JERUSALEM (AP) — Israel built and exploded so-called “dirty bombs,” explosives laced with nuclear material, to examine how such explosions would affect the country if it were to be attacked by the crude radioactive weapons, the Haaretz daily newspaper reported Monday.

Israeli defense officials and scientists refused to comment on the report when reached by The Associated Press. However, Israel has what is widely considered to be an extensive nuclear weapons program that it has never declared.

The Haaretz report, which included photographs, said the project conducted 20 detonations with explosives laced with a radioactive substance. Mini-drones measured radiation levels and sensors logged the force of the explosions, Haaretz reported.

Researchers quoted in the Haaretz report said the Israeli tests were for defensive purposes only. They said high radiation was found at the center of blasts while small particles carried by wind didn’t pose serious danger, except for the psychological effect of such an attack.

The newspaper said the project, code-named “Green Field” and conducted by staff from Israel’s nuclear reactor in the southern town of Dimona, ended in 2014 after four years of tests. Most were conducted in Israel’s Negev Desert and one in a closed facility, it said.

Another experiment, called “Red House,” tested the consequences of a radiological substance left in a crowded area without being detonated, the newspaper said. The article said Israeli officials put a radioactive material mixed with water in the ventilation system of a building that simulated a shopping mall.

The report said scientists found such an attack would be ineffective as a majority of the radiation remains in air conditioning filters. Results of the experiments were presented at unspecified scientific forums, it said.

The international community long has feared that extremists like the Islamic State group or al-Qaida could make such weapons to attack civilian areas, potentially rendering them inhospitable.

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