Safety scenario and integrated thermofluid test

Fusion Engineering and Design 42 (1998) 37–44

Safety scenario and integrated thermofluid test

Yasushi Seki *, Ryoichi Kurihara, Satoshi Nishio, Shuzo Ueda, Isao Aoki,Toshio Ajima, Tomoaki Kunugi, Kazuyuki Takase, Mitsuhiko Shibata

Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, 801-1 Mukouyama, Naka-machi, Naka-gun,Ibaraki-ken 311-0193, Japan

Abstract

The largest mobilizable radioactive material inventory in the form of tritium and activated dust in a fusion reactoris estimated to be located in the vacuum vessel. The accident scenarios of postulated thermofluid transients such asingress of coolant inside the vacuum vessel and the loss of vacuum boundary leading to the release of radioactivematerial are introduced. The accuracy of the present analysis method and database for evaluating the radioactivematerial release in such accident scenarios is assessed. The areas where the data and methods seem to be mostuncertain are identified, such as the condensation of steam under vacuum condition, the activated dust mobilizationand transport in and out of the vacuum vessel in the event of the transients. An approach to experimentally reducesuch uncertainties in the evaluation of radioactive material release are presented. A combination of a number ofspecific test devices to reduce uncertainties in such areas as dust mobilization and transport, and an integratedthermofluid test facility to establish the evaluation methodology are proposed. © 1998 Elsevier Science S.A. All rightsreserved.

1. Introduction

The largest radioactive material inventoryresides in the vacuum vessel (VV) of a tokamakfusion reactor. Although there are still large un-certainties, the amount of tritium and activateddust in the W are assumed to be in the order ofkilograms and tens of kilograms, respectively, inthe case of ITER [1]. The containment of suchradioactive material is the key to achieve fusionsafety. The release of the radioactive materialmust be kept as low as reasonably achievableduring the normal operation including mainte-nance operations. In the event of accidents, the

possibility of radioactive material being mobilizedand released outside the vacuum boundary andeventually to the environment has been consid-ered by postulating various accident scenarios.Among the accident sequences considered theones which resulted in the largest amount ofradioactive material release to the environmentare the in-vessel LOCA or ICE (Ingress ofCoolant Event) leading to the LOVA (Loss ofVacuum Event) and LOVA occurring alone.

In view of the importance of the ICE andLOVA events, preliminary ICE and LOVA exper-iments have been conducted in JAERI since 1994and more recently as ITER Safety R&D Tasks[2–6]. In the preliminary experiments, ICE andLOVA are tested separately to clarify the basic* Corresponding author.

0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.

PII S0920-3796(98)00123-9

Y. Seki et al. / Fusion Engineering and Design 42 (1998) 37–4438

Fig. 1. Concept of the integrated thermofluid test facility.

phenomena of ICE and LOVA and to developcalculational model of these thermofluid eventsinside the VV. Some interesting results from thesetwo experiments have been obtained and will bepresented in this symposium [7–9]. An integratedthermofluid test facility has been planned and theconcept design of the facility is in progress asshown in Fig. 1 [10]. This figure shows an inte-grated test facility capable of testing ICE leadingto LOVA sequence. The main objectives of thefacility are to investigate the consequences ofpossible interaction of ICE and LOVA and tovalidate the analytical model of thermofluidevents in the VV of a fusion reactor. The pressureand temperature transient characteristics insidethe VV, and the mobilization and release charac-

teristics of accumulated dust in the VV are someof the parameters to be measured in the facility.The facility also aims at providing design data forreliable safety systems for the thermofluidtransients.

This paper presents further break-down of theroles of the integrated test facility together withthe supplemental specific test devices. An ap-proach to establish the evaluation methodologyfor thermofluid transients in the VV is presented.In Section 2, the event sequences of the ICEleading to LOVA (ICE/LOVA) and LOVA occur-ring alone are followed. In Section 3, the factorsaffecting the radioactive material release duringthe sequence and the calculation codes for theevaluation of these accidents are tabulated to-

Y. Seki et al. / Fusion Engineering and Design 42 (1998) 37–44 39

gether with the possible cause of uncertainties inthe evaluation. The break-down, of the roles ofthe experimental test devices needed for reducingthe uncertainties is also proposed. A summaryand the future plan of research are given in thelast section.

2. Accident sequences of ICE/LOVA and LOVA

The accident sequences of the ICE/LOVA areshown in Fig. 2.

(1) An Ingress of Coolant Event (ICE) in theVV could occur due to plasma anomalies such asrunaway electrons or by an ex-vessel LOCA lead-ing to overheating of in-vessel components suchas the first wall or divertor. Steam from theevaporation of water in contact with the hotin-vessel components pressurizes the vacuumboundary. Here the vacuum boundary is defined

as the vacuum boundary consisting of the vacuumvessel and penetrations in the vessel. It is postu-lated that the vessel itself does not fail but rela-tively weak penetrations could fail. Somechemical reactions between steam and the hotsurface of the in-vessel components could producehydrogen or other combustible gas. The steamwill also react with the dust in the vacuum vesseland also mobilizes the dust in the form of wetdust or aerosol.

(2) The pressurization of VV could be con-tained within the design pressure with the use of apressure suppression system. If this pressure sup-pression is assumed to fail in the extremely un-likely event, the loss of vacuum boundary couldtake place. It has also been postulated that someweak penetrations could break at some pressurebelow the design pressure as the result of the ICE.Even with the loss of the vacuum boundary, if thesecondary containment is intact, either pressur-ized steam could blow down into the cryostatvacuum or into the region filled with inert gas andrelease of activation material outside the sec-ondary containment will be prevented.

(3) It is only in the case of extremely unlikelyevent of simultaneous loss of the secondaryboundary that the steam and radioactive materialare released to the reactor room and some frac-tion is eventually released to the environment. Ithas been postulated that some penetration by-pass to the air in room could occur.

In addition to the above so called ICE/LOVAsequence, LOVA is also considered as an indepen-dent design basis event caused by a failure ofsome penetration leading to room air. The case ofLOVA event sequence is followed in Fig. 3.

(1) The loss of the vacuum boundary in thepenetration by-pass to the air is postulated. Thisis an extremely unlikely event caused only bysomething like a very severe earthquake or inter-nal explosion.

(2) Air leaks into the vacuum vessel. If thebreach size is significantly large, the air inleakcould be a violent jet flow, in which case it is notso easy to simulate with the present calculationcodes. In such a case, air rushes into the VV in afraction of a second and the dust in the VV couldbe significantly mobilized. As the pressure nears

Fig. 2. Ingress of Coolant Event (ICE) leading to Loss ofVacuum Event (LOVA), the dotted arrow shows the simulta-neous occurrance of ICE and LOVA.


Fig. 3. Loss of vacuum event.

in the integrated test facility [10] is shown in thethird column. The candidate calculation codes forthe analysis and the possible cause of uncertain-ties in the calculation are listed in columns 4 and5, respectively. In columns 6 and 7 are shown,respectively, the roles of specific test devices andthe integrated test facility to establish the evalua-tion methodology.

In the second line of Table 1, the pressure riseby the steam generation will be affected by thefailure mode of the cooling pipe, the heat transferfrom the in-vessel components and the amount ofcondensation of the steam. The difficulty in theexperiment is the simulation of the heat transferfrom the in-vessel components which will dependon the initial temperature, material, heat capacity,geometry of the in-vessel component and the heatbalance within the VV and beyond. To providesufficient heat capacity and the relatively coldsurface for condensation, the scale of the testfacility needs to be sufficiently large. The TRAC-BF1 [11] and MELCOR [12] codes, modified totreat fusion specific conditions such as near vac-uum, water injection in horizontal direction, arebeing used for the ICE/LOVA analysis. Theanalyses of preliminary ICE experiments indicatesome uncertainties in the evaluation of condensa-tion in the near vacuum condition if some non-condensable gas is present. By measuring thetemperature distribution and pressure change inthe VV of the integrated test facility, the evalua-tion methodology for the pressure rise can beestablished.

In a similar manner, the effecting factors, mea-surement items, calculation uncertainties and thetest means to reduce uncertainties are listed foreach of the event sequences of ICE/LOVA.

From this study summarized in Table 1, thefollowing results have been obtained.

(1) Specific test devices to supplement the inte-grated test facility are proposed to measure thereaction rate equation of steam and hot surfacesat high temperature, mobilization of wet dust, andfloatation/retention of wet dust.

(2) Sufficiently larger scale of the integrated testfacility is required in order to provide sufficientheat capacity in the in-vessel components, therelatively cold surface for condensation, and

the equilibrium pressure, the air flow becomesslower and could be modelled with present calcu-lation codes. If the temperature is significantlyhigh, the air could react chemically with the hotin-vessel components and produce some com-bustible gases.

(3) As the air is heated in the VV by the heatcapacity of the in-vessel components or decayheat, it expands and the pressure in the VV couldbecome larger than that of the room and airoutleakage will occur. The tritium and activateddust could be released to the room with the air.

3. Evaluation plans of the ICE/LOVA andLOVA sequences

The experimental evaluation plans for themethodology to evaluate radioactive material re-lease by the ICE/LOVA and LOVA sequences areshown in Tables 1 and 2, respectively. For eachstep of the accident sequences, the factors affect-ing the sequence and the release of radioactivematerial are listed in column 2. The factors whichaffect the scale of the experimental device areshown in bold letters. What should be measured


Tab

le1

Eva

luat

ion

plan

ofIC

E/L

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Ase

quen

ce

Aff

ecti

ngfa

ctor

sM

easu

rem

ent

item

sE

vent

sequ

ence

Cal

c.co

deC

alcu

lati

onun

cer-

Inte

grat

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stfa

cil-

Spec

ific

test

devi

ces

ity

tain

itie

s

Con

dens

atio

nin

vac-

(1)

ICE

(Ing

ress

ofSu

rfac

ete

mpe

ratu

re/

Fai

lure

mod

e/H

eat

TR

AC

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L-

Est

ablis

hed

eval

ua-

uum

Pre

ssur

eti

onm

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gyby

CO

Rtr

ansf

erof

in-v

esse

lC

oola

ntE

vent

)/C

oolin

gtu

befa

ilure

/co

mpo

nent

s(T

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ra-

tem

pera

ture

and

ture

cont

rol)/C

onde

n-[

Pre

ssur

eri

sepr

essu

rem

easu

re-

sati

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ents

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ctio

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teeq

uati

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mic

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Con

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ace

area

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stab

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ogy

bym

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ring

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ace

tem

pera

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high

tem

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tion

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and

CO

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nera

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stri

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bem

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acte

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cle

size

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tobe

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ence

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ndar

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Tab

le2

Eva

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plan

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easu

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ach

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mob

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tion

data

biliz

atio

nas

afu

nc-

mob

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data

totu

reis

tics

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pass

[(2

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mea

sure

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mea

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ingr

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aliz

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reac

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dens

ity/

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preservation of height difference of the pressuresuppression system [9].

(3) The experimental study of blow-down ofsteam or inert gas should be considered if theanalytical valuation results in some seriousconsequences.

As in Table 1, the LOVA sequence is evaluatedin Table 2. In the case of LOVA, a three dimen-sional thermofluid analysis code called STREAMcode, which is being jointly developed by JAERIand Software Cradle, Ltd. [13] will be used for theanalysis. Following results have been obtainedfrom the study.

(1) Specific test devices to supplement the inte-grated test facility are proposed to measure thereaction rate equation of steam and the hot sur-faces at high temperature, mobilization and trans-port of dry dust through the penetration.

(2) The factors requiring sufficiently larger scaleof the integrated test facility include the height ofthe VV to reproduce a turbulent flow by a naturalflow of air inside the VV.

4. Summary and future plans

Experiment plan to establish the evaluationmethodology of two major thermofluid transients,namely ICE/LOVA and LOVA are consideredand the following conclusions have been obtainedfrom studying the experimental procedures andcalculational uncertainties in the each step of theevent sequences:

(1) The combination of an integrated test facil-ity and a number of specific test devices for testingsteam-material reaction rate at high temperature,dry and wet dust mobilization and transport hasbeen proposed for effective establishment of theevaluation methodology.

(2) The factors requiring scaling of the inte-grated test facility include the need to providesufficient heat capacity in the in-vessel compo-nents, the relatively cold surface for condensation,preservation of height difference of the pressuresuppression system, and the height of the VV toreproduce a turbulent flow by the natural flow ofair inside the VV.

It is planned to proceed with the followingsteps:

(1) Become further acquainted with the calcula-tion codes of ICE and LOVA, namely the MEL-COR, TRAC and STREAM codes through theanalysis of preliminary ICE and LOVA experi-ments and to show where the largest uncertaintiesare located.

(2) Begin with the specific tests to reduce thecause of largest uncertainties and to modify thecalculation codes based on experimental results ofthe specific tests.

(3) Design and construct the integrated testfacility based on the results of (1) and (2).

(4) Proceed with the integrated tests to establishthe evaluation methodology.

Acknowledgements

Authors thank Dr Hajime Akimoto of JAERIfor his valuable comments to this work.

References

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[3] M. Ogawa, T. Kunugi, Thermohydraulic experiments onwater jet into vacuum during ingress of coolant event in afusion experimental reactor, Fusion Eng. Des. 29 (1995)1–6.

[4] K. Takase, T. Kunugi, Y. Seki, Effects of breach area andlength to exchange flow rates under the LOVA conditionin a fusion reactor, Fusion Technol. 30 (1966) 1459.

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[9] R. Kurthara, et al., Numerical analysis of ingress ofcoolant event in vacuum vessel, presented at ISFNT4.Fusion Eng. Des. (in press).

[10] R. Kurihara, et al., Proposal of integrated test facility forin-vessel thermofluid safety of fusion reactors, 6th IAEA-TCM on Development in Fusion Safety, 1996. J. FusionEnergy (in press).

[11] J.A. Borkowski, N.L. Wade, et al., TRAC-BFl/MODl: anadvanced best-estimate computer program for BWR acci-dent analysis, NUREG/CR-4356, 1992.

[12] R.M. Summers, et al., MELCOR 1.8.0: a computer codefor nuclear reactor severe accident source term and riskassessment analyses, NUREG/CR-5531, 1991.

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Safety scenario and integrated thermofluid test