BENCH-SCALE TESTS OF A YUCCA MOUNTAIN 

HEATED DRIFT ENVIRONMENT 

Prepared for

U.S. Nuclear Regulatory Commission 

Contract NRC–02–07–006 

Prepared by

T. Mintz 

R. Pabalan 

C. Manepally 

Center for Nuclear Waste Regulatory Analyses 

San Antonio, Texas 

September 2011
ii 

ABSTRACT 

The lifetime of a waste package emplaced in a potential repository can be estimated by 

evaluating the expected degradation modes and their corresponding degradation rates. The 

susceptibility of a material to a particular degradation mode and the severity of that degradation 

will depend upon the environment to which it is subjected. Key parameters of the environment 

include both the temperature and chemical constituents. The variation in these two quantities 

can significantly affect lifetime prediction of the component or structure. The evolution of this 

environment may be influenced by several factors including quantity and chemistry of the 

seepage water. In addition, temperature and relative humidity are environmental factors that 

will modify the degradation types and severity for a waste package placed in a potential 

repository drift. However, the difficulty in determining the actual environment is a challenge due 

to the heterogeneity of the natural system, interactions between the engineered and natural 

components, the unsaturated conditions at the repository horizon, the potential for natural drift 

degradation in a thermally perturbed environment, and evolution of the near-field environment. 

To evaluate this complex system, in 2007 a scaled model of a drift environment for a potential 

repository was developed to estimate how heat in a drift may modify the environmental 

condition surrounding a waste package. The relationships between the evolution of the water 

chemistry as a function of temperature, infiltration flux, relative humidity, and corrosion rates 

were evaluated. The experimental setup consisted of a cuboid container filled with quartz sand. 

A stainless steel tube was placed in the middle of the container simulating the underground drift. 

The environmental conditions were monitored using corrosion and temperature sensors placed 

in the stainless steel mesh and in the surrounding porous media. These sensors allowed for the 

interpretation of the evolution of environmental conditions in the porous media. The only test 

that was conducted before the experiment was stopped consisted of allowing deionized water to 

percolate through the system during a 3-month test period. Solution was extracted from various 

regions in the experimental system and analyzed to understand the evolution of environmental 

conditions. The report discusses the experimental setup and the results, which initially indicate 

that there is a progressive change in the chemical activity and thermal conditions in the region 

surrounding the emplacement drift. 
iii 

CONTENTS 

Section Page 

ABSTRACT ................................................................................................................................... ii 

FIGURES ..................................................................................................................................... iv 

TABLE ........................................................................................................................................... v 

ACKNOWLEDGMENTS .............................................................................................................. vi 

1 INTRODUCTION ................................................................................................................ 1-1

2 EXPERIMENTAL DETAILS ................................................................................................ 2-1

2.1 Experimental Setup ................................................................................................... 2-1

2.2 Experimental Procedure ............................................................................................ 2-5

3 RESULTS ........................................................................................................................... 3-1

3.1 Temperature Sensors ................................................................................................ 3-1

3.2 Corrosion Sensors ..................................................................................................... 3-1

3.3 Chemical Analyses .................................................................................................... 3-7

4 DISCUSSION ...................................................................................................................... 4-1

5 CONCLUSIONS .................................................................................................................. 5-1

6 REFERENCES ................................................................................................................... 6-1
iv 

FIGURES 

Figure Page 

1-1 Schematic Diagram of the Waste Package in Emplacement Drift ................................. 1-2 

2-1 Scaled Model System Prior to Addition of Porous Quartz Media, Sensors, and 

 Heaters ........................................................................................................................... 2-2 

2-2 Scaled Model System Showing the Location of the Corrosion and Temperature 

 Sensors .......................................................................................................................... 2-3 

2-3 Corrosion Sensors Used to Estimate the Corrosivity of Local Environment .................. 2-4 

3-1 Temperature Sensor Profile During the Deionized Water Infiltration Test ..................... 3-2 

3-2 Relative Current Values From Corrosion Sensors Placed 15 cm [6 in] Vertical of 

 the Stainless Steel Mesh ................................................................................................ 3-3 

3-3 Relative Current Values From Corrosion Sensors Placed 3 cm [1 in] Vertical of 

 the Stainless Steel Mesh ................................................................................................ 3-4 

3-4 Relative Current Values From Corrosion Sensors Placed in the Stainless Steel 

 Mesh Structure ............................................................................................................... 3-5 

3-5 Relative Current Values From Corrosion Sensors Placed 8 cm [3 in] Horizontal of 

 the Stainless Steel Mesh ................................................................................................ 3-6 

3-6 Relative Current Values From Corrosion Sensors Placed 8 cm [3 in] Horizontal of 

 the Stainless Steel Mesh (Expanded) ............................................................................ 3-7 
v 

TABLE 

Table Page 

3-1 Solution Composition Analyses From Four Bottom Chambers ...................................... 3-8 
vi 

ACKNOWLEDGMENTS 

This report describes work performed by the Center for Nuclear Waste Regulatory 

Analyses (CNWRA®) for the U.S. Nuclear Regulatory Commission (USNRC) under 

Contract No. NRC–02–07–006. The activities reported here were performed on behalf of the 

USNRC Office of Nuclear Material Safety and Safeguards, Division of High-Level Waste 

Repository Safety. This report is an independent product of CNWRA and does not necessarily 

reflect the view or regulatory position of USNRC. 

The authors gratefully acknowledge X. He for her technical review, K. Axler for his 

programmatic review, L. Mulverhill for her editorial review, and A. Ramos for his administrative 

support. Special thanks to Mr. B. Derby for his assistance in conducting the experiments 

described in this study. 

QUALITY OF DATA, ANALYSES, AND CODE DEVELOPMENT 

DATA: All CNWRA-generated original data contained in this report meet the quality assurance 

requirements described in the Geosciences and Engineering Division Quality Assurance 

Manual. Sources for other data should be consulted for determining the level of quality for 

those data. All data and calculations related to this report have been recorded in CNWRA 

Scientific Notebook 768 (Dunn and Mintz, 2011). 

ANALYSES AND CODES: No codes were used in the analyses of the work described in 

this report. 

Reference 

Dunn, D. and T.S. Mintz. “Integrated Tests—Initial Testing.” Scientific Notebook No. 768. 

San Antonio, Texas: Center for Nuclear Waste Regulatory Analyses. 

pp. 55–89. 2011. 
1-1 

1 INTRODUCTION 

Nuclear waste disposal packages are currently used and are potentially going to be used in 

many disposal applications. Some potential deep geological disposal sites include the Finnish 

repository that has proposed using buffer and backfill and the potential Japanese repository that 

is considering a similar concept (NEA, 2004). In addition, the U.S. Department of Energy (DOE) 

proposed to dispose nuclear waste at Yucca Mountain using a waste package consisting of an 

outer layer manufactured from Alloy 22, as seen in Figure 1-1. 

To predict the lifetime of the waste canister material used in a potential repository, it is essential 

to understand the potential types and rates of degradation that could occur in the emplacement 

drift. Most forms of degradation are affected by environmental conditions. Localized corrosion, 

which can be in the form of crevice corrosion or pitting, is affected by both temperature and the 

chemical environment (Pardo, et al., 2000). For example, crevice corrosion on Alloy 22 in 

proposed Yucca Mountain repository conditions was shown to be a function of the exposure 

conditions (Dunn, et al., 2005). Another form of localized corrosion that may affect underground 

structures is stress corrosion cracking. Stress corrosion cracking susceptibility was also shown 

to be a function of environmental exposure conditions for Alloy 22 (Chiang, et al., 2005). 

The types and rates of degradation can be measured in a laboratory setting; however, one must 

first know the environmental exposure conditions that the waste package could experience. 

Several environmental factors are important to lifetime calculations, including temperature, 

quantity and chemistry of seepage water, and relative humidity. The primary factor affecting 

temperature and relative humidity inside a drift is heat load in the postclosure period. The 

evolution of the surrounding environment is a complex process due to the heterogeneity of the 

natural system. Surrounding porous media properties may affect the fluid flow regime, pore 

water chemistry, and relative humidity, which in turn, will affect corrosion rates. Depending on 

the chemical constituents of the surrounding porous media, the solution that may reach the 

waste package could potentially range from acidic to basic. Two key features in porous media 

that may affect the corrosion severity of the drift environment are the amount and type of soluble 

salts and the moisture content. Understanding the evolution of these environmental conditions 

is necessary in gaining a perspective on the initiation and propagation of various degradation 

modes for an underground structure. 

The subject research described in this report is the second phase in an approach to estimate 

the composition of the salts that form during the thermal period that would potentially occur in a 

repository. This system was not used to simulate an exact condition in a repository, but provide 

a simple setup to start identifying key parameters that must be understood. The first phase of 

this project was conducted in 2006 and examined the effect of dripping solutions onto a heated 

Alloy 22 sample. In this phase, an Alloy 22 sample was heated to roughly 115 °C [239 °F]. 

Different types of pore water solutions were allowed to drip onto the surface of the heated Alloy 

22 sample. The composition and properties of the salts and solutions that formed on the heated 

Alloy 22 surface were measured. The results of this test, described in Dunn, et al. 
1-2 

Figure 1-1. Schematic Diagram of the Waste Package in Emplacement Drift (DOE, 2008) 

(2006), showed that the final salt deposit depended on the initial composition of the dripping 

solution. The results also indicated that deliquescent salt deposits or brines were not likely to 

form under the tested conditions. The following phase of this testing was conducted in 2007 

and is described in the remaining sections of this report.
2-1 

2 EXPERIMENTAL DETAILS 

2.1 Experimental Setup

A scaled model of an underground drift was developed to replicate the surrounding 

environmental conditions a waste package may experience. The experimental setup consisted 

of a polycarbonate cuboid container, as shown in Figure 2-1. The container was filled with 

quartz sand as the porous media because it is chemically inert and should minimize any 

modification of the water chemistry that is introduced to the scaled model system. In 

subsequent tests, which did not occur, the quartz sand was to be replaced with media more 

closely related to those found at Yucca Mountain. The polycarbonate container was 122 cm 

[48 in] high, 50 cm [20 in] wide, and 50 cm [20 in] deep. A hollow stainless steel mesh tube was 

placed in the middle of the container simulating the emplacement drift. The stainless steel 

mesh restrained the porous quartz media while allowing liquid to penetrate the mesh. The key 

attribute of the test system design is the dimensions, which provide sufficient height so that 

liquid flow through the system can be influenced by the heat source within the stainless 

steel structure. 

The temperature of the system was controlled using two sources of heat, as shown in 

Figure 2-2. There was an internal heater centered inside the stainless steel mesh structure. 

This internal heater simulated a heat source in the emplacement drip from a waste package. In 

addition to the central heater, there were heaters located around the exterior of the system and 

covered by insulation. The exterior heaters maintained a constant temperature gradient from 

the central heater and the outer walls of the experimental system. This gradient removed any 

cold spots where the water could condense on the outer walls and flow straight into the bottom 

of the unit. There were no heaters on the top or bottom of the scaled model. All the heaters 

were powered by controllers and temperature feedback to allow for better control of the 

system temperature. 

To monitor the evolution of environmental conditions in the porous media, corrosion and 

temperature sensors were placed at specific positions in the scaled model, as shown in 

Figure 2-2. The corrosion sensor consisted of a dissimilar metal coupling that could be a 

combination of either Alloy 22/A516 Grade 60 carbon steel or Alloy 22/Type 304 stainless steel. 

Each type of corrosion sensor was placed at four locations. The two types of corrosion sensors 

were placed in the stainless steel mesh (i) 3 cm [1 in] above the stainless steel mesh, (ii) 15 cm 

[6 in] vertical of the stainless steel mesh, and (iii) 8 cm [3 in] horizontal of the stainless steel 

mesh. The two types of corrosion sensors were separated in the depth direction of the scaled 

model by roughly 30 cm [1 ft] at each of the four locations. Each sensor was approximately 

2.54 × 1.27 × 0.318 cm [1 × 0.5 × 0.125 in], as shown in Figure 2-3. The Alloy 22 samples had 

two 0.64-cm [0.25-in] holes drilled into them, as shown in Figure 2-3. The A516 carbon steel 

and Type 304 stainless steel were isolated from the Alloy 22 by a film of polyimide film (Kapton®

Tape). Two areas were not covered by the polyimide film: (i) two holes on the A516 carbon 

steel or 304 stainless steel material that matched up with the holes on Alloy 22 And (ii) the area 

on the top of the sensor, which allowed moisture to reach both 
2-2 

Figure 2-1. Scaled Model System Prior to Addition of Porous Quartz Media, Sensors, 

and Heaters. 

Note: SS = Stainless Steel, PVC = Polyvinyl Chloride. 

Bottom 

Infiltration 

Collection Bins 

Perforated Mesh 

Plate 

Polycarbonate 

Container 

Central 

Collection 

Hose 

19.5 in 

Deep 

PVC 

Tube 

SS Mesh 

19.5 in 

Wide 

48 in 

High 
2-3 

Figure 2-2. Scaled Model System Showing the Location of the Corrosion and 

Temperature Sensors. Distances Shown Are With Respect to the Center of the 

Stainless Steel Central Structure. 

Note: SS = Stainless Steel. 

All measurements are from the 

centerline of the stainless steel 

mesh 

48 Inches 

2”

4”

2” 4” 8”

8”

3” SS Mesh 

6”

Thermocouple 

Corrosion sensor 

19.5 Inches 

Support 

Heaters 

Infiltration of Water 

Infiltration Collection Bins 

Vertical 

Direction 

Horizontal 

Direction 

1”

Central 

Collection 

Hose 
2-4 

Figure 2-3. Corrosion Sensors Used to Estimate the Corrosivity of 

Local Environment 

materials. However, both materials were electrically isolated from each other. After placing the 

two materials together, the system was wrapped in the same polyimide film as shown in Figure 

2-3. During the test, the sensors were held at a biased voltage of 0.5 volts with the A516 

carbon steel or Type 304 stainless steel as the anode and the Alloy C22 as the cathode. If 

liquid accumulated in the Alloy 22 holes, this accumulation would be sensed by a change in the 

measured current. In addition, the sensor response should be consistent with the corrosivity of 

the accumulated moisture. 

Each corrosion sensor location, as shown in Figure 2-2, consisted of the two types of sensors. 

These two corrosion sensors were separated in the depth direction by roughly 30 cm [12 in]. 

The two types of corrosion sensors provided different sensitivities to the corrosivity. The 

Alloy 22/A516 carbon steel sensor provided a much larger current, whereas the Alloy 22/304 

stainless steel corrosion sensor provided a more sensitive measurement. The difference in 

sensitivities is due to the difference in the galvanic interactions. There is a much larger 

difference in the electrode potentials between the Alloy 22 and A516 carbon steel than between 

the Alloy 22 and the Type 304 stainless steel. Therefore, because there is a larger driving force 

on the Alloy22/A516 carbon steel sensor, if the same solution electrically connects these two 

materials, there will be a much larger resulting current for this sensor than for the Alloy 22/304 

stainless steel sensor. 

In addition to the corrosion sensors, the model system was implemented with six temperature 

sensors, as shown in Figure 2-2. Three of the temperature sensors are located vertically above 

the stainless steel mesh structure, while the other three are located horizontal of the stainless 

steel mesh structure. The distances vertically and horizontally from the stainless steel central 

structure were the same at 5, 10, and 20 cm [2, 4, and 8 in]. These temperature sensors were 

initially recorded manually; however, it was determined that more frequent measurements were 

Alloy 22 

A516 Grade 60 

or 

Type 304 SS 

1-Inch 

0.5-Inch 

0.125-Inch 

0.5 Volts 

Across 

Polyimide 

Coating 
2-5 

needed, and the temperature recording was automated with computerized data acquisition 

during the experiment. 

Water chemistry was measured from two different areas. One area was at the bottom of the 

scaled model system. As illustrated in Figures 2-1 and 2-2, there are collection bins on the 

bottom of the scaled model system. The collection bins were separated from the quartz media 

by a perforated mesh plate. The plate allowed the liquid to pass through while holding the 

quartz sand in place. Liquid that percolates through the quartz sand that does not reach the 

center stainless steel mesh structure was collected in these bottom infiltration chambers. Any 

liquid that reaches the center stainless steel structure will collect in the bottom of the stainless 

steel mesh, where there is a polyvinyl chloride tube that has been bifurcated down the axial 

length. A hose attached to the bottom of the bifurcated polyvinyl chloride tube collects the water 

that reaches the stainless steel structure. 

By measuring the temperature, moisture, and chemical content from various points in this 

system, one can gain an understanding of the relationship between the starting drip water and 

evolution of the water chemistry as a function of temperature and time. 

2.2 Experimental Procedure 

The scaled model was initially dried out after adding the quartz sand and installing all of the 

corrosion and temperature sensors. The scaled model dry out was conducted by raising the 

outer support heaters to 80 °C [176 °F] and holding the system at that temperature for 

3 months. After the dry out was complete, an initial test of the system was performed using 

deionized water to establish base-line performance of the scaled system. The infiltration rate of 

the deionized water was set at 1 L/day [0.264 gal/day] as a point source directly in the center of 

the top surface of the quartz sand, shown in Figure 2-2. The top of the test system was covered 

and insulated to reduce the escape of water due to evaporation. The experimental setup was 

located in an environmental chamber that reduced the ambient external temperature and 

relative humidity variations caused by diurnal and seasonal changes by controlling 

these properties roughly between 60 and 70 percent relative humidity and 23 and 25 °C 

[75 and 76 °F]. The internal central heater was held at 110 °C [230 °F], which provided a 

temperature gradient of 30 °C [54 °F] between the outer walls and underground structure. The 

corrosion sensor currents were measured using a data acquisition system. The temperature 

measurements were initially measured manually, and after 60 days of testing, they were 

measured by a data acquisition system. The overall duration of the test was approximately 

3 months. The collected solutions were analyzed for chemical composition. 
3-1 

3 RESULTS 

The following sections describe the results of the deionized water infiltration testing analysis. 

The interpretation of the temperature sensor data, corrosion sensor data, and chemical analysis 

indicates how the water content in the system evolved over time. 

3.1 Temperature Sensors 

There were six temperature sensors placed into the surrounding quartz porous media. These 

sensors showed the change in temperature, which is affected by the evolution of the thermal 

conductivity of the porous media. The change in the thermal conductivity is a result of the 

evolution of the moisture level throughout the porous media. The temperature profiles for all 

six sensors over the period of the test are shown in Figure 3-1. Day 0 is when the infiltration of 

deionized water started. The conductive heat transfer process dictates the temperature 

distribution in the surrounding porous media. This effective conductivity in the porous media is 

dependent on the degree of liquid saturation in the media that is dependent on the infiltration 

rate of the deionized water. Convective heat transfer dominates in the air gap between the 

central heater and the stainless steel mesh wall. This explains the significant temperature 

difference between the central heat source set point temperature and the temperature sensor 

located at 5 cm [2 in]. Higher temperatures were observed at the temperature probes closest to 

the stainless steel structure. As can be seen in Figure 3-1, there was a larger temperature 

gradient in the vertical direction than in the horizontal direction. This is attributed to the support 

heaters laterally surrounding the test system. The top boundary of the experimental setup was 

open to external ambient temperature {approximately 27 °C [81 °F]} and resulted in a higher 

temperature gradient in the vertical direction compared to the lateral direction. This resulted in a 

lower temperature at the sensor located 20 cm [8 in] vertically above the central heater 

compared to the sensor located 20 cm [8 in] horizontally from the central heater. However, 

even in the horizontal direction, the temperatures were warmer toward the central heater, as 

would be expected. Once the test was initiated, all of the temperature sensors equilibrated to 

the new moisture level in the surrounding porous media. The 20-cm [8-in] vertical sensor, which 

was the coolest, showed an increase in temperature, while the other 5 hottest sensors showed 

a decrease in the temperature as moisture was added to the system.

3.2 Corrosion Sensors 

Eight corrosion sensors were placed in the test system as shown in Figure 2-2. Figures 3-2 

through 3-6 indicate the relative change in currents from the moisture addition to the test 

system. The relative current was calculated by taking the base current value at Day 0 and then 

subtracting this from all of the following data measurements. The base current for each 

corrosion sensor ranged from −10 μA to −50 μA. Note that in Figure 3-3, the Alloy 22/304 

stainless steel sensor data only goes to roughly 25 days because the sensor malfunctioned at 

that time. 

Figure 3-2 shows the response of the corrosion sensors located 15 cm [6 in] above the central 

stainless steel mesh. Both sensors behave similarly in that there is an initial increase in the 

relative current that corresponds with the change in temperature observed in Figure 3-1. The 

maximum positive relative current observed for the Alloy 22/A516 carbon steel sensor was 

0.75 μA, while the Alloy 22/304 stainless steel sensor reached a maximum relative current of 
3-2 

Figure 3-1. Temperature Sensor Profile During the Deionized Water Infiltration Test 

roughly 0.5 μA. The relative current then started to slowly decrease for both corrosion sensors 

and finally ended up at a negative relative current value. The final equilibrium current value for 

the Alloy 22/A516 carbon steel sensor was roughly −0.10 μA, while the Alloy 22/304 stainless 

steel sensor equilibrated at −0.5 μA. 

Figure 3-3 shows the relative current for the corrosion sensors located 3 cm [1 in] above the 

central stainless steel mesh. As mentioned previously, the Alloy 22/304 stainless steel 

corrosion sensor malfunctioned roughly 25 days into the test. However, the data indicate that 

both sensors saw an overall increase in the relative current with time. The Alloy 22/A516 

carbon steel corrosion sensor reached an equilibrium relative current value of 1.5 μA at roughly 

45 days into the test. This corrosion sensor indicated some spikes in the relative current, 

indicative of potential localized corrosion events. These spikes were on the order of only 1 μA. 

Secondly, there was a large trough in the relative current value for the Alloy 22/A516 carbon 

steel corrosion sensor, indicating either a relative dry period or a change toward a more 

benign chemistry. 

Figure 3-4 presents the relative current values for the corrosion sensors that were located inside 

the central stainless steel mesh structure. The results of these data also indicate an increase in 

the relative current for both sensors. The Alloy 22/A516 carbon steel corrosion sensor reached 

a maximum relative current of roughly 6.5 μA at 70 days of testing. The Alloy 22/304 stainless 

steel corrosion sensor reached a maximum relative current value of 3 μA at roughly 10 days. 

There are no relative current spikes indicating any localized corrosion; however, these 

equilibrium relative currents are the largest observed by any of the corrosion sensors and are 

attributed to a more corrosive (concentrated) solution. 

45

50

55

60

65

70

75

80

85

90

95

0 20 40 60 80

Time (Days)

Temperature ( oC )

8-inch Vertical

4-Inch Vertical

2-Inch Vertical

2-Inch Horizontal

4-Inch Horizontal

8-Inch Horizontal
3-3 

Figure 3-2. Relative Current Values From Corrosion Sensors Placed 15 cm [6 in] 

Vertical of the Stainless Steel Mesh 
3-4 

Figure 3-3. Relative Current Values From Corrosion Sensors Placed 3 cm [ 1in ] Vertical 

of the Stainless Steel Mesh 
3-5 

Figure 3-4. Relative Current Values From Corrosion Sensors Placed in the 

Stainless Steel Mesh Structure 

Finally, Figures 3-5 and 3-6 show the relative current values for the corrosion sensors placed 

8 cm [3 in] horizontally from the central stainless steel mesh structure. As can be seen in 

Figure 3-5, there is a large relative current spike for the Alloy 22/A516 carbon steel corrosion 

sensor to roughly −425 μA. It is unclear whether this indicates a change in the corrosivity at this 

sensor or an electronic error in the sensor. A closer image of the current values relative to 

base-line is provided in Figure 3-6. As can be observed in the figure, the current behaved 

similarly to the corrosion sensors placed 15 cm [6 in] vertical of the stainless steel mesh as 
3-6 

Figure 3-5. Relative Current Values From Corrosion Sensors Placed 8 cm [3 in] 

Horizontal of the Stainless Steel Mesh 

shown in Figure 3-2. The relative current initially increased and then decreased. The maximum 

relative current was roughly 2.2 μA and 2 μA for the Alloy 22/A516 carbon steel corrosion and 

Alloy 22/304 stainless steel corrosion sensors, respectively. The final equilibrium relative 

current for both corrosion sensors was roughly −0.3 μA and 0.3 μA for the Alloy 22/A516 carbon 

steel and Alloy 22/304 stainless steel corrosion sensors, respectively. 
3-7 

Figure 3-6. Relative Current Values From Corrosion Sensors Placed 8 cm [3 in] 

Horizontal of the Stainless Steel Mesh (Expanded) 

3.3 Chemical Analyses 

As previously mentioned, solution was collected from both the bottom of the testing system and 

the central stainless steel mesh structure. The bottom infiltration collection bins were separated 
3-8 

into four different chambers. The liquid in each chamber was collected and chemically 

analyzed. The composition of the liquid in each of these four chambers and the results of the 

liquid collected from the stainless steel structure are shown in Table 3-1. The liquid analyzed 

was from the total accumulated liquid collected during the whole test. 

As can be seen from Table 3-1, the liquid collected at the bottom of the test system is mostly 

uniform in composition. From the data presented in Table 3-1, it appears that leaching did 

occur from the quartz sand as deionized water passed through the system. However, it is the 

differences between the liquid that was collected in the bottom chambers and that collected 

from the stainless steel central structure that is of interest. While the concentration of silicon 

from the bottom chamber 1 was much higher, the concentration of boron, calcium, sodium, and 

sulfur were very similar in all the bottom chambers. Comparing the bottom chambers to the 

values collected from the central stainless steel structure was quite different. There was a much 

higher level of sulfur, sodium, boron, and phosphorus in the liquid collected from the stainless 

steel central structure. This increased concentration in the central stainless steel structure is 

likely due to the higher temperatures, which may have let to evaporation and a 

concentrating effect. 

Table 3-1. Solution Composition Analyses From Four Bottom Chambers* 

Locations Boron 

(mg/L) 

Calcium 

(mg/L) 

Silicon 

(mg/L) 

Sodium 

(mg/L) 

Sulfur 

(mg/L) 

Phosphorus 

(mg/L) 

Bottom 

Chamber 1 4.91 8.25 54.4 6.32 1.72 <0.200 

Bottom 

Chamber 2 5.18 1.64 16.1 5.97 <1.00 <0.200 

Bottom 

Chamber 3 2.07 1.64 14.3 3.14 1.03 <0.200 

Bottom 

Chamber 4 5.18 2.39 18.0 6.43 <1.00 <0.200 

Stainless 

Steel Mesh 25.2 <1.25 17.8 38.7 108 1.21 

*Solutions were analyzed for additional chemical species other than those listed. Only the species listed had 

reportable results.
4-1 

4 DISCUSSION 

The corrosion performance of a waste package emplaced in a drift environment will be 

dependent upon the surrounding porous media conditions. Underground metallic structures in a 

porous media environment can suffer from general corrosion, stress corrosion cracking, and 

localized corrosion (pitting or crevice). Changes in pH, moisture, temperature, and chemical 

content will affect the corrosion resistance of an underground metallic structure or system. 

A scaled model of a heated emplacement drift was designed to examine how interactions with 

the surrounding porous media may affect the local environmental conditions. In the current 

scaled model, the inner stainless steel structure had a thermal heater held at 110 °C [230 °F]; 

however, the outer walls of this stainless steel structure were somewhere below this value. The 

outer walls of the scaled model were held at 80 °C [176 °F] by support heaters, while the top 

and bottom of the scaled model was in equilibrium with the room temperature air. 

For the initial test, deionized water was added to the top of the system and allowed to 

equilibrate over a period of 3 months. Measurements were made from temperature sensors 

and corrosion sensors located in the test chamber. In addition, solution was collected from 

bins at the bottom of the test system and from the underground structure region and 

chemically analyzed. 

The results of the temperature profile over the testing period showed that the temperatures 

changed quickly (roughly 10 days) and then slowly reached equilibrium at 25–30 days of the 

test. These results correspond to the peak relative current observed in both the vertical 15-cm 

[6-in] and horizontal 8-cm [3-in] corrosion sensors. Depending on the initial temperature at a 

particular sensor location, the temperature either increased or decreased with time. This 

difference in temperature was due to the different temperature gradients in the vertical and 

horizontal direction. Different gradients occurred because the support heaters on the lateral 

walls were held at 80 °C [176 °F], while the top of the system was held in equilibrium with the 

room temperature of 27 °C [81 °F]. However, the results indicate that the temperature of the 

media surrounding the stainless steel structure decreased as moisture was added to the 

system. While the decreased temperature may lead to less severe corrosion, the added liquid 

can lead to higher porous media conductivity and more corrosive solution, which can lead to 

higher corrosion rates. 

The corrosion sensors, which indicate corrosivity, were placed in and around the stainless steel 

structure. The corrosion sensors that were 15 cm [6 in] vertically and 8 cm [3 in] horizontally 

away from the stainless steel structure showed an initial increase in the relative current that 

could be taken as an increase in moisture and, as such, corrosivity. However, the relative 

current decreased roughly 10 days after the infiltration was initiated. The results suggest that 

the environmental conditions in the outer porous media reached equilibrium quickly. This is 

based on the initial relative current increase observed for the corrosion sensors. Once the 

equilibrium condition had been obtained, an oxide film likely continued to build on the materials, 

leading to the lower relative currents observed for the corrosion sensors. 

Both the 2.5-cm [1-in] corrosion sensors and corrosion sensors in the central stainless steel 

structure showed an increase in the relative current to a much higher degree. This indicates a 

much more corrosive environment, which is consistent with the results from the chemical 

analysis showing a much more concentrated solution at the central stainless steel structure. 

The additional time needed to reach an equilibrium relative current value for the corrosion 

sensors at 2.5 cm [1 in] and in the stainless steel mesh structure is thought to be related to the 
4-2 

evaporation and a concentrating effect. Because of the higher temperatures at the stainless 

steel mesh structure, it will take a longer time to reach an equilibrium condition. However, the 

Alloy 22/304 stainless steel sample does not take long to reach equilibrium, because it builds up 

a chromium oxide film inhibiting further degradation. 

The chemical analysis examined the variation of the chemistry in the surrounding porous quartz 

media compared to the chemistry of liquid around the central stainless steel structure. The 

quartz sand should have been composed only of silicon-oxygen tetrahedra particles and inert. 

However, the quartz must have contained other constituents, which was dissolved into the 

deionized water. Therefore, the original assumption that the quartz would be inert was 

incorrect. The results indicate that there was a difference in the chemistry surrounding a 

stainless steel structure versus the solution that was collected at the bottom of the scaled 

model. In this case, there was a large increase in the concentration of sulfur in the liquid near 

the stainless structure. Sulfur is a chemical element that has been known to accelerate 

corrosion in various materials (Ahn, et al., 2008; Yang, et al., 2004). Therefore, observed 

changes in chemistry explain the higher corrosion rate for the corrosion sensor located closed to 

the central heated structure. Note that even though quartz sand was used and was expected to 

be inert, there was an interaction between the porous media and dripping deionized water.
5-1 

5 CONCLUSIONS 

Waste canisters that are being considered for use in underground repositories of high-level 

radioactive waste and spent nuclear fuel are susceptible to various forms of degradation. The 

severity of these degradation modes is dependent upon the local environment. The evolution of 

the environment in the near field is a complex process due to the heterogeneity of the natural 

system. The subject research described in this report is the second phase of an approach to 

estimate the composition of the salts that form on a waste package in a proposed repository drift 

during the thermal period. This first phase, conducted in 2006, examined the salt composition 

that would form when repository ground water was dripped onto the surface of heated Alloy 22 

material. This second phase, conducted in 2007, examined the chemical changes of an 

aqueous solution as it moves through heated porous matrix. 

A scaled model of an underground drift was developed to mimic the surrounding environmental 

conditions that a waste package may experience. Temperature and corrosion sensors were 

placed in this scaled model to characterize the evolution of the environmental conditions as 

solution was allowed to percolate through the system. The first test consisted of deionized 

water dripping directly in the center of the top surface of the quartz sand that was used to fill the 

scaled model. The pseudo-drift temperature was maintained at 110 °C [230 °F], while the walls 

of the scaled model were kept at 80 °C [176 °F] (not including the top and bottom of the scaled 

model). This control test was used to observe how the scaled model behaves. This was the 

only test conducted, because the experimental work was put on hold after completing this initial 

control test. 

The results of the deionized water test in the scaled model indicate that the porous media on the 

walls of an emplacement drift will differ in moisture level, temperature, and chemical species 

from the main porous matrix during the thermal period. The composition of the solution that 

reached the pseudo-emplacement drift in the scaled model was concentrated in boron, sodium, 

phosphorus, and sulfur compared to solution that percolated through the system. The corrosion 

sensors indicated that the concentrated environment at the pseudo-drift was much more 

corrosive than the environment farther from the internal heater.
6-1 

6 REFERENCES 

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