Supporting waste activities at Harwell

MAGNOX HARWELL’S LIQUID EFFLUENT TREATMENT plant (LETP) was used for decades to process liquid effluent from the ‘main site’ nuclear research operations, and over recent years has undergone decommissioning works to reduce above-ground structures to base slab. What remains are many sub-surface tanks with associated pipe-runs, hard standings, roadways and vegetated areas, which have to be removed and the land remediated. The work entails excavation of all materials within the site boundary into bulk bags, which will be assayed to determine their radiological content and the waste disposal route.

The end point is to provide information to Magnox to demonstrate that the site is below regulatory concern, so that nuclear licence and environmental permits can be terminated and the land released. It was critical to be able to provide real-time data and a system for managing it to enable radioactive and out-of-scope materials to be sent off-site daily.

Magnox required waste segregation at the point of production, to satisfy obligations under the ‘waste hierarchy’ and maximise use of better management options, and as part of its ‘duty of care’ over non-active waste. This meant Nuvia had to optimise current waste characterisation, such as the Gamma Excavation Monitor (GEM) system and the High Resolution Assay Monitor (HIRAM), but also develop additional systems, including an information management system to speed up the data handling process, improve quality controls and reduce transcription errors.

Scope of work

Nuvia had to design and plan the waste assay process to support the remediation works undertaken by Magnox. It also had to provide health physics, radiation protection and radioactive waste advice to the remediation contractor. Magnox required 555 waste bags to be processed through the assay area per week — a significantly higher throughput than Nuvia had achieved for any other project. It required a great deal of preparation and some innovation to track waste bags as they were received into the assay area and to manage and process the data to facilitate consignment off-site. That had to be done on the same day for out-of- scope wastes, and within three days for active wastes.

To track waste bags and manage the data, Nuvia designed and developed an information management system (IMS) comprising a database and associated software. Bags generated in the remediation zone have barcode labels applied. On receipt into the waste assay facility, tablets with barcode reader technology are used to scan and log the bags into the database. The tablets are also used to interrogate the IMS, enabling the operators to locate bags and confirm their status. Following assay, using high resolution gamma spectrometry (HRGS), results are recorded in the IMS and can be accessed by the characterisation consultants in the on-site office, where they perform quality checks before making the data live in the system.

To achieve the throughput of 555 bags per week, Nuvia proposed to use four HRGS systems. The operators in the remediation zone are responsible for applying the barcode, and other information on the label such as the material content, weight and fingerprint zone. This allowed the HRGS to be fully automated, so that at the end of a five-minute count time, the results could populate a formulated spreadsheet providing radionuclide activities with the appropriate fingerprint factored in. This facilitated reporting of the waste category and additional reporting requirements, such as delicensing criteria status, European Waste Code (for out of scope disposal purposes), waste acceptance criteria, nuclear material and transport calculations. A metric was also developed to check the validity of the applied fingerprint automatically, based on gamma emitting radionuclide ratios.

The HRGS systems are shielded in four concrete units, and telehandler activity around the units was a health and safety issue. To optimise throughput and improve safety, Nuvia designed a light signalling system, which tells the HRGS operator when a count has finished, and tells the telehandler drivers when they can change the waste package on the turntable. As there are two telehandlers serving the four turntables, this has the added benefit of keeping people and plant separated. In addition, the lighting system control box also controls the resting position for the turntable, so that it always lines up in the correct position for the telehandler to access the bag handles.

Out of scope bags are transferred and scanned into the appropriate skip, based on their material content. Once the skip is full, the office consultants generate the ‘duty of care’ paperwork for Magnox to approve. Following approval, the skip is collected and driven through a vehicle monitor, operated by Nuvia, where the skip barcode is scanned and registered in the IMS as having left site. OOS materials are either recycled (e.g. concrete), or sent to landfill. Up to 12 skips of OOS material is transported off site per day.

Nuvia developed a methodology, based on MCNP calculations, to investigate the reason in the event of an alarm, which could be triggered by an elevated amount of naturally occurring radioactivity associated with bricks.

Active bags are transferred to the waste storage facility, where they are overpacked in PacTec bags and prepared for disposal. Prepared bags are scanned into consignment bays and once full, the office consultants generate the consignment paperwork for Magnox to approve and submit for authorisation for disposal. Nuvia loads the consignment onto trailers for the carrier to collect, and a maximum of four trailers can leave site per day. From assay to consignment off-site takes approximately three days.

To optimise segregation at source and to facilitate reuse of materials, for ramps and batters, in the remediation zone, Nuvia provided the GEM system to enable rapid screening of excavated materials. The GEM was designed for land remediation projects, to provide a rapid screening tool for excavator buckets of material. The presence of the excavator bucket, lowered over the detector, automatically triggers proximity switches, which initiate the counting routine. At the end of a short count time — typically 10 seconds — the material category is indicated by means of a ‘traffic light’ system: red for LLW, amber for VLLW or green for OOS, based on the activity in the Cs-137 photo-peak. The GEM is also used to validate the remediation end-point, i.e. provide a real-time indicator to the operators that the end-point — delicensing criteria — has been attained.

Conclusion

The IMS has engineered-out error traps, such as transcription errors, and has facilitated the requirement for a large throughput. Developing the IMS and optimising existing technologies provided Magnox with a quality- controlled waste management solution. It enabled them to consign 555 bags of waste off-site per week and satisfy obligations under the waste hierarchy, use better management options and fulfil its Duty of Care for non-active wastes.