As global energy needs continue to increase, many believe that electricity will be supplemented with other energy carriers, e.g., hydrogen. A hydrogen based economy could allow expansion of energy production while improving environmental quality. However, this vision of the future relies upon developing hydrogen production methods that are competitive economically and that meet growing environmental concerns. Nuclear-based hydrogen production using advanced reactor technologies may hold the solution. Nuclear energy is particularly advantageous because:

• Nuclear technology for hydrogen production is proven,
• High temperatures are available,
• Nuclear power yields large thermal energy generation for relatively low cost, and
• Nuclear power has very low pollution emissions.

Three nuclear-based hydrogen processes currently in development around the world are particularly promising.

Electrolysis

The simplest and cleanest way to produce hydrogen is by electrolysis of water. The actual electrolytic step of splitting water molecules with electricity is very efficient (0 to 90 percent). However, when electricity generation is accounted for, overall thermal efficiency drops to 25 to 45 percent. In general, electrolysis is considered expesive and is only used for small production facilities.

Use of new advances in high-efficiency electrolytic cells may significantly reduce costs, making it a viable hydrogen production method when cheap, off-peak electricity can be used.

When coupled with a nuclear power plant, electrolysis is an extremely clean method of producing hydrogen.

The Sulfur-Iodine Cycle

Full Image 2-Phase Sulfur-Iodine Process (Adapted from JAERI figure)

The Sulfur-Iodine Cycle uses thermochemical processes to obtain hydrogen and oxygen from water. The figure right depicts the cycle. In the initial stage, water reacts with iodine and sulfur dioxide to form intermediate products. The intermediate products break down to constituents upon heating, releasing hydrogen and oxygen. The iodine and sulfur are recycled in the system.

This cycle, powered by a HTGR (High Temperature Gas Reactor), may one day supply hydrogen efficiently, without any dependence on fossil fuels. As shown, the cycle uses only energy (as heat) and water as inputs. The only products are hydrogen and oxygen and some reject heat. With a predicted efficiency of ~50 percent, the cycle is more attractive than electrolysis according to research by the Japan Atomic Energy Research Institute (JAERI). JAERI has successfully conducted continuous production experiments using S-I technology. Scale up will require significant additional research and development in the following areas.

• Large scale components will have to be constructed of materials able to resist the highly corrosive and high temperature environments en-countered with the S-I process.
• Solution concentrations are critical to the success of the cycle, and these parameters currently are difficult to control in large scale.
• Process improvements using membrane technology for the HI decomposition step need to be refined.

Steam Reforming

This process currently is the most common and cost effective means of producing hydrogen today. Methane and steam are reacted at high temperature to produce hydrogen and carbon monoxide. Although a fossil-fired heat source can be used, a high temperature nuclear reactor also can produce the needed heat. The heat is transferred via an intermediate heat exchanger, so the primary coolant remains isolated.

Full Image JAERI Concept for Nuclear-Based Steam Reforming Hydrogen Plant (Adapted from JAERI schematic)

Out - of - pile tests of a prototype at JAERI have proven that a steam reforming hydrogen production unit works. Using this process (illustrated left), JAERI is on schedule to operate the world’s first nuclear powered hydrogen production facility in 2008.

Steam reforming of methane is significantly more feasible than the S-I Cycle methodology in the short term, but the process remains dependent on hydrocarbons and produces green-house gas (CO2). Nevertheless, using hydrogen from nuclear-based steam reforming would result in significantly lower emissions than current fossil-burning engines presently used for transportation. The knowledge gained in the steam reforming plant also provides vital information to support eventual realization of the S-I Cycle.

Current Status

On an efficiency basis, thermo-chemical processes are superior to electrolysis. However, electrolysis has the advantage of much greater freedom of location, as the hydrogen plant does not need to be co-located with the power plant. Electrolysis can allow production close to customers, reducing the costs associated with hydrogen storage and distribution.

It appears that nuclear power uses will expand and evolve to include hydrogen production. However as the application of nuclear power to new energy distribution opportunities such as hydrogen evolves, MPR is ready. Our continuing research and development projects help us maintain and increase our technical expertise enabling us to stay at the forefront of the energy industry.

R&D tasks create a culture of excitement that becomes a part of every challenge MPR faces. Our goal is to provide our clients with excellent and innovative engineering solutions.

Contact
For further information on this article, a copy of the latest MPR Profile or our engineering services, contact Larry Cundy.