IEEE Transactions on Nuclear Science, Vol. NS-29, No. 1, February 1982

LONG-TERM MONITORING OF CLOSED LOW-LEVELWASTE AND URANIUM MILL TAILINGS

DISPOSAL SITES

by

A.A. SutherlandV.C. RogersP.J. Macbeth

Rogers and Associates Engineering CorporationP.O. Box 330

Salt Lake City, Utah 84110

ABSTRACT

The goals of radioactive waste disposal arereviewed and their impact on long term monitoring ofclosed low-ievel waste and uranium mill tailings dis-posal sites are described. It is shown that existinginstruments are adequate for long-term monitoring andthat advances in instrumentation can be accommodatedas they become available. The important role of long-term monitoring in improving predictive models ofsystem performance is described.

INTRODUCTION

While high-level radioactive waste has receivedthe major share of attention in the past, recent eventsand government actions have focussed public awarenesson low-level waste and uranium mill tailings. Low-level waste is defined as all radioactive waste otherthan spent reactor fuel, primary wastes from reprocess-ing spent fuel, material with more than 10 nanocuriesof transuranic isotopes per gram and uranium mill tail-ings. This broad definition means that low-level wastecan take a diverse range of forms--including contamin-ated clothing, wastes from radiopharmaceutical applica-tions, contaminated equipment and materials fromdecommissioning of nuclear facilities--and can containa wide range of radionuclides. Uranium mill tailingson the other hand, tend to be relatively uniform incharacter and isotopic content. They consist chieflyof sand or slime residues from the milling process andhave concentrations of thorium and radium ranging fromhundreds to several thousands of picocuries per gram.

The National Low-Level Waste Policy Act (PL 96-573)has given primary responsibility to the states and com-pacts of states to develop low-level waste disposalsites. After January 1, 1986 states or compacts withlow-level waste sites can refuse to accept out-of-region wastes. This will give impetus to the develop-ment of new low-level waste disposal facilities. At thesame time, the Uranium Mill Tailings Radiation ControlAct (PL 95-604) has outlined steps leading to remedialaction and final disposal for numerous uranium milltailings piles in the U.S. during the 1980's. When thelow-level waste and mill tailings disposal sites ceaseoperations they will require monitoring for many yearsfollowing closure. Considerations that influence the-methods and instruments used in long-term monitoring ofsuch facilities are the subject of this paper.

Long-term monitoring of closed low-level waste(LLW) and uranium mill tailings (UMT) disposal facil-ities must be carried out with the generally acceptedgoals and criteria for disposal of these wastes inmind. Four major goals for adequate waste disposalare:

* Disposal will protect the public healthand safety for all times.

* Disposal will not require activeparticipation by future generations.

* Disposal will not deprive futuregenerations of needed resources.

* The cost of disposal will be justifiedby the benefits derived.

The generally accepted method for disposing of LLWand UMT in the forseeable future is burial in relativelyshallow facilities which lie above the highest aquifer.Figure 1 illustrates this form of shallow land burialand the pathways by which the radionuclides in the wastemay reach man. Plants and animals may enter the waste,bringing some of it to the surface and providing channel!for water to enter the waste. The nuclides can diffuseupward in moisture in the ground, radon gas can migrateupward and cause large concentrations in buildings con-structed over the disposal site at a later date, andgamma radiation can expose people working or living overthe site. Finally, the waste can move down into anaquifer and be introduced into the food chain throughwells and surface water. The four goals stated abovemust be met in the context of the shallow land disposalmethod shown in Figure 1.

_- - - u

- AQUIFER

FIGURE 1 SHALLOW LAND DISPOSAL AND PATHWAYS OFRADIONUCLIDES TO MAN

The purpose of monitoring is to assure in some waythat the first goal listed above is achieved. However,the second goal is in conflict with the use of activemonitoring to achieve the first goal, since the wastescan be toxic for thousands of years. The NRC's proposedregulations for low-level radioactive waste disposal1provide a convenient reference point for deciding howlong "long-term" may be. In applying for a license fora low-level waste disposal facility, the applicant can-not take credit for plans to monitor or control the sitefor more than 100 years after burial operations cease.While this is not necessarily binding on mill tailingsfacilities, 100 years is rapidly gaining acceptance as areasonable maximum period over which post-closure controland monitoring may be expected to take place. Therefore,100 years will be assumed to be the duration of long-termmonitoring and control in this paper.

Figure 2 puts the 100-year monitoring period inperspective. Uranium mill tailings don't begin to losetheir radiotoxicity until about 100,000 years. Mostlow-level waste begins to show a significant reductionin toxicity around 1000 years. But the "half-life" ofthe state of the art in instrumentation may be about adecade, and sensors that can be buried and operate for adecade without service may be rare.

It can be seen that the long-term monitoring problemis complicated at both the near-term and far-term endsof the spectrum. At the near-term end it is highly un-likely that instruments can be placed around the disposalsite and expected to last for the 100-year period. How-

0018-9499/82/0200-0242$00.75w 1982 IEEE242

ever, even if such durability could be achieved, ad-vances in the state-of-the-art would render the in-struments obsolete well before the end of long-termmonitoring. Both of these observations can impact ourability to measure underground movement of radionuclides.In the long term, no reasonable active monitoring pro-gram can be expected to last the entire time the wastesare toxic without placing an undue burden on manyfuture generations. The usual approach to the latterproblem is to project the behavior of the disposal sys-tem through computer modeling.

INSTRUMENT END OF LLW UMTTECHNICAL MONITORING REMAINS REMAINS

OBSOLESENCE AND CONTROL TOXIC TOXIC

TIME SINCE CLOSURE (YR)

FIGURE 2 TIMES OF INTEREST IN DISPOSAL OF LOW-LEVEL WASTE AND URANIUM MILL TAILINGS

Cost considerations also play a strong role in thedevelopment of long-term monitoring systems. Wastedisposal sites generate no new revenue after they areclosed. Any money used to monitor the sites must comefrom current revenue (taxes) or from an escrow accountestablished at closure. Also, the sites will usuallybe in remote areas where monitoring costs will be highbecause of the travel involved. These two economicdisincentives dictate that long-term monitoring besimple and relatively infrequent. Fortunately anadequate monitoring program can meet these requirements.

SURFACE MONITORING

Monitoring at the surface of the waste disposalsite will take two distinct forms: observations andmeasurements. Observations will be made periodically,perhaps once a year. Reference 2 suggests that thisfrequency will be sufficient for mill tailings sites.Observations will seek to identify subsidence orerosion of the cover placed over the waste, animal in-trusion and intrusion by vegetation on the cover. Theywill also verify the integrity of fences and identifi-cation markers. If the cover has subsided to any degree,the waste volume may become host to large volumes ofwater that can quicken the dissolution process in thewaste and increase the speed with which the radio-nuclides enter the groundwater system. If rapid erosionis observed, the assumptions of the facility safetyanalysis may be invalid and there is a danger that thewaste will be released to surface waters while it isstill highly toxic. Burrowing animal intrusion or thegrowth of deep-rooted plants may provide paths forrapid water seepage that can speed movement of the wastedownward into the aquifer or upward to the surface. Inall the cases described above remedial action may becalled for.

The surface measurements to be taken during long-term monitoring seek to identify anomalous behavior ofthe waste disposal facility that can portend undesirablerisks to the public after monitoring and control overthe site cease. In other words, they seek to verifythat, over the monitoring period, the site behaves asexpected. Several quantities should be measured, in-cluding surface elevations, possible contamination ofsurface water and soil at the site, gamma radiation and,if radium bearing wastes are present, radon seepage.

The latter two quantities are concerns for people whomay some day live or work over the site. The measure-ments can be made or samples collected at the same timesobservations of subsidence, etc. are made.

Surface elevation changes can be detected usingstandard land survey instruments. Surface water andsoil samples can currently be processed in laboratoriesusing a variety of methods such as atomic absorption,x-ray flouresence, neutron activation, alpha, beta andgamma counting, etc.

At the present time in situ gamma radiation ismeasured by carrying a gamma scintillation device inproximity to the suspected source. Instruments ofsufficient accuracy are marketed by several manufacturersfor under $1000. Gamma monitoring could be performed bywalking over the site during the time observations arebeing made. An alternative is to leave ThermoluminescentDetectors (TLD's) in place between visits to the site.The TLD's could be located on a predetermined grid andcould be changed with each visit. They will give anaverage measurement for the period they are in place.The TLD chips themselves cost only a dollar or two anddevices to process them in the laboratory cost around$5000.

Current methods for measuring radon fluxes fromthe ground are simple and take only minutes or, at most,several days to accomplish. The primary methodssuggested are charcoal canisters and flux cans. Theseare described in detail in Reference 3. The fluxmeasurements could be taken during the infrequent monitor-ing visits to the site. Equipment for processing themeasurements3 is portable. The equipment needed to takethe measurements is simple and inexpensive.

There is no need to separately measure airborneradon levels at the sites, since almost any anomalousbehavior would be detected first by the flux measure-ments and the main concern is for radon seeping intobuildings placed on the site after monitoring and in-stitutional control cease.

Monitoring for airborne particulates should not benecessary since conditions that would allow significantquantities of particulates to be blown from the sitewill easily be identified by personal observation. Atthe time observations and gamma and radon flux measure-ments are made, soil and surface water (if any) samplescan be taken and analyzed in a laboratory.

It can be seen that instrument lifetime is not amajor concern in surface monitoring since instrumentscan be replaced easily. Also, full advantage can betaken of advances in the state of the art over theduration of the long-term monitoring period.

SUBSURFACE MONITORING

Monitoring subsurface migration of radionuclidesfrom the waste is by far the more difficult part oflong-term monitoring. The purpose of subsurface measure-ments is to identify unexpectedly fast movement throughthe region around the burial site. Of particular in-terest is migration to and through any underlying aquiferwhich can lead to the public via wells or discharge tosurface water.

Ideally, appropriate sensors that measure radiationcould be placed below the surface and unusually high con-centrations of nuclides could be detected. However, forsome of the heavier nuclides (uranium, thorium and rad-ium for both LLW and mill tailings plus neptunium,plutonium and americium for LLW) the measurement problemis complicated because their movement will be highly re-tarded at any properly chosen disposal site. Even radium,

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the least retarded of those mentioned above, can be ex-pected to move no more than one meter during the100-year monitoring period. Figure 3 illustrates theproblem for several key nuclides.

266Ra,137Cs 14C.90Sr 3HP9Tc,129

10 102 1O3

MIGRATION DISTANCE IN 100 YR (M)(GROUNDWATER VELOCITY * 10 M/YR)

FIGURE 3 DIFFICULTIES IN MEASURING SUBSURFACEMIGRATION OF HEAVY ISOTOPES

In order to detect this movement sensors wouldhave to be placed very close to the region of soil near

the waste that is disturbed when the site is constructed.This region is not representative of media throughwhich the waste must ultimately move to reach the publicand any measurement of migration that took place prim-arily in the disturbed region would be of little valuein forecasting the long-term performance of the site.

At the other end of the spectrum, radionuclidespresent in low-level waste such as 3H, 99Tc and 1291,are not retarded, so they move with the water undersaturated conditions and diffuse relatively quickly inunsaturated media. The fact that all three nuclidesare beta emitters makes them somewhat harder to detectthan other isotopes, which may be gamma-emitting.Also, much of the original inventory of 3H will decayin 100 years. All of these nuclides can be sought bythe monitoring system around a low-level waste site buta reasonable measure of their speed of movement remainsdifficult. Significant increases in the concentrationof one or more of them could be detected at a particularpoint near the disposal site. But it would be difficultto know when those nuclides were released, given thediverse nature of low-level waste and conscious attemptsto make it less soluble in water. If they are releasedearly enough in the long-term monitoring period it maybe possible to judge how fast they are moving byobserving the tinies they first arrive at two monitoringlocations some distance apart.

In summary, the heavier nuclides move too slowlyto detect with any reasonable probability in 100 years.For low-level waste sites, lighter nuclides could besought, but their strong presence could only be con-

strued to show the disposal system is not working asplanned. Their absence would not necessarily prove thatit was working properly. Consequently, a major functionof long-term subsurface monitoring is simply to act as

a trip-wire to detect substantial failure of the system.In spite of the above, it is desirable to monitor sub-surface conditions at both LLW and UMT disposal sites.

Appropriate instrumentation for subsurface monitor-ing must detect beta (and possible gamma) emissions,last for 100 years and be able to take advantage ofadvances in the state of the art. Since the primaryconcern of subsurface monitoring is migration throughan underlying aquifer, a satisfactory instrument doesexist. That "instrument" is a well drilled into theaquifer.

By drilling wells at places that are beyond thevolume of soil disturbed when the site is excavated but

easily within 100 years' water travel time, all of theabove requirements for appropriate instrumentation canbe satisfied. It is possible to maintain wells for atleast 100 years and the water withdrawn periodicallycan be analyzed on the surface for radionuclides,chemicals characteristic of the waste, pH, Eh, etc.,using state of the art techniques. The frequency ofmeasurements can be the same as that of observationsand surface measurements.

At the same time water samples from wells are

being analyzed for radionuclides, measurements ofaquifer height can be made to check assumptions usedin the site performance analysis.

In addition to water samples, infrequent soilsamples can be taken from the bottom of wells slopedto reach under the waste. This will permit measurementsof isotope concentrations above the aquifer when it isunlikely that the radionuclides will reach the aquiferduring the monitoring period. This is particularlyuseful in arid climates where downward movement ofgroundwater is slow and the aquifer may be far belowthe waste.

EXPERI ENCE

It is instructive to review how monitoring hasbeen conducted on currently closed low-level wastesites and what nuclides may have been measured in aboveambient concentrations.

The monitoring program for the West Valley, NewYork low-level waste site is described in detail inReference 4. Figure 4 shows a crossection of thefacility. The waste is buried in the till. Any waterflowing from the surface will move through the graveland sand layers to Buttermilk Creek. The clay acts asa lower boundary on the underground water migrationsystem. Note the geologic complexity and proximity tosurface water of the West Valley site compared to thegeneric site shown in Figure 1. Careful site selectioncan help to make future disposal facilities more likethe generic site.

-480

-450

-420

-390

-360

-330

-300

-2T0

0 120 240 360

METERS

FIGURE 4 CROSS SECTION AT WEST VALLEY BURIALGROUND

The monitoring program at West Valley includesmeasurements on surface water, the soil cover over the

waste, soil below the waste and water gathered from

within the waste trenches themselves. The surface water

measurements have been made using periodic samples from

the three small streams that drain the site, as well as

two larger off-site streams. Samples were analyzed for

total alpha, total beta and 3H activity. Some sampleswere also analyzed for specific nuclides. To be re-

presentative, the samples were taken at various stagesof stream flow. Tritium, as tritiated water, was the

most abundant radionuclide detected, with highest con-

centrations coming during periods of low rainfall.

Strontium -90 was the highest beta-emitter detected in

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23OTh238U60co

a mI

surface water samples.

Samples were removed from the waste cover usinghand-driven Shelby tubes and machine-driven core samplerswere used to reach below the trenches. In the wastecover 3H concentrations above background have beenobserved 20 cm below the land surface and deeper. Belowthe trenches elevated concentrations of tritium extenddownward 2 to 3 meters from the waste. The lattermovement has taken place over periods of 7 to 10 years.Since tritium is the most mobile of the radionuclidesburied as low-level waste, the problem of detection andmeasurement in a reasonable period of time is illus-trated by this experience.

Where detectable amounts of 14C were found belowthe waste the concentrations dropped much more sharplywith depth than those of 3H. This reflects some retard-ation of 14C in the soil and demonstrates the fact thatmigration is driven by concentration gradients(diffusion) as well as bulk movement of water. Stron-tium -90 was found only within the first meter belowthe bottom of two trenches.

Table I illustrates the magnitudes of concentra-tions of certain radionuclides detected in surfacewaters at West Valley. The sampling station was in adrainage feature within 100 meters of the nearest trench.The values shown were observed within decade of closure.For comparison, the inventory of each nuclide known tobe buried at West Valley is shown.

TABLE I

Since the chief purpose of the waste disposalsystem is to delay the release of radionuclides andslow their movement to the environment it is notacceptable to release trace amounts of nuclides orconduct migration experiments at the disposal site.However, it may be desirable to set up separate ex-periments some distance from the waste disposal sitebut in a location with similar hydrogeology. At theselocations trace amounts of key nuclides in knownchemical forms could be released in the saturated andunsaturated zones and their movements determined moreprecisely. The information gained from these modelverification experiments could go a long way towardcompensating for the practical constraints imposed onthe active monitoring period.

FIGURE 5 DUAL FUNCTIONS OF LONG-TERM MONITORING

SURFACE WATER CONCENTRATIONSNEAR WEST VALLEY

Range ofMeasured Concentrations*

Nuclide (pCi/k)3H 1,800 - 88,000

14C 1 - 100

60Co < 2 - < 60

90Sr

1291

137CS

ReportedBuried Activity**

(pci)1017

4xlO 14

1017600 - 1,000

< 1.6 - < 5

< 3 - < 50 2xlO13Source: Ref 4

* Source: Ref 5

MODEL VERIFICATION

The monitoring program described above covers onlya short fraction of the time for which the waste istoxic. While practical limitations preclude monitoringfor much more than 100 years, information about the per-formance of the site beyond the end of monitoring can begathered. In the broadest sense long-term monitoringcan be accomplished by performing the two functionsshown in Figure 5, During the 100 or so years in whichmeasurements are taken efforts can be made to verify theparameters used to model performance beyond the end ofthe measurement period. Some measurements already des-cribed, such as the movement of tritium at the site,will be of value. But they may not be available if mostof the waste is in a form that does not readily releasethe tritium. Also, other toxic isotopes that move moreslowly may not be observed during the period of activemeasurement because they are released from the wastelate.

SUMMARY

The period over which measurements of disposalsite performance can be taken is constrained to be asmall fraction of the period of toxicity of low-levelwaste and uranium mill tailings. Adequate instrumen-tation exists today to perform the necessary measure-ments. For above surface measurements advances in thestate of the art can be accommodated and instrumentfailures can be easily overcome. The same comments aretrue of subsurface measurements if the instrumentsthemselves are kept at the surface and soil and watersamples are withdrawn for processing.

It is possible to monitor the underground behaviorof only the fastest moving radionuclides (found only inlow-level waste) by taking measurements near the wastedisposal areas. Uncertainties introduced by the diversenature of the waste complicate this process. Creationof controlled experiments nearby can help correct thisproblem. For many nuclides the monitoring system is atrip-wire to detect anomalous behavior. But failure toobserve an unusual presence of radionuclides is notperfect proof the disposal system is working as designed.

REFERENCES

1. "Final Draft Licensing Requirements for Land Dis-posal of Radioactive Waste,"' 10CFR61, U.S. NuclearRegulatory Commission, June 1981.

2. "Final Generic Environmental Impact Statement onUranium Milling," U.S. Nuclear Regulatory Commission,NUREG-0706, September 1980.

3. Nielson, K.K., et al, "Laboratory Measurements ofRadon Diffusion Through Multilayered Cover Systemsfor Uranium Tailings,"' Rogers and AssociatesEngineering Corp., RAE-9-2, September 1981 (to bepublished as a Battelle Pacific Northwest Laborator-ies document).

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4. Dana, R.H., et al, "General Investigation of Radio-nuclide Retention in Migration Pathways at the WestValley, New York Low-Level Burial Site," U.S. NuclearRegulatory Commission, NUREG/CR-1565, October 1980.

5. "Western New York Nuclear Service Center StudyCompanion Report," U.S. Department of Energy,TID-28905-2, Circa 1979.

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