atw - International Journal for Nuclear Power | 01.2020

atw Vol. 65 (2020) | Issue 1 ı January
Thermal-Hydraulic Analysis for
Total Loss of Feedwater Event in PWR
using SPACE Code
MinJeong Kim, Minhee Kim, Junkyu Song, Bongsik Chu, Jae-Seung Suh and Hyunjin Lee
After the Fukushima nuclear accident, in Japan, comprehensive safety assessments for nuclear power plants are
performed by regulators around the world. As a part of the safety enhancement effort, additional failure of the safety
components are considered and to maintain safety margin, review and improve emergency procedures. In Korea, a new
regulatory requirement is introduced, which requires all nuclear power plants to submit Accident Management Plan
(AMP) that covers design basis accidents, multiple failure accidents and severe accidents.
Total Loss of Feedwater (TLOFW)
event is one of the main multiple
failure accident which assumes failure
of both main feedwater and auxiliary
feedwater system. Since there is no
feedwater supply to steam generators,
heat cannot be removed through
steam generators. In TLOFW event,
primary side feed and bleed operation
is manually performed to remove
heat. Feed and bleed operation continues
until reactor coolant system
(RCS) is cooled and depressurized to
the point where shutdown cooling
system can be used to remove heat
from RCS.
In this paper, thermal-hydraulic
analysis of TLOFW event for OPR1000
plants is performed to evaluate the
validity of RCS cool down strategy
using Safety and Performance Analysis
Code for Nuclear Power Plants
(SPACE). Hanul units 3&4 are selected
as the reference plants and analysis
results show that the RCS cool down
strategy through the feed and bleed
has sufficient core cooling capacity
which prevents core damage.
1 Introduction
After Fukushima nuclear accident, in
Japan, nuclear regulators of around
the world launched a comprehensive
check for their nuclear power plants.
They concluded that nuclear power
plants should consider accidents of
Design Extension Condition (DEC).
Considering beyond design basis
accidents has become very important
for developing cool down strategies
for the Reactor Coolant System (RCS)
and recovery actions. It is also necessary
to consider additional failure of
the safety components in terms of
sufficient safety margin with applying
of proper emergency operating procedures
[1].
The revision of the nuclear safety
act in June, 2015 required all nuclear
power plants in Korea to submit
Accident Management Plan (AMP)
that covers design basis accidents,
multiple failure accidents and severe
accidents.
The Total Loss of Feedwater
(TLOFW) event is one of the multiple
failure events. TLOFW assumes that
the feed water supply is completely
stopped by failure of both main feedwater
and auxiliary feedwater(AFW)
due to pump failure, pipe break or
other. Since there are two motor
driven AFW pumps and two turbine
driven AFW pumps, probability of
TLOFW occurring is very low. There
are several safety limits related to the
TLOFW event. Core damage from fuel
heat-up should not occur during the
event. The maximum allowable fuel
cladding temperature is 1,204 °C
(2,200 °F ). To maintain fuel cladding
temperature below the limit, it is
necessary for the primary system to
have sufficient core cooling capability.
When heat removal through the
secondary system is not available, the
decay heat of the core should be
removed by rapid depressurization of
the primary system and operation of
the Emergency Core Cooling System
(ECCS). According to recent studies
on the TLOFW event [2-6], the feed
and bleed operations has been one of
the useful strategies for removing the
decay heat. The OPR1000 plants have
been designed to manually operate
the feed and bleed strategy during the
TLOFW event by using the Safety
Depressurization System (SDS). The
SDS valves provides rapid depressurization,
which is connected to the
top of the pressurizer with two flow
paths. The feed and bleed operations
can be started after the pressure of
the primary system reaches safety
injection actuation point.
In this paper, we present thermalhydraulic
analysis for the TLOFW
event assuming the loss of the secondary
cooling function by the failure of
main feedwater and auxiliary feedwater
system. We use the Safety and
Performance Analysis Code for
Nuclear Power Plants (SPACE) code
which is an advanced thermal hydraulic
analysis code with two-fluid and
three-field governing equations [7].
The comparative study covers three
cases according to operations of the
SDS and safety injection system
during the TLOFW event. We also
examine the effectiveness of the RCS
cool down strategy through the feed
and bleed operations in accordance
with the emergency operating procedure
(EOP). The reference plants
for this study are the Hanul units 3&4.
2 Analysis information
2.1 SPACE code
The Korea Hydro & Nuclear Power Co.
through collaborative works with
other Korean nuclear industries and
research institutes has been developing
the thermal-hydraulic analysis
code for the safety analysis of the
Pressurized Water Reactors (PWRs),
which was named the SPACE. The
SPACE is the best-estimate two-fluid
and three-field analysis code for
analyzing the safety and performance
of the PWRs. The code has been
developed to improve the prediction
accuracy of the thermal hydrodynamic
behavior of the nuclear reactor
system in transient conditions. The
semi-implicit scheme has been used
for the time integration method. The
SPACE code consists of the package of
the input and output package, the
reactor kinetics model, the hydrodynamic
model, and the heat structure
model.
The hydrodynamic model package
is composed of hydraulic solver, constitutive
models, special process
models, and component models. The
hydraulic solver is based on two-fluid
and three-field governing equations
25
RESEARCH AND INNOVATION
Research and Innovation
Thermal-Hydraulic Analysis for Total Loss of Feedwater Event in PWR using SPACE Code ı MinJeong Kim, Minhee Kim, Junkyu Song, Bongsik Chu, Jae-Seung Suh and Hyunjin Lee