NDE & inspection: waste monitoring

A watching brief

15 August 2011



The need to ensure the integrity of interim waste packages over a longer-than-envisaged design life has led to an integrated approach to identify effective waste package monitoring solutions. By Jonathan Cox


Interim storage facilities at the UK’s various nuclear licensed sites were envisaged to be required for a 20-40 year life. But following the UK Committee for Radioactive Waste Management report on Managing Radioactive Waste Safely, and the recognition that a Geological Disposal Facility (GDF) may not now be available until 2040, government policy on radioactive waste management has evolved. For interim waste storage, this has resulted in a need to extend the design life for stores, up to 100-125 years.

The long-term performance and safe functionality of the containers within these facilities for storage, transport and acceptance at the GDF, on the 100-150 year scale, is unproven. Moreover, these facilities are currently managed by the Site Licence Companies (SLCs) on a site-by-site basis which, while meeting the regulations of the Nuclear Installations Inspectorate (the UK regulator), also provides an opportunity for standardisation and sharing of best practice.

With this in mind, a collaborative research project has been ongoing, funded by the NDA Direct Research Portfolio (DRP) and linked into a cross-industry integrated project team on interim storage of Higher Activity Waste (HAW). The objective is to increase confidence in the durability of the waste packages, and to identify promptly any unforeseen degradation mechanisms so that corrective action may be taken if necessary. The best of industry expertise has been brought together for this project, including Babcock, National Nuclear Laboratory (NNL), and Hyder Consulting, to review and provide tools and techniques that could be implemented in current and future stores to identify precursor indicators for package failures.

Evaluation of practices

To begin the programme, current practices in the UK and internationally (France, Belgium, Switzerland, Netherlands and Japan) were reviewed. This identified that in the UK interim waste is typically stored in 304L/316L stainless steel or mild steel containers (generally four- or two-metre boxes, 500 litre drums, or 3m3 box/drums), and that air circulation controls vary from forced or natural, filtered or not, and with or without temperature and humidity controls. UK and international monitoring and inspection practices included none, remote-controlled cameras, dummy drums with manually-inspected coupons, limited manual inspection (commonly visual and swabbing), non-destructive examination (NDE) testing (primarily focused on the wasteform), and destructive testing.

The next stage was to identify the data that is needed to monitor packages for safety, and identify where technology gaps exist. Thirty monitoring data requirements were reduced (with industry input) to six. These are: external corrosion (having the potential to lead to package failure if allowed to continue); salt/chloride deposition on the waste package (a precursor to external corrosion); package lifting feature condition (to ensure that the package can be moved or lifted when required); identification of the waste package (to ensure a suitable data management strategy and maintenance of the audit trail); deformation of the waste package (likely to lead to package failure if not addressed); and internal crack formation (a requirement for some waste packages, potentially requiring remedial action).

Assessment of technologies to obtain these data requirements was approached holistically. For example, consideration was given to whether the power required by a given sensor can be delivered, whether it can be installed, how applicable it is across the industry platforms, and so on. Of 50 potential technologies, five were taken forward at this stage. The five investigated in 2009-2010 included: inductive coupling, SMART coupons, salt deposition, in-situ monitoring and lifting feature monitoring.

Inductive coupling in practice involves presenting a probe containing a ferrite core coil close to the waste container. The probe would then receive whatever data the embedded sensor was monitoring. The principle is identical to that used by RFID tags in stores and warehouses. It facilitates non-destructive measurement of the wasteform using embedded sensors within the waste package, while maintaining container integrity. The technology allows long-term in-situ monitoring as well as providing a reliable means for unique identification. Previous work (in 2009) has investigated the functionality of the technology in response to accelerated ageing by gamma radiation. Outstanding technical challenges to be addressed include the identification of power matched sensors, and the method of locating the external circuit to negate the need to physically move the container to an inspection bay.

SMART coupons are suspended from the top of the drum on specially designed hangers (Figure 1). These are manufactured from the same material as the drum skin, and so can be inspected either in-situ or externally for corrosion, weight loss and chloride by swabbing and subsequent chemical analysis. The SMART coupon method to monitor local environmental conditions in-situ, would, importantly, address the labour-intensiveness of current coupon-based monitoring methods. Since it is self-measuring, it thereby avoids the need for physical extraction for visual inspection and chemical analysis. This method aims to measure local corrosion and factors influencing the likelihood of corrosion, including temperature and humidity using thermal conductivity humidity sensors, and corrosion using field signature method monitoring technology, to provide proactive, real-time, remote monitoring for the store and its contents. [The field signature method maps changes in the electrical field induced in the surface of a metal object; changes relative an initial reference point (the ‘signature’ in the name) can indicate the presence of cracks, corrosion or other material changes [1]. The outstanding technical challenges here revolve around how to remotely power and install such systems inside a shielded store.

The salt deposition investigation sought to identify whether in-situ salt deposition monitoring (being a precursor to the onset of external corrosion) is achievable within the cost and deployability constraints of most waste stores, and what might be the most suitable method for the nuclear industry. Current commercially-available devices can transmit and receive Surface Acoustic Waves (SAW) that propagate over the surface of an elastic body. It concluded that SAW sensors could be integrated into SMART coupons. Another technology, laser-induced breakdown spectroscopy (LIBS), ablates a sample of surface material, and then analyses the contents of the subsequent plasma plume. The investigation concluded that LIBS could provide container deposition measurements. LIBS is field-deployable, with fibre optics allowing access to obscured surfaces. Additionally, it has the benefit of being able to withstand harsh environments (temperature, dust, shock and vibration), and is operated by wireless remote control. Among the potential issues to address is this system’s reliance on line-of-sight optical principles, which could prove challenging in a shielded store. An additional consideration is the extent to which the LIBS technique affects the passivating chromium oxide layer on the surface of the stainless steel containers (although this would not be a factor for mild steel mini-store storage solutions).

In-situ monitoring research examined means of monitoring packages stacked with minimal interstitial spacing (as in existing stores) without moving them. Two technologies were considered: magnetically-coupled robots and low-impact sensor arrays. The project concluded that robotic technology was not suitable as a generic solution, although in some instances the technology may be suitable for limited camera inspection in vault-type stores, and other potential niche applications.

With regard to low-impact sensor array methods, ten system concepts were developed based on commercially-available solutions, and two selected based on installation and operational practicalities. The two selected systems were based on wirelessly-powered sensors that are small enough to fit between closely-packed container columns and which do not require a line of sight to the receiving/transmitting antenna. In the first system, shielded wireless sensor units use standard communications methods to relay data to an antenna in the roof of the store (Figure 2). Sensor lifetimes could be maximised by the use of radio frequency (RF) trickle-charging of an on-board power source (battery or capacitor). Since this approach primarily integrates readily- available technologies, it could be fairly rapid to deploy. The use of an expandable core electronics platform allows additional sensors (such as those requiring more development time) to be added easily to future generations of the unit.

In the second system, an array of passive resonant RF sensors are deployed (Figure 3). Cables and waveguides can be used to couple signals to and from particular sets of sensors in a local area. Antennas are then installed in the ceiling to interface with the sensors. Where this is not practical, a robot running on the overhead crane tracks could be used to interface with the sensors. This concept requires sensors to be simple, robust and intrinsically radiation-hard. Sensing methods such as surface acoustic wave, microwave resonators and LC (inductor-capacitor) resonators are likely to be suitable for this purpose.

Structural integrity of lifting features

A further project looked at monitoring the structural integrity of waste package lifting features, for which there is currently no baseline technology routinely used (Figure 4). The structural integrity of the waste package lifting features is of paramount importance to the safety functionality of waste packages. From a storage perspective, a key benefit to the NDA and associated store operators lies in eliminating the potential for lifting features to degrade to the point of mechanical failure, thereby eliminating the occurrence of ‘un-movable’ waste packages.

This project combined work undertaken by both the National Nuclear Laboratory with two universities, and by Babcock with the support of cross-industry technology transfer organisation The Technology Partnership (TTP). With Bristol University, NNL looked at the suitability and performance of two-dimensional ultrasonic array inspection for three-dimensional inspection of the lifting feature. With Imperial University, it looked at guided acoustic wave inspection of the lifting feature welds. NNL reported that the 2D array technology has shown potential for being suitable for inspection of the lifting features, but requires significant further development and test work to be undertaken at Bristol University. Bristol University has a good understanding of the 2D array technology for the imaging through an isotropic homogeneous material, but imaging through the weld itself is not trivial, and requires further work on data processing methods and inspection strategies (such as positioning of the array). The guided acoustic wave inspection method, on the other hand, was determined not to be suitable for this application.

The work carried out by Babcock with TTP assessed the suitability and performance of non-linear analysis as a sensor technique for the detection of corrosion and cracks in waste packaging lifting features. It also compared non-linear analysis to existing approaches, including visual inspection, liquid- or dye- penetrant testing and eddy current sensing. The technique was found to offer a number of advantages. The required antenna size, for example, is likely to be only a few centimetres, and the technique is likely to work with operational distances of several centimetres or more. The antenna can be separated from the necessary sensitive electronic equipment with cabling, allowing non-radiation-hardened equipment to be shielded. Moreover, while the technique might initially be tested in the inspection area of Intermediate Level Waste (ILW) stores, there is no reason why it could not later be used in crane-mounted or similar in-situ monitoring situations.

The greater likelihood of identifying cracks using this method is another advantage. Because the system is intended to illuminate a local area with radio signals and listen for the characteristic frequencies created by cracks and corrosion in that area, it is less likely to miss a crack than direct millimetre-by-millimetre inspection of the surface. During development, it will be important to optimise the trade-off between detecting every crack and obtaining false positive signals from detecting corrosion and/or cracks that are not relevant. A further advantage lies in the reduced vulnerability to signal reflections from surroundings. An intermodulation- (or harmonic-) based system has a detector signal tuned to a different frequency than the probe signal. As a result, reflections from surrounding, undamaged drums will not cause interference since they will be filtered out along with the driving probe signal. This feature promises improved results compared with other methods in crowded storage areas.

The issue of interference between intermodulation products and communications networks is being addressed by advanced analysis technology in the telecoms sector. The lowest commonly-used telecommunications frequency band is around 400 MHz (for Tetra emergency services radios). The storage containers have a skin depth of around µm at this frequency. With custom filters, commercial detection equipment may operate down to around 100 MHz, where the skin depth is around µm. Based on these skin depth values, currently-available commercial equipment could examine near-surface corrosion.

Non-linear analysis has been found to offer potential to be suited to the passive inspection of waste package lifting features. Its benefits include operational potential in close-packed stores; potential for crane mounting; radiation robustness; relatively long range of several centimetres from the container surface; relatively short antenna length of just a few centimetres. Further, the technology’s commercially availability (to a 22 micron depth) significantly simplifies initial tests and potentially provides a low-cost development route. The system is now, therefore, being taken forward for further investigation and development.

Further investigations

A joint development programme will combine Babcock’s knowledge of the UK nuclear industry and TTP’s knowledge of the technology. The programme will proceed in a phased approach, to address the largest technical risks early on in the development, while allowing good control of costs and timescales. The exact schedule depends on the availability of further funding.

Initial stages will include securing provision of samples, and experimental proof-of-principle, to test and refine the measurement capability of the technique. As the technology is not established for the required nuclear application at present, there is a risk that its sensitivity may be too low to be useful, or that the detector could be swamped with signals from unimportant sources. These issues can only be answered by setting up a physical trial with off-the-shelf equipment from the communications industry in a mock-up environment.

Development of a prototype test system will follow. After successful proof-of-principle experiments and initial risk reduction, a prototype test system can be specified, designed and built. This stage is expected to include further consultation with potential users of the system, along with further development and testing of different antenna arrangements and signal processing schemes.

Further stages will include organising site visits to assess constraints on the practical application of the system in store environments (such as space limitations); liaison with store operators to build industry awareness of the technique; and facilitating visits to TTP’s test facility for site operators to see the techniques in operation. Stress analysis modelling may also be undertaken to assess the potential management action required as a consequence of faults found in lifting features. Ultimately, trials of the prototype system in an ILW store environment will be undertaken to investigate longer-term performance, usability, and radiation hardness, and to allow further comparison of results with those obtained by other methods.

In summary, of the five projects reviewed in 2009/10, four are now being taken forward in 2010/11: lift feature degradation monitoring using non-linear analysis, inductive coupling for wasteform monitoring, laser-induced breakdown spectroscopy to monitor salt deposition, and the SMART coupon to monitor local environmental conditions. Phased results of these investigations will be available from the UK Nuclear Decommissioning Agency from spring 2011.



References

[1] “Weld Root Corrosion Monitoring With A New Electrical Field Signature Mapping Inspection Tool” NACE International - 1995 Conference on Corrosion & Infrastructure, November 28-30, 1995, Baltimore, Maryland, Downloaded from www.atlas-dt.co.jp
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Fig. 1: Drum with SMART coupons Fig. 1: Drum with SMART coupons
Fig. 2: Powered wireless sensing Fig. 2: Powered wireless sensing
Fig. 3: Passive RF array concept Fig. 3: Passive RF array concept
Fig. 4: Gantry cranes lift radwaste packages from above Fig. 4: Gantry cranes lift radwaste packages from above


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