Q72 — Subcritical SR Counts and SUR After Rod Withdrawal
T4 192008 K1.03 (4.0)
Given:
- Salem Unit 1 is performing a reactor startup with the reactor slightly subcritical.
- 2N31 and 2N32 Source Range count rates are stable.
Then:
- The RO performs a short control rod withdrawal.
Which of the following describes the response on the Source Range (SR) counts and startup rate after the rod withdrawal has stopped (assume the reactor remains subcritical)?
- Salem Unit 1 is performing a reactor startup with the reactor slightly subcritical.
- 2N31 and 2N32 Source Range count rates are stable.
Then:
- The RO performs a short control rod withdrawal.
Which of the following describes the response on the Source Range (SR) counts and startup rate after the rod withdrawal has stopped (assume the reactor remains subcritical)?
A. SR counts will slowly increase then gradually increase and stabilize at a higher value; startup rate will rapidly increase then gradually decrease to a slightly positive value.
B. SR counts will rapidly increase then stabilize at a higher value; startup rate will rapidly increase then gradually decrease and stabilize at zero.
C. SR counts will rapidly increase then gradually decrease and stabilize at the previous value; startup rate will rapidly increase then gradually decrease and stabilize at zero.
D. SR counts will rapidly increase then stabilize at a higher value; startup rate will not change until criticality is achieved.
▶ Show Answer & Explanation
✓ B. Correct. The reactor will have a stable count rate in the SR while subcritical (keff < 1.0). When the operator stops rod motion in a subcritical reactor, the source range count rate will achieve a new higher equilibrium level due to the insertion of positive reactivity (keff increases, but still not critical). With each rod pull positive reactivity is inserted and the startup rate will increase then decay to zero indicating subcritical multiplication has reached equilibrium.
✗ A. First part is incorrect. Plausible because the operator may believe that the SR counts will continue to increase sometime after the rod pull has stopped. Incorrect in that this describes the SR count rate when the reactor is near critical conditions. Second part is incorrect. Plausible because the operator may believe that a stable positive SUR is necessary to achieve a higher equilibrium SR count rate. Incorrect in that in subcritical multiplication, SUR will initially rise then decay to zero with a new stable SR count rate.
✗ C. Plausible because the operator may believe that if the SUR is not constant, then the SR count rate will not achieve a higher value.
✗ D. Plausible if the operator believes that the SR startup rate will not change until the reactor is critical.
Ref: INPO Reactor Operational Physics (R3) | LO: NOS05RXOPER-02, ELO 1.2 Reactor Startup Nuclear Instrumentation Response | Source: New | Cognitive: Fundamental/Memory
Connections
- Related systems: RPS/SSPS, Rx Vessel & Internals
- Related exam: 2023 NRC Written Exam