PRINCIPLES OF NAVAL ENGINEERING 



the investigation of energy released in this 

 reaction we find: 



Mass of uranium-235 atom = 235. 0439 



Mass of neutron 



Original mass 



Mass of molybdenum-95 



atom 

 Mass of lanthanum- 139 



atom 

 Mass of 2 neutrons 

 Total mass of fission 



fragments 



Mass defect = 236. 05284 - 235. 82978 



0. 22306 amu/ fission 



= 1. 00894 



= 236. 05284 amu 



= 94.9058 



= 138.9061 

 = 2.01788 



= 235. 82978 



Hence, 



0.22306 amu/fission x 931 Mev/amu 

 207.7 Mev/fission 



Thus we find that from each fission ap- 

 proximately 200 Mev of energy is released, 

 most of which (about 80 percent) appears 

 immediately as kinetic energy of the fission 

 fragments. As the fission fragments slow down, 

 they collide with other atoms and molecules; 

 this results in a transfer of velocity to the 

 surrounding particles. The increased molecular 

 motion is manifested as sensible heat. The re- 

 maining energy is realized from the decay of 

 fission fragments by beta particle and gamma 

 ray emission, kinetic energy of fission neutrons, 

 and instantaneous gamma ray energy. 



In a nuclear reactor, the two neutrons 

 liberated in the above reaction are available, 

 under certain conditions, to fission other ura- 

 nium atoms and assist in maintaining the re- 

 actor critical . A nuclear reactor is said to be 

 critical if the neutron flux remains constant. 

 Neutron flux is defined as the number of 

 neutrons passing through unit area in unit 

 time, A neutron flux of 10^3 neutrons per 

 square centimeter per second is not uncommon. 

 If the neutron flux is decreasing, the reactor 

 is said to be subcritical; conversely, a reactor 

 is supercritical if the neutron flux is increasing. 



NEUTRON REACTIONS 



Neutrons may be classified by their energy 

 levels. A fast neutron has an energy level of 

 greater than 0.1 Mev, an intermediate neutron 

 in the process of slowing down possesses an 

 energy level between 1 ev and 0.1 Mev, a 

 thermal neutron is in thermal equilibrium with 



its surroundings and has an energy level of 

 less than 1 ev. 



Neutrons lose their kinetic energy by inter- 

 acting with atoms in the surrounding area. 

 The probability of a neutron interacting with 

 one atom is dependent upon the target area 

 presented by that atom for a neutron reaction. 

 This target area (which is the probability of a 

 neutron reaction occurring) is called cross 

 section . The unit of cross section measure- 

 ment is barns. The size of a barn is 10"" 

 square centimeters. Four of the different cross 

 sections that an element may have for neutron 

 processes are as follows: 



Scattering cross section is a measure of the 

 probability of an elastic (billiard ball) coUison 

 with a neutron. In this type of collision part of 

 the kinetic energy of the neutron is imparted to 

 the atom and the neutron rebounds after col- 

 lision. Neutrons are thermalized (reduced to an 

 energy level below 1 ev) by elastic collisions. 



Capture cross section is a measure of the 

 probability of the neutron being captured with- 

 out causing fission. 



Fission cross section is a measure of the 

 probability of fission of the atom after neutron 

 capture. 



Absorption cross section is a measure of the 

 probability that an atom will absorb a neutron. 

 The absorption cross section is the sum of the 

 capture cross section and the fission cross 

 section. 



The cross section for any given element 

 may vary with the energy level of the ap- 

 proaching neutron. In the case of uranium-235, 

 the absorption cross section for a thermal 

 neutron is 100 times the cross section for a 

 fast neutron. 



REACTOR PRINCIPLES 



A nuclear reactor must contain a critical 

 mass. A critical mass contains sufficient fis- 

 sionable material to enable the reactor to 

 maintain a self-sustaining chain reaction, there- 

 by keeping the reactor critical. A critical 

 mass is dependent upon the species of fis- 

 sionable material, its concentration and purity 

 the geometry and size of the reactor, and the 

 matter surrounding the fissionable material." 



For a thorough discussion of the aspects of reactor 

 design, see Samuel Glasstone, Sourcebook on Atomic 

 Energy (2d ed.; Princeton: D. Van Nostrand Company, 

 Inc., 1958). 



620 



