One pcm corresponds to what relative change in k/k?

• A. 1 × 10^-5 Δk/k
• B. 1 × 10^-4 Δk/k
• C. 1 × 10^-6 Δk/k
• D. 1 × 10^-3 Δk/k

Answer: (A)

One pcm is a very small reactivity change, equal to 1×10^-5. In reactor theory, reactivity is defined as ρ ≡ (k_eff − 1)/k_eff, which for k_eff near 1 is essentially the fractional change in k_eff, Δk/k. So a change of 1 pcm corresponds to Δk/k ≈ 1×10^-5. For example, if k_eff goes from about 1.00000 to 1.00001, the relative change is 1×10^-5, i.e., one pcm.