Natural uranium contains only 0.7 % 235U. The remaining 99.3 % is non-fissile material. Light water reactors (LWRs), operated under the thermal spectrum, mainly utilize 235U as nuclear fuel.

A U nuclide is converted to a 9Pu nuclide by capturing a neutron. 9Pu is a new nuclear fuel. The 239Pu nuclide is further converted to 241Pu through 240Pu by capturing a neutron twice. 241Pu is also a new nuclear fuel. By utilizing those

Y. Oka (ed.), Nuclear Reactor Design, An Advanced Course in Nuclear Engineering 2, DOI 10.1007/978-4-431-54898-0_4, © Authors 2014

Fig. 4.1 Neutron yields per absorption (n) [1]

239 241

conversions, breeder reactors where production of Pu and Pu is larger than their consumption have been developed.

The configuration of a nuclear reactor, which pursues breeding of nuclear fuel, is determined by fundamental phenomena: nuclear reactions. The neutron yield per absorption n becomes

(4.1)

where v, oc, Of, a are neutron yield per fission, capture cross section, fission cross section and the ratio oc/of. Among n neutrons, one neutron is absorbed by a fissile nuclide to keep the fission chain reaction going. L neutrons are consumed by being absorbed by structures and coolant or leaking to the outside of the reactor. The remaining n~ (1 + L) neutrons are captured by fertile materials ( U, Pu)

239 241

and produce new fissile materials ( Pu, Pu). For breeding the fissile materials, condition (4.2) is necessary.

П -(1 + L)> 1 (4.2)

As L cannot be zero, breeding is fundamentally impossible unless n is over 2. Figure 4.1 shows n. It depends on neutron energy. Among fissile materials, 239Pu gives the highest n for fast neutrons and hence the highest breeding perfor­mance. A nuclear reactor that breeds nuclear fuel by utilizing fast neutrons is called a fast breeder reactor (FBR) [2, 3]. On the other hand, a nuclear reactor where fission reactions are mainly caused by thermal neutrons is called a thermal reactor such as a LWR. Figure 4.2 shows neutron spectra of typical fast and thermal reactors.

For the reactor core concept of a fast reactor, the following conditions are essential.

(a) Remove materials that moderate fast neutrons.

(b) Make the fraction of fissile fuel and fertile materials in the core as large as possible in order to efficiently utilize neutrons for breeding.

The breeding ratio (BR) and doubling time (DT) indicate the characteristics of a fast reactor.

Fissile materials produced per unit time Fissile material destroyed per unit time

DT = Time for doubling mass of fissile materials

Initialloadingmassoffissile materials (44)

(BR — 1) x (Reactorpower)

As BR becomes higher or the specific power, which is the ratio of the reactor power and the initial loading mass of fissile materials, becomes higher, DT is shorter [3, 5].

Liquid metal fast breeder reactors (LMFBRs) have been developed by giving shape to the fast reactor concept described above. The liquid metal (mainly sodium) was selected in consideration of the following conditions.

(a) Moderation of neutrons is as little as possible.

(b) The breeding ratio is kept as high as possible.

(c) Cooling performance is very high in order to realize a high power density.

Despite not satisfying(c), gas cooled fast reactors have also been researched

because their characteristics associated with (a) and (b) are good [3].

In the following subsections, core design mainly for the LMFBR is introduced.