Neutron economy

Neutron economy in graphite occurs because pure graphite has a neutron capture cross section of only 0.0032 0.002 x lO " cm. Taking into account the density of reactor grade graphite (bulk density 1.71 g/cm ), the bulk neutron absorption coefficient is 0.0003/cm. Thus a slow neutron may travel >32 m in graphite without capture. 

CANDU power reactors are characterized by the combination of heavy water as moderator and pressure tubes to contain the fuel and coolant. Their excellent neutron economy provides the simplicity and low costs... 

This neutron economy leads to low fuelling costs. Also, the production of about 3 g plutonium per kilogram of the fuel at the end of the cycle is an added advantage.f ... 

The loss of neutrons to non-fissile absorption represents a significant problem for the reactor designer, particularly near the end of the reactor run, when the fuel is starting to become used up. While very careful attention to neutron economy may allow a reactor to be designed to run on natural uranium (e.g. the UK s Magnox and Canada s CANDU reactors), most commercial reactors use enriched uranium as the fuel. 

Nuclear properties of the three principal fissile nuclides are summarized in Table 3.1. The property 7 given in Table 3.1 is of interest in relation to the possibility of using these fissile nuclides in a breeder reactor. If a reactor is designed carefully for neutron economy, it is possible under certain conditions to generate fissile material at a rate equal to or greater than the consumption rate of fissile material. Such a reactor can be operated as a true breeder if the newly formed fissile material is returned to the reactor. The minimum requirements of a fuel to... 

The deuterium and the tritium can react as shown by equations (17.15)—(17.19). If the temperature and pressure is high enough the D-D reaction can also contribute. Natural uranium, depleted uranium, or enriched uranium must probably be used to inqirove the neutron economy when the charge contains LiD. Secondary fission in uranium also leads to an increased energy production in the weapon. The high energy of the neutrons from the fusion reactions make them very effective for fission of U. The fission of U produces... 

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 ... 

The nuclear properties of a material must be the first consideration in the selection of a suitable nuclear fuel. Principle properties are those bearing on neutron economy absorption and fission cross sections, the reactions and products that result, neutron production, and the energy released. These are properties of a specific nuclide, such as Th, and its product during breeding,... 

Recent findings on nuclear reactor stability are discussed and the value of computing machines in theoretical studies is noted. A commentary on current concepts in reactors is presented the flux trap reactor, boiMng reactors, gas-cooUng, and breeders using beryllium or for possible improvement of neutron economy. 

One of the distinctive features of a fast reactor is its good neutron economy. Utilizing the excess of neutrons enables us to constract flexible cores such that they breed or bum plutonium in consideration of plutonium balance, incinerate MA and long lived fission products for reducing radio toxicity and improve safety. 

Stainless-steel cladding results in parasitic absorption of neutrons, which reduces the neutron economy of the reactor. 

When developing transmutation scenarios for the minor actinides (MAs), consideration of their transmutation yield in reactors using either thermal or fast neutrons is impoi4ant, but not the only factor that must be taken into account. Of equal or even more importance is the impact of loading of MAs on the operation of the reactor. For example, the neutron economy over the reactor volume must be considered as well as issues related to reactivity such as spikes or transients and the associated safety parameters covering situations such as loss-of-coolant incidents. For a discussion of the impacts on reactor performance caused by the loading of minor actinides, see Chap. 2 in ref. OECD/NEA (1999). 

A nuclear reactor is a device in which the fission process is controlled, either to produce power, radionuclides, or both. All nuclear reactors depend upon an initial load of fuel that contains fissile materials. Absorption of a neutron by a fissile nucleus produces another fission event with high probability, accompanied by the emission of more neutrons. If one of the neutrons emitted in each fission induces another fission, the number of neutrons in each succeeding generation will remain constant and the neutron economy is balanced. This is referred to as a self-sustaining chain reaction, and is the normal operating condition of a nuclear reactor. (See O Chaps. 57 and O 58 in this Volume.)... 

If the reactor runs long enough that a significant fraction of the initial is transmuted into Pu, the neutron economy must be corrected for fissions arising in neutron reactions with Pu. is a fertile material under these conditions, since it produces fissile fuel under neutron irradiation. The other common fertile material is Th, which produces in much the same way ... 

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