Fuel fission product escape rates

The release of fission products to the plant environs d.U require a sequence of events In which (l) some portion of the fuel Is over-heated/ releasing fission products (either as a result of Inadequate cooling or excessive heat generation rate), (2) the primary loop Is ruptured to admit fission products Into the building spaces and (3) the fission products escape from the building to the atmosphere. 

This paper deals mainly with the condensation of trace concentrations of radioactive vapor onto spherical particles of a substrate. For this situation the relation between the engineering approach, the molecular approach, and the fluid-dynamic approach are illustrated for several different cases of rate limitation. From these considerations criteria are derived for the use of basic physical and chemical parameters to predict the rate-controlling step or steps. Finally, the effect of changing temperature is considered and the groundwork is thereby laid for a kinetic approach to predicting fallout formation. The relation of these approaches to the escape of fission products from reactor fuel and to the deposition of radon and thoron daughters on dust particles in a uranium mine is indicated. 

The escape rate coefllcients of the iodine and noble gas isotopes identified in one PWR or BWR plant can be directly applied to other plants of the same type, provided that the essential conditions, for example, fuel rod linear heat ratings, are comparable or can be corrected for. By this means it became possible to evaluate the number of failed fuel rods in the core of an operating reactor on the basis of the fission product activity concentrations measured in the coolant early attempts in this area were reported by Schuster et al. (1977). Although these estimates were based only on empirical data, they permitted a rather trustworthy prediction, as can be seen from Fig. 4.6., where the predicted numbers of failed fuel rods are compared with those detected in the course of post-cycle examinations. These techniques have been considerably improved, on the basis of experimental results as well as of model development and calculations. Because of the great number of parameters influencing the escape of fission products from defective fuel rods, evaluation of the number and type of defects from the measured activity concentration of fission products in the primary coolant is a difficult task which can be performed reliably only by specialists with considerable experience in this field. Attempts were, therefore, imdertaken to develop computerized expert systems that could be applied routinely. Lewis et al. (1992) described the development of such a system by means of which information can be obtained on the number of defects and their... 

The maximum level of the activity concentrations in the coolant is probably reached at the moment when the gap of the failed fuel rod is filled with water and when there is no further movement of the liquid front and no convection within the liquid phase. After this point, additional fission products may reach the leak position and escape to the coolant by diffusion in the liquid phase only, which within the gap is probably a comparatively slow process and does not cause a significant further increase of the activity concentrations in the coolant, which are already high at this moment. Therefore, the activity concentrations in the coolant begin to decrease at a rate which corresponds to that effected by the action of the purification system. Following a reduction in the coolant pressure, however, an additional fraction of the liquid phase can be transported from inside the rod to the coolant by the action of temporary pressure differences, leading to the formation of the secondary depressurization spikes as shown in Fig. 4.9. When the reactor is started up again after the shutdown period, water which still remained in the gap of defective fuel rods, containing dissolved fission products, is transported out to the coolant, forced by the increase in temperature of the fuel pellets. 

Despite the large fission product and actinide inventory of the fuel, the leakage rate from the fuel pins may be kept to a very small value by adequate design and careful quality control in manufacture. The coolant circuits of reactors which are refueled on-load are particularly easy to maintain at a low level of activity, on account of the ease with which defective elements can be removed. In addition to any small escape of fission products from cracks or pinholes in the cladding, some activity will generally arise from slight contamination of the outsides of the fuel pins with fuel while they are being loaded. 

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