WASHINGTON – Resting atop the Trident II missile, the W88 warhead is among the mainstays of the country’s submarine-based nuclear arsenal. For years, however, testing the warhead’s components to ensure the weapon produces the intended blast instead of a fizzle has been complicated by a lack of replacement plutonium triggers.

Last summer, the first replacement plutonium trigger in 18 years received “diamond stamp” approval signaling it was ready for use in a warhead. To scientists at Los Alamos National Laboratory in New Mexico, that was a milestone to celebrate. It meant the warheads, after testing that makes the original trigger unsuitable for reuse, could be reassembled with a new trigger and put back into service.

A watchdog group now is raising questions about whether the replacement triggers, also known as pits, can be guaranteed to be as reliable as those already in some 400 W88 warheads. The original triggers were made with the benefit of underground nuclear testing, which the U.S. halted in 1992, and through a different process than the replacements. The last of the original triggers were manufactured in the late 1980s.

The Project on Government Oversight says it was told by some Los Alamos scientists that the trigger certified last July and known as the W88 pit needed 72 waivers from the specifications used for the original triggers, including 53 engineering-related changes.

“With this large number of waivers, how is it possible to objectively tell whether the pit will even work?” said Danielle Brian, executive director of the group that monitors nuclear weapons-related activities. She posed that question in a letter last Friday to Energy Secretary Samuel Bodman.

The government acknowledges differences between the old triggers and their replacements.

The new ones were made by using a mold to cast the grapefruit-size plutonium sphere. The original triggers, all made at the now-closed Rocky Flats facility in Colorado, were hammered into precise form. This process is viewed by metallurgists as producing a stronger product.

Because the U.S. no longer conducts underground nuclear tests, the Los Alamos scientists had to rely on other sources to replicate the original triggers and guarantee that the replacements would be as reliable as the old.

In a warhead’s detonation, a conventional explosive packaged around the pit compresses the plutonium inward, creating enough pressure for an atomic chain reaction. That, in turn, creates the high temperatures and pressure to ignite a “secondary” nuclear component. The result is a massive hydrogen blast. Any variation or flaw in the pit could cause a warhead not to detonate properly or to detonate with less explosive power than expected.