The production of Np237 and Pu238 was calculated for present-gene ration reactors. Results indicate that Np237 production now averages about 3 kg. per year per 1 000 Mw(th) (at 0.8 plant factor); it was estimated that 1 00 kg. per year would be produced in 1975. Because the Pu 238 capture cross section is about three times that of Np237, high efficiency for conversion of Np237 to Pu238 requires removal of the product after relative short exposures. The maximum conversion of Np 237 to Pu238 in a single exposure is about 20%, but by recycling the Np, over-all conversion of 50 to 60% can be realized. Thk use of Pu238 as a heat source for space power applications is of interest because of its very desirable nuclear decay properties relative to more conventional radioisotopes. Pu238 emits 5.4 m.e.v. alpha particles and has a half life of 89.6 years. It is worth nearly as much per unit weight as Sr90. Pu238 would also last about three times as long as Sr90 which has a 28year half life.Pu238 is produced in a reactor by the following reactions: ß ß U233 (n. y) -* U236 (n, y) -*-U237 |-*Np237 (n, y) -* Np238 7 Pu238 yrf 6.7 d 2.1 d VOL. 3 NO. 4 OCTOBER 1964 293
between neutrons and the nuclei of atoms. These reactions can occur in a variety of ways. In the most common nuclear reaction, the nucleus &dquo;captures&dquo; the neutron forming an isotope of higher atomic weight. This isotope is usually radioactive and decays to form a new element. In the &dquo;fission&dquo; reaction, the nucleus splits into two parts forming a variety of lighter elements and releasing a number of additional neutrons. Although many heavy nuclei undergo both fission and capture reactions with neutrons, only a few have a higher probability of fission than capture. Since these few generate excess neutrons and are capable of sustaining a nuclear chain reaction, they are called &dquo;fissionable&dquo; nuclei. The most common fissionable materials are uranium 233, uranium 235, and plutonium 239. Of these, only the uranium 235 occurs in nature as 0.71 1 per cent of normal uranium. The remaining 99.3 per cent is uranium 238, which has a very low fission-tocapture probability and does not fission enough to sustain a nuclear reaction.If normal uranium is bombarded with neutrons, two primary reactions occur.These are:The first reaction is the nuclear fission reaction, and the second is a neutron capture reaction. Only the first produces the energy for power and neutrons necessary for continuing the reaction.If the nuclear fuel consists of pure U-235, there will be 1.5 excess neutrons created per fission, and in a mixture of U-235 and U-238, less than this, depending on the relative amounts of these elements present. In natural uranium, the above reactions occur in approximately equal amounts, which means that there are only 0.5 neutrons excess per fission.Although the neutron reaction with U-238 uses up a neutron, this capture reaction produces an unstable atom U-239 which decays to neptunium 239, and this in turn to plutonium 239.Since Pu-239 is fissionable, each neutron capture in U-238 produces a new atom of nuclear fuel. In a like manner, thorium 232 will capture a neutron and produce U-233 as follows (Pa-233 means protactinium 233 ) :The large amount of energy released from the fission reaction arises from the fact that the combined masses of the fission products do not quite equal that of the fissioned nucleus, the difference being converted to energy. This manifests itself primarily in the form of kinetic energy of the fission products. These create heat on impact with surrounding atoms of the nuclear fuel. REACTOR COMPONENTSA nuclear chain reaction will be initiated spontaneously in a block of pure fissionable material if the number of at NORTH DAKOTA STATE UNIV LIB on June 15, 2015 ann.sagepub.com Downloaded from
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