Criteria Derived for Geologic
Disposal Concepts
Roald A. WigelandManager, Engineering Analysis Department
Nuclear Engineering Division
Contributions from Edgar E. Morris and Theodore H. Bauer,
Nuclear Engineering Division
OECD/NEA 9th Information Exchange Meeting on Actinide andFission Product Partitioning and Transmutation
September 28, 2006
Nimes, FranceGeologic Disposal of Hazardous Nuclear Waste
 In the United States, the geologic disposal of spent nuclear fuel and highlevel waste is planned for the Yucca Mountain repository Disposal of these hazardous nuclear wastes may be constrained by atleast three major factors– Peak dose rate for repository releases to satisfy regulatory limits– Temperature limits for parts of the repository system to provide
greater assurance on long-term performance predictions– Volume of the waste materials
 For the current planned disposal, the loading of the Yucca Mountainrepository is constrained by the temperature limits due to the high decay
heat from the spent nuclear fuel– Estimates of peak dose rate have been calculated, but there are noregulations in place at this time to judge the acceptability of theplanned waste disposal– There are spaces between the waste packages in order to limit thelinear heat load in the emplacement drifts (tunnels), so waste
package volume is not an issue2

Current Yucca Mountain Thermal Limitations
 Current temperature limits for the repository design include:– Peak temperature below the local boiling point (96 oC) at all times
midway between adjacent drifts• To ensure adequate drainage of water at all times– Peak temperature of the drift wall below 200 oC at all time
• Structural integrity of the repository– Limits are still evolving, in response to concerns related to the longterm performance of the repository

3

Thermal Characteristics Limiting Repository Usage To determine the characteristics of spent nuclear fuel that limit geologic
repository usage by exceeding thermal limits, a study examining direct
disposal of spent PWR fuel was performed– Yucca Mountain is used as the example for a geologic repository– 51 GWd/MTIHM discharge burnup– Placement in the repository 25 years after discharge
 The response of the repository is modeled as follows– A drift near the center of the repository is used since the highest
temperatures would be expected in this region• allows a small section of a 3-D model to be used
– Forced ventilation for 75 years
 Maximum drift loading is 1.15 MTIHM / m, representing material that
produced 58.6 GWd of energy being stored per meter– Initial linear heat load is 1.45 kW / m– No margin is provided for uncertainties; used as reference point

4

Repository Response to Direct Disposal of Spent Fuel
2500

51 GWd/MTIHM Spent PWR Fuel,
Emplaced 25 Years After DischargeDrift Loading = 58.6 GWd/m(1.15 MTIHM/m)

Temperature, C

160

120

2000

1500
Heating in Drift →Water Boiling, 96 C
Waste Package
Surface (average)

801000Decay Heat, W/m200

Drift Wall

40500

Between Drifts

Airflow
Turned Off01101001000

010000Time after Disposal, years

5Repository Loading Limitation for Direct Disposal
 As the figure shows, the loading of the repository is constrained by thetemperature limit of 96 oC for the location midway between adjacent drifts– allows water to drain through the repository at all times
 Peak temperature is reached at about 1500 years after disposal. The
temperature midway between adjacent drifts only rises substantially after
the repository is closed– most of the heat to raise the rock temperature is the integrated decay
heat from the time of repository closure, 75 years after waste
placement (100 years after reactor discharge), to the time of peaktemperature at about 1600 years– forced ventilation reduces the importance of shorter-lived isotopes
 Examination of the spent fuel decay heat identifies the cause of thisdecay heat as plutonium and americium– Am-241, from the decay of Pu-241– Pu-238, existing at discharge, and other plutonium isotopes

6Spent PWR Fuel Decay Heat
1.000E+02
Time of Disposal
Time of Closure
PU238

Decay Heat, Watts/GWd of Energy Produced

Time of Peak Temperature

PU239
PU240

1.000E+01

PU241
AM241
AM243
CM242
CM243
1.000E+00

CM244
Act. sum
SR
Y
CS
BA

1.000E-01

EUFP sum
Total

1.000E-02
101001000

10000

Time after Discharge, Years

7

Chemical Elements to Remove from Spent Nuclear Fuel
 With the removal of plutonium and americium, it is possible to more
densely load the repository with the resulting process waste– lower decay heat from the process waste– uranium is also removed to reduce the volume of the process waste With these elements removed from the process waste, the limiting
temperature occurs much earlier, shortly after the repository is closed– ventilation still removes most of the decay heat prior to closure Examination of the decay heat for the remaining process waste identifies
the cause of this decay heat as barium and yttrium, decay products ofcesium and strontium– relatively short-lived fission products– can be removed from the process waste and placed in separate
storage for decay instead of being placed in the repository Additional removal of cesium and strontium allows for much larger
increases in loading density

8Disposal of Process Waste, Pu & Am Removed
Waste Package
Surface (Average)

Drift Wall

51 GWD/MTIHM Spent PWR Fuel,
99.9% Pu, Am & Np Removed
Drift Loading = 298 GWd/m(Process waste from 5.85 MTIHM/mbeing stored per meter)

160

Temperature, °C

2500

Rock Temperature Limit120

2000

1500

Water Boiling, 96 C80100040500
Between Drifts

Airflow
Turned Off

Heating in Drift →0110

Decay Heat, W/m

200

100

1000

010000Time after Disposal, years

9Process Waste Decay Heat w/o Pu & Am
1.000E+02
Time of Disposal

Decay Heat, Watts/GWd of Energy Produced

Time of Closure

1.000E+01
CM243
CM244
Act. sum
SRY
CS
BAEU
FP sumTotal

1.000E+00

1.000E-01

1.000E-02
10100

1000

10000

Time after Discharge, Years

10

Disposal of Process Waste, Pu, Am, Cs, & Sr Removed
250

6500

Temperature, °C

200

Drift Wall

150Waste Package
Surface (Average)100

Rock Temperature Limit

5200

51 GWd/MTHM Spent PWR Fuel,99.9% Pu, Am & Np Removed
Cs, Sr and Dtrs. (Y, Ba) Rem.
Drift Loading = 3010 GWd/m(Process waste from 59.0 MTIHM
being stored per meter)

3900

Water Boiling

2600

50Decay Heat, W/m

Heating in Drift →1300

Between Drifts

01

10100

Airflow
Turned Off

1000

010000Time after Disposal, years

11Potential Increase in Utilization of Repository Space With the processing of spent PWR fuel to remove the elements
responsible for the decay heat that cause temperature limits to bereached, large gains in utilization of repository space are possible– The amount of gain is related to separation efficiency– Only considers thermal performance, not dose rate
 Pu, Am, Cs, Sr, & Cm are thedominant elements– The recovered elements
must be treated• separate storage of Cs& Sr for 200-300 years
 Recycling of Pu, Am, & Cm fortransmutation and/or fission– Irradiation in reactors
 No direct disposal of any spent
fuel

Assumptions
Burnup: 50 GWd/MT
Separation: 25 years
Emplacement: 25 years
Closure: 100 years

225.0175.0

Limited by 96 ºC
Mid-Drift Temp.
>1600 yrs

Limited by 200 ºC Drift WallTemp. at Emplacement

94.0

91.0

Limited by 200 ºC DriftWall Temp. at Closure

54.0
44.0
10.5
1.00.001

10.3

1.00.01

Fraction Cs & Sr
in Waste

10.0
4.4

1.0
0.1

5.5

1.0
0.1
11

5.7
0.01

0.001

Fraction Pu, Am, & Cm
in Waste

12Impact of Processing and Loading Increase on Dose Rate The only function of a geologic repository is to isolate the hazardous
materials from the public and the environment– Actinides are responsible for the peak dose rate in a repository like
Yucca Mountain– Removal of actinides will decrease the peak dose rate
– Increasing the loading of the repository will increase the peak dose rate
• Inventory of elements not separated from the waste is greatlyincreased, such as technetium and iodine• Residual actinide inventory depends on the separation efficiency
 Peak dose rate is also affected by the waste form
– Direct disposal of spent fuel with intact cladding– Process waste in a vitrified waste form such as borosilicate glass• Chemical conditions as the waste forms degrade affect waste form
degradation and the dissolution and mobility of hazardous materials
 Task is to identify the processing required to lower the dose rate from spent
PWR fuel– Results normalized to peak dose rate for direct disposal of spent fuel13

Direct Disposal of Spent PWR Fuel
10

Total
1237

230

Normalized Dose Rate

NpTh

0.1

PbRa

242

Pu

210

226

231

Pa

227

Ac

229

Th

0.01
239

Pu

233

U

129

I

99

Tc

238

U

0.001
234

U

236

U0.0001
0200,000

400,000

600,000

800,000

1,000,000

Time, years
14

Disposal of Spent PWR Fuel Inventory in Glass
10

Total
1230

Th 226
Ra

Normalized Dose Rate237

Np210

Pb229

Th

0.1
242

231

Pu

227

PaAc

0.01

239

Pu

99

Tc

129

I

234

236

U233

UU

0.001
238

U

239

Ic

0.0001
0200,000

400,000

600,000

800,000

1,000,000

Time, years
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Dose Rate Contributors
 The main contributors to the dose rate for releases from the repository areactinide isotopes– Np-237• Np-237 is present in spent PWR fuel, but additional amounts are
created by the decay of Pu-241 and higher actinides– Th-230, Ra-226, and Pb-210• created by the decay of uranium, plutonium, and higher actinides• 238U → 234Th → 234Pa → 234U → 230Th → 226Ra → 222Rn →218Po → 214Pb → 214Bi → 214Po → 210Pb → 210Bi → 210Po →206Pb
 The difficulty in reducing the dose rate is that each of the dominant isotopes
has more than one parent isotope– removal of individual chemical elements is not very effective– only group separation of the actinide elements provides large reductions

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Dose Rate Reduction from Processing Spent Fuel
Element(s) RemovedNormalized Peak Dose Rate
None
1.550
Uranium
0.852
Plutonium
0.835
Americium
1.473
Neptunium
1.525
Uranium &Plutonium
0.199
Uranium & Americium
0.765
Uranium & Neptunium
0.823
Plutonium & Americium
0.749
Plutonium & Neptunium
0.806
Americium & Neptunium
1.395
Uranium, Plutonium & Americium
0.092
Uranium, Plutonium & Neptunium
0.163
Uranium, Americium & Neptunium
0.700
Plutonium, Americium & Neptunium
0.666
All Actinides
0.011

Peak Dose Rate
Reduction Factor*
0.65
1.17
1.20
0.68
0.66
5.03
1.31
1.22
1.33
1.24
0.72
10.9
6.131.43
1.50
91.0*The reduction factor is for the peak dose rate in each case compared to the peak dose rate for thereference case of direct disposal of PWR spent fuel assemblies. Reduction factors less than 1.0
indicate that the peak dose rate is higher than the reference case.

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Process Waste with 99.9% Actinide Removal
0.1

Total
0.01

99

Tc
129

I

237

Normalized Dose Rate

Np

0.001

226

Ra

230

Th
210

Pb

0.0001

242

Pu

239

Pu
234

U229

Th

233

U0.00001

236

U
227

Ac

231

Pa
238

U

239

Ic

0.000001
0200,000

400,000600,000

800,000

1,000,000

Time, years
18Waste Management Options
 The use of a geologic repository like Yucca Mountain can be greatly
increased by processing spent nuclear fuel to remove all actinides alongwith Cs & Sr and by recycling the transuranic elements– Large increases in the utilization of space by a factor of about 100
can be achieved on a peak dose rate and thermal basis, or– Large reductions in peak dose rate (about a factor of 100) are
possible with 99.9% removal of all actinide elements
 Three basic options– Can be used to greatly lower the peak dose rate associated with arepository intended for a specific amount of spent fuel– Can be used to greatly increase the amount of waste placed in therepository without increasing the estimated peak dose rate ascompared to the direct disposal of spent fuel– Can be used to both reduce peak dose rate (but less than themaximum reduction) and increase the amount of waste (but less thanthe maximum increase) The need for additional repository space or a second repository can besignificantly delayed if there is no direct disposal of spent fuel19

Example of Increased Repository Utilization
10Normalized Dose Rate

Remove 99.9% of Actinides
Remove 99.9% of Cesium and Strontium
Increase Loading by a Factor of 100

Total

199129

Tc

I

0.1

230

Th
210

Pb237

Np

226

Ra

237

Np

0.01

242

Pu

229

Th

227

Ac

0.001

231

Pa

239

Pu

0.00010200000

4000006000008000001000000

Time, years
20Waste Volume Considerations
 The third major factor in determining repository usage is waste volume– Waste forms have chemical loading and temperature limits• to ensure that the waste form can be produced• to ensure long-term stability of the waste form
– At this time, the ability to produce waste forms that would allow thepotential densification of the waste products is unknown However, waste volume can be addressed by altering repository design– If the waste form loading is lower than desired, the waste form will
also have lower decay heat generation than would be allowed• Drift spacing can be reduced to allow more drifts in a given
repository area (about a factor of 3 for Yucca Mountain)• Depending on the heat load, multilevel placement can also beconsidered The overall conclusion is that waste volume should not be a primary
determinant for repository usage, and less than optimal waste formloading can be accommodated by increasing waste package loading perunit area, consistent with the applicable peak dose rate and thermal limits
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Conclusions
 The use of space in a geologic repository like Yucca Mountain can begreatly improved by processing spent nuclear fuel and recycling thetransuranic elements
 The amount of improvement is controlled by the separation efficiencies
for spent fuel processing– Increasing the drift loading by a factor of about 100 can be achieved
while satisfying thermal limits with essentially no change in theestimated peak dose rate as compared to direct disposal of spent fuel– waste form volume appears to be a secondary issue, with less than
optimal waste form loading compensated by repository designchanges while still satisfying peak dose rate and thermal limits
 Continuous recycling of the recovered transuranics is essential– no direct disposal of spent fuel– transuranics remain in the fuel cycle
 The need for additional repository space or a second repository can bedelayed for at least a century, probably much longer

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