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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

Our civilization has grown-up fossil-powered. A sincere tribute must be paid to coal, oil and gas. Our lives absolutely depend on them.

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?. It is perfectly known that all existing hydrogen in our planet is oxidized (mainly as water). Nowadays, the cheapest way to generate H₂ is natural gas reforming, but in a scenario of sustainable development, this alternative can not be accounted for. Even for paving the road to the Hydrogen Economy, natural gas seems not to be the most suitable choice. Natural gas is useful for many applications, both through direct combustion and chemical reactions to produce new molecules. Moreover, when H₂ is extracted from CH₄, CO₂ is also produced, which is connected with the global warming problem. This problem, besides the anticipated scarcity of natural combustible fluids in one or two generations, points out the need of finding other energy alternatives to produce hydrogen (with freshwater as raw material). Our civilization has grown-up fossil-powered. A sincere tribute must be paid to coal, oil and gas. Our lives absolutely depend on them. If one wants to move from a fossil-powered system to a hydrogen economy, energy has to be applied first. In the very long term, it would come from fusion and solar energy, whose gross resources are indeed extremely large. However, they are still far away, both in technology and economy. 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.

The foregoing statements are to some extent connected to the Generation Four initiative, which addresses in general those aims of developing safer and more competitive reactors.

On the other hand, on November 2003, the International Partnership on the Hydrogen Economy, IPHE,  was summoned in Washington D.C. to try and start a worldwide roadmap towards a hydrogen-dominated future. In the IPHE, emphasis is put on technology and safety of hydrogen applications, as fuel cells, for instance. If the question of primary energy to produce hydrogen does not get the first priority, hydrogen will have to be produced from natural gas, whose resources are slightly bigger than those of oil (of the order of 150 Gtep). This is not a sensible alternative. If hydrogen has to reach a significant share in the combustion fuels basket, it means that more than 2 Gtep per year would have to be provided by hydrogen. Current consumption of fossil fuels is about 8 Gtep/year.

Let us presume that Nuclear Fission is finally, and logically, invited to the hydrogen production panel. Its present power level is about 0.7 Gtep/year (equivalent, in heat gross production). In order to produce 2 Gtep of H₂ per year, the thermal gross power would have to be about 10 Gtep/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.

Note that H₂ is storable, and should be stored anyhow for commercialization. So, discontinuous operation of the electrolytic facilities is a possibility to be considered

5.1. Electrolytic production of H₂

In this case, an energy carrier -electricity- is used to connect (and to geographically decouple) the NPP and the hydrogen facility. Although many scenarios can be devised about this idea, a very sound one would rely on dedicated NPP. This means that those NPPs would not sell their kWh to the general grid, but to a specific site (or pool of sites) of hydrogen facilities. In this way, the nuclear-chemical system could be better optimized as a unified entity.

However, the opposite scenario would also be acceptable, with the hydrogen facilities just working during hours of very low electricity prices. Note that H₂ is storable, and should be stored anyhow for commercialization. So, discontinuous operation of the electrolytic facilities is a possibility to be considered, although continuous operation seems to be more adequate from the electrochemical point of view.

In both cases, no special safety feature does appear in relation to H₂ production, provided there is enough physical distance between NPP and the chemical plants. As already said in relation to the Generation Four initiative, those scenarios of H₂ nuclear production would really need very safe and economically competitive reactors. In turn, this requires a suitable nuclear fuel cycle, as studied in AFCI (Advanced Fuel Cycle Initiative of the USA Dpt.of Energy). 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 standars, ruling out prompt-criticality states (the Chernobyl syndrome) and sizeable radioactive product releases.

  Ways and means to cope with those challenges are being envisaged in new conceptual designs and new reprocessing techniques (as UREX+). It could be said that the aforementioned principles have inspired the Nuclear Fission development since its beginning (as was seen, for instance, in the INFCE study, 1978-1980). However, they must be followed more tightly in the quest for a new, much broader deployment of Nuclear Energy, in a context of Sustainable Development, including the Hydrogen Economy.

Very likely, reactors dedicated to H₂ production will be very different from current LWR. In fact, reactor types considered for Generation Four, for instance, do not include at all LWR of any kind.

4.1.           Thermochemical production of H₂

There are several methods to produce molecular hydrogen using endothermic reactions. In general, they convey a multi-stage chemical reactor, but what actually counts here is the need of heat at very high temperature (in the range 700 ⁰C – 900 ⁰C). This implies a physical and thermodynamical connection between the reactor and the hydrogen plant, although an intermediate heat exchanger (IHX) is devised in almost all concepts.

Thermally insulated pipes would obviously be used for transferring heat from the reactor (or the IHX) to the hydrogen production facility (the chemical reactor). The distance can be of several hundred meters, which is not enough distance to consider that they are independent facilities. On the contrary, they must be considered as belonging to the same installation (or site) and accidents in one of them can affect the another. For instance, fires have been a safety issue since the very beginning of Nuclear Energy (the Windscale accident, UK, 1957, started with a fire; the Vandellos accident, Spain, 1989, was propagated by a fire; and many other instances could be cited). In such a nuclear-chemical compound, fires will still be more important in its Safety Analysis. Moreover, hydrogen fireballs and detonation waves will pose additional problems to be taken into account. The complexity of this subject is out of the scope of this paper, but it does not mean that it will be unmanageable. The expertise in Nuclear Safety (which does include hazards from hydrogen produced in accidents, as was reviewed in a previous section) is more than enough to deal with this problem.

Very likely, reactors dedicated to H₂ production will be very different from current LWR. In fact, reactor types considered for Generation Four, for instance, do not include at all LWR of any kind. In order to meet principles of Nuclear Energy sustainability (in particular, better efficiency in the use of natural nuclear resources) reactors have to be designed in other ways. However, all the experience gained in LWR will be very relevant for the future, particularly regarding safety. This is indeed true for how to study and manage hydrogen-related risks.

Last, but not least, 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 is some types of accidents with de-pressurization.

Molten metals and salts can work at atmospheric pressure, but they can present severe problems of corrosion. Neutronic and thermalhydraulic advantages of molten lead, for instance, are well known, and this is why it has been proposed for some futuristic reactors (including accelerator-driven reactors, as the Energy Amplifier by Carlo Rubbia and co-workers at CERN). 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.

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