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Safety Issues on Nuclear Production of Hydrogen 
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José M. Martínez-Val, Jesús Talavera^(*), Agustín Alonso
ETSII, Universidad Politécnica de Madrid- Getafe, Spain
^(*) Refinería de Puertollano, REPSOL YPF, Spain

4. Risk analysis on hydrogen facilities

At present, 65% of the hydrogen is consumed in oil refineries. Fertilizer production facilities, food industries and other chemical specialities are also relevant in hydrogen consumption. In most of the cases, it is the so called captive hydrogen, produced in the same place where it is consumed. Hydrogen for fuel cells still is a very minor fraction of the total flow rate in the world.

According to the foregoing statements, it could be said that the very high safety records in he hydrogen industry are not totally meaningful for the future hydrogen economy, where hydrogen will have to be stored, transported, distributed and used in huge amounts and through very  many channels. It is true that hydrogen is also used nowadays in very hard equipment, as alternators and rockets, but hydrogen is manipulated in those cases under very strict safety rules, including inert atmospheres (of CO₂ in the first case, He in the second one). From that viewpoint, it can be said that hydrogen manipulation still suffers from the "Hindenburg" syndrome, named after the large dirigible that exploded in Germany when being refuelled (in the period between the First and the Second World Wars).

In some refineries, particularly those working with heavy oil and under very strict limits of sulphur contents in the refined products, the hydrogen consumption can reach 4 kg per tonne of crude oil. Part of it is generated within the refining processes (catalytic reforming) and part of it (50% or more) is obtained from steam reforming of natural gas. The state of the art of all these operations is very high, and hydrogen is considered as another (not particularly nasty) flammable fuel (as methane, propane and the like). In order to establish any operation involving hydrogen, the following steps are compulsory:

In some refineries, particularly those working with heavy oil and under very strict limits of sulphur contents in the refined products, the hydrogen consumption can reach 4 kg per tonne of crude oil.

·        Tested results of research and development.

·        Development and approval of safety standards

·        Specific standard implementation

·        Certified testing (quality assurance)

·        Education and training

Although each oil refinery defines and implements own safety procedures, most of them follow the standards approved by several organisations (ANSI, ASME, CGA, GAMA, ISO, NFPA, NIOSH, NHA, OSHA, all of them from the USA, TÜV from Germany, KHK from Japan, and others). Some of those standards (for instance, NFPA) are addressed to fire protection, and others apply to pressurized thanks (35 MPa and 70 MPa) as ISO-11439 and NGV-2 (USA). Tanks tested at the Institute of High Pressure for Gas Applications (KHK, Japan) achieved 160 MPa before rupture (i.e., 2.35 times as high as the reference value, 70 MPa).

In order to implement the safety standards, any hydrogen facility must be evaluated by a systematic safety study using a well-established methodology (FM &EA, Hazop, AFO…). In the evaluation process, any  national safety rule must be taken into account, because they can require special permits for professionals, for instance. Nevertheless, specific standards on hydrogen safety are taken from the professional organisations cited above, plus some agencies (as OSHA) with a long tradition in physical protection and safety. All those standards have many common points, as stated in the following list:

- Hydrogen production and manipulation must be done in single-story buildings. Multiple-story buildings are not recommended at all. Hydrogen accumulation must be avoided. Venting is a fundamental point to be stimulated. Natural convection can help obtain a sufficient flow in order to prevent accumulation over the lean flammability limit.

- Storage of hydrogen must be done in certified tanks, either as a gas at high pressure or liquefied in cryogenic vessels, that must be properly cooled and thermally insulated.

- Storage can also be done by absorption or chemical reaction, for instance as a metallic hydride, or in coal/zelite beds.

- Metals for the storage walls must be properly chosen. Contents of some chemical  elements (S,P) in the steels are severely limited. Thermal treatment is typically required for steels to be used in hydrogen applications.

- Hydrogen detectors are mandatory in suitable places (f.i., ceilings above valves).

- Fire detectors are also mandatory. It must be noted that most of the hydrogen fires are jet-type, produced by a leakage in a pipe or a tank. Hydrogen flames are not visible, but they radiate heat and are very noisy because of the deflagration process is the flame front.

- Accessibility of fire brigades is a fundamental point in the lay-out of any hydrogen plant. Fire extinguishers must be placed in proper points. Cooling capability of nearby equipments is also mandatory. Water or cryogenic foam can be used for that purpose. Water (steam) explosions produced by contact with very hot surfaces must be taken into account in the cooling process, that must not produce electrical disruptions.

In order to implement the safety standards, any hydrogen facility must be evaluated by a systematic safety study using a well-established methodology

- Safety distances are a major point in the lay-out of hydrogen facilities. Particularly, hot points that can trigger an explosion must be away enough for hydrogen dilution by vertical diffusion. There are limits for many equipments (electrical sparks, for instance) storages (of any other flammable material) and activities (even the parking lot!). Compressors and high pressure components must be also at a safety distance (between 5 to 15 meters, typically).

- Safety distances are particularly important for pipes running in parallel. The one with hydrogen must be placed above any other. Colour code is mandatory for identification of the fluids.

- Electricity is a major concern in hydrogen production facilities. They must not be placed in close vicinity to high-voltage lines and grids. All electrical equipment used in the facility must have continuity (no sparks). All the system must be properly grounded (with a resistance to ground lower than 10 W).

- Explosion effects must be anticipated in the safety evaluation process. Walls and ceilings collapse has to be assessed for not to increase the damage by hurting sensitive components and structures.

- All valves must be regulated by teleorder. This is particularly strict for isolation valves, in order to safely cut any hydrogen flow moving into an area under accident or hazard.

The foregoing list is by no means complete. It embodies some safety guides of primary importance. The quoted standards from different organisations have gathered all the relevant information to deal with hydrogen safely. More than 4,000 institutions, universities, companies and professional associations have worked with hydrogen and have contributed to a substantial knowledge on hydrogen safety. This is a very positive experience. However, most of those institutions and companies have a very high professional level. Laymen have not participated so far in the hydrogen industrial sector. If the hydrogen economy initiative aims at developing hydrogen applications down to the laymen level, new safety standards will be required. Filling an alternator with hydrogen, for instance, is a routine task for specialized professionals, but it would be an impossible mission for untrained personnel. A key point in hydrogen safety for the future is how to cope with this problem of extending hydrogen application to everyone. Although it is a challenging task, technology development and mental ingenuity will do it.

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