Small nuclear reactors are being hailed as "the future of nuclear power" and as a complement or alternative to renewable energy. They should be safer and cheaper than large systems. But nuclear waste is also generated when they are used - quite a lot in comparison to their performance.
Small nuclear reactors can produce significantly more radioactive waste per amount of energy generated than large ones. This is the result of a recent study in which researchers analyzed the operation and life cycle of three reactor types. Small nuclear reactors are repeatedly brought up for discussion as a supplement or alternative to renewable energies as a measure against climate change. The study by the group led by Lindsay Krall from the Swedish Nuclear Fuel and Waste Management Company in Solna (Sweden) has been published in the journal "Proceedings of the National Academy of Sciences" ("PNAS").
'Small modular reactors proposed as the future of nuclear power are said to have cost and safety advantages over existing gigawatt-scale light-water reactors,' the researchers write. However, few studies have analyzed the handling and disposal of their nuclear waste streams. Using license and patent documents from smaller reactors, Krall's team has now investigated what type and amount of radioactive waste is produced during operation and decommissioning.
As a benchmark, the scientists used a pressurized water reactor, the most common commercial reactor type, with an electrical output of 1,100 megawatts (1.1 gigawatts). The smaller reactors considered were an integrated pressurized water reactor, a fast breeder reactor with sodium as the coolant and a molten salt reactor. They are to be built predominantly as smaller units and then interconnected to form larger systems. For the integrated pressurized water reactor, around twelve such units are planned in a common reactor basin.
Fast neutrons (nuclear particles) trigger the fission chain reaction, which is used to generate energy in nuclear reactors. With smaller reactors, more effort is needed to shield these fast neutrons from the environment: while less than three percent of the free neutrons escape in a large pressurized water reactor (3400 megawatts of thermal energy), it is more than in an integrated pressurized water reactor (160 megawatts of thermal energy). seven percent, the researchers report. The effort required for shielding is correspondingly greater, and the amount of radioactive waste that is produced is correspondingly greater.
The researchers calculated the radioactive waste of the different reactor types in relation to the heat energy generated. In the conventional large pressurized water reactor, the waste amounts to about five cubic meters per gigawatt of heat output per year. With the integrated pressurized water reactor, the amount is about 2.5 times as large, with the molten salt reactor (400 megawatts) about 5 times as large. Because of the large amounts of coolant (sodium) produced, the amount of radioactive waste in the fast breeder reactor is even 30 times greater.
The scientists are also critical that the burnup of the fuel rods is lower in the small reactors than in the large ones. As a result, more radioactive isotopes are concentrated in the fuel residue than in conventional pressurized water reactors. There is therefore more of a risk that the critical mass for a new nuclear chain reaction will be reached in the waste. Accordingly, new casks would have to be developed for disposal or the existing casks would have to be filled with less radioactive waste.
"Future studies should address the question of whether safe interim storage of waste streams from small modular reactors is credible in the context of an ongoing delay in the development of a geological repository in the United States," the researchers write, referring to the situation in the United States States where interest in small reactors is high.