atw 2018-02

atw Vol. 63 (2018) | Issue 2 ı February
ENERGY POLICY, ECONOMY AND LAW 82
reactor core. It is used to support the reactor core and
locate the reactor core components. On June 8, 2017, the
installation of the ceramic components inside the second
reactor core was finished, which means half of the
installation progress of the main facilities in the nuclear
island has been done. Before August 11, 2017, the fuel
production line has produced 250,000 pebbles, which met
the requirement of connecting to the grid for HTR-PM.
The project is planned to be completed and put into operation
at the end of 2017/beginning of 2018, but probably
it will be delayed (Figure 3). The design lifetime of
HTR-PM is 40 years.
| | Fig. 3.
The construction of Shidao Bay HTGR conventional island was finished
on June 27, 2015 (photo credits: Shidao Bay NPP).
3 Safety features of HTGR
One of the most important safety issues for nuclear power
plant is decay heat removal. In the Three Mile Island and
Fukushima Daiichi nuclear accidents, the reactor cores are
overheated and melt down due to the failure of decay heat
removal. In Chernobyl accident, the failure of decay heat
removal system caused the resulting sequences after the
initial exploration due to the fission power increment.
So developing a highly reliable emergency core cooling
system with reliable water and electricity supply is very
important for a light water reactor (LWR).
But for HTGR, inherent safety can be achieved based
on three physical ideas: 1. using silicon carbide (SiC),
which has very good heat-resistance, as the fuel cladding;
2. lowering the volumetric power density of the reactor
core significantly; 3. using identical small reactor modules
to replace a large reactor in order to make sure that the
reactor core won’t be heated to the temperature limit [7].
Besides physical ideas, the safety of HTGR can be
protected from three engineering designs:
1. Multiple barriers to prevent the release of
radioactivity
The HTGR has three safety barriers to prevent the release
of radioactivity. The first barrier is the fuel particles coated
with SiC. The maximum temperature of the fuel particles
is designed to be limited to 1,600 °C under any operation
or accident conditions. Less than 1,600 °C, the coat of the
particles can maintain integrated [8]. The second barrier
is the pressure boundary of the primary circuit, which
contains the reactor pressure vessel, the steam generator
pressure vessel and the hot gas duct pressure vessel which
connects the previous two vessels. The likelihood for
these three vessels to have ruptures can be neglected. The
third barrier is the bounding volume, which contains the
primary circuit cabin, Helium purification cabin as well as
fuel loading and unloading cabin. They can prevent the
radioactive gas to be released into the atmosphere.
2 Passive decay heat removal system
The thermal design of HTGR has already considered that
in case of any accidents, the cooling of the reactor core
doesn’t need any active decay heat removal system. The
decay heat in the reactor core can be removed from the
core to the surface cooler outside of the reactor pressure
vessel passively through heat conduction and radiation.
Then the heat can be passed to the atmosphere from the
surface cooler by nature convection. If the primary circuit
lost pressure and the main and the auxiliary decay heat
removal system are out of work, the decay heat can still be
removed from the core to the outside. The reactor core
meltdown can be avoided. Under accident conditions,
because the decay heat cannot be removed by the main
decay heat removal system, the temperature of the pebbles
will be increased. In order to make sure the maximum
temperature of the pebbles will not exceed 1,600 °C, some
restrictions to the power density and geometry of the
reactor core are necessary. That’s the reason why the
capacity of the HTGR is usually small.
3 Negative temperature coefficient has good reactivity
compensation
The reactor has a relatively high negative temperature
coefficient for the fuel and moderator and if it is under
normal condition, the margin between the maximum
temperature of the pebbles and its limit is large. The
negative temperature coefficient can give a good reactivity
compensation. When a positive reactivity is introduced
into the reactor, it can be automatically shut down thanks
to the reactivity compensation from the negative temperature
coefficient [9].
The long term operation of HTR-10 and different
safety experiments have proved the inherent safety of
HTGR, which improved the public acceptance of nuclear
reactors.
4 Fuel technology
In 2005, INET built a prototyping fuel-production facility
with a capacity of 100,000 fuel elements per year. In order
to solidify the fabrication level, INET started to construct
HTGR fuel-production factory in Baotou, Northern China
in 2013. The fuel-production equipment was installed in
2014. In 2015, they started the commissioning and trial
production. Some experiments have been done in Petten,
the Netherlands. The irradiation test of five fuel spheres of
the HTR-PM started in October 2012 in the high flux
reactor (HFR) and finished on December 30, 2014. The
fuel sphere quality, which is one of the key technologies in
HTR-PM project, has been proved to meet the requirements
[7].
On August 15, 2016, the construction of the fuel
production line in Baotou was finished and the fuel pebble
production started. By July 17, 2017, the fuel production
line has already produced 200,000 pebbles. It means
that the fuel production of HTGR has shifted from trial
production to industrial production. It also means that the
fuel production technology of HTGR in China is leading
the world, which has great significance for achieving
commercialization and export of HTGR [10].
When a fuel element is discharged from the bottom
of the RPV to the fuel handling system, its burn-up is
measured immediately. If its burn-up does not reach the
design burn-up limit, it will be recharged into the reactor
Energy Policy, Economy and Law
Development of High Temperature Gas Cooled Reactor in China ı Wentao Guo and Michael Schorer