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Research Papers

Thermomechanical Safety Analyses for a 238Pu Production Target at the HFIR

[+] Author and Article Information
Christopher J. Hurt

Research Reactors Division,
Oak Ridge National Laboratory,
P. O. Box 2008,
Oak Ridge, TN 37831-6399
e-mail: hurtcj@ornl.gov

James D. Freels

Research Reactors Division,
Oak Ridge National Laboratory,
P. O. Box 2008,
Oak Ridge, TN 37831-6399

Prashant K. Jain

Reactor and Nuclear Systems Division,
Oak Ridge National Laboratory,
Oak Ridge, TN 37831

G. Ivan Maldonado

Department of Nuclear Engineering,
University of Tennessee,
Knoxville, TN 37996-2300

1Corresponding author.

Manuscript received July 12, 2016; final manuscript received August 17, 2018; published online March 15, 2019. Assoc. Editor: Jay F. Kunze. The United States Government retains, and by accepting the article for publication, the publisher acknowledges that the United States Government retains, a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States government purposes.

ASME J of Nuclear Rad Sci 5(2), 021004 (Mar 15, 2019) (15 pages) Paper No: NERS-16-1070; doi: 10.1115/1.4041295 History: Received July 12, 2016; Revised August 17, 2018

Safety analyses at the high flux isotope reactor (HFIR) are required to qualify irradiation of production targets containing neptunium dioxide/aluminum cermet (NpO2/Al) pellets for the production of plutonium-238 (238Pu). High heat generation rates (HGRs) due to a fertile starting material (237Np), low melting temperatures, and previously unstudied material irradiation behavior (i.e., swelling/densification, fission gas release) require a sophisticated set of steady-state thermal simulations in order to ensure sufficient safety margins. Experience gained from previous models for preliminary target designs is incorporated into a more comprehensive production target model designed to qualify a target for three cycles of irradiation and illuminate potential in-reactor behavior of the target.

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References

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Figures

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Fig. 1

An NpO2/Al pellet (left) and target rods in target holder (right)

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Fig. 2

Simple diagram of the experiment safety review process at the HFIR

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Fig. 7

Measured pellet stress–strain curves at varying temperatures

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Fig. 6

Thermal expansion measurement data, averaged data, and the COMSOL probe results

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Fig. 5

TLB EOC-1 radial temperature profile at hot-spot showing gradient across gap

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Fig. 4

Fit and moving average of PIE dimensional data with two different sets of observation confidence intervals, nonsimultaneous (left) and simultaneous (right)

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Fig. 3

General trend and contributions to pellet dimensional irradiation behavior

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Fig. 8

Pin 1 pellet mass-specific heating rates versus axial position for the polynomial fits and input data at 0, 5, 10, 15, 20, and 26 days into the third cycle

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Fig. 16

2-D R-Z (distorted, y-axis grid in cm) pellet temperature profiles (°C) showing radial gap for EOC-1, 2, and 3 where gap width shown is approximately 25 μm

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Fig. 17

Pin 1, cycle 1 best-estimate pellet centerline temperatures as a function of axial position

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Fig. 18

Pin 1, cycle 2 best-estimate pellet centerline temperatures as a function of axial position

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Fig. 14

Maximum target temperatures over three irradiation cycles (78 days) for all seven pins in the hot VXF position and pin 1 in the colder VXF position (“Pin 8”)

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Fig. 15:

2-D R-Z (distorted, spatial grid in cm) pellet temperature profiles (° C) for EOC-1, 2, and 3

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Fig. 23

Three-dimensinal center pellet von Mises stress profile (in MPa) at EOC-3 for TLB, SLB-1, and SLB-2 cases with 100× deformation (grid dimensions in cm)

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Fig. 24

Three-dimensinal pellet stack von Mises stress profile (in MPa) at EOC-3 for SLB-1 and SLB-2 cases with 100× deformation and 20× distortion in R direction (grid dimensions in cm)

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Fig. 25

EOC-3 target bulk temperature as a function of axial position for all cases

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Fig. 26

EOC-3 target surface temperature as a function of axial position for all cases

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Fig. 27

Mesh refinement results for EOC1/EOC3 and linear/quadratic bases using an averaged error norm

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Fig. 28

Mesh refinement results for EOC1/EOC3 and linear/quadratic bases using the maximum temperature error norm

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Fig. 9

Representation of production model target pin components

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Fig. 10

Radial cut-view of prototypical target holder with seven prototypical target “pins”

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Fig. 11

Extra-coarse, finer, and extremely fine triangular meshes in the pellet/cladding region

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Fig. 12

Extra coarse and extremely fine meshes in the upper region near the support tube

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Fig. 13

Extra coarse and extremely fine meshes in the lower region near the bottom cap, lower spacer tube, lower expansion spacers, and housing

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Fig. 19

Pin 1, cycle 3 best-estimate pellet centerline temperatures as a function of axial position

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Fig. 20

Pin 1, cycle 3 best-estimate pellet side temperatures as a function of axial position

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Fig. 21

Pin 1, cycle 3 best-estimate pellet side pressures as a function of axial position

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Fig. 22

Pin 1 maximum material region temperatures as a function of irradiation days over three cycles for the thermally limiting basis

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