> Grupo de apoyo: Laboratorio Energético del Hidrógeno

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

Tables 1 and 2 gather some relevant information about gaseous and liquid hydrogen. One of the most important data are the limits of flammability in air. In molar (volume) percentage, the lean limit is 4.1% H₂, and the rich limit 75%. This is the widest range of flammability for all combustible gases. (Cavendish was very accurate when he named it flammable air). It is also worth pointing out the very wide detonability range, between 18,3% and 59%, because it is related to the combustion speed and the overpressure in the wavefront, which can reach 2 MPa (~ 20 bar).

Liquid hydrogen requires either very low temperatures or very high pressures, and it is the second coldest cryogen after liquid helium. In this context it can be said that inert atmospheres must be made of helium when transferring liquid H₂, because any other inert fluid (as CO₂ or N₂) would already be condensated.

Liquid H₂ has a very small specific heat of vaporization (less than 0.5 MJ/kg) which means that evaporation will suddenly occur after any liquid hydrogen spill or leak. In a fire of liquid H₂, the heat from the flames will strongly boost the boiling process, and the main preventer against the acceleration of the fire would be the lack of oxygen, because it will take time for the oxygen to diffuse (enhanced by convection) and approach the fire front from the non-burning air.

Table 1. Gaseous H₂ properties

Table 2.  Liquid Hydrogen Properties

In general, three types of combustion processes can be distinguished: catalytic ones, direct diffusion flames and pre-mixture flames.

One of the means to characterize the potential consequences of a hydrogen explosion is by the Fireball model. Of course, those models can not predict the behaviour of hydrogen explosions in all geometries and ventilation/confinement conditions. It is obvious that the absence of an enough amount of oxygen, the hydrogen explosion will not occur. In the classical combustion triangle, three things are needed: a fuel (hydrogen), an oxydizer (oxygen) and an igniting point, or hot point. Flammable mixtures H₂/air needs very small amounts to heat to start ignition.

In order to have a high-speed deflagration (250-320 m/s) or even worse, a detonation (~ 2.000 m/s) an H₂/air mixture is needed. The worst case can happen in closed rooms capable to keep some overpressure as the mixture is formed. Then, a hot spot can start ignition, and a detonation wave can be launched, with overpressures that would destroy the surrounding walls, pipes and structures.

In general, three types of combustion processes can be distinguished: catalytic ones (that can happen at very low temperatures and are used, for instance, in hydrogen detectors even below the lean flammability level); direct diffusion flames (typical for solid and liquid fuels, as a coal furnace or a simple candle); and pre-mixture flames, where fuel and oxydizers become mixed-up before ignition starts.

The latter always happens in a explosive way, although the front propagation speed can be very small. This is the case, for instance, of gas burners in any standard boiler (or in any kitchen). A hypostoichiometric mixture of gas plus air is formed in the tube feeding the burner, and a slow deflagration takes place in the burner mouth, in such a way that the combustion front velocity equals the speed of mixture feeding from the tube. In that way, the combustion  front (the flame) is at steady state in the laboratory (or kitchen) system, but there is a true deflagration of the combustion process into the gas-air mixture. In those household examples, the deflagration speed is usually much lower than 1 m/s.

Hydrogen is one of the very few combustible gases that can detonate with air at atmospheric pressure, particularly in closed rooms.

Within the same combustion structure, detonation waves can appear, at propagation speeds over the speed of sound in the reacting media. This is the case, for instance, of internal combustion engines fed by gasoline. A mixture of gasoline/air is injected and compressed by the piston. At the right point, an electric spark ignites the compressed mixture, and a detonation wave moves across it in a very short time, so producing the effective power of the engine, that must be assembled to withstand the strong overpressure produced by the detonation wave, which is converted (part of its energy) into mechanical energy.

Hydrogen is one of the very few combustible gases that can detonate with air at atmospheric pressure, particularly in closed rooms. It is presumed that in most of the cases ignition would start as soon as achieving the lean detonation limit, and a fast and strong deflagration would develop, but at speed somewhat below 340 m/s. Overpressures are in this case lower than 1 bar, which is enough for severe destruction of the surrounding structure. (Of course, consequences are much more benign than in detonation explosions, where overpressure of 20 bars can be reached). Besides that, the thermal effect can also be very deleterious, not only inside the fireball but in places nearby, where the radiation flux can be harmful for human beings (and even for some non-organic materials, such as instrumentation detectors of several kinds).

The maximum (or adiabatic) temperature produced in hydrogen explosions is not very different from those of other combustible gases, for stoichiometric mixtures (and it is above 2000 ⁰C). Besides that, note that a deflagration can be triggered in a mixture with just 4.1% of hydrogen molar concentration, which means that is very hypostoichiometric, and there are large numbers of N₂ and O₂ molecules that must share the energy released in the combustion process. It means that in this case the adiabatic temperature will be much smaller than in the stoichiometric case. Nevertheless, consequences would also be deleterious, but at a smaller scale (in fireball size, overpressure, and radiation intensity).

There are several models to predict the macroscopic effects of deflagration and detonation waves in hydrogen/air mixtures, but a detailed study of this problem is very case-dependent. It is typically said that the size of the fireball (in diameter) scales at M^0.32, where M is the hydrogen mass (in SI units). In a first approach, the time duration of the fireball also scales at M^0.32, and the fireball power as M/t, i.e., M^0.68 (always in SI units). Nevertheless, more precise calculations are needed in the safety analysis of a given project, because actual hydrogen diffusivity will depend on the geometry, wind conditions and other factors. Another fundamental point in the calculation hypothesis is the moment in which ignition happens. For instance, if there is a source of heat near the leakage, it can be presumed that ignition will start as soon as a ~ 4% hydrogen molar concentration is achieved near the heat source. The rest of the process, including the deflagration-to-detonation transition, will depend on the H₂ concentration distribution in the environment of the accident.

There is a well-known set of safety principles and rules to minimize the hydrogen risk to the levels of any other industry. There are three main areas of work in this context: inherently safe design, personal training, and instrumentation and control.

There is a well-known set of safety principles and rules to minimize the hydrogen risk to the levels of any other industry. There are three main areas of work in this context: inherently safe design, personal training, and instrumentation and control. Hydrogen facilities safe design must include the fail-safe rule for any part of the equipment (valves, pumping system, vacuum devices…) and must also include other principles as limiting guaranties, adequate ventilation, elimination of potential ignition points and some other items that will specifically be addressed in the following sections of this article.

Some of the safety analysis techniques and principles are to some extent general (for instance, the use of Fault Tree Analysis and Consequences Analysis) but some issues are very proper of the hydrogen industry, as the embritlement produced by hydrogen absorption in metallic components. This is a well-known fact, and can be characterized by analysing samples of the same metal which undergo the same exposition to hydrogen. The metallic bond is damaged by the appearance of hydrogen atoms inside the solid lattice. To some extent, this is also known in some nuclear applications, mainly due to generation of protons inside a metal, by (n,p) reactions particularly.

Operators training is also a fundamental requisite in all activities with some level of risk, as is the case considered here. In the field of nuclear production of hydrogen, there is not any doubt about the capability to train selected personnel to do the job. Although human errors have been very important in nuclear accidents (Harrisburg TMI-2, Chernobyl-4…) as propagating events in the catastrophic route of those accidents, and human errors also are the primary cause of concern in the hydrogen industry, there is a general confidence in improving the safety performance of any industrial installation by improving operator training.

Last, but not least, monitoring and control of hydrogen facilities is very likely the main point in operational safety. As radioactivity, hydrogen can be detected easily. The response characteristic time of radioactivity detection is much shorter than the response time of hydrogen detection, but early warnings can be triggered by H₂ concentrations well below the lean limit of flammability, or by identifying that a small flow rate leak is taking place. In both cases, monitoring of the evolution of the accident is essential for a suitable emergency response. In many accidents, natural or enhanced ventilation is the preferred option. For closed domains, catalytic combustion can be a better option.

Hydrogen detection can be based in different mechanisms, as catalytic combustion in small detectors with temperature sensing; thermal conductivity variations; electrochemical reactions; semiconductor oxide sensors relying on surface effects; gas absorption in microelectronic devices...

Hydrogen detection can be based in different mechanisms, as catalytic combustion in small detectors with temperature sensing; thermal conductivity variations; electrochemical reactions; semiconductor oxide sensors relying on surface effects; gas absorption in microelectronic devices; gas chromatography and, probably the most sensitive one, up to 1 ppmv, mass spectrometers. New detectors are based on small changes in the electric/electronic properties of palladium as it becomes palladium hydride by sudden hydrogen absorption. The University of California at Irvine (USA) and the University of Montpellier (France) have developed palladium detectors that provide both reliability and robustness.

Many of the quoted mechanisms need sampling of the gas under surveillance (for instance, not all the gas in a large room can pass through a mass spectrometer). This is not a main drawback if the sampling plus detection times are not long as compared to the hydrogen flow rate through the leak. In fact, leakage detection can be a complement to H₂ detection. For instance a sudden pressure loss can help identify very rapidly that some part of the flow is appearing in a wrong place, and hydrogen production can be discontinued, or some valves can be closed, or the hydrogen flow can be sent to a by-pass with some storage capability. Ultrasonic leak detection and bubble testing with soapy films around pipes can also help to this purpose. The same can be said about loss of vacuum in double-shell pipes, with vacuum in between.

Early fire detection and quenching are other fundamental objectives. Glow plugs can burn hydrogen leakages at a concentration level well below 4%. Doing so, the thermal effect is very limited and a big deflagration (or even a detonation) can be prevented. Nevertheless, it is very difficult to burn all the hydrogen in an accidental leakage by glow plugs alone, because ignition does not propagate around them, unless the concentration value reaches the flammability limit. Another possibility to prevent big fires in closed room is by restricting the access of oxygen (by restricting the access of air). If the oxygen concentration is lower than 5%, there will be no deflagration. Hence, addition of N₂ or any other inert gas can make things safe after an accident.

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