Fission products gaseous, activity

Table 2.8 shows the relative activities of fission products, and plutonium, in air filters. About a quarter of the 1311 reaching western Europe was particulate, the remainder gaseous or desorbable from particles during collection. Except where stated, the results quoted in Table 2.8 refer to measurements with sampling packs designed to trap inorganic and organic vapour as well as particulate iodine. 

If fission products are released by faulty fuel elements, the gaseous ones appear most rapidly in the cooling water Xe, Xe, Kr, etc. The noble gases can be delayed in sand-filled hold-up tanks (permitting shortlived activity to decay) and the remaining... 

A containment gaseous radiation monitor is provided to measure the gamma radioactivity levels in the containment atmosphere by continuous sampling. Leakage is detected by this method, and to the extent practicable quantified, with a response time dependent on various factors such as the fraction of failed fuel, the fission product inventory in the core, and time of transit from the origin of the leak to the monitor. The activity is indicated in the Control Room by the DPS and averaged hourly. 

The Mark 111 primary containment consists of drywell which is a cylindrical, reinforced concrete structure with a removable head and suppression pool. The drywell and wetwell are connected via the weir wall and the horizontal vents. The suppression pool contains a large volume of water for rapidly condensing steam directed to it. A leak-tight, cylindrical, steel containment vessel surrounds the drywell and the suppression pool to prevent gaseous and particulate fission products from escaping to the enviromnent. The containment system has water spray system at top of the drywell that activates when the containment pressure increases from a set point. 

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). 

The half-lives and methods of production of some of the more important fission or activation products are given in Table 12.4. For the BWR, the main gaseous or volatile species include the isotopes of the noble gas fission products krypton and xenon, the isotopes of iodine, the nitrogen isotopes... 

If the fuel assembly cladding is leak-tight according to the results of fuel leak testing, then the leakage of isotopes is very small. Using conservative assumptions, 10% is a permissible leak of activity from the fuel in the case of gaseous fission products [222,224]. 

The recommended release rates for determining the activities of gaseous fission products inside the transport container but outside of the fuel rods are ... 

Based on this, it is possible to establish the releasable activities of gaseous fission products from the 30 or 48 fuel assemblies. 

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