The funny thing about radioactive materials is that there's a big tradeoff in terms of short-term vs long-term danger.
Say we have two samples of radioactive isotopes, both of equal mass and which produce the same particles when they decay. Sample A has a very short half-life (let's say minutes). Sample B has a very long half-life (let's say centuries). Sample A is much more dangerous right now but, in a few weeks it will be mostly gone. Sample B, having hardly decayed at all, will be the bigger threat then. In a single catastrophic release, as in the case of Fukushima, you'd probably prefer to have a lot of B sent out. It'll be around for a while, but it's not too deadly and will disperse, effectively raising the ocean's background radiation by an imperceptible amount. A huge release of A might do a lot of localized damage. If you have a slow, continuous leak, B might actually be worse though, since it could build up while A will not.
Cesium 137 and 134 have moderate half-lives of 30 and 2 years respectively. Iodine 131 has a shortish half-life of 8 days. It's been 666 days (Gee, I wonder why this is popping up today!) since the Fukushima incident, so the Iodine 131 had now decayed to 0.5^(666/8) or roughly 1e-25 of it's original strength. It's basically gone. The Cesium will be around for a while, but it's a lot less dangerous than the Iodine was.
Funny Aside: Cs-137 is very commonly used in undergraduate student laboratories. Samples of it are relatively safe to handle without safety equipment and it doesn't need to be replaced very often. I once TA'd a lab course where students were asked to do some experiments with CS-137 and, in their write-up's, calculate the effective dose they received. Most got the right answer, but a few messed up their units and wound up with ginormous doses that would have killed a zombified rhinoceros! Some of these students noticed this and reasoned they must have made a mistake, since we probably wouldn't try to kill our students. A couple did get very concerned though! Most just didn't notice, which was a tad depressing for me.
Say we have two samples of radioactive isotopes, both of equal mass and which produce the same particles when they decay. Sample A has a very short half-life (let's say minutes). Sample B has a very long half-life (let's say centuries). Sample A is much more dangerous right now but, in a few weeks it will be mostly gone. Sample B, having hardly decayed at all, will be the bigger threat then. In a single catastrophic release, as in the case of Fukushima, you'd probably prefer to have a lot of B sent out. It'll be around for a while, but it's not too deadly and will disperse, effectively raising the ocean's background radiation by an imperceptible amount. A huge release of A might do a lot of localized damage. If you have a slow, continuous leak, B might actually be worse though, since it could build up while A will not.
Cesium 137 and 134 have moderate half-lives of 30 and 2 years respectively. Iodine 131 has a shortish half-life of 8 days. It's been 666 days (Gee, I wonder why this is popping up today!) since the Fukushima incident, so the Iodine 131 had now decayed to 0.5^(666/8) or roughly 1e-25 of it's original strength. It's basically gone. The Cesium will be around for a while, but it's a lot less dangerous than the Iodine was.
Funny Aside: Cs-137 is very commonly used in undergraduate student laboratories. Samples of it are relatively safe to handle without safety equipment and it doesn't need to be replaced very often. I once TA'd a lab course where students were asked to do some experiments with CS-137 and, in their write-up's, calculate the effective dose they received. Most got the right answer, but a few messed up their units and wound up with ginormous doses that would have killed a zombified rhinoceros! Some of these students noticed this and reasoned they must have made a mistake, since we probably wouldn't try to kill our students. A couple did get very concerned though! Most just didn't notice, which was a tad depressing for me.