An agreement involving the United States, Canada and Mexico will help convert fuel at Mexico's nuclear research reactor to a lower grade of uranium unsuitable for nuclear weapons, the White House said Tuesday.

In a statement at the Nuclear Security Summit convened by President Obama, the White House said the highly enriched uranium fuel at the reactor would be converted to low enriched uranium, which would eliminate all highly enriched uranium in Mexico.

The International Atomic Energy Agency will oversee the project, according to the statement.

In the statement, Mexican President Felipe Calderon said the agreement contributes to reducing the risk of nuclear materials such as highly enriched uranium being sold illegally and possibly ending up in the hands of terrorists.

The agreement follows announcements Monday by Ukraine saying it would get rid of its highly enriched uranium and Canada saying it will send highly enriched uranium from an Ontario power plant to the United States to be converted into another form.

Obama opened the main working session of the 47-nation summit earlier Tuesday with a call for unified action against nuclear terrorism.

Obama said the gathering was an opportunity to act against what he called one of the "greatest threats to global security."

"It will require a new mindset -- that we summon the will, as nations, as partners, to do what this moment in history demands," Obama said.

"I believe strongly that the problems of the 21st century cannot be solved by any one nation acting in isolation -- they must be solved by all of us coming together."

The two-day summit that began Monday seeks to safeguard nuclear weapons and materials, both old and new, to prevent them from falling into the hands of terrorists and rogue states.

On Monday, Obama's goal of getting specific commitments and working plans from participating nations received a boost when Ukraine announced it would get rid of its highly enriched uranium within the next two years.

Ukrainian President Viktor Yanukovich announced the decision on enriched uranium after a side meeting with Obama before the summit began with a welcoming statement and working dinner at the Walter E. Washington Convention Center in the nation's capital.

On Tuesday, the participating leaders gathered for a group photo before entering the meeting hall for daylong plenary sessions around a large circular table.

The session began with a moment of silence in honor of the victims of last weekend's plane crash that killed Poland's president, first lady, military leaders and other top officials.

Referring to the changing global situation regarding nuclear weapons, Obama said it was "a cruel irony of history" that the risk of nuclear confrontation between nations has decreased while the risk of nuclear terrorism has risen.

He noted that terrorists only needed "plutonium about the size of an apple" for a weapon capable of causing widespread death and destruction.

"Were they to do so, it would be a catastrophe for the world," Obama said.

Obama convened the summit as part of a broader strategy to decrease the threat of nuclear weapons and technology from reaching terrorists. It is considered an unprecedented effort to rally global action on securing vulnerable nuclear materials.

"It's an indication of how deeply concerned everybody should be with the possibilities of nuclear traffic, and I think at the end of this we're going to see some very specific, concrete actions that each nation is taking that will make the world a little bit safer," Obama told reporters Monday.

In an exclusive interview with CNN, Yanukovich said Monday that most of his nation's highly enriched uranium would be gone by the end of 2010.

"The Ukraine has quite a number of power plants and various universities and research institutes where we are trying to understand to best utilize nuclear in a peaceful means," Yanukovich said.

"We are working on it. However, we are already taking the necessary step to remove highly enriched uranium from the country. By the end of this year, Ukraine is going to have the larger part of this uranium taken out of the country."

A White House statement said Obama "praised Ukraine's decision as a historic step and a reaffirmation of Ukraine's leadership in nuclear security and nonproliferation."

"Ukraine joins the United States in the international effort to convert civil nuclear research facilities to operate with low enriched uranium fuel, which is becoming the global standard in the 21st century," the statement said.

In the interview, Yanukovich indicated that the final destination of the highly enriched uranium could be Russia. However, White House press secretary Robert Gibbs told reporters the final destination had yet to be determined, though he said Russia was likely to be one of the countries to receive such nuclear materials.

The White House statement said the United States will provide financial and technical assistance to Ukraine to get rid of its highly enriched uranium.

According to Gibbs, the uranium that Ukraine will give up is enough to construct several nuclear weapons. He said Ukraine would convert its nuclear energy facilities to operate on low-enriched fuel.

Later Monday, Canadian Prime Minister Stephen Harper announced the plan to return highly enriched uranium from the Chalk River Laboratories in Ontario to U.S. facilities for conversion into a form "unusable in nuclear weapons," according to the Canadian government Web site.

Security concerns for the meeting locked down part of Washington's downtown core, with streets closed and temporary fencing erected around the convention center. Parked military vehicles and city hauling trucks blocked access at some points to all but official vehicles.

In the run-up to the summit, Obama met with 10 of the visiting leaders on Sunday and Monday, including his session with Yanukovich and a meeting with Chinese President Hu Jintao.

The bilateral talks were intended to bolster a summit that included a working dinner Monday night and Tuesday's plenary session chaired by Obama.

The summit also is the centerpiece of a major Obama objective aimed at preventing the spread of nuclear weapons and technology. As it takes place, the United States is negotiating with the four other permanent members of the U.N. Security Council on tougher sanctions against Iran over its nuclear ambitions.

In addition, Obama signed a new treaty with Russia last week to reduce the nuclear stockpiles of both nations, and his administration issued a revised U.S. nuclear arms strategy intended to reinforce the nation's nuclear deterrent while isolating terrorists and rogue states that fail to comply with international regulations.

Of particular concern are older nuclear weapons and materials that aren't safeguarded as well as they should be, particularly in Russia and other states of the former Soviet Union, according to U.S. officials. A goal of the summit is to shift the world's attention to what used to be considered a U.S.-Soviet issue, they said.

Obama has made clear he wants to prevent Iran from developing nuclear weapons and to halt North Korea's program. Neither nation was invited to the summit.

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