Radiolytic oxidation of graphite

The simplest description of the reaction responsible for the radiolytic oxidation of graphite is ... 

Chemical interaction between fuel and cladding, or between coolant and cladding or structural materials, must be limited to a level giving an acceptably low probability of fuel element or plant component failure. The normal problems of chemical compatibihty are increased by the effect of irradiation in enhancing the rate of chemical attack. The radiolytic oxidation of graphite in a C02-cooled reactor, for example, raises problems of graphite erosion and mass transport to cooler regions of the primary circuit. 

B.T. Kelly, P.A.V. Johnson, P. Schofield, J.E. Brocklehurst, M. Birch, UKAEA northern division studies of the radiolytic oxidation of graphite in carbon dioxide. Carbon N.Y. 21 (1983) 441-449. 

It is well known that for a given weight loss, thermal oxidation of graphite causes a larger reduction in strength and elastic modulus than radiolytic oxidation. Pickup et al. [78] showed the decrement in dynamic elastic modulus, E, due to thermal oxidation fitted an exponential relationship ... 

The radiolytic oxidation of porous reactor moderator graphite in C02 represents another effect induced by neutron irradiation in which the radiation-induced formation of structural defects is proposed to play an important role [245]. It has, however, been demonstrated that neutron irradiation also affects the electronic properties of various carbons [246] and one interpretation placed on such effects was that a dose of 1 neutron per cm2 created four electronic holes in the valence band per cm3. The... 

Radiolytic oxidation alters most of the important properties of graphite, including strength, elastic modulus, work of fracture, thermal conductivity, permeability, and diffusivity but does not affect the thermal expansion coefficient or Poisson s ratio. The effects of radiolytic oxidation on the properties of a wide range of graphites have been studied in the U.K. [7,73,74] where it was found that, to a first approximation, they can be described by similar relationships ... 

Radiolytic oxidation is important to the design and operation of reactors because it adversely affects key graphite properties and, by removing moderator material, may bring about the need for increased fuel enrichment. As mentioned earlier, an inhibitor (methane) is added to the coolant to reduce radiolytic oxidation to acceptable levels. However, access of the inhibitor to the inner portions of the moderator brick must be assured. Two approaches have been adopted in the AGRs to provide this access. Vertical methane access holes are provided in the fuel bricks and in the later stations, Heysham II and Tomess, a pressure drop from outside to inside the brick was established to cause an enhanced flow through the brick. The amount of inhibitor added must be restricted, however, because the carbon inhibition reaction product deposits on the fuel pin and restricts heat transfer to the coolant, thus reducing reactor efficiency. 

In support of the development of graphite moderated reactors, an enormous amount of research has been conducted on the effects of neutron irradiation and radiolytic oxidation on the structure and properties of graphites. The essential mechanisms of these phenomena are understood and the years of research have translated into engineering codes and design practices for the safe design, construction and operation of gas-cooled reactors. 

Kelly, B.T., The effect of radiolytic oxidation on the graphite moderator brick strength in advanced gas-cooled reactors, Nucl. Energy, 1984, 24(3), 265 272. 

The authors also draw attention to specific issues related to high-temperature graphite moderated reactors such as thermal oxidation in the event of an accident. Oxidation represents the loss of graphite mass, which if significant could limit its ability to moderate the fast neutrons. Thermal oxidation is not a problem in the current reactors such as the AGRs. However, current reactors suffer firom a different type of oxidation, namely radiolytic oxidation. This type of oxidation is not relevant to Generation IV, because the proposed cooling composition is inert. 

In-pile studies. Radiolytic-gas production and recombination rates were determined in the ORNL Graphite Reactor using a slurry of Th02 containing approximately 2.8% uranium which w as approximately 93% enriched in The mixed oxide was prepared by coprecipitation... 

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