- Email: [email protected]
Safety issues of nuclear production of hydrogen
Energy Conversion and Management 47 (2006) 2732–2739 www.elsevier.com/locate/enconman
Safety issues of nuclear production of hydrogen Mireia Piera a, Jose´ M. Martı´nez-Val
b,*
, Ma Jose´ Montes
b
a
b
E.T.S.I. Industriales, UNED, Ciudad Universitaria, s/n-28040 Madrid, Spain E.T.S.I. Industriales, Madrid Polytechnical University UPM, J.Gutie´rrez Abascal, 2, 28006 Madrid, Spain Available online 23 March 2006
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 known chemical element currently produced by steam reforming of natural gas, and other methods (such as coal gasification). In the not-too-distant future, hydrogen will have to be produced (by chemical reduction of water) using renewable and nuclear energy sources. In particular, nuclear fission seems to offer the cheapest way to provide the primary energy in the mediumterm. Safety principles are fundamental guidelines in the design, construction and operation both of hydrogen facilities and nuclear power plants. When these two technologies are integrated, a complete safety analysis must consider not only the safety practices of each industry, but any interaction 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 substances (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. No significant roadblocks are identified that could hamper the deployment of this new industry, but some of the hydrogen production methods will involve very demanding safety standards. 2006 Elsevier Ltd. All rights reserved. Keywords: Nuclear reactions; Hydrogen production; Safety
1. Introduction and background In the very long term, nuclear energy will come from fusion reactors, but the dominating nuclear energy source for many years (maybe the whole 21st century) will be Fission. Therefore, this paper mainly deals with Nuclear Fission Reactors, which produce heat, that can be converted into electricity. In the following, it will be seen that both heat and electricity will be useful for H2 production. Although some of the performance analysis will be based on Fission reactors, the general considerations could also be applied to Fusion reactors in the long term future. However, safety concerns are very different in the
*
Corresponding author. Tel.: +34 91 336 3078; fax: +34 91 336 3079. E-mail address: [email protected] (J.M. Martı´nez-Val).
0196-8904/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.02.002
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
2733
Fission and Fusion case, and the safety assessments presented in this paper will specifically refer to Fission reactors, which are already an industrial reality supported by a sound theory and practise of Nuclear Safety. In 1766, the very famous and reclusive scientist Henry Cavendish discovered a gas that he called ‘‘flammable air’’. Five years later, he was able to demonstrate that ‘‘flammable air’’, in combination with oxygen, produced water, and Lavoisier proposed the name ‘‘hydrogen’’ for Cavendish’s ‘‘flammable air’’, placing it in the emerging systematic context of chemistry. Some decades later, hydrogen became accepted as the lightest of all the chemical elements, and it was evident that all the hydrogen in our planet is within compound molecules, mainly water. To set it free, it must be chemically reduced from those molecules. Once isolated, hydrogen clearly deserves Cavendish’s name, ‘‘flammable air’’, because its combustion is easily initiated, is extremely energetic, and releases a huge amount of heat. This creates very high temperatures and can produce explosive overpressures. Warnings about it started in the pioneering work by Cavendish and Lavoisier but hydrogen found its early way to industrial applications in the first decade of the 19th Century, in the form of manufactured gas. A typical way to produce gas for lighting and heating was based on the anaerobic reaction between water vapour and very hot coal. After condensing excess 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 overall efficiency. (So far, the most powerful plant of this technology is ELCOGAS, in Puertollano, Spain, with 315 MWe [1]). For many years and in many places, different types of manufactured gases containing H2 where widely distributed and used for industrial, commercial and residential applications, but most of these have now been displaced by natural gas. Nevertheless, more than 400 billion m3 of hydrogen is produced (and consumed) every year in the world [2]. Petroleum refineries use the biggest share of it, for de-sulphurization and for hydrogenation and cracking of heavy molecules. It is also used in making fertilizers and in food industries. In all these applications, the safety record is good, and the existence of technical legislation (for instance, 29CFR 1910) [3], standards and safety guides is a guarantee for future developments. Nowadays, hydrogen is used just by specialists. Most of it is the so-called captive hydrogen, ‘‘captive’’ in the sense that it is produced in the same facility where it is used, as in 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 industrial consumers. However, the hydrogen pipelines total length (in all the world) is still below 1000 km, which is trivial compared to the total length of natural gas networks. There are two hydrogen applications where safety is a primary issue: cooling of large alternators and, even more, propulsion of large rockets (including NASA shuttles). A 500 MVA alternator typically has a hydrogen gas charge of 70 Nm3 (6.25 kg) 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 large hydrogen contents is a hazard, and safety measures are taken both in design of the machine and during operation to avoid risks of fire or explosion. Big rockets for space missions use very large 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 in space missions [4]. The use of hydrogen in rockets is explained in terms of weight and payload. One kilogram of H2 has a heat content equal to 2.8 kg of gasoline or 2.4 kg of methane. However, in other applications, volume can be more important than weight, and H2 is less attractive. For instance, in energy content, 1 l of liquid H2 is equivalent to 0.27 l of gasoline, but liquid hydrogen has the additional drawback of needing ultra-low temperatures (see Table 1). Similarly, 1 l of gaseous H2 at 350 bar (and room temperature) is equivalent to 0.1 l of gasoline or 0.3 l of methane (also at 350 bar). In the very long future, hydrogen can play a substantial role in the energy sector, mainly within the context of Sustainable Development. Of course, the most important challenge to play such a role is hydrogen production from water. At present, the main source of H2 is methane, and the production method is the so called Steam Reforming. This method competes with the rest of applications of natural gas, and cannot be accepted in a Sustainable Development scenario because it relies on fossil hydrocarbons.
2734
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
Table 1 Gaseous H2 properties Property
Value
Molecular weight Boiling point (K) Critical temperature (K) Density of gas at NTPa (kg/m3) Viscosity of gas at NTP (g/cm s) Minimum energy for ignition (mJ) Limits of flammability in air (vol.%) Limits of detonability in air (vol.%) Flame temperature (K) Detonation velocity (km/s) Detonation overpressure (kPa) Lower heating value (kJ/g) Higher heating value (kJ/g) Burning velocity at NTP (cm/s) Percent thermal energy radiated (%) Heat release rate (kJ/cm2 s) Energy of explosion (kgTNT/m3) Buoyant velocity at NTP (cm/s) Diffusion coefficient at NTP (cm2/s)
2.02 20.3 33 0.0838 8.9 · 10 5 0.02 4.1–75 18.3–59 2318 1.48–2.15 1470 120 142 265–325 21 1.53 · 10 2 2 1.2–9 0.61
a
NTP: 1 atmosphere and 298 K.
In the long run, Nuclear Fusion and Renewables are seen as the main sources of energy to satisfy the demand from mankind. In the quest for full sustainability, coal and Nuclear Fission are regarded as fundamental tools, although CO2 sequestration, in the former, and nuclear safety, in the latter, would have to be managed in a acceptable level from all points of view (including waste toxicity reduction and non-proliferation issues). 2. Hydrogen properties related to safety Tables 1 and 2 gather some relevant information about gaseous and liquid hydrogen. Among 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 very low temperatures and it is the second coldest cryogen after liquid helium. Helium must be used as an inert atmosphere over liquid H2, because any other inert fluid (as CO2 or N2) would condense. Besides that, it 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 Table 2 Liquid hydrogen properties Property
Value
Temperature of liquid at NBP (K) Heat of vaporization (MJ/kg) Density of liquid at NBP (kg/m3) Density of vapour at NBP (kg/m3) Viscosity of NBP liquid (g/cm s) Vaporization rate of liquid pools without burning (cm/min) Energy of explosion (g TNT/cm3 NBP liquid fuel)
20.3 0.46 71 1.34 13.56 2.5–5.0 1.71
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
2735
flames will strongly boost the boiling process, and the main inhibitor against the acceleration of the fire will 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. In order to have a high-speed deflagration (250–320 m/s) or a detonation (2000 m/s), an H2/air mixture is needed. The worst case can happen in closed volumes capable of confining an overpressure as the mixture is formed. Then a hot spot can start ignition and a detonation wave can be launched, with overpressures that can 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 in a coal furnace or a simple candle); and pre-mixture flames, where fuel and oxidizers become mixed before ignition starts. The latter always happens explosively, although the front propagation speed can be very small in some cases, as in gas burners in any standard boiler (or in any kitchen). Hydrogen has a large potential to 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 still enough for severe destruction of surrounding structures. Of course, the consequences are much more benign with deflagrations than in detonation explosions, where overpressure of 20 bar 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 nonorganic materials, such as instrumentation detectors of several kinds). 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, personnel training, and instrumentation and control. Some of the safety analysis techniques and principles are general (for instance, the use of Fault Tree Analysis and Consequences Analysis) but some issues are specific to the hydrogen industry, such as the embrittlement produced by hydrogen absorption in metallic components. 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. 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. 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. Dilution is another way to cope with an accident. H2 has a very large diffusion coefficient, and can be ventilated very rapidly so that the lean flammability limit is not reached. 3. H2 production methods Hydrogen is not a newcomer in the energy scientific community. In the seventies of the previous century, a huge effort was done in several laboratories for more than one decade in order to make a first evaluation of hydrogen as an energy carrier in a context dominated by the oil crisis of 73–74 and 79–80. Between 1975 and 1985 more than 400 international papers were published about H2. Most of them were about production
2736
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
Table 3 Producing hydrogen methods from nuclear power Hydrogen production process
Heat conversion factor (%)
Energy source Raw material Production process
Nuclear electricity (Elect./Heat. 32–50%) Water Electrolysis
Nuclear electricity and heat Water Hot electrolysis
Nuclear heat (high temperature) Water Thermo-chemical
Methane combustion Methane and steam Steam reforming
H2 heat/nuclear heat H2 heat/methane heat
20–40% –
50% –
55% –
– 85%
methods, and more than 200 alternatives were considered at that time, most of them within the thermo-chemical branch. After that effort, interest on H2 declined a lot, and most of the relevant laboratories (notably the Ispra JRC centre of the European Union) discontinued the work on this subject, which was revisited after the USA administration started its initiative on the Hydrogen Economy in 2001. It must be noted, however, that the OECD/NEA agency had convened a first Information Exchange Meeting on Nuclear Production of Hydrogen in 2000, and an important scientific review was already published by one of the pioneers on the subject (Funk) in 2001[5]. Seventeen years earlier, in 1984, Weirich et al. [6] had already presented their review on thermo-chemical processes driven by heat from nuclear reactors. Last but not least, in 2003, the American Nuclear Society presented its position on Nuclear Energy for Hydrogen Generation [7]. These are a few indications of the importance of the subject, re-visited nowadays from the viewpoint of Sustainable Development, according to the principles established about this concept, including concerns about the increase of the atmospheric greenhouse effect and its potential impact on the climate change [8]. It is worth pointing out that hydrogen can be considered as the essential chemical fuel for combustion applications in any scenario of sustainability, and the main challenge is to produce it by clean and cheap methods consuming clean, cheap and long-lasting sources of energy. Two main branches have identified for hydrogen production, plus some hybrids alternatives. One branch is based on electrolysis, the another in high temperature (thermo-chemical) processes. Hybrid methods are mainly thermo-chemical ones, but include a step carried out by electrolysis. Table 3 presents a summary of the methods, including a reference to the most frequent one at present, which is Steam Methane Reforming (SMR). The latter is the cheapest for oil refineries and other industries, and it is mature at commercial level, although additional research efforts are producing very interesting results to make it cheaper and more effective, including H2 separation at medium temperatures (600 K) by suitable membranes. From the previous viewpoint, and taking into account the need to pave the road to the future in an acceptable way by using the available technologies and resources in the short and medium term, Steam Reforming of fossil fuels should not be discarded from the methods to be scaled-up for the deployment of the Hydrogen Economy. Nevertheless, the really interesting H2 producing methods must use only water as raw material, and this fact will guide the rest of this article. This means that our analysis will be focused on the capability of developing nuclear reactors for meeting the energy demands of those methods, namely electro-chemical and thermo-chemical. 3.1. Nuclear electrolysis Electricity for electrolysis can be produced in any type of nuclear reactor. From this viewpoint, the main requirement for the power plant would be to reach a very high efficiency [9] in order to have a low production cost. This is connected to a main advantage of this method, namely the possibility to place the electrolytic factories close to the high consumption zones, while the NPP could be very far from the high populated areas, with an electric grid connection between both facilities. However, standard electrolysis at room temperature will not have the highest efficiency within this branch. Very high temperature steam electrolysis can produce better results in the overall energy balance, because the electricity toll decreases as temperature increases. This fact points out the suitability of dedicated reactors, in order to optimise the fraction of primary energy finally contained as H2 molecules.
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
2737
As in many futuristic applications, HTR (high temperature reactors) are very appealing facilities because of the high temperature they can reach, particularly in the coolant. Research on this reactor family was halted some 20 years ago (in the wake of the INFCE initiative, see Ref. [10]) and a lot of new effort will be needed to reach the technology levels that were already at hand in those days [11]. 3.2. Nuclear heat for thermo-chemical processes A very large number of chemical methods has been proposed so far to produce hydrogen from water, but theoretical analysis and technological considerations have reduced that number to a few. In fact, current developments are mainly based on five systems [5,6]. The maximum temperature required in each case is also given. • • • • •
The The The The The
SI method, 870 C. UT3 process, 780 C. hybrid W scheme, 900 C. Ispra hybrid scheme, 900 C. ZnO/Zn cycle, 1500 C.
In all cases, water is split indirectly. Direct hydrolysis requires extremely high temperatures (of the order of 3000 K) and it is therefore beyond any conceivable reactor concept. Even the ZnO/Zn cycle needs ultra high temperature, over 1500 C, and is not usually considered in the Nuclear realm, although it conveys the highest efficiency for H2 production. In general, the smaller the number of steps in the cycle, the higher the efficiency. A general feature of all the methods is that they require several chemical reactors at very high temperature (and pressure, in some cases) with large amounts of potentially harmful chemicals, as HI or H2SO4.
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. 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 also available [2,3]. All those standards have many common points, as stated in the following list: 1. 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. 2. 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. 3. Electricity is a major concern in hydrogen production facilities. 4. Safety distances are a major point in the lay-out of hydrogen facilities. 5. 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. 6. Hydrogen production and manipulation must be done in single-story buildings. 7. Storage of hydrogen must be done in certified tanks. 8. Metals for the storage walls must be properly chosen.
2738
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
9. Hydrogen detectors are mandatory in suitable places (f.i., ceilings above valves). 10. Fire detectors are also mandatory. 5. Integrated safety in nuclear production of hydrogen If the world has to go towards a Hydrogen Economy, the primary question is where will the energy to produce hydrogen come from? One of the most suitable ways to actually pave the road to the H2 Economy is Nuclear Fission. Of course, if NPPs had to participate in a prominent way in hydrogen production, the current installed capacity would have to be multiplied by a large factor. In such a huge deployment of Nuclear Fission, safety would still be more important (if possible) than today, and inherently safe reactors would be needed to comply with the anticipated standards of the Hydrogen Economy. Let us presume that Nuclear Fission is finally, and logically, invited to the hydrogen production panel. Its present power level is about 0.7 Gtoe/year (equivalent, in heat gross production; 1 toe = 11.63 MW h thermal). In order to produce 2 Gtoe of H2 per year, the thermal gross power would have to be about 10 Gtoe/year, or even more. That means to multiply the current power level by 14. That would be a challenge which needs a very good answer in terms of safety [12]. Further development for Nuclear Fission to become relevant in the Hydrogen Economy does require: • • • •
proliferation resistance of the whole fuel cycle; high efficiency in exploiting nuclear ores (natural uranium, thorium); minimization of radiotoxicity contained in the long-term nuclear wastes; very high safety standards, ruling out prompt-criticality states (the Chernobyl syndrome) and sizeable radioactive product releases.
Some specific safety issues will appear in relation to the reactor coolant needed to reach temperatures as high as required for hydrogen production. Candidate coolants are gases (particularly inert gases) molten salts and molten metals (particularly lead). In the case of gases, very high pressures needed for an effective cooling can present a threat to mechanical integrity of some containment barriers. Moreover, afterheat cooling is difficult to achieve in some types of accidents with de-pressurization. Molten metals and salts can work at atmospheric pressure, but they can present severe problems of corrosion [13]. Neutronic and thermalhydraulic advantages of molten lead, for instance, are well known, and this is why it has been proposed for some futuristic reactors. However, corrosion rates increase enormously with temperature, and cladding and structural materials (including IHX) can suffer from that. Another advantage of molten lead (and similar coolants with a very low Prandtl number) is the possibility of using natural convection for cooling, at least to remove residual heat. Passive safety features will play a very important role in the development of new reactors for futuristic needs, as those of the Hydrogen Economy. 6. Tentative conclusions Hydrogen can become an important energy vector in the economy of the future, but primary energy will be needed for that. This is a job that Nuclear Fission Energy can do in the first stages of this development. There are several types of reactors to be studied and computer simulation codes to carry out those studies. Some of those codes will specifically be used to analyze the safety levels required in the whole cycle of nuclear production of hydrogen. It is impossible to declare that all possible safety issues have been anticipated in the analysis of nuclear production of hydrogen, for all possible methods. It can be stated, however, that there are principles, criteria, rules and methodology to make the safety assessment of any nuclear installation devised to produce hydrogen, either by electrolysis or by thermo-chemical reactions. Nuclear safety methodologies and practices can be extended to hydrogen operations in order to reach safety levels as high as reasonably achievable.
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
2739
Two different cases can be considered in this context, depending on the hydrogen production method: • pure electrolysis, in which NPP and hydrogen plants can be placed in different sites, so that an accident can not propagate from one installation to the other; • methods requiring high T fluids, which needed a short distance between the reactor and the chemical complex (where the H2 producing reactors are placed). The later presents very much challenging safety issues than the former. In fact, Seveso Directives I and II (Ref. [14]) (applicable in the European Union to any chemical facility), would have to be reviewed in order to obtain permission for an integrated construction of a nuclear reactor within a chemical industrial complex. By all accounts, safety will be the first priority in this evolutionary revolution. In spite of TMI-2 and Chernobyl-4 accidents [15], safety records in the nuclear industry have been very good in the Western World [16]. Even so, there is a lot of room for improvements by using new materials (new fuels, new coolants) and new designs (very stable reactors with strongly negative reactivity feedback; emergency passive cooling; unconditional subcriticality in the case of accidents and so on). All these improvements have to be embodied in the designs of the new reactors, which must also meet the requirements for H2 production, particularly the production of high temperature fluids. New coolants and new structural materials will be needed for that quest, that could be materialize if, and only if, a commensurate R&D effort is carried out in that direction. Acknowledgements This paper was largely inspired by the document ‘‘Safety issues on Nuclear Production of Hydrogen’’ elaborated by J.M. Martı´nez-Val, J. Talavera and A. Alonso (www.energiasostenible.net). References [1] Trevin˜o M. Tecnologı´a de gasificacio´n integrada en ciclo combinado. Madrid: ELCOGAS, Club Espan˜ol de la Energı´a; 2003. [2] Bose TK, Ohi J, Hay R. Sourcebook for hydrogen applications. Canada: TISEC Inc.; 1998. [3] CFR Title 29. Occupational health and safety standards. Code of Federal Regulations. Parts 1900 to 1910, US Government Printing Office, Washington; 1993. [4] NHB 7320.1. NASA facilities engineering handbook, revision B. Washington, DC: National Aeronautics and Space Administration; 1982. [5] Funk JE. Thermochemical hydrogen production: past and present. Int J Hydrogen Energy 2001;26:185. [6] Weirich W et al. Thermochemical process for water-splitting-status and outlook. Nucl Eng Des 1984;78:285. [7] ANS Position Statement 60: Nuclear energy for hydrogen generation; June 2003. [8] Brundtland G, Chairman. World Commission on Environment and Development, (1987), ‘‘Our common future’’ (The Brundtland Report). Oxford: Oxford University Press (Reino Unido, 1987). [9] Miller AI. Electrochemical production of hydrogen by nuclear energy, Nuclear productino of hydrogen-technologies and perspectives for global deployment, American Nuclear Society; 2004 [chapter 4, Table 6]. [10] INFCE. International nuclear fuel cycle evaluation, International Atomic Energy Agency. Agency Report, INFCE/PC/2/9; 1980. [11] Inagaki Y et al. Present status and future plan of HTTR Project, presented at OECD/NEA 2nd information exchange meeting of nuclear production of hydrogen, Argonne, IL; 2–3 October 2003. [12] Hori M. Role of nuclear energy in the long-term global energy perspective. In: Proc OECD/NEA 1st information exchange meeting on nuclear production of hydrogen, Paris, France; 2–3 October 2000. [13] Shreier LL, editor. Corrosion. 3rd ed. Knovel Library, Elsevier; 1994. [14] Seveso Directives: Directive 82/501/EEC, Directive 87/216/EEC and Directive 88/610/EEC (European Union, Brussels). 1988. [15] Martı´nez-Val JM et al. An analysis of the physical causes of the Chernobyl accident. Nucl Technol 1990;90:371. [16] NEA annual report on occupational exposures at nuclear power plants, 2004 (NEA-OCDE, Parı´s, 2005).
Safety issues of nuclear production of hydrogen Mireia Piera a, Jose´ M. Martı´nez-Val
b,*
, Ma Jose´ Montes
b
a
b
E.T.S.I. Industriales, UNED, Ciudad Universitaria, s/n-28040 Madrid, Spain E.T.S.I. Industriales, Madrid Polytechnical University UPM, J.Gutie´rrez Abascal, 2, 28006 Madrid, Spain Available online 23 March 2006
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 known chemical element currently produced by steam reforming of natural gas, and other methods (such as coal gasification). In the not-too-distant future, hydrogen will have to be produced (by chemical reduction of water) using renewable and nuclear energy sources. In particular, nuclear fission seems to offer the cheapest way to provide the primary energy in the mediumterm. Safety principles are fundamental guidelines in the design, construction and operation both of hydrogen facilities and nuclear power plants. When these two technologies are integrated, a complete safety analysis must consider not only the safety practices of each industry, but any interaction 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 substances (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. No significant roadblocks are identified that could hamper the deployment of this new industry, but some of the hydrogen production methods will involve very demanding safety standards. 2006 Elsevier Ltd. All rights reserved. Keywords: Nuclear reactions; Hydrogen production; Safety
1. Introduction and background In the very long term, nuclear energy will come from fusion reactors, but the dominating nuclear energy source for many years (maybe the whole 21st century) will be Fission. Therefore, this paper mainly deals with Nuclear Fission Reactors, which produce heat, that can be converted into electricity. In the following, it will be seen that both heat and electricity will be useful for H2 production. Although some of the performance analysis will be based on Fission reactors, the general considerations could also be applied to Fusion reactors in the long term future. However, safety concerns are very different in the
*
Corresponding author. Tel.: +34 91 336 3078; fax: +34 91 336 3079. E-mail address: [email protected] (J.M. Martı´nez-Val).
0196-8904/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.02.002
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
2733
Fission and Fusion case, and the safety assessments presented in this paper will specifically refer to Fission reactors, which are already an industrial reality supported by a sound theory and practise of Nuclear Safety. In 1766, the very famous and reclusive scientist Henry Cavendish discovered a gas that he called ‘‘flammable air’’. Five years later, he was able to demonstrate that ‘‘flammable air’’, in combination with oxygen, produced water, and Lavoisier proposed the name ‘‘hydrogen’’ for Cavendish’s ‘‘flammable air’’, placing it in the emerging systematic context of chemistry. Some decades later, hydrogen became accepted as the lightest of all the chemical elements, and it was evident that all the hydrogen in our planet is within compound molecules, mainly water. To set it free, it must be chemically reduced from those molecules. Once isolated, hydrogen clearly deserves Cavendish’s name, ‘‘flammable air’’, because its combustion is easily initiated, is extremely energetic, and releases a huge amount of heat. This creates very high temperatures and can produce explosive overpressures. Warnings about it started in the pioneering work by Cavendish and Lavoisier but hydrogen found its early way to industrial applications in the first decade of the 19th Century, in the form of manufactured gas. A typical way to produce gas for lighting and heating was based on the anaerobic reaction between water vapour and very hot coal. After condensing excess 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 overall efficiency. (So far, the most powerful plant of this technology is ELCOGAS, in Puertollano, Spain, with 315 MWe [1]). For many years and in many places, different types of manufactured gases containing H2 where widely distributed and used for industrial, commercial and residential applications, but most of these have now been displaced by natural gas. Nevertheless, more than 400 billion m3 of hydrogen is produced (and consumed) every year in the world [2]. Petroleum refineries use the biggest share of it, for de-sulphurization and for hydrogenation and cracking of heavy molecules. It is also used in making fertilizers and in food industries. In all these applications, the safety record is good, and the existence of technical legislation (for instance, 29CFR 1910) [3], standards and safety guides is a guarantee for future developments. Nowadays, hydrogen is used just by specialists. Most of it is the so-called captive hydrogen, ‘‘captive’’ in the sense that it is produced in the same facility where it is used, as in 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 industrial consumers. However, the hydrogen pipelines total length (in all the world) is still below 1000 km, which is trivial compared to the total length of natural gas networks. There are two hydrogen applications where safety is a primary issue: cooling of large alternators and, even more, propulsion of large rockets (including NASA shuttles). A 500 MVA alternator typically has a hydrogen gas charge of 70 Nm3 (6.25 kg) 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 large hydrogen contents is a hazard, and safety measures are taken both in design of the machine and during operation to avoid risks of fire or explosion. Big rockets for space missions use very large 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 in space missions [4]. The use of hydrogen in rockets is explained in terms of weight and payload. One kilogram of H2 has a heat content equal to 2.8 kg of gasoline or 2.4 kg of methane. However, in other applications, volume can be more important than weight, and H2 is less attractive. For instance, in energy content, 1 l of liquid H2 is equivalent to 0.27 l of gasoline, but liquid hydrogen has the additional drawback of needing ultra-low temperatures (see Table 1). Similarly, 1 l of gaseous H2 at 350 bar (and room temperature) is equivalent to 0.1 l of gasoline or 0.3 l of methane (also at 350 bar). In the very long future, hydrogen can play a substantial role in the energy sector, mainly within the context of Sustainable Development. Of course, the most important challenge to play such a role is hydrogen production from water. At present, the main source of H2 is methane, and the production method is the so called Steam Reforming. This method competes with the rest of applications of natural gas, and cannot be accepted in a Sustainable Development scenario because it relies on fossil hydrocarbons.
2734
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
Table 1 Gaseous H2 properties Property
Value
Molecular weight Boiling point (K) Critical temperature (K) Density of gas at NTPa (kg/m3) Viscosity of gas at NTP (g/cm s) Minimum energy for ignition (mJ) Limits of flammability in air (vol.%) Limits of detonability in air (vol.%) Flame temperature (K) Detonation velocity (km/s) Detonation overpressure (kPa) Lower heating value (kJ/g) Higher heating value (kJ/g) Burning velocity at NTP (cm/s) Percent thermal energy radiated (%) Heat release rate (kJ/cm2 s) Energy of explosion (kgTNT/m3) Buoyant velocity at NTP (cm/s) Diffusion coefficient at NTP (cm2/s)
2.02 20.3 33 0.0838 8.9 · 10 5 0.02 4.1–75 18.3–59 2318 1.48–2.15 1470 120 142 265–325 21 1.53 · 10 2 2 1.2–9 0.61
a
NTP: 1 atmosphere and 298 K.
In the long run, Nuclear Fusion and Renewables are seen as the main sources of energy to satisfy the demand from mankind. In the quest for full sustainability, coal and Nuclear Fission are regarded as fundamental tools, although CO2 sequestration, in the former, and nuclear safety, in the latter, would have to be managed in a acceptable level from all points of view (including waste toxicity reduction and non-proliferation issues). 2. Hydrogen properties related to safety Tables 1 and 2 gather some relevant information about gaseous and liquid hydrogen. Among 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 very low temperatures and it is the second coldest cryogen after liquid helium. Helium must be used as an inert atmosphere over liquid H2, because any other inert fluid (as CO2 or N2) would condense. Besides that, it 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 Table 2 Liquid hydrogen properties Property
Value
Temperature of liquid at NBP (K) Heat of vaporization (MJ/kg) Density of liquid at NBP (kg/m3) Density of vapour at NBP (kg/m3) Viscosity of NBP liquid (g/cm s) Vaporization rate of liquid pools without burning (cm/min) Energy of explosion (g TNT/cm3 NBP liquid fuel)
20.3 0.46 71 1.34 13.56 2.5–5.0 1.71
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
2735
flames will strongly boost the boiling process, and the main inhibitor against the acceleration of the fire will 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. In order to have a high-speed deflagration (250–320 m/s) or a detonation (2000 m/s), an H2/air mixture is needed. The worst case can happen in closed volumes capable of confining an overpressure as the mixture is formed. Then a hot spot can start ignition and a detonation wave can be launched, with overpressures that can 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 in a coal furnace or a simple candle); and pre-mixture flames, where fuel and oxidizers become mixed before ignition starts. The latter always happens explosively, although the front propagation speed can be very small in some cases, as in gas burners in any standard boiler (or in any kitchen). Hydrogen has a large potential to 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 still enough for severe destruction of surrounding structures. Of course, the consequences are much more benign with deflagrations than in detonation explosions, where overpressure of 20 bar 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 nonorganic materials, such as instrumentation detectors of several kinds). 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, personnel training, and instrumentation and control. Some of the safety analysis techniques and principles are general (for instance, the use of Fault Tree Analysis and Consequences Analysis) but some issues are specific to the hydrogen industry, such as the embrittlement produced by hydrogen absorption in metallic components. 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. 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. 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. Dilution is another way to cope with an accident. H2 has a very large diffusion coefficient, and can be ventilated very rapidly so that the lean flammability limit is not reached. 3. H2 production methods Hydrogen is not a newcomer in the energy scientific community. In the seventies of the previous century, a huge effort was done in several laboratories for more than one decade in order to make a first evaluation of hydrogen as an energy carrier in a context dominated by the oil crisis of 73–74 and 79–80. Between 1975 and 1985 more than 400 international papers were published about H2. Most of them were about production
2736
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
Table 3 Producing hydrogen methods from nuclear power Hydrogen production process
Heat conversion factor (%)
Energy source Raw material Production process
Nuclear electricity (Elect./Heat. 32–50%) Water Electrolysis
Nuclear electricity and heat Water Hot electrolysis
Nuclear heat (high temperature) Water Thermo-chemical
Methane combustion Methane and steam Steam reforming
H2 heat/nuclear heat H2 heat/methane heat
20–40% –
50% –
55% –
– 85%
methods, and more than 200 alternatives were considered at that time, most of them within the thermo-chemical branch. After that effort, interest on H2 declined a lot, and most of the relevant laboratories (notably the Ispra JRC centre of the European Union) discontinued the work on this subject, which was revisited after the USA administration started its initiative on the Hydrogen Economy in 2001. It must be noted, however, that the OECD/NEA agency had convened a first Information Exchange Meeting on Nuclear Production of Hydrogen in 2000, and an important scientific review was already published by one of the pioneers on the subject (Funk) in 2001[5]. Seventeen years earlier, in 1984, Weirich et al. [6] had already presented their review on thermo-chemical processes driven by heat from nuclear reactors. Last but not least, in 2003, the American Nuclear Society presented its position on Nuclear Energy for Hydrogen Generation [7]. These are a few indications of the importance of the subject, re-visited nowadays from the viewpoint of Sustainable Development, according to the principles established about this concept, including concerns about the increase of the atmospheric greenhouse effect and its potential impact on the climate change [8]. It is worth pointing out that hydrogen can be considered as the essential chemical fuel for combustion applications in any scenario of sustainability, and the main challenge is to produce it by clean and cheap methods consuming clean, cheap and long-lasting sources of energy. Two main branches have identified for hydrogen production, plus some hybrids alternatives. One branch is based on electrolysis, the another in high temperature (thermo-chemical) processes. Hybrid methods are mainly thermo-chemical ones, but include a step carried out by electrolysis. Table 3 presents a summary of the methods, including a reference to the most frequent one at present, which is Steam Methane Reforming (SMR). The latter is the cheapest for oil refineries and other industries, and it is mature at commercial level, although additional research efforts are producing very interesting results to make it cheaper and more effective, including H2 separation at medium temperatures (600 K) by suitable membranes. From the previous viewpoint, and taking into account the need to pave the road to the future in an acceptable way by using the available technologies and resources in the short and medium term, Steam Reforming of fossil fuels should not be discarded from the methods to be scaled-up for the deployment of the Hydrogen Economy. Nevertheless, the really interesting H2 producing methods must use only water as raw material, and this fact will guide the rest of this article. This means that our analysis will be focused on the capability of developing nuclear reactors for meeting the energy demands of those methods, namely electro-chemical and thermo-chemical. 3.1. Nuclear electrolysis Electricity for electrolysis can be produced in any type of nuclear reactor. From this viewpoint, the main requirement for the power plant would be to reach a very high efficiency [9] in order to have a low production cost. This is connected to a main advantage of this method, namely the possibility to place the electrolytic factories close to the high consumption zones, while the NPP could be very far from the high populated areas, with an electric grid connection between both facilities. However, standard electrolysis at room temperature will not have the highest efficiency within this branch. Very high temperature steam electrolysis can produce better results in the overall energy balance, because the electricity toll decreases as temperature increases. This fact points out the suitability of dedicated reactors, in order to optimise the fraction of primary energy finally contained as H2 molecules.
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
2737
As in many futuristic applications, HTR (high temperature reactors) are very appealing facilities because of the high temperature they can reach, particularly in the coolant. Research on this reactor family was halted some 20 years ago (in the wake of the INFCE initiative, see Ref. [10]) and a lot of new effort will be needed to reach the technology levels that were already at hand in those days [11]. 3.2. Nuclear heat for thermo-chemical processes A very large number of chemical methods has been proposed so far to produce hydrogen from water, but theoretical analysis and technological considerations have reduced that number to a few. In fact, current developments are mainly based on five systems [5,6]. The maximum temperature required in each case is also given. • • • • •
The The The The The
SI method, 870 C. UT3 process, 780 C. hybrid W scheme, 900 C. Ispra hybrid scheme, 900 C. ZnO/Zn cycle, 1500 C.
In all cases, water is split indirectly. Direct hydrolysis requires extremely high temperatures (of the order of 3000 K) and it is therefore beyond any conceivable reactor concept. Even the ZnO/Zn cycle needs ultra high temperature, over 1500 C, and is not usually considered in the Nuclear realm, although it conveys the highest efficiency for H2 production. In general, the smaller the number of steps in the cycle, the higher the efficiency. A general feature of all the methods is that they require several chemical reactors at very high temperature (and pressure, in some cases) with large amounts of potentially harmful chemicals, as HI or H2SO4.
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. 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 also available [2,3]. All those standards have many common points, as stated in the following list: 1. 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. 2. 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. 3. Electricity is a major concern in hydrogen production facilities. 4. Safety distances are a major point in the lay-out of hydrogen facilities. 5. 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. 6. Hydrogen production and manipulation must be done in single-story buildings. 7. Storage of hydrogen must be done in certified tanks. 8. Metals for the storage walls must be properly chosen.
2738
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
9. Hydrogen detectors are mandatory in suitable places (f.i., ceilings above valves). 10. Fire detectors are also mandatory. 5. Integrated safety in nuclear production of hydrogen If the world has to go towards a Hydrogen Economy, the primary question is where will the energy to produce hydrogen come from? One of the most suitable ways to actually pave the road to the H2 Economy is Nuclear Fission. Of course, if NPPs had to participate in a prominent way in hydrogen production, the current installed capacity would have to be multiplied by a large factor. In such a huge deployment of Nuclear Fission, safety would still be more important (if possible) than today, and inherently safe reactors would be needed to comply with the anticipated standards of the Hydrogen Economy. Let us presume that Nuclear Fission is finally, and logically, invited to the hydrogen production panel. Its present power level is about 0.7 Gtoe/year (equivalent, in heat gross production; 1 toe = 11.63 MW h thermal). In order to produce 2 Gtoe of H2 per year, the thermal gross power would have to be about 10 Gtoe/year, or even more. That means to multiply the current power level by 14. That would be a challenge which needs a very good answer in terms of safety [12]. Further development for Nuclear Fission to become relevant in the Hydrogen Economy does require: • • • •
proliferation resistance of the whole fuel cycle; high efficiency in exploiting nuclear ores (natural uranium, thorium); minimization of radiotoxicity contained in the long-term nuclear wastes; very high safety standards, ruling out prompt-criticality states (the Chernobyl syndrome) and sizeable radioactive product releases.
Some specific safety issues will appear in relation to the reactor coolant needed to reach temperatures as high as required for hydrogen production. Candidate coolants are gases (particularly inert gases) molten salts and molten metals (particularly lead). In the case of gases, very high pressures needed for an effective cooling can present a threat to mechanical integrity of some containment barriers. Moreover, afterheat cooling is difficult to achieve in some types of accidents with de-pressurization. Molten metals and salts can work at atmospheric pressure, but they can present severe problems of corrosion [13]. Neutronic and thermalhydraulic advantages of molten lead, for instance, are well known, and this is why it has been proposed for some futuristic reactors. However, corrosion rates increase enormously with temperature, and cladding and structural materials (including IHX) can suffer from that. Another advantage of molten lead (and similar coolants with a very low Prandtl number) is the possibility of using natural convection for cooling, at least to remove residual heat. Passive safety features will play a very important role in the development of new reactors for futuristic needs, as those of the Hydrogen Economy. 6. Tentative conclusions Hydrogen can become an important energy vector in the economy of the future, but primary energy will be needed for that. This is a job that Nuclear Fission Energy can do in the first stages of this development. There are several types of reactors to be studied and computer simulation codes to carry out those studies. Some of those codes will specifically be used to analyze the safety levels required in the whole cycle of nuclear production of hydrogen. It is impossible to declare that all possible safety issues have been anticipated in the analysis of nuclear production of hydrogen, for all possible methods. It can be stated, however, that there are principles, criteria, rules and methodology to make the safety assessment of any nuclear installation devised to produce hydrogen, either by electrolysis or by thermo-chemical reactions. Nuclear safety methodologies and practices can be extended to hydrogen operations in order to reach safety levels as high as reasonably achievable.
M. Piera et al. / Energy Conversion and Management 47 (2006) 2732–2739
2739
Two different cases can be considered in this context, depending on the hydrogen production method: • pure electrolysis, in which NPP and hydrogen plants can be placed in different sites, so that an accident can not propagate from one installation to the other; • methods requiring high T fluids, which needed a short distance between the reactor and the chemical complex (where the H2 producing reactors are placed). The later presents very much challenging safety issues than the former. In fact, Seveso Directives I and II (Ref. [14]) (applicable in the European Union to any chemical facility), would have to be reviewed in order to obtain permission for an integrated construction of a nuclear reactor within a chemical industrial complex. By all accounts, safety will be the first priority in this evolutionary revolution. In spite of TMI-2 and Chernobyl-4 accidents [15], safety records in the nuclear industry have been very good in the Western World [16]. Even so, there is a lot of room for improvements by using new materials (new fuels, new coolants) and new designs (very stable reactors with strongly negative reactivity feedback; emergency passive cooling; unconditional subcriticality in the case of accidents and so on). All these improvements have to be embodied in the designs of the new reactors, which must also meet the requirements for H2 production, particularly the production of high temperature fluids. New coolants and new structural materials will be needed for that quest, that could be materialize if, and only if, a commensurate R&D effort is carried out in that direction. Acknowledgements This paper was largely inspired by the document ‘‘Safety issues on Nuclear Production of Hydrogen’’ elaborated by J.M. Martı´nez-Val, J. Talavera and A. Alonso (www.energiasostenible.net). References [1] Trevin˜o M. Tecnologı´a de gasificacio´n integrada en ciclo combinado. Madrid: ELCOGAS, Club Espan˜ol de la Energı´a; 2003. [2] Bose TK, Ohi J, Hay R. Sourcebook for hydrogen applications. Canada: TISEC Inc.; 1998. [3] CFR Title 29. Occupational health and safety standards. Code of Federal Regulations. Parts 1900 to 1910, US Government Printing Office, Washington; 1993. [4] NHB 7320.1. NASA facilities engineering handbook, revision B. Washington, DC: National Aeronautics and Space Administration; 1982. [5] Funk JE. Thermochemical hydrogen production: past and present. Int J Hydrogen Energy 2001;26:185. [6] Weirich W et al. Thermochemical process for water-splitting-status and outlook. Nucl Eng Des 1984;78:285. [7] ANS Position Statement 60: Nuclear energy for hydrogen generation; June 2003. [8] Brundtland G, Chairman. World Commission on Environment and Development, (1987), ‘‘Our common future’’ (The Brundtland Report). Oxford: Oxford University Press (Reino Unido, 1987). [9] Miller AI. Electrochemical production of hydrogen by nuclear energy, Nuclear productino of hydrogen-technologies and perspectives for global deployment, American Nuclear Society; 2004 [chapter 4, Table 6]. [10] INFCE. International nuclear fuel cycle evaluation, International Atomic Energy Agency. Agency Report, INFCE/PC/2/9; 1980. [11] Inagaki Y et al. Present status and future plan of HTTR Project, presented at OECD/NEA 2nd information exchange meeting of nuclear production of hydrogen, Argonne, IL; 2–3 October 2003. [12] Hori M. Role of nuclear energy in the long-term global energy perspective. In: Proc OECD/NEA 1st information exchange meeting on nuclear production of hydrogen, Paris, France; 2–3 October 2000. [13] Shreier LL, editor. Corrosion. 3rd ed. Knovel Library, Elsevier; 1994. [14] Seveso Directives: Directive 82/501/EEC, Directive 87/216/EEC and Directive 88/610/EEC (European Union, Brussels). 1988. [15] Martı´nez-Val JM et al. An analysis of the physical causes of the Chernobyl accident. Nucl Technol 1990;90:371. [16] NEA annual report on occupational exposures at nuclear power plants, 2004 (NEA-OCDE, Parı´s, 2005).