Pure beryllium (few enough trace contaminants to not activate) is very very very very expensive. And you need a lot. And it's pretty nasty on its own even compa

Pure beryllium (few enough trace contaminants to not activate) is very very very very expensive. And you need a lot. And it's pretty nasty on its own even compared to irradiated steel or similar. Highly experienced nuclear engineers need regular safe handling training because it's so toxic.