For many years it was assumed that void swelling would not be an issue for the 304 and 316 stainless components comprising the internals of power — producing light water-cooled reactors. Such a conclu­sion was easily accepted for boiling water reactors since steels used in the shroud assembly are separated from the core by a substantial water gap and therefore experience less than 5 dpa over a 40-year lifetime. For pressurized water reactors, however, the steel is much closer to the core and some regions can reach 80-100 dpa over 40 years. Swelling was still not thought to be a problem because swelling was per­ceived to inhabit a temperature range that did not extend down to the 280-290 °C inlet temperatures of PWRs, and based on most fast reactor irradiations, swelling was thought to vanish below 345 °C, the max­imum water temperature in PWRs. It was also thought that the lower dpa rates characteristic of PWRs would reduce vacancy supersaturations and would therefore inhibit void nucleation.

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Figure 57 Voids observed in Tihange baffle-former bolt made with cold-worked 316 stainless steel after irradiation at ~345°C to 12 dpa. Reproduced from Edwards, D. J.; Simonen, E. P.; Garner, F. A.; Greenwood, L. R.; Oliver,

B. A.; Bruemmer, S. M. J. Nucl. Mater. 2003, 317, 32-45.

Unfortunately, gamma and nuclear heating of thick plates can raise the internal temperatures in some areas of the baffle-former plates to temperatures above 400 °C, known to be prime territory for void swelling. Also, as seen in the previous section, void nucleation does not dominate the swelling response to decreasing dpa rate. The shortening of the tran­sient regime at lower dpa rates raised the strong possibility that void swelling might indeed occur in PWR internals. Theoretical considerations based on void nucleation also suggested that the temperature regime of swelling might move to lower temperatures with decreasing dpa rates. Therefore an effort was made to find stainless steels irradiated at lower dpa rates and/or lower temperatures.

The first clear example of void swelling in PWRs was found in a cold-worked 316 baffle bolt removed from the Tihange PWR reactor located in Belgium.39 The bolt was removed in response to an ultrasonic indication of cracking under the bolt head.

Although the bolt shown in Figure 57 was con­structed from cold-worked 316 austenitic stainless steel known to be more resistant to the onset of swelling than the annealed AISI 304 plate in which it was embedded, well-faceted voids of easily resolv­able size were clearly observed in three sections removed along the bolt axis. The doses in the bolt were relatively low and the calculated temperatures were also relatively low compared to typical fast reactor observations, but the swelling exceeded expectations based on fast reactor experience. As cold-worked 316 is known to always swell less than
annealed 304 at the same temperature and dpa rate the worrisome inference is that the 304 plate sur­rounding the bolt might be swelling at higher levels.

Significantly, hydrogen was also found to be stored in the microstructure at unexpectedly high levels that increased as void swelling increased along the bolt length.

Subsequently, voids were observed in other AISI 316 bolts from this same reactor by other research­ers148 often at even lower doses and temperatures, producing lesser but measurable amounts of swelling. An example is shown in Figure 58, but it should be noted that there appear to be two populations of cavities, a few that are recognizable as voids and an exceptionally high population of nanometer-sized cavities that are only visible using a large level of defocusing, similar to the behavior shown earlier in Figure 17.

Voids have been sometimes but not always observed in bolts of various steels removed from US PWRs.149,150 These studies were conducted before the need for defocusing was recognized, however. Small cavities that could be either voids or bubbles have also been observed in thin-walled flux thimble tubes removed from various PWRs.76,151-153 Neus- troev and coworkers also found voids in a thimble tube removed from a VVER operating in the Ukraine, noting that voids were observed at unexpectedly low temperatures and dpa levels.154

The potential for void swelling at PWR-relevant dpa rates and temperatures is best demonstrated in more comprehensive studies conducted in four USSR sodium-cooled fast reactors located in Russia and Kazakhstan. Whereas the inlet temperature of most Western or Asian fast reactors was of the order of 365-375 °C, the Soviet BOR-60 and BN-350 fast reactors had inlet temperatures of the order of 270-280°C. Components from regions below the core or in the reflector region have been extracted for study at dpa rates and temperatures that were

comparable to those of PWRs.155-161

A summary paper containing an overview of these studies shows that in all studies conducted on compo­nents removed from low flux positions in Soviet fast reactors, certain recurrent trends were observed.155 First, whenever the dpa rate was significantly lower at any investigated temperature, swelling was observed at surprisingly very low dpa levels. An excellent exam­ple is shown in Figure 59 where significant void swelling was observed at only 0.64 dpa at 350 °C.156 Second, whenever a comparison could be made within one reactor at a given temperature, the transient dura­tion decreased with lower dpa rate.157-159 Most impor­tantly, whenever temperatures approaching 280 °C could be reached, swelling was observed not only at these low temperatures but also at surprisingly low dpa levels.160,161 Other examples are shown in Figures 60 and 61.