PWR/VVER fuel assembly bowing may occur due to excessive guide tube (GT) growth that will result in larger holding down forces (Fig. 5.1) (Strasser et al, 2010a). The bowing is caused by the complex interaction of a variety of parameters that include the bowing of the skeleton assembly. The param­eters include:

• GT irradiation growth as a function of fluence and temperature, see Section 4.6.1 on irradiation growth.

• GT creep as a function of fluence and temperature, see Section 4.6.2 on irradiation creep.

• GT stiffness and buckling strength as a function of temperature.

• The effect of hydrogen pickup and irradiation on the GT properties, see Section 4.5 on corrosion of zirconium alloys.

• Hold-down force.

• Thermal expansion of the skeleton components, the core plate spacing and their interaction.

• Fuel rod/grid friction force and relaxation over time.

BWR fuel channel bowing was studied by Cantonwine et al. (2009). According to them, channel-control blade interference had been a
challenging issue over the previous 8 years for operating BWR plants where ~2-year cycles are normal and Zircaloy-2 is the standard channel material. The primary reason for this was the unaccounted channel dis­tortion caused by differential hydrogen across the channel resulting from shadow corrosion on the blade side (known as shadow corrosion-induced bow). Zircaloy-2 is particularly susceptible to this distortion mechanism because it has a high hydrogen pickup fraction (HPUF) that increases with exposure.

Several strategies have been developed to combat bow. As an interme­diate resolution to this issue Zircaloy-4 has been reintroduced because it is effectively resistant to shadow corrosion-induced bow and has similar irradiation growth and creep performance to Zircaloy-2. The one disadvan­tage of Zircaloy-4 is that it has less corrosion resistance than Zircaloy-2. However, based on the extensive experience with Zircaloy-4 channels both in the United States and Japan (plus processing improvements have been made specifically to enhance corrosion resistance), the corrosion perfor­mance of Zircaloy-4 is claimed to be adequate for channel applications. Other examples of global nuclear fuel (GNF) publications on channel bow are described by Mahmood et al. (2007) and Cantonwine et al. (2009). Other reasons for BWR fuel channel bowing are (Strasser et al, 2010a):

• Fast neutron flux gradients from a variety of causes including the flux gradient at the core periphery (see Fig. 5.2).

• Non-uniform metallurgical structure (e. g. texture difference between the two opposing channel sides leading to difference in irradiation growth rate) or composition.

• Non-uniform wall thickness.

• As-fabricated bow.

The bowing may result in difficulties in inserting the control rods (a safety issue) and/or in a decrease in thermal margins, the latter from two possible causes. First a departure from nucleate boiling (DNB) value: if the fuel rod surface heat flux becomes large enough, the water film adjacent to the fuel rod will convert into a steam film with a much lower thermal conductivity resulting in a rapid large increase in the fuel cladding temperature which, in turn, will accelerate the oxidation and embrittlement of the fuel cladding. The maximum heat flux at which the water is converted into a steam film is referred to as the DNB value. Second, a loss of coolant accident (LOCA) could, for example be caused by a coolant pipe break in the primary circula­tion system since larger water gaps between assemblies may exist in the core than is accounted for in the core nuclear design. To ensure that the LOCA licensing criteria are met, the fuel rod surface heat flux must be limited.

Core periphery

5.2 Schematics showing fuel outer channel bowing at core periphery due to large fast neutron fluence. Largest degree of bowing in BWRs occurs at the core periphery due to the flux profile. Also the type of FA bow seems to be very dependent on core location (Strasser et al., 2010a).