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G. L. Hofman
Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439 USA
June 1996

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A fuel development campaign that results in an aluminum plate-type fuel of unlimited LEU burnup capability with an uranium loading of 9 grams per cm3 of meat (while at the same time meeting required homogeneity and formability criteria) should be considered an unqualified success. To put this goal in perspective, our current worldwide approved and accepted highest loading is 4.8 g/cm3 with U3Si2 as fuel. This fuel compound has excellent radiation performance to full U-235 burnup, but its modest density limits application for very high loadings.

The 4.8 g/cm3 loading corresponds to approximately 43 vol % U3Si2 in the meat which is, with conventional rolling techniques, an upper limit for commercial fabrication.

Recently several fabricators have reported satisfactory yields with up to 53 vol % U3Si2 achieved through optimized fabrication procedures. Assuming that these new processes prove commercially viable, we have now an upper limit of 6 g/cm3 with a proven fuel compound. Or in other words we are a factor of 1.5 short of our 9 g/cm3 goal. We can not expect to increase the fuel volume fraction significantly, if at all, beyond 53%. Thus our only hope lies in finding a much-higher-density fuel than U3Si2 with, however, similar characteristics such as fabricability, compatibility with aluminum, and stable irradiation behavior.

High-density uranium compounds are listed in Table 1 with, for comparison, U3Si2 and an older stable fuel, UAl2. Many of these compounds offer no real density advantage over U3Si2 and have less desirable fabrication and performance characteristics as well. Of the higher-density compounds, U3Si has approximately a 30% higher uranium density but the density of the U6X compounds would yield the factor 1.5 needed to achieve 9 g/cm3 uranium loading.

Unfortunately, irradiation tests proved these peritectic compounds as a group to have poor swelling behavior, as shown in Fig. 1. The high swelling rate of these compounds is associated with fission-induced amorphization, and, unless we can find a way to stabilize these compounds without reducing their density, it must be concluded that intermetallic compounds are not going to get us to our 9 g/cm3 goal. It is for this reason that we are turning to uranium alloys. The obvious question is, why not use pure uranium, for this would clearly result in the highest possible loading. The reason pure uranium was not seriously considered as a dispersion fuel is mainly due to its high rate of growth and swelling at low temperatures. This problem was solved at least for relatively low burnup application in non-dispersion fuel elements with small (a few hundred ppm) additions of Si, Fe, and Al. This so called adjusted uranium has nearly the same density as pure alpha-uranium and it seems prudent to reconsider this alloy as a dispersant.

Further modifications of uranium metal to achieve higher burnup swelling stability involve stabilization of the cubic gamma phase at low temperatures where normally alpha phase exists. Several low neutron capture cross section elements such as Zr, Nb, Ti and Mo accomplish this in various degrees. As shown in Fig. 2, combinations of Nb-Zr and Mo by itself appear most effective. The density of some of these alloys are given in Table II showing that U-5Mo is equivalent in density with the aforementioned U6X compounds. Alloys around this composition, as well as the lower U-NbZr alloys, would meet our high loading requirements. The challenge is to produce a suitable form of fuel powder and develop a plate fabrication procedure, as well as obtain high burnup capability through irradiation testing.

In summary, as in the case of any new fuel development effort, there is no guarantee that we will reach our goal, but we have enough promising options to hope for a high probability of success.

Table I. Nominal Density, Uranium Content and
Melting Point of Uranium Compounds
  Density U-Density Melting
Compound g/cm3 g/cm3 Point, deg C
UO2 10.9 9.7 2750
U4O9 11.2 9.7 (a)
UC 13.6 13.0 2400
UN 14.3 13.5 2650
UAl2 8.1 6.6 1590
U3Si2 12.2 11.3 1650
U3Si 15.4 14.8 930 (b)
U6Ni 17.6 16.9 790 (c)
U6Fe 17.7 17.0 815 (c)
U6Mn 17.8 17.0 725 (c)
(a) Transforms to UO2 at high temperatures
(b) Peritectoid temperature, (c) Peritectic temperature.
Table II. Density, Uranium Content and Melting Point
Of Gamma-Stabilized Uranium Alloys
  Density U-Density Melting
Alloy W. % g/cm3 g/cm3 Point, deg C
U 19.0 19.0 1135
U-2Mo 18.5 18.1 1135
U-5Mo 17.9 17.0 1135
U-6.5Mo 17.5 16.4 1135
U-8Mo 17.3 15.9 1135
U-9Mo 17.0 15.5 1160
U-4Zr-2Nb 17.3 16.2 1160
U-6Zr-41Nb 16.4 15.8 1160
U-7Nb 17.0 15.0 1160
U-10Zr 16.0 14.4 1160

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