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Radiological

Case Study - Radiological Decontamination Works

Contract In Brief:  
Location: Client:
Sefton, Liverpool Sefton Metropolitan Borough Council (SMBC)
HBR Role: Radiological Constultant:
Main Contractor using
CampbellReith as design team

Main Contractor

AMEC Nuclear UK Limited
Contract Value: Contract Start:
£1.72M October 2009
HBR Site Management    
Construction Manager:

Jeysen Mootien
 

Historical Legacy:

Following a competitive tender process Blackwell were awarded the contract to remediate a former tin ore smelting site in a residential area within the Borough of Sefton, to the north of Liverpool.  

One of the key drivers of the remediation works centred on removing the historical legacy  and “perceived risk” associated with radioactivity from the smelting of naturally occurring radioactive minerals (tin ore) on-site. 

Resultant tin slag deposits which had been backfilled in distinct areas of a former clay pit contained concentrated nuclides of U-238, U-235 and Th-232 and their associated decay chains at levels identified as posing unacceptable risk of irradiation, radon inhalation and contaminant ingestion to future site users.  

Remediation Protocols:

Blackwell contracted the services of AMEC Nuclear to provide radiological support during the specialist remediation works phase. This consisted of specific on-site awareness training, development of site working practices and waste validation procedures. 

Further characterisation (non-intrusive, intrusive and radiochemical analysis) and ionising radiation risk assessment work provided sufficient data to formulate an approved remediation methodology with the Environment Agency. This consisted of waste minimisation and reuse, combined with wholesale removal of source materials to reduce measurable activity at the site to below <0.3mSv/annum. This material would then be removed to landfill under the Radioactive Substances (Phosphatic Substances and Rare Earths) Exemption Order.

Following removal of surface hard standing, delineation of zones of contamination was carried out using surface probe readings to detect gross gamma radiation. Radionuclide concentrations were typically found in the range of 1-10Bq/g with levels occasionally reaching 40-50Bq/g.

Excavation of radiologically unacceptable material then proceeded. Each dispatch volume of material was assessed for compliance under the Exemption Order by undergoing gate monitoring prior to leaving site. This involved each lorry load passing beneath a sodium Iodide probe positioned at a set height above the load and its’ activity measured at 5 predetermined intervals along the length of the lorry.

The disposal procedure relied upon microshield calculations undertaken by AMEC to determine the dose rate from the threshold activity associated with the Exemption Order.

Instrument calibration was then applied to convert the dose rate to a count rate on the probe, thus allowing quick determination of the pass/fail trigger level to be established for each load.

Failure of any load to meet the exempt activity concentration, resulted in the load being returned to the excavation for further homogenisation with remaining slag material. Material was removed in this manner for successive tin slag layers until probe readings indicated no further radioactive material was present.

These procedures were backed up by establishment of contingency measures for heavily contaminated materials, restricted access arrangements, continual radiological monitoring of site plant and site workers, and perimeter assurance air sampling and analysis.

All works were quality assured by the Client’s independent radiological and environmental consultant and the site validation report accepted by the Regulator and other stakeholder parties.

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                 Chemical v Physical v Biologicalv Complex Sorting v Water Treatmentsv Complimentary Processes

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