The National Nuclear Security Administration (NNSA) can only wait five to seven more years before beginning a potentially costly life-extension program on the megaton-class B83 gravity bomb, a senior agency official said Wednesday in a budget hearing.

Sen. Dianne Feinstein (D-Calif.) coaxed the estimate out of Charles Verdon, deputy administrator for NNSA Defense Programs, during a hearing of the Senate Appropriations energy and water subcommittee.

Feinstien, ranking member of the panel that writes the Senate’s first draft of the annual energy budget bill covering the NNSA, firmly opposes the B83, which on Wednesday she called “dangerous and unnecessary.”

In its 2018 Nuclear Posture Reivew, the Trump administration proposed reversing the Obama administration’s decision to retire the B83 and replace it —  according to estimates from the Washington-based nonprofit Federation of American Scientists — with some 480 B61-12 gravity bombs. The NNSA is creating B61-12 from four separate versions of the old B61 .

According to the Nuclear Posture Review, NNSA will keep B83 in warm storage “at least until there is sufficient confidence in the B61-12 gravity bomb that will be available in 2020.”

The NNSA quantified the expense of this about-face for the first time in the 2020 budget request released in March. The agency wants about $52 million for B83 stockpile systems in 2020. That is about 45 percent more than the 2019 budget.

“What is that for?” Feinstein asked in Wednesday’s hearing.

“We’ve upped the surveillance on [B83] in order to meet the annual assessment requirements associated with … keeping it in the stockpile longer,” Verdon said.

California’s senior senator asked Verdon if the Pentagon wanted to keep the B83 in the NNSA’s active stockpile because its destructive yield was potentially so much greater than the B61.

Verdon said the Defense Department decision was “based on their need for targeting: what they need for targets that they’re provided that they have to hold at risk.”

When Feinstein asked how much a full B83 life-extension program would cost, Verdon said he did not have an estimate on hand.