Plutonium is much rarer in nature than uranium and is more commonly used for bombs rather than as reactor fuel. Like uranium, it comes in various isotope forms, with plutonium 239 the most commonly used. It is mixed with the much more unstable plutonium 240 to generate power.
It's my understanding that a horse's knees close around age 2- 2 1/2. My gelding JUST turned 3. A couple days ago in fact. Will a horse's knees almost certainly be closed by that age?
Depends on how well cared for the horse is. I've known a lot of my friends had old horses that died or had to be put down in their early 20s. Meanwhile, I'm riding a 29 year old Thoroughbred at my stables that can only be trusted with more experienced riders because he's so full of it, and he jumps 2 feet with ease.