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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

Abstract

Hydrogen is not an uncommon issue in Nuclear Safety analysis, particularly in relation to severe accidents. On the other hand, hydrogen is a household name in the chemical industry, particularly in oil refineries, and is also a well know chemical element currently produced by steam reforming of natural gas, and other methods (as coal gasification). In the non-distant future, hydrogen would have to be produced (by chemical reduction of water) by using renewable energies and nuclear energy. In particular, nuclear fission seems to offer the cheapest way to provide the required energy for that purpose.

Safety principles are fundamental guidelines in the design, construction and operation both of hydrogen facilities and nuclear power plants. When both of them are assembled to work in connection, a complete safety analysis must consider not only the safety practices of each industry, but any inter-relation that could be established between them. In particular, any accident involving a sudden energy release from one of the facilities can affect the other. Release of  dangerous (chemicals, radiotoxic effluents) can also pose safety problems. Although nuclear-produced hydrogen facilities will need specific approaches and detailed analysis on their safety features, a preliminary approach is presented in this paper, where no significant roadblocks are identified that could hamper the deployment of this new industry.

For many years and in many places, different types of manufactured gases containing H₂ where used for industrial, commercial and residential applications, but they where displaced by natural gas.

A typical way to produce gas for lightning and heating was based on the reaction between water vapour and very hot coal, with no oxygen (no air) nearby.

1. Introduction and background

In 1766, the very famous and reclusive scientist Henry Cavendish discovered a gas that he called "flammable air". In those early times of Chemistry as a Science, Cavendish was an outstanding researcher with a very limited capability to communicate his research results. In 1781 he was able to demonstrate that "flammable air", in combination with oxygen, produced water. Two years after that (1783) Lavoisier proposed the name "hydrogen" for Cavendish's "flammable air", in the way to organise Chemistry as a systematic knowledge. Some decades later, it was accepted that hydrogen was the lightest of all the chemical elements. Its constitution was definitely established early in the 20^th Century, as an electron orbiting a proton, but its simplicity was already identified in Mendeliev periodic table, a generation before. At that moment, it was evident that all the hydrogen in our planet is inside compound molecules, mainly water. It is necessary to chemically reduce it from those molecules in order to set it free. Once isolated, hydrogen clearly deserves its Cavendish name, "flammable air", because its combustion is extremely violent, releasing a huge amount of heat and creating very high temperatures and overpressures. Warnings about it started in the pioneering work by Cavendish (and by Lavoisier, soon afterwards) but hydrogen found its early way to industrial applications in the first decade of the 19^th Century, under the form of manufactured gas.

A typical way to produce gas for lightning and heating was based on the reaction between water vapour and very hot coal, with no oxygen (no air) nearby. After condensing the steam, the non-condensable gas was mainly made of H₂ and CO. Even today, this is the basis of coal gasification, a clean technology to burn coal, where a gas-steam combined cycle can be used to increase the overall efficiency. (The most powerful plant of this technology is ELCOGAS, in Puertollano, Spain, with 315 MWe (Treviño)).

For many years and in many places, different types of manufactured gases containing H₂ where used for industrial, commercial and residential applications, but they where displaced by natural gas. The safety record of the manufactured gas industry was really high, as it is in the natural gas industry. However, in some cases the substitution of the former by the latter was not so safe. In the seventies of the 20^th Century, for instance, the deployment of natural gas in Barcelone (Spain) was plagued with a short but intense series of accidents involving several casualities. Subsequent deployments of natural gas in other Spanish cities did not undergo such a bad experience. Since then, safety records of the natural gas industry and commercialization have been very high. The bad pioneering experience in Barcelone could be related to the lack of well-trained personnel. Safety standards were well developed at that time. However, many workers involved in changing piping, valves and, above all, gas burners, had no previous experience in this field. This example is of general interest because a futuristic deployment of the Hydrogen Economy will have to take very seriously into account that a highly flammable gas will be used by millions of people without previous experience in the field. Nowadays, hydrogen is used just by experts. Most of it is the so called captive hydrogen, in the sense that it is produced in the same facility where it is used, as is the case of petroleum refineries. In some specific areas (Ruhr Valley in Germany, La Porte industrial complex in Texas) hydrogen pipelines are used to connect production installations to consumers. However, the hydrogen pipelines total length (in all the world) does not reach 1.000 km yet, which is a mere nothing as compared to the total length of natural gas networks.

The use of hydrogen in rockets is explained in terms of weight and playload. 1 kg of H₂ has a heat contents equal to 2.8 kg of gasoline

About 400 billion m³ of hydrogen are produce (and consumed) every year in the world [Bose et al]. Petroleum refineries use the biggest share of it, for de-sulphuration and for hydrogenation and cracking of heavy molecules. It is also used in making fertilizers and in food industries (also to increase the contents of hydrogen in molecules that have much less commercial value without it). In all these applications, the safety record is high, and the existence of technical legislation (for instance, 29CFR 1910), standards and safety guides is a guarantee for future developments. However, it is worth noting that more than 40% of the accidents are caused by human errors [Bose]. From this viewpoint, the previous warning is particularly suited about a fostered deployment of the hydrogen economy without a strong effort in training and qualifying personnel.

There are two hydrogen applications were safety is a primary issue: cooling of large alternators and, even more, propulsion of bit rockets (including tripulated shuttles).

A 500 MVA alternator typically has a hydrogen gas load of 70 Nm³ (or even more) acting as main coolant for rotor and stator coils. Both to fill and remove it, an inert atmosphere of CO₂ must be used. Obviously, such a hydrogen contents is a clear hazard, and safety measures are taken both in the machine design and during operation to avoid fire and explosion risks.

Big rockets for space missions use very high amounts of liquid hydrogen (and liquid oxygen) for propulsion, and very secure systems must be designed, constructed and maintained to provide a reliable propulsion mechanism. Besides that, hydrogen is used inside space shuttles to produce electricity (and water) in fuel cells. Again safety is a major concern.

The use of hydrogen in rockets is explained in terms of weight and playload. 1 kg of H₂ has a heat contents equal to 2.8 kg of gasoline or 2.4 kg of methane. However, in other applications, room (volume) can be more important than weight, and H₂ is not so competing there. For instance, 1 liter of liquid H₂ is equivalent to 0,27 liters of gasoline, but liquid hydrogen has the additional drawback of needing ultra-low temperatures (see next point). Similarly, 1 liter of gaseous H₂ at 350 bar (and room temperature) is equivalent to 0,1 liters of gasoline or 0,3 liters of methane (at 350 bar, too).

Methane (natural gas) is becoming increasingly important in town public transportation. Here it must be pointed out again that the safety record is also high (in fact, much higher than butane engines, which have also been used in transportation, although at a much lower scale). A main difference between both gases is that methane is much lighter than air, and it diffuses (mainly vertically) at a very fast speed. On the contrary, butane is heavier than air, and it remains for longer around the leakage point, and can reach explosive concentrations in the vicinity of a hot zone, so producing a fireball, which seems to be much less likely to happen with methane.

Hydrogen is still much lighter than methane. In fact, is the lightest gas in nature, which is a positive feature for safety

Hydrogen is still much lighter than methane. In fact, is the lightest gas in nature, which is a positive feature for safety, because it can be removed easily by venting. However, this feature can also be a drawback, because it can fill a room immediately, with a very high hazard of igniting, launching a deflagration wave and, even in some cases, a detonation wave, at speeds much higher than the speed of sound in air. These features will be briefly reviewed in next point. Before that, it must be recognized that a hydrogen production facility near a nuclear power plant (NPP) presents a major concern from the point of view of safety. It is worth remembering that any type of chemical installation, let alone an explosive material facility, has to be carefully evaluated in the site selection process of the nuclear power plant, and in the Safety Analysis. It is well known that site features related to demography, geology, hydrology, meteorology and the like are fundamental items to be checked in NPP sitting. Additionally, any installation in the vicinity that could represent a potential hazard, must also be evaluated. This is the case for airports, industrial harbours and, particularly, facilities for the production, managing or storage of toxic substances, explosives and combustible fluids.

This is of paramount importance for the Safety Analysis of nuclear production of hydrogen. In order to carry out a systematic approach for this study, a main parameter is the distance between the NPP and the hydrogen facility, which in turn depends on the production method (analyzed in other chapters of this book). In the case of electrolysis, the distance can be as long as needed. In fact, the electric energy feeding the hydrogen facility could theoretically come from any electric plant. However, NPPs will produce the cheapest kWh and can keep the feeding continuously, without stochastic interruptions (as happens with wind and solar energy). This is an important requisite for electrolyzers, that do not work very properly if there are many fluctuations in intensity (power).

A different picture is seen in thermochemical methods to produce hydrogen. They require heat at a very high temperature. Therefore, a close connection must be established for heat transfer between the reactor and the hydrogen facility. This situation will  pose new and non-negligible safety issues. In order to understand them, some facts on hydrogen will be first reviewed, including its flammability and detonability. We will also review the problem of hydrogen in the context of NPP Safety. This is the chosen background to address nuclear-related safety matters. Thirdly, a review will be made on hydrogen safety in a chemical installation. By putting both backgrounds together and using some conceptual designs as references in this analysis, rationale views on this emerging field of nuclear production of hydrogen, as far as safety is concerned, could be presented.

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