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## NC STATE UNIVERSITY NUCLEAR ENGINEERIG DEPARTMENT

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NUCLEAR ENGINEERIG DEPARTMENT

CENTER FOR ENGINEERING APPLICATIONS OF RADIOISOTOPES

RADIOACTIVE PARTICLE TRACKING IN PEBBLE BED REACTORS

By

Prof. R. P. Gardner and Ashraf Shehata

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- Introduction
- Review of RPT Techniques
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- The Nonlinear Inverse Analysis Approach
- Monte Carlo Simulated Results
- Experimental Results
- Introduction To Pebble Bed Reactor
- Critique on Previous Work
- An Alternative Approach for RPT

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- ORIGIN:Developed Primarily by Chemical
- Engineers
- USES:Flow Characterization of Chemical
- and Mineral Processes
- EXPECTATIONS:
- Potential for Benchmarking CFD Calculations
- Potential for Combining RTD and CFD
- Potential for Flow Characterization and Mapping of Fuel Pebbles In a Pebble Bed Reactor
- Potential for Combining Flow Characterization andResidence Time Distribution Calculations

for optimized Fuel Cycles

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- Introduction
- Review of RPT Techniques
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- Experimental Results
- The Nonlinear Semi-Empirical Modeling
- Monte Carlo Simulated Modeling
- Introduction To Pebble Bed Reactor
- Critique on Previous Work
- An Alternative Approach for RPT

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- Extensive Review by Larachi, Chaouki, Kennedy, and Dudukovic: Chap.11: RadioactiveParticle Tracking in Multiphase Reactors: Principles and Applications in NON-INVASIVE MONITORING OF MULTIPHASE FLOWS, Elsevier Science, 1996
- Linearization of Inverse Solution
- First “full-flow-field” particle velocities in multiphase reactors were by Kondukov et al. (1964), Borlai et al. (1967), and van Velzen et al. (1974)

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- University of Illinois System, 1983
- Florida Atlantic University, 1990
- Washington University, 1990
- Ecole Polytechnique of Montreal, 1994
- Gatt at AAEC Research Establishment, 1977, Flow of Individual Pebbles in Cylindrical Vessels, Nuclear Engineering and Design, 42, 265-275 – missed in review

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- CURRENT APPROACHES:
- Based Primarily on “TOMOGRAPHIC” Imaging, in which an Array of Detectors is Assembled Around the System.

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- Introduction
- Review of RPT Techniques
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- Experimental Results
- The Nonlinear Semi-Empirical Modeling
- Monte Carlo Simulated Modeling
- Introduction To Pebble Bed Reactor
- Critique on Previous Work
- An Alternative Approach for RPT

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Top View

10 cm

4 cm

6 cm

6 cm

10 cm

Sources

Detector 1

Detector 3

25.4 cm

25.5 cm

5 cm

5 cm

Detector 2

Detector 4

Four Detectors, Dual Energy RPT Experiment

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- Introduction
- Review of RPT Techniques
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- Experimental Results
- The Nonlinear Semi-Empirical Modeling
- Monte Carlo Simulated Modeling
- Introduction To Pebble Bed Reactor
- Critique on Previous Work
- An Alternative Approach for RPT

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The Dual Energy Approach

- A radioactive particle with two “clean” energies is used – Co-60, Sc-46, Na-24
- Either two SCA’s with a “window” in each or an MCA with two ROI’s can be used for each detector –> doubling the data available from the same number of detectors
- The additional data can be placed directly into the least-squares analysis

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- Introduction
- Review of RPT Techniques
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- Experimental Results
- The Nonlinear Semi-Empirical Modeling
- Monte Carlo Simulated Modeling
- Introduction To Pebble Bed Reactor
- Critique on Previous Work
- An Alternative Approach for RPT

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Y

Detector 4

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Z

Particle Trajectory

Radioactive Particle

Detector 3

Detector 1

Detector 2

Experimental Results:

Experiment Schematic

- System Chosen for Study was Ball Mill
- Four 2” X 2” NaI Detectors were Used
- A Sparrow 4-channel system with MCA’s was Used

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Detector 4

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Particle Trajectory

Radioactive Particle

Detector 3

Detector 1

Detector 2

Experimental Results:

Responses For The Four Detectors

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- Introduction
- Review of RPT Techniques
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- Experimental Results
- The Nonlinear Semi-Empirical Modeling
- Monte Carlo Simulated Modeling
- Introduction To Pebble Bed Reactor
- Critique on Previous Work
- An Alternative Approach for RPT

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Experimental Results:

Analytical Nonlinear Model

Where; Ci=total photopeak counts of ith detector,

ri=distance from tracer to ith detector,

R=attenuation of mill wall and charge,

SR=distance traveled in wall and charge,

D =attenuation in detector,

SD=distance traveled in detector,

B = background,

A = a constant proportional to the source

intensity.

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- Introduction
- Review of RPT Techniques
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- Experimental Results
- The Nonlinear Inverse Analysis Approach
- Monte Carlo Simulated Results
- Introduction To Pebble Bed Reactor
- Critique on Previous Work
- An Alternative Approach for RPT

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Monte Carlo Simulated Results:

- The Expected Value Approach Was Used
- One can Force Every Gamma Ray to be Detected so only a few are required
- Needed Limiting Subtended Angles to Cylinder: Gardner, Choi, Mickael, Yacout, Jin, and Verghese, 1987, Algorithms for Forcing Scattered Radiation to Spherical, Planar Circular, and Right Circular Cylindrical Detectors for Monte Carlo Simulation, Nuclear Science and Engineering, 95, 245-256

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Monte Carlo Simulated Results:

Schematic of a NaI Detector in an Attenuating Column

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Monte Carlo Simulated Results:

Monte Carlo Map of Counting Rates of one Detector VS. Tracer Position

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Monte Carlo Simulated Results:

Monte Carlo Simulated Vs. Experimental Results

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- Introduction
- Introduction To Pebble Bed Reactor
- Review of Previous Work on RPT
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- Experimental Results
- Monte Carlo Simulated Results
- The Nonlinear Inverse Analysis Approach
- Critique on Previous Work
- An Alternative Approach for RPT
- Discussion and Conclusions

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- Discrepancies Between Measurements and Monte Carlo Modeling:

- This is Mainly Due to Information Distortion Due to Counting Losses Typical to High CountingRate Systems (Dead Time, Pulse Pileup,..)

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- Introduction
- Review of RPT Techniques
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- Experimental Results
- The Nonlinear Inverse Analysis Approach
- Monte Carlo Simulated Results
- Introduction To Pebble Bed Reactor
- Critique on Previous Work
- An Alternative Approach for RPT

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MODULAR HIGH TEMPERATURE PEBBLE BED RECTOR

- Helium Cooled
- “Indirect” Cycle
- 8 % Enriched Fuel
- Can be Built in
- 2 Years
- Factory Built
- Site Assembled
- On-line Refueling
- Highly Modular
- (Modules added to meet demand)
- High Burnup >90,000 Mwd/MT
- Direct Disposal of High Level Waste

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MODULAR HIGH TEMPERATURE PEBBLE BED RECTOR

For A 110 MWe:

- 360,000 pebbles in core
- about 3,000 pebbles handled
- by Fuel Handling System Daily
- about 350 discarded daily
- one pebble discharged every
- 30 seconds
- average pebble cycles through
- core 15 times
- Fuel handling most maintenance-intensive
- part of plant

German AVR Pebble Bed Reactor

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MODULAR HIGH TEMPERATURE PEBBLE BED RECTOR

TRISO Fuel Particle; (Microsphere)

- 0.9mm diameter
- ~ 11,000 in every pebble
- 109 microspheres in core
- Fission products retained inside microsphere
- TRISO acts as a pressure vessel

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MODULAR HIGH TEMPERATURE PEBBLE BED RECTOR

International Activities:

- China - 10 Mwth Pebble Bed - 2000 critical
- Japan - 40 Mwth Prismatic
- South Africa - 250 Mwth Pebble Bed- 2003
- Russia - 330 Mwe - Pu Burner Prismatic 2007
- Netherlands - small industrial Pebble
- Germany 300 Mwe Pebble Bed (Shut down)
- MIT - 250 Mwth - Intermediate Heat Exch.

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MODULAR HIGH TEMPERATURE PEBBLE BED RECTOR

Application Of (RPT) To Pebble Tracking:

- Excessive Time Spent In Parts Of The Bed Could Result In Severe Irradiation And Thermal Damage To The Pebble
- The Feasibility Of The Recycling Of Fuel Pebbles In a Pebble Bed Reactor Depends On a satisfactory Pebble Flow Through The Vessel, Its Outlet, and The Pebble Extractor
- So It Is Important To Know Pebble Pathways And Relative Velocities Through The Bed
- There Is Need For technique To study the Dynamics Of Pebbles In a Pebble Bed Reactor
- One Can Track pebbles In a Scaled Prototype

Pebble Bed Reactor (RPT)

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- Introduction
- Review of RPT Techniques
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- Experimental Results
- The Nonlinear Inverse Analysis Approach
- Monte Carlo Simulated Results
- Introduction To Pebble Bed Reactor
- Critique on Previous Work
- An Alternative Approach for RPT

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- Large Number of Detectors is Needed For Reasonably Accurate Tracking :

- Due to Inherent uncertainties Associated With Linear/Nonlinear Map Search, As much As Possible Redundant Information is Necessary For Sufficiently Accurate Particle Tracking. Thus a Large Number of Detectors is Needed Depending on the Size of The System To Be Investigated

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- Expected Modeling Difficulties In Stochastic Heterogeneous Systems, Such As Pebble Bed Reactors.

- Randomly Distributed Pebbles May Produce Large Amount of Contrast in Attenuation Between Different Particle Positions Corresponding To Same Distance From The Detector In Question, Specially At The Container Wall Region

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- Introduction
- Review of RPT Techniques
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- Experimental Results
- The Nonlinear Inverse Analysis Approach
- Monte Carlo Simulated Results
- Introduction To Pebble Bed Reactor
- Critique on Previous Work
- An Alternative Approach for RPT

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OBJECTIVES

- Investigate An Improved RPT Approach to Overcome Current Approach Limitations :
- Eliminate Information Distortion Due to Counting Losses Typical to High CountingRate Systems (Dead Time, Pulse Pileup,..)
- Reduce number of Detectors Needed
- Overcome Modeling Difficulties of Heterogeneous Stochastic Attenuating Media Such as Pebble Bed Reactors

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OBJECTIVES

- Investigate An Improved RPT Approach to Overcome Current Approach Limitations :
- Eliminate Information Distortion Due to Counting Losses Typical to High CountingRate Systems (Dead Time, Pulse Pileup,..)
- Reduce number of Detectors Needed
- Overcome Modeling Difficulties of Heterogeneous Stochastic Attenuating Media Such as Pebble Bed Reactors

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Overview of The Concept

- The Concept is Based on Dynamic Motion Control of a Cluster of Three Very Well CollimatedDetectors
- The Detectors are Mounted on a Platform Whose Height Can be Varied
- The Center Detector (With a Horizontal Slit) is Used to Directly Establish the Vertical Position of the Tracer

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Overview of The Concept

- The Two Outer Detectors are Independently Rotated To Align With The Tracer (Line of Sight Position)
- The Angles 1 and 2 are Instantaneously Recorded Providing The Two Cylindrical Coordinates r and

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Detector Response to Particle Position

- The Accuracy and Resolution of the
- Particle Positions are determined Primarily
- by Two Factors:
- How Well Resolved
- Detector Response
- Maximum.
- The Precession and Accuracy of the Physical Dimensions of the Tracking System.

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Design Optimization Of Collimator

Using Monte Carlo Simulation to Optimize the

Design of the Collimator:

- Effect of Slit Width
- Effect of the Slit Depth

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d

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W

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Effect Of Collimator Parameters On Accuracy and Resolution

- Triangles abc and ade
- are similar. Thus;

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d

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Effect Of Collimator Parameters On Accuracy and Resolution

- ROV is to be Minimized
- Thus Minimize W, and Maximize D

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Effect Of Collimator Parameters On Accuracy and Resolution

- Collimator 1:
- W= 3mm, & D= 25.4mm; Thus for S=100mm:
- ROV= 26.6mm

- Collimator 2:
- W= 1mm, & D= 50.8mm; Thus for S= 100mm:
- ROV= 4.94mm

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Implementation Of The Concept

- Precision Manufactured Platform, and Collimators

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Implementation Of The Concept

- Computer Controlled Micro-Stepping Stepper

Motors (Up to 0.18 Degrees Per Step).

- Programmable Automation and Control System

(National Instruments LabView)

- Development of a Graphical User Interface (GUI)

LabView Program, for Experiment Automation,

Control, and Visualization.

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- Introduction
- Introduction To Pebble Bed Reactor
- Review of Previous Work on RPT
- Review of Previous Work on RPT at CEAR
- The Dual Energy Approach
- Experimental Results
- Monte Carlo Simulated Results
- The Nonlinear Inverse Analysis Approach
- Critique on Previous Work
- An Alternative Approach for RPT
- Discussion and Conclusions

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- The Dual Energy Approach Was Useful As It Uses Additional Already Available Data
- The Nonlinear Inverse Analysis Approach Gave Much Better Accuracy, Than Monte Carlo Mapping
- Discrepancies Between Monte Carlo Simulation and Experiment Raised Due to Information Distortions Pertinent to High Counting Rate (Dead Time, and Pulse Pileup)
- A Simpler And More Efficient Alternative Was Needed Particularly for Tracking Pebbles In A Pebble Bed Reactor Vessel.

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- The Approach Proved to Be Valid In General.
- The Concept Has Remarkable Advantages Over the Current RPT Techniques;
- Much Less Number of Detectors Needed For Tracking (Only 3)
- Only Simple Radiation Detection Needed, Based on Measuring Only The Counting
- Rate of Each Detector, and Identify Their Maxima. This would avoid
- Measurement Complications,
- such as Pulse Pile Up,
- and Dead Time Losses

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- The Concept Eliminates The Need for Complicated, and Time Consuming Modeling and Computations, and Thus It is Most Ideal for Real Time Online RPT.
- However, The Technique Has Some Limitations.
- The Limited Speed of The Tracker, Particularly in
- The Vertical Direction, Because of the Heavy
- Mechanical Load

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