atw 2018-02

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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

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