Less than a month before a tube broke at California's San Onofre nuclear power plant, prompting Southern California Edison to take the plant offline, engineers wrote in a trade magazine about a series of modifications they had made to the plant's generators that they said would improve the plant's dependability and reduce tube wear.

The article, published by top-level Edison engineer Boguslaw Olech and Mitsubishi Heavy Industries engineer Tomoyuki Inoue in Nuclear Engineering International, gives a detailed account of what the company was trying to accomplish with the new generators. But the prime question remains: How did a design intended to reduce tube wear end up increasing it, in some cases rapidly?

In the article, the engineers describe the history of the original generators, which were removed from the plant more than a decade before the end of their expected 40-year life span, in part because of excessive tube wear.

The new generators, the engineers wrote, "include all possible improvements introduced by the industry into the steam generator design and fabrication" and were designed in part based on the experience at San Onofre, where tube wear had been a big problem.

The new generators had to be designed in a way that would require only minor modifications in the rest of the plant, but would also meet a federal test to qualify as "in-kind" -- essentially identical -- replacements, allowing them to be installed without approval from federal regulators, the engineers wrote. The article outlines alterations that were made to meet the benchmarks, including using a different tube alloy in the generators, which was believed to be more resistant to cracking, and designing V-shaped tube supports to cut down on tube wear and vibration (AP/Washington Post, May 13). -- AS