RRFM 2009 Transactions - European Nuclear Society

Figure 1. MCNP geometry model of fuel element with burnable absorber lumped in 36 wires
or 3 slabs in the aluminium stiffeners.
2.3 Optimization of burnable absorber composition
The criteria for choice of optimal burnable absorber material was based on: (i) maximum
efficiency (macroscopic absorption cross section; reactivity worth); (ii) maximum burn up
rate; (iii) the daughter isotopes of the burnable poison should have small absorption
macroscopic cross section compared to the cross section of their precursors; (iv) high
melting point is preferable for the chosen burnable absorber. The best of these options
should have high macroscopic effective absorption cross section in the beginning of cycle
and burn up most rapidly in order to end the cycle with the lowest burn up penalty. Suitable
burnable absorbers are cadmium, gadolinium, boron, samarium; also erbium and
dysprosium. Europium and hafnium have high cross sections of daughter isotopes, therefore
they were excluded as candidates for burnable absorber. Samarium possibly will be included
as additional burnable absorber, mixed in the fuel meat as in the current fuel after the final
selection of main burnable absorber. The burnable absorber samarium burns in ~ 2 days and
it is added to the fuel with purpose to compensate the rapid production rate of xenon-135 at
the reactor start-up.
A comparative analysis of the macroscopic absorption cross sections, burn up rate,
reactivity effects was performed for the selected burnable poisons and presented in [1-2].
Three major candidates as burnable absorber for the new BR2 fuel were selected: Cd,
Gd 2 O 3 , B 4 C. The preliminary evaluations with erbium did not show satisfactory results for the
reactivity excess during the transition cores, therefore it was excluded from further studies.
On the basis of the absorption properties of the considered 3 burnable absorbers it was
concluded that reducing the wire diameter enhances the burn up rate.
2.4 Comparison of BA absorption properties in transition cores
Preliminary studies for the feasibility of the BR2 fuel cycle with different fuel types (HEU, 90%
U5, 1.3 gU tot /cc, UAl x ; LEU, 20% U5, UMo, 8.5 and 7.5 gU tot /cc; LEU, 20% U5, U 3 Si 2 , 4.8
gU tot /cc) and various burnable absorber options have been performed for the transition and
fully converted cores. The reactivity excess during the transition and fully converted cores
was evaluated and compared for the different burnable absorbers. The diameter of the wires
has been varied between 0.2 mm and 0.6 mm with purpose to adjust the burn up rate of the
poison to the desired reactivity excess for the different fuel types. The gain in reactivity is
maximum (+3.30$) for UMo, 8.5 gU tot /cc with Cd-wires with diameter D=0.4 mm. The
reactivity excess in UMo, 8.5 gU tot /cc with borated aluminium stiffeners is similar (+3.22 $) to
the same core with Cd-wires with D=0.4 mm. The gain for UMo, 7.5 gU tot /cc is similar to the
reference load (+0.36$). The preliminary calculations of the reactivity evolution of fully
converted cores with cadmium wires (D=0.4 mm) have shown flattened reactivity evolution
*AREVA-CERCA, a subsidiary of AREVA NP, an AREVA an SIEMENS company
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