Editing Neutron moderator (section)

==Impurities==

Good moderators are free of neutron-absorbing impurities such as [[boron]]. In commercial nuclear power plants the moderator typically contains dissolved boron. The boron concentration of the reactor coolant can be changed by the operators by adding boric acid or by diluting with water to manipulate reactor power. The [[German nuclear weapon project|Nazi Nuclear Program]] suffered a substantial setback when its inexpensive graphite moderators failed to function. At that time, most graphites were deposited onto boron electrodes, and the German commercial graphite contained too much boron. Since the war-time German program never discovered this problem, they were forced to use far more expensive heavy water moderators. This problem was discovered by physicist [[Leó Szilárd]].<ref>{{Citation |last=Weinberg |first=Alvin |title=Memorial Tributes |volume=7 |pages=143–147 |year=1994b |chapter=Herbert G. MacPherson |chapter-url=http://www.nap.edu/openbook.php?record_id=4779&page=142 |publisher=National Academy of Engineering Press |doi=10.17226/4779 |isbn=978-0-309-05146-0}}</ref>