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?
It's hard to give a specific age when horses stop growing, because this varies wildly among different breeds. While most horses reach their full adult height between the ages of 4 and 5, some tall, heavy breeds won't reach their full height until they're around 8 years old. Horses grow much more quickly at first, before greatly slowing down.