Want to understand what nuclear talks with Iran are about but don't know uranium from plutonium, or a centrifuge from a fuel cycle?

Here is a primer for the technical talks in Switzerland as U.S. and Iranian negotiators try to seal a framework agreement before the end of March, scaling back Iranian programs that can lead to nuclear weapons and easing crippling sanctions. The deadline for a final accord is end of June.

The United States' primary concern is Iran's ability to enrich uranium. Iran says its aims are for peaceful energy, medical and scientific purposes, but many governments believe it has nuclear weapons ambitions.

___

WHY THE FOCUS ON URANIUM?

Uranium is a metal found in the ground around the world, most commonly in Australia, Kazakhstan, Canada and Ukraine, that can be used as an energy source. Much works goes into transforming uranium extracted at mills into fuel for a nuclear reactor or material for a weapon. Less than 1 percent of any piece of uranium pulled from the earth is the good stuff — the uranium-235 isotope. For fuel, the level of that isotope must be increased to between 3.5 percent and 5 percent. To produce isotopes for cancer treatment, 20 percent is required. For weapons, the concentration must reach 90 percent. This is what nuclear experts talk about when they speak of enrichment levels. Although plutonium can also be used in bombs, the focus is on enrichment because Iran's uranium program is far more advanced.

HOW DOES ENRICHMENT WORK?

There are different methods to enrich. All concern isolating the good stuff from the less useful. For Iran's purposes, the process starts at the mill, where the uranium is separated from other metals and pounded into a fine yellow powder called yellowcake. At a conversion facility it is then transformed into gas that can be put in centrifuges. Inside the centrifuge, a rotor spins gas around the tube at thousands of times per second. That pushes the heavier uranium-238 out off to the side of the tube, and more of the slightly lighter uranium-235 to the inside. Physicists connect many of these centrifuges together, enriching the material a bit more as it goes through each machine. Such a setup is called a cascade.

WHAT DOES IRAN HAVE?

Iran has a couple of uranium mines but is not blessed with rich deposits. It has two main enrichment facilities. The oldest and largest is about 160 miles southeast of Tehran, built underground and surrounded by anti-aircraft batteries. Another site is built into a mountainside south of Tehran, its construction kept secret by Iran for years. The area is heavily protected by the Revolutionary Guard. Since an interim accord between Iran and world powers in late 2013, U.N. nuclear inspectors have had daily access to both sites. Iran's basic centrifuge represents 1970s technology, involving an aluminum tube about 6 feet tall. It has more advanced models sitting offline or frozen in development since the deal 16 months ago.

SO WHY THE CONCERN?

By 2013, Iran was enriching uranium to 20 percent. That is only a few steps away from weapons-grade material. Iran says it has no interest in producing such material. But Washington wants to impose enough limits on Iran's nuclear activity so any effort by the country to "break out," or "sneak out," toward weapons development would take at least a year. Right now, that period is about two or three months. To that end, the U.S. and its partners are hoping Iran will take thousands more centrifuges offline, halt development of more efficient machines, ship out most of its enriched stockpiles and embark on other steps so that its program moves further away from the bomb. American officials believe a year of breakout time provides enough of a window to discover and interrupt any Iranian push toward the bomb.

HOW DOES THE WORLD KNOW IRAN DOESN'T HAVE MORE GOING ON?

It doesn't. But the United States and its negotiating partners are trying to secure several Iranian concessions that would make Iran's program more transparent and closely monitored. Iran has just a couple of known mills processing uranium. The deal would likely compel Tehran to allow inspectors greater access to make sure there are no more. The same goes for enrichment sites and other locations of interest. U.S. officials say the strategy is to ensure that Iran isn't extracting more uranium than is known from the ground and siphoning off some to mystery locations. They also want to be able to sleuth out any suspicions that Iran is doing enrichment-related work secretly. Doing these things would lower chances of what negotiators call Iran's potential "covert" path to a bomb.

____

Associated Press writer George Jahn contributed to this report.