Abstract:
Saeed Ehsan Awan, PhD, Department of Physics & Applied Mathematics, PIEAS, June 2012.
"Kinetic Simulation, Sensitivity Analysis of Fission Product Activity and Source Term
Evaluation for Typical Accident Scenarios in Nuclear Reactors”; Supervisor: Dr. Nasir. M.
Mirza; Co-Supervisor: Dr. Sikander M. Mirza; Department of Physics & Applied
Mathematics, PIEAS, Nilore 45650, Islamabad.
With growing demands of safe and reliable energy resources worldwide, nuclear power
plants present viable option. A two third majority of these plants are PWRs. In
comparison with their competitors, PWRs suffer from significantly higher dose rate due
to radioactivity in the primary circuit which is dominantly contributed by corrosion and
followed by fission products leakage from fuel. There has been extensive investigation in
developing corrosion resistance alloys. But the problem of corrosion product activity in
primary circuit has aggravated in view of trend towards high burn-ups, high temperatures,
and longer-life time reactors. Under this scenario, the significance of fission products
releases becomes even higher. The fission product activity (FPA) is considered to be the
second leading contributor towards prevalent radiation levels in Pressurized Water Reactors
(PWRs). The elevated radiation level results in delay and prolongation of routine
repair/maintenance tasks of reactor’s cooling system, which not only reduces its effectiveness
but also results in several million dollars revenue loss per power plant annually. However the
reliable estimates of fission product activity (FPA) are also significant for the evaluation
of fuel performance, assessment of radiological consequences in case of any accident
releasing radioactivity and scheduling repair/maintenance tasks. The detailed knowledge
about radioactivity build up and sensitivity analysis of fission product activity (FPA) is
essential for reducing the plant maintenance time, which also helps to reduce the dose for
plant operators and general public.
In this work, first a model is developed for dynamic and static sensitivity analysis of
fission product activity in primary coolant of typical pressurized water reactor (PWR). It has
been implemented in the FPCART based computer program FPCART-SA that carries out
sensitivity analysis of fission product activity (FPA) using both static as well as dynamic
approaches. For long steady power operation of reactor, the computed values of normalized
static sensitivity have been compared with the corresponding values obtained by using the
dynamic sensitivity analysis. The normalized sensitivity values for the reactor power (P),
failed fuel fraction (D), Coolant leakage rate (L), total mass of coolant (m) and the let down
flow rate (Q) have been calculated and the values: 1.0, 0.857, -2.0177 × 10-6, 2.349 × 10-4,
-2.329 × 10-4 have been found correspondingly for Kr-88 with the dominant values of fission
product activity (FPA) as 0.273 μCi/g.
In the second part of this study, evaluation of time dependence of source term has
been carried out for a typical reactor system. The modeling and simulation of release of
radioactivity has been carried out by developing a computer program FPARA which uses
the ORIGEN2 code as subroutine, for core inventory calculations. Time dependent release
of fission product activity to the containment and air has been simulated for loss of coolant
accident scenarios. For noble gases, iodine and for aerosols, the release rate studies have
been carried out for different leakage rates from containment. Effects of fraction of source in
the coolant that is directly available after the accident on volumetric fission product activity
were studied. Results show that volumetric activity in the containment air for different
fission products remains strong function of decay constants, leakage rates, retention factors,
deposition rates and fractional release rates.