Small modular reactors, long touted as the future of nuclear energy, will actually generate more radioactive waste than conventional nuclear
It’s difficult to understand why anyone would ever say that small nuclear reactors would lead to the production of less waste, and yet we have seen the claim made, repeatedly. For a given reactor type, the smaller the core, the greater the loss of neutrons by leakage. This means that the initial fuel charge must have a greater proportion of fissile material, and less of it is consumed before the operating reactivity margin falls too low and it must be replaced.
This study, however, doesn’t make a great deal of sense. The authors concentrate on two factors which are both probably irrelevant. The first is neutron activation of steel — specifically the steel of the reactor pressure vessel. The first reason that this is surprising is that the main constitutents of steel, iron and carbon, do not generally become transformed into radioactive isotopes by interaction with neutrons, and especially not long-lived, energetic radioisotopes. About the only substance commonly found in steel that does become so activated is cobalt, and so that element is typically excluded from reactor construction. (There is also some possibility of neutron absorption in molybdenum to form technetium.) Since the half-life of cobalt-60 is less than 6 years, irradiated stainless steels and other nickel alloys containing traces of cobalt can, if necessary, be held for 60 years for the activity to decay, before being mixed with other scrap steel.
Now, neutron collisions move atoms out of their places in the crystal lattice of a solid material. This happens much more often than the absorption of neutrons to create new (and sometimes radioactive) nuclei. As a result, inside the typical reactor pressure vessel you will find something called a “thermal shield”. This is a steel liner, which is under no structural load, so that changes in its mechanical properties as a result of such displacements, known as “neutron embrittlement”, don’t hurt anything. In other words, its whole function is to stop neutrons from getting to the pressure vessel (which is frequently lined with stainless steel, which in turn may contain traces of cobalt). And since this thermal shield is constructed of materials which do not become strongly and long-lastingly radioactive under neutron bombardment, it can be treated as normal scrap steel after a moderate cooling-off period.
The second factor they consider is radiotoxicity of plutonium in the fuel wastes. This, it seems to us, reflects a fundamental misunderstanding of the role of the small reactor. The large nuclear power reactor is very economical in meeting the energy needs of large cities. In the absence of anti-nuclear political pressure, the demand for such reactors tends to be strong. While there are many potential applications for small reactors, relatively few of them are so economically or technically compelling that they are likely to be pursued, absent a strong commitment to shifting the overall energy supply towards fission.
A heavily-nuclear energy economy requires a closed, regenerative nuclear fuel cycle. In other words, small reactors are not likely to account for more than a very small amount of the nuclear fuel consumed (and thus the fuel waste produced) unless discharged fuel is going to reprocessing plants and into breeder reactors, not to geological repositories for disposal. Therefore the question of “disposing of plutonium” from such small reactors is probably irrelevant.
