Fuel pins, fast reactor failure

I0.6.8.I Cladding failure in oxide fuel pins of nuclear reactors. The long-term operational performance of nuclear fuel pins is critically governed by the reactions that occur in the gap between the fuel and its cladding. Ball et al. (1989) examined this for the cases of (1) Zircaloy-clad pellets of U02+, in a pressurised water reactor (PWR) and (2) stainless-steel-clad pellets of (U, P)02+, in a liquid-metal-cooled fast-breeder reactor (LMFBR). In particular they were interested in the influence of O potential on Cs, I, Te and Mo and the effects of irradiation on the gaseous species within the fuel-clad gaps. 

The experimental fast reactor JOYO at the Japan Nuclear Cycle Development Institute s Oarai Engineering Center attained initial criticality in April 1977 and was the first liquid metal cooled fast reactor in Japan. From 1983 to 2000, JOYO operated with the MK-II core as an irradiation test bed to develop the fuels and materials for future Japanese fast reactors. Thirty-five duty cycle operations and thirteen special tests with the MK-II core were completed by June 2000 without any fuel pin failures or serious plant trouble. The reactor is currently being upgraded to the MK-III core. This paper provides a review of the operational experiences obtained through the JOYO s operation. 

The period 1985 - 1998 has seen substantial technical advances. Chief among these has been the demonstration of reliable operation by the BN-600 plant, and the reliable operation of fuel at high bumup. In PFR large numbers of mixed-oxide fuel pins reached more than 15 % bumup without failure, and several reached 20 % bumup with an irradiation dose in excess of 130 displacements per atom (dpa) in the cladding. These results have been confirmed and surpassed by irradiations in Phenix to more than 160 dpa. The fuel cycle, based on mixed oxide fuel and PUREX reprocessing, has been closed in that plutonium fi om irradiated fuel has been separated, fabricated into new fuel and recycled to the reactor for further use. In the USA bumup of up to 20 % has been achieved in ternary alloy U - Pu - Zr metal fuel. The basic technology of the use of fast reactors to breed and recycle plutonium in a commercially acceptable manner has thus been demonstrated. 

To summarize, the suitability of UO2 as a reactor fuel for both thermal and fast reactors has been confirmed by a wealth of experience, and the failure rate of fuel pins can be made acceptably low (down to about 1 in 10" ) by careful quality control in manufacturing. Particular problems which have arisen, such as fuel densification and damage due to pellet-clad interaction, have been solved by adjustment of the initial fuel density and pin gas pressure, and by limiting the rate at which large power increases are allowed to occur. Alternatives to the oxide fuel, such as uranium carbide and uranium nitride, have been investigated less extensively, but hold promise of superior performance in fast reactor systems. 

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