Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGthe investigation of energy released in thisreaction we find:Mass of uranium-235 atom = 235. 0439Mass of neutron= 1. 00894Original mass= 236. 05284 amuMass of molybdenum-95atom= 94.9058Mass of lanthanum- 139atom= 138.9061Mass of 2 neutrons= 2.01788Total mass of fissionfragmentsMass defect = 236. 05284 -= 235. 82978235. 829780. 22306 amu/ fissionHence,0.22306 amu/fission x 931 Mev/amu207.7 Mev/fissionThus we find that from each fission approximately200 Mev of energy is released,most of which (about 80 percent) appearsimmediately as kinetic energy of the fissionfragments. As the fission fragments slow down,they collide with other atoms and molecules;this results in a transfer of velocity to thesurrounding particles. The increased molecularmotion is manifested as sensible heat. The remainingenergy is realized from the decay offission fragments by beta particle and gammaray emission, kinetic energy of fission neutrons,and instantaneous gamma ray energy.In a nuclear reactor, the two neutronsliberated in the above reaction are available,under certain conditions, to fission other uraniumatoms and assist in maintaining the reactorcritical . A nuclear reactor is said to becritical if the neutron flux remains constant.Neutron flux is defined as the number ofneutrons passing through unit area in unittime, A neutron flux of 10^3 neutrons persquare centimeter per second is not uncommon.If the neutron flux is decreasing, the reactoris said to be subcritical; conversely, a reactoris supercritical if the neutron flux is increasing.NEUTRON REACTIONSNeutrons may be classified by their energylevels. A fast neutron has an energy level ofgreater than 0.1 Mev, an intermediate neutronin the process of slowing down possesses anenergy level between 1 ev and 0.1 Mev, athermal neutron is in thermal equilibrium withits surroundings and has an energy level ofless than 1 ev.Neutrons lose their kinetic energy by interactingwith atoms in the surrounding area.The probability of a neutron interacting withone atom is dependent upon the target areapresented by that atom for a neutron reaction.This target area (which is the probability of aneutron reaction occurring) is called crosssection . The unit of cross section measurementis barns. The size of a barn is 10""square centimeters. Four of the different crosssections that an element may have for neutronprocesses are as follows:Scattering cross section is a measure of theprobability of an elastic (billiard ball) coUisonwith a neutron. In this type of collision part ofthe kinetic energy of the neutron is imparted tothe atom and the neutron rebounds after collision.Neutrons are thermalized (reduced to anenergy level below 1 ev) by elastic collisions.Capture cross section is a measure of theprobability of the neutron being captured withoutcausing fission.Fission cross section is a measure of theprobability of fission of the atom after neutroncapture.Absorption cross section is a measure of theprobability that an atom will absorb a neutron.The absorption cross section is the sum of thecapture cross section and the fission crosssection.The cross section for any given elementmay vary with the energy level of the approachingneutron. In the case of uranium-235,the absorption cross section for a thermalneutron is 100 times the cross section for afast neutron.REACTOR PRINCIPLESA nuclear reactor must contain a criticalmass. A critical mass contains sufficient fissionablematerial to enable the reactor tomaintain a self-sustaining chain reaction, therebykeeping the reactor critical. A criticalmass is dependent upon the species of fissionablematerial, its concentration and puritythe geometry and size of the reactor, and thematter surrounding the fissionable material."For a thorough discussion of the aspects of reactordesign, see Samuel Glasstone, Sourcebook on AtomicEnergy (2d ed.; Princeton: D. Van Nostrand Company,Inc., 1958).620