Early Leak Detection 

External to Structures 

at Nuclear Power Plants

Office of Nuclear Regulatory Research

NUREG-2151
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Early Leak Detection 

External to Structures 

at Nuclear Power Plants

Manuscript Completed: December 2012

Date Published: April 2013

Prepared by:

Mark Fuhrmann and Joseph Kanney

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001

Iouri Prokofiev, NRC Project Manager

Office of Nuclear Regulatory Research

NUREG-2151

iii

ABSTRACT

Concerns about inadvertent releases of radioactive liquids to the environment from nuclear 

power plants have prompted consideration of ways to provide early leak detection in the 

subsurface external to the structures of the facilities. Approaches to this include the use of 

single-point sensors to detect changes in moisture content in the vadose zone. While many of 

these sensors are sensitive and relatively durable, they only interrogate about 1 liter of soil. 

Arrays of single-point sensors could provide adequate coverage of larger areas. 

Two-dimensional and three-dimensional geophysical methods provide information for much 

larger areas. These methods sense moisture or other parameters that may be related to leaks, 

such as changes in conductivity/resistivity, permittivity, or temperature. Use of cross-borehole 

geophysics may provide coverage of vertical planes of soil while horizontal boreholes (or 

horizontal tubes installed during construction) can be used to interrogate planes underneath 

areas of concern. Other techniques include detection of tritium in soil vapor and temperature 

changes using coaxial cables. Some of these methods can be made autonomous. The 

methods are critically reviewed and discussed with emphasis on practical application at nuclear 

power plants. The U.S. Nuclear Regulatory Commission (NRC) held a public workshop on

February 15, 2012, to discuss many of these methods with experts and interested parties. 

Recommendations are made for NRC’s Long-Term Research Program.

v

CONTENTS

ABSTRACT..........................................................................................................................iii

CONTENTS.........................................................................................................................v

LIST OF FIGURES.............................................................................................................. vii

LIST OF TABLES................................................................................................................ vii

ACKNOWLEDGEMENTS....................................................................................................ix

1. INTRODUCTION.......................................................................................... 1

1.1 Objective.......................................................................................... 1

1.2 Approach........................................................................................................... 1

1.3 Background and Regulatory Context.................................................... 2

1.4 Relation to Ongoing Industry Initiatives................................................. 3

2. CHARACTERISTICS OF LEAKING SYSTEMS AND LEAKED FLUIDS 

IMPORTANT FOR LEAK DETECTION........................................................ 5

2.1 Structures, Systems, and Components................................................. 5

2.1.1 Spent Fuel Pools................................................................... 6

2.1.2 Cooling Water Discharge Systems.......................................... 6

2.1.3 Primary Cooling Water Treatment and Storage Systems.............. 6

2.1.4 Liquid Radioactive Waste Systems.......................................... 6

2.1.5 Storm Drain Systems............................................................ 6

2.1.6 Septic System Piping and Leach Fields...................................... 7

2.2 Small Modular Reactors................................................................... 7

2.3 Potential Signals............................................................................. 7

2.3.1 Radioactivity........................................................................ 7

2.3.2 Moisture.............................................................................. 7

2.3.3 Temperature........................................................................ 7

2.3.4 Chemistry............................................................................ 8

2.3.5 Geophysical Properties.......................................................... 8

2.4 Challenges of Leak Detection............................................................. 8

3. TECHNOLOGY DESCRIPTIONS................................................................... 11

3.1 Single-Point Methods....................................................................... 12

3.1.1 Porous Block Sensors........................................................... 12

3.1.2 Tensiometers...................................................................... 13

3.1.3 Capacitance Probes.............................................................. 13

3.1.4 Heat Dissipation Sensors....................................................... 13

3.1.5 Dual-Probe Heat Pulse Sensors.............................................. 14

3.1.6 Psychrometer...................................................................... 14

3.1.7 Time Domain Reflectometry................................................... 14

3.1.8 Examples of Arrays of Single-Point Moisture Sensors................... 15

3.2 Two-Dimensional and Three-Dimensional Methods................................ 16

3.2.1 Electromagnetic Methods....................................................... 16

3.2.1.1 Frequency Domain Electromagnetic Methods................ 17

3.2.1.2 Time Domain Electromagnetic Methods....................... 18

3.2.1.3 Ground-Penetrating Radar....................................... ... 19

3.2.2 Electrical Resistivity Methods ................................................ 21

3.2.3 Some Approaches for the Use of Electrical Resistivity Methods for Leak 

Detection.............................................................................. 23

3.2.3.1 Leak Rate and Volume Detection on a Tank Farm.............. 23

3.2.3.2 Moisture Location, Imaging, and Impact of Buried Pipes...... 25
vi

3.2.4 Nuclear Magnetic Resonance Sensing........................................ 27

3.2.5 Fiber-Optic Distributed Temperature Sensing............................... 28

3.3 Nuclear Methods for Water Content...................................................... 30

3.3.1 Neutron Probe....................................................................... 30

3.3.2 Gamma Ray Attenuation.............................................................. 31

4. DETECTION METHODS FOR RADIONUCLIDES ..............................................33

4.1 In Situ Tritium Vapor Detection............................................................... 34

4.2 Down-Well Tritium Detection.................................................................. 34

5. DISCUSSION............................................................................................. 37

5.1 Comparison of Single-Point Methods....................................................... 37

5.2 Comparison of Two-Dimensional and Three-Dimensional Methods................... 37

5.3 Other Uses of Techniques of Interest to the NRC....................................... 42

5.4 The Infrastructure Problem..................................................................... 42

6. LONG TERM RESEARCH RECOMMENDATIONS............................................ 45

7. REFERENCES.......................................................................................... 49

APPENDICES

APPENDIX A. ABSTRACTS FROM THE FEBRUARY 15, 2012, WORKSHOP............ A-1

APPENDIX B. PARTICIPANTS IN THE FEBRUARY 15, 2012, WORKSHOP.................B-1
vii

FIGURES

1. Data sets from advanced tensiometers showing long-term drying trends at 

different locations at INEEL (Hubbell et al., 2005)............................................... 16

2. Transmitted and received signals from TDEM instruments (U.S. Army Corps 

of Engineers, 1995).................................................................................... 18

3. Schematic of TDEM process showing the transmitter and receiver antennae and

the EM image propagating away from the transmitter antenna over time (U.S. Army

Corps of Engineers, 1995)............................................................................. 18

4. Three configurations for downhole GPR (Lane et al., 2004) ..................................20

5. (a) Plan of Hanford mock tank and the locations of wells used as electrodes

and (b) After Rucker, 2012, data from the four wells showing volumes of 

leak tests (derived from voltages). Leak rates can be determined from the 

slopes of voltages. ..................................................................................................... 24

6. Analysis of various events for resistivity monitoring of Tank C-104 at Hanford for 

about 5 months............................................................................................ 25

7. ERT showing infiltration of rain over a 9-week period at the Hanford 300 Area............26

8. ERT-derived subsurface monitoring for the vadose zone desiccation at the

Hanford B-C cribs (Johnson et al., 2012)............................................................ 27

9. Transform analysis at two periods.................................................................... .30

10. Output from detector tests showing separation of counts from technetium-99, 

strontium-90 and yttrium-90 (Akers et al., 2010).................................................. 35

11. Modeled response of ERT detection of a leak with the absence and presence of 

a pipe...............................................................................................................................43

12. One approach to using zero-offset ER monitoring..........................................................46

TABLES

1. Systems, Components, and Potential Leak Signals.............................................. 9

2. Comparison of Large-Volume Interrogation Methods.......................................... .41

ix

ACKNOWLEDGMENTS

We are grateful for comments from staff of the U.S. Geological Survey, particularly Bill Ally, 

Fred Day-Lewis, and John Lane. We thank Professor Ty Ferré of the University of Arizona,

Professors Lee Slater and Kristina Keating of Rutgers University, and Shuh-Haw Sheen of 

Argonne National Laboratory for their comments. Thank you also to Yakov Pachepsky, 

Andrey Guber, and Bill Rowland of the Agricultural Research Station of the U.S. Department of 

Agriculture; Tim Johnson, Mike Truex, Dawn Wellman, and Andy Ward of Pacific Northwest 

National Laboratory, and Bob Goldstein from Technical Associates for their useful discussions. 

1

1. INTRODUCTION

Between 2006 and 2010, nine nuclear power plants (NPPs) identified inadvertent releases of 

radioactive liquids to the environment (NRC, 2010). Several of these plants had multiple 

releases. Others had leaks that did not contain radionuclides. The U.S. Nuclear Regulatory 

Commission (NRC) maintains an historical listing of leaks of water containing tritium from power 

plants (NRC 2011, 2007). While none were determined to affect safety or human health, these 

leaks caused considerable concern on the part of some stakeholders, including Congressional, 

State, and local officials. The common approach to environmental monitoring external to facility 

structures, systems, and components (SSCs) is to use ground water wells. The drawback to 

this method is that the wells may be some distance from the leak and therefore are a very late 

indicator of contaminants entering the ground water. In addition, experience has shown that 

contaminants can move through unexpected pathways that may not be observed by wells. 

From this, it is clear that a means of providing early detection of leaks from subsurface pipes 

and structures, before contaminants reach the ground water, is desirable.

1.1 Objective

This report presents the results of a scoping study undertaken to identify and assess subsurface 

monitoring methods for detecting early indicators of leaks at new and existing NPPs, as well as 

proposed new reactor concepts (e.g., small modular reactors (SMRs)). A number of inadvertent 

releases from NPPs of liquids containing radionuclides prompted this study. This information 

will be used as input to decisionmaking for the Long-Term Research Program conducted by the 

NRC’s Office of Nuclear Regulatory Research.

The scoping study focused on the feasibility of using sensors and monitoring techniques to 

promptly detect abnormal releases of liquids from the engineered facility SSCs to the 

surrounding subsurface environment (either engineered backfill materials or native materials). 

The objective was to study those sensors and techniques that could fulfill a critical inspection or 

monitoring need identified as early detection of a leak in close proximity to, but external to,

SSCs (e.g., before it would be detected in a conventional monitoring well network). Such early 

detection of leaks may constitute an important element in any strategy for minimizing 

contamination during NPP operation and reducing the technical and financial uncertainties of 

decommissioning.

The regulatory driver is compliance with the proposed Title 10 of the Code of Federal 

Regulations (10 CFR) 20.1406(c) that requires operation of existing facilities in a manner that 

minimizes the introduction of radioactive materials into the site (including the subsurface). Early 

detection of leaks will be an important tool when implementing the Decommissioning Planning 

Rule to assess the need to remediate subsurface contamination, as discussed in Draft 

Regulatory Guide 4014, “Decommissioning Planning during Operations” (NRC, 2011). Early 

leak detection would facilitate decommissioning and remediation such that the site could be 

returned to unrestricted use at license termination. Related to this, the amended 

10 CFR 20.1501(a) explicitly includes the subsurface in requiring surveys to evaluate residual 

radioactivity at the site.

1.2 Approach

The approach to early leak detection is to monitor the subsurface (especially the unsaturated 

zone) for moisture, radionuclides (especially tritium), temperature changes, or other indicators of 
2

leaks from nuclear facilities. Methods of interest include (1) detection of anomalous water 

content of soils surrounding SSCs, (2) radionuclides contained in the leaking water or soil vapor, 

and (3) secondary signals such as changes in temperature or chemical conditions. Ideally the 

methods should be in situ, sufficiently robust to maintain their useful life over some years, and 

sufficiently sensitive that they provide an early signal of leaks. Detection systems may be most 

important in the vadose zone but some certainly have applications in the saturated zone.

1.3 Background and Regulatory Context

Discovery of inadvertent, unmonitored releases (leaks and spills) of radionuclide-containing 

fluids that have affected ground water at several operating and decommissioning power 

reactors over the past decade have resulted in substantial costs during decommissioning, led to 

considerable public concern, generated widespread media attention, and motivated both 

licensees and the NRC to initiate activities aimed at minimizing future incidents. The NRC’s

response to ground water contamination incidents at power reactor sites has been multifaceted. 

On July 10, 2006, the NRC issued Information Notice 2006-13, “Ground-Water Contamination 

Due to Undetected Leakage of Radioactive Water,” which summarized its review of radioactive 

contamination of ground water at multiple facilities as a result of undetected leakage from facility 

SSCs that contain or transport radioactive fluids. The NRC instructed licensees to review the 

information for applicability to their facilities and consider actions, as appropriate, to avoid 

similar problems. The NRC also formed a Liquid Radioactive Release Lessons Learned Task

Force (LLTF), composed of NRC staff members, to assess the inadvertent release of 

radioactive liquid to the environment at power reactor sites. The LLTF issued its final report on 

September 1, 2006 (NRC, 2006). The report included 26 recommendations for additional 

consideration by the NRC. Several staff recommendations urged development of guidance for 

early detection of leaks and for onsite ground water monitoring. Other recommendations urged 

review of NRC regulations regarding minimization of contamination (10 CFR 20.1406, 

“Minimization of Contamination”) and adequate decommissioning funding (10 CFR 20.1501, 

“General”). 

In 2012 the NRC issued Information Notice 2012-05 (ML120410213). It addresses the sources 

and causes of recent abnormal, unmonitored releases of radioactive materials in liquids to the 

ground that could potentially migrate to groundwater. It summarizes NRC requirements related 

to radioactive liquid effluents, related information notices, provides some examples of recent 

abnormal releases, and identifies those plant systems and causes most frequently associated 

with these abnormal releases.

Proposed changes to NRC regulations for decommissioning planning (SECY-09-0042, “Final 

Rule: Decommissioning Planning (10 CFR Parts 20, 30, 40, 50, 70, and 72; RIN-3150-AI55,” 

dated March 13, 2009) have recently addressed the last two points in the preceding discussion 

of LLTF recommendations. The proposed final rule adds 10 CFR 20.1406(c), which will require

licensees to conduct their operations to minimize the introduction of residual radioactivity into 

the site, including subsurface soil and ground water. This rule also amends 10 CFR 20.1501 to 

require licensees to survey and evaluate residual radioactivity that may require remediation 

during decommissioning to meet the unrestricted use criteria of 10 CFR 20.1402, “Radiological 

Criteria for Unrestricted Use,” including in subsurface areas, and to keep records of surveys of 

subsurface residual radioactivity identified at the site with records important for 

decommissioning. In conjunction with the proposed rule changes, NRC staff is preparing new 

guidance (Regulatory Guide 4.22). These rule changes and regulatory guides will augment 

existing regulations (10 CFR 20.1406(a) and (b)) and guidance (Regulatory Guide 4.21, 
3

“Minimization of Contamination and Radioactive Waste Generation: Life-Cycle Planning”) that 

address requirements for minimization of contamination for new facilities.

1.4 Relation to Ongoing Industry Initiatives

The Nuclear Energy Institute (NEI) has coordinated an industrywide response to the ground

water contamination problem in the form of the Groundwater Protection Initiative (GPI) (NEI, 

2007). The GPI identifies actions necessary for the implementation of a timely and effective 

ground water protection program. Among other actions, the GPI directs power plant operators

to establish an onsite ground water monitoring program to ensure timely detection of inadvertent 

radiological releases to ground water. Through a staff requirements memorandum dated 

August 15, 2011, the NRC has approved a recommendation not to incorporate the voluntary 

industry initiative on ground water protection into the regulatory framework. The Commission 

has directed that the staff should, instead, monitor the effectiveness of the industry initiatives. 

Another industry initiative is the NEI Underground Piping and Tanks Integrity Initiative (NEI, 

2010), which is intended to “provide reasonable assurance of structural and leakage integrity of 

in-scope underground and buried piping and tanks” with “special emphasis on components that 

contain licensed radioactive materials.” The Electric Power Research Institute (EPRI) is actively 

examining issues related to ground water protection and leak detection. EPRI (2008) provides 

guidance for designing and implementing a ground water protection program that is specific to 

site hydrologic conditions and plant SSCs.

Recently the consensus standard American National Standards Institute/American Nuclear 

Society (ANSI/ANS) 2.17-2010, “Evaluation of Subsurface Radionuclide Transport at 

Commercial Nuclear Power Plants” (ANS, 2010), was published. This standard establishes 

requirements for evaluating the occurrence and movement of radionuclides in the subsurface 

resulting from abnormal radionuclide releases at commercial NPPs. This new consensus 

standard provides technical guidance and references for site characterization, mathematical 

modeling, performance assessments, performance confirmation monitoring, and information 

management.

Two recommendations put forward in ANSI/ANS 2.17 are particularly relevant to this report. 

Section 6.2.2 suggests that “Surface and subsurface resistivity arrays may be used for detecting 

subsurface changes in soil moisture content or electrical conductivity.” Section 6.3 suggests the 

location of monitoring stations, stating that “monitoring locations shall be close enough to the 

locations where a release to the subsurface might occur to ensure that the source of the 

detected contamination is identified.” While ANSI/ANS 2.17 is not a regulatory requirement, it 

does provide a certain expectation, as an industry standard, of a greater level of monitoring.

The scoping study reported here identifies methods that might prove beneficial in realizing that 

expectation.
4
5

2. CHARACTERISTICS OF LEAKING SYSTEMS AND LEAKED 

FLUIDS IMPORTANT FOR LEAK DETECTION

Design of a subsurface monitoring network aimed at reliable early detection of radioactive liquid

leaks requires adequate knowledge of the following:

• SSCs that store or transport liquids that contain radioactive material

• SSCs that have leaked in the past or have significant risk of leaking in the future

• construction details of SSCs that may influence the nature of leaks and the ability to 

detect them (i.e., SSC materials, depth of burial, backfill material)

• characteristics of the radioactivity-containing fluids that, when leaked into the 

subsurface, may be detected by a sensor (i.e., moisture, temperature, or chemical 

characteristics that differ from ambient subsurface conditions)

The hydrogeological setting of the SSC will be a very important aspect of leak detection. Leaks 

into the unsaturated zone provide more opportunity for early detection than those into the 

saturated zone. In addition, the location of leaking systems with respect to the water table may 

not be a static property because of water table fluctuations on various time scales.

For buried piping or below-grade structures, the properties of the backfill will be a significant 

factor in leak detection. However, with the exception of structural backfill around 

safety-significant SSCs, detailed information about the backfill used is generally not readily 

available. 

The material properties of the leaking systems are extremely varied (e.g., metal tanks and 

pipes, concrete-lined metal pipes, polyvinyl chloride pipes, concrete sumps, and vaults). 

However, one useful generalization that can be made is that a large number of components are 

made entirely of metal or contain metal reinforcement. This will be a significant detail for some

geophysical survey methods.

2.1 Structures, Systems, and Components

The LLTF report (NRC, 2006) describes a variety of specific cases of leaks from NPPs. The 

report points out that some of the components that leaked are not subject to surveillance, 

maintenance, or inspection. As required by 10 CFR Part 50, “Domestic Licensing of Production 

and Utilization Facilities,” safety-related SSCs of NPPs must be designed, fabricated, erected, 

and tested to quality standards commensurate with the importance of the safety functions to be 

performed. The regulations at 10 CFR 50.55, “Conditions of Construction Permits, Early Site 

Permits, Combined Licenses, and Manufacturing Licenses,” and the guidance in Regulatory 

Guide 1.26, “Quality Group Classification and Standards for Water-, Steam-, and 

Radioactive-Waste-Containing Components of Nuclear Power Plants” (NRC, 2007), define 

these categories and standards. No American Society of Mechanical Engineers (ASME) Code 

Class 1 and 2 components (the most important for safety) are buried. Class 3 and unclassified, 

nonsafety-related components may be buried and contain relatively low pressure, low 

temperature water and are not necessarily subject to inspection.
6

In addition, the LLTF report states that “Leakage that enters the ground below the plant may be 

undetected because there are generally no NRC requirements to monitor the ground water 

onsite for radioactive contamination.” It is also clear from the LLTF report that it is very difficult 

to generalize about construction details of systems that have leaked or have the potential to 

leak. A recent presentation at the EPRI Groundwater Workshop (Coker, 2011; Riley, 2012)

provides some details specific to buried pipe systems that have leaked in 2009 and 2010. (See 

Appendix A to this report and the NRC workshop slides at Agencywide Documents Access and 

Management System (ADAMS) Accession No. ML120541054 for additional information.) More 

than 25 percent of the reported leaks were from potable water systems. About 5 percent of 

leaks contained radioactive material. Also of special concern were the 7 percent of the leaks 

that occurred in safety-related systems, the 8 percent that contained environmentally sensitive 

fluids (e.g., diesel fuel), and those leaks that occurred in systems necessary for fire prevention. 

A detailed list of SSCs that store or transport radioactivity-containing fluids will vary from plant to 

plant, but several classes of SSCs that have leaked into the subsurface at NPPs or present 

plausible risks for leaks that may reach the subsurface are discussed below and summarized in 

Table 1.

2.1.1 Spent Fuel Pools

Several plants have experienced spent fuel pool leaks. In addition, it is plausible for 

components comprising the fuel pool cooling and cleanup systems to leak. Leaks and spills 

from the refueling water storage tanks have occurred at several plants. A few plants have also 

experienced leaks in fuel transfer canals. Thus, the entire system for managing water used in 

the refueling process should be considered with respect to leaks that may affect soil or ground

water.

2.1.2 Cooling Water Discharge Systems

Cooling water discharge pipes and canals are often used for permitted discharges of radioactive 

fluids. Leaks along these lines have resulted in ground water contamination at several plants. 

In addition, cooling water may also become contaminated by primary to the secondary side

leaks in steam generators (pressurized-water reactors) or condensers (boiling-water reactors).

2.1.3 Primary Cooling Water Treatment and Storage Systems 

Primary cooling water (and in some cases secondary cooling water) is run through treatment 

systems (e.g., demineralizers) to remove impurities, including certain radionuclides. These 

systems and associated piping have leaked at several plants. Water used in the primary 

cooling system is stored in a variety of ways (e.g., condensate storage tanks, borated water 

storage tanks). Leaks from these systems have affected ground water at a number of plants.

2.1.4 Liquid Radioactive Waste Systems

Radioactive waste storage tanks and associated piping have leaked at several plants. Drains 

and tanks containing liquid waste from onsite analytical laboratories and other waste-handling

facilities at nuclear power plants can also be a source of soil and ground water contamination.

2.1.5 Storm Drain Systems

There have been many recorded instances in which spills, leaks, and sump overflows have 

caused contaminated fluids to flow into storm drain systems. In some cases contaminated 
7

water was then discharged to a storm water detention pond. The detention ponds, as well as 

joints in drainage pipes, can introduce contaminated water into the subsurface. 

2.1.6 Septic System Piping and Leach Fields

Septic systems are designed for sanitary disposal of waste water, and fluids containing 

radioactivity are not intentionally disposed of via septic systems. However, experience has 

shown that prohibited wastes are sometimes mistakenly disposed of in both domestic and 

industrial septic systems. Thus, it is plausible that accidental disposal of radioactivity containing 

fluids could happen, and such events actually have occurred at a small number of NPPs.

2.2 Small Modular Reactors

In some ways, SMRs may present a special case for leak detection technologies. Some of the 

major differences between existing plants and SMRs are size, inclusion of some systems and 

components within the pressure vessel, some advanced safety features, and, most importantly

for our purposes, subsurface emplacement of the reactor containment. Many SSCs that are 

described above for existing plants can be expected to be similar in SMRs. For example, some 

tanks and waste-handling facilities will be at the surface and have the potential to leak into the 

vadose zone. New designs are expected to incorporate measures to minimize undetected 

leaks, in accordance with 10 CFR 20.1406(c) (e.g., pipe chases, avoiding buried pipe, and new 

pipe materials). However, a very large part of the facility may be underground, with some 

excavations being well over 100 feet deep. Installed systems for leak detection around these 

buried structures may be useful. 

2.3 Potential Signals

2.3.1 Radioactivity

Appropriate selection of devices that measure radioactivity of sampled material can detect leaks 

of water-containing radionuclides. Remote detection of radioactivity in the subsurface is difficult, 

if not impossible, for tritium. However, in many cases, radioactivity may not be the optimal 

signal to use for leak detection. In many cases, tritium is the radionuclide of concern (or at least 

a leading indicator of leaks containing other radionuclides). Detecting tritium is sometimes very 

challenging because it is a weak beta emitter. In addition, most of the readily available radiation 

detection instruments provide point measurements and typically require analysis of samples 

taken from the subsurface. 

2.3.2 Moisture

For SSCs located in the vadose zone, leaks will generate a moisture anomaly that may be 

detected by a variety of methods.

2.3.3 Temperature

In many instances the temperature of the leaking fluid may differ significantly from the ambient 

soil water or ground water. Thus, temperature anomalies in the unsaturated or saturated zone 

may be a basis for leak detection.
8

2.3.4 Chemistry

In most cases, the chemical constituents of the leaked fluid will be very different than that of the 

native ground water. In the case of primary-side cooling water, the concentration of dissolved 

solids may be much lower than native water. Certain chemical components (e.g., boron) may 

provide a “fingerprint” to distinguish the leak from native water. On the other hand, blowdown 

water or radioactive waste water may have a much higher total dissolved solids concentration 

than native water.

2.3.5 Geophysical Properties

Several of the properties of leaking fluid discussed above (moisture, temperature, chemistry) 

may also create changes in the bulk geophysical properties of the subsurface (e.g., electrical 

conductivity, dielectric permittivity) that can be detected by geophysical survey methods.

2.4 Challenges of Leak Detection

The objective of early identification of leaks external to SSCs presents significant challenges. 

One challenge is that the distribution of leaking fluid in the subsurface can take several general 

forms (e.g., plume morphology). In the case of coarser grained material, fluid will move 

downward in a relatively small area as if in a conduit. In other cases in which backfill is well 

sorted and contains more fines, fluid will be distributed over a larger area within the vadose 

zone. In reality, leakage will find areas where it is distributed in or on lower permeability 

materials, as well as preferential flowpaths. This is hypothesized to be the case in the tritiated 

water leak at the High Flux Beam Reactor at Brookhaven National Laboratory (Sullivan et al.

2011, Figure 2-4). Preferential flowpaths present the substantial challenge of detecting leak 

signatures under unknown flow and dispersion regimes.

The challenge in the vadose zone is to detect anomalous water content of soil. In some 

circumstances, such as under buildings or pavement, the soil may have dried and attained a 

relatively steady state of moisture content. Leak detection under this condition should be 

relatively easy. However, many areas will be subject to influx of precipitation and experience 

substantial and rapid changes in moisture content. The challenge then is to discern a leak from 

background moisture levels that can change rapidly. Discussions with EPRI have indicated that 

it is working on identifying characteristics of water from different systems based on chemical 

composition and other features to help identify sources of leaks. This approach may help 

distinguish natural from anthropogenic water inputs. With regard to radionuclide detectors, the 

ideal would be an in situ low-energy beta detection system that is sufficiently robust that it can 

be left in place. Alternatively, a probe that can be placed in a borehole would be useful, but in 

both cases very low detection limits are needed. Another alternative could be vapor 

collection/samplers that provide information at discrete depths. EPRI is exploring this option.

Another challenge is the need for interrogation under foundations of building or pads. There are 

several documented cases in which leaks took place inside buildings and were only detected 

when contaminants emerged in the ground water from under the building (e.g., the tritium plume 

from the High Flux Beam Reactor at Brookhaven National Laboratory). Cross-well tomography 

may be well suited for this application. Another possibility is the use of existing buried pipes to 

act as electrodes for geophysical survey methods to interrogate the subsurface beneath certain 

susceptible SSCs.
9

Table 1. Systems, Components, and Potential Leak Signals

System Component(s) Potential Signal(s)

Spent Fuel Spent fuel pool

Fuel transfer canal

Heat exchangers

Filters and demineralizers

Piping

Radioactivity

Moisture

Temperature ≠ Ambient GW

Chemistry ≠ Ambient GW

Reactor Water Cleanup Filters

Demineralizers

Piping

Storage tanks

Radioactivity

Moisture

Temperature ≠ Ambient GW

Chemistry ≠ Ambient GW

Cooling Water Blowdown/discharge lines Radioactivity

Moisture

Temperature ≠ Ambient GW

Chemistry ≠ Ambient GW

Liquid Radioactive Waste Piping

Storage tanks

Radioactivity

Moisture

Temperature ≠ Ambient GW

Chemistry ≠ Ambient GW

Storm Water Detention ponds

Ditches

Pipes and culverts

Radioactivity

Moisture

Temperature ≠ Ambient GW

Chemistry ≠ Ambient GW

Septic Pipes

Leach field

Radioactivity

Moisture

Temperature ≠ Ambient GW

Chemistry ≠ Ambient GW

Microbes ≠ Ambient GW

GW = Groundwater

11

3. TECHNOLOGY DESCRIPTIONS

The focus of this report is not on detecting leaks in structures, such as pipe races, but rather on 

detecting the leaked liquid once it has left the SSC and entered the soil. The following 

paragraphs outline some of the characteristics that are important in leak detection.

The volume of soil that is interrogated by the leak detection method is a critical factor. In situ, 

single-point sensors are only able to access information from the relatively small volume of soil

near the sensor—perhaps 1 liter (L). Consequently, the sensor must be in exactly the right 

place or part of a larger array to ensure that a leak is detected. Methods in which instruments 

measure moisture as they are moved up and down in a well (e.g., neutron probes) sample a 

somewhat larger volume—from a few centimeters (cm) to perhaps 15 cm into the soil over a 

distance from the ground surface to the water table. This method describes a vertical line that is

constrained by well location. Assuming a 5-cm ring of soil interrogated and a 20-meter (m)

depth to water, this method might interrogate about 500 to 1,000 liters of soil. Other methods 

(e.g., geophysical survey methods) can interrogate much larger volumes. For example, two 

passes of ground-penetrating radar (GPR) examines a volume of perhaps 20 m deep by 1 m 

wide by the length of the path (say 10 m), giving a volume of 200,000 L. In this case, the 

position can be moved as needed unless constrained by structures. The large volume sampled 

provides greater assurance that a zone of anomalous moisture in the soil will be detected.

The advantages of many methods that provide larger sampling volumes are offset because they

often require well-trained personnel to conduct periodic surveys and to perform detailed 

analysis. In contrast, simple in situ point measurement of soil moisture can be automatic and 

essentially untended for relatively long times. While this sort of measurement needs little 

analysis, interpretation of causes and distribution of elevated moisture is not possible. The use 

of a network of in situ point sensors, since it provides better areal coverage, would help develop 

a clearer picture of changing moisture distributions.

A comparison of the various methods must consider the interpretation of the signal and its 

sensitivity, as well as the measurement uncertainty. Simple moisture detectors, assuming they 

are calibrated and working properly, provide relatively unambiguous data with measurement 

errors of about plus or minus 2-percent volumetric water content. More complex techniques 

such as those based on electromagnetic radiation (e.g., GPR) have similar errors (about 

2-percent root mean squared (RMS) based on surface wave measurements) but are potentially 

open to several interpretations, with penetration depth varying substantially with moisture 

content and sediment texture. As a result, it is typically the case that some other method (e.g.,

neutron logging of a borehole) is needed to interpret geophysical signals. In addition, surfacebased geophysical methods can be subject to electromagnetic interferences which may be 

prohibitive at an NPP. 

Frequency of data taking is another important consideration. A traditional GPR survey, for 

example, is conducted in a campaign that provides an image of the subsurface at a single point 

in time. These types of methods would need to be repeated periodically (e.g., time-lapse

surveys) to detect leaks. Other methods, especially those using in situ probes could provide 

frequent measurements, providing essentially real-time data. Related to this is the cost of 

staging periodic measurements with outside personnel as opposed to an in situ method that 

only needs a data logger system.
12

Based on this brief discussion, favorable attributes of a moisture-sensing system to detect leaks 

at an NPP include the following:

• The sampling volume should be large, especially in cross-section, or the area of concern 

is small.

• Measurements can be automated and interpretation is straightforward.

• Interferences are minimal.

• Sensitivity is adequate

• Measurements can be performed frequently.

This section briefly discusses the advantages and disadvantages of various methods for 

detection of moisture in soils, particularly with regard to their application to NPPs. For useful 

overviews of soil moisture measurement by various methods, consult Robinson et al. (2008a 

and b), Young et al. (1999), and the following Web sites:

• http://www.sjrwmd.com/floridawaterstar/pdfs/SMS_field_guide.pdf

• http://www.sowacs.com/sensors/tdr.html (the Soil Water Content Sensors and 

Measurement Web site)

• http://water.usgs.gov/ogw/bgas/ (the U.S. Geological Survey (USGS), Geophysics 

Branch Web site)

The paper by Robinson et al. (2008b) describes more methods than discussed in this 

report, such as large-scale airborne methods.

3.1 Single-Point Methods

While these small volume, single-point soil moisture methods only interrogate about 1 L of soil, 

they may be used effectively in several situations. They present several advantages, including 

making direct measurements of some parameter. (Most geophysical methods provide indirect 

measurements). These sensors can be used in areas where leakage tends to accumulate and 

be preferentially transported, such as in backfilled trenches. To overcome the disadvantage of 

interrogating a small volume, they can be assembled in arrays that provide much greater 

coverage. All of the methods discussed need to be placed into the soil to be measured, 

therefore soil has to be excavated to a selected depth in such a way that installation is possible. 

Wires and, in some cases, tubing need to be run to the surface. These methods may be 

installed when buried pipes are excavated for repair. Young et al. (1999) discuss the strengths 

and weaknesses of several of these methods.

3.1.1 Porous Block Sensors

The homogeneity, mineralogy, and structure of soil can affect measurement of moisture content 

by electrical properties, such as capacitance or resistivity. One means of solving this problem is 

to insert porous blocks containing sensors, typically a pair of electrodes, into the soil. The 

porous material equilibrates with soil moisture, and electrical properties can then be measured. 
13

These electrical properties are a function of moisture content within the block, which is 

representative of soil moisture. Calibration is necessary for the specific soil being measured. 

Some time is required for the block to come to equilibrium as its wetting and drying profiles will 

likely be quite different from the soil. As a result, these methods do not respond well to sudden 

changes in moisture content. Some sensors use gypsum blocks so that dissolution of gypsum 

provides a consistent ionic strength solution, the quantity of which is controlled by soil moisture.

These blocks can function for up to 2 years in situ. Other approaches use inert porous 

materials.

3.1.2 Tensiometers

Tensiometers measure the energy with which water is held by soil by measuring the difference 

in pressure between the soil and a saturated porous cup. Dry soil will exert more suction on the 

cup imparting a negative pressure inside the tensiometer. This is measured with a vacuum 

gauge or pressure transducer. Conversely, wet soil will exert a positive pressure. The range of 

useful measurements by tensiometers extends from about +100 kilopascals (kPa) to 

−160 kilopascals, having limited utility under dry conditions. However, this type of sensor

typically provides the most precise measurements of the instruments under consideration. The 

measurement of soil suction needs to be calibrated for specific soils to obtain moisture content.

Tensiometers need to be refilled with water periodically, although newer gel-type probes may 

need less maintenance.

3.1.3 Capacitance Probes 

Capacitance probes provide an estimate of soil moisture (as volumetric water content) by an 

inferred measurement of dielectric constant (Starr and Paltineanu, 1998; Bosch, 2004). When 

the probe is installed in soil, the soil becomes part of an electric capacitor while the probe 

comprises another part, with soil moisture determined by time required to charge the system. 

Pure water at 25 degrees Celsius (C) has a dielectric constant of 80, while air is close to 1 and 

dry soil ranges from 4–10. As a result, this measurement, while affected by temperature and 

salinity, is mostly influenced by water content. The sensors need to be embedded in the soil

and calibrated with site-specific soil; they can be connected to wireless data collection systems. 

A variety of configurations of capacitance probes exist, including two or more parallel rods 

attached to a probe head (Topp et al., 2008). Another configuration consists of one or more 

pairs of metallic, cylindrical rings separated by an insulator. These may be arranged on a rod 

that contains a number of sensors so that measurements can be made at several depths in a 

single hole, which can be several meters deep. This type of sensor reads a wide range of 

moisture content. Measurements are observed in an approximately 10-cm radius around each 

individual sensor. These multisensory capacitance probes are installed in an access tube, 

helping durability. Capacitance sensors have been extensively used in agriculture and appear 

to be robust. In fact some have been installed in a research field at the U.S. Department of 

Agriculture’s Agricultural Research Station in Beltsville, MD, for 10 years (personal 

communication to M. Fuhrmann from Y. Pachepsky, 2012). Commercial systems are available 

that allow many probes to be read frequently, and the data transmitted wirelessly. Some are 

capable of measuring temperature and conductivity as well as soil moisture.

3.1.4 Heat Dissipation Sensors

A porous medium containing air will heat up and dissipate a pulse of heat more rapidly than the 

same medium containing water. These sensors consist of a porous material, such as a 

ceramic, that contains a sensitive temperature detector and a small heater. The porous 
14

material, being in equilibrium with soil moisture, provides a defined medium for observation. A 

short heat pulse is applied and the temperature change is observed. The rate of dissipation of 

heat can be directly related to water potential. These sensors are commonly used and appear 

to be quite rugged.

3.1.5 Dual-Probe Heat Pulse Sensors

Another approach to detecting moisture is through determining specific heat of the combined 

soil and moisture (Campbell et al., 1991). Dual-probe heat sensors consist of two small parallel 

needles, one containing a heater and the other a thermistor. Knowing the quantity of heat

delivered, the change in temperature, and the distance between the heater and the temperature 

sensor allows determination of volumetric heat capacity. From this and the specific heat of soil 

solids, the volumetric water content can be estimated. These probes tend to have a small bias 

toward greater water content; however, the response appears to be linear and corrections have 

been published (Basinger et al., 2003; Ochsner et al., 2003). The volume of soil interrogated is 

small. Depending on needle length and separation, the sensed volume will be a few tens of 

centimeters.

3.1.6 Psychrometer

A number of designs exist for these sensors, with a common basis in the use of the “wet 

bulb/dry bulb” method of measuring relative humidity. The sensor contains two thermocouples. 

One is sealed within the sensor and is the “dry” bulb. The other is contained in an air chamber 

formed within a porous medium, such as a ceramic that can come to equilibrium with soil vapor. 

One thermocouple measures the ambient dry temperature while the other measures the 

temperature as lowered by evaporation of moisture in contact with the thermocouple. The rate 

of evaporation, as determined by the temperature difference, can be related to relative humidity 

and therefore the moisture potential of the soil. These sensors are commonly used in research,

especially for drier soils, and may not perform very well near saturation. While they can be left 

in situ for some time, these sensors often have the drawback of requiring maintenance, 

including replenishing the water supply. One design uses Peltier (thermoelectric) cooling to cool 

a thermocouple so that ambient moisture condenses on it in place. In this way, the single 

thermocouple provides the wet bulb and dry bulb measurement.

3.1.7 Time Domain Reflectometry

Time domain reflectometry (TDR) is an electromagnetic measurement of the apparent dielectric 

constant of a material. A probe is inserted into the material to be measured and the two-way 

travel time is measured of an electromagnetic pulse sent through the probe and reflected back 

to a measurement instrument. Moisture content is calculated based on the apparent dielectric 

constant (Topp et al., 1980). American Society for Testing and Materials (ASTM) Standard 

Method D6565, “Standard Test Method for Determination of Water (Moisture) Content of Soil by 

the Time-Domain Reflectometry (TDR) Method,” describes moisture quantification in soil by the 

TDR method (ASTM, 2000). Robinson et al. (2003) provide a detailed review of TDR. Many 

commercial instruments and probes of various configurations are commercially available. Some 

of these can be automated, multiplexed to provide measurements at many points, and the data 

transmitted. TDR requires the use of probes with two or more waveguides. These range in 

length from a few millimeters (mm) with multiple probes on printed circuit boards (Ito et al., 

2010) to several meters. The moisture content is averaged over the length of the probe. To 

obtain profiles with long probes, switching diodes can be used to separate sections of the 
15

waveguides so that specific segments of the probe can be made active or inactive. TDR senses 

a relatively small volume of material with a 1-m-long probe interrogating about 2 L of material. 

A number of factors influence the calculated moisture content, but the accuracy of the time 

delay measurement and calibration are primary. TDR instruments should be calibrated with the 

specific soil to be tested. Attenuation of the returned pulse by high ionic strength soil solutions 

or by high clay content can limit measurements, but this effect, within limits, can be used to 

estimate electrical conductivity. Temperature influences measurement by changing the cable 

length and by decreasing the dielectric permittivity of water with rising temperature, leading to 

underestimates of moisture content (Gong et al., 2003). 

3.1.8 Example of Arrays of Single-Point Moisture Sensors

The radioactive waste subsurface disposal area at the Idaho National Engineering Laboratory 

(INEEL) (Hubbell et al., 2005) provides an example of the use of arrays of moisture sensors. 

For deep observations (i.e., from 16 to 385 feet), advanced tensiometers were developed, which 

Hubbell et al. (2005) describe. For shallow measurements, less than 25 feet in depth, two types 

of sensors were used—combined soil-moisture, resistivity, and temperature (SMRT) sensors 

and direct-push Type B tensiometer (DPT) sensors. Ninety-five SMRTs were installed with 

73 percent providing data, and 66 DPTs were installed with 52 percent providing data. Many of 

these were located directly in the waste to provide measurements of wetting/drying trends. The 

DPTs required periodic additions of water, and this disturbed measurements for at least several 

weeks.

Hubell et al. (2005) and Meyer et al. (2005) evaluated the performance of these various types of 

sensors and found that the advanced tensiometers were preferred for long-term monitoring 

because they provide data year round during both saturated and unsaturated conditions. (See 

Figure 1 for an example of data from a set of these instruments.) McElroy and Hubbell (2004) 

provide additional information on using these instruments to evaluate the conceptual model of 

the deep vadose zone. SMRTs were suitable for long-term monitoring and were preferred over 

the DPTs, but were affected by temperature variations. The DPTs required extensive 

maintenance and were not able to measure in the dry range of moisture conditions. 
16

Figure 1. Data sets from advanced tensiometers showing long-term drying trends at 

different locations at INEEL (Hubbell et al., 2005)

3.2 Two-Dimensional and Three-Dimensional Methods

A number of methods, primarily geophysical, have been used to interrogate large volumes 

(relative to the small volume, single-point methods discussed earlier) of soil for moisture content

and other parameters. Some of these can be used to generate data in a two-dimensional plane,

while others can be configured to also provide three-dimensional tomographic information. The 

U.S. Environmental Protection Agency (EPA) provides useful background and describes many 

of these methods at http://www.epa.gov/esd/cmb/GeophysicsWebsite/index.html. Some of the 

configurations use probes or antennae at the ground’s surface, while others are borehole to 

borehole or surface to borehole. Typically, vertical boreholes have been used, but horizontal 

boreholes or pipes installed during construction may present even more useful configurations. 

Murray et al. (2005) reviewed and compared the state of the art of a variety of geophysical 

techniques that can be used to assess properties of the subsurface, such as stratigraphy, 

moisture content, porosity, permeability, geochemical properties, and flow patterns. The 

technologies reviewed are focused on those that would be most appropriate for use at the 

Hanford site and that are “minimally invasive,” meaning that they can be installed at the surface 

(upper 1 foot of material), using preexisting monitoring wells, or can be installed in the 

subsurface with inexpensive techniques. Murray et al. (2005) evaluated the technologies on the 

basis of functionality, state of development, and costs, among other factors. 

3.2.1 Electromagnetic Methods

Similar to the electrical resistivity (ER) methods discussed in Section 3.2.2, electromagnetic 

induction (EM) methods measure the apparent electrical conductivity (or resistivity) of a bulk 

volume of subsurface material (although using a different operating principle). In the EM 
17

approach, a time-varying current in the transmitter generates a primary magnetic field. Time 

variation of the primary magnetic field induces an electromotive force (EMF), which drives 

electromagnetic eddy currents in the subsurface. The decay of these eddy currents, which is 

governed by the subsurface electrical conductivity, generates a secondary electromagnetic field. 

The voltage measured in the receiver is proportional to the time rate of change of the secondary 

electromagnetic field. Thus, the receiver voltage can be related to the apparent subsurface 

electrical conductivity. Zhdanov and Keller (1994) provide a more detailed discussion of the 

underlying theory.

There are a wide variety of EM methods and several ways to classify them. Standard 

geophysics textbooks, such as Reynolds (2011), Sharma (1997), and Telford et al. (1990), 

provide an overview of the available types of EM surveys. EM methods may be classified 

according to the nature of source (i.e., controlled or uncontrolled), characteristics of the 

time-varying signal produced by the transmitter (i.e., time domain or frequency domain 

methods), and configuration of the transmitter and receiver. The following discussion will focus 

on controlled source electromagnetic methods (CSEM), since they are more appropriate for the 

application addressed in this paper. CSEM methods are defined as those which incorporate a 

transmitter device so that the user has control over the electromagnetic field transmitted. This 

distinguishes CSEM methods from those that rely on uncontrolled sources, such as natural 

electromagnetic fields or very low frequency radio signals. CSEM methods may be very useful

for shallow subsurface investigations. CSEM methods can image deeper into the subsurface 

than ground-penetrating radar (GPR) and typically provide better resolution of small features 

than direct current resistivity (Everett and Meju, 2005). Similar to capacitive resistivity (CR) 

methods discussed in Section 3.2.2, CSEM methods do not require electrodes to be inserted 

into the subsurface and can therefore be mounted on a mobile platform, allowing for relatively 

rapid surveys of large areas. Everett and Meju (2005) and McNeill (1980a, b) discuss CSEM 

methods in more detail. In particular, Everett and Meju (2005) discuss techniques for dealing 

with noise in CSEM signals, which will be a concern at operating power plants.

3.2.1.1 Frequency Domain Electromagnetic Methods

In frequency domain electromagnetic methods (FDEM), the transmitter uses a fixed-frequency 

oscillating current. The amplitude and phase differences between the primary and secondary 

electromagnetic fields are then used, along with the transmitter-receiver intercoil spacing, to 

calculate an apparent soil electrical conductivity (or resistivity). An advantage of this approach 

is that equipment is available that allows one to select from multiple frequencies (typically 

ranging from 100 hertz (Hz) to 50 kilohertz (kHz)) to tailor the subsurface investigation to certain 

depth ranges and to avoid known noise sources. FDEM methods have proven to be well suited 

to rapid reconnaissance mapping of lateral changes in near-surface conductivity (Everett and 

Meju, 2005). 

In practice, this approach is generally applied as terrain conductivity meters. Measurements are 

made in relative conductivity, and interpretation is qualitative and best applied to searching for 

anomalies. Surveys are conducted by moving a transmitting and a receiving antenna across 

the surface of the ground at a fixed distance to each other. Some systems are small enough 

that they can be carried manually along a transect. Measurements are taken periodically, 

perhaps every 20 feet. The depth of investigation depends on antenna spacing—a depth of 

49 feet for 20-foot spacing and a depth of 98 feet for 40-foot spacing. An extensive bibliography 

of applications is available at http://www.dualem.com/abib.html, and at

http://www.cflhd.gov/resources/agm/ . The technique has been used to map soil and ground

water salinity in agricultural fields at depths of about 1 m (McNeill, 1980a). Other applications
18

have included surveying subsurface acid mine drainage and saltwater intrusion in a coastal 

setting (see the EPA Web site cited above for additional information). Commercial systems are 

available. 

3.2.1.2 Time Domain Electromagnetic Methods

In time domain electromagnetic methods (TDEM), the typical transmitter wave form is a rapid

rise to a steady value followed by a rapid shutoff, as shown in Figure 2, with a cycle of about 

1 millisecond. The transmitter current is sent through a large loop of wire (transmitter antenna) 

and induces an electromagnetic image that is propagated through the media above and below 

the antenna. As this image (Figure 3) moves through the medium, perturbations in conductivity 

generate eddy currents that can be measured at the receiver. A depth profile of conductivity 

can then be determined. 

Figure 2. Transmitted and received signals from TDEM instruments (U.S. Army 

Corps of Engineers, 1995)

Figure 3. Schematic of TDEM process showing the transmitter and receiver antennae 

and the EM image propagating away from the transmitter antenna over time 

(U.S. Army Corps of Engineers, 1995)
19

The advantage of this approach is that the relatively weak secondary magnetic field is 

measured during the transmitter off time. In addition, various transmitter and receiver 

configuration and spacing combinations may be used to image depth zones of interest to a 

particular application. The transmitter antenna is generally a square that can have dimensions 

of perhaps 50 or 100 m. A single measurement may require about 100 microseconds, and 

perhaps 1,000 measurements are stacked to provide usable data. The depth to which 

conductivity measurements can be made depends on the current used in the transmitter,

antenna size, and the material being interrogated. However, depths of 100 m or much more 

can be assessed. Surface or aerial surveys can be conducted to develop one-dimensional, 

two-dimensional, or three-dimensional images of the subsurface (Everett and Meju, 2005). 

Induced interferences from power lines present a problem as does the presence of metallic 

objects not only in the subsurface but the surface as well. 

3.2.1.3 Ground-Penetrating Radar

Ground-penetrating radar (GPR) is a geophysical survey method that uses radiofrequency 

electromagnetic radiation (40–1,500 megahertz (MHz)) to detect objects and changes in soil 

properties. Transmitting antennae in contact with the ground produce an EM signal. Receiving 

antennae observe the direct or reflected signal, from which the time delay and intensity can be 

determined. This imaging technique uses differences in wave velocities of returned signals that

are a function of the dielectric constant of the subsurface media, which in general, is related to

the moisture content of the medium (e.g., soil). (See Huisman et al. (2003) for details about the 

method and a variety of applications.) In addition to moisture content, GPR can be used to 

determine the electrical conductivity of the medium by measuring the attenuation of the radar 

signal (Lane et al., 2004).

The typical configuration for GPR is an antenna array at the ground surface. This configuration 

is used to search for buried objects or to assess the moisture content of agricultural soils. The 

antennae are moved to produce readings over a large volume. Spacing of antennae can be 

important, and an initial test of antenna configuration is common during a survey. Another 

configuration has been tested in which the antennae are located down wells several meters 

apart (Alumbaugh et al., 2002). Figure 4 illustrates several configurations for downhole GPR. 

These same arrangements can also be applied to some other geophysical methods. 

GPR is of interest because it appears to be sufficiently sensitive to changes in soil moisture 

such that it could readily observe anomalous moisture zones in the soil. The RMS error in 

volumetric water content taken by tomographic GPR is estimated to be 2–3 volume percent

(Alumbaugh et al., 2002). Grote et al., (2003) reported the RMS for 29 estimates as between 

2.2 and 1.5 volume percent. Use of site-specific soil moisture and texture measurements to 

calibrate GPR is necessary for high-quality measurements. 
20

Figure 4. Three configurations for downhole GPR (Lane et al., 2004). Level-run 

geometry is also called “zero-offset.” VRP geometry refers to vertical radar 

profile geometry. These configurations can also be used with some other 

geophysical methods, such as resistivity profiles or tomography.

The volume sampled by GPR depends on a number of factors that include antenna 

configuration; the frequency of use; and the texture, homogeneity, and moisture content of the 

soil. Under ideal (dry, homogenous) conditions, penetration depth can be up to 30 m, but more 

commonly depths are only a few meters. Nevertheless, this method can describe a plane that 

covers a substantial area. Several scans spaced 1 m apart can interrogate a large volume of 

soil with a measurement error of about 2–3 volume percent. 

A serious limitation of GPR, especially for surface-deployed systems, is that it is subject to 

electromagnetic interferences. This is not necessarily an issue for many applications, but at a 

power generation plant it could in fact limit the use of this method.

In cross-borehole GPR, the antennae are placed down boreholes to sample a plane between 

two wells. This has several advantages, including shielding from electromagnetic interference 

and the ability to interrogate to greater depths than surface methods. In the downhole mode,

GPR can be used in two ways (Lane et al., 2004). One approach, the zero-offset profile,

produces simple measurements of average moisture content at discrete depths. With this 

technique, the transmitter and receiver (in different wells) are moved simultaneously so that they 

are at the same depth, providing a simple measurement of travel time on the line between the 

two antennae (Binley et al., 2001). The other approach uses sets of ray paths between 

receivers and transmitters at different depths. This computationally intense method is used to 

generate tomographs. Alumbaugh et al. (2002) compare the moisture content from 

cross-borehole GPR and neutron probe for five wells. In most cases, the general trends of 

moisture content with depth are similar for the two methods. The GPR system tends to average 

out local peaks that are observed by the neutron probe. However, in this case the resolution of 

GPR could be improved by smaller intervals. 

Cross-borehole GPR can be applied to horizontal subsurface wells to observe vertical moisture 

movement in certain suspect areas. Because the cross-borehole approach is used in a fixed 

location in a periodic monitoring mode, the soil could be well characterized and the time series 

of measurements would be available for comparisons, making interpretation easier. This 

method is especially sensitive to signals from dense materials, such as pipes in the subsurface. 
21

As a result, while this method may be good for finding pipes, the signal from them may mask 

subtler signals from changing moisture content.

3.2.2 Electrical Resistivity Methods

Electrical (direct current) resistivity methods have been widely applied in fields such as mining, 

oil and gas exploration, construction, water resources engineering, and ground water 

remediation (Telford et al., 1990). Many of these applications have focused on the 

measurement of subsurface water content. In its most elementary form of application, the ER

method comprises propagation of an electrical current between two electrodes (“current” 

electrodes) inserted at the ground surface, while information on the induced electric field is 

obtained by measuring the voltage between a second pair of electrodes (“potential” electrodes) 

also inserted at the ground surface The applied current and the resulting measured voltage, 

together with knowledge of the electrode spacing and arraignment, allow determination of the 

bulk resistivity of a subsurface soil volume. Resistivity measurements can be conducted in a 

variety of configurations. Useful information can be obtained from time series of voltage values,

or data can be used to generate images. These can be relatively simple two-dimensional

interrogations of a plane through the subsurface to much more complex three-dimensional or 

tomographic imaging techniques that require sophisticated computational analysis. For

three-dimensional imaging, electrical resistance tomography (ERT) is a method that calculates

the subsurface distribution of ER from a large number of resistance measurements made from 

electrodes on the ground surface or in boreholes, or both, to produce images of vertical or 

horizontal sections. Depending on the application, resistivity variations can then be related to 

geology, subsurface moisture content, porosity, temperature, and pore fluid chemistry.

For soils comprised of sands and silts (i.e., negligible clay and organic fractions), bulk soil 

resistivity will be directly proportional to the soil solution resistivity and vary inversely with 

porosity and saturation (i.e., inversely with volumetric water content). The soil solution resistivity 

is, in turn, a function of ion concentration, ion type (since different ions have different mobilities), 

and temperature. One should note that, because the soil solution resistivity depends on ion 

concentration, it is often interrelated with soil moisture content. For example, as a soil dries 

(moisture content decreases), the dissolved ions become more concentrated in the soil solution 

that remains and the soil solution resistivity decreases.

Seasonal to daily fluctuations in temperature and moisture may cause resistivity variations 

greater than 50 percent of mean values. For example, the resistivity of frozen sand or silt is 

extremely high since ion mobilities are reduced to near zero. However, in spite of the significant 

natural temporal variability, the spatial patterns of soil resistivity are often remarkably stable. 

Even small weight percentages of clay and organics can have a significant influence on 

resistivity because clay minerals and organic matter often coat the surfaces of sand and silt 

particles, reducing bulk soil resistivity by providing exchangeable ions that lower solution 

resistivity. 

For leak detection at NPPs, ERT methods can be used to detect changes in subsurface 

conditions rather than precise measurement of water content. Several different changes in the 

subsurface conditions associated with a leak could produce detectable changes in bulk soil 

resistivity in the soil volume affected by the leak. Soil moisture change (for leaks above the 

water table) is one obvious signal that could be detected. The challenge with respect to soil 

moisture changes will be differentiating between natural soil moisture changes and those 

caused by leaks. In this regard, looking at changes in spatial patterns of soil moisture rather 

than focusing only on a time series of moisture measurements at a point or in a small volume 
22

will be beneficial. Time-series data will be useful, but only after a sufficient length of record has 

been collected to determine natural patterns and establish correlations with other observations,

such as local precipitation events. 

If the concentration or type of dissolved ions in the leaking fluid differs significantly from the 

ambient soil water, then there may be an observable change in bulk soil resistivity. This type of 

change should be detectable whether the leak has occurred above or below the water table. 

Similarly, if the leaking fluid differs significantly in temperature from ambient soil water, it may 

produce detectable changes in bulk resistivity. 

A variation on the ERT method that may have applications at NPPs is cross-borehole ERT (e.g., 

see Daily and Owen, 1991; Binley et al., 2002). In cases in which electrodes cannot be left at 

the surface, two or more boreholes can be drilled on either side of the structure. Electrodes are 

installed at several depths in the boreholes and computerized control provides an array of 

measurements. A two-borehole arrangement will allow for a two-dimensional image of the 

subsurface. Three or more boreholes will allow for a three-dimensional survey, but these are 

computationally very intensive. The acceptable distance between boreholes can be limited by 

signal attenuation and may be only a few meters. 

Using electrodes to contact the ground is called “galvanically coupling.” Another approach, 

discussed later, is termed “capacitively coupling,” in which electrodes are not inserted into the 

ground but can be moved over the surface. Capacitive coupled resistivity (CR) is a geophysical 

technique designed to extend the scope of the conventional methodology to environments in 

which galvanic coupling is notoriously difficult to achieve, such as pavement, dry soils, or 

snow/ice cover (Douma et al., 1994; Kuras, 2002; Kuras et al., 2006). CR systems use a 

capacitive-coupling approach to introduce electric current into the ground and to measure 

potential differences at the soil surface. This capacitive-coupling is accomplished using the 

capacitance of an antenna (commonly a coaxial cable) to couple an alternating current (ac)

signal into the ground (Geometrics, 2001). Existing CR systems use a fixed, high-frequency ac

current (10–20 kHz) to induce an ac current in the soil (Allred et al., 2008). In this way, no direct 

contact is needed between the instrument and the ground. Essentially, a large capacitor is 

formed by the coaxial cable and the soil surface. The metal shield of the coaxial cable is one of 

the capacitor plates, and the soil surface is the other capacitor plate, with the outer insulation of 

the coaxial cable acting as the dielectric material separating the two plates. The system 

transmitter applies ac to the coaxial cable side of the capacitor, in turn generating ac in the soil 

on the other side of the capacitor. 

With regard to the receiver, a similar phenomenon occurs, except in reverse. The ac in the soil 

charges up the capacitance of the coaxial cable, which is measured to determine the potential 

difference (voltage) generated by the electric current within the soil (Allred et al., 2008). The 

transmitter and receiver are usually deployed in a dipole-dipole configuration in which the 

transmitter and receiver are placed in line and separated by an integer number of dipole lengths 

(Geometrics, 2001; Allred et al., 2008). By using line electrodes with different lengths and 

changing the separations, it is possible to vary the penetration depth, which can extend down to 

approximately 20 m (Geometrics, 2001) The CR methodology allows the use of towed sensor 

arrays, thus enabling the rapid collection of high-resolution resistivity data. Tomographic 

imaging is possible using such datasets acquired with moving arrays (Allred et al., 2006; Kuras 

et al., 2007).

Resistivity measurements, as mentioned earlier, can be set up with lines of electrodes at the 

ground’s surface or with electrodes inserted at discrete depths in the subsurface (electrodes 
23

have been developed for direct push-hole boring techniques). Resistivity survey techniques that 

take advantage of existing subsurface infrastructure (e.g., steel-cased wells, metal pipes) by 

using them as current electrodes are a recent advance that may prove useful at NPPs (Ramirez 

et al., 1996; Calendine et al., 2011). ER measurements can be conducted from systems that 

consist of an electrode array and associated electronics that can be towed along the ground 

surface behind a small vehicle, allowing rapid measurement along transects without the need to 

insert and remove electrodes. These are continuous resistivity measurement techniques that 

use either galvanic contact or capacitive-coupling approaches. It is likely that electromagnetic 

interference may be a problem for this approach, but in an NPP environment this type of survey 

may be appropriate for use away from paved areas (e.g., for long effluent discharge lines). 

Paved areas (e.g., concrete, asphalt, gravel) will clearly pose difficulties as will very dry surface 

soils. In addition, winter conditions (e.g., ice-covered, snow-covered, or frozen ground) may 

prevent sufficient galvanic contact for the method to work properly. 

3.2.3 Some Approaches for the Use of Electrical Resistivity Methods for Leak Detection

The following examples illustrate approaches that may be used for leak detection. Some are 

based on presentations and discussions that occurred during the NRC’s Workshop on Early 

Leak Detection, held on February 15, 2012 (see the abstracts in Appendix A to this report and 

slides from that workshop at http://pbadupws.nrc.gov/docs/ML1205/ML120540481.html). 

3.2.3.1 Leak Rate and Volume Detection on a Tank Farm

As part of the Hanford work to address leak detection, monitoring, and mitigation (LDMM) from 

high-level waste (HLW) tanks, researchers subjected six technologies, five of which are 

geophysical, to preliminary field testing. Evaluations of these methods (Bratton, 2002) are 

based on the results of field work on a mockup of a tank with simulated leaks. This report also 

discusses the maturity of the techniques and describes various projects using the methods. 

Rucker (see Appendix A to this report and the presentation slides) presented results of ER

measurements for tests conducted at a mock tank, as well as subsequent tests at actual tank 

farms at Hanford. These sites include buried pipe and other infrastructure. Figure 5(a) shows 

the configuration of the mock tank; note the different locations and arrays of electrodes for ER. 

Water injection took place from a well screened at 20–30 feet below ground surface at rates that 

ranged from 4–16 gallons per hour. Some methods use steel wells as electrodes; others use 

electrodes that were emplaced by cone penetrometer. Figure 5(b) shows data for the tests in 

terms of leak volume (in gallons) based on conversions of voltages using the high resolution 

resistivity-steel casing resistivity technique (HRR-SCRT). Leaks were readily detectable by this 

method and both leak rate and volume could be estimated. Later tests detected leak rates as 

low as 2 gallons per hour; typical leak volumes were about 1,000 gallons. Figure 5(b) presents

four data traces for measurements made using the four steel well casings around the tank 

(indicated in Figure 5(a)) as potential electrodes. The injection well or the tank itself was used 

as the excitation electrode. This method does not provide spatial information for leak location 

but is sensitive to volumes and rates.

Later tests at the Hanford HLW tank farms were conducted to assess the ability of geophysical 

monitoring systems, especially ER methods, to detect leaking HLW and to function in an 

industrial setting. Figure 6 (Calendine et al., 2011) shows the effects of a number of events and 

environmental changes from August 2010 to January 2011. Particularly interesting is the effect 

of cathodic protection on the noise level of the signal compared to a short interval when the 
24

protection was turned off. No leaks were detected during this period.

(a)

(b)

Figure 5. (a) Plan of Hanford mock tank and the locations of wells used as electrodes

and (b) After Rucker, 2012, data from the four wells showing volumes of 

leak tests (derived from voltages). Leak rates can be determined from the 

slopes of voltages.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

TEST DAY

2800

3000

3200

3400

3600

3800

4000

4200

4400

4600

4800

5000

5200

CALCULATED VOLUME (HRR-SCRT Voltage Units)

Well Casings

NE

SE

SW

NW

artificial "flood"

event

sheet piling

disconnected

Rate=4 GPH

Vol.=2100 Gal

Rate=19 GPH

Vol.=900 Gal

Rate=15 GPH

Vol.=800 Gal

Rate=10 GPH

Vol.=1050 Gal

Rate=13 GPH

Vol.=650 Gal

Rate=13 GPH

Vol.=1200 Gal

Rate=9.5 GPH

Vol.=1000 Gal

Rate=10 GPH

Vol.=1150 Gal

Rate=14 GPH

Vol.=800 Gal
25

Figure 6. Analysis of various events for resistivity monitoring of Tank C-104 at 

Hanford for about 5 months. Of particular interest is the difference in the 

data when cathodic protection is on and when it is off (Calendine et al., 

2011).

3.2.3.2 Moisture Location, Imaging, and Impact of Buried Pipes

Perhaps the most NPP-relevant example of using ERT for leak detection is the Hanford Tank 

Farm (Rucker et al., 2008; Calendine et al., 2011; Johnson, 2012), at which an ERT-based 

program for monitoring and leak detection has been deployed since 2004. Many of the 

challenges presented by the industrial nature of the Hanford site (e.g., extensive subsurface 

piping, grounded electrical infrastructure, cathodic protection), as well as natural features such 

as diurnal and seasonal fluctuations in temperature and soil moisture, are similar to those that 

might be expected at a NPP. The problem of drilling in an infrastructure-rich location can be 

overcome by using existing steel monitoring wells as long electrodes (Rucker et al., 2012). ER

imaging techniques can provide detailed spatial information on subsurface electrical properties. 

For leak detection, this technique can be applied in time series and examined for differences in 

moisture content. Figure 7 illustrates infiltration of rainwater into the subsurface at 
26

Hanford, WA, over a 9-week period. Also shown is the surface array of electrodes used to 

generate the data. 

Figure 7. ERT showing infiltration of rain over a 9-week period at the Hanford 300 

Area. Also shown is the array of electrodes used for the ERT. Dimensions 

are in meters.

Figure 8 shows the results of ERT used to determine changes in soil moisture as a result of a 

desiccation process intended to retard the transport of technetium-99 under old liquid waste 

discharge cribs at Hanford. The waste was an aqueous solution high in nitrate salts providing a 

target of high (180 microsiemens per centimeter) electrical conductivity. Nitrogen is injected into 

the subsurface through a well and then extracted from another well 15 m away, carrying 

moisture with it. Imaging indicates that moisture is removed from zones of high permeability,

but is retained in stringers of finer grained sediment. Images were autonomously produced 

twice per day and show a growing area of lower soil moisture over time. 
27

Figure 8. ERT-derived subsurface monitoring for the vadose zone desiccation at the 

Hanford B-C cribs (Johnson et al., 2012)

ERT now can be run autonomously and frequently. Advances such as computer-controlled

multielectrode systems greatly simplified the task of conducting surveys along long transects or 

large grids. Electrodes can be installed in cement and have been used for 11–12 years. A soil 

moisture monitoring system has been installed at a site at Hanford, WA, where a treatability test 

for moisture removal is underway. A variety of soil moisture and water potential probes are 

used, as well as cross-hole GPR and daily ERT surveys. The ERT system consists of 

15 electrodes in each of nine wells, giving daily images at submeter resolution (personal 

communication, Tim Johnson and Mike Truex, March 7, 2011).

At the Brandywine Superfund site in Maryland, remediation of volatile organic compounds 

(primarily trichloroethylene) used bioaugmentation to degrade the contaminants. As part of the 

monitoring system, an autonomous time-lapse ERT (downhole and surface electrodes) system 

was used that automatically acquired field measurements, managed and processed the data,

and provided images. This system gave near real-time results showing the distribution of 

1,000-gallon injections of molasses into the subsurface to stimulate microbial degradation of the 

volatile organic compounds (http://water.usgs.gov/ogw/bgas/estcp/bioremed-monitor.html,

Versteeg et al., 2010). 

3.2.4 Nuclear Magnetic Resonance Sensing

Magnetic resonance surveys (MRS) measure the magnetic resonance response of protons in 

bulk water molecules after excitation by an ac current that induces a change in magnetic field. 

Unlike other geophysical methods that are indirect methods with nonunique interpretations (e.g.,
28

they can be influenced by variations in mineralogy, temperature, conductivity of the solid, and 

ionic strength of the ground water), MRS is a direct measurement of the presence of bulk 

protons. Typically this means water, although organic fluids and hydrocarbons also respond to 

MRS. Lubcynski and Roy (2004) discuss the theory of the process; USGS also provides 

additional information at http://water.usgs.gov/ogw/bgas/mrs/. The one commercial instrument, 

the NUMIS system, is described at www.heritagegeophysics.com.

Briefly, bulk protons normally align themselves to the local magnetic field of the earth. Using a 

loop of electric cable (often a square from 20–150 meters on a side) on the ground, an ac pulse 

is applied that indices a magnetic field that realigns the protons. The frequency of the pulse is 

important and needs to be tuned to the Larmor frequency, which is a function of the local 

strength of the earth’s magnetic field. When the excitation current is turned off, the protons 

return to their original orientation, and this signal is measured in the same loop of wire. Two 

components of the signal are used for interpretation; amplitude indicates the number of protons 

excited and thus the water content and the decay rate which indicates the pore size (Vouillamoz 

and Legchenko, 2010). Faster decay rates result from frequent collisions of water with the solid 

phase (small pores); even faster rates (i.e., less than 30 milliseconds) indicate water bound to 

the solid (e.g., in clays). These fast decay times are typically filtered out so that only free water 

is detected.

MRS can interrogate a large volume of soil and can provide information on both the vadose and 

saturated zones. The volume is a function of antenna size; for a 100-meter antenna, the 

volume analyzed is about 2 million cubic meters. Depth of analysis is 1 to 1.5 times the antenna 

length; field experience indicates that the depth of shallow, horizontal strata can be placed with 

an accuracy of about 1 m. Sensitivity of the method for water content is not clear, but typical 

graphic data show variation of water content of about 1 percent. Usually MRS data are 

interpreted as one-dimensional models of water content versus depth. A two-dimensional

tomographic inversion is reported to provide more accurate data than the one-dimensional

approaches (Hertrich et al., 2007).

This method is susceptible to electromagnetic noise from a variety of sources (Lubczynsi and 

Roy, 2004). Such sources can be both natural and artificial and include lightning strikes (some 

at a large distance), noise from power lines, ground cables and working electrical machinery,

and conductivity of subsurface media. Interference may be especially problematic when it is 

close to the Larmor frequency.

3.2.5 Fiber-Optic Distributed Temperature Sensing

Distributed fiber-optic temperature sensing (DTS) systems are commonly used to detect leaks in 

pipelines for oil, gas, and brines (Nikles et al., 2004) and for fire detection. Recently these 

systems have been applied to a variety of near-surface hydrologic processes, including 

measuring the positions and dynamics of lake thermoclines, determining energy exchanges at 

stream/atmosphere interfaces, and finding underwater springs where ground water flows into 

river water. In other cases, they have been used to detect illegal discharges into sewer lines 

(Hoes et al., 2009). Down-borehole and mining applications include detecting locations of water 

flow into wells and mines and temperatures in oil wells. Selker et al. (2006) offer several 

examples.

With DTS methods, temperatures are commonly measured in 1-m increments over many 

kilometers (km) to an accuracy of 0.1 degree C. Greater spatial and temperature resolution are 

possible with longer integration times and better lasers and detectors. The fiber-optic cables 
29

used for temperature detection are the same as used for telecommunications; no sensors along 

the cable are needed. Temperature and distance measurements are obtained by measuring 

scattered light from a laser pulse. Distance is determined by timing scattered light returns. 

Raman scattered light is shifted slightly in wavelength from the original pulse. The returned 

scattered light contains both Stokes (lower frequency) and anti-Stokes (higher frequency) 

components. The amplitude of the anti-Stokes component changes as a function of 

temperature while the Stokes component does not. As a result, temperature can be determined 

from the ratio of the Stokes and anti-Stokes peaks. Tyler et al. (2009) provide useful 

observations on applying and deploying DTS systems.

Measurement of temperature may apply directly to leak detection, assuming that the 

temperature of the leaking water differs from ambient temperatures and that the sensor is close 

enough that the difference is still measurable. However, slow leaks or those at ambient 

temperatures may not have a clear thermal signature. Direct measurement of moisture content 

by DTS has been attempted by measuring changes in soil thermal properties as a function of 

moisture content. Conceptually, this is a standard approach to soil moisture measurement at 

single points, but the distributed sensor system allows large areas to be assessed. One attempt 

used diurnal temperature differences of a shallowly buried cable to determine soil moisture 

(Steele-Dunne et al., 2010). This attempt was not especially successful. However, an active 

DTS method can be used in which a heat pulse is applied to the soil and the response of 

temperature over time is observed. Laboratory tests have been conducted in a sand column 

measuring 0.61 m x 1.46 m using 31.5 m of fiber-optic cable. This commonly used type of cable 

contained, in addition to the two optical fibers, stainless steel strands, which were used as an 

electrical resistance heater (Sayde et al., 2010). By heating the cable in a short pulse and then 

measuring the change in temperature over time, the moisture content was calculated.

DTS has been used to assess the location and quantity of ground water discharging into the 

subsurface of water bodies. This approach may have application in assessing ground water 

discharges to water bodies close to nuclear facilities, where the ground water may contain 

contaminants. USGS has demonstrated the use of a fiber-optic distributed temperature sensor 

in evaluation projects (http://water.usgs.gov/ogw/bgas/fiber-optics/) for freshwater and estuarine 

systems. For example, at Waquoit Bay, MA, the contrast of temperature between bay water 

(16–29 degrees C) and ground water (11 degrees C) was used to assess input of freshwater to 

the bay. A 1.3-km-long cable was used and was deployed in a grid pattern of 60 m x 80 m. 

Measurements were taken at about 1-minute intervals over a 2-week period (Day-Lewis et al., 

2006).

Discharge of uranium-bearing ground water to the Columbia River at the Hanford site has been 

investigated by a combination of DTS and continuous waterborne electrical imaging (CWEI)

methods. The purpose of these investigations was to define specific locations where ground

water discharges to the river. Data generated by DTS using four cables, each laid parallel to a 

1.6-km section of the river showed cool water anomalies during summer when the river was 

warmer than the ground water and warm water anomalies when the river was colder than the 

ground water (Mwakanyamale et al., 2012). These anomalies are only observed at the low river 

stage (which varies diurnally by control from a nearby dam) because the elevated water stage 

actually drives water back into the bank. DTS was able to indicate ground water discharge 

locations using a two-dimensional, time-frequency analysis with 4-day periods (see Figure 9). 

CWEI was used to determine changes in lithology under the river, as well as to estimate specific 

surface area normalized to pore volume of the different geologic units outcropping under the 

river. The combined methods showed that ground water exchange with river water is strongly 
30

controlled by the thickness of the transmissive lithological unit, with the exchange being focused 

at springs that are sometimes controlled by paleochannels (Slater et al., 2010).

Figure 9. Transform analysis at two periods. The 4-day period shows strong 

correlation between stage changes and focused exchange of ground water 

with river water, while the 1-day period shows no discrimination (Slater et 

al., 2011). The central panel shows the thickness of transmissive layers 

where ground water exchanges with river water.

3.3 Nuclear Methods for Water Content

3.3.1 Neutron Probe

This well-developed technology is often used to provide reference measurements of soil 

moisture against which other methods are compared. ASTM Standard Method D6031, 

“Standard Test Method for Logging In Situ Moisture Content and Density of Soil and Rock by 

the Nuclear Method in Horizontal, Slanted, and Vertical Access Tubes,” is a standard test 

method that includes thermal neutron moisture content and bulk density by backscattered 

gamma rays (ASTM, 2010). The instrument operates by emission of fast neutrons from a 

sealed source that are slowed (thermalized) by elastic collisions with atoms in the ambient 

environment. Hydrogen atoms are by far the most effective (although boron, cadmium, iron, 

and chlorine can interfere to some extent because of their high cross-sections for neutrons). 

Typically water contains by far the greatest quantity of hydrogen in soils; in the absence of 

hydrocarbons, the flux of thermal neutrons is proportional to water content of the soil.

This is a downhole method. The instrument consists of two parts—the downhole probe and the 

counting system. The probe contains a sealed source of fast neutrons, typically a mixture of 

americium-241 and beryllium powder, with activities of americium-241 ranging from 30 to 
31

100 millicurie. Alpha particles interacting with beryllium produce fast neutrons that range in 

energy from 2 to 10 million electron volts. The probe also contains a detector that observes 

thermal neutrons but not fast neutrons. The counting system remains at the ground surface and 

contains electronics and power supplies 

The method is used with a cased borehole. Aluminum casing is essentially transparent to 

neutrons, but steel is also acceptable. As little air gap as possible between the casing and the 

probe is desirable. The casing must be in direct contact with the ambient soil. The sensing 

volume of the method is greater than many other methods, with a radius of 10 to 40 cm,

depending on moisture content providing an interrogated volume at each point of as much as 

100 L (ASTM, 2010). An essentially continuous set of measurements can be conducted down a 

well. The accuracy of volumetric water content measurements is about plus or minus 

0.005 ft3

/ft3. For high precision information, site-specific soils with known moisture content 

should be used for calibration. 

3.3.2 Gamma Ray Attenuation

Gamma-rays passing through a heterogeneous earth material are adsorbed and scattered by 

the materials. The degree to which a collimated beam of gamma rays is attenuated by a given 

thickness of material depends on the energy of the gamma rays, the bulk density of the 

medium, and the elements of which it is comprised. This method therefore requires a source of 

monoenergetic gamma rays and a detector with the sample between the two. This means that 

the source and the detector are lowered into separate but nearby boreholes. Two source 

radionuclides (typically containing on the order of 100 millicuries) are commonly used—

cesium-137 at 661 kiloelectron volts (keV) and americium-241 at 61 keV. The detector (e.g., a 

sodium iodide detector) must be able to distinguish between the two energies. This method is 

often used in the laboratory because it does not disturb the soil and has high precision; it is 

infrequently used in the field because of the high activity radionuclides and the inconvenience of 

needing two boreholes.

33

4. DETECTION METHODS FOR RADIONUCLIDES

While this report thus far has focused on the detection of moisture as a means of early detection 

of leaks in the subsurface, detection of anthropogenic radionuclides can be a more direct and 

sensitive indicator of leaks from NPPs. The radionuclide that has been most frequently 

observed at leaks has been tritium, although comparatively small amounts of other 

radionuclides, such as cobalt-60, strontium-90, and cesium-137, have been observed in soil and 

water as a result of leaks. A few radionuclides, tritium, iodine-129, and carbon-14, are 

sufficiently volatile that they can be observed in soil gases (ANS, 2010). Tritium (half-life of 

12.35 years) is the radionuclide of interest because it is the radionuclide that most readily 

migrates in the subsurface and therefore can be a leading indicator of other contaminants.

The typical method for tritium quantification in water is by liquid scintillation counting of samples 

in a laboratory instrument. This method is based on detection of light emitted by a scintillator 

material when the material is excited by beta radiation. Standard analytical methods (i.e.,

ASTM D4107, “Standard Test Method for Tritium in Drinking Water” (ASTM, 2008)), state that,

with appropriate instrumentation, sample size, and count times, the detection limit for tritium is 

less than 1,000 picocuries per liter (pCi/L), with the maximum contaminant level for tritium in 

drinking water being 20,000 pCi/L. The method stipulates chemical treatment and distillation of 

a water sample. The distillate is collected, added to scintillation fluid, and then counted. 

However, a variety of other methods have been developed for tritium detection, some of which 

are for surface contamination and others for analysis of gases or liquids. In some cases, silica 

gel is used to sorb moisture from vapor samples for tritium analysis. Typically, moisture is 

desorbed from the gel, collected, and analyzed by liquid scintillation counting. Many flowthrough systems have been developed using any of a number of scintillator materials, such as 

some organic crystals or coatings (with anthracene producing the most light), plastics, ceriumactivated lithium glass, calcium fluoride doped with europium, yttrium glass, and yttrium silicate. 

Some flow-through systems mix liquid scintillation fluids with the water to be analyzed. If the 

approach is taken to test water samples for tritium, then several commercial systems are 

available that provide flowthrough cells (Marsh et al., 2007) with detection limits of about 

100,000 pCi/L. 

In another approach, helium-3, the stable daughter of tritium can be analyzed by mass 

spectrometry and the ratio of helium-3 to helium-4 is used to indicate the presence of tritium. 

This method requires sampling of soil vapor into pressurized gas sample “bombs” and relatively 

sophisticated analytical methods. 

Our interest is in situ methods. Radionuclide detectors should have the following characteristics 

ideal for this application:

• They should to be able to detect low-energy beta emissions (i.e., tritium).

• They need to be sufficiently robust that they can be left in situ for long periods or provide 

measurements of the subsurface remotely.

• They should be able to interrogate large volumes of soil or soil vapor.
34

In addition, correction for background radiation is necessary, and the use of expendables such 

as gas is undesirable. Of the many types of radionuclide detectors, few if any have all of these 

characteristics. Because of the low-energy beta radiation of tritium (average of 5.6 keV with a 

maximum of 18.6 keV), remote detection is not possible. The material to be counted must be 

very close to or in contact with the detector material since the range of beta rays from tritium is

about 6 micrometers in water. Most detector systems consist of electronics that are too delicate 

and expensive to be considered for in situ use in soils. Moreover, for detectors placed in the 

soil, the volume of detection is very small. As a result, options for early detection of tritium leaks 

into the subsurface are limited; they seem to be constrained to sampling and analysis of the 

subsurface material and to downhole methods. 

4.1 In Situ Tritium Vapor Detection

Since tritium is generally in the form of tritiated water and is volatile, one approach to 

maximizing the volume of material sampled from the subsurface is to have an array of gas 

sampling boreholes in the vadose zone connected to an air pump, a manifold, and then to a 

detector. Sampling could be automated to rotate among the wells. Vapor is fed through the 

manifold to any one of several commercially available systems designed for tritium detection in 

air. These detectors, typically ionization chambers or proportional counters, have detection 

limits of about 1,000 pCi/L and have reference chambers that provide compensation for 

background radiation (Marsh et al., 2007). A common problem with these detectors is retention 

of tritium (a memory effect), especially on organic materials in the system. Selection of 

materials can help, as may providing the capacity to heat susceptible portions of the system. 

However, it is not clear how well tritium in ground water becomes distributed to water vapor in 

the subsurface.

Olsen et al. (2000) sampled tritium in moisture from soil vapor in two areas at Hanford, WA, 

underlain by extensive tritium ground water plumes. While the ground water contained 

117,000 pCi/L, tritium in the nearby soil vapor was below the detection limit of 240 pCi/L. This 

suggests that soil moisture is dominated by the downward transport of moisture from 

precipitation; tritiated water vapor does not move upward. However, analysis of helium-3, the

daughter of tritium, using the ratio of helium-3 to helium-4 indicated substantial enrichment of 

helium-3 that increased with depth. This suggests that sampling soil vapor may not be very 

useful for tracking tritium ground water plumes, although this method may be useful for tritium 

that is entering the vadose zone and is moving downward.

4.2 Down-Well Tritium Detection

Canadian nuclear facilities have used passive tritium samples. A glass vial half full of water (or 

a 1:1 water/glycol mix) is positioned in a sampling location, which could be in a well in the 

vadose zone. The vial has an opening in the cover to allow tritiated water moisture to diffuse 

into the vial and to come to equilibrium with the liquid. After about 1 month, the liquid is 

sampled and counted by liquid scintillation. This method is falling out of favor because it has 

been found to have extremely variable results.

Detectors that can be used down a borehole are an option. Several detectors of tritium are 

designed for downhole applications. A commercially available system, Model SSS-33DHC from 

Technical Associates (Canoga Park, CA), has a detector sonde that is lowered down a borehole 

while other electronics remain at the surface. The downhole system pumps water through filters 

and deionizers and then through a flow cell containing scintillation crystals coupled to 

photomultipliers. Data are then sent to the processor unit above ground. The lower limit of 
35

detection is 1,000 pCi/L in a 30-minute count. This instrument is designed for water analysis 

only. A variation puts only the pump system downhole, and it can detect 1 microcurie per liter 

(μCi/L) in a 20-minute count and 0.135 μCi in 3 hours. Product literature states that these 

instruments can be used in continuous operation and that they are “not influenced by other 

radionuclides.”

Recently, researcher at Idaho National Engineering Laboratory developed a downhole beta 

detection system (Akers et al., 2010). Skorska et al. (2010) reports the results of an evaluation 

of the proof-of-concept detector, as well as a subsequently enhanced detector. This detector 

has a diameter of 1.906 inches, a length of about 19 inches, and a 12-inch length of the 

scintillator section. This system is designed for beta detection. It has a demonstrated sensitivity 

to technetium-99 at 1.2 picocuries per gram for a 3-minute count time. The counting system is 

able to discriminate between technetium-99 and strontium-90 (Figure 10), but it has not been 

demonstrated for the much lower energies of tritium. It can be used in water as well as in air.

Figure 10. Output from detector tests showing separation of counts from 

technetium-99, strontium-90, and yttrium-90 (Akers et al., 2010)

This system is in the development stage and requires demonstrated ability to detect tritium and 

to discriminate between tritium and soil background radionuclides.

Work conducted for the U.S. Department of Energy at Mc Dermott Technologies (Berthold and 

Jeffers, 1998) investigated the development of a tritium detector based on the use of 

fluor-doped optical fibers. U.S. Patent # 5793046 was filed. The low-energy beta particle 

emitted by tritium is completely absorbed by coating materials typically used on 

radiation-detecting fibers. With this system, however, a polystyrene fiber has a thin coating of 

fluorescent material which is in direct contact with water containing tritium and therefore is able 

to interact with beta particles. The light generated can then propagate down the fiber to a 

photomultiplier tube. This work was stopped after the initial phase because of detection limit 

challenges, as well as questions about the stability of the coatings. 

Recently, a fiber-optic tritium detector was developed (Jang et al., 2010) that consists of a 

scintillator in the form of a 10-mm-diameter disc, a bundle of 40 plastic fiber-optic cables feeding 
36

into a photomultiplier tube, and an amplifier which is read by a LabVIEW system. The 

scintillator was a 0.1-mm-thick disk of Gd2O2S:Tb in optical epoxy (1:1 by volume). The optical

fibers were 1 m in length. The system was tested with metal hydride/tritium sources containing 

0.05, 0.2, and 0.4 curies of tritium. The response of the detector to the different sources was 

linear, but count rates and efficiencies are not given. Nevertheless, the feasibility of coupling a 

scintillator and fiber optics for observing low-energy beta rays was shown.

Several other approaches to detection of tritium have been published and may provide the basis 

for useful means of subsurface detection. One is the use of avalanche photodiodes. Willms et

al. (2005), at Los Alamos, developed a device for measuring tritium surface contamination that 

is capable of measuring down to 1,000 disintegrations per minute per 100 square centimeters,

which is a free release limit. To obtain these detection limits, count times were very long and no 

window was used between the sample and detector. This work follows from earlier use of PIN 

diodes as detectors for x rays. 

Researchers at Savannah River (Hofstetter and Wilson, 1992) developed a system for 

continuous monitoring of tritium in effluent. This system consists of a U-tube analysis cell 

containing plastic scintillator beads (0.1 mm in diameter) and a commercial (Berthold) 

radio-High Pressure Liquid Chromatography system using two photomultipliers as detectors. To 

minimize fouling and interferences, it was necessary to clean up the effluent with complex inline 

water treatments. A detection limit of 800,000 pCi/L was obtained for a 10-minute count and 

300,000 pCi/L for 1 hour.
37

5. DISCUSSION

Table 2 compares the methods that can be used to monitor for early signals from leaks in the 

subsurface at NPPs. It includes methods for measuring moisture content, temperature, and 

tritium vapor. However, as discussed below, challenges are associated with many of these 

methods, especially the geophysical techniques using electromagnetic signals. While many 

technologies are available to detect and quantify the moisture content of soil, their practical 

application, especially at large complex facilities, is not necessarily straightforward. The 

following sections discuss some assumptions and observations that may apply to attempts to 

provide early leak detection at NPPs.

5.1 Comparison of Single-Point Methods

Several methods of measuring soil moisture lend themselves to being installed for long-term 

applications. These methods, although they only interrogate small volumes, could be used in 

specific areas where leaking water will accumulate or be channeled. For example, they could 

be emplaced into backfilled trenches during construction or repair work. Durability is the key 

requirement for these sensors. Sensitivity seems less important since changes of moisture 

content in a confined space would likely be rapid and readily apparent. Placement even within a 

constrained area could be important, and sensors should be positioned at the lower interface of 

backfill and native soil. 

Since these sensors would be inaccessible, they cannot require maintenance (such as periodic 

addition of water) and should have low failure and degradation rates. TDR probes and perhaps 

capacitance probes seem to provide these characteristics, although there is no experience with 

in situ sensor functionability beyond about 10 years.

As part of the work to identify technologies that may be applied to treating wastes in the deep 

vadose zone of the central plateau at Hanford, WA, in situ sensors were identified as an 

important tool in determining moisture conditions in the vadose zone. Truex et al. (2011)

evaluated in situ moisture sensors for use in field applications to desiccation as a remediation 

technology. These researchers conducted two laboratory experiments with a large flow cell into 

which a variety of sensors were installed—thermistors for soil temperature (evaporative 

cooling), heat dissipation units for soil matric potential between −0.1 and −5 megapascals, 

thermocouple psychrometers for soil matric potential between −0.2 and −8 megapascals, dual 

probe heat pulse for water content, and humidity probes for relative humidity. In the flow cell,

moist soil was dried by air injection and extraction after which water was reintroduced. All 

sensors detected the passage of a desiccation front, but the thermocouple psychrometers and 

dual probe heat pulse sensors could not be reactivated upon rewetting.

5.2 Comparison of Two-Dimensional and Three-Dimensional Methods

In many circumstances, methods that interrogate large volumes of the subsurface provide a 

great advantage over point measurement techniques. The use of vertical cross-borehole

methods that allow interrogation of a “panel” or plane of subsurface material has several 

advantages. The area interrogated between wells is consistent in location and solid material 

characteristics. As a result, measurements repeated over months and years can be used for 

simple comparisons instead of needing more complex analysis. In this configuration, the plane 

that is interrogated could be below a suspect area and therefore very likely to intercept 

moisture. Downhole arrays can be permanently installed, can be set up with wireless 
38

communications, and are out of the way of plant operations. In some cases, existing monitoring 

wells can be used. These boreholes do not need to be finished wells; polyvinyl chloride casings 

are used and, for some methods, 2-inch inside diameters are acceptable. Perhaps more 

advantageous is the possibility of using horizontal boreholes, especially for new construction. In 

either configuration, a simple analysis, rather than a complete imaging process, should be able

to detect change. 

The use of boreholes has limitations. First, locating the boreholes in appropriate areas could be 

difficult, not only because of the uncertainty in suspect areas but also because of the presence 

of subsurface pipes and cables that would preclude drilling. This is a concern at NPPs but 

drilling is often done, albeit with care to avoid infrastructure and with sufficient justification 

(personal communication; Zigmund Karpa to M. Fuhrmann, February 15, 2012). There is a 

limited distance between boreholes at which signals can be detected as a result of attenuation 

by the solid medium. Typically this separation distance is between one and two times the depth 

of the borehole. 

Electromagnetic interference at an NPP is expected to be intense because of the operation of 

generators, motors, electrical cables, switches, and galvanic corrosion protection for 

underground pipes. This may preclude the use of some surface-based electromagnetic 

systems, such as MRS. However, EM noise comes in many forms, geological and cultural, with 

causes ranging from distant lightning strikes to the coherent noise from power lines (Everett and 

Meju, 2005). Fortunately, some EM methods are tunable so that they use only a small segment 

of the EM spectrum. In this case, it may be possible to use parts of the spectrum that are 

relatively free of noise or to filter out the interference. Surveys of a site can be conducted to 

assess the degree of interference. In addition, downhole methods provide some shielding from 

electromagnetic interference, but it is not clear whether this shielding effect is enough to allow 

use of these methods. Combining the two approaches may be effective. Figure 6 illustrates the 

influence of cathodic protection on resistivity measurements. Segment B is a period when 

cathodic protection at a tank was turned off. The signal is much less noisy than it is with it on, 

but the noisy signal still allowed detection of rain events (Calendine et al., 2011). 

Challenges exist in identifying moisture movement resulting from precipitation as opposed to 

leak detection. Combinations of methods may allow better discrimination by sensing 

conductivity, temperature differences, and tritium, as well as moisture. In addition, techniques 

will be needed in which the site-specific behavior of natural processes (precipitation) can be 

“learned” so as to differentiate between leaks and infiltrating precipitation. 

Of the methods shown in Table 2, DTS is consistently noted as favorable. As described earlier,

this technique is primarily intended for temperature sensing but can be adapted for moisture 

sensing. Nevertheless, for many reactor systems, leaking water may provide a temperature 

signal. The sensor for DTS is a cable that can be very long; as a result it can be arranged as 

needed to provide coverage of an area. As discussed earlier, these systems are commercially 

available and configured for a variety of applications, including detection of leaks and fires.

The use of an array of single-point sensors has also been rated favorably on a consistent basis. 

While point measurement methods can in many cases provide high precision measurements of 

moisture content, they suffer from a lack of areal continuity. Typically they measure moisture in 

a volume of about a liter. However, arrays of point measurement instruments may be useful in 

small, well-defined areas that are especially subject to leaks. Many of these methods can be 

automated and permanently installed. Computer-controlled arrays of sensors (e.g., ER or TDR) 

have been used for extended periods (in excess of 10 years). They can be easily installed in 
39

simple boreholes with wireless communications. On Table 2, in the row entitled “Arrays of 

single-point sensors,” the topic of “Adequate Sensitivity” is marked with “F ?.” While the 

sensitivity of the sensors themselves is very good, sensitivity of the entire array is a different 

matter and will depend on sensor spacing.

As discussed earlier, ER can be used in a downhole zero-offset configuration for detection of 

changes in moisture content. This differs from ERT because of the shorter pathway (see 

Figure 3) between transmitter and receiver (when compared to tomography) and better signal to 

noise ratio. ER systems can be installed in the subsurface and operated for long times. Data 

computation is much simpler in the zero-offset configuration than in tomography, as well. 

Because of these differences, Table 2 treats zero-offset ER, which is the most favorably rated 

geophysical method, separately from ERT.

HRR-SCRT has been demonstrated with mock leak tests at Hanford; Table 2 rates this method

highly. This time-lapse method seems to be sufficiently sensitive to low flow rate subsurface 

leaks (a few gallons per hour) that it is interesting for industrial applications. Calendine et al.

(2011) discuss the method’s application to long-term monitoring, including data required for a 

complete data set, anthropogenic changes, factors causing false positives, and automation of a 

leak detection system.

Cross-borehole ERT and tritium vapor analysis both are rated as favorable, but in each case 

there are substantial unknowns associated with the method’s applicability to leak detection at 

NPPs. For tritium vapor detection, a topic of considerable interest to EPRI, a key issue is the 

volume of soil from which vapor is drawn. This will likely be site specific and is subject to gas 

movement through preferential flowpaths that may draw low activity vapor from unintended 

areas, such as along well casings. Analytical equipment for continuous tritium detection is 

commercially available and has been in use for years for air monitoring. Most require a gas 

supply and appear to be sufficiently sensitive for application to leak detection. However, the 

partitioning of tritium between the vapor and the liquid and solid phases in the subsurface may 

need to be better understood to inform models of contaminant migration.

Cross-borehole ERT is also a well-developed technology that has been applied to a variety of 

sites, primarily in the area of geochemical remediation where near continuous and autonomous 

monitoring was done. It is possible that ERT using surface probes will suffer from EM 

interference to such an extent that it is not useful. Downhole ER methods are less prone to 

anthropogenic interference than many other methods; however, it is not clear if the technology, 

in its downhole configuration, will be usable in the presumed high EM interference environment 

of an NPP. A limitation to cross-borehole ERT is the need for wells that are within a few meters 

of each other. Both cross-borehole ERT and tritium vapor analysis have promise, but are likely 

to require additional development specific to the question of leak detection. This report further 

addresses this issue in a later section.

MRS is a relatively new means of assessing moisture content noninvasively. While it is very 

sensitive to EM interference, such that the presence of local powerlines or distant lightning 

strikes present problems, there is a downhole Nuclear Magnetic Resonance tool that could be 

less subject to interference. As a result, while surficial configuration appears to be inappropriate 

for leak detection at NPPs, it may be possible to use another configuration.

GPR, like ERT, can be configured for surface or downhole interrogations, or a combination of 

the two. Surface-based methods are probably seriously affected by EM interference, but the 

downhole approach may be less degraded. Of the two cross-borehole configurations,
40

tomographic imaging does require sophisticated computer processing, while the zero-offset (or 

level-run) approach is much simpler. This latter technique appears to be well suited to provide 

change detection information. However, GPR systems are not suited to continuous or 

autonomous operation, requiring skilled operators and complex data processing. GPR 

techniques may also have particular difficulty overcoming the presence of infrastructure. 

In summary, the methods that appear to be most applicable for early leak detection are DTS, 

arrays of a single-point method, tritium vapor sampling with real-time analysis, and 

cross-borehole zero-offset ER. One method is primarily for temperature, one is for tritium, and 

two focus on moisture content (but may also be used for temperature and fluid conductivity). 

Combining these methods lend themselves to a more robust monitoring strategy by using 

methods that interrogate different properties. This allows a double check of anomalous signals 

or may allow identification of the leaking pipe system.
41

Table 2. Comparison of Large-Volume Interrogation Methods

F = favorable U = unfavorable ? = more information is needed

1. Interference may be caused by the presence of pipes and other materials or structures at the surface, such as pavement, or at depth.

2. Sensitivity consists of two parts: spatial resolution and ability to observe the analyte.

3. For the column entitled “Installed”: F = can be permanently installed at site or U = must be deployed for each use.

4. Method does not require expert interpretation of data.

Method Subject to EM 

Interference

Subject to Physical 

Interference1 Autonomous Continuous

Operation

Adequate

Sensitivity2

Installed3 Simple

Interpretation4 Comment

Cross-hole GPR 

tomography F ? F U U F ? U U Subject to EM but downhole may 

shield

Surface

GPR U U U U F? U ? U Sensitive to

EM interference

Cross-hole

zero offset GPR F ? F U U F U F Subject to EM but downhole may 

shield

Cross-hole

ERT F ? F F F F ? F U Subject to EM but downhole may 

shield

Surface

ERT U U F F F ? F U Sensitive to

EM interference

Zero-offset

cross-hole ER F ? F F F F F F Higher S/N than ERT

Less subject to EM

Magnetic

resonance U? F U U F U ? Very sensitive to EM

EM

methods U? U U U F U ? Some methods tunable

to avoid noise

DTS F F F F F F F Only detects temperature in 

areas where cable is placed

Tritium vapor F F ? F ? F ? F ? F F Volume sampled is not clear. H3 or He-3?

Array of single-point 

methods F F F F F ? F F

High-resolution resistivity F? F? F F F F F May be able to use existing wells 

(SCRT)

41
42

5.3 Other Uses of Techniques of Interest to the NRC

Some of the geophysical methods described previously could be applied in a proactive 

approach to identifying pathways that leaking water would take in subsurface systems. This 

method, suggested by John Lane of USGS (personal communication J. Lane to M. Fuhrmann, 

September 30, 2011), entails the use of a geophysically identifiable tracer (e.g., salt water) 

introduced at an area of interest, perhaps a structure that tends to leak. Geophysical methods 

would be used to follow this tracer as it moves through the subsurface. In this way, flow rates, 

dispersion, and preferential pathways could be identified. Using this approach, optimal 

locations and screen depth for monitoring wells could be established. This approach would be 

especially valuable for sites with large areas of underground infrastructure and backfill that may 

provide unexpected pathways for leaking water. The concept of using electrical imaging to 

follow a saline tracer was explored in a tracer release in an experimental tank (Slater et al., 

2000).

Aside from detecting leaks at NPPs, the NRC has several other applications in which the 

techniques discussed in this report may be useful. The presence of water, changes in its 

concentration, or its movement through a system can be key indicators of performance of waste 

disposal systems. Engineered cover systems for low-level radioactive waste trenches, for 

waste incidental to reprocessing, and for the very large covers for uranium mill tailings disposal 

facilities are intended to manage water intrusion into the waste. This may be done by limiting 

infiltration of precipitation, enhancing and controlling runoff, and providing for 

evapotranspiration. Understanding the behavior of these covers is an important factor in 

assessing the long-term performance of these disposal systems. Many of the methods outlined 

in this report can be applied to determining the behavior of moisture in covers and how that 

behavior changes over time. Especially interesting are methods that allow large volumes of 

material to be interrogated in a horizontal plane. This can allow identification of areas with 

higher and lower permeabilities. High resolution arrays of vertical sensors within the covers can 

provide information on fluxes. 

Several of the techniques discussed here are currently used to monitor the integrity of earthen 

berms and dikes. They are used to provide periodic, autonomous measurements through 

wireless communications systems of the water content or phreatic surface within earthen dams. 

This is important to the NRC for several reasons. At some NPPs, berms are used to channel 

and impound cooling water supplies. At others, earthen dams on water bodies upstream of 

NPPs have the potential to be an issue for flooding. As a result, their integrity may need to be 

monitored.

5.4 The Infrastructure Problem

The problem of subsurface infrastructure (e.g., pipes) interfering with geophysical methods is 

substantial but is being addressed as these methods mature. The greater density and electrical 

conductivity of metal pipes present problems for radar and electrical methods. Resistivity 

methods, which are the most commonly used for leak detection, determine changes in the 

resistance of the flow of current within a medium. While soil is relatively resistive, moisture and 

salts (e.g., from waste leakage) will decrease the resistivity, and this difference can be detected 

and models used to produce two- and three-dimensional images of their distributions. However, 

the presence of a pipe made of steel will perturb these measurements such that any other data

are masked by the intense signal of the pipe. Rucker et al. (2012) discuss this problem for work 

done at a Hanford tank farm. Johnson also discussed this problem at the February 15, 2012,

42
43

workshop (see Appendix A to this report and the workshop slides at ADAMS Accession 

No. ML120540481). Figure 11 is a synthetic modeling exercise illustrating the effect of a pipe 

on ERT results assuming the use of surface electrodes. This result occurs because the ERT 

models are not able to simulate the behavior of the pipe, the model meshes are too small to 

incorporate the pipe, the very large contrast in resistivity causes numerical instability in the 

models, and smoothing constraints are inconsistent with reality (Johnson, 2012). However, 

interference from infrastructure can be reduced by using electrodes that project below the 

infrastructure. The use of monitoring wells as long electrodes allows the distribution of some of 

the current beneath the infrastructure. Problems are associated with this approach since 

vertical resolution is lost when using a well as an electrode and lateral coverage is limited 

(Rucker et al., 2010; Rucker et al., 2012). However, coverage and resolution can be improved 

by using wells as electrodes in combination with surface electrodes positioned away from the 

infrastructure or by using subsurface electrodes emplaced at discrete depths using direct push 

techniques. It may be possible to use the pipe itself as an electrode. Advances in 

computational capabilities, models, and understanding of the behavior of various configurations 

of electrodes have rapidly improved plume detection capabilities (Rucker et al., 2012).

Figure 11. Modeled response of ERT detection of a leak with the absence and 

presence of a pipe.

45

6. LONG-TERM RESEARCH RECOMMENDATIONS

Table 2 suggests that the following large volume methods are the most applicable for early leak 

detection: 

• DTS

• array of single-point methods

• high-resolution resistivity

• zero-offset cross-hole ER

• tritium vapor sampling with real-time analysis

Each of these methods is worth further investigation, to a greater or lesser degree, through the 

long-term research program. Of these methods, only DTS has no “?” notations on the table, 

indicating that this technique could be applied, as it exists, with some confidence that it will 

perform as expected. In fact, this technique is commercially available and is used for leak 

detection along pipelines. Additional information is needed for each of the remaining four 

methods.

The use of single-point small volume probes has application in certain areas, such as backfilled 

trenches. These sensors are well developed and some, such as TDR probes, have been used 

in situ over periods approaching 10 years. For the “array of single-point” method, there is a 

question of sensitivity. As discussed earlier, each probe (such as TDR probes) can have very 

good sensitivity for moisture content. However, the emplacement of the probes, particularly

their spacing and number, will determine how readily a leak will be detected. An assessment of 

the area that needs to be interrogated and the number and arrangement of probes will be

application specific.

Analysis of tritium vapor from the subsurface is envisioned as a real-time method in which 

sampling locations are periodically pumped by a computer-controlled system to an almost 

autonomous tritium detector. The detector and pumping control portions of this system are 

routinely available. The questions rest in achieving adequate sensitivity and knowing exactly 

where the vapor originates. Are there preferential flowpaths that short circuit the collection 

process? Are mechanical ways needed to ensure that a volume of soil vapor is obtained from 

its intended location? How much pumping is optimal? How readily does tritium partition from 

ground water to the vapor phase? Does it also become associated with the solid? Does 

tritiated water vapor segregate in the subsurface because of the mass difference with light 

water? How can measured concentrations of tritium in water vapor be modeled to assess 

concentrations in ground water? These questions are beyond the scope of this report and 

laboratory and field programs may be required to resolve them.

For moisture detection, zero-offset cross-hole ER is perhaps the simplest version of ER 

geophysical methods (Figure 12). While it appears that this method has advantages over other 

geophysical techniques, several unknowns are associated with its use for routine monitoring at 

an NPP. While this approach is the least affected by anthropogenic EM interference, the 

expected variety and intensity of interferences still may challenge the method. Sensitivity is also 

a question, primarily because of EM interference. Spatial sensitivity can be controlled by 

vertical spacing of electrodes, which can be as close as every 10–20 cm. As mentioned earlier, 

one issue is the optimal spacing of wells; distances may need to be fairly close together and this 

spacing is a function of the depth of the downhole array of electrodes.
46

Figure 12. One approach to using zero-offset ER monitoring

Any moisture sensing system will have the problem of trying to distinguish between leaks and 

natural moisture infiltration. As a result, the system will need to be able to assess the difference 

in behavior of moisture during the two types of events. It is thought that a monitoring system 

could “learn” the difference in behavior of moisture through a neural network or other similar 

techniques (F. Day-Lewis and T. Ferré to J. Kanney, personal communications February 15, 

2012). One system, developed at Argonne National Laboratory and known as the multivariate 

state estimation technique (MSET), is a software system for real-time process monitoring. 

While the system appears to be focused primarily on equipment monitoring, it may be applicable 

to the question of changing moisture regimes. A second function of MSET is its ability to detect 

anomalous sensor readings that indicate failure of the sensor

(www.ne.anl.gov/codes/mset/#nuclear). Both issues, EM interference and discrimination of 

leaks versus rainfall, will require some research activity to resolve.

Some of the moisture measurement methods discussed are well developed and commonly 

used. Others are relatively new. Nevertheless, few if any have been used at NPPs. None have 

ever been applied to leak detection at NPPs. Consequently, a number of questions about these 

techniques and their application to NPPs may need to be explored, perhaps with a long-term 
47

research evaluation, before any decision can be made about their usefulness. General points 

about leak detection at NPPs that would need to be considered include the following:

• Leak detection for an entire plant is completely impractical. The possibility exists to 

install detection systems in local areas but these areas need to be defined. The NRC 

lessons-learned reports are helpful. EPRI has developed a procedure to rank specific 

systems for risk of leaks and to determine the characteristics of those leaks. It is 

available as a software package (BPWORKS).

• The complexity of NPPs may limit the installation of boreholes and other detection 

system equipment. This complication leads to a question about the optimal 

configuration and distance between wells for downhole applications and for surface 

electrodes.

• The electrical environment of an NPP may seriously affect many moisture-sensing 

systems, especially geophysical methods. It is unclear if this interference will be 

dampened in the subsurface and if galvanic pipe protection will also interfere. As a 

result, it will be important to assess the extent that the electrical environment interacts 

with selected methods of leak detection. Some methods can be tuned to certain 

frequencies that avoid interference. Survey instruments are available to allow sites to be 

checked before any field deployment.

It is likely that these conditions are location and plant specific. As such, it will be necessary to 

perform some onsite tests to provide a better understanding of the benefits and limits of the 

various methods.

Based on the discussion above, a research program will likely be needed to properly assess the 

feasibility of early leak detection methods. While much of the burden of proof should be placed 

on utilities, it is also necessary for the NRC to have sufficient independent information that a 

reasonable judgment can be made about the value of early leak detection methods. In addition, 

some of the moisture detection methods are applicable to evaluating processes at other types of 

sites of interest to the NRC, including the long-term performance of earthen covers for low-level 

waste and uranium mill tailings. Some of these methods also have the potential for monitoring 

the integrity of dams.

An initial assessment should consist of a feasibility study based on modeling of simple 

scenarios of leak propagation. Generic soil properties and parameters can be used to model 

water distribution under varying leak conditions based on evidence from the LLTF report (U.S. 

Nuclear Regulatory Commission, 2006) and on engineering properties of backfill materials. The 

modeling should be used to determine how sensors might be arranged and distributed for 

optimal effect. This initial study should also evaluate interferences generated by an operating 

power plant.

A second stage of investigation will be necessary to ascertain the utility of various methods 

under different conditions of leaks. We envision this to occur in several steps:

(1) The first step would involve the conduct of a systematic set of field test-bed experiments

to assess the sensitivity of methods with respect to changing conditions of leak volume, 

backfill types, liquid conductivity, temperature, and background EM noise.
48

(2) Next, the selected methods would be installed at a power plant for tests following EM 

noise surveys.

(3) Finally, geophysical methods would be used to follow a tracer (e.g., salt water) that is 

introduced to some area of concern. This approach could be used to assess the 

pathway of leaking water from a specific structure to allow proper positioning of 

monitoring wells.

It would be beneficial to collaborate with several organizations for these tests, including EPRI, 

USGS, and universities. It should be possible to arrange a test-bed location locally, perhaps at 

the Agriculture Research Station in Beltsville, MD. 
49

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A-1

APPENDIX A

Abstracts from the February 15, 2012, Workshop

Interagency Workshop on Monitoring for Early Detection of

Underground Leaks at Nuclear Facilities

This appendix contains the abstracts for the presentations made at the workshop. Also included 

is the contact information for the speakers, as well as links to most of the presentations. This 

latter information is provided at the following web site:

http://pbadupws.nrc.gov/docs/ML1205/ML120540481.html

An Agencywide Documents Access and Management System (ADAMS) accession number is 

provided after each abstract title. Paste this number (e.g., ML120541054) into the search box 

on the U.S. Nuclear Regulatory Commission ADAMS Web page and it should take you to the 

presentation. Unfortunately, we were unable to post the slides for the talk by Karen Kim of the 

Electric Power Research Institute because of copyright issues, but please contact her, or any of 

the other speakers, for more information.
A-2

ABSTRACTS

Interagency Workshop on Monitoring for Early Detection of 

Underground Leaks at Nuclear Facilities

February 15, 2012

Underground Piping and Tanks Integrity Initiative (ML120541054)

James Riley

Nuclear Energy Institute

1776 I St., N.W., Suite 400

Washington, DC 20006

jhr@nei.org (202) 739-8137

The nuclear power industries Buried Pipe Integrity initiative will be described. The goal of the 

program is reasonable assurance of structural and leakage integrity of in-scope piping and 

tanks with special emphasis on components containing licensed material. Information will be 

presented on the principal buried systems of nuclear power plants and a summary of the leaks 

that have occurred in different systems. Observations of causes of these leaks, the materials 

involved, and their recent frequency will be discussed. 

Key Concepts for Early Leak Detection and Technical Questions (ML120541066)

Mark Fuhrmann, Joseph Kanney, Tom Nicholson, and Jacob Philip

U.S. NRC, Office of Nuclear Regulatory Research

11555 Rockville Pike

Rockville, MD

mark.fuhrmann@nrc.gov (301) 251-7472

Early detection of leaks from nuclear facilities could provide substantial benefits if leaks are 

detected close to structures, systems, and components before fluid reaches the water table. 

Methods that may be useful for early leak detection have been briefly described and evaluated 

in a draft NRC white paper entitled, “Monitoring for Subsurface Leaks at Nuclear Power Plants: 

External to Structures.” We have focused on detection and distribution of moisture and tritium,

but other parameters, such as conductivity and temperature, are also important. Some moisture 

sensors that are commercially available and durable are able to interrogate small soil volumes 

(about 1 liter) but could be arranged in arrays to attain greater volumes. Others, such as 

continuous downhole neutron probe moisture measurements interrogate a greater volume (e.g.,

several cubic meters) but are limited to soil borings. Other methods, primarily geophysical, can 

assess electrical properties from which moisture distributions, conductivity, and temperatures 

may be determined. These methods can interrogate large volumes of the subsurface, hundreds 

to thousands of cubic meters. Real-time downhole beta detection and tritium in soil vapor are 

also described. The practicality of using these methods in the environment of different nuclear 

facilities will be the focus of this workshop and will be valuable input to the final white paper and 

to the NRC’s Long-Term Research Program.
A-3

Soil Physics of Leak Detection Using Geoelectrical Methods (ML120541082)

Lee Slater

Department of Earth & Environmental Sciences

Rutgers University

101 Warren Street, Smith 136

Newark, NJ 07102

lslater@andromeda.rutgers.edu (973) 353-5109

Electrical geophysical methods are sensitive to changes in pore fluid composition and moisture 

content of porous media. These methods therefore offer opportunities for imaging and 

monitoring of leaks from storage and containment facilities. The direct detection of an existing 

leak from a single resistivity survey may be complicated by the fact that electrical resistivity is a 

property that depends on multiple chemical and physical properties of a soil. However, the 

monitoring of time-lapse changes in resistivity associated with the transport of leaking fluids is 

likely to provide more confirmatory evidence of a leak. In theory, it may also be possible to 

estimate leak volumes or concentrations of a constituent in the leak from petrophysical relations 

relating resistivity to moisture content and/or chemical composition of the pore fluid. However, 

such petrophysical relations are inherently uncertain and likely to be spatially variable. 

Therefore, the transformation of resistivity to quantitative descriptors of the leak composition 

should be treated with caution. Despite these limitations, numerous successful applications of 

the detection of leaks from landfills and chemical storage facilities have been documented in the 

geophysical literature. These examples offer insights into the potential for electrical geophysical 

methods for monitoring and early detection of underground leaks at nuclear facilities.

http://www.ncas.rutgers.edu/lee-slater

Industrial Applications of Real-Time Electrical Monitoring (ML120541098)

Dale Rucker

HydroGEOPHYSICS

2302 N. Forbes Boulevard

Tucson, AZ 85745

drucker@hgiworld.com (520) 647-3315

The three main objectives for leak detection and monitoring are to determine onset, rate, and 

location. The electrical resistivity geophysical method has been shown to be the most 

promising in meeting these objectives because resistivity is highly sensitive to changes in 

saturation and concentration of ionic constituents. Unfortunately, resistivity cannot be directly 

measured and instead is required to be indirectly determined by (1) electrical current transmitted 

into the earth to create an electrical field, (2) voltage measured at multiple locations, and (3) the 

resulting dataset inverse modeled to create the spatial distribution of resistivity over the 

domain. In our work, we have shown that all three steps can be used to address the leak 

detection objectives, provided sufficient temporal detail exist to discern minute changes in any 

measurement. Additionally, the electrical current and voltage data can be evaluated in real-time 

to help reduce time in determining the leak parameters. We showcase a couple of leak 

detection examples from mining and nuclear industries, where infrastructure and sources of 
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noise are prevalent. Lastly, we provide information on the long-term leak detection program in 

place at Hanford to monitor underground waste storage tanks.

www.hgiworld.com

3D Time-Lapse Electrical Resistivity Imaging: Field Examples and Application Potential 

for Leak Detection at Industrial Sites (ML120541124)

Timothy Johnson 

Pacific Northwest National Laboratory 

P.O. Box 999, MSIN: K3-61

Richland, WA 99352

TJ@pnnl.gov (509) 372-4715

Recent developments in autonomous, multichannel electrical resistivity data collection hardware 

have enabled monitoring of subsurface processes with high resolution in space and time in 

terms of the bulk electrical conductivity alterations governed by those processes We give 

several examples of how three-dimensional time-lapse electrical resistivity tomography (ERT) 

has been successfully used to monitor a variety of subsurface processes, including surface 

infiltration, groundwater-river water interaction, subsurface bioremediation, and subsurface 

desiccation. ERT monitoring also has potential for imaging changes in subsurface conductivity 

caused by leaking infrastructure within industrial environments. However, this application is 

problematic because of the deleterious influence infrastructure has on ERT measurements, 

which is usually referred to as “noise.” In this talk we investigate the influence electrically 

conductive buried pipes have on ERT data. We show that in the presence of pipes and under 

typical conditions, ERT data are likely to be sensitive to leak-induced changes in subsurface 

bulk conductivity, and therefore can be used to image leaks with the appropriate modeling 

approach. We then show how typical ERT imaging algorithms break down in the presence of

infrastructure because they are not designed to accommodate the sharp electrical conductivity 

contrasts in both space and magnitude arising from electrically conductive infrastructure. We 

show with a synthetic example how this problem may be rectified by explicitly modeling the 

infrastructure and allowing for sharp contrasts within the imaging algorithm. Finally, we discuss 

computational requirements of such an approach and the corresponding feasibility in 

commercial applications.

https://inlportal.inl.gov/portal/server.pt/community/inl_science_focus_area_project/700/sensing
A-5

Fiber-Optic Distributed Temperature Sensing:

Theory and Application to Monitoring Problems (ML120541137)

Fred Day-Lewis

U.S. Geological Survey

WRD/Office of Groundwater, Branch of Geophysics

11 Sherman Place, Unit 5015

Storrs, CT 06269

daylewis@usgs.gov (860) 487-7402, ext. 21

Fiber-optic distributed temperature sensing (FODTS) is an emerging technology with potential 

for diverse applications in hydrology, petroleum geoscience, reservoir management, and 

geotechnical and environmental engineering. Depending on instrument settings, FODTS can 

measure temperature at high spatial resolution (approximately 1 meter), high precision 

(approximately 0.1 degree Celsius), and high frequency (about every minute) along cables up to 

several kilometers long. Higher resolution, precision, and frequency are possible using 

specialized cables and/or instrumentation or setting tradeoffs between resolution, precision, and 

sampling frequency. Laser light is transmitted down one or more fiber-optic cables, and 

backscatter is analyzed to estimate temperature all along the cables. Although FODTS 

technology has been commercially available for over a decade, recent decreases in instrument 

costs and expansion of capabilities have led to much wider use. Common applications include 

studies of ground water/surface water exchange, snowpack monitoring, dam/levee seepage, 

and leak detection in pipelines. This presentation reviews FODTS technology, instrument 

capabilities, case-study applications, and the potential for monitoring leaks at nuclear power 

plants and associated infrastructure. 

http://water.usgs.gov/ogw/bgas/fiber-optics/waquoit.html

Finding Leaks Using Hydrogeophysical Data and Numerical Models

(ML120541140 and ML120550567)

Stefan Finsterle

Earth Sciences Division

Lawrence Berkeley National Laboratory

Berkeley, CA

SAFinsterle@lbl.gov (510) 486-5205

Geophysical methods are capable of providing high-resolution images of a specific geophysical 

attribute of the subsurface, which may reflect spatially variable, but stagnant geological features, 

time-dependent changes in fluid content and fluid properties, or a combination thereof. To 

successfully use such images for leak detection and specifically for predicting the system 

behavior in support of risk assessment, it is essential to (1) unravel the various factors that 

contribute to the variations in the geophysical attribute, (2) relate the geophysical attributes to 

hydrogeological properties that affect flow and transport, and (3) reduce ambiguities, 

nonuniqueness, instabilities, and other inversion artifacts. These issues are typically resolved 

(in part) by choosing an appropriate geophysical method with optimized measurement 

configuration, inverting the data using regularization techniques, and eventually transferring the 

geophysical image to a hydrogeological model using some petrophysical relationship. This 

approach can be enhanced by performing a joint inversion of geophysical and hydrogeological 
A-6

data using a flow and transport simulator that is coupled to a geophysical forward code within an 

optimization framework. The main contribution of hydrogeological modeling is that (1) additional 

data can be included to constrain the inverse problem, (2) regularization is based on physical 

rather than somewhat arbitrary geometrical criteria, and (3) the parameters determined are 

more directly related to features and processes that are of interest for leak detection and 

ultimate contaminant transport prediction. We will discuss the role that flow and transport 

simulations may play for early detection of underground leaks at nuclear facilities and the 

advantages and limitations of a joint hydrological-geophysical approach for characterization of 

the subsurface and monitoring of contaminant movement. 

Assessing the Likely Value of Geophysical Data (ML120541144)

http://pbadupws.nrc.gov/docs/ML1205/ML120541144.pdf

Ty Ferré

Department of Hydrology and Water Resources

University of Arizona

Tucson, AZ 85721

tyferre@gmail.com (520) 621-2952

Geophysical methods offer unprecedented opportunities to image subsurface structures and to 

monitor temporal changes in property distributions. The methods range in their resolution, 

coverage, and sensitivity. But all are subject to limitations because of their indirect nature. A 

previous talk highlighted the importance of understanding and defining petrophysical 

relationships and properties. Others have shown the importance of selecting a geophysical 

method that is sensitive to processes of interest and then designing a survey to make the best 

use of the instrument’s spatial sensitivity. Finally, we have heard about the value of combining 

process and geophysical instrument models to perform coupled hydrogeophysical 

interpretation. In particular, we have seen the value of added context that results from 

interpreting direct and geophysical data in a common framework. All of these concepts and 

approaches must be considered when designing monitoring networks for specific applications. 

I will present an approach that allows for assessment of the likely value of proposed direct and 

indirect (geophysical) measurements in the context of all known site information and in the 

context of user-defined cost or risk functions. We refer to the approach as the 

Discrimination/Inference to Reduced Expected Cost Technique (DIRECT). I will present a 

simple contaminant treatment example in the talk in the hopes that it will be useful for the 

general discussion of designing monitoring networks.

https://sites.google.com/site/tyferre/
A-7

EPRI Project on Advanced Technologies for Groundwater Protection: Automatic Tools 

and In-Situ Sensors for Groundwater Monitoring

Karen Kim

EPRI Project Manager 

3420 Hillview Avenue

Palo Alto, CA 94304

kkim@epri.com (650) 391-5610

In 2011, the Electric Power Research Institute investigated technologies for the automatic and 

in situ detection of ground water contamination in monitoring wells. Such an automatic 

technology would facilitate the early detection of ground water contamination. EPRI assessed 

the state of science and technology for detecting radiological and nonradiological (e.g., chemical 

and physical parameters) signatures in ground water to identify technologies that would be 

applicable to nuclear power plant implementation. These technologies were assessed for their 

maturity, ability to detect tritium at levels typically found in the environment and at nuclear power 

plants, ability to detect nonradiological signatures that can be correlated to a potential leak or 

spill, and the functional capabilities of the technology. An EPRI Technical Report will document 

the results of this study.

Sensors for Tritium Detection (ML120620402)

Shuh-Haw Sheen

Argonne National Laboratory

9700 South Cass Avenue

Argonne, IL 60439

sheen@anl.gov (630) 252-7502

The presentation covers the state-of-the-art measurement techniques for detecting tritium in 

underground water at a nuclear power plant. Routinely used sampling techniques and the need 

of a real-time continuous monitoring system are discussed. Recent developments in sensor 

and sensing technique for tritium detection are also highlighted, and future research needs are 

suggested.

B-1

APPENDIX B 

Participants in the February 15, 2012, Workshop

Interagency Workshop on Monitoring for Early Detection of

Underground Leaks at Nuclear Facilities
B-2

Participants at the Rockville Office

Name Organization Email

Alley, David NRC/NRR David.Alley@nrc.gov

Arlt, Hans NRC/FSME Hans.Arlt@nrc.gov

Beam, Paul DOE-EM12 Paul.Beam@em.doe.gov

Bodnar, Imor ANL bodnar@anl.gov

Conatser, Richard NRC/NRR Richard.Conatser@nrc.gov

Day-Lewis, Fred USGS daylewis@usgs.gov

England, Louise Progress Energy Louise.England@pgnmail.com

Esh, David NRC/FSME David.Esh@nrc.gov

Ferré, Ty University of Arizona tyferre@gmail.com

Finsterle, Stefan LBNL SAFinsterle@lbl.gov

Fuhrmann, Mark NRC/RES Mark.Fuhrmann@nrc.gov

Gunter, Paul Beyond Nuclear paul@beyondnuclear.org

Hardies, Bob NRC/NRR Robert.Hardies@nrc.gov

Johnson, Tim PNNL tj@pnnl.gov

Kanney, Joseph NRC/RES Joseph.Kanney@nrc.gov

Karpa, Zigmund Exelon Zigmund.Karpa@exeloncorp.com

Kim, Karen EPRI kkim@epri.com

Lesmes, David DOE-SC David.Lesmes@science.doe.gov

Lindley, Roy ANL lindley@anl.gov

Malama, Bwalya SNL bnmalam@sandia.gov

Marble, Justin DOE-EM Justin.Marble@em.doe.gov

Martin, Aaron University of Maryland martinaj@geol.umd.edu

Nicholson, Tom NRC/RES Thomas.Nicholson@nrc.gov

Noggle, James D. NRC Region I James.Noggle@nrc.gov

Ott, Bill NRC/RES William.Ott@nrc.gov

Pantazes, Jeff PSEG Jeffrey.Pantazes@pseg.com

Prokofiev, Iouri NRC/RES Iouri.Prokofiev@nrc.gov

Reed, Wendy NRC/RES Wendy.Reed@nrc.gov

Riley, Jim NEI jhr@nei.org

Rucker, Dale HGI drucker@hgiworld.com

Sheen, Shuh-Haw ANL sheen@anl.gov

Slater, Lee Rutgers University lslater@rutgers.edu

Tregoning, Rob NRC/RES Robert.Tregoning@nrc.gov
B-3

Remote Participants in the Workshop on Early Leak Detection at Nuclear 

Facilities

Name Organization E-mail

Allred, Barry OSU allred13@osu.edu

Carlson, Norman Zonge International norman.carlson@zonge.com

Cassidy, John NRC Region III john.cassidy@nrc.gov

Chang, Jeannie APS jeannie.chang@aps.com

Dionne, Bruce NRC Region I bruce.dionne@nrc.gov

Donahue, Patrick Entergy pdonahu@entergy.com

Drake, James NRC Region IV james.drake@nrc.gov

Dykes, Carmen NRC carmen.dykes@nrc.gov

Eckert, Timothy EPRI teckert@epri.com

Gary Bailey, Gary Entergy gbaile3@entergy.com

Golliet, Matthew Westinghouse golliem@westinghouse.com

Lesan, Harvey APS harvey.lesan@aps.com

McGovern, Matthew Xcel Energy matthew.mcgovern@xenuclear.com

Merritt, James U.S. Dept. of Transportation james.merritt@dot.gov

Nimitz, Ronald NRC Region I ronald.nimitz@nrc.gov

Parks, Leah NRC leah.parks@ntc.gov

Ptasznik, Liz AMEREN eptasznik@ameren.com

Rejcek, Milton NINA STP 3 & 4 mfrejcek@stpegs.com

Ricketson, Larry NRC larry.ricketson@nrc.gov

Shaikh, Atif NRC atif.shaikh@nrc.gov

Smith, David EPRI dsmith@epri.com

Snyder, Ken NIST kenneth.snyder@nist.gov

Stewart, Craig American Nuclear Insurers cstewart@amnucins.com

Swickley, Robert LANL rswickley@lanl.gov

Vesselinov, Velimir LANL vvv@lanl.gov

Wright, Martin PGE martin.wright@pge.com

Zhang, Henry Canadian Nuclear Safety 

Commission henry.zhang@cnsc-ccsn.gc.ca




UNITED STATES 

NUCLEAR REGULATORY COMMISSION

WASHINGTON, DC 20555-0001

--------------------

OFFICIAL BUSINESS
NUREG-2151 Early Leak Detection External to Structures at Nuclear Power Plants