The bad zone of radioactive byproducts is the half life between 10 and 1000000 years. Shorter and you can wait it out. Longer and the activity is low enough to

The bad zone of radioactive byproducts is the half life between 10 and 1000000 years. Shorter and you can wait it out. Longer and the activity is low enough to ignore when it's spread out. When designing a fusion reactor, you can usually choose components that won't generate these undesirable nuclides. But in fission, many products are generated, so persistent contamination is practically impossible to avoid.