SNAP-50 Experiments

construction:UN assessments under active modification

All nitride assessment cases are undergoing active modifications. At this time, the documenation here does not fully reflect the models in the input files. New documation that will be available soon will include improvements due to the new UNSifgrs model implementation.

Overview

The SNAP-50 project included irradiation testing of UN fuel for space reactors at the Materials Testing Reactor (MTR). The principal focus was on the performance of UN fuel at central temperatures of 1100 to 1300 $var element = document.getElementById("moose-equation-b0d8259a-2e86-47c0-9b4e-0b05574726b5");katex.render("^{\\circ}", element, {displayMode:false,throwOnError:false});$ C, the effect of higher burnups than usually studied for space applications (up to 4.6 at. % U), and long irradiation times (up to 12,000 hours) Weaver et al. (1969). Claddings were either PWC-11 (Nb-1%Zr-0.1%C) or Nb-1%Zr with a tungsten liner. Most of the information used to set up this assessment case, i.e., pin geometry, irradiation conditions, fuel swelling, fission-gas release, and microstructural data, were obtained from Weaver et al. (1969) and DeCrescente et al. (1965).

Test Description

The SNAP-50 tests selected for these assessment cases correspond to the capsule 57-669 due to its detailed post-irradiation measurements and its lower temperatures than other capsules. The capsule contained three different pins: top test specimen (TTS), middle test specimen (MTS), and bottom test specimen (BTS). These helium-bonded UN pins were the same in design and differed only in their operating conditions and irradiation history resulting from their different positions in the capsule. These pins were destructively examined and swelling and fission gas release were measured. These three pins, SNAP-50-57-669-TTS, SNAP-50-57-669-MTS, and SNAP-50-57-669-BTS, are listed as assessment cases in BISON for UN fuel.

Rod Design Specifications

The fuel was solid cylindrical pellets of UN. The capsule contained three pins in which the pressed and sintered UN pellets were encased in PWC-11 or Nb-1%Zr cladding (PWC-11 for pins in capsule 57-669) with a chemical-vapor-deposited (CVD) tungsten reaction barrier. Note that rather than rest at the bottom of the pin, the fuel stack is maintained at the center of the pin using tungsten spacers. These spacers are not included in the current pin design used in the assessment case, but accounted for in defining the position of the pellets during the simulation. This might result in an over estimation of bottom gap and plenum volume. Table 1 lists the rod design specifications, common for pins SNAP-50-57-669-TTS, SNAP-50-57-669-MTS, and SNAP-50-57-669-BTS, specifying if the value comes from a direct measurement or was unknown and had to be estimated or calculated.

Table 1: SP-1 Rod Geometry

Parameter	Value	Units	Source
Clad material	PWC-11		Weaver et al. (1969)
Bonding	He		Weaver et al. (1969)
Pin length	40.16	mm	Weaver et al. (1969)
Clad OD	6.35	mm	Weaver et al. (1969)
Clad thickness	0.635	mm	Weaver et al. (1969)
Liner thickness	0.127	mm	Weaver et al. (1969)
Pellet diameter	4.775	mm	Weaver et al. (1969)
Fuel stack	16.51	mm	Weaver et al. (1969)
Top/bottom plug	4.88	mm	Weaver et al. (1969) (Fig. 4)
Bottom gap under pellet	6.94	mm	Calculated using pin length, fuel stack length, and top/bottom plugs to center the fuel stack in the pin
Plenum height	6.94	mm	Calculated using pin length, fuel stack length, and top/bottom plugs to center the fuel stack in the pin
Plenum pressure	12.4	MPa	Unknown, fixed equal to what is used in SP1
Fuel density	95	%TD	Estimated based on data from DeCrescente et al. (1965)
Smear density	40.25	%TD	Weaver et al. (1969)

Note that a since very limited amount of data is available on PWC-11, and because its composition is very close to Nb-1%Zr, we will assume PWC-11 to behave like Nb-1%Zr.

Operating Conditions and Irradiation History

Being tested in EBR-II, an irradiation test vehicle was used for the SP-1 tests to increase the obtainable temperature of the fuel pin cladding (Dutt et al., 1984). Cladding temperatures of 1300 to 1500 K were therefore obtained to be relevant to space propulsion conditions.

The actual power history for specific these specific experiments is still being determined. Therefore, a simplified power history containing an initial ramp to power and hold for a given amount of time with a final power down is being used. The power density is calculated based on the desired final burnup and irradiation time. The peak power density is calculated to be around 8.1 W/m $var element = document.getElementById("moose-equation-1d1fbaa9-adc2-4783-b233-22dc91b23e44");katex.render("^3", element, {displayMode:false,throwOnError:false});$ . The average burnup of the fuel at the end of the simulation is used as a check that the power history is reasonable. The parameters for the irradiation and temperature history are detailed in Table 2. The final burnup reached in the BISON simulations is 2.71 at.% U, very close to the burnup listed in Weaver et al. (1969).

The exact temperature history is also still being determined. In the meantime, the initial temperature is fixed to 298 K, and the temperature ramps up to the desired profile for the irradiation time before ramping down at the end of the simulation.

Table 2: SP-1 Operating Conditions

Parameter	Value	Units	Source
Burnup	2.72	at.% U	Weaver et al. (1969)
Irradiation duration	10352	h	Weaver et al. (1969)
Average cladding temperature TTS	1108.15	K	Weaver et al. (1969)
Average cladding temperature MTS	1253.15	K	Weaver et al. (1969)
Average cladding temperature BTS	1203.15	K	Weaver et al. (1969)
Calculated fuel center temperature TTS	1213.15	K	Weaver et al. (1969)
Calculated fuel center temperature MTS	1348.15	K	Weaver et al. (1969)
Calculated fuel center temperature BTS	1308.15	K	Weaver et al. (1969)

Model Description

The SNAP-50-57-669-TTS, SNAP-50-57-669-MTS, and SNAP-50-57-669-BTS pins from the SNAP-50 project are modeled in BISON using a base input file titled SNAP50_base.i (either with the full input file or using the action to create the necessary blocks), a file titled SNAP50_Pin_options_57-669.i containing the information common to the three pins (e.g., rod design, irradiations conditions, etc.), and files containing the information specific to each pins, titled SNAP50_Pin_options_57-669_TTS.i, SNAP50_Pin_options_57-669_MTS.i, and SNAP50_Pin_options_57-669_BTS.i.

The specifications and conditions for all three pins are described below. Note that only the pins are being modeled, not the capsule.

Temperature Profile

Ref. Weaver et al. (1969) provides the temperature profile for a different capsule than 57-669, but it provides information about the shape of the temperature profiles in the SNAP-50 tests. The temperature profile can be approximated with a second order polynomial $var element = document.getElementById("moose-equation-e9b22507-bb06-46f3-bc44-68147108b800");katex.render("T(x) = ax^2 + bx + c", element, {displayMode:false,throwOnError:false});$ with $var element = document.getElementById("moose-equation-fce8d79d-2a50-4785-ad66-4ef66da3046d");katex.render("x", element, {displayMode:false,throwOnError:false});$ being the position along the cladding, and $var element = document.getElementById("moose-equation-179a6ad3-383c-485e-983d-5aeb61e1f498");katex.render("a", element, {displayMode:false,throwOnError:false});$ , $var element = document.getElementById("moose-equation-7e0d879e-a23c-4f71-999d-dc4e570c5ad9");katex.render("b", element, {displayMode:false,throwOnError:false});$ , and $var element = document.getElementById("moose-equation-0bf12ec4-2375-442b-ac3b-ec383704974f");katex.render("c", element, {displayMode:false,throwOnError:false});$ constants to be determined. Since the information provided about the pins temperatures in the 57-669 capsule is the average temperatures of each pin cladding surface (see Table 2 ) (Weaver et al., 1969), the constants $var element = document.getElementById("moose-equation-fceab297-52eb-4b16-8e5b-87fb80aded5f");katex.render("a", element, {displayMode:false,throwOnError:false});$ , $var element = document.getElementById("moose-equation-f990fa2d-bdd3-4a7a-abff-943f5de47b08");katex.render("b", element, {displayMode:false,throwOnError:false});$ , and $var element = document.getElementById("moose-equation-7e5be35f-9e38-487a-84f2-5b635cef2267");katex.render("c", element, {displayMode:false,throwOnError:false});$ are defined in a way that reproduce these average cladding surface temperature. This is done by solving $var element = document.getElementById("moose-equation-66420b10-a3b9-48f8-aa80-30755a1aeb7d");katex.render("T_{avg,i} = \\int_{x_{start,i}}^{x_{end,i}}{T(x)dx}", element, {displayMode:true,throwOnError:false});$ with $var element = document.getElementById("moose-equation-a4d0ef66-a0f2-4d02-b4bb-f5c51492ff25");katex.render("i", element, {displayMode:false,throwOnError:false});$ denoting TTS, MTS, and BTS, $var element = document.getElementById("moose-equation-930d7b02-a51d-4fd7-87e8-f4308d3313fa");katex.render("T_{avg,i}", element, {displayMode:false,throwOnError:false});$ being the provided average cladding surface temperature, and $var element = document.getElementById("moose-equation-e084ba6d-4335-4852-9898-b7dea424cc59");katex.render("x_{start,i}", element, {displayMode:false,throwOnError:false});$ , $var element = document.getElementById("moose-equation-f828750e-1ccd-4390-8694-08776e201ab7");katex.render("x_{end,i}", element, {displayMode:false,throwOnError:false});$ being the starting and end positions of the pin $var element = document.getElementById("moose-equation-a53e2fce-3234-4e51-b435-a82628e11204");katex.render("i", element, {displayMode:false,throwOnError:false});$ . These calculations are performed in the file titled SNAP50_Pin_options_57-669.i. To define the position of the pins within that temperature profile, we fix the bottom of the lowest pin (i.e., SNAP50_Pin_options_57-669_BTS) at 0.0, and then stack the pins on top of each other for a total height of three times the pin height.

Power Profile

Concerning the power profile, the SNAP50 tests were performed in MTR. However, since the exact power profile is still unknown, it is assumed to be homogeneous along the fuel stack for all three pins.

Pressure Profile

The coolant pressure is unknown for now. The coolant is made out of Lithium, and the pressure is assumed to be equal to 15.1 MPa.

Geometry and Mesh

The 2D-RZ mesh for the assessment case is generated with the internal smeared pellet meshing capability in BISON FuelPinMeshGenerator, which is able to model the PWC-11 cladding with the tungsten liner.

All of the dimensions and meshing details are contained in the [Mesh] block.

Material and Behavioral Models

The following material and behavioral models for the UN fuel were used:

MNElasticityTensor: Computes Young's modulus and Poisson ratio for MN fuel
MNThermalExpansionEigenstrain: Computes an eigenstrain due to thermal expansion for MN fuel
MNCreepUpdate: Calculates creep mechanical properties and deformation behavior for MN fuel
MNThermal: Calculates the thermal conductivity and specific heat for MN fuel
MNThermalExpansionEigenstrain: Calculates thermal expansion coefficient and isotropic expansion for MN fuel
MNVolumetricSwellingEigenstrain: Computes swelling due to solid and gaseous fission products for MN fuel

The following material and behavioral models are used for the PWC-11 cladding, even though these correspond to Nb-1%Zr (ASTM B391 Grade R04251) properties:

ComputeIsotropicElasticityTensor: Computes isotropic elastic mechanical properties for generic material (elastic modulus of 68.9 GPa (ASM, 1993), Poisson's ratio of 0.4).
ComputeThermalExpansionEigenstrain: Computes eigenstrain due to thermal expansion with a constant coefficient (7.54 10 $var element = document.getElementById("moose-equation-bc933fca-afb7-46a6-b284-24480305cda3");katex.render("^{-6}", element, {displayMode:false,throwOnError:false});$ K $var element = document.getElementById("moose-equation-84864d49-dcc6-486f-98b7-ef0430a4805b");katex.render("^{-1}", element, {displayMode:false,throwOnError:false});$ in the temperature range of $var element = document.getElementById("moose-equation-e97f2fb8-963a-4822-a1fe-d8cdb4910c00");katex.render("[293.15, 673.15]", element, {displayMode:false,throwOnError:false});$ K (Cverna and Committee., 2002; ASM, 1993)).
StrainAdjustedDensity: Computes density for a generic material (8590 kg/m $var element = document.getElementById("moose-equation-ea47f885-6b21-47fe-af7a-2f0053076c17");katex.render("^3", element, {displayMode:false,throwOnError:false});$ at room temperature (Robbins and Finger, 1991; Cverna and Committee., 2002; ASM, 1993)).
HeatConductionMaterial: Computes thermal conductivity and specific heat capacity for generic material. In the case of Nb%Zr, the thermal conductivity is set as a constant 41.9 W/(m.K) (measured at 298.15 K) (Cverna and Committee., 2002; ASM, 1993) and the specific heat capacity is defined as a constant 270 J/(kg.K) (measured at 293.15 K) (Cverna and Committee., 2002; ASM, 1993)).

The following material and behavioral models for the tungsten liner were used:

ComputeIsotropicElasticityTensor: Computes the elasticity tensor of tungsten
TungstenThermalExpansionEigenstrain: Computes eigenstrain due to thermal expansion of tungsten
TungstenThermal: Computes the thermal properties of tungsten
StrainAdjustedDensity: Computes density for a generic material (19300 kg/m $var element = document.getElementById("moose-equation-c9ac73e7-6bc6-41ea-9047-57817e822517");katex.render("^3", element, {displayMode:false,throwOnError:false});$ at 293.15 K (Cverna and Committee., 2002)).

Note that thermal and mechanical contacts are modeled using mortar contact.

Input files

The input files for these assessment cases are located at bison/assessment/nitride/MTR/SNAP50/analysis.

To run the assessment cases, the input files and option files need to be combined into one by listing them in the command. For example, to run the assessment case for pin SNAP50_Pin_options_57-669_BTS, one should list SNAP50_Pin_options_57-669_BTS.i SNAP50_Pin_options_57-669.i SNAP50_base.i, or SNAP50_Pin_options_57-669_BTS.i SNAP50_Pin_options_57-669.i SNAP50_Pin_base_action.i to use the action.

Results Comparison

The post-irradiation examinations (PIE) of the SNAP-50-57-669-TTS, SNAP-50-57-669-MTS, and SNAP-50-57-669-BTS pins provide the measurements listed in Table 3, Table 4, and Table 5, respectively.

Table 3: PIE data for SNAP-50-57-669-TTS

Quantity	Measurements	Units	Source
FGR Xe	Unknown
FGR Kr	Unknown
FGR total	0.05	%	Weaver et al. (1969)
Fuel swelling $var element = document.getElementById("moose-equation-2b369a0c-f8b9-4fe5-803f-8869fd283cae");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-b4003719-6c61-42cb-aebf-05dd1c5dd501");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel swelling $var element = document.getElementById("moose-equation-49c877b5-d684-49b2-8d8e-86c07124f869");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ V/V $var element = document.getElementById("moose-equation-b6792921-84a4-42b9-ae5d-f45694278c15");katex.render("_0", element, {displayMode:false,throwOnError:false});$	6.5	%	Weaver et al. (1969)
Cladding strain $var element = document.getElementById("moose-equation-a82e071a-1cc4-4904-9d5c-ce4fedd98d7c");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-1b11273e-8dde-45bb-9712-04e332a1bfd9");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel density decrease	6.1	%	Weaver et al. (1969)
Change in cladding length	Unknown
Change in cladding OD	0.52	%	Weaver et al. (1969)

Table 4: PIE data for SNAP-50-57-669-MTS

Quantity	Measurements	Units	Source
FGR Xe	Unknown
FGR Kr	Unknown
FGR total	0.13	%	Weaver et al. (1969)
Fuel swelling $var element = document.getElementById("moose-equation-01ff3d9f-f785-4a62-9fc5-8e8915384e0d");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-34338758-1518-4a7d-8854-e5e8452c442b");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel swelling $var element = document.getElementById("moose-equation-73875190-21b5-4de0-b433-cd2332353240");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ V/V $var element = document.getElementById("moose-equation-abcfc80a-6ada-4081-b896-a7beb987a62b");katex.render("_0", element, {displayMode:false,throwOnError:false});$	7.4	%	Weaver et al. (1969)
Cladding strain $var element = document.getElementById("moose-equation-f92be9c9-456c-4680-8492-6fc45593171e");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-9fdd6e89-4ee1-4114-ba0e-be5510963821");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel density decrease	6.9	%	Weaver et al. (1969)
Change in cladding length	Unknown
Change in cladding OD	0.36	%	Weaver et al. (1969)

Table 5: PIE data for SNAP-50-57-669-BTS

Quantity	Measurements	Units	Source
FGR Xe	Unknown
FGR Kr	Unknown
FGR total	0.02	%	Weaver et al. (1969)
Fuel swelling $var element = document.getElementById("moose-equation-f45c96f2-2a00-412b-b4ad-a0d3679b7d4c");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-e864a92f-ddea-474c-aec5-b47d0d057ffc");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel swelling $var element = document.getElementById("moose-equation-f3bf190a-5b00-4285-a07c-f58d20ecbf68");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ V/V $var element = document.getElementById("moose-equation-c8d86ceb-5e76-4441-9fbf-ae87945c31a9");katex.render("_0", element, {displayMode:false,throwOnError:false});$	6.0	%	Weaver et al. (1969)
Cladding strain $var element = document.getElementById("moose-equation-71b0debb-bcf0-4b30-8ecb-914397ab9f39");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-4ee9de27-2de3-48ea-b90e-5af05f3c65a6");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel density decrease	5.6	%	Weaver et al. (1969)
Change in cladding length	Unknown
Change in cladding OD	0.36	%	Weaver et al. (1969)

Discussion

The BISON simulation results have not yet been compared to the post-experiment examinations data.

References

ASM. ASM handbook Volume 2 - Properties and selection: Nonferrous alloys and special-purpose materials. Volume 2. ASM International, 1993. ISBN 978-0-87170-378-1.

@book{ASMHandbook1993,
    author = "ASM",
    isbn = "978-0-87170-378-1",
    issn = "08170379",
    publisher = "ASM International",
    pages = "1300",
    title = "ASM handbook Volume 2 - Properties and selection: Nonferrous alloys and special-purpose materials",
    volume = "2",
    year = "1993"
}

Fran. Cverna and ASM International. Materials Properties Database Committee. ASM ready reference. Thermal properties of metals. ASM International, 2002. ISBN 978-1-68015-944-8.

@book{CVERNA2002,
    author = "Cverna, Fran. and Committee., ASM International. Materials Properties Database",
    isbn = "978-1-68015-944-8",
    pages = "560",
    publisher = "ASM International",
    title = "ASM ready reference. Thermal properties of metals",
    year = "2002"
}

M A DeCrescente, M S Freed, and S D Caplow. Uranium nitride fuel development, snap-50. Technical Report, Technical Information Center, 10 1965. URL: http://www.osti.gov/servlets/purl/4324037/, doi:10.2172/4324037.

@TECHREPORT{DECRESCENTE1965,
    author = "DeCrescente, M A and Freed, M S and Caplow, S D",
    city = "U.S. Department of Energy",
    doi = "10.2172/4324037",
    institution = "Technical Information Center",
    month = "10",
    title = "Uranium nitride fuel development, SNAP-50",
    url = "http://www.osti.gov/servlets/purl/4324037/",
    year = "1965"
}

D S Dutt, C M Cox, and M K Millhollen. Performance of refractory alloy-clad fuel pins. In 2. Symposium on Space Nuclear Power Systems. 12 1984.

@inproceedings{DUTT1984,
    author = "Dutt, D S and Cox, C M and Millhollen, M K",
    city = "Albuquerque, NM, USA",
    booktitle = "2. Symposium on Space Nuclear Power Systems",
    month = "12",
    title = "Performance of refractory alloy-clad fuel pins",
    year = "1984"
}

W. H. Robbins and H.B. Finger. An historical perspective of the nerva nuclear rocket engine technology program. Technical Report NASA Contractor Report 187154, AIAA-91-3451, Analytical Engineering Corporation, Lewis Research Center, 7 1991.

@TECHREPORT{ROBBINS1991,
    author = "Robbins, W. H. and Finger, H.B.",
    city = "Ohio",
    institution = "Analytical Engineering Corporation, Lewis Research Center",
    month = "7",
    title = "An Historical Perspective of the NERVA Nuclear Rocket Engine Technology Program",
    year = "1991",
    number = "NASA Contractor Report 187154, AIAA-91-3451"
}

S C Weaver, J L Scott, R L Senn, and B H Montgomery. Effects of irradiation on uranium nitride under space-reactor conditions. Technical Report, Technical Information Center, 10 1969. URL: http://www.osti.gov/servlets/purl/4467048/, doi:10.2172/4467048.

@TECHREPORT{WEAVER1969,
    author = "Weaver, S C and Scott, J L and Senn, R L and Montgomery, B H",
    city = "U.S. Department of Energy",
    doi = "10.2172/4467048",
    institution = "Technical Information Center",
    month = "10",
    title = "Effects of irradiation on uranium nitride under space-reactor conditions",
    url = "http://www.osti.gov/servlets/purl/4467048/",
    year = "1969"
}

Overview
Test Description
Model Description
Input files
Results Comparison
Discussion
References