Experiment Introduction

Integral Fast Reactor (IFR) X447/A Experiment

X447/A was designed to test the high-temperature performance of HT9 cladding and its compatibility with binary U-10Zr fuel (Pahl et al., 1993). The experiment utilized MK-D61 inner blanket driver type subassembly hardware. The coolant was orificed in this experiment to intentionally raise peak HT9 cladding temperature beyond 650 C. MK-D61 hardware adopted in X447/A usually contained 61 fuel pins (see Figure 1).

Figure 1: Schematic showing the U-10Zr/HT9 pins locations in X447/A subassembly (data available in Carmack (2012)).

However, as EBR-II enforced a subassembly mean coolant outlet temperature of 544 C, 12 solid dummy fuel pins were included to reduce the subassembly total power. Then 19 HT9 clad and 30 D9 clad U-10Zr fuel pins were irradiated in the subassembly X447 to a burnup around 4.7 at.%. Then four HT9 clad pins were replaced with fresh fuel pins before the irradiation experiment continued as subassembly X447A to achieve an ultimate burnup of approximately 10 at.%. In the IFR X447/A experiment, all the HT9 clad pins were located in the inner region of the subassemblies to be exposed to high cladding temperature, while the D9 clad pins were located in the peripheral region along with dummy pins. In the presence of adjacent dummy pins, all the D9 clad pins experienced much lower irradiation temperatures and thus had no issues surviving throughout the experiment. At the end, among the 15 HT9 clad and U-10Zr fueled pins irradiated to approximately 10 at.% burnup, the cladding of two pins (DP70 and DP75) were found to have breached during the steady-state high-temperature irradiation. The cladding failure occurred near the top of the fuel slugs, corresponding to the peak cladding temperature locations. More importantly, severe fuel-cladding chemical interaction (FCCI) wastage formation and prominent coolant-cladding chemical interaction (CCCI) wastage formation were observed in those HT9 clad pins irradiated to ∼10 at.% burnup at elevated temperatures. In some cases, over one-third of the cladding thickness was consumed by FCCI wastage formation. Thus, the HT9 cladding failures observed in X447/A were determined to originate from a combined effect of FCCI wastage-induced cladding degradation and thermally-activated creep rupture caused by internal plenum pressure buildup. The X447/A experiment was a valuable experiment for FCCI/CCCI study because the wastage formation effects were prominent. In general, this experiment was important for HT9 cladding as it defined the operating condition envelope of applying HT9 cladding in a Sodium-cooled Fasted Reactor (SFR). Therefore, only the HT9 clad pins in X447/A experiment were investigated in this evaluation effort. In particular, four pins with most Post-Irradiation Examination (PIE) data available (i.e., DP04, DP11, DP70, and DP75) are the focus here, as highlighted in Figure 1. These four pins are usually regarded as the pins that are most worth investigating within the IFR X447/A experiment (Nam et al., 1998).

Fuel Pin DP11

As mentioned above, Pin DP11, irradiated in IFR X447/A experiment, is one of the four pins of interest in this study. Three of these four pins (i.e., DP04, DP11, and DP70) were the only three pins located in the central subassembly region that consists of 7 pins throughout the experiment, resulting in higher cladding temperatures compared to those near the peripheral region. Although Pin DP75 was located off the central subassembly area, the subassembly orientation during the X447A irradiation made this pin experience relatively higher linear power and consequent high cladding temperature. Among these four pins, Pins DP70 and DP75 experienced cladding breaches near the end of irradiation, while Pins DP04 and DP11 maintained their cladding integrity throughout the ∼10 at.% burnup. Given that pin breaching is a stochastic process, Pins DP04 and DP11 are considered the sibling unbreached pins of DP70 and DP75. This is particularly important, as post-irradiation examination (PIE) techniques are limited for breached pins due to compromised structural integrity and contamination concerns. Consequently, Pins DP04 and DP11 have the most comprehensive PIE data among all X447 pins. More importantly, as these two pins, especially Pin DP11, have been extensively studied and analyzed, all essential data for their fuel performance analysis is available in open literature. Therefore, Pin DP11 was selected as the focus of this VTB case, while the simulation of the other X447 pins and statistical analyses are covered by BISON's X447 assessment case.

References