Activation products of the coolant, its additives and impurities

The activation products of the coolant, with the sole exception of N, are not of substantial importance in plant operation in some cases, however, they have to be taken into consideration environmentally following release of off-gas or waste water. The fission products and the fuel activation products represent by far the greatest proportion of the radionuclide inventory in the reactor, from the viewpoint of radioactivity as well as from that of radiotoxicity. However, with the exception of severe accidents (which will be treated in Part C), during plant operation they are reliably confined within the fuel rods, so that only the very small amounts released from failed rods to the primary coolant are of interest in this context. Finally, the activated corrosion products are the origin of the buildup of radiation dose rates at the surfaces of the circuits, which potentially complicate the performance of operational procedures, in particular of inspection and repair work. 

In the following sections, the production mechanisms of these radionuclides will be discussed, as well as their chemical and transport behavior in the plant during normal operation phases. 

During its circulation in the primary system, each volume element of the coolant is exposed for approximately 10% of the recirculation time (i. e. for about 2 seconds, followed by about 18 seconds residence time outside the reactor core) to the neutron field inside the reactor pressure vessel. During this time interval, a number of nuclear reactions can take place, the most important of which are listed in Table 4.1. 

When a nitrogen atom freshly produced by a nuclear reaction is broken away from an H2O molecule, the thermalized nitrogen atom may react with various radicals, ions and molecules in the immediately surrounding area of the aqueous phase. In a high radiation field, such as exists in the core of a light-water cooled reactor, the principal reaction partners will be the products of water radiolysis. In earlier laboratory studies (Schleiffer and Adloff, 1964) the following chemical forms and distribution of N produced in pure water were identified  

A schematic presentation of chemical reactions and steam transport of nitrogen species in a BWR primary system is shown in Fig. 4.1., demonstrating that the yields of the different products depend on the ambient conditions, i. e. on the redox potential of the aqueous system. Earlier measurements in a BWR with forward- 

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