Uranium-plutonium fuel cycle

Fission energy can be obtained from uranium, using the uranium once-through option and the uranium-plutonium fuel cycle, and from thorium, by the thorium-uranium fuel cycle. Each fuel cycle offers a number of alternative routes with respect to reactor type, reprocessing, and waste handling. Although the uranium based cycles are described with special reference to light water reactors, the cycles also apply to the old uranium fueled gas cooled reactors. 

The heavy arrows in Fig. 21.1 indicate the steps in the nuclear fuel cycle pres dy used on a large commercial scale. The cycle stops at the spent fuel interim storage facility from here two alternative routes are available, one leading to the uranium-plutonium fuel cycle (reprocessing of the spent fuel elements, as described in the next section) and another leading to final storage of the unreprocessed spent fuel elements. The latter is referred to as the once-through fuel cycle (UOT) option. 

Problems relevant in the long-term are the radiotoxicity of the fuel and the long-term risk related to a final repository, which can be steered by an adequate choice of fuel cycle and reactor type. Moreover, it is possible to transmute very long-lived actinides and fission products into less toxic or stable nuclei by means of specific nuclear reactions. Following figure summarises these options for the back-end in the case of the uranium-plutonium fuel cycle. 

The MSR (see Fig. 2.7) embodies the very special feature of a liquid fuel. MSR concepts, which may be used as efficient burners of transuranic elements from spent LWR fuel, also have a breeding capability in any kind of neutron spectmm ranging from thermal (with a thorium fuel cycle) to fast (with a uranium—plutonium fuel cycle). Whether configured for burning or breeding, MSRs have considerable promise for the minimization of radiotoxic nuclear waste. 

The fast breeder reactor cycle in this cycle, the spent fuel is similarly reprocessed and the uranium and plutonium fabricated into new fuel elements. However, they are recycled to fast breeder reactors, in which there is a central core of uranium/plutonium fuel surrounded by a blanket of depleted uranium (uranium from which most of the uranium-235 atoms have been removed during the process of enrichment) or to burner reactors. This depleted uranium consists mostly of uranium-238 atoms, some of which are converted to plutonium during irradiation. By suitable operation, fast breeder reactors thus can produce slightly more fuel than they consume, hence the name breeder (see Fig. 7.1). 

In fast reactors, the uranium-plutonium fuel of equilibrium composition is unfit for production of nuclear weapons. Its breeding properties are worse than those of uranium 20% enriched in (the IAEA limit). Absence of plutonium and uranium separation in all stages of the fuel cycle guarantees proliferation resistance. 

The capability of fissile self-sustainable regime (core breeding ratio 1) in a closed nuclear fuel cycle with mixed uranium-plutonium fuel (oxide or nitride) ... 

The by-product of any uranium based fuel cycle is plutonium, which is generically an attractive material for a weapon programme ... 

Pebble bed and prismatic reactor are the two major design variants. Both are in use today. In either case, the basic fuel construction is the TRISO-coated particle fuel. Uranium, thorium, and plutonium fuel cycle options have been investigated and some have been operated in the reactors. Spent fuel may be direct disposed or recycled. The unique constmction and high bumup potential of the TRISO fuel enhances proliferation resistance. 

The second class of innovative concepts is liqnid metal-cooled fast-spectrum reactors ( fast-spectrum refers to the energy of the neutrons in the reactor core). In a typical reactor, a moderator (usually water, which pulls double-duty as both neutron moderator and reactor coolant) is nsed to slow down neutrons because slower neutrons are more efficient at causing fission in U-235. In a fast-spectrum reactor, there is no moderator. Instead, it relies on higher energy neutrons, which are less effective at causing uranium to fission but are more effective at causing fission in plutonium and other heavy elements. For this reason, these reactors are not ideal for a uranium-based fuel cycle but they are quite suitable for use with a fuel cycle based on plutonium and the other heavy... 

Mill Wastes. The uranium-containing wastes from milling are mounded and covered with earth. This earth cover prevents erosion and delays for decay the 14-hour radon gas, the gaseous decay product of uranium. These mill waste repositories are located near the mines and mills and are not a very different hazard from the original naturally occurring uranium deposits. The depleted uranium from the enrichment operations is stored in cylinders as uranium hexafluoride for future use in the uranium-plutonium breeding cycle. Other uranium wastes from enrichment and fuel fabrication go to the low-level repositories. 

The recycle weapons fuel cycle rehes on the reservoir of SWUs and yellow cake equivalents represented by the fissile materials in decommissioned nuclear weapons. This variation impacts the prereactor portion of the fuel cycle. The post-reactor portion can be either classical or throwaway. Because the avadabihty of weapons-grade fissile material for use as an energy source is a relatively recent phenomenon, it has not been fully implemented. As of early 1995 the United States had purchased highly enriched uranium from Russia, and France had initiated a modification and expansion of the breeder program to use plutonium as the primary fuel (3). AH U.S. reactor manufacturers were working on designs to use weapons-grade plutonium as fuel. 

The plutonium-uranium fuel cycle has particular advantages in fast spectrum... 

Research and development activities for thorium fuel cycles have been conducted in Germany, the USA, India, Japan, Russia and the UK during the last 30 years at a much smaller scale than uranium and uranium-plutonium cycles. Nowadays, India, in particular, has made the utilisation of thorium a major goal in its nuclear power programme, as it has ambitious nuclear expansion plans and significant indigenous thorium resources. 

Vigneron et al. [90] applied ANNs to analyse XRF spectra to dose uranium and plutonium at various stages of the nuclear fuel cycle. 

Why, after a uranium fuel rod reaches the end of its fuel cycle (typically 3 years), does most of its energy come from the fissioning of plutonium ... 

The use of the thorium-uranium fuel cycle in the HTGR provides improved core performance over the plutonium/uranium low-enrichment... 

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