Uranium-235 has a half-life of 704 million years.

It emits so slowly that the main hazard is heavy metal poisoning, which you'd also get from the depleted uranium that's a waste product from enrichment.

From the numbers I see on https://www.iaea.org/topics/spent-fuel-management/depleted-u..., if a dirty bomb with 1kg of 100% pure Uranium-235 exploded and evenly deposited a fine dust over a 170 by 170 meter square of typical soil, the total Uranium-235 (and specifically that isotope) of the top meter would only double; the total mass of uranium in that volume would only go up by 0.7%.

  The average concentration of natural uranium in soil is about 2 parts per million, which is equivalent to 2 grams of uranium in 1000 kg of soil. This means that the top metre of soil in a typical 10 m ´ 40 m garden contains about 2 kg of uranium (corresponding to about 50,000,000 Bq of activity just from the decay of the uranium isotopes and ignoring the considerable activity associated with the decay of the progeny. Concentrations of uranium in granite range from 2 parts per million to 20 parts per million. Uranium in higher concentrations (50 - 1000 mg per kg of soil) can be found in soil associated with phosphate deposits. In air, uranium exists as dust. Very small, dust-like particles of uranium in the air are deposited onto surface water, plant surfaces, and soil. These particles of uranium eventually end up back in the soil or in the bottom of lakes, rivers, and ponds, where they mix with the natural uranium that is already there. Typical activity concentrations of uranium in air are around 2 µBq per cubic metre. (UNSCEAR 2000).