Как выбрать гостиницу для кошек
14 декабря, 2021
There is insufficient up-to-date information and experience available to develop a meaningful cost projection for thorium-fuelled reactors at the present time. Nevertheless, it should be possible to make a few comparisons with the equivalent uranium-fuelled systems. The main cost item for an LWR system comes from the up-front expense of preparing the site and building the reactor and its associated cooling circuits. In general, the capital cost and operation and maintenance cost of reactors will hardly be affected by the type of fuel being used.
Fuel constitutes a small but still significant component of the overall cost of nuclear power — the most recent OECD-NEA study 1 3 indicates that the fuel cycle represents around 20% of the cost of a typical nuclear generation operation. This is not insignificant, of course, and, because of past efforts to minimize capital and operational costs, it is of increasing importance the remaining cost component that is influenced by essentially external factors such as uranium price,
front-end and back-end fuel cycle service costs, etc. The uncertainties associated with these external factors (e. g. higher and more volatile uranium prices) make it necessary to consider possible alternatives, of which the thorium cycle is one. Within this 20%, the costs can be further broken down:
Uranium
Uranium conversion Enrichment Fuel fabrication Back-end activities Total
where the ‘back-end activities’ include interim storage, reprocessing and waste disposal.
Considering the items in this list and evaluating the possible impact of a change from uranium to thorium, we can see that, at the present time, as a by-product of rare-earth production and, with no great demand, thorium is essentially ‘free’. This would change, of course, if the thorium cycle were to be widely adopted. Nevertheless, in comparison to uranium, we would expect its greater availability and easier mining conditions to allow it to be produced at no greater cost.
Moving down the list, it is clear that, once a closed thorium cycle is established in which U-233 was the fissile material, conversion and enrichment would not be needed at all and this may represent a significant cost saving. Before that situation could be reached, however, a supply of fissile material — U-235 and/or Pu — would be needed to supply the neutrons needed to transmute thorium to U-233. In the case of U-235, conversion and enrichment will, of course, be needed. Indeed, if medium enriched uranium (MEU) is used, five times more SWU (Separative Work Unit) would be needed than for conventional uranium fuel (enrichment 4 to 5%) although the smaller amount of MEU in U/Th-fuels will compensate for this. If Pu is used as seed material, then conversion and enrichment would not be needed, but Pu-based fuel fabrication is approximately three to five times more expensive than for UOX-based fuel. One may conclude from this that, while a closed thorium cycle will not incur conversion and enrichment costs, the establishment of the cycle will entail costs for these processes (or their equivalent for Pu seed fuel) that are likely to be greater than for a conventional once-through cycle.
As explained, because of the need for remote handling, fuel fabrication with U-233 will be more expensive than U-235. The refabrication cost will therefore be higher than MOX fuel fabrication where glove boxes are sufficient. Similarly, the greater technical difficulties of the THOREX, as compared with the PUREX, process will also increase costs. It is, however, difficult to assess the extra-cost since no industrial feedback is available regarding back-end operations. Regarding waste management, the THOREX process might generate 50-70% more vitrified waste than PUREX (cf. Section 8.3.3), so that the interim storage and long-term repository costs might also engender extra costs.
Given the wide variety of thorium-fuel options being investigated today, each of these variants has its own specific economic potentials and limitations and a general statement on the economic performance of thorium-fuel options in LWRs is difficult because the specifics of the individual options would need to be considered, which is beyond the scope of this chapter. Nevertheless, today’s renewed interest in thorium-fuel options is specifically directed at the formulation of a well-founded technical-economic assessment of its industrial viability. This work relies on updated knowledge and experience resulting from R&D undertaken worldwide during the last decades.
Thorium is not a direct competitor to uranium since thorium does not contain fissile isotopes. It thus must be used in combination with fissile isotopes from another source (enriched uranium, plutonium or U-233). Nevertheless, thorium has always been considered as an attractive fuel cycle option for future development of nuclear energy for the following main reasons, which have been discussed and assessed in this chapter:
• the enhancement of fuel resources by producing a new fissile isotope, U-233, which is moreover the best fissile isotope for thermal neutrons
• the existence in some countries of domestic thorium and, conversely, shortages of natural uranium, combined with the knowledge that thorium natural resources in the world are probably greater than those of natural uranium
• the good in-core neutronic and physical behaviour of thorium fuel, allowing it to reach high burn-ups, high conversion factors compared to U-233 and even breeding (i. e. a conversion factor superior to 1) in thermal reactors
Today, these benefits are more relevant than ever in the context of the nuclear renaissance, and possible uranium scarcity in the decades to come. In addition, new priorities have also stimulated renewed interest in thorium-based fuels. Among them, two main reasons are to be cited: (a) the fact that the thorium cycle strongly reduces the global inventory of long-lived minor actinides (and thus the long-term radiotoxic inventory of the finally disposed waste), (b) the fact that the use of thorium allows very efficient plutonium burning.
Another argument, which is sometimes quoted in favour of thorium, is its ability to be more proliferation resistant. This argument is not very compelling because certain physical properties of U-233 make it attractive to potential weapon use. Nevertheless, the discussion presented here shows that several routes do exist to impede such utilization and that, overall, thorium fuel may be no less proliferation-resistant than uranium fuel.
Despite the benefits of thorium, its use presents technical challenges that were described in this chapter. To support thorium industrial implementation at a large scale, infrastructures need to be developed, (i. e. mining, milling, fuel fabrication, transport and reprocessing of thorium-based fuel). Reprocessing will be required if it is intended to recover and reuse the U-233 that is generated from the fertile thorium. Fuel fabrication using the recovered U-233 with its inseparable sister isotope U-232, and the build-up of U-232’s gamma-emitting daughters, will probably require a shielded facility. And the fabricated fuel will also need to be shielded from that point on.
Beyond these considerations, this review has shown that significant experience has been gained on thorium-based fuel in both test reactors and power reactors, but not on an industrial scale. The feasibility of the front-end fuel cycle technologies (mining, fuel fabrication) has been successfully demonstrated with generally rather old technologies. For the back-end of the cycle (reprocessing and recycling), however, experience is practically non-existent. Therefore, the use of thorium on an industrial scale would still entail quite significant R&D efforts and costs, to master and optimize all the steps of the fuel cycle (including a better knowledge of thorium resources and dedicated extraction processes). Nonetheless, modern technological breakthroughs such as remote fuel fabrication techniques already applied to MOX fuels, should lower the perceived technological hurdles of the past to allow the complete implementation of the thorium cycle, including U-233 recycling, which is required if this cycle is to be used to best advantage.
To sum up, it is clear that thorium-based fuel shows useful characteristics but they do not appear sufficient to justify an industrial development in the shortterm, all the more so as these potential advantages are offset by some real drawbacks. On the other hand, in the term of a few tens of years, thorium could help to lower the radiotoxicity of radioactive waste to be disposed and, if U-233 is recycled, could reduce demand for uranium. In this latter respect, the possibility of achieving near breeding or even breeding conditions in thermal reactors represents a very attractive feature of the thorium cycle.
Finally, the future is unwritten. The appearance of new, currently unanticipated, constraints will, no doubt, modify the current context and lead to unexpected developments. This indicates the need to maintain and develop the thorium cycle as a credible option.