Depleted uranium has a few features that make it useful for modern projectile weapons:
- It's one of the densest substances known. For high-speed impacts, one of the key parameters for penetration depth is the ratio of the projectile's density to the target's density.
- It's self-sharpening. As a DU projectile penetrates a target, it tends to fracture in a way that maintains the shape of the projectile.
- It's flammable. Uranium dust tends to spontaneously ignite, and after shooting an armored vehicle, there's plenty of dust flying around.
- If you've got a nuclear weapons program, it's dirt-cheap. DU is a waste byproduct of uranium enrichment, so if you're willing to spend the money to build nuclear weapons, you get a large supply of depleted uranium essentially for free.
DU is used for armor because of the density and cost factors mentioned above.
Now, let's look at it in the context of medieval combat.
Swords, arrows, and other weapons are moving at slow-enough speeds that the strength of the material matters. Uranium isn't very good for that: it has a tensile strength of around 500 MPa and a yield strength of 170 MPa; for comparison, basic carbon steel is around 900 MPa/500 MPa, good spring steel is 1400 MPa/1000 MPa, and even bronze is better at around 700 MPa/650 MPa.
The self-sharpening ability is nice, but your arrows and swords probably aren't hitting metal often enough for it to matter.
Flammability is bad if it's your armor or your sword that is catching fire.
Density? You can get some seriously effective maces that way, by concentrating a huge weight into a small area. For weapons such as swords, where strength comes as much from the shape as from the material, a denser material simply means a heavier weapon, causing the user to tire faster.
If your Vikings decide to concentrate on depleted-uranium weapons, they'll probably be remembered as suicidally-aggressive warriors who tire easily and whose weapons shatter with a good blow.