Patent Number: 045004889
Section: summary

BACKGROUND OF THE INVENTION Uranium of different isotopic ratios has long been used in research throughout the world to support the nuclear reactor development program. The uranium may be natural, containing the normal ratio of .sup.235 U and .sup.238 U; enriched, where the .sup.235 U content has been raised; or depleted, where the .sup.235 U content has been reduced. In any case, bare uranium as a solid metal will oxidize when exposed to air. The rate of oxidation depends on several factors, including the degree of humidity in the atmosphere; and the oxide builds up on the surface of the parent metal and is radioactive. The oxide is easily brushed or scraped off the parent metal, so much so that anything coming in contact with the uranium becomes contaminated. This then requires an expensive decontamination process. Finding a suitable protective coating has been a serious and ongoing problem. Many research programs limit the extraneous material that can be introduced into the reactor with the uranium. Hydrogeneous materials are particularly undesirable due to their strong interaction with the neutron flux in the reactor. This precludes the use of most plastics as a coating. Some nonhydrogeneous coatings are available, notably some of the chloro-, fluoro-, bromoethylene polymers. However, this type of coating has a limited useful life (two years or so) whereupon the uranium must be cleaned and recoated. Use and handling can also shorten this life and risk contamination of the area. Often, the uranium fuel is only between 1/64" and 1/8" thick, and otherwise is shaped as rectangular plates from between 1" and 2" wide and 2" and 4" long. Attempts to encapsulate or clad the uranium plate(s) with stainless steel have not been totally successful either, because even when such encapsulating or cladding structures are formed of thin 15-20 mil (0.015-0.020") sheets, they generally introduce excessive extraneous material to adversely affect the sensitivity of the research. Moreover, manufacture of such structures with the narrow thickness-to-width ratios and with thin sheets of stainless steel has been difficult and unreliable, due to thermal warpage and dimensional instability. The problem of uranium contamination is even more burdensome in tests involving both uranium and plutonium. Plutonium is almost always cladded or enclosed in a sealed container. Both uranium and plutonium are radioactive, releasing alpha radiation; however, plutonium is also biologically toxic. Thus, preliminary radiation tests are made to detect "leakers" (where the cladding or enclosing container of the plutonium is imperfect), but such tests cannot distinguish between a plutonium container having exterior contamination induced by rubbing against an oxidized uranium fuel plate and a "leaker" container. It then becomes necessary to double check the suspect plutonium container with more costly and sophisticated tests, and it yet further necessitates the decontamination of the container. SUMMARY OF THE INVENTION This invention relates to an improved stainless steel structure for holding uranium fuel formed as plates as small as 1/64" thick, to an improved method of forming such a structure, and to the combination of the structure having fuel plate(s) encapsulated therein. This encapsulated unit is formed initially as a tube-like housing of very thin (5 mils-0.005") stainless steel sheet material, having an open/through dimension only slightly larger than the uranium fuel plate(s) that will fit inside the housing. End caps are designed to be welded to the housing to close the open ends thereof; one end cap being welded in place after the fuel plate(s) have been loaded into the housing so as to physically confine the fuel plate(s) therein. The end caps are preferably formed of a slightly porous metal having micron size pores, allowing the confined uranium to breath slightly while yet precluding the release of contaminating oxide from the uranium. The technique for fabricating the encapsulating structures involves beam welding two C-shaped channels together along opposite side seams where the channel legs butt against one another. For uniform fixturing of the channels and for proper heat distribution during welding, interior and exterior chill blocks of ground and hardened tool steel are used to sandwich and hold the channels therebetween. The interior chill block is composed of two wedges (1) that fit exactly thicknesswise between the two channel pieces when the ends of the channel legs just butt together, and (2) that can be expanded widthwise to snug up flush against the insides of the channel legs before welding and contracted widthwise to be easily removed from the tube-like housing after welding. A positioning pin is used to lock the widthwise expanded wedges quickly and accurately and with little operator effort. The channel pieces must butt exactly along the opposite side legs or they cannot be welded together. The electron beam weld is actually made in an evacuated chamber or in an inert atmosphere. The parameters such as the feed speed along the seam, beam power, beam focusing, etc., are initially set by trial and error but thereafter are regulated automatically. As practiced now, several complete sets of interior and exterior chill blocks are fixtured as a stack on a table in the welder and the table is automatically moved and indexed to make the seam welds successively on one side only of each tube-like housing. Thereafter, the chill blocks are flipped over as a stack and refixtured, etc., to make seam welds on the opposite side of the housing.