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HW18-8g-i

# HW18-8g-i - Solution For a given material koo is fixed and...

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10-7 is the breeding of new 239pu fissile fuel from 238U.However, unless the reactor is a breeder reactor, the former effectsdominate and koodecreases to less that one and the power will decrease. (g) When the reactor is in space there will be less neutron reflection than when it was on the launch pad where the ground and atmosphere act as a (poor) reflector. Thus, in space pJ and ~~ decrease, and the reactor becomes subcritical and the power decreases. (h) Although cadmium is a strong thermal neutron absorber, it does have a small scattering cross section. Thus, the cadmium acts as a reflector (although a very poor one) and keff becomes> 1 and the power increases. (i) As the fuel enrichment increases, 11 increases causing koo to increase, thereby making the reactor supercritical. The power thus increases. 9. For a given amount of multiplying material with koo > 1, what is the shape of a bare core with the smallest mass of this material? Explain.
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Unformatted text preview: Solution: For a given material, koo is fixed, and the geometry affects only the nonleakage probabilities ~~ and P~. These nonleakage probabilities both increase as the critical buckling Bc decreases. For a given volume, B; is smallest for a sphere (see Table 10.6). Thus, a sphere has the smallest critical mass. Alternatiely, for a given volume, a sphere has the smallest surface area and hence the least amount of neutron leakage. Thus, if the sphere is critical, any other shape must have a greater volume (mass) of material to be critical. 10. If the uranium fuel enrichment in a reactor is increased, what is the effect on koo? Explain. Solution: As the uranium enrichment increases, only the thermal fission factor 1/ is af-fected. The thermal fission factor is As the enrichment increases, the ratio N238/ N235 decreases, 11 increase, and koo increases. July 24, 2002...
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