As an example, 1.7×109 years ago the NA of 235U was 3.1% compared with today's 0.7%, and for that reason a natural nuclear fission reactor was able to form, something that cannot happen today.
例如， 1.7×109年前，235U的天然丰度NA为3.1％，而今天为0.7％，因此可以形成天然核裂变反应堆，而这是今天不可能发生的事情。