The form and location of the moderator can greatly influence the cost and safety of a reactor . Classically , moderators were precision-machined blocks of high purity graphite with embedded ducting to carry away heat . They were in the hottest part of the reactor , and therefore subject to corrosion and ablation . In some materials , including graphite , the impact of the neutrons with the moderator can cause the moderator to accumulate dangerous amounts of Wigner energy . This problem led to the infamous Windscale fire at the Windscale Piles , a nuclear reactor complex in the United Kingdom , in 1957
Windscale fire
Windscale fire
Neutron moderator