Caesium-137, with a half-life of 30.17 years, is one of the two principal medium-lived fission products, along with strontium-90, which are responsible for most of the radioactivity of spent nuclear fuel after several years of cooling, up to several hundred years after use.
sˈiːsiəm wˈʌnhˈʌndɹɪd θˈɜːɾi sˈɛvən, wɪð ɐ hˈæflˈaɪf ʌv θˈɜːɾi.sˈɛvəntˌiːn jˈɪɹz, ɪz wˈʌn ʌvðə tˈuː pɹˈɪnsɪpəl mˈiːdiəmlˈɪvd fˈɪʃən pɹˈɑːdʌkts, ɐlˈɔŋ wɪð stɹˈɔntiəm nˈaɪnti, wˌɪtʃ ɑːɹ ɹᵻspˈɑːnsᵻbəl fɔːɹ mˈoʊst əv ðə ɹˌeɪdɪoʊæktˈɪvᵻɾi ʌv spˈɛnt nˈuːklɪɹ fjˈuːəl ˈæftɚ sˈɛvɹəl jˈɪɹz ʌv kˈuːlɪŋ, ˌʌp tə sˈɛvɹəl hˈʌndɹɪd jˈɪɹz ˈæftɚ jˈuːs.