Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 Page 7 Page 8 Page 9 Page 10 Page 11 Page 12 Page 13 Page 14 Page 15 Page 16 Page 17 Page 18 Page 19 Page 20 Page 21 Page 22 Page 23 Page 24 Page 25 Page 26 Page 27 Page 28 Page 29 Page 30 Page 31 Page 32 Page 33 Page 34 Page 35 Page 36 Page 37 Page 38 Page 39 Page 40 Page 41 Page 42 Page 43 Page 44 Page 45 Page 46 Page 47 Page 48 Page 49 Page 50 Page 51 Page 52 Page 53 Page 54 Page 55 Page 56 Page 57 Page 58 Page 59 Page 60 Page 61 Page 62 Page 63 Page 64 Page 65 Page 66 Page 67 Page 68 Page 69 Page 70 Page 71 Page 72 Page 73 Page 74 Page 75 Page 76 Page 77 Page 78 Page 79 Page 80 Page 81 Page 82 Page 83 Page 84 Page 85 Page 86 Page 87 Page 88 Page 89 Page 90 Page 91 Page 92 Page 93 Page 94 Page 95 Page 96 Page 97 Page 98 Page 99 Page 100 Page 101 Page 102 Page 103 Page 104 Page 105 Page 106 Page 107 Page 108 Page 109 Page 110 Page 111 Page 112 Page 113 Page 114 Page 115 Page 116 Page 117 Page 118 Page 119 Page 120 Page 121 Page 122 Page 123 Page 124Nuclear Science User Facilities 60 Collaboration with first-class material scientists and access to PNNL advanced facilities resulted in the successful completion of the project. — Mehdi Balooch, Co-principal Investigator with X-ray microanalysis (SEM/ energy dispersive X-ray spectroscopy [EDS]) for detection of swelling, void formation, corrosion of cladding, dissolution of zircaloy and fuel in LM gap, and finally LM chemical changes due to tempera- ture cycles during irradiation. Accomplishments In this progress report, we present the post-irradiation analysis of one of the rodlets, the 0.29% FIMA, using high‑resolution optical and SEM/EDS at two height-level axial positions (mid- and lower‑level). Comparing the unirradiated with irradi- ated fuel rod images by high-resolution optical microscopy (Figure 1) we have observed extensive thinning, mainly from the outside surface of the cladding at the mid-level cross section, where the cladding prematurely failed during irradiation at the MIT reactor.This persuaded, but to much lesser extent, at lower‑level cross section where it was limited to ~8%. Limited availability of the LM for dissolution of Zr in the fuel‑cladding gap impeded thinning of the inside surface of the cladding to negligible amount. In contrast, the larger outer LM reservoir was a more- efficient sink for dissolution of the cladding, which resulted in massive and surprising thinning of the cladding at mid-level.This zircaloy loss is an artifact of the experiment. In actual use, the outer cladding surface is contacted by water, not by LM. The loss of cladding material, suggests the dissolution of Zr in LM passes beyond the solubility limit. But it continued by generating new phases such as ZrBi2, ZrBi, Zr3Bi, Zr5Pb3, and possibly Zr3.33 Pb0.67. U phase of UBi2 may also be formed. High-resolution elemental imaging of unirradiated and irradiated fuel (Figures 2 and 3) reveal the dissolved Zr was distributed uniformly in the LM gap.The fuel surface interaction with the LM gap was limited—some evidence of dispersed micron-size uranium phase of the fuel in the LM gap adjacent to the fuel surface and its aggregation close to the inner cladding surface was found. The bulk of the hydride fuel appeared intact—no detectable swelling of the fuel, or its microstructural changes within the capabilities of the instru- ments was observed. The dissolution of 8% of zircaloy-2 cladding in lead-bismuth eutectic Bi-Pb LM during 1000 hours of exposure was found excessive for use as a gap-filler in a nuclear reactor. However, the addition of Figure 2. Elemental map including inner liquid metal, fuel, and cladding of the unirradiated fuel rodlet.