HT Reactor Associated to Thermo-chemical Cycles

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Introduction

There are actually around 440 nuclear power reactor used worldwide in 31 countries. Currently, about 27 reactors are built and more than 151 are projected to be built Those reactors (known as Generation II or III) use enriched uranium as fuel and a classical steam loop with a steam turbine to generate electricity. The overall energy efficiency is quite low : around 33%.

The main advantages of nuclear power is that it uses a non-carbon fossil fuel : as a consequence there are almost no GHG associated with the nuclear electricity generation. On the other hand, the nuclear waste management is still a matter of concerns : up to now, no solution other than long term underground storage can be proposed for radioactive elements with lifetime that can go from several hundred years to up to several thousand years.

The current nuclear reactor technology cannot be used for thermo-chemical cycles because the operational temperature is too low : typically around 500 to 600 °C in comparison with the 850°C that are needed for the I-S cycle that is the most cited thermo chemical cycle. To be able to have those high temperature it is necessary to use new type of nuclear reactors  : meaning at least an high temperature loop (for example using helium) associated with a thermal-spectrum reactor or a generation IV reactor (fast neutron reactor associated with a high efficiency loop for the electricity generation in itself).

The so called generation IV reactor refers to solutions that are studied in the generation IV consortium launch in 2002 between 10 countries plus the European Union. The goals of this consortium are the following :

• To establish a sustainable nuclear energy, meaning first to extend the nuclear fuel supply into future centuries by recycling used fuel to recover its energy content and by converting ²³⁸U to new fuel; second to have a positive impact on the environment through the displacement of pollutant energy in particular in the transport sector through nuclear produced hydrogen; third to substantially reduce the amount of nuclear wastes and their decay heat; forth to largely reduce the lifetime and toxicity of residual radioactive waste sent to repositories for final geological disposal in order to facilitate the demonstration of safe repository for long time periods (> 1000 years).
• To build a competitive nuclear energy, meaning first to enhance economic life-cycle and energy production costs through a number of innovative advances in plant and fuel cycle efficiency, design simplification and plant sizes; second to reduce economic risk linked to nuclear projects through the development of plants built with innovative fabrication and construction techniques and possibly modular designs; third, to allow the co-production of electricity and hydrogen, fresh water (desalinization), district heating, and other energy products to be used by industrial process.
• To have safe and reliable nuclear systems, meaning first to increase the use of inherent safety measures, robust designs and transparent safety features that can be understood by non-experts.
• To limit the proliferation of nuclear materials and improve the physical protection, meaning provide improved design features and increase the robustness of new facilities against terrorist attacks.

To reach those goals, six nuclear reactor concepts have been selected:

• Gas-Cooled Fast Reactor System (GFR),
• Lead-Cooled Fast Reactor System (LFR),
• Molten Salt Reactor System (MSR),
• Sodium-Cooled Fast Reactor System (SFR),
• Supercritical-Water-Cooled Reactor System (SCWR),
• Very-High-Temperature Reactor System (VHTR).

Not all of those systems can be used for thermo-chemical cycles as it can be seen from the figure below:
Even if some of those technologies have already been demonstrated with quite large prototypes in the past (as an example the Superphénix nuclear plant of 1GW has been shut down in 1998 in France, it was a Sodium fast reactor) those technologies will not be commercialized before 2030 at the best. Major R&D efforts are still needed for all these concepts.

Possibility of using Generation IV concept for thermo chemical cycles

Source : A technology roadmap for Generation IV Nuclear Energy System

The idea of using a thermo-chemical reaction to produce hydrogen is the result of the low efficiency of classical alkaline water electrolysis run on nuclear electricity: the total efficiency is around 20% (30% for electricity x 65% for water electrolysis). Theoretically, when coupled to a high temperature reactor, the efficiency to produce hydrogen through thermo-chemical reaction could be as high as 50%. Hydrogen thermo-chemical cycles can be defined as a series of chemical reactions activated by heat where the net results is the splitting of water into oxygen and hydrogen. The use of chemical reactions allows to operate at much lower temperature than for direct thermal water splitting (>2000°C). Typically temperatures around 750°C to 1000 °C are the most suited for these chemical reaction.

Thermo-chemical processes have been studied since the 70's. They have been considered as promising because of potential high energy efficiency, scaling to large capacities and potential for lower costs than electrolysis. For this last point, the basic idea is that thermo-chemical processes will, if ever industrialized, produce hydrogen in large quantities directly and more easily than when using modular electrolysis.

Finally, since the 70’s more than 1000 thermo-cycles have been studied and most of the work ended in the 80’s. The technology, particularly if associated with a high temperature nuclear reactor has be to seen as immature. Large R&D effort are needed to make this hydrogen production pathway an industrial reality, meaning that it has to be considered as a long term technology (possible first application beyond 2030 to 2040).

State of the Art

There are more than 1000 thermo-cycles that are identified and have been studied. Then, when considering this option making viable, a selection has to be made from this huge amount of possibilities to finally work on thermo-cycle that appears to be the most promising.

The first major European program of that type was performed at the JRC (Ispra) beginning in the late 60's continuing trough 1983. The goal was to identify the most suited thermo-chemical cycle to couple with a high temperature gas-cooled reactor. This 3 phases program investigated 24 cycles :

• In phase 1, thermo-chemical cycles were developed based on the chemistries of mercury, manganese and vanadium.
• In phase 2, thermo-chemical cycles were developed based on iron chlorides.
• In phase 3, multiple sulfur-based cycles were studied with a laboratory demonstration of the sulfur-bromine hybrid process. In parallel to the laboratory testing, design of large scale equipment and development of industrial flow sheets were also completed.

In the USA, the Gas Technology Institute funded a nine year program in which 200 distinct thermo chemical cycles were examined. From those, 125 were extracted because considered as thermodynamically feasible. Then 80 of the most promising cycles were tested in a laboratory. 15 cycles were found to be operable using batch techniques with reagent grade chemicals and eight cycles were operated successfully with recycled materials to achieve proof -of-principle. The source of energy that can be used for those cycles could be either nuclear heat or solar systems.

From the conclusions that were drawn for those different activities, it can be extracted that experimental verification is required to determine if a thermo-chemical cycle is viable. Analysis appears not to be sufficient. Most of the proposed cycles where eliminated in the laboratory because the chemical reactions were too slow, unwanted chemical reaction products were produced or no efficient methods were found to separate chemical reaction products. Recent development in catalysts or separation techniques could make some of those processes viable. Another conclusion is that there may be very large differences between theoretical efficiencies and efficiencies based on initial process flow sheets; thus, processes that have high theoretical energy efficiencies may not work in practice. Finally, it appears that relatively few cycles investigated were promising for further development. The three that were most highly ranked were the hybrid sulfur process, use of sulfur iodine, and the hybrid copper sulfate process.

The largest single-process development effort was conducted by Westinghouse Corporation to develop the hybrid sulfur process. This effort progressed through a laboratory demonstration with the final product being a conceptual design report for a pilot plant. Two conclusions are derived from this work : first, the hybrid sulfur process with the 1970s technologies seems to be able to work and second, given sufficient R&D effort, many improvement can be foreseen. Process efficiencies above 40% were calculated fort these hybrid sulfur experiments; however, potential improvements were also identified that could significantly increase energy efficiency. The decrease of energy price with the counter oil shock of 1986 stopped a lot of R&D investment in this field.

More recently in the USA, General Atomics, Sandia National Laboratories and the University of Kentucky jointly conducted a literature evaluation of thermo chemical processes. From the 115 initially selected cycles, only four were finally selected : hybrid-sulfur, hybrid sulfur bromide, UT-3 and sulfur iodine.

The general conclusion of these studies is that there are currently only a small number of processes that can be considered as candidates for large scale development. Given the scope of research in the 1970s, it is judged relatively unlikely that a significant number of new cycles with more promising chemistry will be identified in the near term. It is recognized that new technologies (catalysts and separations techniques) may make previously unattractive thermo chemical cycles viable. Although many cycles should be re-evaluated in light of these new technologies, they cannot yet be considered candidates for near-term development. Therefore, the cycles that were examined in the previous efforts and that survived the laboratory confirmation process formed the starting point for many R&D publicly funded programs.

The sulfur-based cycles were commonly identified in all studies: sulfur-iodine, hybrid sulfur, and sulfur bromine hybrid. These cycles were demonstrated to have high efficiencies and were among the least complex (even if they are still very complex in comparison with actual used technology for hydrogen production). They have also been extensively demonstrated at a laboratory-scale to confirm performance characteristics. A calcium-bromine (Ca-Br) cycle (UT-3), developed later in Japan, has also been demonstrated and appears as a promising cycle. Researchers demonstrated high efficiencies at lower temperatures than the sulfur-based cycles. But it has to be noted that the gas solid reactions involved in this cycle present a very high technical challenge.

These studies also identified many other cycles that had potentially promising features (higher efficiency, lower temperature, less complex configurations) that either had not been or could not be demonstrated as workable at that time. In most cases, there is a process step that makes the process non competitive with the leading sulfur processes. Examples of these potentially promising but high-risk cycles include: the hybrid copper sulfate cycles and the iron-based and copper-based cycles (with lower temperature and less toxic materials).

Sulfur family of thermo chemical cycles

Source : "Nuclear hydrogen R&D plan", DOE of the USA, March 2004

There are actually 3 EU funded actions in this field. They are:

• RAPHAEL (IP), acronym for "Reactor for Process Heat, Hydrogen, and Electricity generation". The IP consisting of 35 partners started in April 2005 and has a duration of 48 months. Its main objectives are on the one hand a study of advanced gas-cooled reactor technologies, which are needed for industrial reference designs, but also taking benefit from the existing demonstrator projects in Japan and China. On the other hand, it will explore options for the new nuclear generation with "very high temperature" applications, i.e., at coolant exit temperatures of 1000°C and above. The proposal comprises efforts in all VHTR sections including reactor physics and thermodynamics, fuel, back-end, materials and components development, and safety.

• HYTHEC (STREP),HYTHEC is a specific targeted research project (STREP) with the objective to evaluate the potential of thermochemical processes, focusing on the sulfur-iodine cycle and on the Westinghouse hybrid cycle. Nuclear and solar will be considered as the primary energy sources with a maximum temperature of the process limited to 950°C. In particular, a large scale solar furnace will be used as an experimental tool to study the high temperature reactions.

• INNOHYP-CA which is a coordinated action on innovative thermo-chemical cycles for the production of hydrogen.

The most promising cycles are the sulphur family described here in the figureabove.

The status of various thermo chemical cycle are given in the table 1 below.

Table 1: Status of various thermo-chemical cycles

Source : "Nuclear hydrogen R&D plan", DOE of USA, March 2004 and IFP.

Main Metrics

The metrics as defined by the WP0 are given in the following table.

In most of cases, the metrics for the thermo-chemical hydrogen production are not available or difficult to evaluate because the thermo-chemical hydrogen process are still in a very early stage of development. Thus, the metrics indicated here above just represent the current state of the art that may evolve in the years to come, depending greatly on the results of R&D programs that are on-going.

Market/Diffusion

There are actually no industrial capacities of production of thermo-chemical hydrogen.

Main Industrial Players

The main industrial player in the field of thermo-chemical process appear to be Westinghouse. But his experience is still very limited since no industrial facilities has been constructed using the Westinghouse hybrid sulfur process.

References

• Nuclear production of hydrogen
  Third information exchange meeting, Oaraï, Japan, 5-7 October 2005, OCDE 2006.

• F. Werkoff & al. (CEA)
  Process of hydrogen production, coupled with nuclear reactor
  Paper 227 ENC 2005

Notes

1. ↑ A.D. Henderson, Hydrogen from Nuclear, Presentation at National Academy of Sciences Committee Meeting, US DOE Office of Advanced Nuclear Research, Washington D.C. December 2, 2002; T.E. Lipman, What Will Power the Hydrogen Economy? Present and Future Sources of Hydrogen Energy, UCD-ITS-RR-04-10, Institute of Transportation Studies, University of California, Berkeley