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In the core of BWRs, the second-phase particles (SPP) of Zircaloy-2 and Zircaloy-4, the Zr(FeCr)2 and the Zr2(FeNi) phase, release Fe and dissolve. The degree of dissolution depends on initial size and fluence. These SPP, however, are important for the corrosion behavior of Zircaloy. Zircaloy shows an increase of corrosion at a certain burnup, depending on the initial SPP size and fast neutron fluence. Only Zr alloys with irradiation resistant SPP avoid this type of increased corrosion completely.
Two types of irradiation resistant materials were considered. One is a Zr-Sn-Fe alloy containing the Zr3Fe phase, which is irradiation resistant under BWR conditions. The other material is a Zr-Sn-Nb alloy containing the irradiation resistant β-Nb phase. In-BWR tests have shown that a Sn content of >0.8% is mandatory to minimize the nodular corrosion. Two prototypes of irradiation resistant alloys, Zr1.3Sn0.25-0.3 Fe and Zr1Sn2-3Nb, were irradiated in a BWR for 1372 days to a fast fluence of 9 x 1021n/cm2 (E > 1 MeV).
These irradiation tests showed that Zr1.3Sn0.25-0.3 Fe has a little lower resistance against nodular corrosion than optimized LTP (Low Temperature Process) Zircaloy-2/4 and revealed that Zr1Sn2-3Nb is superior to LTP Zircaloy-2/4 with respect to nodular and shadow corrosion resistance.
The BWR corrosion resistance of Zr1Sn2-3Nb depends on heat treatment. The lowest corrosion was observed with material fabricated completely in the α-range, but also material manufactured in the lower (α+β)-range exhibits low corrosion. Material fabricated in the upper (α+β)-range showed a somewhat higher corrosion, a corrosion behavior similar to LTP Zircaloy-2/4. As far as final annealing is concerned, a long time annealing at 540°C is superior to a standard recrystallization treatment (e.g., at 580°C), which still leads to a corrosion behavior that is better than stress relieved Zr1Sn2-3Nb.
Zr1Sn2-3Nb is resistant to shadow corrosion, when fabricated completely in the α-range. In other conditions, it shows a similar shadow corrosion behavior as Zircaloy-4.
BWR, Zr alloy, Zircaloy-2/4, Zr-Sn-Fe alloy, Zr-Sn-Nb alloy, increased corrosion, in reactor corrosion, shadow corrosion, SPP, fabrication
Van Swam, L
Framatome ANP GmbH, Erlangen,