PROGRESS REPORT ON FUEL ELEMENT DEVELOPMENT AND ASSOCIATED PROJECTS (1960)

; 9 < 4 6 9 7 ; 6 8 7 6 sting Deactor (MTR) has sought to develop improved, economical, long-life fuel assemblies through a comprehensive study of various fuel compositions, enrichments, claddings, burnable poisons, fuel and poison distributions, and fuelelement geometry optimization. The core materials, including uranium -- aluminum alloys, uranium oxide -aluminum cermets, thorium, thorium oxide, boron, gadolinium, dysprosium, and iridium, are tested in pilot-plant scale by irradiating them as sandwich type sample fuel plates. In the procurement of these sample plates, fabrication techniques were developed and evaluated for incorporation of all the fuels and poisons (except Ir/sub 2/O/sub 3/) into cores of aluminum or aluminum alloys. Methods were developed to minimize "dog-boning" and to produce graded fuels. Some of the sample plate compcsitions have been irradiated to high burn-up, i.e., over 50% of the U/sup 235/ content, and have operated successfully in the MTR for seven or more cycles. The irradiated uranium-- aluminum alloy and uranium oxide-- aluminum cermet fuel plates have shown excellent dimensional stability and good corrosion resistance to long-term irradiation. However, some of the thorium oxide fuel plates failed during one cycle of irradiation because of blistering, rupturing, or forming of pinholes in the cladding. The isostatic bonding procedure used to bond aluminum plates to the ThO/sub 2/ cores is apparently not adequate for reactor use. The sample fuel plate work has demonstrated the suitability of high wt.% uranium oxide--aluminum fuels for testing reactors, indicated the potential of systematically varying the fuel loading within a single plate, and experimentally verified the applicability of burnable poisons for reducing reactivity changes resulting from fuel burnup. The Deactivity Measurement Facility has proved to be an excellent nondestructive analytical tool for determination of fuel and poison burn-up. This program has stimulated several new developments and revealed many interesting facts in the fabrication and testing of reactor fuel materials. For example: (1) ultrasonic inspection has proved to be an excellent nondestructive method for determination of small voids in the core and unbonded cladding not otherwise detected by radiographing, (2) the ultrasonic inspection of irradiated fuel plates in the MTR canal is feasible, and (3) analytical procedures were developed for the determination of the small quantities of gadolinium added to the cores. The prototype studies consisted of theoretical and experimental evaluations of the hydraulic and heat- transfer characteristics, the structural properties, the economics and the reactor operating characteristics of various full-sized fuel assemblies and shim rods. The results of the sample fuel plate studies were incorporated in these prototypes to obtain optimum practical designs for testing reactors. The fuel element geometries investigated include plates, tube bundles, hexagonal honeycomb, and concentric cylinders. A MTR shim rod with renewable fuel and poison sections was designed, tested hydraulically, and is now considered ready for final in-pile testing. This rod outlasts the existing shim rods, is cheaper, and allows more operational flexibility. A theoretical analysis, hydraulic tests, and a mechanical evaluation have shown that an improvement can be made in plate type fuel elements by using an increased number of thinner high-strength fuel plates in the fuel element. An in-pile prototype test of such an element is now planned. An analysis of roughened surfaces indicates that economy or increases in reactor power may be gained through the use of roughened heat- transfer surfaces in nonboiling watercooled reactors. Preliminary hydraulic tests were performed and indicate that practical roughened surfaces may be formed. Out-of-pile heat-transfer tests are now planned. The theoretical analysis of geometries indicates that tube bundles, honeycomb, and concentric cylinder de