconcentration mechanism is oflimited efficiency because the organic substrates have such high affinities for uranium. These high affinities are reflected in the large distribution coefficients calculated from uranium activities in leachate solids and liquids reported in the 1995 Field Work Report (ARCO /B&W,1996). The large distribution coefficients greatly retard the movement of uranium through the

Q trenches and would essentially preclude the reconcentration of uranium into a compact U

mass. This is likely the reason uranium has not migrated significant distances from the trenches over a period of 20 to 30 years (ARCO /B&W,1995). More important for the present discussion, the large distribution coefficients also imply uranium has not migrated significant distances withm the trenches over the last 20 to 30 years. In fact, data on U-235 activities with depth in individual TWSPs (see Table 1) support this conclusion. If uranium were mobile within the trenches, a more homogeneous distribution of uranium activities would be expected than that reflected in the data presented in Table 1. An important issue that must be addressed is whether this retardation mechanism will continue to inhibit uranium migration in the trenches in the future. Essentially, this is a j question of how long organic substrates survive in the trenches. The rate of oxidation (i.e., degradation) of organic matter in subsurface environments such as landfills is very sensitive to the water content in this environment. According to Rathje and Murphy (1992), biodegradation of organic debris in landfills not saturated with water is a very slow process. These authors note that perishable items such as food stuffs and clothing were easily recognizable in 50-year-old garbage enclosed in a sanitary landfill that had not been water-saturated. These observations suggest that organic substrates will likely remain in the trenches at the Parks SLDA for a long time under the unsaturated conditions associated with the SIP +B remedial alternative. p Under the No-Action altemative the trenches will be at least in part water-saturated, and V under the degraded SIP +B scenario, some of the trenches may become resaturated with water. In these cases, the degradation of organic constituents will likely proceed more m o m co m uooco m n w e 25

n rapidly than under unsaturated conditions. Garbage in a water-saturated landfill of () similar age to that discussed above had been degraded to a stable " gray slime" with no further biologic activity. An important question is, "What is formed when organics degrade under water-saturated conditions?" Studies of the natural degradation of organic compounds suggest likely products (Killops and Killops,1993). Solid organic starting materials (e.g., plant remains) are progressively modified into various intermediate (solid) organic compounds before eventual conversion to humus and humic compounds that are chemically stable under conditions representative of the trenches at the Parks SLDA. This degradation process occurs under aerobic as well as anaerobic conditions and is commonly mediated by microorganisms. The humus forms solid materials while humic acids that are formed can be solubilized and transported by water (Killops and Killops, 1993). Studies of the sorption of uranium on humic substances have been reported by various authors including Szalay (1964), Borovec et al. (1979), Nash et al. (1981), and others. These studies indicate that uranium is strongly sorbed by these substances. In fact, a humate-based model for the formation of uranium ore deposits has been formulated by Turner-Peterson (1985). Therefore, the conclusior: that uranium will remain bound to organic substrates in the trenches even under water-saturated conditions appears incontrovertible. The results of Zielinski and Meier (1988) indicate that uranium is strongly bound onto degraded organic substrates such as peat even under oxidizing conditions. This leads to the conclusion that uranium in the trenches will remain strongly (") bound to solid organic substrates in the future. Some uranium mobilization may occur as a result of the transport of humic acid / uranium complexes in water. However, the amounts transported by this mechanism will be small and the mechanism will not lead to reconcentration of uranium into compact masses. In fact, humic acids are strongly sorbed to clays (Nash et al.,1981). Further, as noted by Turner-Peterson (1985), high salinity solutions are required to precipitate humic acids. Because such solutions are not anticipated in the flow system associated with the trenches at the Parks SLDA, this mechanism is not expected to be operative in the trenches. This leads to the conclusion that most of the uranium inventory will remain immobilized on organic substrates and unavailable to reconcentration processes within high-hydraulic conductivity pathways. Oxidation / Reduction Mechanisms If, for some reason, organic substrates were absent in the trenches, uranium :ould become dissolved in oxygenated waters close to the ground surface and later be precipitated lower in the trenches if reducing conditions were present. Data on oxidation / reduction potentials (ORP) presented in the report " Studies for Geochemical Parameters," suggest the trenches are generally not sufficiently reducing to allow the precipitation of tetravalent uranium compounds (ARCO /B&W,1995). Therefore, the oxidation / reduction mechanism for reconcentration is not expected to dominate potential reconcentration processes in the trenches. However, it is conceivable that there could be local volumes in 3 the trenches where conditions are more reducing that those indicated by the ORP analyses (0 t mowco-socmu noc 26

_________....m i i i i l of leachate removed from the TWSPs. Therefore, a hypothetical model for uranium [ reconcentration by oxidation and reduction / precipitation is discussed below, i l The dissolution and reconcentration process hypothesized above is not unusual in nature j and is in fact thought to be responsible for the formation of a type of uranium ore deposit i known as roll-front deposits (Dahl and Hagmaier,1976). As shown in Figure 14, i t uranium in these deposits is leached under oxidizing conditions from the rock mass .I hydrologically upgradient of the deposit and precipitated under reducing conditions j further downgradient. In effect, the uranium is dissolved and reconcentrated along an j oxidation / reduction interface. This interface migrates in the direction of flow as oxygen in incoming waters reacts with organic debris and other reduced material, oxidizing tetravalent uranium to the mobile hexavalent state. The hexavalent uranium migrates l only a short distance downgradient into the' reducing zone' before being reduced to l . tetravalent uranium and precipitating as an insoluble oxide. The fact that uranium is j reconcentrated at the interface and not in the larger volume of reduced soil / rock l downgradient is significant because it implies the geometry of the reconcentration zone will mimic that of the oxidation / reduction interface. In the trenches, the shape of the I j interface will be irregular due to the heterogeneous distribution of wastes, precluding reconcentration in a compact form such as a sphere. Because of the comparatively greater amount of uranium found in limited portions of j some trenches (e.g., Trenches 2, 5, and 6), the model trench shown in Figure 12 was j divided into six sections to provide a worst-case scenario for reconcentration (e.g., the j shortest flowpath to a compact mass). Figure 15 shows a cross section of the trench. The i inferred flow lines are superimposed on it to represent water flow in a trench section that 1 contributes water to a hypothetical high-hydraulic conductivity pathway. The shape of i the flow lines in this figure reflect the assumption that the trench contents can be represented by a homogeneous porous medium. Note that unsaturated conditions would quickly develop above the portion of the trench section draining into the high-hydraulic j conductivity pathway. Unsaturated conditions lead to oxidizing conditions because j atmospheric oxygen is in contact with the matrix under unsaturated conditions. This would cause the oxidation / reduction interface to move downward more rapidly in some ponions of the trench section than others, as shown schematically in Figure 16. With time, the oxidation / reduction interface would continue to migrate downward in a fashion l similar to that shown in Figure 16. It is obvious from Figure 16 that oxidizing conditions will reach the zone of reconcentration well before the uranium has been swept from the bulk of the trench section. Because the trench contents are heterogeneous in nature, there likely will be preferential flowpaths through the waste (e.g., Zeiss and Major, 1992-93). This would cause the oxidation / reduction interface to reach the sink even more quickly (see Figure 17) than would be the case for the hydrologic model reflected in Figure 16. Figures 15 through 17 and the associated discussion imply that the requirement of O, persistent reducing conditions in the reconcentration zone will not be met. Instead, oxygenated waters will likely reach and oxidize any reconcentration zone in the trenches mammu""" 27

i (~] long before the uranium can be swept from the bulk of the trench contents. This will V preclude the formation of a critical mass by oxidation and precipitation reactions. The discussion presented above assumes saturated conditions exist within the trenches. This would be the case for No-Action altemative and possibly for the SIP +B altemative with degraded engineering components as noted in Section V. The SIP +B alternative l will lead to unsaturated conditions. As discussed above, unsaturated conditions lead to oxidizing conditions which will prevent the reconcentration of uranium by this mechanism. i B. Conclusion The chemical constraints on uranium mobilization and transport preclude the efficient reconcentration of uranium in high-hydraulic conductivity pathways. This implies the formation of a critical mass of uranium in the trenches is also precluded. VIL TECHNICAL ANALYSIS AND CONCLUSIONS i The analysis demonstrates that criticality within the trenches at the Parks SLDA at some fq time in the future is, in the practical sense, not credible. In summary, criticality is C/ impossible except in an extremely narrow range of conditions. The conditions making criticality possible are the combined occurrence of (1) the presence of a sufficient j quantity of U-235, (2) convergent flow of water in a trench system to a hydraulic sink in a i small area of the trench, (3) mobilization of uranium to a localized zone of potential reconcentration, (4) the presence of oxidizing agents (oxygenated water) along the convergent flow path so that uranium can be mobilized, and (5) the existence of a reconcentration mechanism by which an adequate quantity of uranium is precipitated or adsorbed. Furthermore, these combined factors must operate in such a way that the uranium would deposit in sufficient concentration and within a limited range of geometric configurations. The combined presence or operation of all of these complex j factors, with concerted action in an exquisitely configured and coordinated fashion, is considered incredible. VIIL REFERENCES ARCO /B&W,1993. Site Characterization Report, October, revised May 1995 (1995h). ARCO /B&W,1995. Stabilization in Place: Description and Performance Report, December. ARCO /B&W, 1996.1995 Field Work Report, February. meermumvsn"* 28 j

~_ 1 i i Borovec et al. (Z. Borovec, B. Kribek, and V. Tolar),1979. " Sorption of uranyl by humic O acids," Chem. Geol., 22, 39-46. Dahl, A. R., and J. L. Hagmaier,1976. " Genesis and characteristics of the southern Powder River Basin uranium deposits, Wyoming," in Twenty-Eighth Annual Field Conference,1976 Wyoming Geological Association Guidebook, p. 243-252. 1 l Economy, K., and R. S. Bowman,1992. " Preferential flow effects on chemical transport and retardation in soils," New Mexico Water Resources Research Institute, WRRI Report No. 272. Hsi, C-K, D. and D. Langmuir,1985. " Adsorption of uranyl onto ferric oxyhydroxides: application of the surface complexation site-binding model," Geochim. Coemochim Acta,42,1931-1941. Killops, S. D., and V. J. Killops,1993. An Introduction to Organic Geochemistry, Longwell Scientific and Technical, London. Langmuir, D.,1978. " Uranium solution-mineral equilibria at low temperatures with i application to sedimentary ore deposits," Geochimica Cosmochim Acta, 42, 547-570. gV 1 Nash et al. (K. S. Nash, A. M. Friedman, and J. C. Sullivan),1981. " Redox behavior, i complexing, and adsorption of hexavalent actinides by humic acid and selected clays," Environ Sci. Tech,.11,834-837. 1 Rathje, W., and C. Murphy,1992. Rubbish - The Archeology ofGarbage, HarperCollins, New York. Szalay, A.,1964. " Cation exchange prope ties of humic acids and their importance in the ) r 2 ' and other cations," Geochimica Cosmochim geochemical enrichment of UO2 Acta,28,1605-1614. j Turner-Peterson, C. E.,1985. " Lacustrine-humate model for primary uranium ore deposits, Grants uranium region, New Mexico," Am. Assoc. Petrol. Geol. Bull. 62,1999-2020. Zeiss, C., and W. Major, 1992-1993. " Moisture flow through municipal solid waste: ) patterns and characteristics," J. Environmental Systems,22, 211-231. Zielinski, R. A., and A. L. Meier,1988. "The association of uranium with organic matter in Holocene peat: an experimental leaching study," Applied Geochem.,3,631-643. 2,

4 10 CFR Part 20, Standardsfor Protection Against Radiation, U.S. Nuclear Regulatory Commission (1993). i ) + i i 1 e i i, l i 4 i 1 4 n uor m u m u m e m u n m e 30

_... ~.... _... _... _. _ _.. _. - _. _ _ _ _. _ _.. _ _ _ _ _ _ _. _... _... _. -. _. 1 4 4 i t i d a f J i l 4 ll t i ) t i j i 1 4 B 4 1 d 3 1 1 4 .i ); 1 1 1 i-i FIGURES E 4 i 1 i J 4 i 1 ,f ] l i I i 1 1 1 1 i

i i LIST OF FIGURES gQ 1 Elgium i 'l Site Base Map i 2 Frequency Histogram - 1993 - Two-Inch TWSPs 3 Frequency Histogram - 1995 - Two-Inch TWSPs 4 Frequency Histogram - 1995 - Four-Inch TWSPs 5 The Volume of the Uranium-Bearing Zone Over Which Criticality Is a Possibility for Idealized Spherical Geometry 6 Ratios of Cylindrical to Spherical Critical Masses of.U(93)O F Solutions, 22 Unreflected and With Water Reflector, as a Function of Cylinder Height to Cylinder Diameter Ratio '7 K rrvs. Slab Thickness for Constant Area (725 Square Meters) e 8 Volume Over Which Criticality Is a Theoretical Possibility for Idealized Spherical l Shell Geometry for the Base Case UO / Soil / Water Medium (76 kg U-235) 2 9 Volume Over Which Criticality Is a Theoretical Possibility for Idealized Spherical Geometry for UO / Soil / Water Medium 2 10 Minimum Spherical Critical Mass for UO / Soil / Water Medium at 35 Volume Percent 2 HO 2 11 ' Volumes Over Which Criticality Is a Theoretical Possibility for Idealized Spherical Geometry for UO / Iron Oxide / Water Medium ] 2 12 ModelTrench 13 - Average Groundwater Elevations in the Soil Zone,11/95 Through 9/96 14 Idealized Cross Section of Ore Roll 15 Model Trench Section Hydrology - Groundwater Flow Lines ] 16 Model Trench Section Redox Boundaries - Homogeneous Porous Medium 17 Model Trench Section - Redox Boundaries 1 O

1 \\ l/ / ~' \\ .s 1 y ,<~ 'N i [/ 'Ns x: x. N ( [;lLA ( ~.. . s,c ss_ q- . 4 s, ~ %...~., f ,' w N,' a s , g'^, N. x --,s v ;.< - n.- '~s s. +s. .s- -- g. ; q, ._,s. s% e.,. %. . 7..,-.s,N, < -s t \\ s '+ f yQ 1 .-r, ~ ~.. ..m ~ , x-4 .- ) .,g - / - s.,. .,'*.s. / j N ~\\ ,./ .'.L k,,[N N..., s i s ..,/ ,e ~ g s.,, s.,'N J %,.,N. [N s, N ,.<j ps' s N sN ( 3.N ( / x.- N.,N x,'x ~. s .:r' / c's t, j N s 0 f ~. Q (- s x,,' s ,{/ ,J, N, s,N 4 s..,"%(N y \\ s s / '! [, 'e,L./ ~~~ N,.'N. .,( '- - N i. / / ~ s ~.'N-- ,4 g - i .. ~~~"' .-==-a- ,,,,,,- s.~~.___ v- .p-- s -,5 .; 4 ,/ / l, f 4 w -, :... s-t / ( ..,f. ,,.%~.. s. 1 l-f 3 - ~ . w....} } l,;:.y ~ Mo.. J,.j, .n - ~ l.9 y ^9 I* i, jil r i l I l ~" 'l .s I l I i .), +l l l i I 't I I i j j j ,I i i $$d.k r fI t \\ ~ ~ ~... i l; ,.l t + i i g ,I ,1 4, i ,/ i ( n i

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\\ i s il \\ \\ \\ \\ < t N 'N l l l ( ( i l \\, s, H i 2 4 u \\ s 3 s x \\ y to '\\ i. I i i 1 d ) i ^jV -__ r c. y i t 3 s i 1 i 1 ,/'.! s / s.. y __J,_., q.. s -- _.. %a f 7 __.._...._._.4 l g,,. s i jsy {--,;---g y '.,/- p y - / /.+---- % - % _ _.,, _ _. n __,r. l {. i //. "-) _ _,- - - --..*** * ** * -~+ ----.--... *. 9 2T71-f,-a - . __,. j--- ] ~--~~f - ~- g _. _.g( i Y11 t -.-C...._A.__..._.(~~--~-----%..____r^------.- -d A g __ _._- - L s s .. y.;___ \\ g 4 h. LEGLE NOTES: -1 y 910' TOPOGRAPHIC CONTO, TRENCH # = 1 CONTOUR INTERVAL = 10 FEET DRY RUN ) g p. 1 February 21,1997

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GV FIGURE 2. FREQUENCY HISTOGRAM 1993 TWO-INCH TWSPS - g ...g i i , g i . g i l 4QQ .j..................i.................j..... ..... j.. _ j 4 g . Distribution Fit: Exponential. ' ~ 300 -' .f i G-ll m iii . ll O ll D . l ll D v u , i .8 200 -!a i-E l, 3Z i i w l l l I I { I i I r w Lt. i-100 4-l I i t l t g 0 -il - r - - - - - - r - - - - -- !- - - -- - ' - r I t I 1 I i -10 90 190 290 390 Distribution Intervals,5 pCi/g C r \\ obruary 21,1997

) 4 1 c i ( 0 I FIGURE 3. FREQUENCY HISTOGRAM 1995 TWO-INCH TWSPS g i e i g. ..g ... g i ii g g. i. l i.. ..4... .....s..............4... f% -.4....... .......5..... ....g..... ......5..... j Distributibn Fit: Exponential ; - i 300 g-m g l i n. il a en a ,l 16 g r, I 'l -{ - j-i- i i-200 E i

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l i i -I i E -i i u. 100 l-i g I t t \\ 0 -Y r - - !- - - - +- 1 I 1 ,iiI I l i,, ,, i .. i -10 30 70 110 150 190 230 Distribution Intervals. 5 pCi/g Februar/ 21.1997

l T i FIGURE 4. FREQUENCY HISTOGRAM 4 l 1995 FOUR-INCH TWSPS J 4 i ...i...i...i..., 240 -5 i-y j- -l - i-1 i 1 'l 200 l I i Distr bution Fit: F,xponential a. 160 +- m .i lii O i E i l 120 -t-r i-i- s 1 I i ) 80 E I i w LA. i 1 40 s"--.- t g t i \\ O ". -- -- _ _ A ~

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• • • • • ~~,,,..............- ------------------------------------------------------

f 1 10" = Possibly Criticd i j ~ i 10 Subcriticd Regime ~~"'*******----........,,,,,,,,,, t i f i i i I 1 i i i 10 15 20 25 30 35 l f Volume Fraction of Water (V/%) i i i l t FIGURE 5 THE VOLUME OF THE URANIUM-BEARING ZONE OVER WHICH CRITICAUTY IS A POSSIBILITY FOR IDEALIZED SPHERICAL GEOMETRY i t I L February 21,1997 l I t

.~.. _ _. _ _. _... _. _, _. _.. _., _ _.. _ O O O I 6 H/2350 Atomic Ratio V 44.3 Q329 O 52.9, a 43.9 0 320 5 r

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) Urveflected j g Q tt i ~ s 3 N k l o water Reflector [ ~ N ~ k W ~ I O i i r O 0.05 0.1 1 10 Cylinder Height /Cytnder Diameter Ref: Los Alamos Nationd Laboratory Report LA-10860-MS (1987). { FIGURE 6 RATIOS OF CYLINDRICAL TO SPHERICAL CRITICAL MASSES OF U(93)O,F, SOLUTIONS, UNREFLECTED AND WITH l WATER REFLECTOR, AS A FUNCTION OF CYLINDER HEIGHT TO CYLINDER DIAMETER RATIO Fetwuery 21,1997

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O O O t i l i 4 2-r Subcritical Regime i 10 - 9-8- 7-i 6_ ~ s-t "E Possibly Critical w 4-o 3-o> 2-10 - 9-t 8-7- i i i i i l i i 0.0 0.1 0.2 0.3 0.4 l Inside Radius (m) FIGURE 8 VOLUME OVER WHICH CRITICALITY IS A THEORETICAL POSSIBILITY FOR IDEALIZED SPHERICAL SHELL GEOMETRY FOR THE BASE CASE UO,/ SOIL / WATER MEDIUM (16kgU-235) [ 1 Fetruary 21,1997 i i

O i Subcritical Regime 2

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j 8 kg '"U y 4 kP "'O s 4 I I I I i i i 3 10 15 20 25 30 35 Volume FracHon of Water (V/7.) 4 FIGURE 9 VOLUME OVER WHICH CRITICALITY 15 A THEORETICAL POSSIBILITY FOR IDEALIZED SPHERICAL GEOMETRY FOR UO / SOIL / WATER MEDIUM '9 February 21,1997

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i i i i i i l i i i I g 0 100 200 300 400 500 Volume (111ets) i FIGURE 10 MINIMUM SPHERICAL CRITICAL MASS FOR UO / SOIL / WATER MEDIUM AT 35 VOLUME PERCENT H 0 6 February 21, t997 i

O O O 2 (42) 10, Subcritical Regime (98) (125) (79) 3 (135) (118) 4 (134) (134) 3 (119) (l37) (33) ~ (127) -o (74L 'Y6I) 1 I I E [84), # ' 10,i Possibly Critical -( (79) 2 i-s O' 8 (30) (69), ' (88) l i 5 16 kg '"U f II 4 kg '"U t 3(20) 3 (22) s 2 (3) (14)w's (4) -i i I I I i i i l i i i 1 ig 30 40 50 60 70 80 90 100 j Volume Fraction of Water (volume percent) NOTE: Atom ratios of iron to '"U shown in parentheses. l t FIGURE 11 VOLUMES OVER WHICH CRITICALITY IS A THEORETICAL POSSIBILITY FOR IDEALIZED SPHERICAL GEOMETRY FOR UO/lRON OXIDE / WATER MEDIUM Fetruary 21.1997

..._m m.. '\\ r 200' - 4' n _.-~_m.m -___.m ...:_.m_ _......___.m_ ..,__._._________,,__.,_..._.m g e an n mu n g = me = = = g = nn = nn = = = mn un = = = mu =- nn = mn = = = un = nn = =n = mn = nn =- an = me = m' = = = = = = =

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../..,-.. ~,,....,-...__... ~., - -~..,.../,- ... / /- .-.~~../,-.~..~..~~.... ~... ~,.., - ~ . ~.. .._..- ~..._ / ~.._ .._...~._.- ~..._. ~... _,.- _E.D. ROC K.......- ~.... . -...._ - ~. ~... .... B BE.DROCK ~. ~.. - .,. H10._. ~ ~.... . ~.. ..~..,....,._...-.._/,-....._...._.._.... . o -~../. .,..... ~... - ..-....-~~.~.-_- .(S.k).. - /.../..., - ~ ~.. _. p i G,, 25 50 75 o n SCALE IN FEET Maximum Critical Volume: 24 kg = U/2.2m3 O. (>10 Vol.% H2O) TOTAL VOLUME: 2000-3000 m3 Minimum Critical Mass / Volume: 4 kg5 U/0.2m2 TOTALINVENTORY: 2-24 kg"U (>35 Vol % H2O) FIGURE 12 MODELTRENCH February 21,1997

__aw_4J 4FAA4..J-4.me*4 A,-4 A .ma 4uz .u..-m_. aM_.._MC A&4We 4 4 M-Mee-eeMw-.-.a4.e J- ,Aar e.agr.,,-aseama.1m.w ss w see

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• Eo."7['~ ' $0$ND A E E CVATIONS February 21,1997

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. _... ~ _. _ _ _ _ l 1 LIST OF TABLES IRblt 1 Downhole Gamma Logging Activities in Two-Inch TWSPs 2 Statistics for Gamma Log Data ) i 3 U-235 Inventory Estimates from Gamma Logs i 4 Total Uranium Activity in Boring Samples 1 5 ' Composition of Nominal Soil ~ 6 Sensitivity Studies of K, gat 35 v/%, Water Content 4 l 7 Water Level Elevation on Date Indicated 4 J O l l O

___._____._7.____ l Table 1 j Downhole Gamma Logging Activities in Two-Inch TWSPs I 1993 1995 1993 1995 { Count Activity Activity Count Activity Activity Depth (ft) (pCi/s) (pCi/g) Depth (A) (pCi/s) (pCi/g) TWSP01 1 1 4.7 TWSP02-1 1 4.1 TWSP011 2 2.4 2.3 TWSP02-1 3 3.5 i TWSP01 1 4 1.2 TWSP021 4 0.8 11.9 f TWSP01-1 6 0.6 TWSP02-1 5 16.6 51.2 i TWSP011 7 0.2 2.5 TWSP02-1 6 31.8 3.8 TWSP011 9 1.4 TWSP021 7 15.4 TWSP011 11 1.3 TWSP021 8 12.5 TWSP01-2 2 2.2 2.8 TWSP02-1 9 0.7 j TWSP012 3 0.5 _TWSP02-1 10 0.5 TWSP012 5 68.4 TWSP02-1 11 4.3 3.9 i j TWSP012 6 45.1 13.6 TWSP02-1 12 14.7 7.6 j TWSP012 7 7.9 8.0 TWSP02-2 3 5.6 1 TWSP012 8 6.1 6.5 TWSP02 2 5 8.3 2.6 TWSP012 9 1.5 2.4 TWSP02 2 6 9.2 14.2 TWSP012 10 1.7 TWSP02 2 7 27.1 18.3 TWSP013 4 1.0 TWSP02-2 8 6.0 24.6 TWSP013 5 3.5 TWSP02 2 9 4.7 20.8 TWSP013 8 0.1 TWSP02-2 10 10.0 12.9 TWSP013 10 2.0 TWSP02 2 11 20.5 O TWSP013 11 8.9 0.4 TWSP02 2 12 21.3 TWSP014 1 3.6 TWSP02-2 13 10.0 TWSP01-4 2 2.0 0.6 TWSP02-3 1 3.0 TWSP01-4 4 0.7 TWSP02 3 2 43 TWSP01-4 5 4.8 TWSP02 3 3 16.6 TWSP01-4 6 33 12.5 TWSP02 3 4 0.8 TWSP01-4 7 43 TWSP02 3 5 3.1 1.9 TWSP014 9 5.2 TWSP02-3 6 13.5 53 TWSP01.4 2 2.2 TWSP02 3 7 10.0 14.8 TWSP01-5 4 1.5 TWSP02 3 8 17.7 13.0 TWSP015 5 0.5 TWSP02-3 9 27.1 12.5 TWSP015 7 8.2 7.9 TWSP02 3 10 275.0 160.0 TWSP015 8 21.9 25.2 TWSP02-3 11 10.7 18.6 TWSP015 9 3.7 3.0, TWSP02 3 12 15.5 10.8 TWSP01-6 0 2.2 TWSP02 3 13 23.7 33.7 TWSP014 1 0.2 TWSP02 3 14 38.7 TWSP01-6 2 1.3 3.9 TWSP02-4 3 5.3 TWSP01-6 3 2.5 TWSP02-4 5 3.9 12.4 TWSP01-6 5 2.3 0.6 TWSP02 4 6 41.2 16.9 TWSP01-6 6 6.5 3.0 TWSP02 4 7 11.4 40.9 TWSP016 7 23.1 5.0 TWSP02-4 8 51.6 27.7 TWSP01-6 8 41.9 64.2 TWSP02-4 9 38.8 203 TWSP01-6 9 11.4 44.5 TWSP02 4 10 24.9 23.9 TWSP01-6 10 13.4 TWSP02-4 11 12.7 O TWSP021 0 4.8 TWSP02-4 12 1.8 9.2

• 8 U N sworacosmimocersu ooc T-1

. ~. _ Tcble 1 Centi:ued i Downhole Gamma Logging Activities in Two-Inch TWSPs i l 1993 1995 1993 1995 Count Activity Activity Count Activity Activity g, I Depth (ft) (pCUg) (pCUg) Depth (ft) (pCUg) (pCUg) j TWSP02-4 13 6.0 TWSP06-3 14 0.6 TWSP02-4 14 1.8 TWSP06-3 15 6.8 TWSP031 3 0.4 TWSP06-3 16 6.7 TWSP03-1 4 0.6 TWSP07-1 2 3.4 i TWSP03 2 8 13.0 1.7 TWSP071 6 5.2 TWSP03 2 9 32.9 55.5 TWSP07-1 7 0.6 113 3 TWSPO41 1 2.9 TWSP07-1 8 0.1 43.7 TWSPO4-1 7 1.7 TWSP071 9 41.7 11.4 i TWSPO4 2 0 1.3 TWSP07-1 10 38.3 22.0 TWSPO4 2 1 1.4 TWSP07-1 11 2.97 TWSPO4-2 2 2.5 TWSP07-1 14 10.29 TWSPO4 2 3 2.6 TWSP07-2 4 0.7 J TWSPO4-2 4 1.7 TWSP07-2 5 1.0 TWSPO4-2 7 0.5 TWSP07-2 7 0.5 l TWSPO4 2 8 1.8 TWSP07-2 9 0.4 1 TWSPO4-2 9 0.7 TWSP07-2 10 2.4 f TWSP04-2 10 4.8 TWSP07 2 11 5.3 j TWSP04 2 11 12.4 38.0 TWSP07 2 12 1.1 0.6 TWSP04-2 12 35.5 31.4 TWSP07-2 14 1.7 i TWSPO4-2 13 39.8 11.7 TWSP07 2 15 0.1 TWSP04-2 14 0.9 0.5 TWSP07 3 0 2.0 TWSP04-2 15 0.7 7.6 TWSP07-3 6 15.5 14.2 TWSP04 2 16 36.6 TWSP07-3 7 6.7 23.4 j TWSP051 9 17.2 TWSP07 3 9 0.7 i TWSP05-1 10 $ 1.0 TWSP07-3 10 1.5 TWSP05-1 11 24.5 43.7 TWSP07-3 11 9.5 1.2 TWSP051 12 64.2 23.2 TWSP07 3 12 21.9 16.8 TWSP051 13 80.5 28.9 TWSP07 3 13 15.6 10.5 l TWSP05-2 2 1.0 TWSP07-3 14 9.4 TWSP05 2 6 8.6 TWSP07 3 16 1.1 2.8 I TWSP05 3 8 7.8 TWSP07 3 17 3.1 TWSP05-3 9 7.0 50.6 TWSP08-2 17 2.2 TWSP05-3 10 33.7 20.5 TWSP08 2 19 0.2 TWSP05-3 11 10.5 0.5 TWSP091 6 5.0 i TWSP05-3 12 0.4 TWSP09-1 7 2.5 l TWSP06-1 7 153.2 TWSP09-1 8 23 1 TWSP06-1 8 345.3 37.1 TWSP091 9 10.7 i TWSP061 9 31.1 181.7 TWSP091 10 3.9 TWSP06-1 10 70.3 12.0 l'WSP09 2 1 1.0 TWSP06-1 11 25.9 15.4 TWSP09-2 3 2.4 l TWSP061 12 11.2 44.9 TWSP09 2 6 5.6 TWSP06-1 13 85.4 TWSP09-2 7 03 TWSP06-2 12 27.7 TWSP09 2 8 6.1 TWSP06-2 13 3.6 TWSP09-2 9 8.2 4 ) TWSP06 2 14 2.0 TWSP09-3 7 2.5 TWSP06-3 13 13 TWSP09-3 8 1.5 4 n aotAnco m u m e m m s m e T-2 cm

• 5 :nv i,

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Tcble 1 Centinued I Downhole Gamma Logging Activities in Two-Inch TWSPs 4 1993 1995 1993 1995 Count Activity Activity Count Activity Activity h ion Depth (ft) (pCUg) (pCi/g) Depth (ft) (pCi/s) (pCi/g) i TWSP101 3 1.5 TWSP10-5 2 0.4 TWSP10-1 7 27.5 TWSP10-5 11 48.3 TWSP10-1 8 22.5 54.7 TWSP10-5 12 853 22.0 TWSP10-1 9 29.4 2.8 TWSP10-5 13 11.1 4.7 TWSP10-2 1 2.2 TWSP10-5 14 1.0 TWSP10-2 10 4.5 T W SP10-5 15 1.9 i TWSP10 2 11 1.1 TWSP10-5 18 1.9 l TWSP10-4 3 0.7 TWSP10-6 9 0.4 TWSP10-4 4 1.5 TWSP10-8 2 0.7 T W SP10-4 8 0.2 TWSP10-8 6 0.3 0.7 TWSP10-4 17 0.1 TWSP10-8 7 0.4 l TWSP10-8 19 0.3 I LeSend: Bold location indicates new data point. Bold 1993 data point indicates new or revised data. Bold 1995 data point indicates new or revised data. Note: MDA for U 235 for 1993 and 1995 measurements is 3.5 pCi/g. !O

l l

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9 l Table 2 i Statistics for Gamma Log Data i Variable 1993 Two-inch TWSPs 1995 Two-Inch TWSPs 1995 Four-inch TWSPs Sample Size (Number) 503 453 310 4 } Average

• 4.85 5.07 3.93 Standard Deviation
• 22.2 17.4 14.7 j
• U 235 pCi/g E

l l 4 1 i i 9 4 1 1 i, i i l i I PAOfARCollNALDOCf tNTBLE DOC T4

Table 3 U-235 Inventory Estimates from Gamma Logs 0.5 Fraction of MDA* 2-INCH TWSP DATA 4-INCH TWSP DATA 1993 1995 U-235 1993 1995 U-235 Trench TWSP G U-235"8 G U-235"' % ENR Trench TWSP G U-235"' G U-235"' % ENR 01 1 281 281 4.9 01 7 2027 11.5 01 2 1329 2051 7.6 01 8 34 7.9 01 3 460 281 6.5 01 9 23 7.9 01 4 333 628 7.8 01 10 -30 8 01 5 820 841 9 01 11 -4 8 4 01 6 2006 2426 7.6 01 12 20 6.1 02 1 2045 2331 29.1 02 5 1093 17.6 02 2 2356 2784 13.8 02 6 -67 14.3 02 3 10247 6447 13.5 02 7 4131 27.6 02 4 4249 4083 4.1 05 4 2648 14.1 03 1 88 88 0.72 05 5 4954 37.4 i i 03 2 103 103 0.72 06 4 157 5.2 j 04 1 444 444 1.3 07 4 751 42.3 04 2 2425 3288 21.6 07 5 1313 7.4 05 1 2510 2389 8.3 07 6 974 7.8 j 05 2 194 288 3.4 09 4 310 11 05 3 878 1261 1.8 09 5 -14 11 06 1 15020 11764 24.2 10 9 -235 0.72 06 2 1272 547 2.3 10 10 -171 0.72 06 3 854 593 1 10 11 -76 0.72 07 1 1812 3582 23 7 10 12 -375 0.72 1 07 2 427 365 11.4 10 13 499 0.72 j j 07 3 1476 1565 8.3 J 08 1 359 359 2.5 1993 2. inch + 1995 4-inch 1995 2-inch + 1995 4-inch j 08 2 383 383 2.6 G" G 09 1 323 159 5.5 Trench U-235 INV ENR U-235 INV ENR 01 7300 9.40 8.67 8581 11.56 8.55 09 2 346 179 7.] 02 24054 30.96 15.80 20803 28.02 16.46 ~ 09 3 159 159 8 03 191 0.25 0.72 191 0.26 0.72 10 l 1496 2150 1.3 04 2868 3.69 18.46 3731 5.03 19.19 l 10 2 512 453 2.5 05 11183 14.40 21.97 11541 15.54 21.29 10 3 491 491 3.1 06 17302 22.27 21.27 13061 17.59 22.00 l 10 4 528 528 0.72 07 6752 8.69 16.16 8549 11.51 17.68 10 5 2569 20/0 0.72 08 742 0.96 2.55 742 1.00 2.55 10 6 604 604 0.72 09 1124 1.45 7.80 794 1.07 8.42 10 7 642 642 0.72 10 6165 7.94 1.20 6260 8.43 1.22 10 8 679 679 0.72 mal 77683 100 74252 100 i G U-235-grams of U-235 in trench %INV-percent of the total U-235 inventory present in trench i % ENR-inventory-weighted enrichment in trench Assumed U-235 concentraction for 2-inch TWSP results <MDA: i ' Minimum Detectable Activity (MDA) of 3.5 pCi/g U-235 iV mn 23 ru s norucomanocmats poc T-5

Table 3 Continued U-235 Inventory Estimates from Gamma Logs 0.0 Fraction of MDA* 2-INCH TWSP DATA 4-INCH TWSP DATA Trene TWSP 1993 1995 U-235 Trench TWSP 1993 1995 U-235 h G U-235 G U-235 % ENR G U-235 G U-235 % ENR 01 1 0 0 4.9 01 7 2027 11.5 01 2 1188 1946 7.6 01 8 34 7.9 01 3 249 0 6.5 01 9 23 7.9 01 4 86 452 7.8 01 10 -30 8 1 01 5 679 665 9 01 11 -4 8 01 6 1936 2286 7.6 01 12 20 6.1 02 1 1880 2208 29.1 02 5 1093 17.6 02 2 2273 2619 13.8 02 6 -67 14.3 02 3 10164 6324 13.5 02 7 4131 27.6 02 4 4043 4001 4.1 05 4 2648 14.1 03 1 0 0 0.72 05 5 4954 37.4 1 03 2 0 0 0.72 06 4 157 5.2 04 1 0 0 1.3 07 4 751 42.3 04 2 2021 3005 21.6 07 5 1313 7.4 05 1 2340 2268 8.3 07 6 974 7.8 05 2 0 119 3.4 09 4 310 11 05 3 708 1091 1.8 09 5 -14 11 06 1 14837 11581 24.2 10 9 -235 0.72 O 06 2 816 0 2.3 10 10 -171 0.72 06 3 352 0 1 10 11 -76 0.72 07 1 1569 3399 23.7 10 12 -375 0.72 07 2 92 0 11.4 10 13 -499 0.72 07 3 1203 1291 8.3 08 1 0 0 2.5 1993 2-inch + 1995 4-inch 1995 2-inch + 1995 4-luch 08 2 0 0 2.6 G" % C' G '" % C' Trench t!-235 INV ENR L1-235 INY ENR 09 1 223 0 5.5 01 6210 9.25 8.98 7420 11.66 8.80 09 2 226 0 7.1 02 23519 35.03 15.83 20309 31.93 16.47 09 3 0 0 8 03 0 0.00 0 0 0.00 0 10 1 1119 1773 1.3 04 2021 3.01 21.60 3005 4.72 21.60 10 2 97 0 2.5 05 10651 15.86 22.85 11081 17.42 22.00 10 3 0 0 3.1 06 16162 24.07 22.40 11738 18.45 23.95 10 4 0 0 0.72 07 5901 8.79 16.49 7727 12.15 18.16 10 5 2079 1617 0.72 08 0 0.00 0 0 0.00 0 10 6 0 0 0.72 09 746 1.11 8.17 297 0.47 11.00 10 7 0 0 0.72 10 1938 2.89 1.14 2033 3.20 1.19 10 8 0 0 0.72 63610 100 Total 67148 100 G U 235-grams of U-235 in trench % INV--percent of the total U 235 inventory present in trench % ENR-inventory weighted enrichment in trench Assumed U-235 concentraction for 2-Inch TWSP results <MDA: ' Minimum Detectable Activity (MDA) of 3.5 pCi/g U-235 Os "2 ' * ' 2' "" morecomeocurats me T-6

Table 4 Total Uranium Activity In Boring Samples Boring Total Uranium (pCi/g)- Lab Measured Trench 1: 01U06* 1107,588 01U09* 154,98,37,48 l 01U13* 24,72,30,29,6,32 OlU15* 83,29 01U16* 130,51,171 Treneh 2: 02UO2* 31,264,73,7,6 02U0t.' 43,3,4 02UO8* 356,626,180 02U12* 69,141,49,62 02U 13

• 60,900,79,136,12,15 Trench 3:

03UO3 34 03UO4* 6,16 1 03UO6* 15,92 Trench 4: 04UO2 8, 9 Trench 5: 05UO4* 32,14 ) 05UO5 10 05UO7 6 Trench 6: 06U0l* 38 06UO3 17 Trench 7: 07UO4 07UO5' 12,17,20,86 07 06* 16 \\ 07UO7* 29 07U08 13 Trench 8: 08UO6 3,8,7 ' Obvious waste debris present. O senounconmoceronns mc T7

Table 5 l } p Composihon ofNominalSoil 4 Constituent MassFraction Cabon' O.0429 Oxygen' O.48999 Sodium' O.0068 Magnesium' O.0060 4 Aluminum' O.0710 Silicon' 03300 Potassium' O.0136 Calcium' O.0137 hon' O.0260 2 Cadmium 0.000005 2 Gadolinium 0.000005 4 O) ' Nominal soil representative of Pennsylvania soils. Obtained from NRC during a V meeting held 5 December 1996. 2 Site-specific soil concentrations (ARCO /B&W,1993). O anonAncom4uxxvsrets poc T-8 4

Table 6 Sensitivity Studies of K rr t 35 v/%, Water Content a e 16 kg U235 16 kg U235 3.99 kg U235 Parameter 1850 liters 54 liters 140 liters U(20) Base Case 0.97 0.97 0.97 U(l0) 0.95 0.93 0.93 l U(30) 0.98 0.99 0.98 2 X Gd 0.94 0.97 0.96 Zero Gd 1.00 0.97 0.98 2 X Fe 0.94 0.97 0.96 Zero Fe 1.00 0.97 0.98 2 X Cd 0.97 0.97 0.97 i J Zero Cd 0.97 0.97 0.97 1 1.1 g/cc core, 0.99 0.96 0.96 1.5 g/cc reflector 1.3 g/cc core, 0.98 0.96 0.96 1.5 g/cc reflector 1.7 g/cc core, 0.96 0.97 0.97 1.5 g/cc reflector Legend U(20) = U-235 at 20 percent enrichment. U(10) = U-235 at 10 percent enrichment. U(30) = U-235 at 30 percent enrichment. 2 X Gd, Fe, Cd = 2 times gadolinium, iron and cadmium present in base case. Zero Gd, Fe, Cd = zero gadolinium, iron and cadmium. 02mm s 2nu a norecosmemocemrats noe T-9

~ Table 7 i l O V Water Level Elevation on Date Indicated LARAllON 9/11/95 40/4'93 II/20/95 42/7/9$ 1/17.96 2/19/96 3/21/96 $/20/96 6/27/96 8 19/96 9/2$/96 A % i.R Abt, IDENTinCATION %ATER LEVEL ELEVATION Dil.131 P/ 894 50 892 60 493 80 892 10 893 84 894 10 9u0 40 897 10 av6 60 $wd 95 Oil 171P1 896 98 896 98 896 98 896 98 896 98 8 % 98 896 98 896 98 896 98 896 98 896 94 896 98 Ull'291PZ 892 26 892 26 892 26 893 14 894 72 892.26 894 42 897.90 897.20 892.26 894.24 894 OM 02L i1 'l PI 900 % 9uo 90 900 90 900 90 900 90 900 90 900 90 900 90 900 90 900 90 Wo 90 Wo w 02L'l3 TPZ 910 40 908 63 910 92 912 40 911 30 912 76 909 23 412.23 913 23 918.07 911 40 984.23 43t '0$ I PZ 906 65 m 65 906 65 906 65 906 6$ 906 65 916 to 906 6$ 906 65 907 78 0$l)C71 PZ 930 7$ 928 $$ 910 $$ 92885 930 $8 93607 93367 912 67 93307 930 63 930.20 96 06 LIM 1 PZ 925 34 926 74 92794 926 89 928 74 929 42 932 47 93l97 91197 928 30 928 32 928 9; 07 ties TP/, 922 10 926 80 928 80 927 07 928 91 921 00 932.28 932 00 932 00 928 46 928 R 927 98 14l 04 TPZ 920 89 92649 924 60 922 99 924 19 925 40 929 79 929 79 929 59 923 74 924 47 92513 00l2051 P/, 924 11 922SI 924li 92251 924 $1 926 76 929 76 929.20 928 78 923 92 924 16 925 47 09t071PZ 90$ 60 9u5 60 90$ 60 9u$ 60 905 60 vuS 60 90$ 60 905 60 90$ 60 9us60 905 60 905 60 P/ ol 892 ll kuv 91 890t5 891 O6 hvi69 892 63 8v5 55 893.73 894 90 891.27 89137 kv2 40 P/,-02 892 17 89697 892 00 891 97 89L97 89$ 90 896 72 8v8 28 898 09 av3 49 893 ko 8v4 28 P/ 03A 900 58 90u 28 8v9 63 90u 28 906 78 900 79 901 64 W2.75 902 78 901 87 901 78 901 29 P/ 04 899 80 899 77 499 77 899 77 899 89 899 77 899 77 8v9 60 89977 899 77 899 77 Pl.05 908 72 908 $2 908 38 908 32 908 72 9u838 908 18 908 35 908 22 9u8 38 908 38 908 43 PI 06 A 91646 929 7v 929 18 929 65 933 76 924 43 93844 937 74 937 60 932 86 933 04 932.54 P/ 07 9049 930 48 929 av 930 66 932 59 934 39 938 66 937 66 937 Ol 931 98 932 99 933$7 P4 04 417.11 916 68 916 46 917 86 919 81 913 24 922 02 92281 922.28 918 78 920 16 91884 Pl 09 92912 9lb$6 920 12 92248 925 07 926 56 928 92 929 92 928 96 923 90 926 79 924 49 1 % hP 01 1 895 29 894 25 497 vi 897 66 8v8 42 89805 89841 899 41 av8 96 896 49 8v8 53 897 63 I M kP 012 Vu2 37 904 37 902 37 902 37 902 47 904 37 9u6.$ $ 90$ 54 90$.76 902 $7 9u3 Su 903 84 1 % hP ol.3 90171 900 25 907.21 906.26 905 31 9u6 51 9u7 67 m 99 m 87 904 63 m36 9uS 42 "l % %P ol.4 90v 23 908 73 90405 910 $$ 910 35 911 05 912 78 912 46 912.23 909 90 911.52 910 1'/ ^ 'l % $P ol-S 9 t l.22 909 40 907 05 409 22 916 68 909 31 960.22 9l 102 911.22 940 14 909 62 910 01 1% hP Cl4 982 64 91146 916 88 916 14 912 54 91576 917 14 916 39 916 14 413 76 91534 414 8k TM hr 017 9u648 901 18 von 78 9ul48 603 Su 903 83 903 73 90$ 87 vuS 48 902.20 vu) 94 90334 1 % hP 01-4 899 77 899 77 902 91 903Ii 904 23 9u4 81 906 v6 906 81 906 49 904 95 90570 9u414 'l % hP 01-9 902.28 402.25 903 US VuS 65 908 00 908 65 90936 909 92 909 93 9u8 63 90k77 90714 'i% NP 01 10 906 76 9u6 76 vuo 76 906 76 vuo76 906 76 W616 90736 907 93 908 16 908 14 907 17 1 % hP 01 il 910 41 910 4l 940 4l 940 41 910 44 9105l 910 41 918.29 914 89 912.20 91$ l7 911.23 1 % hP ol-62 908 64 908 64 910 96 913 44 913 93 914 60 965 $1 915.24 96533 911.69 910 $7 912.$' 1 % hP C21 916 12 965 92 919 $2 918 92 917 96 919 04 920 69 919 48 918 92 917 42 918 33 918.39 'l % hP 02 2 916 12 9:2.$7 912 81 911 92 964 92 91230 912.52 914 92 91$ 92 915 92 915 64 914 44 l%hP 02 3 9 8 0.$ $ 9u9 3$ 940 9$ 910 6$ v l2.23 9126$ 914 6$ 913 90 91431 914.6$ 913.38 912.21 l'W hP 02-4

91) 26 910 81

91) 06 91826 912 76 913 03 947 al 964.26 914 45 912 48 91351 91316 j

1 % hP 02 5 914 64 963 56 91 $ 04 964 24 914 84 915.24 946 24 965.24 966 80 915 12 91687 915 19 i% hr 02 6 908 79 909 47 9u8 79 922.27 908 79 9ua 79 9i0 27 9 n.27 9n 88 on 61 9is av 1 % SP 021 910 38 911 88 911 38 915.22 912 70 966 43 913 89 914 16 911 85 913.27 913 09 'I% hP 03-l 909 US 909 US 909 OS 90905 we76 909 U$ 909OS 909 05 909 03 909 05 909 05 909 02 1 % hP O3-2 910 8) 9tO t) 910 83 910 83 982 94 910 81 910 83 910 83 91u s3 91473 91093 911 11 1 % $P 04 6 923 90 923 2u 926 30 923.70 923 69 926 62 92840 928.20 928 35 926 4I 926 39 926.29 'l % hP 04-2 930 27 928 $3 930 67 928 97 930.25 931 67 M4 47 9B.25 933 27 930 46 930 04 930 99 1%hP048 929 09 928 64 929 79 429 16 927.93 931 3 { 934 44 933 51 933 45 930 54 930.23 910 ?4 1 % hP 052 925$4 927 89 930 64 929$0 92982 929 9 6094 940 60 930 29 929 94 929 $7 929.52 1% hP043 938 44 928 81 931 16 929 24 930.24 9M ' d3 L 99 931 96 932 On 928 54 929 87 93i L7 l%SP0%4 929 00 929 00 93l 90 929$6 932 20 93Q, 932 30 932 14 932 30 930 8$ 930 18 930 94 1%hPOSS 928 96 92866 929.28 929 16 930 16 6 934 35 93.1 lb 933 46 930 16 930 19 930 80 TM kP 06-1 926 26 925 $8 926 38 925 78 927 46 tN 927 46 928 88 928 46 927 46 927 34 927 Ol i TM hP A2 428 42 927 92 928 42 927 92 910.22 T301 ' 935 14 933 77 9077 929 92 9297l 930 5' I % hP 06-3 930 72 929 49 929 36 928 92 932 82 9D 60 94848 936 42 937.26 932 49 931 94 932 99 1% hP N4 93367 930 06 929 87 929 32 929 40 931 92 93731 93647 93567 94167 9314$ 93230 1 % hP 01-1 924 13 9D 83 926 83 925 08 925.67 92$83 92783 928 23 92846 924 83 92546 926 02 I % $P 07-2 929 87 92943 929 49 929 29 93129 930.29 933 56 933 47 933.29 936.29 930 98 93l Il 'I % SP 07-3 932 08 931 63 93 I i5 930 83 933 61 93508 938 $8 938 23 937 98 933 26 932 69 934 to '1 % hP 07-4 914 10 93936 9H $2 933 44 934 10 934 $3 934 35 938 00 91787 934 9u 43132 935 41 1% hP 07 $ 926 92 926.22 927 87 926 22 926 37 927 74 91089 9%l 45 930 81 928 22 928 03 928 27 I % hP 074 924 84 418 30 923 84 923 14 923 64 923 44 926 39 928 16 927 62 925 66 928 au 924 91 I W hP 04-1 9298v 929 to 929 43 929 89 93499 90 4v 936 42 935 89 9M 66 931 $6 93438 932 23 TMhP002 932 75 932 08 931 68 9)l.38 934 08 936 08 939 78 939 08 939 33 9043 933.29 434 83 Tu hP 09-l 912 Il 912 43 92040 912.13 91 $ 67 962 13 912 13 912 45 942 il 926.82 918 13 914 71 c::i et $ 23 Pu hPROFARCOff%LDOCflNTBL5 DOC N0

Table 7 Continued Water Level Elevation on Date Indicated \\ "I% hP 09-2 911 43 911 4) 914 91 964 03 918 9) 91241 961 64 914 64 911 64 914 4) H 7 98 913 0$ T% hP 09-3 911 $7 911 40 91787 98 $ 17 91507 916 17 916 17 945 8v 916 67 916 62 915 67 915 21 '! % hP 094 912 $9 912 $9 913 64 912 $9 912 $9 912 $9 912.59 912Sv 913.24 912.$v 913 02 912 78 T%hP 09-A 912 l$ 91245 915 64 912 29 912 14 962 15 91215 912 39 913 14 913 14 913 42 912to LST% L 923$ thTw l-933 $ L$T% L 913.$ th1 % L 918$ L%TW L 913 $ L%i% L 913 $ LS1M l. 933 $ L%I M L 903 $ %H%h 890 %H%% 6k4 4%) av" th1% 1 943 $ %-4 890 %IktAM 880 MM huu Didi 912 %42 912 %1 kLAM 092 %1kl AM h2 %IktAM 894 %I kL Ahl % r> DIOi IIi %I kLAM 890 %IklAM 8 91 %)HLAM 900 00 bl( H 940 51 RLAM 902 %Ikl AM 904 tilt H 909 Ul( H km siklAM 9% %IkLAM 906 SlkLAM 940 %1kLAM 914 sikl AM 912 % I kl.A M 9 6 t. %iLD 922.$ %I kLAM 918 %IRLAM 920 00 siktAM 922 siktAM 924 %1ktAM 92r> %iktAM 926 siREAM 930 i Note If a well was reputed as dry, the Imttom of the well zone was used as the readmr fo o it date Sotne wells presiously augned to the soil umt could not be used because they were actually mstalled entuely m the top of the weathered bedrock. 4

• Nch were installed partially m the bedrock and partially in the seil zone were generally used unless they indicated an obviously at.amalous hydrauhc head mluded wells were all from the PZ and TPZ senes (TPZ 01007,17Z 02Ull,TPZ 07005 TPZ 08UO5, TPZ 09U07 PZ 2 PZ-3, PZ 4, PZ-5, and PZ-8 lhe TPZ wells consisted of I ft screens mstated m the bottom of soil bonngs Many of these tonngs extended a faen or mme mio the weathered bedrock. The PZ wells were installed across the soiWedrock mterface Both sets of wells produce wster levels lower than neartiy wells mstalled entirely in the soil rone ben the TPZ and PZ wells retamed for the analysis appear to exhibit water levels lower than the neartiy wells as shown m the comour mar tsee Figure 13) lo imprese the extrapolatinn of contour imes outside the area of well data, surface elevations were mput for locations along the stream and m the permanent wetlands near Trenches 4 and 5 In addition, estimated water levels were input for locations uphill of the trenches dented from the surface elevanons and the averaFe depths to groundwater These locatmns are also indicated on Figure 13 I tRotARCotth ALtxxvlNTBL5 DOC T.I1

!O a \\ ) a i \\ i APPENDIX A 1 1 l Explanation for Revisions to Table 7, Voluine 2 of 5 of the 1995 Field Work Report, Appendix A, Rev. 0,2/5/96 1 (Table 1 of Criticality of Safety Evaluation Report)) 4 i j i i 4 3 1 4 ) a

During the last week of 1996, MJW was requested by GCX, Inc. to review the 1993 and (V) 1995 TWSP data as compiled in Table 7 of Appendix A of the 1995 Field Work Report. This request was prompted by the present criticality calculations being performed and concern over some of the discrepancies seen between the 1993 data and those recorded in 1995. The 1993 data were first checked for accuracy. The following data problems were identified as a result of this review: The value reported for temporary waste sampling point (TWSP) 02-4 at 11 feet (ft) was reported in error. Review of 1993 2-inch TWSP gamma logging data shows that for TWSP 02-4 at 11 ft that no data are listed for a peak in the region of spectrum from channel 165 to 211 at this depth. Peaks were identified in this general region between depths of 6 to 10 ft in TWSP 02-4. The spectrum at 11 ft was reevaluated and it was confirmed that the net integral for the region of the spectrum between channels 168 and 206 was 0. Therefore, the new value for TWSP 02-4 at 11 ft is 0 versus the originally reported number of 728.3 picoeuries per gram (pCi/g). Further, a value of zero is more consistent with the 1995 measured value of 12.7 pCi/g for TWSP 02-4 at 11 ft. The protocol for TWSP measurements in 1993 was to count the first four 1-ft intervals for 5 minutes and each 1-ft interval below 4 ft for 10 minutes. This data (V collection procedure was followed for TWSP 011 through TWSP 02-4. The 3 counting protocol was modified at TWSP 03-1 in 1993 to count the first 4 ft at 5 j minutes and all 1-ft intervals below 4 ft for 20 minutes. The original activity calculations for Rev. O of the 1995 Field Work Report for TWSPs 03-1 through 10-8 used 10 minutes as the count time instead of 20 minutes. Therefore, the values reported in Table 7 for these TWSPs are a factor of 2 above what the actual value should be. Therefore, all of the 1993 activity data calculated for this group of TWSPs listed in Table 7 are actually one-half of the value given. The newly revised Table 7 corrects this error, All spectra collected in 1993 were reevaluated to determine if, even in the absence of e a discernible peak, a net positive integral would result over the same region where other spectra had shown an identifiable peak. The results of this reevaluation show that 91 additional data points have been added to Table 7 for the 1993 data. Of these 91 additional points, however,63 or approximately 70 percent are reported at an activity level less than the calculated minimum detectable activity (of 3.5 pCi/g) for the detection system. For the 28 additional data points which are above the minimum detected activity (MDA), the data range from 3.51 to 27.7 pCi/g with an average of 7.3 5 pCi/g at one standard deviation. in a few instances, it appears that the depths for the 1993 data are off by I ft when e compared to the 1995 data. For example, the data for TWSP 05-3 at 9,10, and 11 ft b,o e2 2isi sa ru a nor.Aacomu.oocmarrs ooc A-1 r/

l for 1993 appear to correlate with the 1995 data for 8,9, and 10 ft. Similarly, the 1993 data for TWSP 06-1 from 8 to 13 ft match well with the 1995 data for TWSP 06-1 from 7 to 12 ft; and for TWSP 01-2 from 5 to 6 ft for 1995 seems to match well with i the 1993 data for TWSP 01-2 for 6 to 7 ft. It is believed that the 1993 data were measured at the end of the 1-ft measurement mterval whereas the 1995 data were measured at the mid-point of the interval. The probe location would therefore be approximately 6 inches off between the two data sets and would account for the discrepancy.

• The data reevaluation for the 1995 remeasurement of selected depths for the 2-inch TWSPs shows that an additional 49 data points have been added to the original Table

7. Of these,49 additional data points (41 or 83.7 percent) are reported at an activity level below the MDA. For the eight data points which are greater than or equal to the MDA, the data range from 3.5 to 7.6 pCi/g with an average value of 4.9 i 1.3 pCi/g at one standard deviation.

In general, the 1993 TWSP gamma log data were predictive of locations of increased gamma activity when compared with the remeasurements of these same TWSPs in 1995. More importantly, the 1993 data measured above background levels are of the same magnitude as those calculated for the 1995 2-inch TWSP remeasurement data, thus adding increased validity to and showing increased consistency with the earlier 1993 measurements.

Reference:

ARCO /B&W,1996.1995 Field Work Report, February. l l l '. O l LJ morneomecocoes poc A-2 1

~ O J d i ) l i APPENDIX B Validation of Computer Codes and Cross Section Libraries O

~

l l l ,A Several computer code / cross section set combinations were employed in this validation d study to provide confidence in the accuracy of the calculations. These were: ONEDANT and KENO with the Hansen-Roach 16 group cross section set; ONEDANT and KENO with the 27 group SCALE 4.3(ENDF/B-IV) cross section set; ONEDANT and KENO with the 41 group SCALE 4.3(ENDF/B-V) cross section set; MCNP with a continuous energy (ENDF/B-V) cross section set. The primary source of the benchmark cases was the Intemational Criticality Safety i Benchmark Evaluation Project (ICSBEP). Other sources were used based upon need and review of the experimental data. If the critical experiment was evaluated and approved by the ICSBEP, no further investigation was done. If the experiment was included from another source, a detailed study of the experimental data was conducted. An experiment was not used if the uncertainties in the data were such that the uncertainty in the calculated K,g was greater than 1.0%. The experiment was not used if the experimental data was not complete, i.e. the critical experiment could not be modeled from the information contained in the reference. A summary description of the critical experiments used in the validation calculations is (q provided in Table Bl. j Table B1 Critical Experiments Used for Validation Uranium Solution / Compound Systems Esperiment Description Reference Case Number U(93.17)OfNO )2 Cylinder HEU SOL-THERM-001 CASE 1 3 1 inch U(30)F, cubes reflected by poly IEU-COMP THERM-001 CASE 2 U (4.89)O,/Sterote" LA 10860-MS CASE 3 3 Bare U(4.98)O F Solution ORNL-3714 CASE 4 2 2 Ut93.2)OfNO )s Solution NS&E Vol 12 No 3 CASE 5 2 Bare U(93.2)O (NO 13 Solution NS&E Vol 12 No 3 CASE 6 2 2 Bare U(93.2)O (NO ), Solution NS&E Vol 12 No 3 CASE 7 2 2 Results of the validation runs are provided in Table B2. The major point to be noted is that the Hansen Roach results average slightly in excess of 1.0, with none of the individual results less than 0.98. Thus, the subcritical limit of 0.97 used in the studies l l represents the validated critical condition plus an additional, arbitrary margin of reactivity of about 3%. This is a reasonable margin in light of the well characterized critical l experiments which were all able to be accurately reproduced. Further, since the ( important features of the landfill will not be known to near this accuracy, it would be unreasonably conservative to impose a larger, arbitrary margin on the validation. i l l mormonn.oonmms me B-l en s2 m i

l Additional conservatisms have been appropriately associated with the parameter ranges O discussed in Section IV of the report. Table B2 Validation Case Calculational Results Cme ONEDANT ONEDA!YT ONEDANT KENO KENO KENO MCh? ENDF4 H-R ENDF5 H-R ENDF4 ENDF5 TEDF5 CASEI 0.9993 0.9822 1D073 0.9835 1.0101 1.0082 0.9981 CASE 2 1.0029 ID155 1.0142 0.9995 CASE 3a 7D053 ID017 1.0124 1.0086 CASE 3b ID033 1.0056 ID103 1.0099 CASE 3c 0.9840 0.9910 0.9975 0.9967 CASE 3d ID078 1.0199 ID181 CASE 3e 1.0010 1.0172 1.0094 CASE 3f 1.0073 ID149 ID156 CASE 3g 0.9983 1.0113 1.0054 CASE 3h 1.0163 1D174 1D273 CASE 3i 1.0074 1.0143 1.0143 CASE 3j 1D200 ID162 1.0273 CASE 3k ID2N 1.0227 1.0274 CASE 31 1.0176 1.0190 1.0248 CASE 3m 1.0261 1.0318 ID331 CASE 4 1.0019 0.9922 1.0098 0.9952 1.0022 1.0056 1.0048 i CASESa ID028 1.0090 1.0076 1.0085 0.9974 1.0043 0.9987 h CASE 5b 1.0112 0.9881 1.0131 0.9901 1.0103 1.0102 1.0059 CASE 5e 1.0063 0.9845 1.0083 0.9833 1.0050 1.0070 1.0015 CASE 5d 1.0N8 1.0080 1.0092 1.0093 1.0017 1.0059 1.0025 CASE 5e 1.0076 0.9890 1.0099 0.9898 1D112 1.0086 1.0N4 CASE 5f 1.0187 1.0073 1.0218 1.0114 1.0209 1.0226 1.0161 CASE 5g 1.0079 0.9836 1.0109 0.9801 ID127 1.0150 1.0037 CASE 5h 1.0240 1.0N8 1.02M 1.0076 1.0245 ID322 1.0210 CASE 5i I.0404 1.0281 ID436 ID262 1.0388 1.N51 1.0387 CASE 6a 1.0010 1.0071 1.0057 1.0089 0.9977 1.0032 1.0131 CASE 6b 1.0079 1.ONO 1.0060 1.0087 1.0059 1.0082 0.9987 CASE 6e 0.9983 0.9982 1.0025 1.0006 0.9965 0.9975 ID016 CASE 6d 1.0000 0.9988 1.0040 0.9979 0.9991 1.0010 1.0009 CASE 7 0.9976 1.0143 0.9995 1.0152 0.9964 1.0036 1.0025 In spite of this relatively good agreement between critical experiment results and all of these several code / cross section combinations. It should be noted that for very undermoderated, high neutron leakage (i.e., small volume) systems such as the 4 kg minimum critical mass point and the lower branches of the curves in Figures 5 and 9, that no benchmark critical experiments exist. It has been noted that for extreme systems involving significant quantities of elements such as iron and zirconium, that all available cross section sets exhibit significant departure, one from the next (Ref.1,2). This scatter in calculational results is also evident for undermoderated, high leakage, soil and iron j systems, as would be expected, and is shown in Table B3. i O l "" 52 nnoracomu.ooemms ooc B2 m

/"] Case 1 Case 2 Case 3 V Reference k,r 0.9696 0.9694 0.9694 Scale 27 group +0.0069 +0.0853 +0.067 i Scal:44 group +0.0014 +0.0888 +0.0630 McNP CE endf/b-v +0.0278 +0.0220 +0.0253 g Case 1 is representative the upper branches of the etnves in Figures 5 and 9, i.e., a large volume, low neutron leakage system in which the vast majority on the neutrons are slowed down to thermal energies before absorption. Here the agreement among the Scale libraries and the Hansen-Roach library is excellent, with only the MCNP continuous i energy library showing a few percent departure. On the other extreme are cases 2 and 3, small volume systems for which there are relatively few neutrons which slow down and i are absorbed at thermal energies. This emphasizes the resonance energy regime where neutron cross sections are generally less well understood and where benchmark critical experiments, particularly with major fractions of soil or iron, are nonexistent. In spite of the several percent scatter in the calculated results from the various cross section sets, this uncertainty is relatively small compared to the uncertainties in fissile mass and enrichment, and the volume, shape, and other constituents of the uranium bearing zone. Thus, the calculational tools used in this study are judged to be more than adequately validated for the systems under consideration. O v waouaco<mumermes we B3

4 i APPENDIX C 1 f Tabular Results Supporting Chapter IV and Selected l Computer Code Input Files 4 e 4 4 4 4 i J a! i . =,

n() The values listed in Tables Cl through C7 are all based on interpolation of results of individual multiplication factor calculations. That is, in order to generate these results, the multiplication factor as a function of volume and soil water content was evaluated over a large range and for a relatively fine grid with ONEDANT and the Hansen-Roach cross sections. From these results, volumes (for Kerr = 0.97) at specific soil water contents were estimated. This process introduced only a fraction of one percent uncertainty irao the overall results and was thus negligible. Table C1. Minimum Critical"U Mass, Nominal Soil and 35% Water, Reilected Sphere Volume U 235 Mass (liters) (kg) 500 6.30 400 5.54 ) 300 4.82 200 4.18 150 4.0 140 3.99 130 4.01 120 4.05 100 4.32 80 5.2 75 5.7 50 20.00 mouncormavocwmms we C-1

' ' * ' ' ' ' ' ' ' ' ' " ' ' ' ' ' ' ' ' ' ' ' " " " " " " ' ' ' ' ' ' ' " ~ ' ' ' " ' " " " " ' * " ' ' ' O VF.,, Volume (m') 0.13 0.9 0.15 OA5 135 020 021 1.7 025 0.125 1.85 030 0.085 1.9 035 0.06 1.85 1 Table C3. Calculated (k,n = 0.97) Volumes for 12 kg U-235 in Soil and Water l ) VF. Volume (mi 0.17 0.75 020 025 1.05 025 0.135 12 j 030 0.09 125 035 0.065 12 Table C4. Calculated (k,g= 0.97) Volumes for 8 kg U-235 in Soil and Water VF,,,, Volume (m') 022 0.4 0.25 0.18 0.59 030 0.12 0.68 035 0.07 0.72 I l '2'5*' * $2 '" s noincosm41.oocvm4rrs noe C-2

l... l l Table C5. Spherical Shell Calculation Results l Radwa (cm) -+ Volume of Shell(m3) 4 0 10 20 30 35 37 0.06 0.9691 0.063 0.9705 0.115 0.9713 0.26 0.9682 0.55 0.9704 0.80 0.9708 1.05 0.9725 1.35 0.9709 1.65 0.9710 1.8 0.9709 1.85 0.9704 i O 017197 4 52 PM J fROFARCOTIN ALDOC7(N APPS DOC C.)

Table C6. Calculated (k,g = 0.97) Volumes for 16 kg U-235 in Iron and Water , F,,,,, Vakune(m') 035 0.06(29.6) 0.1209.7) 0.40 0.05 (22.1) 0.16(913) 0.45 0.027(63) 024(1193) 0.50 0.0225(3.1) 030(127.0) 033(133.9) 0.55 0375(137D) 0.60 0.42(135.1) 0.65 0.50(134.4) 0.70 0.55(124.6) 0.75 0.65(117.7) 0.80 0.73 (98.1) j 0.85 0.90(79.1) j 0.90 1.N(41.9) 0.95 O Table C7. Calculated Volumes (k,n = 0.97) for 4 kg U-235 in Iron and Water VF,,,,, Volume (m') 0.60 0.035 (34.5) 0.65 0.026(19.8) 0.06(68.9) 0.70 0.020(142) 0.0825 (87.7) 0.75 0.0125(4.0) 0.095 (83.6) 0.125G8.8) 0.80 0.14 03.6) 0.85 0.185 (63.7) 0.90 0214(333) 0.95 The following six pages provide computer input listings for the various code / cross section combinations were used in developing Figures 5 through 10. The particular model was the 4 kg minimum critical mass for 20 percent enriched uranium with nominal soil at 35 percent water content. NN N N J tPROTMCOffN ALDOCTIN APPS MC C.4

.m.._ l l ONEDANT Hansen-Roach input File 2 j l Volume = 150 VF = 0.3500 j Mass U235= 4000 / block I l-t- -r:. ngroup=16 isn=48 niso=167 j me=2 nzone=2 im=2 it=399 t l / block 2 q xmesh=0.0 32.%10132.%I0 .J xints=99 300 - zones =1,2. t / block 3 J lib-bxslib t i / block 4 matis= core j u25le4 0.0000334519 j u256c3 0.0000348716 1 u284c3 0.0000020323 l u282c3 0.0002678098 o 0.0006763313 - hDE 0.0233996608 0 0.0116998304 e: 0.0020525314 o 0.0175996518 l na 0.0001699754 i mgOR ' O.0001418624 i al 0.0015121794 .'siOR 0.0067521716 k 0.0001998908' caOR - 0.0001964285 fe 0.0002675378 cdOR 0.0000000256 gd 0.000000N83;.. ref1 hDE 0.0233996608 o 0.0116998304 c 0.0032264133 o 0.0276652279 na 0.0002671875 mgOR 0.0002229962 al 0.0023770235 siOR 0.0106138672 k 0.0003142121 l' caOR 0.0003087697 j fe 0.0004205478 j cdOR. 0.0000000402 I gd 0.0000000287 assign =matis t I' / block 5 J chi = 0.204 0.344 0.168 0.180 0.090 0.01410z icvt=1isct=1t Figure Cl, input file for ONEDANT with the Hansen-Roach cross sections 4 ' l 4 I I i l l '"'*" 52 " namacerm4umesmesme C5-l i

l t I l [T MCNP Continuous Energy ENDF/B V I b i l D studies Vol-150 VF water = 0.35 Mass U235= 4000 l l 1 0.0650062610 1 imp:n=1 2 2 0.0805158053 1 2 imp:n=1 3 0 2 impm=0 l I so ' 32.%10 2 so 132.9610 kcode - 1000 1.0 10 110 ksrc 0 0 0 mI ' 92235.50c 0.0000683235 92238.50c 0.0002698421 i 1001.50c 0.0233996608 6000.50c 0.0020525314 8016.50c 0.0299758135 11023.50c 0.0001699754 12000.50c 0.0001418624 13027.50c 0.0015121794 14000.50c 0.0067521716 19000.50c 0.0001998908 20000.50c 0.0001964285 26000.55c 0.0002675378 48000.50c 0.0000000256 64152.55c 0.0000000183 m2 1001.50c 0.0233996608 6000.50c 0.0032264133 8016.50c 0.0393650583 O 11023.50c 0.0002571875 12000.50c 0.0002229962 13027.50c 0.0023770235 14000.50c 0.0106138672 19000.50c 0.0003142121 20000.50c 0.0003087697 26000.55c 0.0004205478 48000.50c 0.0000000402 64152.55c 0.0000000287 mtl Iwtr.Olt mt2 lwtr.Olt prdmp j i 0 2 print Figure C2. MCNP input file with continuous energy cross sections based on ENDF/B-V i i f mouncoimu.oocimms ooc C-6 l l

.m .1 SCALE 4.3 ENDF/B-IV 27 Group acias! Vol=1501, VF=0.35, M(u25)=4000 g 27 roupndf4 infhommedium - 3 u-23510 0.0000683235 end u-23810 0.0002698421 and j-h -- 10 0.0233996608 and r o 10 0.0299758135 end - c 10 0.0020525314 and na 10 0.0001699754 end mg i O 0.0001418624 end ) l al 10 0.0015121794 end j si 10 0.0067521716 end k i O 0.0001998908 end ca ' 10 0.0001964285 end fe 10 0.0002675378 end cd 10 0.0000000256 end i sd.10 0.0000000183 end l h 2 0 0.0233996608 end .c 2 0 0.0032264133 end l o 2 0 0.0393650583 end l na 2 0 0.0002671875 end l mg 2 0 0.0002229962 end [ st 2 0 0.0023770235 end i si 2 0 0.0106138672 end J k 2 0 0.0003142121 end j l ca 2 0 0.0003087697 end -1 fe 2 0 0.0004205478 end cd 2 0 0.0000000402 end gd 2 0 0.0000000287 end i end comp. more data axs=7 end data end j Figure C3. SCALE 4.3/ KENO.Va input file with the ENDF/B-IV 27 group cross sections i l. t i I-1

woonacosmeocemors noe C-7 l

-. _. _ -. _. _ _ _.~.__. , ~ _. _... _ ~.. __..m., _, _ _ _ _m i. i 1 ?- l i ONEDANT with 27 group cross sections 4 ? 2 Volume - 150 liters, VF (water) = 0.35 U 235 mass = 4000 grams - i- / BLOCK 1 igeom=1 ngroup=27 isn=48 niso-2 mt=2 nzone=2 im-2 it=399 miniprt yes t j /DLOCK 2 xmesh-C.0,32.%10,132.%10 l sints=99,300 zones =1,2 : / BLOCK 3 lib =xs27 i i maxord=3 ihm=42 iht=3 ihs=l6 ititl=1 if'$w212lpt=1 t / BLOCK 4 I matis-isos assign =rnatts t 8 / BLOCK 5 chi =.020.195.219.126.164.172.088.014.001 18z; i levt=1 iset=3 t - 1 Figure C4. Input file for ONEDANT with the ENDF/B IV,27 group cross section. 1 l p.U ] l i 1 j snosucomunocmors toc C-8

~ _. - = - ~.. F f SCALE 4.3 ENDF/B V 44 Group i l-(/ =csasi u3os u 23510 0.0000683235 end u 23810 0.0002698421 end i -h I O 0.0233996608 end o 10 0.0299758135 end 'l ~ l 0 0.0020525314 end c na 10 0.0001699754 end l b i mg 10 0.0001418624 end al 10 0.0015121794 end si 10 0.0067521716 end k 10 0.0001998908 end ca 10 0.0001964285 end fc 10 0.0002675378 end cd 10 0.0000000256 end gd 15210 0.00 W000003 end gd-15410 0.00000000035 end 4 gd-15510 0.00000000265 end gd-15610 0.00000000372 end gd 15710 0.00000000285 end gd 15810 0.00000000460 end gd 16010 0.00000000410 end h 2 0 0.0233996608 end c 2 0 0.0032264133 end o 2 0 0.0393650583 end na 2 0 0.0002671875 end mg 2 0 0.0002229%2 end p al 2 0 0,0023770235 end ] 1 si 2 0 0.0106138672 end k 2 0 0.0003142121 end ca ' 2 0 0.0003037697 end fe. 2 0 0.0004205478 end cd 2 0 0.0000000402 end gd-152 2 0 0.0000000005 end gd 154 2 0 0.000000006I end gd 155 2 0 0.0000000419 end gd 156 2 0 0.0000000583 end gd 157 2 0 0.0000000448 end gd 158 2 0 0.0000000716 end gd-160 2 0 0.0000000638 end end comp more data axs=7 end data end Figure C5. Input file for SCALE 4.3/ KENO-V.a with the ENDF/B-V 44 group cross sections 1 I i O naonacomeooemors mc C-9 j

ONEDANT with 44 group cross sections b 2 I V Volume = 150 liters, VF (water) = 0.35 U 235 mass = 4000 grams / BLOCK I igeom=1 ngroup=44 isn=48 niso=2 mt=2 naone=2 im=2 it=399 miniprt yes t / BLOCK 2 xmesh=0.0,32.%!0,132.%10 aints=99,300 zones-l.2 ~ / BLOCK 3 lib =xs44 maxord=3 ihm=68 iht=3 ihs=25 ititl=1 ifido=212lpl=1 t / BLOCK 4 matts-ises - assign =matis t / BLOCK $ chi =.00509.01402.04390.15256.08520.02411 .11475.12658,16461.17280.08288.01178.00076 .00091.00007 29z icvt=1 iset=3 t Figure C6. Input file for ONEDANT with the ENDF/B.V 44 group cross sections O V O s notacarma.coewmms noc C.10

f t i i l s' ) i f a 1 1 i .t APPENDIX D 4 1 I Trench 6 Water Balance Calculation i 1 i l I i f i,

1 I l APPENDIX D ) l Trench 6 Water Balance Calculation 1 ' \\ The calculations, summarized here, were performed by first defining a cell encompassing the j section of Trench 6 represented by TWSP 06-1. To simplify the calculations, lateral boundary faces were selected that were either parallel or perpendicular to the direction of groundwater flow. The upper and lower boundary faces were the top of the water table and the base of the soil zone, respectively. The lateral upgradient inflow (21.0 m'/yr) was calculated by multiplying the cross-sectional area l of the cell face by the hydraulic gradient and the hydraulic conductivity. The lateral downgradient outflow from the selected volume (2'4.1 m'/yr) was determined using the same formula and specific values of area and gradient for the downgradient cell face. By definition, I there is no flow in or out of the selected volume through the two lateral faces parallei to groundwater flow. The flow into the top of the volume due to infiltration (23.4 m'/yr) was i calculated by multiplying the site-specific infiltration rate by the area of the top face. The calculations are shown in Table 7. The outline of the cell is shown on Figure 13. l The cross-sectional area of the lateral faces were derived from the saturated thickness, indicated by wells in the vicinity of Trench 6, and the width of the cell. The hydraulic gradients were obtained from the groundwater contour map. The upgradient face was divided into five subfaces to account for the minor variations in thickness and hydraulic gradient. The downgradient face was more uniform and was not divided. The geometric mean of measured hydraulic conductivity values for the soil was used for the lateral flow calculations. 4 The lateral outflow was subtracted from the total inflow (lateral upgradient and top) to calculate the outflow through the bottom of 21.4 m'/yr. This is appro'ximately 8 percent lower than the j site-wide average. Therefore, there is no indication of any preferential pathway which could cause convergent flow in this area of Trench 6. l i 1 I l l O moraconunocners voc p.g l l

5 t 4 UPGRADIENT [ EAGE l O I SATURATED WIDTH GRADIENT (1) VOLUMETRIC FLOW (MNR) VOLUMETRIC KiA m3/yr THICKNESS (H) (W) F!.OW = l 3.28 1.1 0.0820 2.3 K = 7.41 m/yr I l 3.12 1.4 0.0740 2.4 A = H'W m2 l 2.97 1.5 0.0660 2.2 j 2.82 1.9 0.0570 2.3 2.67 8.5 0.0710 11.9 1 TOTAL 21.0 m3/yr UPGRADIENT LATERAL INFLOW = DOWNGRADIENT FACE: SATURATED WIDTH GRADIENT (1) VOLUMETRIC FLOW (MNR) THICKNESS (H) (W) 2.44 12.9 0.099 23.1 i TOTAL 23.1 m3/yr DOWNGRADIENT l LATERAL OUTFLOW = SURFACE INFILTRATION: INFILTRA-AREA m2 VOLUMETRIC FLOW (MNR) TlON RATE m/yr ~ ~ 0.12 195.1 23.412 TOTAL 23.4 m3/yr INFILTRATION l INFLOW = WATER BALANCE CALCULATIONS: TOTAL INFLOW = 44.4 m3/yr (Lateralinflow + infiltration) l FLOW OUT OF 21.4 m3/yr (Totalinflow -lateral outflow) BOTTOM = l l i i O I fnotARCOEINALDoct!% APPS DOC D-2 I . _.}}