Fission product noble gases

A summary description of fission gas behavior under the conditions of light water reactor nuclear power plant operation was given in review papers by Ass- 

In annealing experiments of irradiated UO2 fuel specimens, the release of fission gases shows a complex behavior, starting with a steep gradient (high release rate) and approaching an almost constant level with low release rates. When the temper- 

The fission gas bubbles present at the grain boundaries can be liberated by mechanical impact. Ruhmann et al. (1987) reported that by impact testing of typical LWR fuel pellets (irradiated to a burnup of 33 MWd/kg U) at ambient temperature with a specific load of about 1000 N m/g, 6 to 7% of the total Kr inventory of the pellet was released. Since these investigations were directed to the study of fission gas release from fuel caused by accidental mechanical impact, the analytical technique used did not aim at a quantitative liberation of the gases present at the grain boundaries. For this reason, the reported values are assumed to represent only a small fraction of the total grain boundary inventory. 

Due to the dependence of fission gas release on the fuel temperature, the advanced fuel assembly configurations (PWR 18 x 18, BWR 9x9 arrays) show only low fission gas release to the plenum when operated at correspondingly low heat ratings, as compared to the older 16 x 16 or 8 X 8 assembly configurations. 

High-burnup fuel pellets are also depleted in fission gases within the fuel grains at the p ellet rim zone. From the investigations it was concluded that the missing fraction was accumulated in gas bubbles in the UO2 matrix, but that no measurable release from these zones into the free volume of the fuel rod occurred even at high burnup levels. 

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Admissible condition means the presence of microcracks in claddings through which gaseous and volatile fission products (noble gases, iodine isotopes) are released to coolant. 

The gases released from the primary coolant in the degasification system mainly contain the fission product noble gases which, with the sole exception of Kr, are comparatively short-lived nuclides. In order to prevent release to the environment, therefore, it is sufficient to store them for a certain time until these isotopes have decayed. In most of the US PWR plants as well as in the plants built by Frama-tome, gas decay tanks are used for this purpose. In the plants designed and built by Siemens/KWU, decay lines are employed which are equipped with a series of charcoal beds in which the noble gases are delayed relative to the carrier gas flow by a dynamic adsorption-desorption equilibrium. Under normal operation conditions, delay times on the order of 60 hours for the krypton isotopes and 60 days for the xenon isotopes are obtained, which are sufficiently long for nearly complete... 

Fission product noble gases and other volatile fission products. 

As can be concluded from this figure, the apparent ionic radii of La ", the tervalent lanthanides and Ce" " are almost identical to that of The radii of most of the other ions and also of the atoms of the platinum metals are within a range of about 30 M> around the calculated value of the lattice vacancy position thus, one can expect that their incorporation into the lattice will be possible without major difficulties. The same apphes for both the neutral atoms and the tetrava-lent ions of molybdenum and technetium, which means that the question of lattice compatibiUty will give no preference to one of the two valency states. On the other hand, the atomic radii of the fission product noble gases krypton and xenon are... 

Summarizing these crystal chemistry considerations one can conclude that the fission products firmly fixed in the fuel are those with higher atomic charges and of dimensions compatible to those of the matrix lattice. Neutral atoms or monovalent fission products, both of which having larger dimensions (e. g. fission product noble gases as well as iodine), are more mobile in the fuel, in particular when they... 

The pathway of fission gases from the interior of the fuel pellet to the rod plenum leads through a network of cracks and tunnels which are formed in the course of fuel operation. The comparatively long time needed to cover this distance results in an extensive decay of the shorter-lived isotopes, which means that the gas mixture reaching the plenum is mainly composed of stable krypton and xenon isotopes, of Kr and of a fraction of the initial amount of Xe. Short-lived isotopes of the fission product noble gases which reach the pellet - cladding gap by fission-induced recoil are superimposed upon this mixture. This isotopic composition is of interest in the event of fuel rod failure when fission products are released from the fuel rod to the primary coolant, a topic that will be treated in more detail in Section 4.3.2. 

The differences in behavior between noble gases and iodine isotopes can be clearly seen from Fig. 4.4. (according to Schuster et al., 1981). When the R B values of the fission product noble gases are corrected for the isotopes Xe, "Kr and Kr coincide very well and, simultaneously, show a pronounced dependence on the linear heat rating. In contrast, I and 1 in this kind of presentation are distinctly separated from each other however, when their R B values are corrected for k, the curves of these two isotopes coincide, too. (More recent investi-... 

An alternative choice to the wet sipping techniques described above is the so-called dry sipping technique, for which the fuel assembly is placed into a test chamber filled with air after a short time, the fission product noble gases (in particular Kr and/or Xe) released from the defective fuel rod during heatup are measured in a gas counter. Reliability and detection limit of this technique are virtually identical to that of the wet sipping procedure. 

In a flowing gas atmosphere this adsorption-desorption equilibrium results in a delayed transport of the fission product noble gases compared to that of the carrier gas. The mean delay time tm (i. e. the time span between the entry of a gas volume element into the adsorption bed and the moment at which half of the radioactivity has passed the column exit) can be calculated according to... 

Due to the delay times achieved on the charcoal beds, the shorter-lived fission product noble gases are completely retained within the plant. Kr is the only one of the radioactive fission gases which passes the delay line and is released completely to the atmosphere, where it contributes to the gobal Kr inventory of the atmosphere. As this contribution is very small compared to the remainders of nuclear weapons testing and to the releases from spent fuel reprocessing plants (as far as they are not equipped with a noble gas retention system) it can be ignored. 

Fission product noble gases entering the water-steam circuit in the event of a tube leak are completely volatilized and transported with the steam to the main condenser where they are extracted and released via the off-gas stack. This release is monitored by a continuously operating detector device located in the condenser off-gas line. Non-volatile fission and activation products which are transported over the leak to the water-steam circuit remain completely in the water phase of the steam generator by the action of the blowdown purification system their activity concentration is kept at a level which is controlled by the injection rate on the one hand and by the purification rate on the other. Because of the very low vapor pressures of these elements and their chemical compounds (dissolved ions or insoluble oxides/hydroxides), their transport to the steam under the prevailing conditions (270 °C, 7 MPa) is only possible by droplet entrainment. This means that partitioning between liquid and steam phases is proportional to the steam moisture content, which is usually well below 0.1%. 

In spite of the incompleteness of the measured data, caused by the above-mentioned failures of part of the instrumentation, it was shown that during the burst and refill phases (both phases could not be separately evaluated due to difficulties in instrumentation) the release fractions from the failed fuel rods amounted to 1.3% of the Xe, 0.8% of the and 0.2% of the Cs inventories of the failed rods. For the fission product noble gases, this fraction represented more than 90% of the total amount released over the course of the entire experiment, for iodine about 60% and for cesium about 10% the remainder of these radionuclides was transported to the coolant during the subsequent leaching phase, which in this experiment was conducted for about 12 hours. 

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