MATERIALS COMPATIBILITY OF SNAP FUEL COMPONENTS DURING SHIPMENT IN 9975 PACKAGING Page: 5 of 15
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P. R. Vormelker WSRC-STI-2006-00140 Rev. 1
November, 2006 Page 1 of 10
Materials Science and Technology has evaluated materials compatibility for the SNAP (Systems
for Nuclear Auxiliary Power) fuel for containment within a 9975 packaging assembly for a
shipping period of one year. The evaluation included consideration for potential for water within
the convenience can, corrosion from water, galvanic corrosion, tape degradation, and thermal
expansion risk. Based on a review of existing literature and assumed conditions, corrosion
and/or degradation of the 304 stainless steel (SS) Primary Containment Vessel (PCV) and the
304 stainless steel convenience cans containing the SNAP fuel is not significant to cause failure
during the 1 year time shipping period in the 9975 packaging assembly. However, storage
beyond the 1 year shipping period has not been validated.
2.0 BACKGROUND AND ASSUMPTIONS
SNAP fuel rods will be shipped in 9975 packaging from the Los Alamos National Laboratory
(LANL) to the Nevada Test Site (NTS).(1) SNAP fuel was developed for compact, lightweight
nuclear reactor heat sources that would provide long-term auxiliary power for spacecraft.
Shipment and subsequent storage of the fuel beyond one year is not planned at this time and was
not considered during this evaluation for materials compatibility.
A maximum of nine individual SNAP fuel rods will be contained within two 304 stainless steel
convenience cans (Vollrath No. 88020) in a nesting pattern of eight rods (1.25 in OD) around the
ID (approximately 4.625 in.) of a convenience can and one rod in the center, as represented in
Figure 1. The second convenience can is used as a lid with the two convenience cans being
taped together with PVC tape. The convenience cans with nine SNAP fuel rods will be placed
into a 304 stainless steel primary containment vessel (PCV). The standard 9975 packaging
assembly will contain the PCV.
The dimensions of individual SNAP fuel rods are 1.25 inches OD and 13.19 inches long per
Figure 2. The minimum inside height of the combined Vollrath cans is 13.22 inches. This will
be a snug fit for the 13.19 inch long fuel rods. If any swelling of the fuel has occurred, the fit
may be even tighter. Aluminum foil packing will fill the vacant space to prevent fuel movement
during shipment. Nominal compositions of the metal alloys (304 stainless steel, Hastelloy N,
and aluminum) used in this package are shown in Table 1.
The SNAP fuel rods contain a solid homogeneous mixture of a fuel (235U) and a moderator
(hydrogen as ZrHx ), designated as ZrUHx or UZrHx. A small amount of carbon (0.15 wt. %)
was added as a grain refiner to produce an optimum microstructure to increase yield strength
during hydriding. The fuel element is clad with a Hastelloy N tube (0.015 in. wall thickness), a
nickel alloy composition per Table 1. Surfaces of the cladding tube are chromized to provide
enhanced adherence to a ceramic glass coating (Solarmaic, S14-35-SM2).(2) Chromizing (Solar
Aircraft's F3-78 process) was performed on all tube surfaces to provide a diffusion barrier to the
formation of molybdenum trioxide on the nickel alloy during fusing of the ceramic coating.(3-4)
The chromium-rich surface enhances corrosion resistance, thermal stability, and wear-resistant
properties. A metallographic cross-section of Hastelloy N with a chromized layer and
subsequent coating from a developmental version of the cladding is shown in Figure 3. The
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Vormelker, P. MATERIALS COMPATIBILITY OF SNAP FUEL COMPONENTS DURING SHIPMENT IN 9975 PACKAGING, report, November 14, 2006; [Aiken, South Carolina]. (https://digital.library.unt.edu/ark:/67531/metadc880009/m1/5/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.