In a nuclear reactor of current state of technology in the once-through mode – all power reactors operating today are falling in this category – not much more than about 0.6% of the atoms in the natural uranium recovered from its ore can be fissioned. This has to do with the isotopic composition of natural uranium, only a small fraction is fissile. By neutron capture Ð neutrons are set free during fission of uranium-235 atoms Ð the non-fissionable uranium-238 atoms can be transformed into plutonium atoms, most of which are fissile. For that reason uranium-238 is coined a fertile material.
Uranium-plutonium breeder concept
The idea behind the U-Pu breeder reactor is to use the plutonium from a number of LWRs to load a special kind of reactor with a core with plutonium as fissile material surrounded by uranium-238 as fertile material. In this reactor, cooled by a liquid metal and operating with fast neutrons, it would be possible to generate more fissile plutonium from uranium-238 in the blanket than is fissioned in the core during operation of the reactor. For that reason this kind of reactor is called a fast U-Pu breeder reactor, which does not mean that the breeding process goes fast.
The spent fuel from the breeder reactor would have to be reprocessed to separate the fission products, which interfere with the fission process. From the remaining uranium and newly formed plutonium fresh nuclear fuel would be produced and placed into the reactor. By repeating this cycle again and again it would be possible to fission about 60% of the atoms in natural uranium, some sources even claim 100%. This theoretical concept is the source of the dreams and promises of the nuclear industry of 'unlimited energy, too cheap to meter'.
In practice the breeder concept involves a cycle of three processes: breeding in the reactor, reprocessing of spent fuel and fuel fabrication. Each of the three components of the breeder cycle must operate flawlessly, continuously and exactly tuned to the other two components, in order to let the system actually breed more fissile material from non-fissile uranium than it consumes. If one component fails, the whole system fails. In fact, none of the three components have ever demonstrated operation as required, let alone the three components together as one integrated continuously operating system.
Process losses occur in each of the three components, resulting in a breeding ratio less than unit, even if all would operate as planned. A breeding ratio less than unit means that the cycle generates less fissile plutonium than it consumes, so during the operational lifetime of the breeder cycle the reactor has to be supplemented with plutonium from conventional reactors.
During the past 50 years intensive research has been conducted on the U-Pu breeder concept in a number of countries (USA, UK, France, Germany, former Sovietunion and Japan). The investments amount to more than $100bn. The international research was concentrated on the LMFBR, the Liquid Metal-cooled Fast Breeder Reactor, a heavily promoted concenpt during the 1980s and 1990s. All efforts failed.
New names, no new concepts
Presently the nuclear industry avoids the term 'breeder' or 'LMFBR' and uses preferably the terms 'fast reactor' or 'Generation IV reactor' or 'closed-cycle reactor'. When speaking about these reactor types the nuclear world usually has fast U-Pu breeder reactors (LMFBR) in mind. From a publicity point of view this change of name has an understandable reason, because the concept proved to be technically unfeasible and consequently 'breeder' and 'LMFBR' are blacked names connoting a failed concept.
The failure of the breeder concept is not caused by protests of environmental activists, nor by actions of leftist politicians, nor for economic reasons, as the nuclear industry asserts, but is caused by fundamental technical limitations. Implicitely the nuclear world assumes the availability of 100% perfect materials and 100% complete separation processes. None of these two conditions are possible, as follows from the Second Law of thermodynamics. By virtue of this law can be argued beforehand that the breeder cycle likely will not work [more i42, i43].
Figure 33-1. The breeder cycle
The breeder reactor is not a stand-alone system, but part of a cycle of three components: a special reactor, a reprocessing plant and a fuel fabrication plant. The cycle has to be started up with plutonium recovered from spent fuel from conventional reactors. The spent fuel from the liquid-metal cooled fast reactor has to be reprocessed to remove the fission products, activation products and actinides. The recovered uranium and plutonium would be reused. Due to rapidly increasing radioactivity of the spent fuel with each cycle, reprocessing and fuel fabrication become increasingly difficult. The isotopic compositions of the recovered uranium and plutonium become less favourable each cycle. Due to the unavoidable and increasing separation losses, the cycle produces less fissile nuclides than it consumes. For these reasons, among other, the breeder cycle is technically unfeasible.