Scoping Studies of Dopants for Stabilization of Uranium Nitride Fuel

Figures & data

Fig. 1. A Pourbaix diagram of chromium species at 300°C. A vertical dotted line corresponds to a neutral pH at 300°C. The image is based on data from Ref. 10.

TABLE I Proposed Dopants and Basic Neutronic Data from Ref. 15

Element	Reaction	Cross Section (b)
Chromium	${ }^{52}$Cr(n,γ)${ }^{53}$Cr	0.8
Nickel	${ }^{58}$Ni(n,γ)${ }^{59}$Ni	4.6
Silicon	${ }^{28}$Si(n,γ)${ }^{29}$Si	0.17
Titanium	${ }^{48}$Ti(n,γ)${ }^{49}$Ti	11
Aluminum	${ }^{27}$Al(n,γ)${ }^{28}$Al	0.23
Tantalum	${ }^{181}$Ta(n,γ)${ }^{182}$Ta	20.5
Niobium	${ }^{93}$Nb(n,γ)${ }^{94}$Nb	1.15

Fig. 2. The value $k_{\mathrm{\infty }}$ as a function of time, assuming an equal power density of 100 kW/L for all cases. The reference lattice has dimensions typical for a PWR fuel assembly, whereas the adjusted lattice has a higher H/HM ratio.

TABLE II Used Parameters for the Simulated Pin Cell Lattices

Parameter	Notation	Value
Fuel pellet radius		0.438 cm
Gap width		0.009 cm
Cladding thickness		0.066 cm
Cladding outer radius	$r_c$	0.513 cm
UO2 lattice pitch	$p_O$	1.295 cm
UN lattice pitch	$p_N$	1.421 cm
UO2 heavy metal density^Citation20,Citation21	$D_O$	9.04 g/cm^3
UN heavy metal density	$D_N$	12.7 g/cm^3
UO2 volumetric heat capacity^Citation20,Citation21	$c_{pO}$	3.29 $\mathrm{M}\mathrm{J}{\mathrm{m}}^{-3}{\mathrm{K}}^{-1}$
UN volumetric heat capacity^Citation22,Citation23	$c_{pN}$	1.62 $\mathrm{M}\mathrm{J}{\mathrm{m}}^{-3}{\mathrm{K}}^{-1}$
UO2 thermal conductivity^Citation20	$k_O$	4.17 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$
UN thermal conductivity^Citation24	$k_N$	18.7 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$

Fig. 3. Achievable cycle length of the doped UN fuels, relative to that of pure UN fuel, as a function of the fraction of dopant. The relative cycle length achievable with pure UO₂ fuel is also shown for comparison.

TABLE III Parameters for the Production of Doped UN

Dopant	Chromium	Nickel	Aluminum
Concentration of uranium (mol/L)	1.5	1.51	1.51
Concentration of dopant (mol/L)	0.19	0.18	0.22
Dopant weight percent (of total metal)	2.7	2.8	1.5
Dopant molar percent (of total metal)	11.3	10.6	11.9
Dopant oxide volume percent	9.5	7.0	8.9
Molar ratio carbon/total metal	2.21	2.22	2.18
Molar ratio urea/total metal	1.28	1.29	1.25
Molar ratio HMTA/total metal	1.69	1.70	1.66
Milling before pressing	No	Yes	No

Fig. 4. SEM/EDS images showing the elemental distributions on the surfaces of the air-dried spheres of UN doped with (a) chromium, (b) aluminum, and (c) nickel, respectively.

Fig. 5. SEM/EDS images showing the elemental distributions on the surfaces of the nitrided spheres of UN doped with (a) chromium, (b) aluminum, and (c) nickel, respectively.

Fig. 6. SEM/EDS images showing the elemental distributions on the surfaces of the sintered pellets of UN doped with (a) chromium, (b) aluminum, and (c) nickel, respectively.

TABLE IV Densities of the Doped UN Pellets

Dopant	Chromium	Nickel	Aluminum
Density (g/cm^3)	5.7	8.2	7.3

Fig. 7. XRD pattern indicating the presence of different compounds at the surface of the chromium-doped UN pellet (a) before and (b) after the dissolution test.

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