Researchers from Sandia National Laboratories have conducted extensive research on these topics. They have reviewed the current literature, identified knowledge gaps in alternate fuel vehicles and tunnel safety, and developed a generalizable framework to assess tunnel safety for a diverse range of alternate fuel vehicles. Supported by US Department of Energy (DOE) and Federal Highway Administration (FHWA), their research focused on: 

• Reviewing the current research on alternate vehicle type as they relate to hazards in tunnels and identifying knowledge gaps. Their findings are captured in this report 

• Understanding the unique hazards associated different types of vehicles powered by gasoline, diesel, batteries, natural gas, propane, and hydrogen fuel cells

• Developing a computationally inexpensive, adaptable, and generalized framework for assessing safety of alternate fuel vehicles in tunnels 

• The framework incorporates accident scenarios, physics models and consequence models, tank blowdown calculations, parametric sensitivity analysis, studies of tank volume, orifice size, fullness, fuel type, and tunnel geometry characterization with a focus on high-traffic tunnel characteristics. 

Their research was presented at an FHWA webinar on November 9, 2023.  The presentation can be viewed here. The webinar recording is available here and the passcode is mPXM1wk&.

The UN ECE R134 regulation is a good requirement to follow as it copies the language in the UN GTR #13 regulation. The updated version of this UN document (UN GTR #13 Phase 2) is currently in approval review at the GRSP in Geneva and should be approved by the end of 2023. Nevertheless, since the US Department of Transportation’s National Highway Traffic Safety Administration is a contracting party to the UN GTR #13 development process, they have already begun a review of the Phase 2 document and are currently creating the FMVSS standard language for this new NHTSA requirement. It won’t become a Federal Motor Vehicle Safety Standards standard for at least 1.5 years, so for now it should be considered as a due diligence requirement for automotive FCEV applications. Since ECE R134 represents the latest embodiment of UN GTR #13, this is a long way to say that following R134 puts one in compliance with UN GTR #13 and hence would be following best practices in the USA (and Canada). 

From a vehicle design perspective, it is always best to ensure that locations where leaking hydrogen can accumulate are minimized/avoided, and that hydrogen sensors are placed in strategic locations where hydrogen can potentially collect. Potential ignition sources in areas where hydrogen can collect should also be avoided. If this safety concept is followed, then the 3% warning/4% shutdown strategy will work fine, although some original equipment manufacturers (OEMs) may choose to employ a stricter requirement of say, 2% warning/3% shutdown. In the event of a detected hydrogen leak, there is a requirement for the vehicle CHSS to be isolated but there is technically no requirement for the high voltage system to be isolated. Some OEMs may choose to leave HV on so that the vehicle may continue in limp mode to a safe location, while others might choose to isolate HV as well, leaving the operator with very limited remaining battery power to safely pull over. 

There are no requirements for the use of detectors for hazardous material transportation. Use of detection equipment is dependent upon the manufacturer and operator based on risk assessment.
 

It’s not clear if “mobile” in the question refers to vehicle fuel tanks, or vessels used for transportation of hazardous materials. 

1. If it’s a Type 4 vessel for transportation of hazardous materials, it will be built to a special permit that details the inspection requirements. Special permits vary but they are generally consistent on a 5-year requalification period, which also includes a visual inspection by an inspector certified to CGA C-6.2 at that interval. 
2. Since C-6.4 is mentioned, the question might pertain to vehicle fuel tanks. The C-6.4 document  recommends 36 months or less, but there are inconsistent requirements. The tank standard (ANSI/CSA HGV 2) originally required for a visual inspection every 36,000 miles or 3 years, whichever comes first, but the latest version states that the inspection is to be specified by the container manufacturer. There is an annex in HGV 2 that calls CGA C-6.4 or ISO 19078 to be used, although the ISO standard does not yet include hydrogen in its scope and is for CNG tanks only. Commercial vehicles such as trucks, buses and trailers also required an annual U.S. Department of Transportation (DOT) inspection to ensure that it complies with the Federal Motor Carrier Safety Regulations. This is a comprehensive check but does not have specific requirements for vehicle fuel tank inspection.
    

FCEVs usually contain only a minimal amount of hydrogen fuel pressure (several Mpa) to support getting the car on and off car carriers. Panel members are not aware of any hydrogen release incidents during vehicle loading and don’t know what the probability of such a leak is considering the loading operations. A risk assessment accounting for the probability of collisions as well as leaks from new cars off the assembly line would be needed. It may be best to connect with the car manufacturers when considering such an assessment. 

Regarding the questions on the source of leaks and leak rates, Section 5.2.2.2.2 of SAE J2579 (which is harmonized with the GTR) provides the following: The maximum allowable discharge (non-accident) from the compressed hydrogen storage system for passenger vehicles is A*150 Ncc/min where A = (Vwidth+1)*(Vheight+0.5)*(Vlength+1)/30.4 and Vwidth, Vheight, Vlength are the vehicle width, height, length (m), respectively. Simply speaking, the allowable leakage from the compressed hydrogen storage system of a passenger vehicle that fits in a standard garage in the USA is 150 Ncc/min. Note that the 150 Ncc/min limit for a typical passenger vehicle drops to an expected value of 7.5 Ncc/min (maximum) at a partial fill condition of 3 MPa. Potential sources for leaks during vehicle operation could include joints/fittings in the piping or at the fuel cell stack. This piping is typically at lower pressures than the storage vessel as a result of a system regulator.

Additionally, a solenoid valve is located in the tank. The solenoid closes when the key is turned off or the electrical system deenergized. The result is that only a very minimal amount of hydrogen would be in the piping system unless there is a failure of the solenoid. A useful diagram is included here, showing the hydrogen line connections in FCEV with Type 4 tank (source: Update to the 700 bar Compressed Hydrogen Storage System Cost Projection Webinar). This also shows a thermally activated pressure relief device integrated into the in-tank valve/regulator assembly as typical.

Generally speaking, the International Fire Code and NFPA 2 apply to non-transportation use of hydrogen. These are maturing quickly, with NFPA 2 currently having issued its most recent edition in 2023. Standards for both on-board LH2 tanks and LH2 tankers for bulk fuel transport are managed by the U.S. Department of Transportation (DOT) and are well established. DOT transport requirements for the U.S. can be found in 49 CFR. 

There is growing activity regarding the use of LH2 as a vehicle fuel and there are several prototype trucks in operation. The on-board tanks may lack some reference standards, but vehicle fuel storage systems are typically self-certified by the original equipment manufacturers, particularly at the current state of development. While there is risk of impeding the development of a commercial market due to a lack of approved and common hardware, there is also risk of finalizing a standard prior to completion of development and testing. ISO TC 197 has several working groups including WG 1 and WG 35 to further develop the necessary standards for fuel tanks, fueling connections, and filling protocols. Liquid hydrogen transfer is well proven and established for industrial applications. However, consumer use of a cryogenic product has not yet been proven and will require refinement of hardware and processes.

The vehicle manufacturers can also provide guidance as to their efforts to meet required regulations.
 

Hydrogen gas storage and fuel cell systems are typically closed systems with a variety of monitoring and control functions to prevent leaks. Please check with the bus manufacturer and authority having jurisdiction to verify this is acceptable per their direction. However, a good safety practice would be to minimize the time spent indoors for these activities. Hydrogen vehicles maintained in a facility should be provided with separation/isolation and appropriate safety control systems. Further guidance can also be provided by the local Authority Having Jurisdiction with regard to local regulations. 

This is not an easy question since many factors influence how much hydrogen can be transferred from one vessel at a higher pressure to another one at a lower pressure and the rate at which it can be transferred. The pressure in the higher vessel will fall while that in the lower vessel will rise as gas is transferred, so the flow rate will typically slow down and eventually stop as the pressures equalize. Fueling protocols have important upper and lower bounds on the flow rate or pressure change rate that is allowed when filling a vehicle. The SAE J2601 family of standards detail those rates and ending pressures.  Most hydrogen dispensers use a cascade method of fueling (switching between multiple supply tanks) to maintain the desired flow rate and maximize the amount of hydrogen that can be supplied. As a general rule, a higher percentage of gas can be transferred from the supply tanks when filling lower pressure 350 bar tanks than higher pressure 700 bar tanks.  Similarly, as storage pressure Y psi is increased, a higher percentage of the gas will be usable for either fill pressure.

The density of hydrogen does not vary linearly with pressure since it’s not an ideal gas. For reference, the density of gas at 350 bar is 24 g/liter and at 700 bar is 40 g/liter. Thus, doubling the pressure only results in a 60% increase in storage volume. For this reason, 700 bar vehicle tanks are rarely filled from a single tank and either use multiple supply tanks in a “Cascade” type system or are direct filled from compression systems. 

In the U.S., liquid hydrogen fueling stations and dispensing equipment are addressed within NFPA 2, Chapter 11. Dispensing is covered within Section 11.3. When liquefied hydrogen is used as the supply for high pressure gaseous fueling, then Chapter 10 of NFPA 2 would apply.
ISO standards are also being developed for global LH2 fueling protocols.