A Defense Department official has blamed an “incomplete” initial cost estimate last year by the Energy Department’s nuclear security organization for new revelations that a program to revamp the B-61 nuclear gravity bomb has more than doubled in price.

The effort to extend the service life of hundreds of the aircraft-launched weapons will cost $10 billion, in stark contrast to a $4 billion estimate the National Nuclear Security Administration, a semiautonomous Energy arm, had first projected.

“The earlier [figure] was a rough estimate based on incomplete knowledge,” said the senior Defense official, who was not authorized to address the matter publicly and demanded anonymity for this article.

Many program details are secret, including how many of the modernized versions of the weapon -- dubbed the B-61-12 -- will ultimately emerge from the life-extension process. Nuclear weapons expert Hans Kristensen speculates that roughly 400 will be produced -- suggesting the per-warhead cost, including a new guidance system, would be the most expensive in history at $28 million.

Benjamin Loehrke, a senior policy analyst at the Ploughshares Fund, in a Monday Twitter post and blog quipped that each of the warheads would cost only $16.5 million if instead they were cast in gold.

The Defense official said, though, that the price hike should not be considered cost growth in the effort, because nothing has changed programmatically. It is more a matter of who was doing the estimates and how they did the counting, the official suggested.

“It’s hard to say that you had a cost growth because you didn’t have a good baseline,” according to the insider. “So from now on, yeah, now watch what happens.”

The so-called “Mod 12” update is aimed at replacing a variety of B-61 versions fielded over the years.

Today the U.S. Air Force has in its active and reserve stockpiles roughly 900 B-61s, of which 400 bombs could be launched from strategic bomber aircraft. Fewer than 200 of these are believed fielded on bomber bases and thus available for operations with several days’ notice, Kristensen said on Friday.

Of the 900 or so total, there are roughly 500 tactical versions of the B-61 in active and reserve stockpiles, with 200 or so of them fielded at European bases.

About 35 B-61s, dubbed the “Mod 11” version, were updated within the past decade and will remain in the arsenal without the Mod 12 update, Kristensen said.

The first life-extended B-61 warhead should be available by 2019, according to Thomas D’Agostino, who heads the National Nuclear Security Administration. Funded at $369 million in the current White House budget request, the effort is to begin developmental engineering during the coming fiscal year, which begins on Oct. 1.

The updates are expected to lend the bombs another 20 to 30 years of service life.

An initial Air Force estimate this past spring was that the project would cost $6 billion, a revelation confirmed by the Defense official in May and first reported earlier that month by the Federation of American Scientists.

When Pentagon Cost Assessment and Program Evaluation officials weighed the Air Force estimate, they found it still lacking and tacked on another $4 billion to reach the $10 billion projection. Senator Dianne Feinstein (D-Calif.) on July 25 revealed that the Defense Department had briefed her two days earlier on the new estimate, but Pentagon officials have not publicly released the cost report.

Even the $10 billion figure underestimates total B-61 modernization costs, though, because the math does not include a key feature being installed on the new delivery systems.

“In addition to the [life extension] itself comes a new guided tail kit assembly that the Air Force is developing to increase the accuracy of the B-61,” Kristensen, who directs the FAS Nuclear Information Project, said in a recent blog post. “The cost estimate for that tail kit has recently increased by 50 percent from $800 million to $1.2 billion.”

When NNSA program officials tallied their initial estimate, the B-61 modernization effort was in an early programmatic stage called “concept exploration,” the Defense official told Global Security Newswire.

At that time, it was uncertain which of five notional program plans -- some of them technically ambitious, some less so -- the nuclear security agency would pursue for the B-61 bomb update, the official said. It was only after an option enigmatically labeled “3B” was selected that cost details and assumptions were shared with the Pentagon and given closer scrutiny, according to this source.

Once Defense Department program analysts got a hold of the more detailed information they realized that the effort could prove more expensive than the earlier NNSA projection had suggested.

“They went, ‘Oh, that’s really expensive. Oh, that’s really expensive, damn,’” the senior official said in an interview.

The reappraisal identified some faulty assumptions in the NNSA pricing, one of which revolved around how much it would cost to update the old electronics found in today’s B-61 warheads, the official said.

“There was a thought we could reuse” some of the aging systems, the official said. “We’re not going to be able to reuse as much as we thought. … They’ve corroded, they’re aged.”

Cost estimates also vary because various government agencies approach pricing differently, another Defense official said on Capitol Hill this week.

“The way DOD does business and the way NNSA does business is different,” Steve Henry, deputy assistant Defense secretary for nuclear matters, said at a Wednesday breakfast event. He noted that most of the facilities in the U.S. nuclear complex are government-owned but contractor-operated, whereas the military typically operates on bases overseen by government officials and uniformed officers.

Henry said Defense and NNSA officials continue to work together to pin down the cost projections.

Asked if he was familiar with the $10 billion estimate, Henry said, “I’ve heard ranges of everything. … We don’t know what the answer is, as of now.”

An NNSA spokesman on Friday agreed there remains some pricing uncertainty, but declined to offer specifics.

"It is too early in the process to speculate on any final cost changes or schedule impacts," said Josh McConaha, "and we will not comment on numbers or dates cited in any review until the required engineering work has been completed."

Some of the expense in modernizing aging components on the B-61 is associated with optional upgrades to the system, the first Defense official conceded in the May interview.

“We are always charged by national directive, by the president, to improve the safety and security of our nuclear stockpile, as a commitment we have to the American public,” the official said. “These are the most powerful weapons the world has ever known, and it is our duty to make sure that they’re safe, secure and effective. So if we know how to do something better, why wouldn’t we do it?”

Kristensen said safety features are important to avoid accidental or unauthorized detonations, but alleged that program officials have gone overboard. He would prefer to see a cheaper update to the warheads that mainly fixes or replaces ill-functioning or corroded parts.

The B-61 life-extension technology plan “includes new use-control and safety features to increase the surety of what is already the most safe warhead design in the stockpile,” he said.

“Several warhead design options were proposed, ranging from a simple life-extension with the current features to a significantly altered design with new optical wiring and multipoint safety,” Kristensen said. “The Nuclear Weapons Council in December chose the second-most ambitious design without optical wiring and multipoint safety.”

The Pentagon official responded that the Defense and Energy departments would be wrong not to build in the latest safety and security features afforded by today’s electronics.