Nuclear Engineering and Design 218 (2002) 209–214

Design of the exhaust system in the confinement of theHTR-10

Feng Jiang *, Sui-sheng Ye, Jun YangInstitute of Nuclear Energy Technology, Tsinghua Uni�ersity, Beijing 100084, China

Received 11 July 2001; received in revised form 24 February 2002; accepted 11 March 2002

Abstract

The modular high temperature gas-cooled reactor has a vented confinement instead of a gastight pressurizedcontainment due to its passive safety features. The safety class negative pressure exhaust system is used in the heating,ventilation and air conditioning system to fulfill all kinds of safety-related functions at the normal operation andduring accidents. This paper introduces and reviews the design of safety class negative pressure exhaust systems of the10 MW high temperature gas-cooled reactor-test module. © 2002 Elsevier Science B.V. All rights reserved.

www.elsevier.com/locate/nucengdes

1. Introduction

The 10 MW high temperature gas-cooled reac-tor-test module (HTR-10) was built at TsinghuaUniversity and reached its first criticality in De-cember 2000. Spherical fuel elements with coatedparticles are used as fuel elements. As all fissionproducts are retained in the coated particles aslong as the temperature of the coated particles iskept below 1600 °C, a modular high temperaturegas-cooled reactor core is designed such that themaximum allowable fuel element temperature of1600 °C is never surpassed—even in the mostsevere accident. So, the radioactive concentrationof the gas in the primary circuit of the HTR-10 isvery low. Thus, the HTR-10 does not need anadditional concrete pressurized containment out-

side the pressure vessel. As an unpressurizedvented confinement is adopted for the HTR-10(He and Qing, in press) markedly, a safety classnegative pressure exhaust system of the confi-nement, which differs from the ventilation systemin the containment of a pressurized-water reactor,is used in the HTR-10 plant.

2. The confinement

The confinement comprises of five parts: (1) thereactor cavity, (2) the steam generator cavity, (3)the fuel handling system cavity, (4) the operationgas valves cavity and (5) the helium purificationsystem cavity (Fig. 1). The first four cavities arestructurally interconnected. In the helium purifi-cation system cavity, there is a DN325 (in diame-ter) steel tube connected with the operation gasvalves cavity, but a ‘one-way’ rupture disk device

* Corresponding author. Tel.: +86-10-62783761x272; fax:+86-10-69771464.

E-mail address: jf601@tsinghua.edu.cn (F. Jiang).

0029-5493/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.

PII: S0029 -5493 (02 )00192 -9

F. Jiang et al. / Nuclear Engineering and Design 218 (2002) 209–214210

is installed at the end of the tube in the heliumpurification system cavity to isolate two cavities.The pressures in the operation gas valves cavityand the helium purification system cavity are de-signed in such way that this ‘one-way’ rupturedisk device can only burst during a helium releasefrom the helium purification system cavity to theoperation gas valves cavity. Its designed burstingpressure is 10 kPa (gauge pressure). On the wallof the steam generator cavity, there is a rupturedisk device that connects to the pressure reliefduct system and then to a 22.6 m chimney . Theconfinement is isolated from the outside by rup-ture disk devices that are of very gastight.

The walls, doors and closed plates of the confi-nement have favorable gastightness. In order to

keep the confinement at negative pressure, theallowable leakage rate of the confinement is 100%(of the volume) per day for a negative pressure of150 Pa. The maximum design pressure of theconfinement is 30 kPa (gauge pressure).

3. The demand on the exhaust system in theconfinement

The confinement has only the safety class nega-tive pressure exhaust system and, does not havethe supply air system. The helium purificationsystem cavity has supply air ports, which areopened only during person entering. Usually thatcavity should be kept apart from the supply airsystem with a closed and sealed damper.

Fig. 1. The sectional view of the confinement in the HTR-10.

F. Jiang et al. / Nuclear Engineering and Design 218 (2002) 209–214 211

Fig. 2. The flow diagram of the safety negative pressure exhaust system.

During normal operation, the safety class nega-tive pressure exhaust system maintains negativepressure of 150 Pa for all confinement cavitiesexcept for the helium purification system cavitywhere a negative pressure of 50 Pa is kept.

When the tube less than DN10 is ruptured inthe confinement, high temperature and high pres-sure helium gas is leaked to the confinementcavities. In order to reduce the radioactive gasrelease to the outside of the confinement as muchas possible, it requires that the exhaust system canmaintain the confinement to be at negative pres-sure and the primary helium is released to theenvironment through the chimney of 40 m afterthe filtering treatment by the safety negative pres-sure exhaust system.

When a tube larger than DN10 in the primarycircuit is ruptured, the primary helium is quicklyreleased into the cavity. When the cavity pressureis beyond 10 kPa, the rupture disk devices will beburst, the primary helium in the cavity is directlyreleased to the environment without filtering.

It is required that the safety negative pressureexhaust system shuts down automatically whenthe shock pressure in the cavity is more than 2500Pa (gauge pressure).

4. The design of the safety negative pressureexhaust system

The scheme of the safety negative pressure ex-haust system of the HTR-10 is detailed in Fig. 2.The safety negative pressure exhaust system in-cludes three subsystems: the exhaust system fornormal operation conditions, the exhaust systemfor accidents and the exhaust system for humanentrance. In addition, there is a pressure reliefduct system.

The classification of the exhaust system fornormal and accident operation conditions issafety class 3 and seismic class 1. All of theircomponents can resist a pressure up to 2500 Pa(gauge pressure). The pressure surge dampers, theelectric tight dampers, the prefilters, the adsorber,

F. Jiang et al. / Nuclear Engineering and Design 218 (2002) 209–214212

the high effective particle air filters and the ex-haust ventilators are all safety class components.The switching between the two systems is accom-plished by controlling remotely the tight dampersin the main control room. Any one of the twoventilators plays the stand-by role of the other.

The protective components in the system— thepressure surge dampers (Fig. 2) are produced byKRANTZ Company in Germany. The manufac-ture standard of this kind of damper correspondsto the specification QS E-7051/30 by SiemensKWU, which is valid for the components for theheating, ventilation and air conditioning systemsin German nuclear power plants. During normaloperation the pressure surge damper is open. Atthe event of a shock wave the pressure surgedamper will close automatically and rapidly in0.04 s. When the shock pressure drops down tothe set value, the blades of the damper jumpautomatically back into the open position. Therequested closing and opening pressure can be setup by a spring pull of the damper.

The exhaust system for normal operation con-dition consists of prefilters, high effective particleair filters, dampers and exhaust fan. Its mainfunction is to maintain the helium purificationsystem cavity at 50 Pa negative pressure and tomaintain the other four cavities of the confi-nement within 150 Pa negative pressure duringnormal operation. The exhaust fan of this systemcontinuously takes out the leaking air from thecavity doors, closed plates, and penetrations tomaintain the negative pressure of each cavity. Theexhaust air is released to the environment throughthe stack after passing the filters.

There are prefilters, adsorbers, high effectiveparticle air filters, dampers and exhaust fan in theexhaust system for accident conditions. It has twomain functions:

(1) When a tube of diameter less than DN10 inthe cavity ruptures, and then the helium in theprimary system leaks slowly out into the confi-nement, the exhaust system for accident condi-tions is started up from the exhaust system fornormal operation conditions, so as to make ad-sorb and filtering treatment, and to keep theconfinement negative pressure state.

(2) After an accident of a large tube ruptures(the DN65 tube), the exhaust system for accidentconditions will be put into operation when thepressure of the cavities has attained atmosphericpressure. One purposes for that is that the confi-nement will be put to the negative pressure state.The other purpose is to resume the ordered ex-haust process with cleaning function according tothe ALARA principle.

The exhaust system for human entrance is nonsafety class and has prefilters, high effective parti-cle air filters, dampers and exhaust fan. When theoperators need to enter the helium purificationsystem cavity to repair the components supply airmust be conveyed to the cavity while the exhaustfan in this system must work. After that, theoperators may enter the cavity, and their safety inwork can be ensured.

The pressure relief duct system consists of tworupture disk devices, a pressure relief air duct, twotight dampers (DN1000) in series and a chimney.Its main function is to protect the integrality ofthe confinement structure from shock waves.When a 65-mm tube of the primary circuit bursts,the mass of the high temperature and high pres-sure helium will blow into the confinement in veryshort time. When the shock wave pressure in thecavity is large than 2500 Pa (gauge pressure), thepressure surge damper in the pipe of the safetynegative pressure ventilation system will close au-tomatically and rapidly. The safety negative pres-sure ventilation system is isolated automaticallyfrom the confinement. When the pressure in theconfinement is more than 10 kPa, the rupture diskdevice is blown up, the helium is released into theDN1000 pressure relief duct and is expelled via achimney of 22.6 m high without any filtering (thereleased radioactive dose is still in the range ofprescribed limited dose). The pressure in theconfinement drops down rapidly. In order toavoid the air flowing backwards to the confi-nement and then into the reactor via the rupturedtube which may cause fuel elements deoxidize,when the pressure in the confinement is reducedto the atmosphere pressure, the operator in thereactor main control room should close two tightdampers in series in the pressure relief duct ac-cording to the accident operation procedure. Then

F. Jiang et al. / Nuclear Engineering and Design 218 (2002) 209–214 213

he has to turn on the safety negative pressureexhaust system for accident conditions to attainnegative pressure in the confinement.

5. The design reference

In the design of the HTR-10, we referred to theconcept of the vented confinement for the HTRproposed by German reactor experts (Altes,1991). In Germany, the vented confinements areused to replace the gastight pressurized contain-ments for HTRs of medium and small power sizessuch as the HTR-Module, It is based on the factthat the reliable retaining of all radio logicallyrelevant fission products in the fuel element isensured to such an extent that environmentalexposure remains below the permissible limits atall accidents (Altes et al., 1983; Breitbach et al.,1984). The reactor protection building withoutliner has a double function in that it protects thereactor against external impacts and ensures con-trolled activity release of the primary system leak-ages into the surrounding (Fig. 3). This meansthat the design of the confinement does not aim atcompletely confining the activity over long peri-ods of time, as it is done for a containment of apressurized-water reactor, but aim at the majorleakages (ranging from helium of 0.5–11.5kg s−1) discharged through the depressurizationsystem and the minor leakages (e.g. primary sys-tem leakages up to a leak size of 2 cm2 (DN16tube broken)) through the exhaust air system withfilters via the stack into the environment. TheGerman Reactor Safety Commission was evalu-ated the concept of confinement. They stated: ‘Itis suited to ensure that the regulations of theRadiation Protection Ordinance for normal oper-ation and design basis accidents are compliedwith.’

6. Conclusion

The safety negative pressure exhaust systemsare used in the confinement of the primary systemin the HTR-10 of Tsinghua University to ensurethe safety of the confinement or even the entire

Fig. 3. The confinement concept of the German HTR-Modu-lar.

reactor main building during normal operationand at accidents. According to ALARA principle,the air in the confinement is filtered as soon aspossible to reduce the contamination of radioac-tive products to the environment. It will fulfill thesafety-related functions of the confinement.

Using the vented confinement to replace thegeneral containment is a new concept in the reac-tor design and construction in China. This designbrought to a great advance in designing andbuilding the Chinese HTR.

References

Altes, J., et al., 1983. Zum Stoerfallverhalten des HTR500,Eine Trendanalyse, Jul-Spez-220, Julich, Germany.

Altes, J., 1991. Containment Concepts for High TemperatureReactors[C], Proceedings of the Fourth International Sem-inar on Containments and Nuclear Reactors, SMiRT II,

4 F. Jiang et al. / Nuclear Engineering and Design 218 (2002) 209–214214

KFA 517-Julich, Germany.Breitbach, G., et al., 1984. Zum. Stoerfallverhalten des HTR-

Modul. Eine Trendanalyse, Jul-Spez-260, Julich, Germany.

He, S.Y., Qing, Z.Y. The primary loop confinement andpressure boundary system of the HTR-10, Nuclear Engi-neering and Design, published in this issue.