Tests for Appropriate Class Determination (18.1)

Tests for Appropriate Class Determination (18.1)

Criticality: Medium
Progress: Addressed, Not Adequately
Score: 12
DOT Relevance: 173 Subpart D

Description of Key Area

DOT hazardous materials regulations for transportation are based on the performance of the material when subjected to various tests. The definitions and requirements classification and packing group assignments are found in 49 CFR 173 of the hazardous materials regulations, specifically 173.50 through 173.156. The classification and packing group assignment are, for the most part, based on tests found in the UN Manual of Tests and Criteria and are essentially open air tests. Testing of this type for the determination of the potential hazards associated with a metal hydride-based hydrogen storage system may not be appropriate.

There are two broad types of metal hydride hydrogen storage systems that are being developed and need to be considered, rechargeable and non-rechargeable systems; where rechargeable systems contain a reversible metal hydride are refilled by applying hydrogen to the system and non-rechargeable systems are refilled by removing the spent hydrogen-depleted material and replacing it with fresh hydrogen-containing material.

For non-rechargeable systems, the current material testing for determination of hazard class and division may satisfactory address the actual hazards presented by the hydrogen storage system. The systems may contain a mixture of hazardous materials, such as a liquid phase that may or may not contain a solid phase in slurry and possibly gaseous hydrogen. Each of the various materials could be tested by the appropriate test methods and the hazard class/division determined; the overall system classification and division set according to 49 CFR 173.2a for mixtures and materials having more than one hazard.

Rechargeable metal hydride systems will normally contain gaseous hydrogen and a solid phase. While it might seem logical to classify them as a mixture of hazardous materials: gaseous hydrogen (a 2.1 flammable gas) and a solid material, which might be non-hazardous, a 4.1 flammable solid, a 4.2 self-heating solid or a 4.3 water reactive solid, this approach might not represent the actual hazard represented by the overall hydrogen storage system. The independent, separate hazards testing of the solid materials in the absence of hydrogen gas does not accurately represent the state of the materials in a hydrogen storage system in the presence of hydrogen gas. When charged with hydrogen, the hydrogen is bonded to the solid phase, forming a distinctly different chemical species. Reversible hydrogen storage materials, by design, decompose under the operational conditions of the storage system to release gaseous hydrogen. This process is normally endothermic, requiring an input of heat, thus the materials cool upon hydrogen desorption. Also the hydrogen gas might provide a barrier, slowing the diffusion of oxygen-containing air to the material. Therefore the reactivity of the solid material in the presence of hydrogen gas will be different than the combustion or oxygen reactivity of the solid material at ambient temperature when in the totally desorbed state in the absence of air. An extreme example of the difference between the hydrided and non-hydrided state could be shown with titanium, which forms a stable hydride, decomposing to release hydrogen only at high (600C (1112F)) temperatures. Dry titanium powder (UN 2546) is a 4.2 self-heating solid assigned to packing group I or II (i.e., either pyrophoric or at least produces moderate heat on air exposure); whereas titanium hydride (UN 1871) is a moderate (packing group II) 4.1 flammable solid.

Additionally hydrogen storage systems that utilize metal hydrides are more complex than simple storage containers. For example two engineered features that these systems will likely contain for proper and optimal operation include a manner to transfer heat between the contained solid phase and an external heat sink and a method of preventing the solid phase from being redistributed within the container. This second feature is to prevent compaction of the solid which could over-stress the container. These engineered features may mitigate potential hazards in case of an accident by minimizing release of material or restricting the ability of air to diffuse to the solid phase. Therefore again the hazard presented by the total system may not be appropriately represented by the individual, open air material tests.

Discussion of Criticality

This item has been assigned a criticality of medium for several reasons. Currently hydrogen storage systems where the hydrogen is absorbed in a metal hydride are allowed for transport under special permits. The UN has approved a listing in the dangerous goods table, UN 3468, and the US DOT issued NA 9279, both of which classifies these systems as 2.1 flammable gas systems. With review and approval of individual systems and manufacturers, potential risks associated with these systems are minimized.

Low power fuel cell systems for portable power applications are entering the commercial marketplace. Today the volumes are relative low with few manufacturers. However it is expected that these applications will be the first to achieve mass market status, with more products, manufacturers, and higher volumes expected within the next few years.

Before packing instructions are put into regulations and some of the materials currently under investigation are introduced into commercial systems, it is recommended that a revised method to determine the true potential hazards presented total system be considered.

Discussion of Progress

In the last several years, the US DOT issued hazardous materials table listing NA 9279, Hydrogen absorbed in metal hydride and the UN SCETDG approved entry UN 3468, Hydrogen in a metal hydride storage system to the List of Dangerous Goods. Both of these listings assign a hazard classification to the systems of 2.1 flammable gas. Currently these identifications can only be used with approval from the OHMS after review and approval of the packaging. No packaging instructions have been adopted in either the US regulations or the international Model Regulations. The OHMS has issued several special permits for metal hydride hydrogen storage systems. All of the systems that allow recharging use NA 9279 and/or UN 3468 with a 2.1 flammable gas classification.

Progress on developing consensus standards that might be used as packaging instructions include:

  1. The ISO technical committee for hydrogen technologies (TC 197) has a working group drafting a standard for transportable reversible metal hydride hydrogen storage systems (document 16111). This document is currently in the approval stage as a committee draft (CD) for advancement to the draft international standard stage (DIS). In parallel to the CD approval, the document is being considered for publication as a technical specification; this will allow publishing of a consensus document at an earlier date than is possible with the International Standard. Once the international standard is approved, the technical specification will be withdrawn.
  2. CGA has also considered developing a standard for portable metal hydride hydrogen storage systems. The current status of this effort is not known at this time.
  3. Once approved and published, either the ISO or CGA document might be used as the basis for packaging instructions for NA 9279 or UN 3468. However it is expected that neither will explicitly address the hazardous classification of the materials or system.

Proposals have been submitted to ICAO and the UN SCETDG for approval of metal hydride hydrogen storage systems of limited size being transported aboard aircraft, both cargo and passenger, including within the passenger cabin. ICAO has approved part of the request to allow transport aboard cargo aircraft. These proposals have included introducing system level tests of the systems and/or reference to ISO 16111 to approve packaging.

ASME's Boiler and Pressure Vessel project team on hydrogen tanks is addressing metal hydride vessel design in a code case to Section VIII-1.


Currently metal hydride hydrogen storage systems can be transported upon review and approval by the OHMS of the packaging. NA 9279 and UN 3468 are available for use as identifications, both with a hazard classification of 2.1 flammable gas. This classification ignores any hazard that might be presented by the solid phase material and/or combination of hydrogen gas with the solid phase material.

Due to the nature of reversible metal hydride hydrogen storage systems, it is recommended that they be considered as articles and system level tests be developed that could predict the potential hazards associated with the total systems under simulated real-life conditions. An example of the testing that could be performed is catastrophic penetrations under several states of charge. This test could include a measurement of the energy that is released and that used to determine restrictions on mode and quantities for transportation. This would not penalize manufacturers that use a material that might appear to be more hazardous according to current test methods but mitigates risk with system design.

While there is a lot of information available about the traditional intermetallic metal hydrides, there is not a lot of public information available about total system performance and their hazards in accident scenarios. Also there are many materials under development that might have very different properties, and thus hazards. By developing system level tests to determine potential hazards, new materials and new designs will be able to be introduced and appropriately classified without the risk of misclassified.