Source strength, fission products from

The level of the activity concentration of a specific fission product in the coolant is determined by the strength of the source feeding to the coolant, on the one hand, and by the extent of its removal from the coolant on the other. For soluble fission products the main mechanisms of removal are radioactive decay, retention in the purification system and, in BWR reactor water, carry-over with the main steam. Under steady-state conditions, i. e. at an activity concentration level which is constant in time, the rates of supply to the coolant and of removal from the coolant must be identical. For a PWR, therefore, the rate of supply, the so-called source strength or release rate R (in atoms per second), under steady-state conditions can be calculated from the measured activity concentration A in the primary coolant according to... 

In BWR plants with forward-pumped heater drains, however, a large fraction of the iodine and other non-gaseous fission products carried by the primary steam is directly returned to the reactor water without passing the condensate polishing system. When calculating source strengths from the activity concentrations in the reactor water, the fission product input with the feedwater has to be taken into account as an additional source for the reactor water (Lin, 1983). 

With regard to long-lived radionuclides that show a low decontamination factor on the purification system (e. g. Cs in PWR primary coolant containing LiOH), the purification constant e also has to include the coolant losses via water exchange or leakages. On the other hand, fission products which form insoluble compounds or can be adsorbed onto non-dissolved corrosion product particulate matter may be removed from the coolant by plate-out onto the primary circuit surfaces. These and other parameters which are liable to affect the activity concentrations of the radionuclides in the primary coolant are the reason why a trustworthy calculation of source strengths can only be made using specific radionuclides such as fission product noble gas isotopes and iodine isotopes. 

On the other hand, when the fission products are generated by nuclear fission of finely dispersed or dissolved fuel traces in the coolant, they will enter the liquid phase instantaneously by recoil this mechanism applies to all cases when the dimensions of the fuel particle are comparable to the recoil range of the fission product. Under such conditions, the source strength R is identical to the production rate B and the ratios R B of different fission product nuclides do not depend on their halflives or decay constants. The same mechanism applies when surface contamination of fuel rods with fuel traces originating from the fabrication... 

According to Hiittig et al. (1990), the amount of uranium released from defective fuel rods and deposited on in-core surfaces can be assessed from the coolant activity levels of various short-lived fission products such as I, I, Cs, calculating their source strengths under the assumption of a direct and instantaneous release to the coolant. Though the releases of these isotopes from failed fuel rods are quite small (due to their short halflives), such data can provide only an upper limit for the uranium contamination if there are simultaneously fuel rod failures in... 

The mechanisms just discussed do not explain the striking shift of the iodine isotope ratio observed in the course of the spiking. As was mentioned above, the source strength ratio I changes from 3 to 4 during constant-load operation to 10 to 15 near the spike maximum the other, shorter-lived iodine isotopes show a depletion which is even more pronounced. This ageing of the fission product mixture which is released from the failed fuel rod can be basically explained by the following ... 

The major cesium isotope in the coolant (like in the fuel) is Cs, accompanied by Cs, which is produced from the stable fission product Cs by neutron capture (thermal neutron cross section 3 10 cm ) therefore, this radionuclide is of greater significance the higher the bumup level of the failed fuel rod. In addition to these two main isotopes, the very long-lived pure P" emitter Cs appears in the coolant as well as produced from it by neutron capture (thermal neutron cross section 8 10" cm ) their activity concentrations in the coolant also depend on the bumup of the defective fuel rods. Finally, the short-lived Cs has to be mentioned from the dependence of its source strength on the decay constant (see Fig. 4.3.) it can be concluded that this isotope is not released itself from the fuel rod, but that it is generated in the coolant by decay of its precursor Xe. Cesium isotopes with mass numbers beyond 138 are usually not detected in the coolant apparently, because of their short halflives they decay completely on the way from the fuel rod to the coolant. 

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