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Free-Release Monitoring Equipment in the UK Nuclear Power Industry

[+] Author Affiliations
M. P. Pottinger, C. H. Orr

BNFL Instruments, Calderbridge, Cumbria, UK

Paper No. ICEM2003-4564, pp. 71-80; 10 pages
doi:10.1115/ICEM2003-4564
From:
  • ASME 2003 9th International Conference on Radioactive Waste Management and Environmental Remediation
  • 9th ASME International Conference on Radioactive Waste Management and Environmental Remediation: Volumes 1, 2, and 3
  • Oxford, England, September 21–25, 2003
  • Conference Sponsors: Nuclear Engineering Division and Environmental Engineering Division
  • ISBN: 0-7918-3732-7 | eISBN: 0-7918-3731-9
  • Copyright © 2003 by ASME

abstract

In the nuclear power industry, many techniques are used to confirm that items are suitable for free-release. These techniques usually involve monitoring of the items with at least one type of radiometric instrument, to ensure that no significant quantity of man-made radioactivity is present. These monitoring techniques depend on the stage in the clearance process, the application and the size of the article being monitored. The UK Radioactive Substances Act has a Substances of Low Activity (SoLA) exemption which allows for articles and waste that have a man-made radioactive content less than 0.4 Bq g−1 , to be classified as non-radioactive and be discharged from site in similar fashion to conventional wastes. Identifying whether the waste meets the criteria is technically challenging due to the low level of this exemption. Consequently the detection limits of most common portable radiation protection instrumentation is higher than this level. Historically portable alpha and beta contamination instruments are used as part of the clearance process; these are reasonably effective at monitoring the surface of an object for the presence of radioactivity, but far less so for bags of waste where alpha and beta emissions are easily absorbed within the waste. Portable gamma only contamination instruments have proved effective where some contaminants emit gamma radiation. However even these instruments have some difficulty in confirming that an article is “free” from man-made radioactivity where the mass of the article is below a critical mass. In this industry, the radioactive fingerprint will typically include an easily detectable gamma emitting radionuclide such as 60 Co, or 137 Cs, which may be used as a tracer for other radionuclides which cannot be easily detected with such an instrument. Installed clearance (bag) monitors are very effective where the user has a good knowledge of the radioactive fingerprint, and where there are significant (more than 10%) gamma emitting radionuclides in the fingerprint. They are ineffective where pure alpha and beta emitters dominate the radioactive fingerprint. These monitors are capable of monitoring to 1/10 of the SoLA exemption level for 60 Co, with a small variation in response across the volume of the measurement chamber. Inevitably these instruments are also sensitive to NORM, so a good knowledge of NORM activity concentrations is each waste stream is required for adequate compensation. Vehicle monitors are useful for reassurance monitoring, although due to the variable nature of each waste consignment and the high background count rate, their high detection limit means they are not suitable as the sole means of free release monitoring.

Copyright © 2003 by ASME
Topics: Nuclear power

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