Page 126 - AERB Annual Report 2020

7.7 REACTOR PHYSICS STUDIES 7.7.3 Estimation of Critical Channel Power in
700 MWe PHWR
7.7.1 Coupled Neutronics-Thermal Hydraulics
Benchmark Regional Overpower Protection System
(ROPS) is one of many FOAK design features
One of the important benets of coupled incorporated in the Indian 700 MWe PHWR that
neutronics and thermal hydraulics modelling of enables reactor trip on local overpower. Critical
reactor cores is relaxation of safety margins Channel Power (CCP) is an important aspect of
without compromising the NPP safety, allowing the ROPS design, which brings out the levels of
higher operating power and extended fuel cycles. local overpower to be considered in deciding the
However, demonstrating credibility of such code trip set points. CCP for a coolant channel is the
system requires comprehensive validation power at which the coolant dry-out or fuel
exercise. Towards this, AERB has oated a melting begins. Determination of minimum CCP
benchmark exercise on coupled code systems. for the core involves solution of fuel and coolant
The benchmark exercise will be carried out in two heat transport equations to determine the Critical
phases and it has been mooted under the ambit of Heat Flux Ratios (CHFR) and fuel temperatures
DAE Steering Committee to coordinate Safety throughout the core for different reactivity device
Research (DAE-SCSR). First phase (1A, 1B and congurations. It is expected that the reactor will
1C) exercises of this benchmark are focused on operate in nominal conguration during most of
stand-alone core neutronics and system thermal its design life. CCPs for nominal core
hydraulics modelling. Phase 1A of the benchmark conguration were independently veried at
exercise is focused on assessment of steady state AERB using the in-house core thermal hydraulics
and transient core neutronics estimation model. It was found that for nominal
capability of the employed models, whereas conguration, CCP occurs due to coolant dry-out
Phase 1B and 1C are focused on assessment of (i.e. due to reduction in mCHFR below 1.1) in all
system modelling. Compilation of specications the channels. The minimum CCP (mCCP) value
and analysis of Phase 1A by AERB participants is obtained in the present analysis has been found to
completed and Phase-2 will be initiated following be in agreement with the design reported value.
conclusion of rst three exercises of Phase-1.
7.7.4 Independent Verication Analyses of
7.7.2 Stability Analysis of 700 MWe PHWR KAPS-3 Start-up Commissioning

Soft neutron spectrum and separate coolant Towards independent verication of
moderator circuit leads to weak reactivity commissioning reports on start-up of KAPP-3,
feedbacks in PHWRs. This deciency is overcome reactor physics calculations were carried out
through reactor regulating system (RRS). Partial using DRAGON-DONJON code system.
coolant voiding near channel exit tends to give Important parameters like neutron multiplication
positive reactivity feedback beyond certain level factor, reactivity device worth, calibration of
of power. However, core stability has been reactivity devices, moderator and primary heat
demonstrated by taking the credit of RRS. An transport (PHT) temperatures reactivity load etc.,
independent verication analysis of core stability were estimated for the test conditions. Analytical
has been taken up for 700 MWe PHWR. A generic ndings show good agreement between design
model was developed for stability analysis of estimates and independent analysis results for
bulk power control loop of 700 MWe PHWR most of the cases.
which was improved by incorporating zonal
models of reactor power, poison dynamics, LZCS, 7.7.5 Sensitivity and Uncertainty Studies in
in-core detectors & FMS detectors. Analysis has Core Physics
been carried out in discrete-time domain by
linearizing the system around its equilibrium Various parameters govern the analytical
points and identifying Eigen values of the closed- uncertainties in core physics. A case study on the
loop system. The dynamics of the system vary same has been performed using IAEA-CRP lattice
widely depending on operating power levels, benchmark on 37-pin fuel bundle as reference
core-fuelling states and cycle time of RRS. problem using DRAGON code. The code has a
Obtained results of gain values for bulk and zonal collection of models for simulating the neutronics
control loop have been found to be in agreement behaviour of a unit cell or a fuel cluster in a
with design reported results. nuclear reactor. The variation of the neutron
multiplication factors (k and keff), isotopic

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AERB Annual Report 2020