Uranium neutrons absorbed

The nuclear reactor is a device in which a controlled chain reaction takes place involving neutrons and a heavy element such as uranium. Neutrons are typically absorbed in uranium-235 [15117-96-17, or plutonium-239 [15117 8-5], Pu, nuclei. These nuclei spHt, releasing two fission fragment nuclei... 

CP-1 was assembled in an approximately spherical shape with the purest graphite in the center. About 6 tons of luanium metal fuel was used, in addition to approximately 40.5 tons of uranium oxide fuel. The lowest point of the reactor rested on the floor and the periphery was supported on a wooden structure. The whole pile was surrounded by a tent of mbberized balloon fabric so that neutron absorbing air could be evacuated. About 75 layers of 10.48-cm (4.125-in.) graphite bricks would have been required to complete the 790-cm diameter sphere. However, criticality was achieved at layer 56 without the need to evacuate the air, and assembly was discontinued at layer 57. The core then had an ellipsoidal cross section, with a polar radius of 209 cm and an equatorial radius of309 cm [20]. CP-1 was operated at low power (0.5 W) for several days. Fortuitously, it was found that the nuclear chain reaction could be controlled with cadmium strips which were inserted into the reactor to absorb neutrons and hence reduce the value of k to considerably less than 1. The pile was then disassembled and rebuilt at what is now the site of Argonne National Laboratory, U.S.A, with a concrete biological shield. Designated CP-2, the pile eventually reached a power level of 100 kW [22]. 

Many of the fission products formed in a nuclear reactor are themselves strong neutron absorbers (i.e. poisons ) and so will stop the chain reaction before all the (and Pu which has also been formed) has been consumed. If this wastage is to be avoided the irradiated fuel elements must be removed periodically and the fission products separated from the remaining uranium and the plutonijjm. Such reprocessing is of course inherent in the operation of fast-breeder reactors, but whether or not it is used for thermal reactors depends on economic and political factors. Reprocessing is currently undertaken in the UK, France and Russia but is not considered to be economic in the USA. 

The equation shows that uranium-235 absorbs a neutron. After absorbing the neutron, the excited uranium nucleus splits and forms barium-141, krypton-92, and three neutrons. Energy is also produced in the reaction. This reaction is only one of a number of different ways that U-235 may split. Several hundred different isotopes have been identified when U-235 undergoes fission. 

Chain reactions do not occur to any great extent in naturally occurring uranium ore because not all uranium atoms fission so easily. Fission occurs mainly in the isotope uranium-235, which is rare and makes up only 0.7 percent of the uranium in pure uranium metal (Figure 4.23)- When the more abundant isotope uranium-238 absorbs neutrons created by fission of a uranium-235 atom,... 

The uranium and thorium ore concentrates received by fuel fabrication plants still contain a variety of impurities, some of which may be quite effective neutron absorbers. Such impurities must be almost completely removed if they are not seriously to impair reactor performance. The thermal neutron capture cross sections of the more important contaminants, along with some typical maximum concentrations acceptable for fuel fabrication, are given in Table 9. The removal of these unwanted elements may be effected either by precipitation and fractional crystallization methods, or by solvent extraction. The former methods have been historically important but have now been superseded by solvent extraction with TBP. The thorium or uranium salts so produced are then of sufficient purity to be accepted for fuel preparation or uranium enrichment. Solvent extraction by TBP also forms the basis of the Purex process for separating uranium and plutonium, and the Thorex process for separating uranium and thorium, in irradiated fuels. These processes and the principles of solvent extraction are described in more detail in Section 65.2.4, but the chemistry of U022+ and Th4+ extraction by TBP is considered here. 

For example, when uranium-235 absorbs a high energy neutron, Jn, it breaks up, or undergoes fission as follows ... 

Nuclear fuel rods consist of uranium oxide pellets contained in zirconium alloy or steel tubes. As the fission process proceeds, uranium is used up and fission products accumulate. A lot of these fission products are good neutron absorbers and reduce the efficiency of the fission process (by absorbing neutrons before they reach uranium atoms) so that the rods are removed for reprocessing before all the content has undergone fission. Fission of a atom produces two lighter atoms of approximate relative atomic masses around 90-100 and 130-140, with the main fission products being the intensely radioactive and short lived I (fi 8 d), °La, Pr, Zr, Ru, and Nb, and longer-lived... 

The primary use for plutonium (Pu) is in nuclear power reactors, nuclear weapons, and radioisotopic thermoelectric generators (RTGs). Pu is formed as a by-product in nuclear reactors when uranium nuclei absorb neutrons. Most of this Pu is burned (fissioned) in place, but a significant fraction remains in the spent nuclear fuel. The primary plutonium isotope formed in reactors is the fissile Pu-239, which has a half-life of 24 400 years. In some nuclear programs (in Europe and Japan), Pu is recovered and blended with uranium (U) for reuse as a nuclear fuel. Since Pu and U are in oxide form, this blend is called mixed oxide or MOX fuel. Plutonium used in nuclear weapons ( weapons-grade ) is metallic in form and made up primarily (>92%) of fissile Pu-239. The alpha decay of Pu-238 (half-life = 86 years) provides a heat source in RTGs, which are long-lived batteries used in some spacecraft, cardiac pacemakers, and other applications. 

When plutonium-239 is produced from urani-um-238 in a breeder reactor, the process occurs in three steps. In the first step, uranium-238 absorbs a neutron. In the second step, a shortlived intermediate element is made through beta decay. In the third step, the plutonium-239 is produced through beta decay. Write a balanced nuclear equation for each of the three steps. 

Uranium-238 absorbs a neutron, making the unstable heavy uranium isotope, U-239. The U-239 emits an electron and becomes neptunium-239. Then another electron is emitted and leaves us with plutonium-239. 

Objective 41 When natural uranium is bombarded by neutrons, the uranium-238 absorbs so... 

Both uranium-235 and uranium-238 absorb fast neutrons, but if the neutrons are slowed down, they are much more likely to be absorbed by uranium-235 atoms than uranium-238 atoms. Therefore, in a nuclear reactor, the fuel rods are surrounded by a substance called a moderator, which slows the neutrons as they pass through it. Several substances have been used as moderators, but normal water is most common. 

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. 

This figure shows that concentration decreases almost exponentially with bumup. a neutron-absorbing isotope of uranium, builds up to a concentration of around 0.4 percent of total fuel. Pu, a fissionable isotope, builds up to a concentration of around 0.6 percent. Pu builds up more slowly to around 0.3 percent. When Pu absorbs a neutron, Pu, another fissionable isotope, is formed. When this absorbs still another neutron, Pu, a neutron... 

A flow sheet like Fig. 4.4 has been used to separate uranium from neutron-absorbing impurities (Chap. 5), and zirconium from hafnium (Chap. 7), by fractional extraction of an aqueous nitrate solution with an organic solution of TBP in kerosene. 

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