Complex sentence: This means that some spontaneous neutron may cause the nuclear chain reaction to take place before the material could be fully joined; it would have caused a "fizzle", a predetonation which would cause the material to blow apart before creating much of an explosion.
Split into simpler sentences: This means that some spontaneous neutron may cause the nuclear chain reaction to take place before the material could be fully joined. Thus there may be a "fizzle", a predetonation which would cause the material to blow apart before creating much of an explosion.