It wouldn't pack much punch. Yellowcake is rather similar in emitting radiation as depleted Uranium...which we use for the armor of our M1 Abrams tanks that our troops ride around in (as well as for some of our tank munitions).
Here's what's going on: Yellowcake is the result of the first level of chemical processing of raw Uranium ore. Statistically, raw Uranium ore tends to be 99.3% U238 and 0.7% U235.
U235 is the good stuff. So to make a powerful dirty bomb or to have the fissionable material for a real nuke, you need lots and lots of U235 (public estimates hit as high as needing over 200 pounds of U235 to get a super-critical mass required for a Boom).
There are two basic ways (and other) of obtaining either U235 or Plutonium: separating the natrual 0.7% of U235 from raw uranium ore, or by bombarding U238 with neutrons (typified with so-called "breeder" reactors).
So what you want to watch out for are nations/groups who are purchasing highly refined aluminum tubes and centrifuges (useful for separation) along with certain chemicals useful for refining and processing uranium, or that are building breeder reactors.
But those breeder reactors and those centrifuges require initial raw materials...in particular, they require things such as yellowcake.
Once you've got raw Uranium refined into yellowcake, then you either separate it or directly enrich it via neutron bombardment.
After separation or bombardment, and once you've processed a lot of yellowcake, then you've got enough fissionable metal for either an atomic bomb or a very pricey dirty bomb.
OK, so what's the math say about 500 tons of yellowcake?
500 tons = 2000 pounds * 500 = 1 million pounds of yellowcake.
1 mil * 0.7% = 7000 pounds of U235.
7000 / 200 pounds per bomb = 35 possible nukes if the entire stockpile of yellowcake was separated (or 5,000 nukes if enriched via a breeder reactor).
5 Legislative Days Left Until The AWB Expires