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Radiation Resilient Ultrasonic Sensors for Nuclear Reactors

Overview

The core of a nuclear reactor poses a challenging environment to implement a non-destructive testing (NDT) inspection in, due to the high radiation levels and the potentially high temperatures.

There are two types of damage, caused by radiation exposure, which can affect the cable or the sensor during an ultrasonic (UT) inspection. The first is a single-event effect (SEE), where an energetic particle interacts with the cable or sensor. However, an SEE is easily filtered from the UT waveform, whereas in the second type of damage - bulk material damage - the material structure is physically altered by the effects of the radiation.

A collaborative project towards the development of radiation resilient ultrasonic sensors (reDRESS) aimed to tackle this problem by constructing a reliable ultrasonic testing (UT) sensor, capable of operating at 350°C for extended periods, whilst exposed to high levels of gamma and neutron + gamma radiation.

Objectives

  • To report on the suitability and degradation levels of different types of materials for the cabling
  • To build radiation resilient UT sensors
  • To identify suitable radiation facilities for testing the sensors that were constructed during the project
  • To expose the sensors to gamma and neutron + gamma radiation
  • To evaluate the radiation damage using ultrasonics, light microscopy, scanning electron microscopy (SEM) and X‑ray diffraction (XRD)
  • Model the radiation exposure
Figure 1. Water tightness testing in TWI’s 7m dive tank
Figure 1. Water tightness testing in TWI’s 7m dive tank

Solution

UT sensors can be manufactured from different piezo-electric materials. As part of the scope, project partner Precision Acoustics Limited (PAL) developed piezo-polymer and fibre optic UT sensors and Ionix Advanced Technologies Ltd (Ionix) created piezo-ceramic based UT sensors. TWI developed a method for attaching UT sensors to stainless steel mineral insulated copper core (MICC) cables. These then underwent thorough testing including water tightness testing, which took place at TWI’s dive tank facility at TWI’s North East Technology and Training Centre in Middlesbrough, as well as impendence testing and signal testing (Fig.1). Project partner, University of Sheffield (UoS), modelled the radiation exposure of the components so that selective radiation hardening could be applied to the electronics where necessary. The model was sufficiently close to the actual exposure that it could be used to simulate the gamma exposure for different components to allow them to be modified to maximise their life before physical testing.

TWI researched European nuclear facilities that would be suitable to test the radiation resilient cables. Two facilities – The University of Manchester’s Dalton Nuclear Institute and The Joseph Stefan Institute (JSI), Ljubljana, Slovenia – were identified as appropriate for testing. The Dalton Nuclear Institute was used to expose the cables to gamma radiation, whilst the Ljubljana facility was used to expose the cables to neutron + gamma radiation (Fig.2). Both types of UT sensors were inserted into the reactors and their operation monitored during exposure to the different radiation types. The gamma exposed sensors (Dalton) were suitable for offsite testing whilst the neutron + gamma sensors (JSI) where too highly radioactive to be removed from site. Samples of piezo-polymer material where exposed to radiation in both facilities and these were suitable for offsite testing once their residual radiation level had decreased to an acceptable level. Testing at Dalton demonstrated that the polymers used in conventional cabling are susceptible to gamma radiation damage and should not be used under radiation conditions.

White light microscopy showed no apparent differences between the working and non-working samples’ fibre optic sensors. SEM examination of sectioned samples showed variations associated with the manufacturing process and TWI were able to advise methods of improving the key process variables.

Samples of the piezo-polymer from PAL were investigated using XRD to characterise their level of crystallinity. The gamma irradiated sample had structural damage and its crystallinity and associated piezoelectric response had been reduced to below a useable level, while the neutron and gamma irradiated sample displayed a single much lower and wider peak, indicating significant structural damage by the neutron bombardment (Fig.3).

The UT sensors from Ionix, utilising a MIMS cable, showed resilience to both gamma and neutron radiation in experiments at both facilities, offering a solution for ultrasonic condition monitoring in a nuclear facility or reactor. There was no observable degradation in performance at doses of 11 MGy gamma, and integrated neutron flux of 2.6 x1018 n.cm-2.

Conclusion

The cabling options were limited, as most manufacturers will not guarantee the integrity of their cables in radiation environments. The Dalton tests were consistent with the literature that states polymers are susceptible to radiation damage and should be avoided for the use of cabling. However, the MICC cable types are water-tight and display no material degradation due to gamma or neutron + gamma radiation.

TWI identified two suitable radiation facilities for testing the sensors that had been constructed by the project partners PAL and Ionix, as well as suitable cabling solutions for connecting the sensors to the UT equipment during testing. Fibre optic, piezo-polymer and piezo-ceramic sensors underwent gamma and neutron + gamma testing at the two facilities.

The piezo-ceramic UT sensors, developed by Ionix, withstood the gamma + neutron and gamma exposure with no observable change in performance.

The piezo-polymer and fibre optic sensors, developed by PAL, survived longer than predicted, however, the polymer sensors were determined to be unsuitable for use under neutron radiation but could be utilised for the long term monitoring of radioactive waste. The fibre-optic sensors, with further development, could be implemented in an environment subject to neutron and gamma radiation.

UoS successfully modelled the radiation exposure at Dalton and were able to advise how to selectively radiation harden the sensor electronics.

XRD investigations revealed that gamma radiation reduced the crystallinity of polymer samples, nullifying the piezoelectric effect, whilst neutron + gamma radiation significantly altered the structure of the polymer, totally diminishing the piezoelectric effect.

The piezo-ceramic UT sensors developed by Ionix with the MICC type cabling were able to withstand the gamma + neutron and gamma exposure with no observable change in performance, allowing fixed point monitoring of asset integrity without shutdown or isolation in a high shine environment.

Figure 2. Radiation exposure testing at Joseph Stefan Institute (JSI), Ljubljana, Slovenia
Figure 2. Radiation exposure testing at Joseph Stefan Institute (JSI), Ljubljana, Slovenia
Figure 3. XRD results of irradiated samples
Figure 3. XRD results of irradiated samples
Figure 4. Signal to noise ratio (SNR) (circles) for two transducers irradiated in the TRIGA reactor and the reactor power (dash) over 100 hrs. Total dose recorded was 11 MGy gamma, and integrated neutron flux 2.6 x1018 n.cm-2
Figure 4. Signal to noise ratio (SNR) (circles) for two transducers irradiated in the TRIGA reactor and the reactor power (dash) over 100 hrs. Total dose recorded was 11 MGy gamma, and integrated neutron flux 2.6 x1018 n.cm-2
Avatar James Kern Senior Project Leader

James graduated from Sheffield University with a B.Eng and PhD in 1992. He worked as a metallurgist at Sandberg consultant engineers, a Senior Technologist at Rolls Royce and as Chief Materials Scientist at APPH (now Heroux Devtek), before joining TWI in 2014. He first specialised in hot forming and bonding processes before transferring to the NDT Group.

He has successfully lead numerous collaborative (Horizon 2020 and Innovate UK) and single client projects, leading research activities and developing manufacturing prototypes. Current areas of interest include radiation effects on materials and sensors, Geothermal Energy, ATEX, manufactured defects for calibration purposes and shearography.

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