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Radioactive waste: monitoring packages in interim storage

An example of a typical intermediate level waste storage facility

Jonathan Cox, Babcock’s Direct Research Portfolio project manager, looks at some of the work being undertaken by a cross-industry project team to identify effective means of monitoring waste packages in interim storage facilities over an extended period, to ensure continued integrity for package disposability.

Jonathan Cox, Babcock International, UK

Monitoring and inspection strategies in place for radioactive waste packages in the interim storage facilities at the UK’s various nuclear licensed sites were developed against a background of disposal on much shorter time-scales than those currently envisaged.

Following the Committee for Radioactive Waste Management 2010 report, 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. Where a 20″40 year requirement for interim storage had been foreseen, this is now extended to up to 100″125 years.

The long-term performance and behaviour of the waste packages or containers within the interim stores are unproven on this extended time-scale, and need to be effectively monitored to ensure continued safe functionality for ongoing storage, and subsequent transport and acceptance at the GDF.

How best to achieve this is the subject of a collaborative research project funded by the Nuclear Decommissioning Authority (NDA) Direct Research Portfolio (DRP) and linked into a cross-industry integrated project team on interim storage of higher activity waste (HAW).

Additionally the Site Licence Companies (SLCs), while meeting all technical safety criteria and legislative requirements, have traditionally managed these facilities on a site-by-site basis with an individual approach to packaging practise and monitoring, and there is now an opportunity for standardization and sharing of best practices.

Ultimately the aim is to ensure a consistent and robust approach to packaging and storage lifetimes across the UK, as well as to enhance the confidence of regulators, government and local stakeholders. The best of industry expertise has been brought together for the collaborative research 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. Specifically, the objective is to increase confidence in the long-term performance and continued durability of the waste packages, and to identify promptly any unforeseen degradation mechanisms so that corrective action may be taken if necessary.

The project is a challenging one, particularly given the significant practical issues involved with using conventional techniques in a radiological environment over the time-scales in question, from identifying appropriate diagnostic equipment to the method of communicating the data to facilitate management action decisions.

Current practices

Typically, UK interim waste is stored in 304L/316L grade stainless steel or mild steel containers: generally 4-metre or 2-metre boxes, 500 litre drums, or 3 m3 box/drums. Environmental controls vary, and may be forced or natural, filtered or not, and with or without temperature and humidity controls.

Monitoring and inspection practices in the UK and internationally, i.e France, Belgium, Switzerland, the Netherlands and Japan, were also reviewed at the start of the programme, and were found to vary, ranging from none to remote controlled camera; dummy drums with coupons manually inspected; limited manual inspection, commonly visual and swabbing; non-destructive examination (NDE) testing, primarily focused on the wasteform; or destructive testing.

Having reviewed current practices, an important initial stage in the project was to identify and prioritize the parameters to be measured or data required to monitor the waste packages to identify any degradation indicators. Through workshops, 30 potential waste monitoring parameters were identified, which were reduced to 12 using a prioritizing process based on criteria such as the significance for remedial action, predictive value and applicability.

These were taken forward and subsequently reduced further, with industry involvement, to six key monitoring needs, namely:

  • External corrosion: applicable to all waste packages, and having the potential to lead to package failure if allowed to continue;
  • Salt/chloride deposition on the waste package: applicable to all waste packages and a precursor to external corrosion, requiring remedial action when combined with certain temperature and humidity conditions;
  • Package lifting feature condition: applicable to all waste packages to ensure that the package can be moved or lifted when required;
  • Deformation of the waste package: required for most waste packages, and likely to lead to package failure if not addressed;
  • Identification of the waste package: applicable to all packages to ensure a suitable data management strategy and maintenance of the audit trail; and
  • Internal crack formation: a requirement for some waste packages, potentially requiring remedial action.

Technology selection

Potential technologies to obtain this required monitoring data were then considered. Some 50 potential technologies at various stages of development were assessed using a holistic approach.

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. This process led to a filtering down to eight technologies, of which five were investigated further during 2009″2010, including: lifting features inspection; in-situ monitoring; salt deposition; SMART coupons; and inductive coupling.

Lifting features inspection

The lifting features investigation, for example, was concerned with the use of inspection technologies to assure the structural integrity of any given lifting feature during storage, inspection and exporting.

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 ‘unmovable’ waste packages. The structural integrity of waste package lifting features is of paramount importance to the safe functionality of waste packages, and there is currently no baseline technology routinely used to monitor this.

A passive RF sensor tag concept

This project combined work undertaken by both NNL with two universities and Babcock with the support of The Technology Partnership (TTP), an organization specializing in cross-industry technology transfer with extensive experience outside the nuclear industry of techniques that can be used for monitoring the integrity of materials and packages. Work carried out by NNL with Bristol University looked at the suitability and performance of 2D ultrasonic arrays inspection for 3D inspection of the lifting feature and, with Imperial College London, guided acoustic wave inspection for weld inspection of the lifting feature.

NNL has reported that the 2D arrays 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 arrays technology for the imaging through an isotropic homogeneous material, but the imaging through the weld itself is not trivial and requires further work on the data processing methods and inspection strategies, such as the positioning of the array. The guided acoustic wave inspection method, on the other hand, has been evaluated as not suitable for this application.

Non-Linear Analysisà‚ 

Work conducted 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, and compared it to existing approaches including visual inspection, liquid or dye penetrant testing and eddy current sensing.

The non-linear analysis 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 antennae is separated from the necessary sensitive electronic equipment with cabling, allowing equipment sensitive to degradation in radiation fields to be located in a shielded environment.

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 optimize 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 its detector signal at a different frequency to the probe signal. A practical result of this is that reflections from surrounding, undamaged drums will not cause a problem for the detector because they will be filtered out along with the driving probe signal. This offers the prospect that this inspection technique could perform better than others in crowded storage areas.

The non-linear analysis technique has been found to be potentially suited to the passive inspection of waste package lifting features, with a number of benefits. These include being operational in close packed stores, having potential for crane mounting, being radiation robust, having a range several centimetres from the container surface, and having an antenna length of just a few centimetres.

Furthermore, the commercial availability of this technique, to a depth of 22 microns, significantly simplifies initial tests and potentially provides a low-cost development route. This is now, therefore, being taken forward for further investigation and development.

Further research

The development programme, which will combine Babcock’s in-depth, specialist knowledge of the UK nuclear industry and TTP’s knowledge of the technology, will be taken forward under a phased approach in order to address the largest technical risks early on in the development, while allowing good control of costs and time-scales.

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

A shielding wireless sensor concept

This will be followed by prototype test system development. Following successful proof-of-principle experiments and initial risk reduction, a prototype test system can be specified, designed and built. This 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 organizing site visits to assess any limitations on the practical application in store environments (such as space limitations); liaising 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 the lifting features. Ultimately, in-store 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.

Other technology investigations

Parallel investigations were undertaken into in-situ monitoring, salt deposition, SMART coupons, and inductive coupling. Briefly, the in-situ monitoring research, targeted at existing stores where packages are stacked with minimal interstitial spacing, sought to identify means of monitoring waste packages without the need to move them.

This was looked at in terms of magnetically coupled robots, and low impact sensor arrays. The project concluded that robotic technology was not suitable as a generic solution for the remote inspection of waste containers, although in some instances the technology may be applicable 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 were selected using a filtering process that considered installation and operational practicalities. The two selected systems were based on wireless powered sensor packets sufficiently small to fit between close packed container columns and not requiring a line of sight between the sensors and the receiving/transmitting antenna.

The salt deposition investigation looked at the achievability of in-situ monitoring of salt deposition, which is a precursor to the onset of external corrosion, within the cost and deployability constraints of most waste stores, and considered the most suitable method for the nuclear industry. It concluded that surface acoustic wave (SAW) sensors could be integrated into SMART coupons, while laser induced breakdown spectroscopy (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 potential issues to address, this system relies on ‘line of sight’ optical principles, which fibre optics may alleviate but their installation in a shielded store could prove challenging. 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 ” not a factor for mild steel mini-store storage solutions.

The SMART coupon method to monitor local environmental conditions in-situ would address the labour-intensiveness of current coupon-based monitoring methods, being self-measuring and thereby avoiding 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 outstanding technical challenges here revolve around how to remotely power and install such systems inside a shielded store.

Inductive coupling facilitates non-destructive measurement of the wasteform using embedded sensors within the waste package, while maintaining container integrity, for long-term in-situ monitoring, as well as providing a reliable means for unique identification. This is a similar principle to the radio frequency identification, or RFID tags, used in stores and warehouses.

Previous work conducted in 2009 investigated the functionality of the technology in response to accelerated ageing by gamma radiation. Outstanding technical challenges to be addressed here 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.

Looking ahead, of the five projects reviewed in 2009-2010, four have been taken forward. Further investigations are being progressed into the 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, and phased results of these will be available from the UK NDA during the course of this year.

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