TLDR: Portals to other places; wizards use them to make nukes-as-fireballs/anti-personal level attacks, how do they survive?
In a system of magic that functions around opening portals, what considerations do these portals need in order to be “safe” when a wizard casts something like a "fireball"?
The natives have portal magic. They don’t know the magic is based on portals. To them, it looks like a wizard chants a spell and a fireball flies out to smite something. Or they wave their hands and gravity is pulling them a different direction, or some other wizardly spell. Mostly they leave the old guys with the robes and staffs alone because no one wants to be turned into a pile of goop.
The mechanics of the magic system: Some people are psychokinetic in a manner. They can open portals with their brains. Not particularly large portals, and not ones that generally go anywhere useful for travel. These portals are controllable but only the most powerful can make the portal loop back to somewhere useful to go to physically and big enough to pass through. Most of the rest of the ‘spells’ lead to other places that are hostile to 3.5d life, possibly higher or lower dimensions or higher or lower energy state places.
This ability manifests itself in the first person as a talent for thinking in a certain way with a certain pattern, flexing the right ‘mental muscles’ to rip the fabric of space-time apart. Often mnemonic chants and hand gestures are found to help with this when trying to achieve a specific effect. It all ‘looks’ very traditional.
However, when ‘casting fireball’, you create the fireball by ripping two very small holes in a very small space in the fabric of space-time, and colliding different things through them. Your fireball is really a continuous nuclear explosion! This is considered an advanced spell because the portals must intersect in such a way that, when matter exits them both, it intersects and reacts. (The “ball of fire” is thrown by tossing the portal pair at whatever after the reaction is started and sustaining.)
The issue; Our wizard is holding an actively nuking nuke between their hands. How does our wizard survive? Can a small nuclear reaction be small enough to produce the effect of a beach-ball-sized fireball? Is the radiation this throws off survivable? Can it be made survivable with different reactive components?
Edit 1: What is meant by ‘nuclear reaction’? Traditionally, to my understanding, nuclear reactions energetic enough to ignite the air around them are generally achieved by smashing some neutrons into something like plutonium. I know other materials can be used, and some may produce less harmful radiation? The “fireball” part of any spell is literally just a fireball. Atmosphere burning because of the “hot thing” in the middle, in this case a small nuclear reaction. The reaction is continuous because it's being constantly fed new material, but small because its reaction surface is “small.” This might become an issue for the “plane of plutonium” if the explosion propagates through the portal to the rest of the plutonium, but we don’t really care about other dimensions when there are orcs to set on fire.
The basis of the question revolves around the idea that you can react just about anything that WILL react when you mash them together on an atomic level. You don’t have to put the energy into making the basic materials because you're bringing them from somewhere else at a low cost. So technically I suppose, you can “loose” energy as long as the observable effect is a burning ball of don’t-touch-it.