Thorium cycle

The metal is a source of nuclear power. There is probably more energy available for use from thorium in the minerals of the earth s crust than from both uranium and fossil fuels. Any sizable demand from thorium as a nuclear fuel is still several years in the future. Work has been done in developing thorium cycle converter-reactor systems. Several prototypes, including the HTGR (high-temperature gas-cooled reactor) and MSRE (molten salt converter reactor experiment), have operated. While the HTGR reactors are efficient, they are not expected to become important commercially for many years because of certain operating difficulties. 

Mumane RJ, Cochran JK, Sarmiento JL (1994) Estimates of particle- and thorium-cycling rates in the northwest Atlantic Ocean. J Geophys Res 99 3373-3392 Mumane RJ, Cocliran JK, Buesseler KO, Bacon MP (1996) Least-squares estimates of thorium, particle and nutrient cycling rate constants from the JGOFS North Atlantic Bloom Experiment. Deep-Sea Res 1 43(2) 239-258... 

Hatcher, S.R., Banerjee, S, Lane, A.D, Tamm, H., Veeder,J.I. "Thorium Cycle in Heavy Water Moderated Pressure Tube (CANDU) Reactors" American Nuclear Society Meeting, San Francisco AECL-5398, 1975... 

Uranium-233. A second fissionable isotope uranium-233, can be produced from naturally occurring thorium. It does not present an economically attractive option at present because of its dependence on highly enriched U-235 to bring the thorium cycle into operation and the large R D expenditures required... 

A similar set of processes has been partially developed for the thorium-uranium system but is not discussed here because it is not expected to be employed in the next several decades. The important feature of the thorium cycle is that it could be used to achieve breeding (to produce more fissionable material than is consumed) in thermal reactors, but nuclear as well as chemical factors have frustrated this development (for more information, see Reference 22). The increasing cost of the natural uranium supply for the ura-nium/plutonium cycle may, several decades in the future, justify development of the thorium cycle. 

It should be noted that breeders would not reduce the demand for uranium ore for many decades because several LWR and/or HWR converters (which produce fissionable material, but less than consumption) are required during the run-in of a breeder cycle to equilibrium. The doubling time of a breeder (the time required for the breeder to produce sufficient fissionable material to start up a second breeder reactor) might be a significant part of its operating life. Furthermore, natural uranium will be required for the thorium cycle, if it is used, and for startup of the fusion cycle. The tritium for the fusion cycle will be made in nuclear reactors, as it now is for nuclear weapons. The nuclear industry will always be dependent on a continuing supply of uranium from ore. 

Sub-criticality is the first essential feature of the concept the second is the use of a thorium cycle, based on the accelerator-induced reactions (Tj/j is the half-life, i.e., the time after which activity has been reduced to half). 

Murnane R. J., Cochran J. K., and Sarmiento J. L. (1994) Estimates of particle-cycUng and thorium-cycling rates in... 

When is used as fissile makeup in the thorium cycle, as in the reference... 

Interest now is centered on the thorium cycle (23) and laboratory studies have continued to investigate both an adaptation of the Thorex process to CANDU fuel and the application of the amine process to recovering uranium-233 from irradiated thorium. The program to develop and fully demonstrate the thorium fuel cycle has been outlined, and would require about 25 years to complete. However, the current research level will not be expanded until a decision can be taken by the Canadian Government when the information from the current International Nuclear Fuel Cycle Evaluation has been assessed. 

The neutron economy makes it feasible to run PHWRs on the thorium cycle with a conversion ratio of 0.9. Such a cycle has to be started and operated either with a mixture of U02, enriched U02 and Th02, or - if isotope enrichment is not used — through the following three step fuel schedule (i) running on natural uranium and extracting Pu ... 

In parallel with the work done in collaboration with the European partners BNFL has conducted studies of the potential role of fast reactors in the UK and elsewhere. It is important to consider the fuel cycle as a whole and to make use of fast reactors in the optimum way to maximise safety and economic advantage while minimising environmental impact and proliferation risks. To this end accelerator-based systems as alternatives to critical reactors, and the thorium cycle as an alternative to the uranium-plutonium cycle, have been examined with particular reference to the implications for fuel fabrication, reprocessing and waste disposal. This work continues but the initial conclusion is that the critical Pu-fuelled fast reactor, properly integrated with reactors of other types, and with optimised arrangements for Pu recycling, has many attractive advantages. 

There have been carried out investigations concerning uranium-thorium cycle at four critical assemblies of the COBRA facility. The central inserts of these assemblies contained U-235, thorium and hydrogen. Hydrogen/U-235 ratio of nuclear density varied from 0.0 to 70.0. These experiments allow to estimate an influence of thorium on neutron spectrum and an accuracy of nuclear data in a wide energy range (from Mev to tens of Kev). In experiment there have been obtained Keff values of various uranium-thorium compositions, cross-section ratios of some nuclear reactions including thorium capture-to-fission ratio, fission cross-section of some TRU. On completion of the experiments the... 

SSET Self-Sufficient Equilibrium Thorium Cycle... 

Nonetheless, many proponents of the thorium cycle promote this feature, and indeed, many detractors of tiie nuclear industry cite the long-term radiotoxicity in spent uranium-based fuels as a major issue. 

In general, thorium cycles are all quite proliferation resistant for a variety of reasons. Prominent among those are that separated is always accompanied by is a... 

Critoph, E., S. Banerjee, F.W. Barclay, D. Flamel, M.S. Milgram, and J.l. Veeder. 1976. Prospects for Self-Sufficient Equilibrium Thorium Cycles in CANDU Reactors, Proceedings of the ANS 1975 Winter Meeting, San Francisco, CA AECL-5501 (1975). 

Dastur, A.R., D.A. Meneley, and D.B. Buss. 1995. Thorium Cycle Options in CANDU Reactors. Proceedings of the 1995 International Conference on Evaluation of Emerging Nuclear Fuel Cycle Systems (GLOBAL-95), Versailles, France. 

Milgram, M.S. 1982. Once Through Thorium Cycles in CANDU Reactors, Atomic Energy of Canada Limited Report, AECL-7516. 

U will only be possible through isotope separation techniques. The high Pu to Pu ratio and the production of gamma emitting Tl in the thorium cycle are hindrances to nuclear proliferation. Pu has a spontaneous fission that contributes to increased residual heat of spent fuel that will complicate the production of nuclear weapons. 

The use of thorium fuel leads to suppression of the generation of minor actinides in the nonplutonium bearing CHTR. Graphite based fuel tubes with low activation and ease in compacting the waste further reduce the amount of wastes generated. An isotope of a certain concern in the thorium cycle is It is formed via (n, 2n) reactions, from Th, Pa and and has a half-life of about 69 years. The daughter products of are hard gamma emitters like T1 (2.6 MeV) with very short half-lives. As a result, the radioactivity increases... 

XXX-12] FURUKAWA, K., et al.. Thorium cycle implementation through Pu — incineration by thorium molten-salt nuclear energy synergetics, lAEA-TECDOC-1319, Thorium Fuel Utilization Options and Trends, Vienna, p.123-137 (November 2002). 

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