Power range detectors are key nuclear instrumentation system for power operation. They monitor neutron flux (reactor power) from zero to about 120% of full rated power along with indication of the axial and radial distribution of that power. They also provide indication, alarms, and reactor trip signals. As neutron flux level increase into the power range, gamma compensation is not a major concern because gamma rays do not contribute much to the total ionization (about 0.1% at 100% power). Therefore, the power range instrumentation usually consists of four uncompensated ionization chambers, each with its own separate detector, cable run, and electronic circuitry. The ionization chamber, also known as the ion chamber, is electrical device that detects various types of ionizing radiation. The voltage of detector is adjusted so that the conditions correspond to the ionization region. The voltage is not high enough to produce gas amplification (secondary ionization). Ionization chambers are preferred for high radiation dose rates because they have no “dead time”, a phenomenon which affects the accuracy of the Geiger-Mueller tube at high dose rates. The detector consists of a single cylindrical chamber whose operation is identical to that of the boron lined chamber of the compensated ion chamber. This uncompensated chamber is sensitive to both gamma rays and neutrons. However, in the power range of operation, the neutron flux level is many times greater than the gamma flux, and therefore, no gamma compensation is required.
All four channels are physically and functionally identical. Each power range channel employs an upper and a lower uncompensated ion chamber detector (tandem detector) which allows for measurement of axial flux difference. Each channel also monitors a “quadrant” of the core. An upper detector and a lower detector are mounted inside the same instrument well. The outputs of both detectors (upper and lower) are combined to produce a channel total power signal. The eight detector outputs (four upper detectors and four lower detectors) are compared to each other to provide power distribution information (AFD and QPTR) to the reactor operator.
The axial flux difference is defined as the difference in normalized flux signals (AFD) between the top and bottom halves of a two section excore neutron detector, will decrease.
AFD is defined as:
AFD or ΔI = Itop – Ibottom
where Itop and Ibottom are expressed as a fraction of rated thermal power.
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.
The power range instrumentation monitors and indicates the neutron flux level of the reactor core and the rate by which the neutron flux changes during a power operation and standard load follow operation. The neutron flux is indicated as percentage of rated power. The rate of change of the neutron population is indicated as startup rate (SUR), which is defined as the number of factors of ten that power changes in one minute. Therefore the units of SUR are powers of ten per minute, or decades per minute (dpm).
Although the nuclear instrumentation system provides prompt response on neutron flux changes and it is irreplaceable system, it must be calibrated. Power range channels are calibrated to indicate percent rated thermal power by a secondary heat balance (calorimetric). The accurate thermal power of the reactor can be measured only by methods based on energy balance of primary circuit or energy balance of secondary circuit.
Special Reference: Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition. NUREG-0800, US NRC.
Neutron Flux and Fuel Burnup
In a power reactor over a relatively short period of time (days or weeks), the atomic number density of the fuel atoms remains relatively constant. Therefore in this short period, also the average neutron flux remains constant, when reactor is operated at a constant power level. On the other hand, the atomic number densities of fissile isotopes over a period of months decrease due to the fuel burnup and therefore also the macroscopic cross-sections decrease. This results is slow increase in the neutron flux in order to keep the desired power level. Therefore, the excore nuclear instrumentation system must be periodically calibrated.
Power Range – Reactor Safety
As was written, the excore nuclear instrumentation system is considered a safety related system, because it provides inputs to the reactor protection system. The power range neutron flux trip provides the core protection for many power excursion accidents in MODE 1 (power operation). Examples of protective action signals provided by the power range include:
- The Power Range Neutron Flux (Low – Setpoint). A reactor trip will occur if power level exceeds the preset value (for example 25%) on two of four channels, and the trip is not blocked.
- The Power Range Neutron Flux (High – Setpoint). A reactor trip will occur if power level exceeds the preset value (for example 109%) on two of four channels to protect the core from an overpower condition and to protect from a positive reactivity excursion leading to DNB during power operations. This trip cannot be blocked.
- Rate Trips. If the rate of change of reactor power exceeds a preset value in either the positive or negative direction, a reactor trip will occur.
- High Positive Rate trip ensures that protection is provided against rapid increases in neutron flux that are characteristic of an RCCA drive rod housing rupture and the accompanying ejection of the RCCA.
- High Negative Rate trip ensures that protection is provided for multiple rod drop accidents. At high power levels, a multiple rod drop accident could cause local flux peaking that would result in a nonconservative local DNBR.