Holtec International Storage Module Underground

HI-STORM UMAXThe underground HI-STORM vertical ventilated module is the latest to join the HI-STAR/HI-STORM family of HLW storage systems. HI-STORM 100U is the underground counterpart of the presently licensed above-ground models – HI-STORM 100 and HI-STORM 100S – which are available in both free-standing and anchored configurations.

The common fuel confinement device used to store fuel and High Level Waste in all HI-STORM models is the Holtec multi-purpose canister (MPC), which is externally identical for storing PWR and BWR fuel, but differs in its internal anatomy depending on the type of fuel being stored. Its principal characteristics in a nutshell are:

The HI-STORM 100U in a Nutshell

  • Fuel is stored vertically in a strength-welded, all-stainless canister (MPC).
  • Fully compatible with all other Holtec dry storage components (MPCs, HI-STAR overpack, HI-TRAC transfer cask, etc.) such that both underground and aboveground HI-STORMs can be deployed at the same ISFSI site using nearly identical loading equipment.
  • MPC stored underground and thus made essentially inaccessible to threats and hazards of any kind.
  • Storage cavity engineered to self-extinguish combustion of any flammable material that is introduced (inadvertently or deliberately) in the MPC storage space.
  • Thermal performance is enhanced, not degraded by floodwater intrusion.
  • Engineered to prevent significant solids deposit in the storage capacity under windborne sand and debris.
  • Configured to be visually inconspicuous (under two feet tall).
  • Designed to allow vanishingly small site boundary dose.
  • Economical to decommission.
  • Loaded MPC can be placed in storage or removed from storage in the matter of a few hours

Essential Data on HI-STORM 100U

Heat rejection capacity/canister34kW in uniform storage; up to 36.8 kW if hot fuel is preferentially loaded in the peripheral cells (called “reverse regionalized” storage)
Maximum seismic levels for general certificationHorizontal: 1.25g’s Vertical: 0.8g’s
Maximum height of the lid (above the crawler riding roadway)23 inches
MPC typesMPC-68 (BWR), MPC-32 (PWR), MPC-24 (PWR)
Maximum enrichment5% PWR fuel, 4.2% BWR fuel
Maximum burnup68.2 GWD/MTU (PWR) 65 GWD/MTU (BWR)
Minimum module center-to-center spacing12 Feet

Fulfilling Industry’s Quest for Zeros

  1. Vanishingly Small Dose in the Vicinity of the Storage Site: The depth of HI-STORM 100U can be increased virtually without limit. Even at its shallowest storage, the dose from a HI-STORM 100U loaded with extremely “hot” fuel (69 GWD/MTU, 5-year cooled) is only about one thousands of a milli-rem per hour at a distance of 100 meters. As the depth is increased, this infinitesimal dose rate reduces to incalculably miniscule values.
  2. Zero Risk of Release of Radioactivity at any Storage Site: Release of radioactivity from the HI-STORM 100U by any mechanical means (crashing aircraft, missile, etc.) is virtually impossible. The only access path into the cavity for a missile is vertically downward, which is guarded by an arched, concrete-fortified steel lid weighing in excess of 10 tons. The lid design, at present configured to easily thwart a crashing aircraft, can be further buttressed to withstand more severe battlefield weapons, if required in the future for homeland security considerations. The lid is engineered to be conveniently replaceable by a later model, if the potency of threat is deemed to escalate to levels that are considered non-credible today.
  3. Zero Risk of Damage from Fire, Flood, Hurricanes, Tsunami, and the Like: Combustible materials, if introduced in the storage cavity, cannot sustain its burn. There is no path of ground water to enter the storage cavity. HI-STORM 100U will continue to maintain the MPC in a cooled state for as long as necessary, even if the site is completely flooded submerging its diminutive body. Even off-the-chart tsunamis, hurricanes, tornados, and other vagaries of nature are incapable of preventing a HI-STORM 100U storage system from rendering its intended function.
  4. Zero Risk of Ground Water Incursion: The underground storage cavity is formed by a thick steel container that has no penetrations or joints and is fortified by a preservative suited to the geological characteristics of the site to prevent corrosion.

Loading and Unloading a HI-STORM 100U

Installing an MPC in a HI-STORM 100U module, or removing it for packaging in the HI-STAR dual-purpose overpack for off-site transport, is simple enough to be completed in half of a work shift. The illustrations show an MPC being removed from a HI-STORM 100U module and packaged in the HI-STAR overpack ready to ship by rail.

MPC bearing HI-STAR outfitted with impact limiter installed on the rail car for off-site transport
MPC bearing HI-STAR outfitted with impact limiter installed on the rail car for off-site transport
HI-TRAC bearing the MPC is staged over the HI-STAR overpack situated in the MPC transfer cavity using the crawler for transfer of the MPC to the HI-STAR overpack.
HI-TRAC bearing the MPC is staged over the HI-STAR overpack situated in the MPC transfer cavity using the crawler for transfer of the MPC to the HI-STAR overpack.


TitleYearPatent No.Status
System and Methods for Storing Spent Nuclear Fuel2022US 9,916,911 B2, US 8,098,790, US 8,625,732, US 7,068,748, US 11,342,091 B2Granted
Ventilated System for Storing High Level Radioactive Waste2018US 10,147,509 B2Granted