What defines a reactor shutdown condition?

What defines a reactor shutdown condition?

• A. The reactor is subcritical by at least 760 pcm with the core at ambient temperature with the reactivity worth of xenon equal to zero and with the reactivity worth of all installed experiments included.
• B. Subcritical by 1000 pcm.
• C. The core is at elevated temperature.
• D. The reactor is defueled.

Answer: (A)

Shutdown is defined by ensuring the reactor is safely subcritical with a predictable, conservative reactivity margin. The best wording sets a specific minimum subcritical level—at least 760 pcm—and pins down the conditions under which that margin must hold: the core at ambient temperature so there’s no extra positive or negative temperature feedback, xenon reactivity is assumed zero so the poisoning effect isn’t contributing to reduce or increase the margin, and the reactivity worth of all installed experiments is included so any foreign-inserted hardware doesn’t push the system toward criticality. This combination creates a robust, worst‑case baseline that guarantees the reactor cannot rebound to criticality under typical perturbations. Subcritical by 1000 pcm lacks the explicit conditioning on temperature and xenon and the experimental hardware, so it doesn’t match the defined shutdown criteria. A condition where the core is at elevated temperature involves a different set of reactivity feedbacks and doesn’t by itself define a definite shutdown margin. Defueled describes fuel removal, which is a state change but not the precise shutdown criterion used to ensure continued subcritical operation.

Shutdown is defined by ensuring the reactor is safely subcritical with a predictable, conservative reactivity margin. The best wording sets a specific minimum subcritical level—at least 760 pcm—and pins down the conditions under which that margin must hold: the core at ambient temperature so there’s no extra positive or negative temperature feedback, xenon reactivity is assumed zero so the poisoning effect isn’t contributing to reduce or increase the margin, and the reactivity worth of all installed experiments is included so any foreign-inserted hardware doesn’t push the system toward criticality. This combination creates a robust, worst‑case baseline that guarantees the reactor cannot rebound to criticality under typical perturbations.

Subcritical by 1000 pcm lacks the explicit conditioning on temperature and xenon and the experimental hardware, so it doesn’t match the defined shutdown criteria. A condition where the core is at elevated temperature involves a different set of reactivity feedbacks and doesn’t by itself define a definite shutdown margin. Defueled describes fuel removal, which is a state change but not the precise shutdown criterion used to ensure continued subcritical operation.