Source range detectors monitor neutron flux (reactor power) at the lowest shutdown levels and provide indications, alarms, and reactor trips. Source range instrumentation usually consists of two or four source range channels, each with its own separate detector, cable run, and electronic circuitry. The detectors utilized are usually high-sensitivity boron-trifluoride (BF3) proportional counters. In general, proportional counters are capable of particle identification and energy measurement (spectroscopy). The pulse height reflects the energy deposited by the incident radiation in the detector gas. It is possible to distinguish the larger pulses produced by alpha particles (produced by (n, alpha) reactions) from the smaller pulses produced by beta particles or gamma rays.
These BF3 detectors produce a pulse rate output proportional to the thermal neutron flux seen at the detector. These channels are typically used over a counting range of 0.1 to 106 counts per second but vary based on reactor design. These excore detectors are usually located in instrument wells in the primary shield (concrete shield) adjacent to the reactor vessel.
The source range instrumentation monitors and indicates the neutron flux level of the reactor core and the rate by which the neutron flux changes during a reactor shutdown and the initial phase of start-up. They are very important for monitoring subcriticality during fuel reload when subcritical multiplication occurs. The neutron flux is indicated in counts per second (cps). The rate of change of the neutron population is called start-up rate (SUR), 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).
There are two principal problems in the source range instrumentation:
- Discrimination. During the reactor shutdown and the initial start-up phase, it is required to distinguish the relatively small number of pulses produced by neutrons from the large number of pulses produced by gamma radiation. Thus gamma discrimination is of particular interest during shutdown after the reactor core reaches a significant level of fuel burnup. This condition produces a high gamma field and a low neutron flux around the detector. Proportional counters allow for discrimination, but they must be calibrated. The discriminator excludes the passage of pulses that are less than a predetermined level. The function of the discriminator is to exclude noise and gamma pulses that are lower in magnitude than neutron pulses (alpha pulses, respectively). Many power plants have found it necessary to place source range proportional counters in lead shielding to reduce gamma radiation at the detectors. This increases the low-end sensitivity of the detector, and it may extend the detector life.
- Dead Time. This instrument can indicate a maximum neutron count rate of 106 cps. Higher count rates are influenced by a phenomenon known as dead-time. The dead-time is the period during which the detector is busy and cannot accept and process pulses. This phenomenon can have serious consequences since dead-time distorts outputs at high activity or high dose rates.
Some power plants have made provisions for moving the source range detectors from their operating positions to a position of reduced neutron flux level once the flux level increases above the source range.
Special Reference: Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition. NUREG-0800, US NRC.
Source 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 source range neutron flux trip provides the core protection for reactivity accidents in MODE 2 (reactor start-up). For example, the source range neutron flux trip ensures protection against an uncontrolled RCCA bank rod withdrawal accident from a subcritical condition during start-up. It also protects against boron dilution accidents and controls rod ejection events.
During refueling, source range detectors also ensure monitoring of reactor subcriticality. They also have an alarm that may serve as a containment evacuation signal if the neutron flux exceeds a preset value. This alarm alerts the control room operators and personnel in the containment of a positive reactivity addition to the reactor during shutdown conditions.