9.4.1 Generation IV Reactors
It should be apparent that an overwhelming concern with respect to nuclear power is the generation of waste. If only we could consume almost all of the troublesome nuclei, then we would not have to concern ourselves with hazardous and expensive reprocessing technologies. Generation IV reactors address this issue along with several other key aspects:
• Improved “burn-up” of nuclear fuel to result in less hazardous waste and a closed fuel cycle
• Improved safety features
• Transmutation of hazardous fission products
• Hydrogen cogeneration from excess process heat leading to improved efficiencies
In general, Generation IV reactors are fast neutron (or fast-flux) reactors: as opposed to their Generation II and III counterparts, the neutrons are not thermalized, so no moderator is needed. The fast neutrons mean that all of the transuranium elements undergo fission, resulting in a higher efficiency and lower radiotoxicity spent fuel. Enriched uranium is not needed and, in fact, a variety of fissile fuels can be used. However, these reactors will operate at much higher temperatures (up to 1000°C) and radiation levels. The demands that will be placed on the coolant are daunting and the coolant is a molten metal, molten salt, or high-pressure gas, depending on the reactor design (see Table 9.2 and recall the hazards associated with the use of a liquid sodium coolant described in Section 18.104.22.168). Yet the demands on the materials of the reactor core are even more formidable, with stress-corrosion cracking and radiation damage leading to hardening and embrittlement likely to be more extreme under the conditions of high temperature, high radiation flux, and high pressure (Zinkle and Was 2013).
Needless to say, these increased safety concerns require increased safety systems, which include things like a built-in “core catcher” should the reactor core suffer a meltdown; double containment walls for containment of radiation leaks; and the so – called passive safety systems. Passive safety systems rely on natural forces—gravity and entropy—to implement the needed safety action (e. g., in the event of a power failure, the control rods are designed to drop into the core under the pull of gravity, not requiring redundant power supplies). While these details make the Generation IV reactors more appealing than, say, a conventional reactor nearing the end of its operating lifetime, their contribution to electrical energy generation is undoubtedly many years in the future.