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HW24-16

# HW24-16 - the water moderator/coolant.have a large number...

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15. Reactors for naval vessels are designed to have very long lifetimes without the need to refuel. Discuss possible techniques that can be used to maintain criticality over the core lifetime as 235U is consumed. Solution: For a long lifetime, a large initial mass of 235U is needed, an amount in excess of the critical mass equal to the anticipated fuel burnup over the design lifetime. Naval reactors are of the PWR type and can employ the following methods to maintain criticality despite the large variation in fissile mass over the core lifetime: .incorporate large amounts of burnable poison in the fuel assemblies so that as the fuel is consumed (a negative reactivity effect) so is the poison (a positive reactivity effect). Thus, the two effects tend to compensate for each other. .use very enriched uranium to obtain a large initial mass of fissile fuel in the relatively small core. .use initially high concentration of soluble poisons, such as boric acid, in
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Unformatted text preview: the water moderator/coolant. .have a large number of control rods to adjust the reactivity over a wide range. 16. A thermal nuclear rocket using hydrogen as the propulsive gas operates for one hour at a thermal power of 4000 MW and a temperature of 2700 K. Estimate (a) the amount (in grams) of fissile material consumed, (b) the specific impulse of the engine. Solution: (a) The fission energy released is Eth = Pth x ~t = 4000 (MW) x (1/24) (d) = 166.7 MWd. Since 1 MWd of fission energy requires the consumption of 1.24 g of 235U (see p 150), the mass of 235U consumed by the rocket is M(235U) = Eth X 1.24 g/MWd = 206.7 g 235U consumed. (b) At an exhaust temperature of 2700 K, the hydrogen is in the form of a diatomic molecule H2. The mass of an exhaust hydrogen molecule is Me = 2 x M(lH) = 2(1.0078 u)(1.661 X 10-27 kgfu) = 3.347 X 10-27 kg. From page 342, the specific impulse is Isp =v~ = ~ = 5326 m/s = 5.3 km/s. July 24, 2002...
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