IEC 61468:2021 pdf free download – Nuclear power plants – Instrumentation systems important to safety
SPNDs are available in a variety of designs with sensitive lengths ranging from a fewcentimetres to full core height.The design of an SPND shall incorporate proper selection ofemitter type and thickness, as well as sheath and insulation material types and dimensions, tooptimize the mechanical design for a specific task.
SPND emitters shall be fabricated with materials resistant to operating conditions ofdetectors，with cross-sections of interaction with neutrons suitable for measured neutronenergy range.
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.
The collector may be fabricated from nickel-based alloys, characterized by excellent corrosionresistance. When choosing the material of the collector (as well as insulator), it shall be bornein mind that the presence of even a small amount of impurities with a large cross-section ofinteraction with neutrons can contribute to the detector reading.
The insulation should have an extremely low probability of interaction with neutrons or gammarays. Under operating conditions，the insulation resistance of the detector should besubstantially greater than the resistance of the signal line conductor and the input resistanceof the measuring device in order to avoid leakage current through the insulation.Ceramicoxides are the preferred materials because they have a high resistivity and can withstand thehostile environment inside a nuclear reactor.
Typically，three materials are used for the insulation in SPNDs employed in power reactorapplications, namely,Al,Og，Mg0 and SiOz.Al,0g has been chosen for most applications asit is readily available and，in powder form, is less sensitive than Mg0 to the effects ofhumidity.However， the currents of the signal line using Al,03insulation may be larger thanthe currents of the signal line with Mg0 insulation due to the formation of the beta-activeisotope 13Al28. MgO is hygroscopic and is therefore sensitive to cable swelling if moisturepenetrates the cable sheath.SiO2 may be used in some applications because its low densityenhances emitter to collector”electron transmission and thus maximizes sensitivitycharacteristics. However, Si0, has lower insulation resistance at high temperature than Mgoor Al2O3.
The SPND connection cable should be a single-wire or two-wire cable with mineral insulationand a metal outer sheath. In order to minimise the contribution to the SPND signal from theinteraction of neutrons and gamma radiation with the signal cable，appropriate materialsshould be selected in the cable design and construction，taking into account the relativegeometric dimensions of the wires and cable sheath.
For SPNDs using a single core cable, the contribution to the SPND signal from the interactionof the signal cable with neutrons and gamma radiation may be minimised by using abackground detector to provide a compensation signal. The background detector should beconstructed without an emitter and using the same cable (see Figure 3).
For SPNDs using a two-wire cable,one wire should be connected to the emitter of the SPNDwith the second wire being used as a background detector. In this case,the identity of thematerial and the geometry of the wires plays an important role.Special attention should begiven to the selection of the materials and the geometry of the wires to minimise effects onthe SPND signal. In order to compensate for possible differences in the irradiation conditionsof the two wires, they should be arranged in a twisted pair arrangement (see Figure 4).