ABOARD THE PAPAL FLIGHT FROM JAPAN (CNS) -- Questions about Vatican finances, especially those involving a real estate deal in London, are serious, but they also are a sign that reforms begun by Pope Benedict XVI are working, Pope Francis said.

"This is the first time the lids have been taken off the pots by someone inside and not outside" the Vatican, the pope told reporters on his return flight to Rome Nov. 26.

Pope Francis spent about an hour with reporters at the end of his weeklong trip to Thailand and Japan. He spoke in general about the two countries and answered eight questions, including two about the recent Vatican finance scandal involving a large loan to develop a London property.

The pope also spoke about nuclear weapons and nuclear energy, nonviolence and the just-war theory and about political unrest in Hong Kong, Chile and several other Latin American countries.

Pope Francis said no one should be bothered by the fact that the Vatican invests the money it collects from Catholics around the world. "The sum of Peter's Pence arrives and what do I do? Put it in a drawer? No, that's bad administration. I try to make an investment."

Peter's Pence is a papal fund used for charity, but also to support the running of the Roman Curia and Vatican embassies around the world. The collection for the fund occurs each year around June 29, the feast of Sts. Peter and Paul.

When handling Vatican funds, the pope said it is best to follow what some people describe as a "widow's investment," which is putting money into a variety of investments, so that if one fails, the entire amount is not lost. And, of course, he said, all of the investments must be moral.

"If you make an investment with Peter's Pence in a weapons factory, the offering is no longer an offering," he said.

"And, yes, you can buy a building and rent it and then sell it," but only when the investment is sound and one is certain that the people who will benefit from it are those Peter's Pence is intended to help, the pope said.

The London deal, though, seems to have involved "things that don't seem 'clean,' but the report did not come from outside." Instead, under finance reform procedures begun by Pope Benedict XVI and continuing under Pope Francis, "it was the internal auditor general, who said, 'Look, here is something that doesn't add up.' He came to me."

When the auditor asked the pope what he should do, the pope said that he told him to go to the Vatican prosecutor with the information. "For that, I am content, because it shows the Vatican administration has the resources" to report and investigate suspicious activity.

The Vatican prosecutor, the pope continued, did a preliminary study and thought some form of "corruption" might be involved, so he asked permission to search several Vatican offices, including in the Vatican Secretariat of State.

"I signed the authorizations myself," Pope Francis told reporters.

One thing he has not signed or even begun to work on, he said, is a proposed encyclical letter on nonviolence.

Asked about the idea of such a letter, Pope Francis said, "The plan exists, but the next pope will do it."

The encyclical is one of many "projects in the drawer" that are "maturing there," waiting until the time is right, he said.

Pope Francis was asked specifically if believed there could be such a thing as a "just war." Catholic tradition has long held that a nation attacked by an enemy could respond morally to that attack under certain conditions, including that the measures taken were proportionate to the damage inflicted and that civilians were not targeted.

"The hypothesis of legitimate defense remains always," the pope said. But in Catholic moral teaching, responding with violence must be "the last resort; the last resort is with weapons." First a nation must try "legitimate defense with diplomacy, with mediation."

As an aside, Pope Francis said he likes the fact that Catholic moral teaching continues to develop. "We are making progress in ethics and I like questioning all these things. It means that humanity is moving forward positively and not only negatively."

Speaking of diplomacy and mediation, the pope praised the United Nations for its peacemaking efforts, but he raised questions about the U.N. Security Council giving a veto power to its permanent members: United States, Russia, China, France and Great Britain.

For example, he said, if "there is a problem with weapons and everyone agrees on resolving the problem" to avoid war, "one with veto power can say no and everything stops."

Pope Francis said he was not an expert on the United Nations, but he thought it would be a good idea if all the members were equal.

He also noted existence of "armaments hypocrisy," which involves "Christian countries, or at least countries with a Christian culture (and) European countries that speak of peace and live by (selling) weapons. This is called hypocrisy."

As for nuclear weapons, the pope reminded reporters that visiting Nagasaki and Hiroshima Nov. 24, "I said again that the use of nuclear weapons is immoral; this must go in the Catechism of the Catholic Church. And not only the use, but also the possession, because if there is an accident or a crazed government, one's madness can destroy humanity. Think about what Einstein said: the fourth World War will be fought with sticks and stones."

As for nuclear power plants, the pope in Tokyo simply pointed out that the country's bishops have called for the abolition of the plants after the meltdown and radiation release in 2011 at the power plant in Fukushima.

Reporters asked for more.

"This is my personal opinion," he said: "I would not use nuclear energy until it is totally safe."

As the pope's plane flew from Thailand to Japan Nov. 23, it crossed the airspace of China and Hong Kong. The pope sent telegrams to Chinese President Xi Jinping and Hong Kong Chief Executive Carrie Lam, wishing their people peace.

Given the ongoing political unrest in Hong Kong, the pope was asked the meaning of the telegrams. Sending the telegrams is "automatic," he said. It is "a courteous way to ask permission to fly over their territory."

"This has no value in the sense of your question," he told the reporter. "It has only a value of courtesy."

As for the unrest in Hong Kong, Pope Francis said he does not know enough about the situation to comment in detail, but he noted that Hong Kong is not the only country with political tensions leading to large-scale demonstrations. Chile, France, Nicaragua, Brazil and other countries of Latin America also are in turmoil.

"What does the Holy See do with this?" he asked. "It calls for dialogue, for peace."

Asked when a papal plane might fly to Beijing, Pope Francis responded, "I'd like to go to Beijing; I love China."