22. A small prototype fast breeder reactor operates at a total power of 200 megawatts at an average power density of 600 watts/cm The core composition in volume percent is as follows: 239PU, 10%; 238U, 20%; iron, 25%; sodium, 45%. The core is surrounded by a blanket 50 in thick of the following composition: 238 U, 45%; iron, 20%; sodium, 35%. Estimate the thickness of a concrete shield surrounding the reactor that reduces the fast neutron dose equivalent rate to 1 mrem/hr. [Note: Take v = 3 and assume oR of plutonium is the same as that of uranium.]