Material considerations neutron absorbing materials

Table 15.2 Some characteristics of the Generation IV projects to be taken into consideration for the choice of the neutron absorber materials... 

A first approximation to Eq. (10.109) may be obtained by introducing the condition, of considerable practical importance, that the resonances in the fuel (or absorber) material are narrow on the energy scale relative to the average energy loss suffered by a neutron in a scattering collision with a moderator atom and, furthermore, are widely separated. In this case we can use the asymptotic expression for the function F which appears in the collision integral for the moderator material and note that it is also consistent to take over the interval of integration. From the analysis of Sec. 4.4b we have q u) = (2i(u) (ti), and it follows that for a normalized source... 

Secondly, and probably equally importantly, it must not be a neutron absorber, which limits the choice of material very considerably. Thirdly, and particularly in a gas-cooled reactor, it must be able to withstand very high temperatures. Fourthly, it should not be affected by intense neutron bombardment. Fifthly, there must be good heat transfer between the fuel, the container, and the coolant which flows through the reactor. Sixthly, it must be chemically compatible with the fuel and the coolant — in... 

Another unique consideration is the prevention of nuclear criticality within the cells. In the dissolver and first cycle, criticality is prevented by the presence of the uranium-238, which absorbs neutrons. Later in the process, the plutonium is separated from the uranium. Criticality is prevented by proper design of the vessels and piping. This includes the cell floor and sumps, where materials would collect in case of leakage from the equipment. To prevent criticality, the vessels are limited in either diameter or thickness. Vessels and piping are placed in arrangements designed to avoid a critical array. 

The cubical lattice that the Columbia football squad stacked in Scher-merhom Hall in September 1941 extrapolated to a disappointing first k of 0.87. Now that is by 0.13 less than one, Fermi comments—13 percent less than the minimum necessary to make a chain reaction go— and it was bad. However, at the moment we had a firm point to start from, and we had essentially to see whether we corild squeeze the extra 0.13 or preferably a little bit more. The cans were made of iron, and iron absorbs neutrons. So, out go the cans. Cubes of uranium were less efficient than spheres next time the Columbia group would press the oxide into small rounded lumps. The materials were impure. So, now, what do these impurities do —clearly they can do only harm. Maybe they make harm to the tune of B percent. Szilard would continue his quest for materials of higher purity. There was some considerable gain to be made. .. there. ... 

Shielding. Any material or obstruction, including terrain, that absorbs radiation and thus tends to protect personnel from the effects of atomic explosion. A moderately thick layer of any opaque material will provide satisfactory shielding from thermal radiation, but a considerable thickness of high-density material—e.g., lead—may be needed for protection from nuclear radiation. Concrete and water absorb the energy of gamma rays and neutrons. 

One system would use pellets of uranium dioxide in stainless steel cans, whereas the other system would use uranium metal and zirconium as the cladding material. Zirconium was the preferred choice as stainless steel absorbed too many neutrons. Testing fuel elements was a lengthy process They had to be designed, fabricated, set up within a channel of a reactor, irradiated for a considerable period of time and then removed. More time had to be given for the fission products to decay, after which the fuel rods could be examined. Depending on availability of reactor space and other factors, this whole sequence could take up to 15 months. Eurthermore, the fuel elements for a submarine reactor would have to be extremely reliable — far more reliable than for a land-based reactor since it would be impossible to remove defective fuel elements whilst at sea. 

---