Neutron continued detectors

In very large reactor plants, the need exists to monitor neutron flux in various portions of the core on a continuous basis. This allows for quick detection of instability in any section of the core. This need brought about the development of the self-powered neutron detector that is small, inexpensive, and rugged enough to withstand the in-core environment. The self-powered neutron detector requires no voltage supply for operation. Figure 29 illustrates a simplified drawing of a self-powered neutron detector. 

Indirect geometry spectrometers have no requirement (within the limitations implied by the use of S (Q,a>), 2.5.1) to calibrate detector efficiencies, on either continuous or pulsed sources (compare 3.4.3). Since the final energy of the neutrons never varies the detection efficiency is constant. Variations arising from differing discrimination levels ( 3.3.2) could play a significant role, except that (on low final energy instruments) all detectors follow almost the same path in Q,o ) space ( 3.4.2.3). Occasionally there is a need to calibrate the detected intensity in respect of the sample mass and standard analytical chemical techniques can be readily adapted to this circumstance. 

In a time-of-flight spectrometer (TOP spectrometer), the energy of the incident neutrons is determined, for example also by diffraction from a monochromator crystal M, but instead of a continuous neutron beam, a neutron pulse or a series of pulses is used, produced by periodic interruption of the neutron beam by a so-called chopper. The energy of the scattered neutron wave then is found from the velocity of the scattered neutrons, which is calculated from the distance and the measured time of flight of the neutrons between the sample crystal and the detector. 

Principal Component REACTOR SERVICES GROUP (Continued) Core and Service Facility Tools Neutron Detector Service Equipment Hot Duct Seirvice Equipment... 

In the refueling mode, the reactor vessel is depressurized. All control rods in the inner and outer reflectors are fully inserted except for two inner and two outer rods which may be removed for refueling a 60 degree sector of the core. The neutron flux level is continuously monitored by the source range detectors. 

In the shutdown mode, the reactor vessel is fully pressurized or, at different times, in various stages of depressurization. Afterheat from fission product decay is generated at rates of up to about 7 percent of the core power level prior to shutdown, depending on the time interval since shutdown. The core decay heat is removed by the HTS. When the HTS is not available, the heat is removed by the Shutdown Cooling System (SCS). The outer control rods are normally fully Inserted during shutdown, and meet the required shutdown margin, with due allowances for uncertainties, even if the maximvim reactivity worth rod remains fully withdrawn. For cold shutdown, the control rods in the inner reflector are also Inserted and for this case, the maximum reactivity worth control rod is in the inner reflector. The neutron flux level is continuously monitored by the source range detectors. 

The MOX fuel assembly capsule assay system (FAAS) determines plutonium content in the final assembly contained in a storage capsule. Coupled to the automated capsule transfer system, it provides information about the movements of fuel into and out of the product storage (Menlove et al. 1993). It is designed to assay the complete active zone of the assembly with plutonium loadings up to 10 kg and can accommodate 5 m long capsules that contain the fuel assemblies. The unshielded detector body has 12 He tubes and an efficiency of 16%. In addition, the continuous mode gives a time history of movements of neutron source material in the vicinity. The FAAS is augmented with a surveillance system to meet verification requirements. 

R. Y. LYON, ITie Use of Neutron Detectors in Continuous Solvent Extraction Control, ISO-SA-28 (October 7, 1966). 

A new detector unit has been designed to add an internal check of radiation detection sensitivity by including a Th source in the detector LiF foil assembly. This provides continuous alpha-particle emission to test the ability of the foil detector to sense the neutron alpha-particle interaction in the LiF foil. An internal clock circuit forces the SCR to trigger if the detector amplifier fails for longer than 1 min. The unit produces a radiation alarm with any detected pulse rate from the LiF foil >4 pulse/s. Mechanical and electrical compatibility of the new detector design with the existing detection system has been a basic requirement not only to maintain the existing radiation sensitivity but also to minimize installation costs. 

Thermal-hydraulics design of the clad failure detection system. This system is used to detect possible clad failures in the fuel subassemblies while the plant is operating. Clad failure causes a release of fission products emitting delayed neutrons, that are transported to the hot plenum and to the detectors. In SPXl the detector itself is placed outside the hot plenum with a continuous poped sampling system for analysis of the primary sodium was set up other systems in which neutron detectors are placed near the intermediate heat exchanger inlets and enabled activity to be measured directly, have been studied. In both cases thermal-hydraulics studies were necessary to measure the hydraulic transfer functions between the various core... 

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