Here is a link for ordering the American Water Works Assn. D100-96.
http://www.awwa.org/bookstore/product.cfm?id=44100
I'm shooting from the hip here but I recall the RT requirements as follows:
Primary stress joints require 1 radiograph in the first 10 feet of weld by each welder then 1 radiograph in each 100 feet (or fraction) of weld without regard to the number of welders. 5 percent of the junctions, with a minimum of 2 per tank are to be radiographed.
Secondary stress joints require 1 radiograph in the first 10 feet of completed weld without regard to the number of welders then 1 radiograph in each additional 200 feet (or fraction) of weld.
Rejected radiographs require tracer radiographs to determine the limits of unacceptable welding.
Don't forget to review the procedure qualification(s) and the qualifications of welders. These follow ASME IX guidelines. By the way, the welder qualifications are contractor specific.