HIGH TEMPERATURE GAS-COOLED REACTOR PROGRAMMEIN CHINA

Yuanhui XuInstitute of Nuclear Energy TechnologyTsinghua University Beijing 100084, Chinainezkf@mail.tsinghua.edu.cn

Introduction

The High Temperature Gas-cooled Reactor (HTGR) is the only reactor type with which the coolanttemperature can be as high as over 700 oC. This unique feature allows it not only for power generation with highefficiency, but also for the supply of process heat in a variety of industrial applications. In the last decade HTGRtechnology has been focused on modular reactor designs which are mostly characterised by favourable features ofinherent safety. China recognises the advantages of Modular HTGRs and has chosen Modular HTGRs as one ofadvanced reactors to be developed for the further intensive utilisation of nuclear power in the next century. Inenergy supply systems of the next century, HTGR is supposed to serve: 1. as supplement to water-cooledreactors for electricity generation and 2. as environmentally friendly heat source providing process heat atdifferent temperatures for various applications like heavy oil recovery, coal gasification and liquefaction, etc..

History of the HTGR Development

The research and development program on the HTGR in China began in mid seventies. In the first phase,beginning from 1973, a target of the construction of a 100 MWt thorium thermal breeder was set and a pebblebed HTGR with core-blanket two zones was designed. A conceptual design was worked out. This conceptualdesign was characterised by its compactness (high specific power), high breeding ratio (approaching unity insuch a small reactor) and operation ability (inherently stable, on-load refuelling property, etc.) and it is designedfor operating on thorium cycle. A series of experiments were carried out parallelly, such as the 1:10 prestressed concrete reactor vessel(PCRV) model test, the 1:10 all-graphite core structure model seismic test, the fuel elements handling and itscomponents test, mechanical strength or force test for the inclined special shaped graphite support beams at thebottom of the core and the thrust force applied to the outermost graphite structure by steel balls served as thebinding force while the structure is allowed to expand and contract freely, the 1: 2.7 and 1:1 control rod and itsdrive model test, the steam generator experiments on two phase flow stability and the vibration-induced wear,the oil lubricated bearing test for helium blowers, the static sealing test, research on chemical reprocessing of thethorium contained spent fuel (including the separation of uranium and thorium from the spent fuel, the trappingof protactinium in high-silicon micro-porous glass and the fluid-bed steam de-nitration of thorium nitratesolution), nuclear graphite development and study on fuel elements technologies. However, the construction of this reactor was postponed indefinitely due to various reasons mainly infinancial problem. The second phase of HTGRs research and development program was in the period of the Sixth Five-YearPlan (1981-1985). The State Science and Technology Commission (SSTC) gave support to continue some basictechnology development and to investigate the possibilities of nuclear process steam and heat for industryapplication. The feasibility study on using HTR-Modules for heavy oil recovery in the Shanjasi section ofShengli oil field and chemical industry application in the Yangshan petrochemical complex was carried outjointly by INET, KFA Juelich and Siemens KWU/Interatom. Meanwhile a modification to the original GermanHTR-Module design was made. The new design almost doubled the thermal output of its original value (i.e.,from 250-500 MWt for a single unit) without sacrificing the inherent safety features and hence improved theeconomy. The essential idea of this design is the adoption of a zoned core with a non-fuelled central column ofgraphite balls. Since the inner core contains no fuel, so the hot point moves outward and the maximum fuelelement temperature after the hypothetical loss of all coolant will never shoot above the safety margin-1600 oC.Prestressed concrete vessel was chosen due to larger core diameter design value. In the third phase the research and development program on the HTGR was integrated in the NationalHigh Technology Program in 1986-2000. In 1986-1990 some key technologies for the HTGR were researchedon, a conceptual design for 10 MW HTR Test Module (HTR-10) was carried out jointly by INET, KFA andSiemens/Interatom. The pre-preparation on its construction was started. Main objectives for the HTR-10 are: 1. to acquire know-how in the design, construction and operation ofHTGRs, 2. to establish an irradiation and experimental facility, 3. to demonstrate the inherent safety features ofModular HTGR, 4. to test electricity and heat co-generation and closed cycle gas turbine technology and 5. to doresearch and development work on the nuclear process heat application. The Institute of Nuclear EnergyTechnology (INET) of Tsinghua University was appointed as the leading institute to be responsible for design,

license applications, construction and operation of the HTR-10. Now the HTR-10 is being constructed at the siteof INET which is located in the North-west of Beijing city. It is expected to be critical in 2000.

HTR-10 Design(1)

The HTR-10 design represents the features of Modular HTGR design. The reactor core and the steamgenerator are housed in two steel pressure vessels which are arranged in a side-by-sideÓ way. These two vesselsare connected to each other by a connecting vessel in which the hot gas duct is designed. All these steel pressurevessels are in touch with the cold helium of about 250 oC coming out from the circulator which sits over thesteam generator tubes in the same vessel. (Figure. 1) The HTR-10 main design parameters are listed in Table 1. Fuel elements used are the spherical fuel elements (6 cm in diameter) with coated particles. The reactorcore contains about 27,000 fuel elements forming a pebble bed which is 180 cm in diameter and 197 cm inaverage height. Spherical fuel elements go through the reactor core in a multi-pass" pattern. Pulse pneumatic fuelhandling system is used for continually charging and discharging fuel elements. Pulse pneumatic driving single-exit gate (or the reducer) is designed instead of mechanical one. It has the advantage of reliability and simplify. Graphite serves as the main material of core structures which mainly consist of the top, bottom and sidereflectors. The ceramic core structures are housed in a metallic core vessel which is supported on the steelpressure vessel. The thickness of side reflector is 100 cm. In the side reflector, cold helium channels are designedin which helium flows upward after entering the reactor pressure vessel from between the connecting vessel andthe hot gas duct. Helium flow reverses at the top of reactor core into the pebble bed, so that a downward flow pattern takesplace in it. After being heated in the pebble bed, helium enters into a hot gas chamber in the bottom reflector,and from there it flows with reactor outlet temperature through hot gas duct to the heat exchanging components.

II

FIG 1. Cross Section of the HTR. Primary Circuit

Table 1 The HTR-10 Main Design ParametersReactor thermal power MW 10Active core volume m3 5Average power density MW/m

32

Primary heliumpressure

MPa 3

Helium inlettemperature

°C 250 / 300

Helium outlettemperature

°C 700 / 900

Helium mass flow rate kg/s 4.3 / 3.2Fuel UO2U-235 enrichment offresh fuel elements

% 17

Diameter of sphericalfuel elements

mm 60

Number of sphericalfuel elements

27,000

Refuelling mode multi-pass,continuous

Average dischargeburnup

MWd/t 80,000

The steam generator is composed of a number of modular helical tubes which are arranged in a circlebetween two insulation barrels inside the steam generator pressure vessel. The place inside the inner barrel isforeseen for an intermediate heat exchanger (IHX) which is to be installed in the second phase of the project. IHXis helical tube type. Nitrogen flows inside the tube while helium flows outside the tube. Decay heat removal of the HTR-10 is designed on a completely passive basis. At a loss of pressureaccident, against which no core cooling is foreseen at all, decay power will dissipate through the core structuresby means of heat conduction and radiation to the outside of the reactor pressure vessel, where, on the wall of theconcrete housing, a surface cooling system is designed. This system works on the principle of natural circulationof water and it takes the decay heat via air coolers to the atmosphere. In fact, this surface cooling system isdesigned to protect the vessel and concrete structures more than the ceramic reactor core from being overheatedby decay power. There are two reactor shutdown systems, one control rod system and another small absorber ball system.They are all designed in the side reflector. Both systems are able to bring the reactor to cold shutdownconditions. Since the reactor has strong negative temperature coefficients and decay heat removal does not require

any circulation of the helium coolant, the turn-off of the helium circulator can also shut down the reactor frompower operating conditions.

Engineering experiments for the HTR-10

In the HTR-10 design some modifications from the HTR-Module were made to meet Chinese conditions.For example, the steam generator is composed of a number of modular helical tubes with small diameter, pulsepneumatic fuel handling system is used for charging and discharging fuel elements as well as step motor drivingcontrol rods are designed. It is necessary to do engineering experiments to prove these new or modified idea.Therefore a program of engineering experiments for HTR-10 key technologies was conducted in INET.

1. Performance test of the hot gas duct

The performance test of the hot gas duct was carried out at the helium test loop (HETL). The detaileddescription of the HETL can be found in references(2). The hot gas duct test section with a triple tube structureincludes an inner electrical heater, a hot gas duct model and a cold-hot helium gas static mixer. The schematicstructure of the hot gas duct test section is shown in Fig. 2.

FIG. 2 Schematic Structure of the Hot Gas Duct Test Section

The flow rate, the temperature, the pressure and the differential pressure are measured. At the crosssections of 610 mm far from two ends of the hot gas duct test section, 16 thermocouples are installed formeasuring the temperature of helium gas, the liner tube, the insulation layer and the outer tube. The temperatureof helium gas at the inlet and outlet of the inner heater as well as helium gas in the annular passage and thepressure tube is also measured by thermocouples. The effective thermal conductivity of insulation layer was measured. A relationship between the effectivethermal conductivity and the average temperature of the insulation layer at helium pressure of 3.0, 2.5 and 1.5MPa is shown in Fig 3. The empirical equation of the effective thermal conductivity were obtained as follows:

Keff =0.3468+0.0003T (°C) W/m/°C

Where T is the average temperature of the insulationlayer.

|Ë He = 0.0003T + 0.15

|Ë ef f = 0.0003T + 0.34

0

0.2

0.4

0.6

0.8

200 2 2 0 2 40 260 280 3 0 0 3 2 0 3 40 360 380 400 4 2 0

A VERAGE TEMPERATURE OF INSULATION(¡æ)

EFFE

CTI

VE T

HER

MAL

CO

ND

UC

TIVI

TY|Ë

eff

(W/m

¡¡æ

)

3 .0MPa 2.5MPa1.5MPa |ËHe (W /m¡ ¡æ|Ëeff (W/m¡ ¡æ)

FIG. 3 Relation Between ¶Àeff and AverageTemperature of Insulation

The measured effective thermal conductivity are 1.5°´ 2 times as large as the thermal conductivity ofhelium gas at the same condition of temperature and pressure. The hot gas duct test section was operated for 258 hours at the temperature of over 700 °C and thepressure of 3.0 MPa, as well as for 98 hours at the temperature of 900 oC and the pressure of 3.0 MPa, and wasborne over 18 times temperature cycle between 300 and 900 °C, and 28 times pressure cycle betweenatmospheric pressure and 3.0 MPa. No any deterioration of thermal performance was detected.

2. Performance test of the pulse pneumatic fuel handling system

In order to avoid operating difficulty with mechanical single-exit gate a pulse pneumatic fuel handlingsystem has been developed. Its features are characterised by pulse pneumatic driving single-exit gate, gas tightmagnetic driving and electro-induction balls counters.

The test at room temperature was carried out. Its main aim is to prove its design concept for fueldischarging. More than 100,000 balls were discharged by pulse pneumatic discharging way. Full scale apparatus for test of whole fuel handling system and its components at helium temperature of150-180 oC and low pressure was installed. The schematic of full scale apparatus is shown in Fig. 4. Maincomponents in this apparatus are prototype ones. It consists of the discharge tube, a reducer, a failed ballseparator, an elevator, graphite ball detectors, pressure reducing valves, adjusting valves, heaters and so on. Itshelium auxiliary system consists of the helium compressor, helium storage tanks, air coolers, filters and vacuumpumps etc.. The test at temperature of 150-180 °C and helium pressure of 0.5 MPa was done. The result shows thatthe spherical ball can be smoothly dropped out from core without any broke up. More than 35,000 balls weredischarged by pulse pneumatic discharging way. Whole system was successfully operated.

M

M

PT

1 06

TE

1 04

PT10

1

TE10

1 M

M

PT1 0

4

PT10

2

M

1 04

TE

1 06

PT

PT

1 01

TE

1 01

1 02

PT

PT

1 04

KZ0 1

KZ03

KZ0 4

M

KZ0 2

KZ05

1

2

3

4

5

6 7 8 9 1 0

9

1 21 1

1 3

1 4

1 5

1 6

1 7

1 9

1 8

2 0

2 1

2 2

2 3

Counter

Heater

PressureMeasurement

Elec t romagnet ismValve

Cut-of fValve

Temperar tureMeasurement

1. Pressure reducingvalve

14. Smallfunnel

2. H. P. tank 15. Pulse gasstorage

3. Check valve 16. Separator 4. Compressors 17. Fragments

tank 5. L. P. tank 18. Elevator 6. Drying cylinder 19. Reducer 7. Air cooler 20. Vacuum

pump 8. Filter 21. Water tank 9. Buffer 22. Pump10. Discharging tube 23. Regulating

valve11. Tank12. Fender13. Large funnelFIG. 4 Schematic of Fuel Handling System

Apparatus3. Test of the control rods driving apparatus(3)

The performance of the control rod at operating temperature and helium atmosphere of low pressure willbe tested at a full scaling apparatus. The schematic of the control rods driving apparatus is shown in Fig. 5. Itconsists of a step motor, a gearbox, a chain-chain wheel, a speed restriction, a rod position indictor, the controlrod and the housing. Following test was done by with this apparatus: control rod movement, maximum fallspeed of the control rod, indictor performance and life time.

4. Two phase flow stability test for the once-through steam generator(4)

The main purposes of the two phase flow engineering test facility are: to study on the flow stability of theHTR-10 steam generator at operational conditions and to determine maximum throat diameter of the throttle, tostudy the flow resistance on the water side and the average heat transfer coefficient on the helium side of thesteam generator with small curvature radius of the helical tube.

The test section is shown in Fig. 6. In the test section helium transfers its heat to the primary loopwater as it does in the HTR-10 steam generator.

Operation of the HTR-10

The HTR-10 project is to be carried out in two phases. In the first phase, the reactor with an coolantoutlet temperature of 700 oC will be coupled with a steam generator providing steam for a steam turbine cyclewhich works on an electricity and heat co-generation basis. In the second phase, the reactor coolant outlettemperature is planned to be raised to 900 oC. A steam reformer cycle or a gas turbine-steam turbine combinedcycle will be coupled to the reactor.

1. Steam reformer cycle

Since the HTR-10 is an experimental reactor, it is impossible to use steam reformer cycle as industrialone. Therefore, it is our design feature of the steam reform system to make it close to form a circuit. If thepassage way of HTR coolant cycle is called the primary circuit and the passage way of the secondary helium

cycle in the intermediate heat exchanger is called the secondary circuit such heat-process application system cycleis called the third circuitÓ.

The schematic flow of the steam reformer system for the HTR-10 is shown in Fig. 7. Its maincomponents are: the steam reformer, the pre-cooler, the steam generator, the condenser, the water separator, themethanol generator, the pump and the turbine, etc..

The produced gas, after reformed in the steam reformer, enters the cooler, the steam generator, and thecondenser to release heat, and then the gas and water will be separated in the water separator. The separatedwater, after pressure raised by the pump, will then enter the steam generator to absorb heat and evaporate; and theseparated process gas, compressed by the blower, is pre-heated by the pre-cooler, it reacts and generates heat inthe methanol generator, then combines with the water from the steam generator, and finally re-enters into thesteam reformer.

2. Gas turbine-steam turbine combined cycle

The HTR-10 design features allow it to accept a gas/gas intermediate heat exchanger in series with thesteam generator, which gives the HTR-10 flexibility for multi-aspect applications. A gas turbine-steam turbine(GT-ST) combined cycle could be connect with the secondary gas circuit for a high efficiency power generationtest. Fig. 8 shows the schematic flow of the GT-ST combined cycle for the HTR-10. The main advantage ofusing the HTR-10 for the GT-ST combined cycle is that the basically well-developed HTGR design andassociated experience provide a secure energy source. For example, the inlet temperature of the reactor core can bekept near 300 oC. Of course, with adoption of IHX and double cycles, the economic competitiveness is decreasedand the system operation and control are more complex. The main design data for the GT-ST combined cycle arelisted in Table 2.

1

2

3 4 5 6

7

8

9

1 0

220

1 1

1 2

H2O

1 3

2 20V

V

1 4

P

T1

T6

T1-T5

1 5

1 2 3 45

6 7 8 9

1. Vacuum pump 2. Helium cylinder 3. Temperature controller 4. Voltage regulator 5. Transformer 6. Insulation 7. Insulation 8. Control rod 9. Step motor10. Synchronometer11. Insulation12. Heater13. Control board14. Temperature controller15. Eddy heater

FIG. 5 Schematic of Control Rods DrivingSystem

1. Electrode2. Helium inlet3. Heater4. Vessel5. Tubeplate6. Steam generator unit7. Water inlet8. Helium outlet9. Steam outlet

FIG. 6 Schematic structure of the test section

Core

SG

IHX

ST

SG

SR PH

SG

CH1

SG

CH2

SHWS

3 Kg /s3 0 0 C

6 2 5 C

5 MW

He 9 5 0C3 .0MPa

3.0 MPa9 00 CHe

5 MW

3 .5MPa4 3 5C

1.61 Kg/ s

3 0 0 1.6 3 Kg/ s 1 04 C

2 .5MW

6 00 C

2 .5 MW

3 .0 MPa

6 0 0C

CH4+ H2O

0 .3 6MW

4 5 0C

0.4 Kg/ s

(114 m/h)

3 00 C

1 .3 6MW

2 5 0 C0.5 9 Kg/ s

0 .9 3MW

0 .3 4Kg /s

1 5 0C

5 0 C

3 5 0C

0 .6 1MW

IHX: int ermediat e hea t exchanger SG: steam generat or SR: steam reformerWS: Wat er separator CH: me thanal genera tor SH: super heat er PH: preheat er

FIG. 7 Schematic Flow of the Steam Reformer System for the HTR-10

G

G

ST

GT LC HC

3 0 0 /3 .05

6 00 /2.95

SG5MW

2 8 7 /2.9 1 .75kg /s

104/ 4.2 deoxidiser heat er

4 1/ 0.007

condenser1 .355MW

4 35 /3.43

1 1.17kg /srecuper ator4 83 /3.2

2 .08MW

850/ 3.0900C/ 3.0MPa 6 0 /1 .57

cooler

144/ 1.62

6 0 /0 .801 6 1 /0 .85

1 44 /3.2

5 5 0 /0 .9

h =h =90% Tout=550 C

IHX5 MW

SC

h =34.4%C

FIG. 8 Schematic Flow of the GT-ST Combined Cycle for the HTR-10

Table 3. Main Data for the GT-ST Combined Cycle

ReactorCore thermal power MW 10Core outlet temperature oC 900Core inlet temperature oC 300Primary pressure MPa 3.0IHXThermal power MW 5Primary helium inlet temperature oC 900Primary helium outlet temperature oC 600Primary pressure MPa 3.0Secondary nitrogen inlet temperature oC 483Secondary nitrogen outlet temperature oC 850Secondary pressure MPa 3.2Nitrogen flow rate kg/s 11.17SGThermal power MW 5Temperature at helium side oC 600/287Temperature at water side oC 435/104Pressure at water side MPa 3.43/4.2PowerPower for gas turbine MWe 2.08Power for steam turbine MWe 1.36Total efficiency % 34.4

Conclusion

The HTGR programme is being carried out in order to solve energy issue in next century in China.The HTGR is supposed to serve as supplement to water-cooled reactors for electricity generation aswell as environmentally friendly heat source providing process heat at different temperatures for variousapplications like heavy oil recovery, coal gasification and liquefaction, etc.. As first step the HTR-10is being constructed in Beijing. Some engineering experiments were carried out to prove the design. Itwill be a nuclear heat resource for the steam reform cycle or the gas turbine-steam turbine combinedcycle.

References

(1) Yuanhui XU and Yuliang SUN Status of the HTR Programme in China IAEA TCM on HighTemperature Gas Reactor Application and Future Prospects ECN, Petten, the Netherlands 10-12November 1997

(2) Zhiyong HUANG, Meisheng YAO, Bing HAN, Yuanhui XU, Jun LI and Xuedong HE HighTemperature Performance Test of Hot Gas Duct of 10 MW High Temperature Gas-cooledReactor Nuclear Power Engineering 1998 Vol. 19 No. 4 Page 326-329 in Chinese

(3) Xingzhong DIAO, Huizhong ZHOU and Caizheng LIU Tests for the HTR-10 Control RodDriving Mechanism MOU meeting between JAERI and INET in Beijing on November 6 1998

(4) Yuanhui XU, Huaiming JU, Zhiyong HUANG and Zhiyong LIU Experimental Study on TwoPhase Flow Stability for the HTR-10 Stam Generator MOU meeting between JAERI andINET in Beijing on November 6 1998