High Temperature Reactors
Commercial reactors generally operate with an outlet temperature of around 200-300°C with the heat produced only used to generate electricity. By contrast, considerably higher temperatures (800-1000°C) can be achieved in a new generation of nuclear reactors known as High Temperature Reactors (HTR). The high temperature reactor is a particularly intriguing reactor system with exceptional safety characteristics combined with high efficiency for conversion of heat to electricity. Furthermore, direct use of the heat produced, via a heat exchanger, are envisaged also for industrial processes. For example, above 400°C the heat generated can be coupled for iron manufacturing and petrochemical processes such as the de-sulfurisation of heavy oil, petroleum refining and production of ethylene and styrene (Fig. 2). One very important product that can be obtained using the heat from a HTR is hydrogen. This element, considered essential for the future transport industry and currently used to produce liquid fuels, ammonia, methanol and other products, is presently obtained from a process called steam methane reforming. In this method, steam and methane are combined to produce hydrogen and CO2, using heat from natural gas. The heat produced by the HTR not only could replace the use of natural gas but also can be used to directly split water to produce H2 and O2 without the necessity to convert large amounts of carbon to CO2. The use of this technology could further reduce green house emissions by replacing the large amount of fossil fuels used today for industrial processes.
Figure 2. Nuclear process heat for the industry. http://www.nextgenerationnuclearplant.com/
The safety of a HTR originates from the characteristics of its fuel and the design of the core. The fuel of the HTR is made of a very small fuel particles (0.5 mm in diameter) covered by four thin layers of ceramic making it no larger than 1 mm (Fig 3). Thousands of this particles, known as TRISO fuel, are then mixed with graphite to make the fuel elements in spherical (pebble bed design) or cylindrical (prismatic core) form depending on the design of the core. One of the main advantages of this type of fuel is that the thin ceramic coatings (aprox. 0.03 mm in thickness each) serve as a miniature fission product containment vessel designed to trap all the fission products. Under off-normal conditions, the fuel in a reactor with a power of less than 200 MW rises to less than 1600°C, and the integrity of the ceramic components is maintained.
Figure 3. The components of the pebble fuel in the High Temperature Reactor. http://www.pbmr.com
Super Critical Light Water Reactors
The supercritical light water reactor (SCLWR) is perhaps more appropriately referred to as the high performance light water reactor (HPLWR). Like the light water reactor (LWR), its coolant is water, but it is maintained under conditions where water becomes a so called super critical fluid. Thus, its characteristics change dramatically. The main advantage of this concept lies in the higher efficiency, with which the heat can be converted to electricity. With a 50% higher efficiency than a conventional system, a major impact on the system economy can be achieved. Furthermore, this leads to a concomitant decrease in the waste generated.
Molten Salt Reactors (MSR)
The final reactor selected is cooled by a molten salt. The salts are mixtures are based on lithium and sodium fluorides (LiF and NaF), which depending on their composition melt at around 700°C. The fuel is also in the form of a fluoride and is dissolved in the molten salt. Unlike all the other reactor systems, the fuel is not static. It is pumped through the reactor core, and is “cleaned” of its impurities (i.e. fission products) in an online treatment facility. This reactor exhibits great flexibility in its operation, and furthermore can be readily fuelled in a sustainable manner by thorium, an element even more predominant than uranium in the earths crust. Challenges lie in the fuel chemistry, separation technologies, and in the qualification of materials resistant to the corrosive molten salts.
The path forward
Current reactor technology will continue to play an important role. The majority of the reactors operating today belong to Generation II light water reactors (LWR). Newest LWR reactors (Generation III) have adopted additional safety technologies.
Safety of the reactor operation is essential no matter what reactors are deployed. The quest for perfection is unremitting. It can cover core design and layout. The behaviour of the fuel with its inventory of fission products during normal and off normal reactor operating conditions, during interim storage (cooling) and reprocessing, and indeed within a long term repository must be known and quantified.
Licensing by national authorities, often known as technical safety organisations (TSOs), is essential. The light water reactor fleets have generated a large quantity of operational experience and data, enabling the establishment of engineering based computer simulation codes, with sufficient predictive capability for licensing. Establishing this competence for future reactor systems will require a similar engineering type effort. Science never stands still. However, theory is developing fast, as is computational power. Though still a fair way off, it can be expected that a paradigm shift for licensing will occur. Improved computational power and modelling will lead to advanced designs, and improved less time consuming and costly paths to best possible fuels.
The recent events at Fukushima will reinforce nuclear industry’s commitment to safety. The unimaginable must be imagined, and measures taken to contain consequences of a plant failure, as successfully achieved at Three Mile Island in 1979.
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