[3] B3. (a) With reference to the neutron reproduction factor, Kao, what is meant by the terms, critical, sub-critical a

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3 B3 A With Reference To The Neutron Reproduction Factor Kao What Is Meant By The Terms Critical Sub Critical A 1 (113.79 KiB) Viewed 65 times

[3] B3. (a) With reference to the neutron reproduction factor, Kao, what is meant by the terms, critical, sub-critical and super-critical? (b) What crucial role do "delayed neutrons" play in the operation of a practical fission reactor? (c) Ignoring the fast neutron loss fraction, the neutron multiplication factor K, for a finite size nuclear reactor is given by: K = K (1-4) [2] [8] where le is the thermal neutron loss fraction. If the thermal neutron loss fraction for a spherical reactor of radius R, is 31°/R”, where 1 is the thermal neutron diffusion length, use the equation above, and the information given in Table 3, to estimate the minimum radius Rimin for a spherical reactor, necessary to sustain a nuclear reaction when fueled with natural Uranium. A smaller reactor can produce a self-sustaining reaction, if the fuel is replaced with enriched Uranium. (d) Estimate the relative concentrations 238U:235U, required to produce a self-sustaining reaction, if the reactor has a radius of only 2 m . (e) In a practical sense, how is the enriched Uranium produced? [5] [2] Table 3: Thermal neutron cross-sections in barns 235 U 238U Fission 580 0.0 Absorption 107 2.8 Mean number of fast neutrons per fission = 2.5 Fast fission factor = 1.03 Resonance escape probability = 0.9 Thermal utilisation factor = 0.9 Thermal neutron diffusion length = 0.5 m The isotopic concentrations of natural Uranium are 99.3% 238 U and 0.7% 235U