1,3 and 5

1. The ASTM Standard lists arsenic and chromium as elements that form nonvolatile and volatile fluorides. Is this an error? Explain. 3-14 CHAPTER 3. REFINING AND CONVERSION 2. Certain contaminants are called "neutron poisons" because they tend to absorb thermal neutrons. These include boron, cadmium, dysprosium, europium, gadolinium, and samarium. Neutron poisons reduce the reactivity of nuclear fuel. The relative probability of absorption is expressed in terms of cross sections, measured in barns. ( 1 barn =1024cm2.) Look up the thermal-neutron absorption crosssections for each of the following: (a) boron; (b) cadmium; (c) dysprosium; (d) europium; (e) gadolinium; (f) samarium; (g) silver. How do these cross-sections compare with that of uranium-235? 3. Older specifications for UF6 required that the concentration of neutron poisons be kept at manageable levels. In the 1970s, for instance, the Department of Energy required that the total thermal-neutron absorption of all neutron poisons not exceed the equivalent of 8ppm boron. Determine the concentration (in ppm or g/kgU ) equivalent to 8 ppm boron: for each of the following: (a) boron; (b) cadmium; (c) dysprosium; (d) europium; (e) gadolinium; (f) samarium; (g) silver. 4. Where are uranium purification and conversion processes performed in North America? 5. Which countries are the leading suppliers of uranium hexafluoride worldwide