2. It is prohibited to employ weapons, projectiles and material and methods of warfare of a nature to cause superfluous injury or unnecessary suffering.
The key words here are "superfluous" and "unnecessary." Military expedience is a defense to each as international law has ever been applied.
The combination makes it a probable violation of article 35 section 2 and section 3 of the 1977 protocols to use DU on persons. Hint: When arguing whether something is or is not a violation, look at primary sources, like the laws or protocols themselves.
And then how they are construed. In my first federal case, our argument was, "This is what the regulations say that the government must do." We knew it was, though not frivolous, a loser. It was a loser because the government could say, "But the cases construing those regulations say we don't really have to."
There are two decisions for any weapons/munitions analysis under the Hangue conventions and their progeny: 1. the legality of the fielding (or modifying) a weapon or munition, and 2. the legality of the use of the weapon or munition in the field.
Extent DU munitions are intended to be armor-piercing. Because of its density, DU is well-suited for this purpose. The nature of these munitions is that they don't tend to stay in the human body. There is incidental exposure to toxic residue from troops who linger around the bulks of vehicles the burned with DU rounds in them. The radioactivity is an unneeded (and indeed unwanted) by-product of the source of the material, and the source is critical to its utility. How this causes unnecessary suffering is beyond me.
Munitions that have been considered to be intended to cause unnecessary suffering are small arms rounds with fragmenting projectiles, and those with projectiles intended to frustrate medical treatment. There is armor that a DU round can penetrate that an equivalent tungsten round cannot; that's why we use it!
Now once the weapons or munitions are fielded, they can be used as is militarily expedient. Military expedience is not synonymous with as originally intended. Thus, if the easiest way of attacking an enemy was to shoot and RPG at him, a HEAP round designed to penetrate vehicular armor, then I may do that, even though it would be illegal for my country to field a HEAP round for a small arm primarily intended for anti-personnel use.
This then raises the obvious question of when can a legally fielded munition be used in an illegal way. These are often difficult to prove, and would involve when a more practical method of attack was consciously avoided in order to adopt one that, while less practical, would cause more suffering. Flame throwers, for instance, are very useful, and certainly cause great suffering. Their use has been accepted for decades, so fielding them is no violation. They are useful for clearing bunkers. However, if a leader stopped troops in a clear position to do so from clearing a bunker with grenades (which also work if you're close enough) for the purpose of bringing forward a flame thrower, solely for the purpose of causing the soldiers int eh bunker to suffer more, that would be a violation.
How to prove that was his intent? That's a hard one; one can imagine myriad reasons he could use to claim military expedience.
This is the reason that we train our soldiers that if it's a legitimate target, you may use any weapon or munition you have been provided, in whatever the most useful way is.
Hint: When arguing whether something is or is not a violation, ask a professional.