IEC 61468-2021 pdf Nuclear power plants – Instrumentation systems important to safety – In-core instrumentation: Characteristics and test methods of self-powered neutron detectors

3.13 photoelectric effect complete absorption of a photon by an atom with the emission of an orbital electron [SOURCE: IEC 60050-395:2014, 395-02-08] 3.14 prompt response signal generation from a self-powered neutron detector based on the (n, γ, e) reaction 3.15 radioactive half-life time required for the activity of a radioisotope to decrease to half of its initial value Note 1 to entry: The radioactive half- life is related to the decay constant λ by the expression: T½= ln2/λ ≈ 0,693/λ. This quantity is expressed in seconds (s). [SOURCE: IEC 60050-395:2014, 395-01-12] 3.16 radioisotope isotope of an element with the property of spontaneously emitting particles or gamma radiation or of emitting X-radiation [SOURCE: ISO 5576:1997, 2.104] 3.17 self-powered neutron detector neutron-sensitive radiation detector that requires no external power supply, consists of three basic elements: an emitter that interacts with neutrons to emit electrons; a collector that collects these electrons and an insulator that isolates the emitter from the collector and converts the neutron fluence rate into electrical signal Note 1 to entry: See Figure 1 and Figure 2. 3.18 self-shielding self-absorption which occurs in the emitter: as emitter diameter increases, the escape probability of an electron born in the interior of the emitter decreases, and current-producing efficiency drops 3.19 in-core detector assembly mechanical arrangement for positioning different detectors inside the core of a nuclear reactor.
5 Self-powered neutron detectors general advantages and disadvantages In self-powered neutron detectors (SPNDs), the interactions of neutrons and atomic nuclei are used to produce a current which is proportional to the neutron fluence rate. The principles and characteristics of SPND design and operation are given in Annex A. When compared to other types of detectors, they have the following advantages: – no need of power supply; – simple and robust construction; – relatively small mechanical “size” well-suited for in-core installation; – good stability under temperature, pressure, radiation and other conditions. The main disadvantages of SPNDs are the low signal level and the inertia of the neutron component of the signal. 6 Composition and construction A typical SPND consists of an emitter, made of a material with a large neutron interaction cross-section, its surrounding insulator, collector and lead cable.