Radiological bombs, or “dirty bombs,” which use conventional explosives together with radioactive material, have been a matter of concern since the terrorist attacks against the United States on September 11, 2001.

Yet a four-year project conducted by researchers at the Dimona nuclear facility has found that the danger posed by dirty bombs is minimal, Haaretz reported on Monday.

Officials at the Negev Nuclear Research Facility in Dimona began working on the Green Field project — a series of tests whose purpose was to determine the outcome of a dirty bomb explosion — in 2010.

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The findings of the project, which ended in 2014, have been presented at scientific conferences and on nuclear science databases, the report said.

The Green Field project involved 20 detonations involving between 250 grams and 25 kilograms of conventional explosives, together with a familiar radioactive substance used for medical imaging known as 99mTc (known also by its trade name, Cardiolite).

The researchers used sensors to measure the force of the explosion and tiny drones to detect radiation levels.

The research found high-level radiation at the center of the explosions, with wind dispersal of radiation particles at a low level, the report said.

Sources at the Negev Nuclear Research Center said that the danger posed was not substantial, and that the main impact of such an attack would be psychological.

The tests also included a trial code-named “Red House,” which probed the outcome of an undetonated dirty bomb left in a public place.

In this test, a substance was mixed with water in the ventilation system of a two-story building on an army base belonging to the Home Front Command that was meant to simulate a shopping mall.

The method was found to be ineffective from the attackers’ standpoint since most of the radiation stayed on the building’s air-conditioning filters.

If a dirty bomb were to explode in a closed space, the area would have to be cordoned off for a long time until the effects had waned.

The radioactive components of a dirty bomb are available in the medical and industrial sectors, though no such bomb has ever been used, the report said.

Israel’s Health Ministry has published guidelines on how to deal with a dirty bomb explosion, and the Israeli army’s Home Front Command website also lists procedures to be followed in such a case.