Possible countermeasures against contamination buildup

From the different topics discussed in the preceding sections it can be concluded that there are different possible ways of keeping the buildup of primary circuit contamination low or of reducing already existing contamination levels. Their common aim is to keep the production of radioactive cobalt isotopes low and/or to minimize the transport of the radionuclides produced from the reactor pressure vessel to other regions of the primary circuit and, in addition, to minimize their plate-out on the surfaces there. Because of the importance of low radiation levels for an undisturbed operation and maintenance of the plants, different measures have been attempted with quite varying success. 

Hold down plates upper core support 112 0.24 m2 Fox Antinit Dur 300 

Centering bolts upper core support 4 0.04 m2 Fox Antinit Dur 300 

When considering the applicability of one or of several of the above-mentioned measures, one has to distinguish between two completely different situations. The first is when a new plant is to be designed and constructed the second relates to plants which have already been in operation for a shorter or longer period of time. In the first-mentioned case, the most effective way certainly is the proper selection of the materials to be used, in order to reduce the cobalt inventory of the primary circuit as far as possible in particular, the high-cobalt materials inside the reactor pressure vessel should be dropped in favor of substitute alloys of equal quality. In the second case, on the other hand, replacement of such materials is only rarely feasible (e. g. in the course of a replacement of components), so that other measures have to be taken into account. 

Similarly, the consequences of the replacement of the high-cobalt bearing materials on the °Co activity levels in the oxide surface layers of the primary circuit 

---

As with PWRs, a detailed knowledge of the relative significance of each of the possible sources of radionuclides described above is a basic requirement for taking effective countermeasures against contamination buildup. Unfortunately, the current state of knowledge on the mechanisms of BWR contamination buildup is not yet as complete as in the PWR case (see Section 4.4.3.). In addition, similar to the PWR situation, cobalt transport and deposition in a BWR is a complex chemical reaction which can be affected by many parameters. This is an essential reason for the considerable variation in the radiation dose levels frequently observed between different plants, which often cannot be explained in a simple manner by differences in design and/or operation. 

---