PWR water - steam circuit

Under normal operating conditions, the water-steam circuit (or secondary circuit) of pressurized water nuclear power reactors is completely free of radionuclides. Activated corrosion products and tritium, which have been reported in very low activity concentrations from the water-steam circuits of some high-temperature reactors and which are caused by the neutron Held reaching into the steam generator or by diffusion through intact steam generator heating tubes, do not appear at the PWR secondary side. 

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 the water—steam circuit, the fission product iodine which is carried by the main steam is distributed between different water and steam flows. A substantial fraction of it is plated out in the cyclone downstream of the high-pressure part of the turbine, and is transported back with the separated condensate to the feedwater storage tank. Similar washdown of iodine occurs at the other locations where condensates are separated from the remaining steam, so that only a small fraction reaches the main condenser. Here as well the major part is plated out to the main condensate water phase and is retained in the ion exchangers of the condensate polishing system the fraction of iodine which passes over to the condenser off-gas represents only a few percent of the amount originally carried by the main steam. 

According to the results reported by Jacob (1978), in a once-through steam generator about 45% of the I activity carried by the main steam is separated in the cyclone downstream of the high-pressure part of the turbine together with the 

The isotopic composition of fission product iodine present in the BWR reactor water in the case of failed fuel rods in the reactor core is quite similar to that in the PWR primary coolant. Since the iodine purification factor of the reactor water cleanup system is on the order of 100, i. e. virtually identical to that of the PWR primary coolant purification system, this similarity in isotopic composition demonstrates that the release mechanisms of iodine isotopes from the failed fuel rods to the water phase are virtually identical under both PWR and BWR operating conditions. On the other hand, the resulting chemical state of fission product iodine in the BWR reactor water is quite different from that in the PWR primary coolant. The BWR reactor water usually does not contain chemical additives (with the possible exception of a hydrogen addition, see below) as a result of water radioly- 

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