aerospace nuclear science & technology division - safety...

ORNL is managed by UT-Battelle

for the US Department of Energy

C.J. Hurt

University of Tennessee Nuclear Engineering Department (This material is based upon work supported under a Department of Energy Nuclear Energy University Programs Graduate Fellowship.)

James D. Freels, Frederick P. Griffin, Randy W. Hobbs, David Chandler ORNL, Research Reactors Division

Robert M. Wham ORNL, Nuclear Science & Engineering

Safety Analysis

Models for 238

Pu

Production at HFIR

Presented at

Nuclear and Emerging

Technologies for Space 2015

February 23-26, 2015 Albuquerque, NM

http://www.utk.edu/

2 Hurt, NETS Conference, Albuquerque, NM, Feb. 23-26, 2015

Presentation Outline

• Experiment Safety Overview

• Model Physics

– Contact/Gap Conductance

– Irradiation Behavior

• Model Inputs

• Steady-State Model

• Transient Model

• Model Outputs


238Pu Supply Requires Integration Across

Existing DOE Facilities

ProcessingORNL

Powder

PlannedPlutonium Fuel Production

IrradiationATR/HFIR

TargetFabrication

LANL

StoredNeptunium

• 238Pu is the fuel source for RTGs that power NASA deep space missions

• The final supply chain requires integration from Oak Ridge National Laboratory (ORNL), Idaho National Laboratory (INL), and Los Alamos National Laboratory (LANL)

• This presentation discusses the safety analyses required for irradiation of preliminary targets at the High Flux Isotope Reactor (HFIR) at ORNL


Target Qualification

• Four phase test program, each phase informing the next phase design

• Post-irradiation examination (PIE) results from each phase serve as a hold point for the following irradiations

pellet dimensional changes pellet clad interaction

fission gas release % 236Pu production

heat generation rates product yields

Post-Irradiation Characteristics


Experiment Safety Review

• Target qualification at HFIR requires a safety review that assures target cooling in off-normal and nominal reactor operating conditions.

• Safety analyses using software that meets DOE requirements for SQA: COMSOL Multiphysics, RELAP5, ANSYS, MCNP5, ORIGEN-S and VESTA.

• Steady-State Analysis in COMSOL

• 50% reduced flow

• 130% overpower Bounding safety condition

• Transient Analysis in RELAP5:

• Small break loss of coolant

• Loss of offsite power

Off-Normal Safety Review Transport and

Depletion Analysis

Steady-State Thermal-Structure Analysis

Transient Heat Transfer

Analysis

Experiment Safety Approval Documentation


Contact/Gap Conductance Physics

• Peak pellet temperatures are driven by heat transfer in the radial gap between the pellets and cladding

• Gap heat transfer (h) is strongly dependent on the pellet dimensional changes and fission gas release

Temperature

increase due to gap

heat transfer

between the pellet

and Al cladding

ℎ = ℎ𝑠 + ℎ𝑔 + ℎ𝑟

hs = constriction

conductance or solid spot

conductance

hg = gas gap conductance

hr = radiative conductance

δ


Contact and Gap Conductance

• Depends on:

– Mean (harmonic) thermal conductivity of mating surfaces, km , Temperature

– Contact pressure (P), surface hardness (H) and elastic modulus (E)

– Function of effective surface roughness σ =√(𝜎12 + 𝜎2

2) and slopes of asperities (m = tanθ)

– Total apparent area of contact, Mean junction temperature

• Predicted by assuming a mode of ‘microscopic deformation’

– Plastic Deformation:

– Elastic Deformation (conservatively assumed):

– Alternatively, a ‘plasticity index’ may be used

• Gas gap conductance

• Portion of thermal contact conductance due to “conduction only” through “real” contact spots


Gas Gap Conductance

• Thermal jump distance

– α = accommodation coefficient, λ = mean free

path

• Large changes in gap heat transfer solution can be caused by small changes in parameters:

– Accuracy of the model equations, inputs, solver, etc.

– Up-to-date pellet material properties (thermal expansion, thermal conductivity, etc.)

– The initial radial fabrication gap between pellet/cladding

– The burnup-dependent pellet radial shrinkage/swelling


-0.10

0.00

0.10

0.20

Vo

lum

e F

ract

ion

Accumulated Fission Density/Burnup

Void Volume

Fission Product Swelling

Total Swelling

Dimensional Irradiation Behavior

• Swelling (+ΔV) increases linearly with the accumulation of fission products.

• Densification (-ΔV) occurs early due to the irradiation-induced sintering of the initial pellet porosity (P0).

• Net behavior is then an initial shrinkage of the pellet gradually overcome by swelling as the irradiation period progresses

• Other parameters (temperature, pressure, pore sizes) can be considered negligible for a simple model dependent on fuel burn up (BU) and constant coefficients (α,β)

∆𝑉𝑠𝑤𝑒𝑙𝑙.= 𝛼 ∗ 𝐵𝑈

∆𝑉𝑑𝑒𝑛𝑠.= 𝑃0 𝑒−𝛽∗𝐵𝑈 − 1

∆𝑉𝑡𝑜𝑡𝑎𝑙= ∆𝑉𝑑𝑒𝑛𝑠. + ∆𝑉𝑠𝑤𝑒𝑙𝑙.

Region of pellet densification/swelling

observed in post irradiation examination results


Model Inputs

• PIE results

• Shrinkage/Swelling vs. burn-up for radial and height dimensions

• Fission gas release fractions

• Temperature- and composition-dependent material property data for the NpO2/Al pellet (and other materials):

• Thermal expansion coefficient

• Thermal conductivity

• Stress/strain curves (i.e. elastic modulus, yield strength)

• Density, Poisson’s ratio, etc.

• Target Design Drawings and Information

• Input heat generation rates, burnup and fission gas production from neutronics calculations


Example Input: Shrinkage/Swelling PIE Curve

PIE data has progressed from reduced length pellet (top left), partial-target (top right) and full-target (bottom right) results for more detailed input to the model

-5

-4

-3

-2

-1

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Dia

mete

r C

han

ge A

vera

ged

by I

nit

al

Po

rosit

y

Ratio to First Cycle Median Burnup

Partially Loaded Target PIE Results

Fully Loaded Target PIE Results


Example Input: Pellet Property Data

0.000015

0.000017

0.000019

0.000021

0.000023

0.000025

400 450 500 550 600

Perc

en

tt T

herm

al E

xp

an

sio

n (

%)

Temperature (degC)

800 C

900c

1200c

Thermal expansion data for pellets sintered at different temperatures

Stress/strain curves for pellets evaluated at different

temperatures

0

2

4

6

8

10

12

14

0 5 10 15 20

Str

ess (

ksi)

Strain (%)

RT 200 °C

300 °C 400 °C

500 °C


Example Input: Neutronics HGR for VXF-15 Cycle 2

0

50

100

150

200

250

300

350

400

450

-25 -15 -5 5 15 25

Mass

-Sp

ecif

ic H

eat

Gen

. R

ate

(w

/g)

Reactor Axial Position (cm)

EOC-1 VXF-15

EOC-1 VXF-3

Day 5

Day 10

Day 15

Day 20

Day 26

EOC-1 VXF-15 Fit

EOC-1 VXF-3 Fit

Day 5 Fit

Day 10 Fit

Day 15 Fit

Day 20 Fit

Day 26 Fit


Steady-State Analysis in COMSOL

• Steady state coupled thermal-structure analysis of an individual Pu-238 capsule or single hot pin

• Primary codes: COMSOL v4.2a, v4.3 and v5.0

• Independent Reviews: Independent Reviews: ANSYS and hand calculations using Excel spreadsheets

• Dimensions: 3-D, 2-D axisymmetric

• Calculations:

• C-HFIR-2012-017, Rev. 1, Bare-Pellet Test Capsules

• C-HFIR-2012-036, Rev. 0, Reduced-Length Pellet Capsules

• C-HFIR-2013-007, Rev. 0, Partially Loaded Targets

• C-HFIR-2013-029, Rev. 4, Fully Loaded Targets


single bare pellet, 2nd irradiation cycle, COMSOL 4.2a, 3D, ¼ pie slice

reduced-length bare pellet, 2nd irradiation cycle, COMSOL 4.2a, 3D, ¼ pie slice

partially-loaded (8 pellets) prototype production target, 2 irradiation cycles, COMSOL 4.3

fully-loaded prototype production target (52 pellets), COMSOL 4.3, 2D axisymmetric

individual pellet at maximum

temperature in stack:

stress contour with 10000x

deformation

Overview of calculations developed in

COMSOL


Transient Analysis in RELAP5

• Transient thermal hydraulics analysis of seven pin Pu-238 target holder: single hot tube plus six average tubes

• Primary code: RELAP5

• Dimensions: 1D, Time-Dependent

• Calculations:

• C-HFIR-2012-015, Rev. 0, Bare-Pellet Test Capsules

• C-HFIR-2013-006, Rev. 1, Partially Loaded Targets

• C-HFIR-2013-030, Rev. 1, Fully Loaded Targets


Model Outputs

• The primary output for the model is the maximum target temperature, which is compared against an assumed melting temperature of 650 °C.

• Other outputs including the pellet and cladding temperature profiles, component temperatures, net thermal expansion.

• Transient analysis outputs also include target surface and bulk coolant temperatures

• Determination of initial fabrication gap and pellet shrinkage values corresponding to the melting temperature:

• For different cycles, irradiation positions, and material property data

• Evaluation at 100% (85 MW) and 130% (110.5 MW) steady state powers

• Evaluation at 100% (5 gpm) and 50% (2.5 gpm) coolant flow rates

• Evaluation at SBLOCA and LOOP events for transient analysis

• A “best-estimate” evaluation was made for the target temperature profiles at various reactor operating times (or burn-ups)


LOOP EOC2 – Target surface temperatures never exceed

adjacent coolant saturation temperatures


RELAP5 Transient Results

Parameter VXF-15, EOC1 VXF-15, EOC2

NpO2/Al pellet thermal conductivity

and thermal expansion

1200-C oxide,

31% non-conductive volume

1200-C oxide,

31% non-conductive volume

Cold fabrication gap (average) 0.5 mil radial 0.2 mil radial

Cold shrinkage gap 2.1 mil radial

(4.2 mil diametric)

1.7 mil radial

(3.4 mil diametric)

Fission gas release fraction 10% Xe/15% Kr 10% Xe/17% Kr

LOOP max. centerline temp. 642.7 C 643.5 C

SBLOCA max. centerline temp. 640.1 C 639.3 C

1

2

3

4

5

6

7

8

9

100 150 200 250 300 350 400

Allo

wab

le P

ellet

Dia

metr

ical

Sh

rin

kag

e (

Mils)

Maximum Pellet Heating (W/g) at 85 MW Reactor Power

RELAP Allowable Shrinkage

COMSOL Allowable Shrinkage

• Maximum pellet temperatures remain below pellet melting point of 650 C

• Boiling does not occur because target surface temperatures never exceed adjacent coolant saturation temperatures


Fully-loaded target qualification for 2nd cycle irradiation at 130% overpower conditions

(estimated based on partial-target PIE results)


Temperature profile during second irradiation cycle Days 0, 5, and 10

t = 10 d, Tmax= 404.1 °C t = 5 d, Tmax= 405.1 °C t = 0 d, Tmax= 219 °C


Temperature profile during second irradiation cycle Days 15, 20, and 26

t = 26 d, Tmax= 413.3 °C t = 20 d, Tmax= 399.8 °C t = 15 d, Tmax= 400.6 °C


Change in gap size from the start of the 2nd cycle

0

0.5

1

1.5

2

2.5

3

-10 -5 0 5 10

Rad

ial

Gap

Siz

e (

mils)

Axial Position w/ respect to reactor midplane (in)

Day 0 Day 5

Day 10 Day 15

Day 20 Day 26

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30

De

cre

ase

in A

vera

ge G

ap S

ize

(m

ils)

Second Irradiation Cycle (Days)

Net Gap -Δ

Swelling

Thermal Exp.Contributions to the close in the gap (right) show that fission product swelling is the driving mechanism after the initial heat up and thermal expansion.

The change in the radial gap along the pellet stack is shown for different irradiation times (left) for an alternative irradiation position.


Safety Factor = Ratio of Housing Failure to

Force Equilibrium

Fully-Loaded Target Final Qualification for Pellet and Housing Stresses and Failure


Summary and Conclusions

• Safety analysis progress through phase 3

– Phase 1 (single pellet) and Phase 2 (partial targets) safety analyses are complete

– Phase 3 safety analysis is complete: Fully-Loaded Targets have been irradiated for 2 cycles and further irradiation is currently underway

– A best-estimate analysis for Phase 3 was made to allow irradiation of previous designs

– The current design is bounded by previous analyses

– Steady-state cases are more thermally limiting than transient cases

• 238Pu throughput optimization will require further analysis

– Using more extensive PIE data and pellet property measurements as inputs, new safety analyses may be developed to examine the potential for 5-20% neptunium loading increases

– Safety analyses considering the effects of filling the HFIR external reflector with targets will need to be made

• A continuous mode of optimization/production is the intended final phase of the project at HFIR


Acknowledgements

• HFIR RRD Staff

• ORNL

• DOE NEUP

• NASA

Questions?


Supplementary Graphics…


Revisions 3 & 4: Pellet Centerline Temperatures in the Fully-Loaded Target 2nd cycle


Revisions 3 & 4: Pellet Centerline Temperatures in the Fully-Loaded Target 2nd cycle & EOC-1

300

320

340

360

380

400

420

440

460

480

-10 -8 -6 -4 -2 0 2 4 6 8 10

Tem

pera

ture

(°

C)

Axial Position along the pellet stack (in)

Day 5 Day 10

Day 15 Day 20

Day 26 EOC-1 VXF-15

EOC-1 VXF-3

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