Possibilities of rapid assessment of norm in oil rig waste products for environmental compliance monitoring

Abstract

Evaluation of Naturally-Occurring Radioactive Materials (NORMs) traditionally takes about a month to accomplish. This
is because two of the major naturally-occurring primordial radioisotopes,238U and232Th, decay into daughter products,
which are themselves serially radioactive. Included in both series is the radon gas, which continuously diffuses away, and
therefore keeps the system in perpetual disequilibrium. The one-month wait is therefore required to seal up the sample
after preparation and allow for the establishment of secular equilibrium within the series. This technical situation could
have enormous socio-economic implications in applications, where NORMs need to be routinely evaluated as rapidly as
possible. One major example is in the tons of waste products from the oil and gas industry, which could be deployed for
other economically-beneficial applications in the public space, provided their compliance with relevant regulatory limits
for radioactivity level could be ascertained in a timely manner. Previous workers trying to resolve this quagmire had
suggested innovative protocols involving sequential counting of the sample on different days. However, these are
generally too complex and laborious for routine applications, especially in an industrial setting involving many samples.
In this work, we have directed our attention primarily on incinerated ash obtained as secondary waste from oil sludge, and
clinkers produced with this ash. We simply evaluated the activity concentrations attributed to the238U and232Th series in
the two matrices at various pre-secular equilibrium waiting periods, and compared with results obtained after secular
equilibrium. By applying basic principles of risk assessment, we could then propose that samples with maximum activity
level of 1,175Bq/kg for238U and 700 Bq/kg for232Th measured at any time pre-equilibrium, will meet the 10,000Bq/kg
limits specified in the Nigeria Basic Ionizing Radiation Regulation 2003, for both radionuclides at secular equilibrium.