Fertile actinides

For the minor actinides, the transmutation process consists of the capture of one or more neutrons until a more fissionable isotope is formed. For the actinides, the most important transmutation reaction is fission because it results in the removal of the isotope from the minor actinide inventory and replaces it with two typically shorter-lived, less toxic fission fragments. With more energetic neutrons, the (n,2n) reaction is also useful because this reaction transforms fertile actinides with low fission probabilities to more fissile actinides with higher fission probabilities. Neutron capture reactions that produce less fissionable isotopes merely add to the inventory of minor actinides. 

Fast neutron capture in fertile actinides is one way of disposing of them, or at least reducing their quantity. Several countries are funding fast reactor R D to develop and evaluate LMR actinide burning capability. 

Over the last six decades, a significant amount of work has been done in this area of nuclear science and technology. In the present review, an attempt has been made to highlight the recent developments in this branch of science concerning the actinide elements with special reference to thorium, uranium, and plutonium, specific isotopes of which are being used as fissile/fertile materials. 

Actinides, particularly the lighter ones, display multiple oxidation states and complex chemical behavior, which makes their chemistry quite fascinating. Some isotopes of these elements, such as 232Th, 233,235,238 and 239Pu, are important for the nuclear industry due to their utility as fissile/fertile materials. Therefore, the separation chemistry of different oxidation states of Th, U, and Pu need to be reviewed with respect to both basic as well as applied aspects. Some fundamental chemical properties of the lighter actinides, including oxidation states, hydrolysis, and complexation characteristics form the basis of their separation. 

The used fiiel elemmts may later be reprocessed to recover the remaining amount of fissile material as well as any fertile material or regarded as waste fertile atoms are those which can be transformed into fissile ones, i.e. " Th and U, which through neutron capture and jS-decays form fissile and Pu, respectively. The chemical reprocessing removes the fission products and actinides other than U and Pu. Some of the removed elements might be valuable enough to be isolated although this is seldom done. The mixed fission products and waste actinides are stored as radioactive waste. The recovered fissile materials may be refabricated (the U may require re-enrichment) into new elements for reuse. This "back-end" of the nuclear fuel cycle is discussed in Chapter 21. 

Figure 19.7 shows the consumption of fissile while new fissile Pu and Pu (as well as some fission products and other actinides) are produced through radiative capture in fertile and Pu. The Figure relates to a particular reactor type, and diffeimit reactors give somewhat different curves. Figure 19.5 shows the differ t capture and decay... 

Neutron capture and /3-decay lead to the formation of higher actinides. This is illustrated in Figs. 16.2, 16.3, 19.5 and 19.7. Pu and Pu also fission, contributing significantly to the energy production (Fig. 19.8). Truly, all plutonium isotopes lead to fission, since the n-capture products and daughters, Am, Am, Cm, Cm, and most other actinides are either fertile, fissible or fissile. 

H, Sm, Ag, "Cd, Nb, Sn, Zr, Tc, Te and Sn. Secondly, neutron capture reactions are occurring. One of these results in the formation of fissile Pu from fertile according to Scheme 1. Some of the Pu produced will also undergo fission, but Pu may represent some 0.5% of the actinide content of the fuel discharged from the reactor. Neutron capture reactions also result in the formation of other plutonium isotopes along with some americium and curium. 

Transuraniums generated hy fuel huming include fissile material (e.g., Pu and Pu) and fertile material (e.g., Pu, Pu, and minor actinides), separated and recovered in a fuel cycle facdity, and then recycled as fuel materials. Basically, transuraniums are assumed to he transmuted into fissUe materials in a reactor at some stage and eventually to undergo fission reactions. 

The following example is an MSR plant with a power level of 1,000 MWe, which has been proposed as a reference GEN-IV MSR in GIF aiming at actinide burning with continuous recycling (US DOE and GIF 2002) (see O Fig. 58.15). The reactor can use U or Th as a fertile fuel dissolved as fluorides in the molten salt. Due to the thermal or epithermal spectrum of the fluoride MSR, Th is favorable for achieving the highest conversion performance. It operates above 700°C of coolant outlet temperature, which affords improved thermal efficiency, i.e., 40—50%. The coolant outlet temperature can he improved up to 850°C for the purpose of hydrogen production. 

The SFR is a sodium-cooled fast-neutron-spectrum reactor designed primarily for the efficient management of actinides and conversion of fertile uranium in a closed fuel cycle. 

The LFR is a fast-neutron spectrum reactor cooled by molten lead or a lead-bismuth eutectic liquid metal. It is designed for the efficient conversion of fertile uranium and the management of actinides in a closed fuel cycle. 

The gas-cooled reactor (GCR) has a gas as the primary coolant, usually helium. With a fast-neutron spectrum, the GCR has the ability to breed fertile uranium and to consume actinides. 

The fast neutron spectrum with transuranic nitride fuel and lead coolant is fissile self sufficient with a core conversion ratio of unity. This enables a closed fuel cycle based upon a fertile feed stream of depleted or natural uranium and a minimal volume waste stream comprised only of fission products. All fissile material including minor actinides is recycled in the fabrication of new fuel cores and is burned as fuel in STAR reactors. 

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