2021:01 In situ NMR condition monitoring of cable insulations in Nuclear Power Plants

SSM perspective

Background

As the nuclear power plants are ageing, it is important to verify that every component installed can function safely in the intended environment. Cables are among the most common components in a nuclear power plant and are required to function during normal operation as well as in the event of an accident. There is a need within the nuclear industry for methodologies which can facilitate on-site measurement for condition monitoring, while having the capability to provide non-destructive in-situ measurements of cable insulation material. Therefore, the aim of this study has been to evaluate if a portable Nuclear Magnetic resonance (NMR) sensor can be constructed for assessing cables and if the prospect exists for assessing the structure profile of a cable.

Results

Portable NMR sensors have never before been constructed for the purpose of evaluating cables. Construction and design of such sensor is therefore state-of the art and a tremendous engineering challenge due to the complex structure of cables. In this report KTH presents extensive simulations made with Comsol Multiphysics, investigating two different magnet system arrangements and the radiofrequency excitation system. These simulations studies have been used for the development of a state-of the art working prototype of the portable NMR sensor, which has been named prototype N1. The prototype has been evaluated by testing on a range of different cables, supplied by Forsmark AB and Ringhals AB. Results indicate that the prototype has the capability to measure the insulation material of unshielded cables (no steel armour surrounding the cable). However, evaluating cables with a shielding is far more a challenge, as the shielding material strongly suppresses the magnetic field and reduces the chance of recording any NMR signal. The engineering challenge for future project will therefore be to investigate the effects from shielding, and hopefully find an engineering solution that can provide sufficientcient capability for NMR experiments. It has also been demonstrated that the relaxation times, recorded by the prototype N1, correlate to an extent with mechanical parameters such as strain at break and indenter modulus. The study shows so far, that the constructed NMR prototype (N1) can be a possible method for investigating material deterioration in cables.

Relevance

The results obtained in this study show that the developed portable NMR sensor, prototype N1, can be used as a non-destructive method to evaluate the condition of commercial cables. Further development of this method would provide data that could be analysed and used to better understand the ageing mechanisms of cables in nuclear power plants. Furthermore, the data could provide knowledge of safety margins, adding valuable information to maintenance programs and achieving safe operations.

Need for further research

The developed technique is not mature and requires further critical evaluation. Foremost the effects caused by cable shielding material (either Aluminium or Stainless steel) need to be addressed and solutions have to be developed.