Small features, long refueling cycle and also beside

Small modular reactors (SMRs)
have benefits like lower initial costs and investment risks, higher safety
features, long refueling cycle and also beside than electricity production,
capability to be used as sea water desalination, district heating and process
heat for industries and small isolated grids, which make these reactor an
alternative option for future of nuclear power industries. Different SMRs all
over the world are in the different stages of design and construction and
different aspects of these reactors are under evaluation by researchers and
engineers. The Korean System Integrated Modular Advanced Reactor (SMART)
categorized as SMR that has received its standard design approval. In this
study Rod ejection accident (REA) that categorized as design basis accident
(DBA) has been evaluated for SMART core according to the standard safety
analysis report (SSAR) of SMART. The DRAGON code has been used for cell
calculation and also for spatial kinetic and thermal hydraulic calculations,
the feedback, transient and TH blocks of PARCS code as a thermo-neutronic code
have been used. Finally the SMART core response to the REA has been evaluated
by comparison the attained results with SMART SSAR that shows proper match. Also
the capability of DRAGON/PARCS codes for predicting SMR behavior during REA has
been approved.

Keywords: Small Modular Reactor (SMR), SMART, Rod
Ejection Accident (REA), DRAGON code, PARCS code.

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1.
Introduction

The concept of integral small
modular reactor (SMR) isn’t new but it seems that the proper time for using
this idea has been coming. According to the international atomic energy agency
(IAEA), the reactors with electrical power lower than 300 MW have been defined
as small reactors, although SMRs are categorized by this fact that more
advantages and design features are attain when intentionally make reactors
small.  In fact these reactors use their
size as advantage to attain more design purposes. The
scalability, modularity (many of major components can be assembled
anywhere far from the sites and then shipped to the main sites), improved safety characteristics and more important than
other, lower up-front cost of the SMRs, offer great advantages over large common
nuclear power plants. Also, many countries and regions (like many of Asian and
African countries) lack suitable sites for producing electricity and water
desalination or totally for the countries with small electric grids,
less developed infrastructure and limited investment capabilities, SMRs
can be the best solution (IAEA, 2016).

According to the
IAEA reports there are many interests all over the world to move toward using
of these kind of reactors. There are many different type of SMRs under
different stages of design, licensing
and construction. Russia (KLT40s), Argentina (CAREM) and china (HTR-PM) have
three type of SMRs under construction now and are
scheduled to begin commercial operation between 2018 and 2020. Korean
System Integrated Modular Advanced Reactor (SMART) has a certified design and
also Russian VBER-300 is under licensing stage. There are many other SMR
designs that will be prepared for near term deployment, although realistically
it seems that the first commercial group of SMRs, start operation near 2025 –
2030 (IAEA, 2016).

Many researchers and
engineers all over the world are trying to assessing and surveying different
aspect of these new reactors like in economical, environmental, nuclear characteristics
and many other fields. Identifying the transient
behavior of the neutron flux in a nuclear reactor in response to either a
planned or unplanned change in the reactor behavior has a great importance on
the safe and reliable operation of the reactor. The most limiting case
over the reactivity initiated accidents (RIA) as a very fast transient is the
rod ejection accident (REA) that has been categorized as design basis accident
(DBA).

Our first objective in this
study is to evaluate an integral SMR reactor during a REA. There are researches
that tried to evaluate REA for different type of pressurized reactors (Selim et al., 2017; Nawaz et al., 2016; Tabadar et al.,
2012; Barrachina et al., 2011; Diamond et al., 2001) but in this study
we are trying to evaluate this accident for Korean SMART reactor as an SMR that
according to the IAEA reports has a certified design (IAEA,
2016).

Many codes and methods have
been used to simulate the REA (Prévot et al., 2017;Song
et al., 2016; Lee et al., 2015; Grgic et al, 2013;) but in this study we
used a combination of DRAGON  as a cell
calculation code (Marleau et al., 2016) and
PARCS as dynamic core calculation code (Downar et
al., 2006). Also another purpose of this study is to evaluate the
capability of DRAGON/PARCS codes for modelling REA for a SMR core by comparison
the results with the REA results of standard safety analysis report (SSAR) of
SMART core. Noori-kalkhoran et al. (2014) used
WIMSD-5b/PARCS for evaluating REA in a WWER-1000 type reactor but we want to
assess DRAGON/PARCS as modelling tools for evaluating REA in SMRs. PARCS code
has been used as neutronic tool coupled with RELAP as a thermal hydraulic code
(Vahman et al., 2016) but in this study we
also used TH block of PARCS code that as an Internal coupling procedure with
powerful spatial kinetics methods of PARCS can simulate the transient behavior
of the reactor (Downar et al., 2006).

The remainder of
the paper is organized as follows. Section 2 presents an overview of the SMART
reactor and its operational parameters. In Section 3, DRAGON and PARCS codes
that have been used for performing our calculations are introduced. The core model
and a validation for steady state condition of SMART core at the beginning of
cycle (BOC) have been presented in Section 4. Section 5 contains the results of
REA calculation from DRAGON/PARCS that have been compared with SMART SSAR for
some results. Finally, conclusion and remarks are given in Section 6.

2. SMART
description

SMART (System-integrated
Modular Advanced Reactor), which is conceptually developed by KAERI (Korea
Atomic Energy Research Institute), is a small-sized advanced integral PWR that
produces 330 MW of thermal energy under full power operating conditions. SMART
is a multi-purpose SMR that furthermore than electricity production can be used
for different applications including: process heat for industries and small
isolated grids, district heating and sea water desalination. This SMR has been
designed with enough output to meet the fresh water and electricity demands of a city with one
hundred thousand populations. As
shown in Fig. 1 major components, including
reactor coolant pumps, steam generators and a self-pressurizer are integrated
within a single pressure vessel, in which the arrangement of components differs
from the conventional loop-type reactors (IAEA,
2011; Lee, 2010).

SMART core overall design
data are presented in Table 1. Cross view of
SMART reactor core configuration is presented in Fig.
2 and also Table 2 describes the
different core configuration quantities. The reactor core has 57 square lattice
fuel assemblies with 2 m active height. Each fuel assembly contains 265 fuel
rods (some fuel rods contains a mixture of UO2+Gd2O3
that known as IFBA (Integral Fuel Burnable absorber)), 24 guide tube and a
central instrumentation channel. Core reactivity in SMART reactor is controlled
only by IFBA rods and soluble poison while most other typical PWRs use fixed
burnable absorber rods (SMART Report, 2012; SMART
SSAR, 2010).

In the SMART core design,
IFBA rods are present in all of the fuel assemblies with different arrangements
(Fig. 3). All IFBA rods have same 8 weight
percent of Gd2O3 to reduce the large initial Keff
value and also flatten the power distribution during the core cycle. The SMART
core fuel assemblies are categorized to A and B according to the presence of
Gd2O3 at the top and bottom of the IFBA rods (Fig.
4). The fuel assemblies placed near the center of core have 2.82% U-235
and other fuel assemblies have 4.88% U-235 fuel enrichment but the IFBA rods
are exception. The IFBA rods have 1.6 w/o U-235 at a part of top and bottom of
the rod and 1.8 w/o U-235 at other parts (SMART
Report, 2012; SMART SSAR, 2010).

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