Holy shit it was cesium-137?
It can be found in natural U-238, but otherwise it's produced in nuclear fission. Stupid radioactive, and has a halflife in your body of like 70 days.
People who consumed this will be at a higher risk of cancer.
How the fuck did shrimp get contaminated with Cesium-137? Did they have like an X-ray machine radiation source blow up over a batch?