Compton Suppressed Gamma Spectroscopy of 

Spent Fuel Inventories

Abdullah Weiss

NCNR, Reactor Operations and Engineering Group
Background

 A little about me:

 Texas A&M University-Kingsville (Junior Fall 2016)

 Mechanical Engineering major, and Nuclear Engineering minor

 Project Origins:

 Idaho National Laboratory (ATR) 

 Penn State University

 Motivation of Project:

 To successfully determine the burnup of highly enriched uranium (HEU) 

fuel in the Neutron Beam Split-core Reactor (NBSR) at the NCNR.

 Benchmark the power and burnup of the NBSR.

2
INL Study (2013)

A Feasibility and Optimization Study to 

Determine Cooling Time and Burnup of 

Advanced Test Reactor Fuels Using a 

Nondestructive Technique by Jorge 

Navarro

 Determined burnup and cooling time of 

fuel using isotopic γ ray peak area, and 

peak area ratios. 

 Relationship was found between burnup 

and isotopes including 137Cs, and the 

ratios of other isotopes correlated with 

the cooling time (such as 144Ce/ 137Cs).

 Isotope ratios were correlated to cancel 

out the geometry of the measurement.

3
Penn State Study (2014)

Study of Compton Suppression for Use in Spent Nuclear Fuel Assay by Sarah 

Bender

 A fuel sample from the Penn State Breazeale Reactor was measured using 

Compton Suppression. 

 Used concrete collimator built into the fuel pool to study aged LEU fuel.

 Eight additional photopeaks were unmasked, allowing for identification of 

more isotopes in the fuel sample.

4
Penn State Study (2014)

5
Compton Suppression

 Compton Scattering

 How do we apply Compton Suppression?

γ

γ’

e

-

∆λ =

h

mec

(1 − cos θ)

θ

I am Arthur H. 

Compton, and 

I won the 

Nobel Prize in 

Physics

θ γ

γ’

Detector

Another Detector

Detectors Run in Anti-Coincidence

6
NBSR Fuel

The Reactor:  D2O Coolant, moderator, and reflector  30 fuel elements with 38.5 day fuel 

cycles

~20 MW Thermal Power

The Fuel:  93% U

3

O

8 + Al

 17 fuel plates per region  2 regions per fuel element

Burnup:  If 10% of an initial isotope underwent 

fission, the burnup is 10%.

In general: MW

∙d

Metric Tonne

=

Power×Time Spent

Initial Mass of Fuel

Fuel 

Plates

7

Burnup:

If 10% of an initial isotope underwent 

fission, the burnup is 10%.

In general: MW

∙d

Metric Tonne

=

Power×Time Spent

Initial Mass of Fuel
We wanted to measure spectra from different spent fuel elements using a HPGe

detector and a BGO detector.

 We had to channel a γ beam from the fuel elements to the detectors.

 The detectors had to be setup in a way that accommodates Compton suppression.

 It had to be safe: health physics, crane lift, and γ beam control.

 Not a single thing besides the collimator and its peripherals was to touch the water.

 Had to be easy to assemble and disassemble for multiple uses for different γ beam 

sizes.

 Mechanical issues to be addressed: structure, buoyancy, material selection, 

manufacturing

 Cost Effectiveness

 Detectors Setup and Calibration

 EVERYTHING MUST WORK!!!!

Results?

Challenges

8

14 15 16 17 18 19 20 21 22

Vertical Distance (ft)

Dose vs Distance From Source to Detector

1048 1049 1050 1051

Radiation Safety

1048

y = a1b1 x

R2 = 0.9976

1049

y = a2b2 x

R2 = 0.9729

1050

y = a3b3 x

R2 = 0.9992

1051

y = a4b4 x

R2 = 0.9986

9

Questions:

1. How far down should we initially measure?

Necessities:

1. Attenuation Coefficients (μ)

2. Average Attenuation Coefficient (μavg)

3. Average Tenth Value Layer (TVL)

???? ft

Collimator

Fuel Element

H2O 

in the pool

Measured by Timothy Barvitskie
Radiation Safety

Attenuation Coefficient (μ):

D = D0e

−μx μ =

−ln(

D

D0

)

x

Tenth Value Layer (TVL):

TVL =

ln(10)

μ

Calculated Results:

μ (1/ft) TVL (ft)

1048 1.8697 1.23

1049 1.6869 1.36

1050 1.8735 1.23

1051 1.8437 1.25

AVG 1.81845 1.27

1.27 ft

D

D

10

THIS IS IMPORTANT!!

10
Where do we put the detectors?

Apparatus Requirements:

 Easy to assemble/disassemble

 Mobile

 Stable

Other places that were considered:

 On the side of the pool

 In the pool (under the lid) HPGe

BGO

Catwalk

The Pool

11

LN2
Where do we put the detectors?

12
Where do we put the detectors?

13
Where do we put the detectors?

14
The Apparatus

8020 Aluminum

Polyethylene

Steel

6061 Aluminum

15
The Collimator

 6’(x4) 

 1” OD – 0.93” ID

 Connected via Tube Locking

Fittings

16
Structural Analysis

wHPGe

wBGO

wCol

Ʃw = 115 lb

Ʃw < 400 lb

 There is a risk of dropping 

detectors in the pool.

 Treat system as a structural beam, 

and find the center of mass for 

the platform.

 Center of mass was within the 

safety zone.



17

C.M.
Buoyancy Analysis

In order for the collimator to work properly, it must sink in.

Original thought: 

2”OD 1.65”ID tubes for collimator

Health Physics Concerns make Collimator smaller (1”OD, 0.93”ID)

FB,applied = ρH2O × g × Vdf,max − wCol ≈ 5 lb

5 lb deficit

But since it’s held in place, would it REALLY bend too much?

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Detector Calibration

 Done using GENIE-2000

 Energy Calibration using Co-60 and Eu-154 Sources

19
20
Initial Results

 Collimator was bending in the water.

 Background spectrum was not very 

different from fuel spectrum.

 Needed a solution

21

Channel

Counts
Initial Results

Background Spectrum (24 hour count)

Fuel Spectrum (1 hour count)

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Initial Results

Background Spectrum

Fuel Spectrum

Burnup can be found using

∆

134Cs

∆

137Cs

23

137Cs

662 keV

t1/2=30 y 134Cs

795 keV

t1/2=2.0 y

110Ag

657 keV

t1/2=249 d
 Peaks were refitted in Hypermet-PC



∆

134Cs

∆

137Cs ratio was calculated with the refitted peaks

Pictures Courtesy of Danyal Turkoglu

24

Peak Fitting
Isotope Buildup

over fuel life

(MCNP)

0 E+0

2 E-3

4 E-3

6 E-3

8 E-3

1 2 3 4 5 6 7 8

Relative Concentration

Cycle #

Cs-137 Isotope vs Fuel Pass

m15 m16 m17 m18 m25 m26 m27 m28

0 E+0

2 E-4

4 E-4

6 E-4

8 E-4

1 2 3 4 5 6 7 8

Relative Concentration

Cycle #

Cs-134 Isotope vs Fuel Pass

m15 m16 m17 m18 m25 m26 m27 m28

25

235U

Other Isotopes

Fission
Experiment efficiency (MCNP)

26

134Cs

137Cs
Burnup of fuel (MCNP depletion study)

0

1000

2000

3000

4000

5000

6000

7000

8000

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

BURNUP (MW-DAYS)

RATIO (134Cs/137Cs)

Burnup vs Cs Ratio

27
BURNUP!!

∆

134Cs

∆

137Cs

=

ε134

ε137

×

CR134

CR137

×

λ137I662

λ134I795

×

e

λ134 tcool

e

λ137 tcool

Experiment Efficiency Count Rate Specific γ emission rate Cooling Time

Fuel Element Isotope Ratio Burnup (MW-day) Power* (MW)

S1025 (July 2015) 0.0751 5329 19.8

S1036 (Jan 2016) 0.0705 5100 18.9

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*38.5 d cycles assumed (7-cycle fuel)

Calculated error due to background corrected gamma peaks area is +/-0.04 MW
Burnup Results

0

1000

2000

3000

4000

5000

6000

7000

8000

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

BURNUP (MW-DAYS)

RATIO (134Cs/137Cs)

Burnup vs Ratio

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Conclusion

 Assembled the entire experiment and proved that it works

 Addressed health physics concerns and completed safety evaluation

 Observed several isotopes from fuel, and was able to determine burnup

 This is the first time this is ever been done

Future Work

 Make a collimator that works better

 Improve stability of apparatus

 Longer count times (overnight)

 Measuring additional elements

 Evolve this experiment into a gamma spectroscopy scanning apparatus

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Acknowledgements

Thomas Newton

Dan Hughes

Daniel Flynn

James Moody

Thomas Johnston

Samuel MacDavid

William Clow

Julie Borchers

Paul Brand

Attila Halacsy

Robert William

Gregory Heller

David Brown

Doug Johnson

Don Lopez

Joe Dura

And all the SURF directors and the SURF Program

31
Acknowledgements

A special thank you to these individuals:

Daniel Mattes

Timothy Barvitskie

Danyal Turkoglu

Ryan Fangmeyer

HE’S A SURF STUDENT TOO!!

Taught me almost everything I know about detectors

He’s a Mechanical Engineer.

It automatically makes him awesome

Guided me through μ and 

TVL calculations (and helped 

out with Genie...and helped 

out with other details of the 

project)

32
Acknowledgments

My Mentor:

Bryan Eyers

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Q/A?