Fertile isotopes

A variation of the classical fuel cycle is the breeder cycle. Special breeder reactors are used to convert fertile isotopes iato fissile isotopes, which creates more fuel than is burned (see Nuclear reactors, reactor types). There are two viable breeder cycles U/ Pu, and Th/ U. The thorium fuels were, however, not ia use as of 1995. A breeder economy implies the existence of both breeder reactors that generate and nonbreeder reactors that consume the fissile material. The breeder reactor fuel cycle has been partially implemented ia France and the U.K. 

Tn a typical fast breeder, most of the fnel is 238I (90 to 9.3%). The remainder of the fuel is in the form of fissile isotopes, which sustain the fission process. The majority of these fissile isotopes are in the form of 23 Pu and 241Pu, although a small portion of 235U call also be present. Normally, the fissile isotopes are located in a central core region that is surrounded by the fertile isotopes in the blanket region. This is illustrated in Fig. 30. 

In a typical fast breeder nuclear reactor, most of the fuel is 238U (90 to 93%). The remainder of the fuel is in the form of fissile isotopes, which sustain the fission process. The majority of these fissile isotopes are in the form of 239Pu and 241Pu, although a small portion of 235U can also be present. Because the fast breeder converts die fertile isotope 238 U into the fissile isotope 239Pu, no enrichment plant is necessary. The fast breeder serves as its own enrichment plant. The need for electricity for supplemental uses in the fuel cycle process is thus reduced. Several of the early hquid-metal-cooled fast reactors used plutonium fuels. The reactor Clementine, first operated in the Unired States in 1949. utilized plutonium metal, as did the BR-1 and BR.-2 reactors in the former Soviet Union in 1955 and 1956, respectively. The BR-5 in the former Soviet Union, put into operation in 1959. utilized plutonium oxide and carbide. The reactor Rapsodie first operated in France in 1967 utilized uranium and plutonium oxides. 

The fissionable isotopes are U-233, U-235, Pu-239, and Pu-241. The fertile isotopes U-238 and Th-232 are converted to fissionable isotopes by neutron absorption (U-238 into plutonium isotopes and Th-232 into U-233). Natural uranium contains 0.71% U-235, 99.28% U-238, and 0.006% U-234. Fuel enriched in U-233 and plutonium must be produced from thorium and U-238, respectively (Fig. 1) by neutron capture the neutrons are provided initially by fission of U-235. 

If the operational conditions and design of a reactor are adjusted to maximize the amount of Pu produced it is possible to operate the reactor to produce more fertile isotopes than were originally used to start the reactor. This operational mode is called the breeder reactor. The breeder reactor can greatly extend the amount of potential energy available from uranium because it is possible to use the 99.3% U present in natural uranium, as fuel. It is also possible to use thorium (Th" ) in a breeder reactor to produce fertile U. The use of the breeder reactor will extend the lifetime of the nuclear fission energy source to several hundred years. 

In the burner fuel cycle, the uranium is cycled one time through the reactor. All the depleted fuel elements and any remaining fertile U or Pu is stored. In the breeder reactor, design and operating parameters are be adjusted to promote the production of more fertile isotopes than are consumed in the reaction. This is the substance of breeder reactor technology. 

Another fertile isotope is 90 Th. Upon capturing slow neutrons, thorium is transmuted to uranium-233, which, like uranium-235, is a fissionable isotope ... 

In addition to fissionable isotopes ( U, or plutonium) and fertile isotopes ( U or thorium), spent fuel from a reactor contains a large number of fission product isotopes, in which all elements of the periodic table from zinc to gadolinium are represented. Some of these fission product isotopes are short-lived and decay rapidly, but a dozen or more need to be considered when designing processes for separation of reactor products. The most important neutron-absorbing and long-lived fission products in irradiated uranium are listed in Table 1.4. 

We shall estimate the resources for nuclear fission, breeder, and fusion reactors by using geological data. In breeder reactors, the fertile isotopes U-238 and Th-232 are converted to the fissile isotopes U-233, U-235, Pu-239, and Pu-241 as the result of neutron capture. Thorium is a very widely distributed element and does not represent a limiting supply when used in breeder reactors with uranium. For this reason, the following discnssion is restricted to uranium resources for fission and breeder reactors. 

A small portion of the 94 39 produced may also be changed to 94 4o by absorption of neutrons. The neutronic reactors referred to above may be called isotope converters in that one thermally fissionable isotope is formed (94 ) as another thermally fissionable isotope (U33B) is used up. However, this conversion is not c6m-plete, and the natural uranium, which acts to supply both the reaction isotope (U s) the absorption isotope (U ), will contain two different thermally fissionable isotopes after the reactor has been started. Certain presently known uranium-graphite reactors have been found to have a conversion factor of. 78, U to 94 3 . However, it may be desirable to form other fissionable isotopes in quantity such, as for example, U . Isotopes such as U 3 and Th , which arc not thermally fissionable isotopes, but which, upon absorption of a neutron, produce a thermally fissionable istotope, are called fertile isotopes. ... 

A primary object of the present invention is to provide a breeder system wherein a nuclear fission chain reaction is utilized to produce fissionable material at a rate 10 greater than the rate of consumption of fissionable material within the chain reacting composition. This is accomplished by neutron bombardment of fertile material adapted to undergo nuclear reaction productive of fissionable material as hereinafter described. Fertile iso-15 topes as herein defined are isotopes such as and U238 which are converted to thermally fissionable isotopes, and Pu 39, respectively, by nuclear reaction under neutron bombardment. These fertile isotopes are fissionable by fast neutrons and substantially nen-fission-20 able by slow neutrons (below about 1000 e.v.) and absorb neutrons fast or slow to undergo the above-mentioned nuclear reactions. 

According to the present invention the novel breeder system comprises a neutronic reactor wherein and 25 heavy water (D2O) neutron moderator are combined in a chain reacting composition surrounded by a neutron reflector of heavy water containing a fertile isotope or isotopes in solunon or in suspension. The fertile material absorbs neutrons emanating from the chain reacting 30 composition and is thus converted to thermally fissionable material. 

While the above invention is described and illustrated as employing uranium and plutonium, it is apparent that fissionable isotopes other than plutonium or a plutonium-uranium mixture may be used as the reactive composi- tion of the core of the reactor and other fertile isotopes 40 than uranium may be used as the reflector-absorber. As an additional example, but not in limitation of the... 

The resonance data of the fertile isotopes are much better known than those of the fissile isotopes. This is because the former have rather widely spaced resonances, which do not overlap up to a few kiloelectron volts, and there are only two partial widths, T and T, and the radiation width varies very little. The data are best known for The most... 

A fast reactor produces more nuclear fuel than it burns during operation, due to irradiation of the fertile isotope. All the depleted uranium that now has no use in the nuclear power... 

There is no change in the view that the ultimate role of fast reactors is to make available the very large reserves of U-238 and other fertile isotopes, and by means of the breeding process to turn them into fissile isotopes such as Pu-239 and then fission them to generate useful energy in the form of heat. However in the medium term, before the breeding role is economically demanding, fast reactors have other purposes. One of these is to consume plutonium. 

As explained in Chapter 2, the delayed neutrons are not emitted from the direct products of the fission, but from nuclei which are formed by subsequent jS decay of these products. While many of the delayed neutron precursors have been identified, it is more convenient in practice to analyze the time behavior of the delayed neutrons by an empirical division into a number of groups, each characterized by a single decay constant, or half-life. It is found that the characteristics of the delayed neutrons from all the fissionable isotopes of interest can be adequately described by the use of six groups. The half-lives and yields of the delayed neutron groups for the fissile isotopes and Pu, and for the fertile isotope are sum-... 

Table 11.2. Values of Cf, (7, v, and Fission Threshold Energy for the Main Fertile Isotopes in a Fast Neutron Spectrum...

The quantity BR — 1 is known as the breeding gain. In the absence of any contribution to the fission rate from fast fission in the fertile isotopes, 1 MW d of power operation would be provided by the fission of almost exactly 10" kg of fuel (taking the recoverable energy per fission for Pu as 215 MeV). This corresponds to 10" (1 + a) kg of fuel destroyed, where a is the capture-to-fission ratio. Taking account of the contribution to power production by fast fission of the fertile isotopes, however, the rate of destruction of fuel will be reduced in the ratio... 

Figure 7.6 Schematic representation of the fuel salt treatment with two loops. On the left is the online treatment with gas bubbhng in the core to extract noble gases and metallic particles (fission products [FPs]). On the right is the mini-batch on-site reprocessing with two objectives removing FPs (Zr, Ln) and adjusting the fuel content in fissile and fertile isotopes.

FERTILE ISOTOPE. A fertile isotope or fertile material is a substance that is not itself fissionable by thermal neutrons but can be converted into fissfle material. This conversion is typically carried out by irradiation in a nuclear reactor. There are two basic fertile isotopes thorium-232 and uranium-238. When these fertile materials capture neutrons, they are converted into the fissile isotopes uranium-233 and plutonium-239, respectively. 

---