MBM UFD Development Plan

Following irradiation in reactor, spent fuel is moved to wet storage where decay heat can be easily removed. After the decay heat is acceptably low, spent fuel is dried and placed in storage casks. Given that the U.S. does not have a spent nuclear fuel facility, used fuel will remain above ground in casks for the foreseeable future. It is desirable that the fuel rods maintain structural integrity during during handling, transportation, and eventual retrieval.
Perhaps the principal concern with respect to spent fuel rods is hydride precipitation and embrittlement. In reactor, fuel rod cladding oxidizes with an associated release of H2. Some of this hydrogen enters into the zirconium alloy. At high temperatures, the hydrogen is primarily in solution but will precipitate as ZrHx at lower temperatures. With significant hydrides, and depending on the hydride orientation, the ductility and fracture toughness of the cladding can change significantly. The orientation of these hydrides has been shown to depend on the stress state in the cladding. In particular, hydrides primarily form perpendicular to the dominant stress field in the cladding. As temperatures drop following irradiation, hydrides form in a circumferential direction. During drying, temperatures are high, and the hydrogen undergoes dissolution. Following drying, lower temperatures and the changed stress state (strong hoop stress due to internal gas pressure) result in radial hydride formation. Radial hydrides are of concern because they significantly weaken cladding, including increasing susceptibility to crack formation and propagation.
Computational modeling of spent fuel is attractive due to both the expense of testing irradiated materials and the long time frames (decades) involved in storing spent fuel. The Used Fuel Disposition Campaign Gap Analysis report [2] identifies understanding the effect of hydrides as a high-priority research issue.