IEC 61468-2000 pdf free download – Nuclear power plants — In-core instrumentation — Characteristics and test methods of self-powered neutron detectors.
SPNDs may be located in the reactor core as single elements，or more typically，inassemblies of several units grouped around a central tube in which the sensor of a calibrationsystem can be located. Examples are described in A.5, showing typical assemblyconfigurations used in light water and heavy water nuclear power plants.
SPNDs are available in a variety of designs with sensitive lengths ranging from a fewcentimetres to full core height. The design of an SPND should incorporate proper selection ofemitter type and thickness, as well as sheath and insulation material types and dimensions,to optimize the mechanical design for a specific radiation measurement.
The material of the collector and lead cable sections shall be resistant to corrosion effects ifin contact with the primary coolant under normal operation and transient conditions. SPNDsmay be installed in assemblies in which the SPND elements are not in contact with theprimary coolant, to eliminate the effects of corrosion and extend the element lifetime.
The collector may be fabricated from nickel-based alloys，characterized by low manganesecontent and excellent corrosion resistance.(Low manganese content of less than 1 % isdesired because manganese produces interfering beta emissions when exposed to neutrons.)
The insulation should be virtually transparent to neutrons or gamma fluence rate，i.e. itshould have an extremely low probability of interaction with neutrons or gamma rays. Itshould ensure that the detector maintains a resistance between centre conductor andcollector in excess of 107 Q in “hot” operating conditions, and in excess of 1012 Q for “cold”nonoperating conditions. Ceramic oxides are the preferred materials because they canwithstand the hostile environment inside a nuclear reactor.
Typically, three materials may be used for the insulation in SPNDs employed in power reactorapplications, namely，AlzO3.Mg0 and SiO2.Al203 has been chosen for most applications asit is readily available and, in powder form,is iess sensitive than Mgo to the effects ofhumidity. Mg0 is hygroscopic and is therefore sensitive to cable swelling if moisturepenetrates the cable sheath. Sio, may be used in some applications because its low densityenhances emitter to collector”electron transmission and thus maximizes sensitivitycharacteristics.However SiO, has lower insulation resistance at high temperature than Mgoor Al2O3.
6 Nuclear and electrical characteristics
For power reactor applications,typical emitter materials used in SPNDs include vanadium,cobalt,rhodium，silver，platinum and hafnia. These materials should be used because theypossess relatively high melting temperatures,relatively high cross-sections to thermalneutrons and are compatible with the SPND manufacturing process.Other emitters such ascadmium, gadolinium and erbium may be used in SPNDs for low temperature experimentalreactors, but are not practical for power reactor applications.
Table 1 gives an overview of some of the important characteristics of SPND emitters used inpower reactor applications.IEC 61468 pdf download.