Simplify the following text for a Grade 10 native English-speaking person.
Original: The becquerel ( symbol Bq ) is the SI derived unit of radioactivity , defined as the activity of a quantity of radioactive material in which one nucleus decays per second .
Simplified: The becquerel ( symbol Bq ) ( pronounced : be - Ë krel ) is the SI-derived unit of radioactivity . One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second .