A power tilt, or flux tilt, is a specific type of core power distribution problem. It is a non-symmetrical variation of core power in one quadrant (PWRs) or in one sextant (WWERs) of the core relative to the others. The power in one quadrant (or sextant) might be suppressed by over-insertion of control rods in that quadrant of the core, which, for a constant overall power level, results in a relatively higher flux in the remainder of the core. This situation can also lead to spatial xenon oscillations (azimuthal oscillations).
Therefore, plant operators define a parameter known as quadrant power tilt (QPT) or quadrant power tilt ratio (QPTR) in the Technical Specifications.
Special link: NRC – Standard Technical Specifications
QPTR is defined as:
The ratio of the maximum upper excore detector calibrated output to the average of the upper excore detector calibrated outputs, or the ratio of the maximum lower excore detector calibrated output to the average of the lower excore detector calibrated outputs, whichever is greater.
QPTR is usually provided by four or six tandem (upper and lower) excore power range neutron detectors, which belong to so called the excore nuclear instrumentation system (NIS). During normal operation, this parameter must be continuously verified. The limit (Limiting Condition for Operation – LCO) for QPRT and SPTR is usually QPTR < 1.02. Operation beyond these limits could invalidate core power distribution assumptions used in the accident analysis.
QPTR and Safety Analyses
In summary, the AFD and the QPTR are direct and continuous measures of the core’s global
power distribution. AFD measures global axial power distribution, whereas QPTR measures global azimuthal power distribution. Staying within their limits and proper operation of the control rods should maintain acceptable peaking factors (FQ(z) and FΔH) on a continuous basis. The AFD and QPTR limits ensure that peaking factors (FQ(z) and FΔH) remain below their limiting values by preventing an undetected change in the gross axial and radial power distribution.
Together, the LCO limits on the AFD, the QPTR, the rod insertion limits and the power distribution (i.e. the Heat Flux Hot Channel Factor (FQ(z)), the Nuclear Enthalpy Rise Hot Channel Factor (FNΔH)) are established to preclude core power distributions that exceed the safety analyses limits.