Pratt and Whitney CANEL SNAP-50 Experiments

Overview

The SNAP-50 project at Pratt and Whitney Aircraft-Connecticut Advanced Nuclear Engineering Laboratory (CANEL) included development and testing of uranium mononitride (UN) fuel for space reactors. Irradiation tests of UN fuel in Nb-1%Zr cladding with a tungsten liner were performed at 2 MW $var element = document.getElementById("moose-equation-838d2c5e-9431-4383-819f-921e7b84ceb5");katex.render("_t", element, {displayMode:false,throwOnError:false});$ and 8 MW $var element = document.getElementById("moose-equation-3e6b76ac-54eb-46b4-9183-ed7d72293bcc");katex.render("_t", element, {displayMode:false,throwOnError:false});$ at different temperatures and for a variety of durations. 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 DeCrescente et al. (1965).

Test Description

The Pratt and Whitney CANEL SNAP-50 tests selected for these assessment cases correspond to the capsule 26-602 due to its geometry, temperature and irradiation conditions, and the relatively detailed post-irradiation measurements. The capsule contained three different pins: W-17, W-18, and W-19. 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, PW_SNAP50_UN_Pin_26-602-W-17, PW_SNAP50_UN_Pin_26-602-W-18, and PW_SNAP50_UN_Pin_26-602-W-19, 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 Nb-1%Zr cladding 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 PW_SNAP50_UN_Pin_26-602-W-17, PW_SNAP50_UN_Pin_26-602-W-18, and PW_SNAP50_UN_Pin_26-602-W-19 and specifies if the value comes from a direct measurement or was unknown and had to be estimated or calculated.

Note that some of these dimensions are listed with different values in DeCrescente et al. (1965) (e.g., cladding thickness listed as 0.635 mm in Fig. 53, but as 0.711 mm in Fig. 2). When this is the case, the values are from Fig. 53 of DeCrescente et al. (1965).

Table 1: SP-1 Rod Geometry

Parameter	Value	Units	Source
Clad material	Nb-1%Zr		DeCrescente et al. (1965)
Bonding	He		DeCrescente et al. (1965)
Pin length	40.157	mm	DeCrescente et al. (1965)
Clad OD	6.35	mm	DeCrescente et al. (1965)
Clad thickness	0.635	mm	DeCrescente et al. (1965)
Liner thickness	0.076	mm	DeCrescente et al. (1965)
Pellet diameter	4.826	mm	DeCrescente et al. (1965)
Fuel stack	16.51	mm	DeCrescente et al. (1965)
Top/bottom plug	4.88	mm	Estimated, fixed as in MTR-SNAP50
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 MTR-SNAP50
Fuel density - W-17	95.4	%TD	DeCrescente et al. (1965)
Fuel density - W-18	95.6	%TD	DeCrescente et al. (1965)
Fuel density - W-19	94.8	%TD	DeCrescente et al. (1965)

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 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 $var element = document.getElementById("moose-equation-3ec1779f-cb26-4e3c-9825-adcb6abd345a");katex.render("^3", element, {displayMode:false,throwOnError:false});$ . 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, being very close to the burnup listed in DeCrescente et al. (1965).

The exact temperature history is 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
Peak power density (W-17)	0.47	kW/cm $var element = document.getElementById("moose-equation-a0f651bd-d714-49f0-a68c-25e41b109251");katex.render("^3", element, {displayMode:false,throwOnError:false});$	DeCrescente et al. (1965)
Peak power density (W-18)	0.44	kW/cm $var element = document.getElementById("moose-equation-de6728d5-710c-41b8-a8a2-8c692d4817d4");katex.render("^3", element, {displayMode:false,throwOnError:false});$	DeCrescente et al. (1965)
Peak power density (W-19)	0.42	kW/cm $var element = document.getElementById("moose-equation-f248fbff-ec7b-4dee-a973-f0102cebb0ac");katex.render("^3", element, {displayMode:false,throwOnError:false});$	DeCrescente et al. (1965)
Burnup (W-18)	0.91	at.% U	DeCrescente et al. (1965)
Burnup (W-19)	0.86	at.% U	DeCrescente et al. (1965)
Burnup (W-17)	0.97	at.% U	DeCrescente et al. (1965)
Burnup (W-18)	0.91	at.% U	DeCrescente et al. (1965)
Burnup (W-19)	0.86	at.% U	DeCrescente et al. (1965)
Irradiation duration	5940	h	DeCrescente et al. (1965)
Average cladding temperature (W-17)	1214	K	DeCrescente et al. (1965)
Max cladding temperature (W-17)	1408	K	DeCrescente et al. (1965)
Average cladding temperature (W-18)	1366	K	DeCrescente et al. (1965)
Max cladding temperature (W-18)	1422	K	DeCrescente et al. (1965)
Average cladding temperature (W-19)	1339	K	DeCrescente et al. (1965)
Max cladding temperature (W-19)	1422	K	DeCrescente et al. (1965)

Model Description

The PW_SNAP50_UN_Pin_26-602-W-17, PW_SNAP50_UN_Pin_26-602-W-18, and PW_SNAP50_UN_Pin_26-602-W-19 pins from the SNAP-50 project are modeled in BISON using a base input file titled PW_SNAP50_UN_Pin_base.i (either with the full input file or using the action to create the necessary blocks), a file titled PW_SNAP50_UN_Pin_options_26-602.i containing the information common to the three pins (e.g., rod design, irradiation conditions, etc.), and files containing the information specific to each pins, titled PW_SNAP50_UN_Pin_options_26-602-W-17.i, PW_SNAP50_UN_Pin_options_26-602-W-18.i, and PW_SNAP50_UN_Pin_options_26-602-W-19.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. DeCrescente et al. (1965) provides the average and max temperature for each pin. However, the exact temperature is currently unknown, as is the relative position of the pins in the capsule. We therefore cannot use the same approach as in MTR-SNAP50. However, assuming that the temperature profile of the pin follows a second polynomial with the max temperature being reach at the center of the cladding (which corresponds to the center of the fuel), we can use $var element = document.getElementById("moose-equation-f4d9dd98-b149-42f5-a35b-23a36df68921");katex.render("T(x) = a(x-x_0)^2 + b", element, {displayMode:false,throwOnError:false});$ with $var element = document.getElementById("moose-equation-7a7fbc44-0ea1-4a75-a538-730ac7b135d8");katex.render("x", element, {displayMode:false,throwOnError:false});$ being the position along the cladding, $var element = document.getElementById("moose-equation-b8c62e94-282a-4190-954e-3c4d6dd0380b");katex.render("x_0", element, {displayMode:false,throwOnError:false});$ being the middle position of the cladding, and $var element = document.getElementById("moose-equation-ee3a7bd9-07c3-4fb4-93ca-c958f7fc57ea");katex.render("a", element, {displayMode:false,throwOnError:false});$ and $var element = document.getElementById("moose-equation-94ef6602-498d-4310-b4d4-9e933ed3c3ca");katex.render("b", element, {displayMode:false,throwOnError:false});$ constants to be determined. $var element = document.getElementById("moose-equation-a692091d-30cd-4a40-882d-d3e67685436f");katex.render("b", element, {displayMode:false,throwOnError:false});$ is actually equal to the maximum cladding temperature, and $var element = document.getElementById("moose-equation-9348ed16-81e4-4abb-8b20-02aeffa52129");katex.render("a", element, {displayMode:false,throwOnError:false});$ can then be determined by solving $var element = document.getElementById("moose-equation-ead89727-5097-4008-ab1b-b2d320e0cf00");katex.render("T_{avg} = \\int_{x_{start}}^{x_{end}}{T(x)dx},", element, {displayMode:true,throwOnError:false});$ with $var element = document.getElementById("moose-equation-9cefde87-e509-47e3-a3c9-c58c9f6c5bf1");katex.render("T_{avg}", element, {displayMode:false,throwOnError:false});$ the provided average cladding surface temperature, and $var element = document.getElementById("moose-equation-fac78cc3-27b3-4908-a686-165b8732016e");katex.render("x_{start}", element, {displayMode:false,throwOnError:false});$ , $var element = document.getElementById("moose-equation-abaffc4a-49a8-4b11-a8aa-afc085aacde7");katex.render("x_{end}", element, {displayMode:false,throwOnError:false});$ the starting and end positions of the pin. These calculations are performed in the file titled PW_SNAP50_UN_Pin_options_26-602.i.

Power Profile

The exact power profile for the Pratt and Whitney CANEL SNAP-50 tests 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 FuelRPinMeshGenerator, which is able to model the Nb-1%Zr 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 mixed nitride (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 Nb-1%Zr (ASTM B391 Grade R04251) cladding:

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 $var element = document.getElementById("moose-equation-5e1a81e2-3d1a-4537-8fa2-9fa6a6463ca0");katex.render("^{-6}", element, {displayMode:false,throwOnError:false});$ K $var element = document.getElementById("moose-equation-7e40c151-e3d4-491c-99d5-4b4b2eb58aa1");katex.render("^{-1}", element, {displayMode:false,throwOnError:false});$ in the temperature range of $var element = document.getElementById("moose-equation-cf321a88-7233-4c00-b2c4-bbf645d8d5a3");katex.render("[293.15, 673.15]", element, {displayMode:false,throwOnError:false});$ K (Cverna and Committee., 2002; ASM, 1993)).
StrainAdjustedDensity: Computes density for a generic material (8590 kg/m $var element = document.getElementById("moose-equation-f12fa37d-1f30-4560-8630-2f8925e9b7bc");katex.render("^3", element, {displayMode:false,throwOnError:false});$ 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 $var element = document.getElementById("moose-equation-a789d4e4-efdd-4a8b-b610-231cf49e03db");katex.render("^3", element, {displayMode:false,throwOnError:false});$ 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/P&W-CANEL/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 PW_SNAP50_UN_Pin_26-602-W-17, one should list PW_SNAP50_UN_Pin_options_26-602-W-17.i PW_SNAP50_UN_Pin_options_26-602.i PW_SNAP50_UN_Pin_base.i, or PW_SNAP50_UN_Pin_options_26-602-W-17.i PW_SNAP50_UN_Pin_options_26-602.i PW_SNAP50_UN_Pin_base_action.i to use the action.

Results Comparison

The post-irradiation examinations (PIE) of the PW_SNAP50_UN_Pin_26-602-W-17, PW_SNAP50_UN_Pin_26-602-W-18, and PW_SNAP50_UN_Pin_26-602-W-19 pins provide the measurements listed in Table 3, Table 4, and Table 5, respectively.

Table 3: PIE data for PW_SNAP50_UN_Pin_26-602-W-17

Quantity	Measurements	Units	Source
FGR Xe	0.02	%	DeCrescente et al. (1965)
FGR Kr	Unknown
FGR total	Unknown
Fuel swelling $var element = document.getElementById("moose-equation-51b1ac85-b37b-47cd-95b3-f5da251fff91");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-9bc4cdfa-061a-48da-8707-fd36bad70569");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel swelling $var element = document.getElementById("moose-equation-fc6f324c-306c-4e8a-b63b-31cf3400cfb4");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ V/V $var element = document.getElementById("moose-equation-70df47d4-fd00-4ddf-bfe4-d8fd2ed2311b");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Cladding strain $var element = document.getElementById("moose-equation-be32a391-9263-423d-801a-6bb4c5cbe3e3");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-a4453444-4415-4c36-a231-367cc8ce569d");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel density decrease	3.0	%	DeCrescente et al. (1965)
Change in cladding length	Unknown
Change in cladding OD	0.12	%	DeCrescente et al. (1965)

Table 4: PIE data for PW_SNAP50_UN_Pin_26-602-W-18

Quantity	Measurements	Units	Source
FGR Xe	0.2	%	DeCrescente et al. (1965)
FGR Kr	Unknown
FGR total	Unknown
Fuel swelling $var element = document.getElementById("moose-equation-ec6ce963-3351-4218-ab4f-a2c99b2ef7ce");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-32bb24c2-be59-49b8-9e1c-3b1bb2db961d");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel swelling $var element = document.getElementById("moose-equation-e054f5d2-f5c7-47c2-b5f7-a81853ddec31");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ V/V $var element = document.getElementById("moose-equation-804741eb-1402-464e-b101-6d27e51e3dcf");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Cladding strain $var element = document.getElementById("moose-equation-686f69f5-4995-4cb9-8813-adf398ff5152");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-6b2f065f-ea07-431d-aa9f-273d1a2c4325");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel density decrease	3.5	%	DeCrescente et al. (1965)
Change in cladding length	Unknown
Change in cladding OD	0.24	%	DeCrescente et al. (1965)

Table 5: PIE data for PW_SNAP50_UN_Pin_26-602-W-19

Quantity	Measurements	Units	Source
FGR Xe	0.08	%	DeCrescente et al. (1965)
FGR Kr	Unknown
FGR total	Unknown
Fuel swelling $var element = document.getElementById("moose-equation-d14be2d8-75ca-4c36-9979-74b0b4be385d");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-856de655-565b-4a09-9777-415ae1658869");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel swelling $var element = document.getElementById("moose-equation-ecad88f8-b8e2-4a2a-bc50-7c6958c33142");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ V/V $var element = document.getElementById("moose-equation-9faf588e-0ed4-4d2f-a515-90d7c52a7713");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Cladding strain $var element = document.getElementById("moose-equation-fca15cad-afa1-4cf3-9e6b-402843b14565");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-1d3fedfa-435e-4867-91ac-7f8e5305d3ce");katex.render("_0", element, {displayMode:false,throwOnError:false});$	Unknown
Fuel density decrease	2.4	%	DeCrescente et al. (1965)
Change in cladding length	Unknown
Change in cladding OD	0.20	%	DeCrescente et al. (1965)

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"
}

Overview
Test Description
Model Description
Input files
Results Comparison
Discussion
References