Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect the nuclear power industry to grow as our world continues to demand energy, especially the non-fossil-fuel kind, the S&P Global Nuclear Energy Index ETF (Nasdaq: NUCL) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The nuclear power ETF's expense ratio -- its annual fee -- is a relatively low 0.48%.

This ETF hasn't been the best performer so far, trailing the S&P 500 badly over the past three years. But it's also very young, with just a few years on the books. It's very small, too, so if you're thinking of buying, beware of occasionally large spreads between its bid and ask prices.

As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

What's in it?
Several of this ETF's components made contributions to its performance over the past year. Despite doubts about nuclear energy's future after the Japanese earthquake, Exelon (NYSE: EXC) did well, rising about 4% over the year. With its $8 billion acquisition of Constellation Energy still moving forward, it will become America's biggest electricity generator, and it offers a solid dividend as well.

FirstEnergy (NYSE: FE), up 19%, has also been buying, acquiring Allegheny Energy for about $5 billion last year and thereby achieving some cost savings. As with many other utility companies, it's a strong dividend payer, recently yielding nearly 5%.

Other companies hurt the ETF's returns last year but could have an effect in the years to come. Shaw Group (NYSE: SHAW) shed 19% over the past year, largely because of weak revenue and falling profit margins, but it has begun to turn that around, sporting strong growth in its order backlog.

EnergySolutions (NYSE: ES) dropped about 41%, but in the eyes of some investors, that just made it more of a bargain. For those who are intrigued by nuclear power but a bit skittish about it, EnergySolutions offers clean-up services, something many peers don't.

The big picture
Demand for energy isn't going away anytime soon, and for better or worse, nuclear energy remains in demand, too. A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.

Learn about the best dividend ETFs. And if you're looking for some great investments beyond ETFs, consider these 10 Stocks for Your Retirement Portfolio.