ML20207T006
| ML20207T006 | |
| Person / Time | |
|---|---|
| Issue date: | 03/04/1987 |
| From: | Matt Young NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Martin D NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| REF-WM-39 NUDOCS 8703230048 | |
| Download: ML20207T006 (6) | |
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54R 4 3 WM-39/MHY/87/02/19- '
MEMORAND'UM FOR:
Dan E. Martin, Section Leader Uranium Recovery Projects. Section Low-Level Waste and Uranium Recovery Projects Branch FROM:
Michael Young, Hydrogeologist-Uranium Recovery Projects Section.
Low-Level Waste and Uranium Recovery
- Projects Branch
SUBJECT:
RECENT TECHNICAL MEETING ON THE TAC INFILTRATION STUDY HELD IN ALBUQUERQUE, NEW MEXICO I recently attended a technical meeting concerning the Technical Assistance Contractor's (TAC) special infiltration study on February 16 & 17, in Albuquerque, New Mexico. Attendees included representatives of DOE, NRC-(HQ & URFO), State of Colorado, the Hopi and Navajo tribes, and the peer review group engaged by the TAC. The meeting was organized by the TAC to present the. latest developments i
of the unsaturated flow model, SOILM0IST, to the peer group and regulators so that constructive criticisms and suggestions on the direction of the project could be communicated.
I attended this meeting to listen to the comments of the peer review panel and to present NRC staff's perspective on-the development and use of S0ILM0IST for designing uranium mill tailings disposal cells.
To briefly summarize the results of the meeting, the peer review panel comended b
the effort of the TAC since last Decembers meeting, but feels that without significant model testing and sensitivity analyses, it will not be possible to determine how effectively SOILM0IST simulates infiltration through tailings cells.
The peer review panel concluded that S0ILM0IST should incorporate additional processes which influence infiltration and should be tested (i.e. validated) prior to implementing disposal designs resulting from the code.
The enclosed trip report provides a more in-depth review of this meeting.
I will be drafting a letter to DOE, for your signature, outlining our concerns regarding this project.
I will be happy to discuss any questions you may have regarding this memo.
31staA&SNEEDW l
l Michael Young, Hydrogeologist Uranium Recovery Projects Section Low-Level Waste and Uranium Recovery i
Projects Branch
- NO f
Enclosure:
WM Record File Dd NU' As stated PDR 2 I
cc: James Anderson, DOE /AL LPDR _
l Andrew Soloby, Jacobs/AL
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q) e MAR 4 W TAC INFIL TRIP REPORT /MHY- '
SUBJECT:
Technical Meeting on Special Infiltration Study DATE:
February 16 and 17, 1987 LOCATION: VMTRA Project Headquarters, Albuquerque, NM PARTICIPANTS:
NRC TAC DOE PEER REVIEW PANEL M. Young (HQ)
J. Brinkman R. Sena GlendonGee(PNL)
E. Hawkins (URF0)
B. Keshian D. Leske Daniel Hillel (U. of Mass.)
.T.Olsen(URF0)
J. Snyder Daniel Stevens (NMIMT)
S. Grace (URFO)
G. Rice B. Deutsch J. Caldwell M. Nelson P. Reckemeyer NAVAJO NATION HOPI TRIBE BIA STATE OF COLORADO M. Zaman L. Shingotewa J. Analla W. Wright T. Begay Jr.
F. Marinelli PURPOSE:
- 1) Attend the technical review meeting where the unsaturated flow model, SOILMOIST, was presented; 2) present NRC staff perspective on the use of the program for designing uranium mill tailings impoundments; 3) provide technical suggestions to the TAC for improving the simulation of infiltration using S0ILM0IST.
BRIEF OVERVIEW 0F S0ILMOIST The unsaturated flow model, 50ILM0IST, is designed to provide the long-term moisture profile for a radon barrier-tailings-foundation soil sequence.
The program solves the Philips Equation for 1-Dimensional infiltration through layered porous media. Also incorporated into S0ILM0IST is a stochastic l
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,D T TAC INFIL TRIP REPORT /MHY climatological generator, which determines future preci itation amounts and
. temperatures based on the statistical characteristics (pi.e. mean and standard deviation)ofpastrecordedevents. The simulated weather patterns are inputted directly into SOILMOIST, which uses the information for determining drainage and evaporation from the filter layer, and infiltration through the radon barrier into the tailings-material. The results including'the predicted volume of leachate and the soil moisture profile, will be used to optimize a design whereby infiltration and radon exhalation are minimized.
A. February 16, 1987 The morning session, chaired by Jim Brinkman (TAC), was devoted to describing the various components and associated a~ssumptions of SOILMOIST. The graphics capabilities were also discussed briefly.
Several components, described in the context of advancements since the recent December meeting, are outlined below:
Surface Boundary Conditions -
Previous versions of SOILM0IST utilized boundary conditions specified by the user. The moisture content for the lower boundary (i.e. the bottom of the tailings) was specified as the design moisture content during construction, and the upper boundary condition (capillary potential) was optimized to infer climatological-conditions. The program now bases the surface capillary potential on the amount of precipitation added to the pile to be infiltrated, or the loss of water from. the pile due to evaporation. The peer review panel concluded that several processes should be included when calculating boundary conditions.
For instance, S0ILMOIST assumed that all water-in the filter layer froze instantly i
during freezing conditions. The peer review panel remarked that'this characteristic of the program can be improved by using a sub-program that calculates the depth of freezing within the filter layer, resulting in more accurate accounting of water within the simulated physical system.
Evaporation -
Evaporation is now calculated using vapor transport equations, whereas previously it was calculated from direct water loss in the filter layer.
The water lost to evaporation will mostly be from the filter layer. The meeting participants agreed that water loss from the radon barrier may be minimal when the filter layer and riprap are thick enough to limit direct exposure to the atmosphere and when the pile is clear of vegetation.
Transpiration is considered negligible because the tailings pile will not-be vegetated, at least under current design practices. This aspect of the program may again have to be modified, because of the likelihood that the cover may become partially vegetated during the design life.
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Climate Simulation -
~ Previous versions of S0ILMOIST inferred climatic events by optimizing soil suction values at the radon barrier-filter layer interface. Recognizing a deficiency in this methodology, 50ILMOIST was modified to stochastically generate a climatological sequence based on the statistical characteristics (mean and standard deviation) of historical precipitation and temperature measurements, which at some UMTRA sites have been recorded for long time periods. The Tuba City site, for example, has almost 70 years of measured rainfall and temperature. The probability of precipitation for a given calendar day, based on historical records, determines future climatic events. Snow is accounted for by using the
. equivalent water content.
4 The peer review panel generally agreed that this was an appropriate direction for determining potential climatic sequences. However, no attempt has been made to ensure that recorded precipitation and temperatures are normally distributed.
Distributions other than normal will invalidate the current method of generating climatic sequences. The TAC will check the distribution of the recorded rainfall events and temperature trends and modify the generating function, if necessary.
1-Dimensional or 2-Dimensional Simulation -
S0ILMOIST utilizes 2-Dimensional (2-D) flow to simulate drainage of water in the filter layer, and 1-Dimensional (1-D) flow for infiltration in the radon barrier, tailings material and foundation soil. Simulating infiltration only in the vertical direction generated more questions and comments than other characteristics of the model, because 1-D simulation of infiltration in the radon barrier and tailings has not been adequately shcwn to represent.known physical processes.
For example, the flank or toe of the tailings pile is considered, by the peer review panel, to be an area where lateral flow is most likely to occur.
Lateral flow of water from the toe of the pile towards the center could result from an increased contact time of water and radon barrier material caused by surface runoff of precipitation. The current 1-D scheme may not adeouately represent these potentially complex flow conditions. The significance of lateral flow can only be ascertained using a 2-D simulation model.
The peer review panel was unified in its conclusion that S0ILM0IST should be tested against an acceptable 2-D model to assess the significance of this lateral flow. Dr. Hillel mentioned that it may be possible for the TAC to model discrete columns iri one dimension across the tailings pile to produce a quasi-2-D simulation model, but this approach was considered 4
secondary to utilizing a fully 2-D model.
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Model Validation -
To date, SOILMOIST has been tested against the analytical Philips Equation for infiltration. -The peer review panel accepted these results.- However, other validation exercises, such as testing SOILMOIST against a 2-D model, l
or using data collected under field conditions frce other research projects have not been performed. Further, SOILM0IST has not yet been executed with.the full climatological dataset or a hypothetical radon.
barrier-tailings-foundation soil profile. Thus, there is no way to evaluate how well the program behaves even with designs similar to past tailings impoundments designs.
The peer review panel considered validation efforts critical to accepting the program and using it for i
design purposes.
Radon Exhalation -
An original objective of S0ILMOIST was to estimate the long-term moisture profile for use in the program RAECOM, which calculates radon exhalation 4
rates. During the study, the objective has shifted towards calculating water flux through the tailings, thus avoiding the radon issue. This is fortunate since the TAC received numerous comments during this meeting, as well as comments through written correspondence from NRC.. The main point
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of contention was the lack of conservatism incorporated into the program code when used for radon; SOILMOIST was designed to be conservative for water flux, causing it to be non-conservative for calculation of radon exhalation rates. The TAC may counteract this non-conservatism by decreasing rainfall 75% of the normal annual amount during the simulation period.
It is unclear whether this will be conservative enough.
In addition to these major points, several hours of discussion were devoted to the fine points of the model and input data, and how potential deficiencies could be minimized.
For instance, the TAC is currently using a method to derive the hydraulic conductivity from soil suction tests. However, the peer review panel stated that the method (developed by Millington and Quirk, 1961) is not very accurate for fine-grained materials. This is important because the model is very sensitive to changes of hydraulic conductivity; small changes of hydraulic conductivity may produce large differences in results. Another i
comment raised by the peer review panel, involved the laboratory measurement of moisture content and soil suction. Much of the data collected during previous laboratory tests were in the dry range of the soil ~, where soil suction approached 10 to 15 bars of pressure. However, since the possibility of drying the radon barrier to 15 bars of pressure, with the current design specifications is unlikely, the panel suggested that most of the data be collected in the zero to one bar range. This would provide better data for 4
conditions likely to be experienced in the field.
B. February 17, 1987 Attendees at the second day of this meeting included representatives from the Hopi and Navajo Tribes, and from NRC/URF0. Jim Brinkman presented the j
developments of S0ILM0IST again, for their benefit.
Several questions raised 0
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- 1 during this meeting were similar to those raised on the previous day. After the first break, the peer review panel presented their conclusions and suggestions for improving S0ILMOIST._ The panel was very candid with their conclusions, which paralleled the concerns discussed on the previous day.
Summarizing an important position of the peer review panel, Dr. Hillel remarked that it will take several months, with the use of outside experts in-house, to fully develop the model prior to using the code for design purposes. Dr.
i Hillel did not say that an off-the-shelf model _would be more accurate, but that SOILMOIST should be more fully developed. Considerable discussion regarding the conservatism of the model ensued. Many comments were similar to those that i
NRC. staff have documented in the past; namely, is SOILMOIST conservative enough so that the designs will withstand worst-case conditions? The panel replied on several occasions that this question could not be answered without additional checking and testing of SOILMOIST. The simplistic nature of SOILMOIST may be adequate-to simulate infiltration into and percolation through tailings piles, but potential errors cannot be evaluated without some level of validation.
During the afternoon session, Jack Caldwell (TAC) and Rich Sena (DOE) presented their interpretation of the essential points of agreement made by the peer review panel. Several of the points appeared to directly contradict the conclusions articulated by the panel, resulting in spirited conversations by the regulators, Indian tribes and DOE. The confusion was cleared up somewhat after additional explanation by Mr. Sena.
It appeared, however, that DOE wants SOILMOIST to be developed while being used for the Tuba City and Gunnison test cases, and that validation and verification of the code may be limited due to monetary and time constraints. Although not explicitly stated, the panel felt that implementing designs resulting from SOILMOIST prior to validating the code would be improper. At the end of the meeting, the peer review panel assured me i
l, that I would receive a copy of their written reconnendations for improving S0ILMOIST, which will then be placed in the PDR.
l The overall purpose of the trip was fulfilled. Due to the importance of this project, maintaining a presence during these types of technical meetings and through the development of S0ILMOIST enhances communication between the agencies and reduces potential future conflicts. The TAC and DOE were made aware that NRC requires a conservative approach when calculating long-term percolation and subsequent water resources impacts, as well as conservative t
calculations of radon exhalation rates. As the NRC technical representative for this study, I will remain abreast of future developments of 50!LM0IST and inform interested parties of these developments.
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