APPLICATION OF SAFETY ASSESSMENT METHODOLOGIES IN DISPOSAL OF DISUSED SEALED SOURCES USING BOREHOLE CONCEPT
APPLICATION OF SAFETY ASSESSMENT METHODOLOGIES IN DISPOSAL OF DISUSED SEALED SOURCES USING BOREHOLE CONCEPT
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Date
2011-07
Authors
ALIYU, SA’ID
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Abstract
This Dissertation develops safety assessment model appropriate for use in calculating
doses to a resident farmer living in the far future in the vicinity of a borehole disposal
facility around Zaria environs. Doses are assumed to arise from potential releases from
the disused sources disposed in the borehole facility having inventories typical to those
found in the African countries. A list of features, events and processes (FEPs) was
developed, relevant to Zaria biosphere and an assumed requirement to calculate doses to
the farmer via abstraction well water. An ‘interaction matrix’ was also developed to
help define the FEPs and to identify the relationships between them. These relationships
define how radionuclides escape from the container, migrate and accumulate in
biospheric media, thereby giving rise to radiation exposure. FEPs were audited for
inclusion in the matrix, and either include with an explanation of how, or excluded with
an explanation of why. The doses to the farmer were calculated using GoldSim
computer model. The model presented is probabilistic in nature that will reflects the
current state of knowledge about the site by using probability distributions to capture
what is expected (average or central tendency) and the uncertainty (standard deviation)
associated with input parameters. The model was then run using Monte Carlo
simulation over 100 realisations in each case. Each realization represents a possible
combination of input parameter value. The output estimated values of contaminant
release rate, concentration in environmental media and resulting doses to human
receptors are obtained. Statistical summaries of the result were then compared with
regulatory performance objectives (dose constraint). Initially the average value was
found to be 0.76mSv/yr, which is greater than the dose constraint (0.3mSv/yr). This was
found to be due to the daughters of 241Am radionuclide especially 239Np which has a
Total Effective Dose Equivalent (TEDE) of 0.761mSv/yr. The 241Am inventory was
then reduced to a value (2.95MBq) that gives a dose limit of 0.296mSv/yr.
Description
A DISSERTATION SUBMITTED TO THE POSTGRADUATE SCHOOL,
AHMADU BELLO UNIVERSITY, ZARIA. NIGERIA.
IN PARTIAL FULFILLMENT FOR THE AWARD OF DOCTOR OF
PHILOSOPHY IN PHYSICS
DEPARTMENT OF PHYSICS
AHMADU BELLO UNIVERSITY, ZARIA.
NIGERIA
July, 2011
Keywords
APPLICATION, SAFETY, ASSESSMENT, METHODOLOGIES, DISPOSAL, DISUSED SEALED, SOURCES, BOREHOLE, CONCEPT