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Regional Basic Professional Training Course in Korea

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<strong>Regional</strong> <strong>Basic</strong> <strong>Professional</strong> <strong>Tra<strong>in</strong><strong>in</strong>g</strong> <strong>Course</strong> (BPTC) on Nuclear Safety<br />

1.3. INTRODUCTION TO REACTOR PHYSICS<br />

1.3.1. Multiplication factor, optimum of moderation, ma<strong>in</strong> k<strong>in</strong>ds of<br />

fission reactors<br />

As was shown <strong>in</strong> sub-section 1.2.3.2 the production of neutrons by fission reactions<br />

allows the possibility to re-use these neutrons to <strong>in</strong>duce new fissions, and so on. We will<br />

exam<strong>in</strong>e here the conditions under which, a susta<strong>in</strong>able fission cha<strong>in</strong> reaction is possible.<br />

Let ν be the average number of neutrons emitted by fission and ωthe probability for a<br />

neutron emitted by fission to <strong>in</strong>duce a new fission. The product k = νω is then the<br />

average number of new fissions after one fission. It is also the number of neutrons<br />

produced by fission for one neutron emitted by fission. This is one of the fundamental<br />

physical quantities <strong>in</strong> reactor physics. It is called the effective multiplication factor.<br />

It is convenient to dist<strong>in</strong>guish between the different neutrons produced and to group them<br />

<strong>in</strong>to generations. Start<strong>in</strong>g with N fission neutrons <strong>in</strong> a generation g, the propagation of<br />

neutron’s population is, accord<strong>in</strong>g to the def<strong>in</strong>ition of k:<br />

Generation Average number of neutrons<br />

g<br />

g+1<br />

❙ 36 ❙<br />

N<br />

k N<br />

g+2 k 2N<br />

g+3 k 3N<br />

g+4 k 4 N

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