Environmental Engineering Reference
In-Depth Information
the alloy changes are the use of the 690 and 800 alloys to replace the 600
series alloys fi rst used in steam generator tubes and the use of enhanced
alloy steels for steam generator support tubes to replace the low alloy car-
bon steels that were previously used.
However, replacement of failed or deteriorating components is not always
a feasible option and replacement is rarely cheap. For instance, a broken
component such as a coolant injection nozzle deep inside a BWR reactor, or
cracked baffl e bolts inside the PWR reactor, cannot require the replacement
of the vessel. Since these issues are occurring in highly radioactive and some-
times hard to access components, remote methods of repair (such as under-
water laser welding) as well as methods for access (tethered robotic welders)
have been developed. Replacement of steam generators costs $40 million to
$50 million each (and there are two to four in every PWR). Replacement of
reactor pressure vessel heads cost about $20 million each (NEI, 2010).
Finally, there is the design of the components. As those who take
Six-Sigma® courses always learn, the root of most problems is in the initial
engineering. For example BWRs have, for many years, had an issue with
cracking of welds in the steam separator (see Fig. 9.4). While not directly a
safety issue, this problem has led to replacement of the steam dryers, and
continued cracking in the welds has been observed. Recently, modeling tools
that provide a more accurate prediction of the vibration stresses affecting
these welds have been developed and deployed on much more powerful
computers. This capability has allowed better designs for the steam dryers
to be made and installed which, so far at least, have led to elimination of
this issue. Of course, better modeling tools alone cannot accomplish better
designs. Better understanding of materials in the relevant chemistry and
radiation environment of the LWR is needed. This is especially true for the
understanding of environmentally assisted crack growth which presents
the potential for catastrophic failure of key boundary components (NRC,
2008b). This understanding can then be translated into more phenomeno-
logical models for use in modeling components and perhaps even successful
prediction of the response of these components to conditions outside those
which were used to originally develop the models.
Another broader issue is the standards to which pressure vessels are
designed. The current fl eet of reactors was designed to the ASME Boiler
and Pressure Vessel Code, Section III, in effect in the 1970s. This code was
based on laboratory environment testing and not on tests using actual
operating conditions (including radiation) (Majumdar, 2011). In the past
40 years, there has been enough data collected in LWR conditions that illus-
trates a need for changes in these codes, but more importantly, how actual
LWR operating conditions have affected the current fl eet. It is important to
know what changes need to be made in current plants to keep them operat-
ing safely for the next 20, 40, or more years. Since the replacement of these
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