SM-2 Flexible Critical Experiments

The cold, clean, steel-reflected, final, SM-2 mock-up containing 36.4 kg U²³⁵ and 61 g B¹⁰ maintained criticality after a seven rod bank withdrawal of 6.974 in. and has an “excess K” (ΔK_E) of 1520 cents. An infinite steel-water laminated reflector is worth approximately +85 cents over the infinite water reflected core. The measured reactivity coefficient, @ 2000 psi, ranges from –1.15 cents/°F @ 150°F to –5.20 cents/°F @ 510°F. The integral reactivity effect of raising the SM-2 core water temperature from 103 to 510°F @ 2000 psi and the water in the reflector coolant graph from 103 to 477°F @ 2000 psi is –889.7 cents. The average measured material coefficients for U²³⁵ and B¹⁰ are 0.157 cents/g and 42.54 cents/g, respectively. Without the benefit of flux suppressors the maximum to average power ratio of 7.28 occurs at the top of the fuel section of control rod C (withdrawn to 7.14 in.), and a ratio of 5.28 occurs at the bottom of stationary element 43 and symmetric elements.