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?
A lot of dogs with Grade 1 or Grade 2 patella luxation early in life will have pretty stiff knee joints when they are 14 or 15 years old that probably are at least partially this way due to arthritis from the years of luxating patellae.