Complex sentence: By August 1946, Sandia Base held electrical and mechanical assemblies for about 50 Fat Man bombs, but there were only nine fissile cores in storage in 1946,13 in 1947, and 53 in 1948.
Split into simpler sentences: By August 1946, Sandia Base held electrical and mechanical assemblies for about 50 Fat Man bombs, but there were only nine fissile cores in storage in 1946. The stockpile of cores grew to 13 in 1947, and 53 in 1948.