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Overview and Status of the Advanced Gas Reactor Fuel

Development and Qualification Program

David Petti Co-National Technical Director, Advanced Reactor Technologies Program

Fuel Element

TRISO fuel is a key part of the “Functional Containment” strategy for licensing modular HTGRs • Functional Containment is the collection of design selections that,

taken together, ensure that:

– Radionuclides are retained within multiple barriers, with emphasis on retention at their source in the fuel

– Regulatory requirements and plant design goals for release of radionuclides are met at the Exclusion Area Boundary

2

• Fuel Kernel

• Fuel Particle Coatings

• Matrix/Graphite

• Helium Pressure Boundary

• Reactor Building

TRISO Fuel is at the heart of the safety case for modular HTGRs

• Fabrication of high-quality low-defect fuel is achievable at industrial scale.

– Defect fractions on the order of 10-5

– Process produces narrow distributions of fuel attributes (standard deviation of thicknesses and densities are small)

– Process is stable and repeatable at industrial scale (batch to batch variation is very small)

• Robust irradiation performance inside design service envelope

– Designer assumes incremental failure fraction of 2x10-4

– Performance is better than designer assumption

• Robust accident performance inside design service envelope

– Designer assumes incremental failure fraction of 6x10-4

– Performance is better than designer assumption

Results in low activity of fission products in helium coolant

For the small level of defective particles, significant retention in fuel kernel, fuel matrix and graphite

Results in low incremental release of fission products to the helium coolants

Significant retention of metallic fission products in graphite and fuel matrix

Results in low incremental release of fission products under accident conditions

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3

Coated Particle Failure Mechanisms

4

Coated Particle Failure Mechanism Control • Structural/mechanical mechanisms

– Excessive PyC irradiation induced shrinkage leading to SiC cracking

– Pressure vessel failure

• Thermochemical mechanisms

– Kernel migration

– Corrosion of SiC

– Thermal decomposition of SiC

• Control of failure mechanism

– Coated particle design

– Fuel specifications

– Product upgrading (sieving, tabling)

– Qualified characterization and acceptance procedures

– Limits on service conditions (burnup, fluence, temperature, temperature gradients)

5

UCO Fuel being Qualified as Improved Fuel

• UCO (UCxOy) is UO2 with UC and UC2 added

• UCO designed to provide superior fuel performance at high burnup

– Kernel migration suppressed (most important for prismatic designs because of larger thermal gradients)

– Eliminates CO formation; internal gas pressure reduced

– Fission products still immobilized as oxides

– Allows longer, more economical fuel cycle

• Reference fuel for NGNP prismatic reactor designs

• Potential higher burnup alternative for pebble bed HTGRs

6

TRISO Particle Fabrication

Overcoating

Furnaces

dry-calcine-sinter

200 – 800 – 1600°C 235U <20% U3O8 Ammonia

Donor

Dissolution

Carbon for UCO

Water-Wash

Gel-Sphere

Kernel

Gelation

TRISO Particle

Matrix + particles

Compaction

Compact

Furnaces

carbonize – heat treat

800 – 1800°C

Insulation

Fluid-Bed Coater

(1300-1500°C) Pyrocarbon, SiC Layers

7

Scaling Up Kernel Production, Coating, Overcoating, and Compacting Processes to Create a Pilot Line

Prepare

Matrix

Overcoat

and Dry Sieve Table

Riffle Compact Carbonize Heat Treat

Granurex Overcoat

and Dry

Dry Mix

and Jet Mill

Matrix

Hot Press

Compact

Carbonize +

Heat Treat in

one Sequential

Process

Lab Scale

Engineering Scale

Lab Scale 2-inch CVD

Coating (60 g charge)

Industrial Scale 6-

inch CVD Coating

(2 kg charge)

Sol-Gel Kernel Production

Kernel Forming

and Drying

8

Fuel Fabrication Evolution

Kernels Coatings Compacts

AGR-1 Engineering scale Lab Scale Lab Scale

AGR-2 Engineering Scale Engineering scale Lab Scale

AGR-5/6/7 Engineering Scale Engineering Scale Engineering Scale

9

AGR TRISO Fuel Fabrication Process Improvements • Removed human interactions in the process

– Eliminated tabling with 3D sieving of coated particles

– Replaced multi-step matrix production (resin solvation, matrix mixing, kneading, drying and crushing) with dry mixing and jet milling of matrix

– Replaced rotary overcoater and upgraded to an automated fluidized bed overcoater that produces highly spherical, uniformly overcoated particles

– Automated die filling and punch travel to form compacts

• Kernel fabrication – Internal gelation to improve sphericity

– Method of carbon addition modified to improve distribution of oxide and carbide phases

• Improved chemical vapor deposition process control – Argon dilution during SiC coating

– Coater “chalice” and multiport nozzle to improve yields (>95%)

– Mass flow controllers to control gas flows during deposition of each coating layer

– Improved MTS vaporizer (leveraging computer chip industry) to evaporate MTS and make SiC layer

• Improved measurement science – Computer measurements of thicknesses

– Greatly improved anisotropy measurements

– Improved density measurements using better density column materials

10

AGR Fabrication Results

• Good tight distributions of particle populations

• Have trouble sometime meeting U.S. prismatic specification on HM

contamination

• Can probably meet German specification of 6E-05

• X-energy can probably have a higher specification here because of low

power level and hence low source term 11

Fuel Fabrication Accomplishments

• Re-established capability to fabricate and characterize TRISO-coated particle fuel in the U.S. after a 10-15 year hiatus

• Developed a significantly improved understanding of how to fabricate high-performing TRISO fuel providing the technical basis for co-location of NGNP in industrial complexes

• Currently fabricating high-quality, low-defect TRISO-coated fuel particles in industry (BWXT). Can meet physical specifications and are almost meeting all defect specifications at 95% confidence. With larger sample sizes and a mature process, should meet the defect specifications in production mode

• Vastly improved quality, reproducibility, process control, and characterization of TRISO fuel. Better control of the process, removal of high variability human interactions in the process, and better measurement technologies all contribute to better quality TRISO fuel

• Establishing a domestic vendor and associated fundamental understanding of key fuel fabrication parameters establishes credibility that the historical industrial experience from Germany in the 1980s is repeatable and has a sound technical basis

• All technologies needed to establish a pilot line are in industrial hands. Qualification fuel for AGR-5/6/7 is being fabricated now

12

Performance Envelope for US AGR TRISO Fuel Program is More Aggressive than Previous German and Japanese Fuel Qualification Efforts

Radar plot of five key parameters of fuel performance

Packing Fraction

50

30

10

Time-averaged

Temperature (°C)

Fast Fluence (x1025 n/m2) Burnup (% FIMA)

Power

Density

(W/cc) 1250

1100 2

10

10 25 3 5

NGNP

German

Japan

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Overview of AGR Program Activities

Moisture and air ingress effects are part of AGR-5/6 PIE

Early Lab Scale Fuel

Capsule Shakedown

Coating Variants German

Type Coatings

Large Scale Fuel

Performance

Demonstration

Failed Fuel to Determine

Retention Behavior

Fuel Qualification

Proof Tests

Fuel and Fission

Product Validation

Fuel Product

Transport/Retention

AGR-1

AGR-2

AGR-3/4

AGR-5/6/7

AGR-8

Lab Tests

AGR-1

AGR-2

AGR-3/4

AGR-5/6/7

AGR-8

Integral Loop

Validation

Update Fuel

Performance and

Fission Product

Transport Models

Validate Fuel

Performance and

Fission Product

Transport Models

Purpose Irradiation Safety Tests

and PIE

Models

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AGR Program Irradiation Experiments and PIE

AGR-1 AGR-2

AGR-3/4

AGR-5/6/7

AGR-1

AGR-2

AGR-3/4

AGR-5/6/7

Early test of lab-scale fuel

performance; shakedown

of test train design.

Fuel qualification proof tests.

Engineering scale particles and

compacts.

Irradiation

(in ATR)

PIE

(INL and ORNL)

Failed fuel to assess

fission product retention

and transport in reactor

graphite and fuel matrix.

Engineering scale particles in lab-

scale compacts. Includes UCO

and UO2 fuel.

15

Capsules

In-core

He Ne He-3

Silver

Zeolite

Particulate

Filters

H-3

Getter

Grab Sample

FPMS

Vessel Wall

Individual capsule assembly

with fuel compacts Completed test train Insertion into INL ATR FPMS system 16

Fuel Irradiations (AGR-1, AGR-2 and AGR-3/4)

TRISO Fuel Irradiation Qualification Accomplishments • Completed most successful U.S. irradiation of

TRISO-coated particle fuel (AGR-1). 300,000

particles tested to peak burnup of 19.6% FIMA, a

peak fast fluence of 4.3×1025 n/m2 and peak

time-average peak temperatures of <1200°C –

peak MHTGR service conditions – with no

failures

– The expected superior irradiation performance

of UCO at high burnup has been confirmed -

no kernel migration, no evidence of CO attack

of SiC, and no indication of SiC attack by

lanthanides

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• AGR-1 95% confidence failure fraction is <1E-5, a factor of 20 better than the design in-

service failure fraction of 2E-4. The more severe AGR-1 irradiation conditions compared to

the vast majority of historic modular HTGR designs suggest substantial fuel performance

margin

• Irradiation of AGR-2 is complete. PIE is underway. No full TRISO failures based on PIE to

date. With 114,000 UCO TRISO particles in the irradiation this corresponds to a failure

fraction of 2.6E-05 at 95% confidence

• Irradiation of AGR-3/4 to study release/retention of fission products from failed TRISO fuel

completed in April 2014. This experiment will provide data needed for source term

evaluations for UCO TRISO fuel and new matrix and graphite. PIE is underway.

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

Kr-

85

m R

/B U.S. TRISO/BISO

U.S. WAR TRISO/BISO

U.S. TRISO/TRISO

U.S. TRISO-P

German (Th,U)O2 TRISO

German UO2 TRISO

AGR-1

AGR-2

U. S. Fuel German Fuel

U.S. German

Irradiation temperature ( C) 930 - 1350 800 - 1320

Burnup (%FIMA) 6.3 - 80 7.5 - 15.6

Fast fluence (1025 n/m2 ) 2.0 - 10.2 0.1 - 8.5

Rp AGR-3/4 & AGR-2 Comparable with Historical Data

Combined fit of AGR-3/4 and AGR-2 data

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AGR-5/6/7 details

Desired Fraction of Particles

per Temperature Range

Minimum Number of Particles

Based on 500,000 total

30% <900C 150,000

30% 900C - 1050C 150,000

30% 1050C - 1250C 150,000

10% 1250C - 1350C 50,000

Total 500,000

Temperature Range Minimum Number of Particles

1350C - 1500C 50,000

AGR-5/6

AGR-7

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Post-Irradiation Examination (PIE) and Safety Testing of TRISO Fuel

• Examine fuel performance:

– Fission product retention:

• during irradiation

• during high temperature accident scenarios (safety testing)

– Fuel kernel and coating microstructure evolution and causes of coating failures

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Key AGR PIE Accomplishments and Results

• Re-established coated particle fuel PIE and safety testing capabilities at both INL and Oak Ridge National Laboratory

• Developed numerous new tools and approaches for analyzing irradiated particle fuel

• Advanced PIE methods are enabling an unprecedented level of understanding of coated particle fuel behavior

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Studying failed particles greatly improves ability to characterize and understand fuel performance

AGR-1 Test Train

Vertical Section

Fuel

Compacts

Plenum

between

Capsules

300,000

particles

in AGR-1

irradiation

Gamma scan

tomography to

identify cesium hot

spot and compact

location

Deconsolidation to

obtain 4,000 particles

from compact

X-ray tomography to

nondestructively locate

defects/fractures

0.15

0.36

0.25

0.00

0.00

0.12

0.00

0.64

1.04

0.51

0.72

0.89

0.49 0.61 0.34 0.19 0.33 0.00 0.24 0.61 0.20 0.31 0.62 0.31

5-2-1 5-2-3

5-2-2

IMGA to find

particles with low

cesium retention

Advanced

microscopy to

study

microstructure

in detail

Capsule

disassembly

22

TRISO Fuel Post-irradiation Examination Accomplishments • Post-irradiation examination is revealing

new understanding of fuel performance and fission product transport

– Characterization of kernel and coating behaviors to better understand performance and potential failure modes

– More complete mass balance of key fission products (Ag, Cs, Sr, Eu, Ce, Pd)

• Very low releases of safety important fission products

• Models overpredict releases

– Lack of significant fission product/SiC interactions

• Gross overprediction by computer models

– Have observed 4 SiC failures out of 300,000 particles corresponding to a SiC failure fraction of 3.1E-05 at 95% confidence

23

Deconsolidated

AGR-1 particles

and matrix

Particle handling and

inspection

Capsule Disassembly

and Non-contact

metrology

Advanced-IMGA

AGR-2 PIE Progress (1/2)

• Disassembly of irradiation capsules complete

• Dimensional measurements of fuel and capsule components complete

• Gamma scanning of compacts and graphite holders from US capsules complete

– Measured burnup of compacts for comparison with physics calculations

– Determined inventory of fission products in compacts; compare Ag-110m inventory with predictions to estimate silver release in-pile

– Map fission products in the graphite fuel holders; determine which compacts may have contained particles with failed SiC based on cesium hot-spots in the graphite

• Analysis of fission product inventory on capsule components is in progress; used to determine total fission product release from fuel compacts

AGR-2 PIE Progress (2/2)

• Destructive analysis of as-irradiated compacts:

– Deconsolidation of two UCO compacts and particle analysis in progress (locate and analyze particles with failed SiC)

– Cross-section analysis of four compacts in progress

• High temperature safety testing

– Completed safety tests on two UO2 compacts (1600°C); post-test analysis in progress, including locating particles with failed SiC

– Completed safety tests on two UCO compacts (1600°C); post-test analysis in progress, including locating particles with failed SiC

– Preliminary results indicate excellent performance of UCO fuel at 1600°C; UO2 exhibits higher cesium release relative to UCO

Results of AGR-2 compact gamma scanning

• Good agreement between burnup measured by gamma spectrometry and burnup determined by physics predictions

6

7

8

9

10

11

12

13

14

15

-30 -20 -10 0 10 20 30 40 50 60

Bu

rnu

p (

%F

IMA

)

Distance from Core Centerline (cm)

Measured Burnup (Cs-134/Cs-137)

Burnup Cs-137 Activity

Predicted Burnup

Capsule 2 Capsule 3 Capsule 5 Capsule 6

0%

20%

40%

60%

80%

100%

120%

950 1050 1150 1250

Re

tain

ed

Ag-

11

0m

Fra

ctio

n

TAVA Temperatture (°C)

Capsule 2

Capsule 3

Capsule 5

Capsule 6

• Ag-110m inventory measured in fuel compacts

as a function of time-average, volume-average

(TAVA) irradiation temperature

Gamma scanning of graphite holders for fission products

• Empty graphite holders gamma-scanned to map the location of fission productts

• Based on areas of elevated cesium activity, a total of 5 compacts have been identified as potentially containing particles with failed SiC

• Compacts are being destructively examined to explore the nature of the SiC failures

Stack 1 Stack 2

Stack 3

Cs-134 activity intensity map of one

level in the AGR-2 Capsule 5 graphite

holder, showing elevated activity near

the location of Compact 5-2-3

AGR-3/4 PIE Progress

• Capsule disassembly completed

• Dimensional measurements of fuel compacts and capsule components completed

• Gamma scanning of graphite and matrix rings to map fission product distributions is in progress

AGR-3/4 ring analysis

• Matrix and graphite rings are gamma-scanned to map the distribution of fission products

• Fission product distributions are compared to model predictions for fission product transport

Co-60

Cs-134 Cs-137

Eu-154

Activity intensity maps of a cross-section of the inner

matrix ring from AGR-3/4 capsule 7

TRISO Fuel Safety Testing

30

INL Furnace ORNL Furnace

AGR-1 Safety Testing Results Highlights

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

-50 50 150 250 350 450

13

4C

s r

ele

as

e f

rac

tio

n

Time at temperature (h)

'3-3-2 '3-2-2 '4-1-2'4-3-3 '5-3-3 '6-2-1'6-4-1 '6-4-3 '3-3-1'4-4-3 '4-3-2 '4-4-1'5-1-3 3-2-3

1800°C 1700°C

1600°C

Level of one particle

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

-50 50 150 250 350

85K

r re

leas

e f

rac

tio

n

Time at temperature (h)

'4-3-3 '5-3-3'6-2-1 '6-4-1'3-3-1 '4-3-2'4-4-1 '5-1-33-2-3 1800°C

1700°C

1600°C

• Fuel compacts were heated to 1600 – 1800°C for 300 h while measuring release of fission products

• No TRISO failures at 1600 and 1700°C; only two failures in a single compact at 1800°C

• Cs release used as indication of SiC layer failure; fuel compacts with SiC failures processed to identify failed particles and characterize the coatings

• Specific mechanism of SiC failure was identified (IPyC failure followed by Pd attack of SiC)

• High temperature fuel performance generally considered very good

31

Eu and Sr Results

32

Safety Behavior Summary

• Accident performance of UCO TRISO is very robust (300 hours at 1600, 1700 and 1800°C). Releases of safety significant fission products are very low at 1600 and 1700°C.

• No full particles failures observed in testing to date at 1600 or 1700°C (10 compacts = ~41,000 particles). This implies a 95% failure fraction of ~ 7.3E-05, a factor of 8 margin relative to the prismatic reactor specification. We expect the margin to be 10× when all testing is complete.

• Recall that only 2-3% of reactor core sees 1600°C in conduction cooldown scenario

• Release from intact particles at 1600 and 1700°C is hard to discern. Release appears to be coming from material that diffused into the matrix under irradiation

• No SiC degradation noted at 1800°C as was seen in German UO2 TRISO

– Releases imply very different physics/chemistry. Lack of CO production in UCO is believed to be the cause

• Cesium releases above the level of one particle are seen infrequently and are related to SiC failures that occurred during heating. Have observed 3 SiC failures out of 33,000 at 1600°C corresponding to a SiC failure rate of 2.4E-04 at 95% confidence.

• Specific mechanism of SiC failure was identified (IPyC failure followed by Pd attack of SiC)

33

Comparison of Key Results to Fuel Performance Design Assumptions (all at 95% confidence)

MHTGR

Prismatic

HTR MODUL

Pebble

AGR Results

Manufacturing Defect Level

Heavy Metal Contamination 2 x 10-5

6 x 10-5

2 to 5 x 10-5

(depending on batch)

SiC Defects

1 x 10-4 3 to 6 x 10-5

(depending on batch)

In-service Performance Requirements

Incremental Full TRISO Failures

Normal Operation

Incremental SiC Failures Normal

Operation

2 x 10-4

-----

1.6 x 10-4

-----

< 1 x 10-5 (AGR-1)

2.6 x 10-5 (AGR-2)

2.6 x 10-5

Incremental Full TRISO Failures

Accidents

Incremental SiC Failures Accidents

6 x 10-4

-------

6.6 x 10-4

------

7.3 x 10-5

2.4 x 10-4

34

Impact of UCO TRISO Fuel as key part of Functional Containment Concept for modular HTGR Licensing

• UCO TRISO fuel is being fabricated at industrial scale (BWXT) to the high quality low defect level required by designer to support the HTGR safety case

– This contributes substantially to the very low source term under normal operation

• The irradiation performance of UCO TRISO is excellent up to 20% FIMA and 1250°C

– The failure rate is 20x below the designer requirement (substantial margin)

– Releases of key safety-relevant fission products (e.g. Cs and Sr) are very low (high degree of fission product retention)

• The safety performance of UCO TRISO fuel is excellent

– Fuel is robust after 300 hours at 1600, 1700 and 1800°C

– Failure rate at 1600°C is 8-10x below designer requirement (substantial margin)

– Releases of key safety-relevant fission products are very low (high degree of fission product retention) at 1600 and 1700°C

35

Future Fuel Activities (next 3 years)

• Complete PIE and safety testing of AGR-2 industrially produced TRISO particles

• Complete PIE and safety testing of AGR-3/4

– This fuel contains designed to fail particles and provides crucial information about fission product release from failed/defective UCO TRISO fuel which is needed to support VHTR source term analysis

• Complete fabrication of final qualification fuel at BWXT for AGR-5/6/7 campaign

• Complete design of AGR-5/6/7 irradiation test train and initiate irradiation of qualification fuel

• Beyond 2017, complete AGR-5/6/7 irradiation, complete PIE and safety testing including moisture and air ingress effects

36