Seismic Anchorage of Dry Storage Casks Using Stretch Length Anchors

For vertical containment structures which do not have significant ductility, a design that utilizes plastic yielding of the anchor bolts used to connect the structure to its foundation can be used to provide structural ductility to the system. One type of vertical containment structure is a dry storage cask (DSC), which is used to store spent nuclear fuel (SNF) rods from nuclear power plants (NPPs). DSCs are a temporary storage solution which may be licensed for operation periods up to 60 years. DSCs are being re-evaluated as a potential mid-term storage solution, where operating periods may be extended to 300 years. The potential extension of DSC compliance period increases the seismic hazard, resulting in very large horizontal accelerations and destabilizing effects from vertical accelerations. DSCs are typically unanchored, but units located in moderate to high seismic regions may slide and/or tip over, damaging the cask internals. This research focuses on the benefits of seismic anchorage of DCSs using stretch length anchors. A steel ring designed to clamp the DSC on a concrete pad is investigated as part of the anchorage system. Two types of anchor bolts are being investigated for the steel ring: (a) conventional bolt details with steel chairs using designs adopted from anchoring liquid storage tanks, and (b) stretch length bolt details with steel chairs, which performed very well during the 2010 Maule, Chile, Earthquake. In the first phase of the study, single anchor tests were performed using both conventional and stretch length bolts under dual actuator cyclic tension and shear. The evaluated stretch lengths are equivalent to four, six and eight bar diameters. Two thicknesses are also considered for the plates of the steel chair: (i) 6.35 mm and (ii) 12.7 mm. In the second phase, a group of anchors using the steel ring to clamp the cask were tested for a 1:2.5 scaled DSC under quasi-static horizontal cyclic loads. The experimental results are presented and compared for conventional and stretch length bolt details, as well as the scaled DSC in terms of load, displacement, and hysteretic energy capacity. It is shown that stretch length bolt anchorage details provide superior seismic performance compared to conventional bolt anchorage details.