TerraPower, a nuclear energy start-up whose backers include Microsoft founder Bill Gates, has raised $35 million in a new round of funding to aid the development of a nuclear reactor that powers itself on its own waste.

The Bellevue, Washington-based company's Series B investors include Charles River Ventures and Khosla Ventures. Previous investors include Gates, and former Microsoft CTO Nathan Myhrvold, the co-founder of Intellectual Ventures, a Washington incubator of which TerraPower is a spin-off. Three-year-old TerraPower puts its own spin on traditional reactors with what it calls travelling-wave reactors, which can generate electricity from smaller amounts of enriched uranium plus recycled uranium from either the same plant or other plants. Since nuclear opponents most often object to the hazards of uranium and its disposal, TerraPower's plans could win it some new fans. (Watch a presentation about TerraPower here.)

"The travelling-wave reactor is an incredibly exciting new design that could really advance the nuclear energy field," Izhar Armony of Charles River Ventures said in a statement.

It's rare for start-ups to be in the business of developing reactors, which are usually the province of global companies (with a boost from government aid). Despite cleantech deals hitting a new record in the first quarter of this year - and having Bill Gates as a backer -  it's still a surprise that TerraPower has pulled in quite so much funding, since it goes against an industry investing trend toward quick turnaround deals rather than manufacturing-heavy ones. "Nuclear is even more pricey and long-term," observed the blog GreenBeat.

In making the case for the funding, Armony said: "A short term approach to investing isn't going to help achieve the goals many have for a clean-energy future."

The company hopes to sell commercial reactors by 2020, Myhrvold told Reuters – though the company likely would go to market with partners. In March, Toshiba confirmed it was in preliminary talks with TerraPower, but no further details have been released.

"We have met most of the companies that could be a potential partner," Myhrvold told Reuters. "What we want to do is take our advanced technology and combine it with a partner's ability to build these things and build a test reactor."