Design criteria for prestressed concrete pressure vessels for High Temperature Reactors

Nuclear Engineering and Design 132 (1991) 53-62 North-Holland

53

Design criteria for prestressed concrete pressure vessels for High Temperature Reactors Klaus Schimmelpfennig

Stangenberg, Schnellenbach und Partner, Gemeinschaft Beratender Ingenieure GmbH, Viktoriastrafle 47, W-4630 Bochum, Germany Received 23 August 1990; revised version 2 January 1991

This paper summarizes the work on design criteria for concrete structures of Prestressed Concrete Reactor Vessels (PCRVs), which has been carried out since 1984 by a couple of competent institutions. After some basic considerations on the safety demands on PCRVs, especially their Prestressed Concrete Structure (PCS), and the consequences for an elevated level of quality to be ensured by the design criteria, an impression is given, first, by what means a higher quality standard is gained with respect to selection of materials and specification of material data in comparison to the usual building industry and what kind of criteria on this behalf should be fixed in a PCRV code. As a further quality increasing feature, the specific demands on design analysis as practised according to the present state of science and as to be treated within a code are discussed. This concerns analyses for steady state and transient temperatures as well as stress and strain analyses for service a n d ultimate load conditions. It is outlined to what degree calculation models should be detailed, which includes statements about admissible idealizations. As a central topic the question is discussed in what way the ultimate load capacity has to be evaluated, thereby presenting results of some investigations pointing out the conditions under which the design is determined by the different kinds of ultimate load conditions. Finally, some reflections on the demands on monitoring the PCS behaviour during its lifetime and on several questions still to be answered in this field are expressed.

1. Introduction

and

basic

safety

considerations

Since 1984, a group of institutions involved in research a n d design of h i g h - t e m p e r a t u r e reactors ( H T R s ) has carried out a joint r e s e a r c h project on design criteria for h i g h - t e m p e r a t u r e loaded metallic and ceramic c o m p o n e n t s of future H T R s including the prestressed c o n c r e t e reactor vessel ( P C R V ) as a highly i m p o r t a n t c o m p o n e n t with respect to safety. T h e s e activities were s p o n s o r e d by the F e d e r a l Minister for R e s e a r c h and Technology with the aim to provide a t h o r o u g h basis for future H T R codes. This article deals with the part of this project concerning the p r e s t r e s s e d concrete reactor vessel ( P C R V ) and especially the p r e s t r e s s e d concrete structure (PCS). T h e s u b g r o u p working on this field a n d chaired by the a u t h o r consisted of m e m b e r s of the following institutions: - Hochtemperatur-Reaktorbau GmbH, Mannheim, - Interatom GmbH, Bergisch-Gladbach, 0029-5493/91/$03.50

- R h e i n i s c h - W e s t f ~ i l i s c h e r T e c h n i s c h e r ldberwachungs-Verein, Essen, -Stangenberg, Schnellenbach und Partner GmbH, Bochum. Derived from the protection objective the d e m a n d is m a d e on P C R V s t h a t the sealing a n d pressure retaining function u n d e r service conditions is ensured, extensive failure is excluded, the structural s h a p e is p r e s e r v e d during accidents, a n d leakage is restricted to a rate which is acceptable for o t h e r c o m p o n e n t s . Furt h e r m o r e , the thick concrete walls serve as shielding against radiation - which should not be neglected w h e n talking a b o u t n e e d of space a n d costs of a P C R V c o m p a r e d with o t h e r a r r a n g e m e n t s . As is well known, P C R V s consist of different components: - the prestressed concrete structure (PCS), - the liner (core liner a n d p e n e t r a t i o n liners), - the closures, a n d - the heat p r o t e c t i o n system (insulation a n d vessel

© 1991 - E l s e v i e r S c i e n c e P u b l i s h e r s B . V . A l l r i g h t s r e s e r v e d

K. Schimmelpfennig / Design criteria for PVRCs .]'or HTRs

54

cooling system), all of which have different functions. What is most important and which is a special feature of PCRVs, is the separation of load-bearing and sealing function, the former fulfilled by the PCS, the latter by the liner. Nevertheless, most of the demands on a P C R V - if not direct task of the PCS - cannot be satisfied without participation of the PCS: Pressure-bearing capability is a direct demand. Gas tightness of the liner during operation can only be guaranteed if the PCS fulfills its load-carrying function. - The same condition exists for restricted leakage in case of accidents. - The design characteristics of the PCS provide a decisive contribution to exclude extensive failure. Preservation of structural shape in case of accidents is effected by the PCS. - The PCS also satisfies the radiation protection demands. That is why, in contrast to other P C R V components, a high safety importance is assigned to the PCS.

2.

Aim

of

PCS

design

criteria

In comparison with other reinforced or prestressed concrete structures, there are some special features of PCRVs which contribute to safety without any more rigorous design criteria than for usual structures being applied: - First, the geometry consisting of a cylinder and circular plates should be mentioned, which is rather simple in comparison with sophisticated structural systems being built now and then for ordinary purposes. - Second, the very thick concrete cross sections cylinders of about 4 m, top and bottom slabs 5 to 7 m - make the structure insensitive against local minor quality of concrete. Most important under this aspect is the extremely high number of single prestressing wires - about 120000 in case of the T H T R - as load-bearing elements. As a further advantage, P C R V s are not exposed to the atmosphere like usual structures, but are in a controlled environment. Furthermore the loads are strictly defined and not at random as for usual structures. Because of the high safety importance of the PCS, design criteria have to be worked out with the aim to make the quality of a P C R V even better than is d e t e r -

mined by its inherent features. That means design rules used for ordinary structures have to be extended and supplemented, where necessai~.,. This mainly concerns the following measures: - use of a concrete mix geared to the special PCS demands, - extended evaluation of material data - especially for the individual concrete mix, - p e r f o r m a n c e of detailed and realistic structural analysis for evaluating the safety against ultimate limit states and limits of serviceability, - application of higher safety coefficients where necessary, - intensified supervision of materials and manufacturc in comparison with common practice, - pressure testing in order to check the proper condition of the vessel, monitoring of the mechanical behaviour throughout the whole P C R V life.

3.

Synopsis

of

investigations

The work of the group concerned with the PCS has, therefore, been focussed on topics in the above sense which are not sufficiently covered by the usual codes with respect to the special structure of PCSs and the special demands on it, and different investigations yielding a basis for such specific design criteria have been carried out. All these are summarized in section CI of the final report of the research project [1]. The main topics of this report - related to sections of a future code - are: (1) Materials: mechanical concrete data regarding actual test results, thermal concrete data regarding actual test resuits, prestressing steel data, - radiation effects; (2) Design: - loads, load categories, design levels, admissible concrete temperatures, - demands on the analysis, permissible stresses, treatment of local stresses, ultimate load analysis, structural details; (3) Manufacture: - evaluation of experiences ( T H T R , abroad); (4) In-service surveillance: - evaluation of experiences ( T H T R , abroad).

K. Schimmelpfennig / Design criteriafor PVRCsfor HTRs This listing - even in this c o n d e n s e d form - shows that it is impossible to give a complete p r e s e n t a t i o n of all these topics within this paper, and, besides, a lot of the results are not p r e d e s t i n e d for a publication like this. Thus, only a couple of subjects being in the fore u n d e r the aspect of defining quality enlarging design criteria for PCSs are outlined in the following.

55

4. C o n c r e t e c h a r a c t e r i s t i c s T h e materials for a concrete to be used for a PCS should be carefully selected with respect to the special r e q u i r e m e n t s of PCRVs. In this sense the following criteria for concrete characteristics to be strived for should be m e n t i o n e d in a code:

1.4 1.2 1.0 uPs e o l ~ 0.8

2~ o.6 0.4 0.2 0

~

0

12

I

m

70 120 temperofure

200

300

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(b)

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Z

T=120%u

T=120%s

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7

28

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02

specimens

doys

90

25 30

trme

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50

%

iI

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(d)

(e)

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1.0

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0.8

T=70° 0,6 O.Z.

7O

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~

d

T=I20°

~

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3led

0./,

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()2 unseoled specimens

0

,

I

7

28

doys time

90

0

20

70

120

200

temperoture

°c

300

__

Fig. 1. Some test results specifying properties of "heat-resistance HTR concrete". (a) Residual compressive strength after 28 days temperature. (b) Compressive strength versus time under different temperatures (unloaded). (c) Compressive strength under sustained load and temperatures. (d) Young's modulus versus time under different temperatures (unloaded). (e) Young's modulus versus sustained temperatures (unloaded).

56

K. Schimmelpfennig / Design criteria/br PVRCsfor HTRs

high compressive strength, low sensitivity to continuous load, low sensitivity to elevated temperatures, - low creep and shrinkage, low thermal expansion, low hydration heat, - good workability (pumping should be possible). Some of these desirable characteristics are contrary. For instance, high strength requires a low w a t e r / cement ratio. Combined with good workability this only can be achieved by a high cement content, which in turn causes higher creep and shrinkage and yields high hydration temperatures. This means that no concrete mix can be called ideal and, hence, be fixed in a code, and careful selection of concrete mix is an indispensable part of planning a PCRV project. In connection with the German H T R 500 project, such a concrete mix development has been performed, which resulted in two definitely specified concrete mixes - one with basaltic, the other with quartzitic aggregates. From the test program for selecting the mixes as well as from the following tests for these specific concretes extensive knowledge about material properties is at hand - especially with regard to temperature dependence. A part of these data has been put to the disposal of the " H T R design criteria" program; fig. 1 shows some diagrams out of this as examples. Although for these specific concretes sufficiently detailed investigations have been done (see, e.g., publications by Becker, Diederichs, Schneider and Weber [2-5]), it has to be asked, to what extent knowledge gained here can become part of a code. Since mixes cannot be prescribed, this depends on - to what extent data are independent of the actual mix, to what extent generalizations in connection with data from literature are possible - e.g., in the form of standardized curves or bands, to what extent the structural behaviour is sensitive to possible deviations or scatter. In this respect no final result could be gained. For a number of properties it has been tried to define temperature-dependent guide values and band widths to be regarded (see, e.g., fig. 2). Recommendations have been made by which single-test calibration for a specific concrete mix would be possible. In addition, the sensitivity against variations of these values has been analysed, such as of variation of thermal conductivity and heat capacity, each from the lower to the upper bound on concrete temperature development during a typical accident (see fig. 3). In this example with a cooling gas temperature increased from operating level -

-

IO_~K£OF is.

........... __

, ........

i (
: - .......

:

0pper b0o,,

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.

.

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.

mean value

-

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600

:

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] (b) 1

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""L

t upper bound meQn v~Iue

-lo

co 1.0 Lower bound E

0.5 o--

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~00

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600

temperafure

Fig. 2. Guide data for HTR concrete properties (examples). (a) Thermal expansion versus temperature for limestone concrete. (b) Thermal conductivity versus temperature for quartzitic concrete.

-

-

(260°C) to 360°C within 5 h and a rapid decrease thereafter, the temperature increase behind the liner deviated up to 37% from the value calculated with the mean values of these parameters. Furthermore, including the possible scattering of the thermal expansion coefficent, temperature stresses may deviate up to 70% from those calculated by use of mean values. Calculations of this kind allow recommendations how accurate the different material parameters have to be defined. As mentioned, investigations like this were only done on a small scale. The code setting committee should use results of such sensitivity studies as a basis for the decision to what extent a code should comprise such guide values and whether possibly studies like this should be promoted.

•zl~a3uo3 ouolsotu!l (q) '~]~aouoo o!].~za~nb (e) :~uop!ooe ue ffu!anp aou!l ~ql pu!qoq soanleaodtuol oloaouo~ uo 'aod Ll!oedeo leoq pue g A1]A!lonpuoo letuJoq% ffU!L~eA Jo oouzn[Ju I "E 'ff!rl owht

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58

K. Schirnrnelp]ennig / Design criteria for PVRCs for HTR~"

5. Design analysis As explained earlier, a more detailed and realistic design analysis in comparison to those for usual civil engineering structures is an essential part of quality improving measures for PCRVs. Thus, requirements in this connection have to be defined in a future code. It has, therefore, been a task within the program, based on existing standards and project-related experience, to settle the question in which manner and how distinctly formulations on this behalf must be or may be written down in a code. Concerning the demands on the analysis of the stand pipe zone of the top cap, definite computational investigations have been carried out. Some appropriate points of view shall be outlined here - first with respect to geometric modelling. It is a strict requirement, because of the thick-walled structural shape, to perform the design analysis based on the continuum theory, which reasonably should be done by numerical methods. The required refinement of discretisation cannot (and should not) be definitely prescribed - e.g., by stipulating numbers or maximum dimension of elements. Criteria for element size to be chosen (regarding the importance of the respective element) are given by the expected steepness of stress or strain gradients in relation to the importance of the individual result. Analysing local problems by modelling separate substructures should always be considered. On the other hand, a code should also specify admissible relieves, e.g., the possibility to use an axisymmetric model if justified, such as for analysing the overall behaviour of a usual single-cavity vessel. In this sense it has been estimated important to check whether it is generally possible to refuse from explicite modelling of the stand pipe zone because of the regular arrangement of penetrations (see fig. 4). Suitable calculations, based on Abdul-Wahab's and Harrop's idea [6], have shown that it is not only possible to describe stiffness and thermal properties of this region by a substitute fictitious homogeneous material when calculating the overall vessel or the whole top cap, but also to calculate stresses of steel tubes and confining concrete in a simple way starting from the "smeared" stress in the homogeneous material. This is also largely valid for ultimate load and accident analyses. The liner should only be included in the PCS analyses where it is of significant influence on the structural behaviour, such as in case of detailed analysis of a penetration zone or in the ultimate limit state.

\

~'~.w

def(ii[

Fig. 4. Idealization of stand pipe zone of a PCRV.

Concerning the efficiency of calculation techniques in modelling material properties, it should be demanded according to the present state of knowledge, that cracking and crushing of concrete in triaxial condition as well as non-linear triaxiat stress-strain relationship must be included in the algorithm. It will always be a problem herein, how to verify the efficiency of the computer code used with respect to the individual task. Also here, no definite criteria can be layed down, and, anyhow, the manufacturer is requested to perform this verification. Obviously, consideration of concrete creep - steady state in case of long-term calculations, transitional in case of accident analyses - must be provided by the program used, in fact - as for most of the other characteristics - in dependence of temperature.

K. Schimmelpfennig / Design criteriafor PVRCs for HTRs

59

6. L i m i t s t a t e d e s i g n

Table 1 Permissible compressive stresses in concrete

The work concerning design rules has been based on the principles used for PCRVs up to now (established, e.g., for the THTR vessel by Zerna [7,8]), which means structural behaviour under service (and upset) conditions being analysed in detail with stresses being checked for permissibility, whereas ultimate load conditions are analysed only with respect to the overall structural behaviour, thereby still being in question how to treat the case of assumed pressurized cracks. Following these principles a system of permissible stresses for the different event categories, materials and if need zones of the structure has been worked out, which is shown, e.g., for concrete in table 1. With respect to ultimate load analysis it has been investigated by simplified calculations under which conditions the analysis with liner assumed to be tight, or the analysis with liner assumed to be leaking becomes the ruling case. In these considerations, the safety factor for the first case was set to 2.25 according to the former HTR 1160 regulations, and to 1.5 for the second case, following the British standard, both re-

Load categories

1.5

J

ultimate [oad with leaking liner Pult =1.50 working p ~

Field stresses

Local stresses

/3r/2.0 /3, / 1.9 ,8,/1.85

/3r / 1.65] /3r/1"6 / 1.2/3(.

/3,/,.55 t < 1.45/3c

/

/3,/2.1

/3,/,.75

/3,/2.0

/3, / 1.651

/3r / 1.7

-<<1.5/3c

/3, / 1.2

/3,/,.4

~ 1.8/3c

/3,/1.1

**

/3r Multiaxial concrete strength - in case reduced due to temperature - at age of loading, at most 90 days. /3c Uniaxial compressive strength - in case reduced due to temperature - at age of loading, at most 90 days. * If design loads cannot be exceeded or in case of short-time load, field stresses up to /3r/1.8 and local stresses up to ,8, / 1.5 are permissible. ** Local stresses need not be established, if admissible with respect to PCRV safety demands.

(a)

ultimate load with liner infect PuLt=2.2S working pressure

- -

Construction * Commissioning Pressure test Normal service conditions Upset service conditions Event category 3 accidents Event category 4 accidents

Permissible compressive stresses in concrete

[ i

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(b)

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Pull = 1.50 working pressure

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81 rn

9.0

/,,.5

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9.0

i

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I 10.8 m 117

fop cap thickness

Fig. 5. Parametric studies of ultimate load capacity (related to minimum prestress for service conditions). (a) Variation of cylinder wall thickness. (b) Variation of top slab thickness.

K. Schimrnelpfennig / Design criteriafor PVRCsJor HTRs

60

lated here to the operating pressure. The result was that for usual wall and slab thicknesses the analysis with intact liner is more unfavourable (see fig. 5). (The

figure also shows that the design for the reference vessel carried out for satisfying the service load requirements may not meet the ultimate load demands:

1

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measured

---

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measured, section 210 °

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measured, section 3 3 0 °

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decrease)

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÷ i

calculated

+

k

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radius

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2

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4,0

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0

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strain Key

¢r

--

strain

+

measured,

section

x

measured, calculated

s e c t i o n 210 °

90 °

eke--

zt_+.~t-'--~ k.~o r

(C)

Fig. 6. Comparison of some THTR pressure test measurements with predicted computational results: (a) Concrete strains versus pressure at mid-height. (b) Concrete strain profiles under proof pressure at mid-height. (c) Concrete strain profiles under proof pressure in bottom slab. (d) Radial deflection of side wall under proof pressure.

K. Schimmelpfennig / Design criteriafor PVRCs for HTRs but the conservativity of the simplified calculation method may also be the reason for this.) When converting all these considerations into a code it has to be (and has already been) questioned, how these design principles fit to those of the future Eurocode, the basic idea of which is that the actions multiplied by partial safety coefficients must be not greater than certain conditions described by different limit states and divided by partial coefficients related to the different materials and to these individual limit states. It is defined that "a limit state is reached by the structure when a specified performance criterion is infringed". Limit states defined in the Eurocode are as follows: (1) Ultimate limit state: - loss of equilibrium, - gross deformation or displacement, attainment of maximum resistance; (2) Serviceability limit state: - deformations or deflections (appearance, use, other damage), - vibrations (discomfort, damage), - c r a c k i n g of concrete (appearance, durability, tightness), -

- microcracking of concrete in compression (durability, level of creep). Obviously, it will be possible to fit the PCS design analyses practised up to now into this concept. Attainment of maximum resistance is the predominating ultimate limit state criterion for PCRVs, the service load analyses of the previous kind may be considered as precaution against untolerable deformations - especially due to creep - that means loss of serviceability. To what extent they also may be interpreted as a kind of local ultimate limit state design is still to be discussed. Basically the system of partial safety coefficients for PCS design has to be established in a way which ensures compatibility with the Eurocode and with regard to the special characteristics of PCRVs. This includes, for example, the question how to take into account the enlarged safety requirements of a nuclear component on one hand, and, on the other hand, how to take advantage of the perfectly fixed loading conditions. In the meantime two work groups have started to prepare a new design concept in this sense. The work done so far allows to say, that despite of the modifications to be introduced the overall level of safety used in PCRV design up to now is obviously high enough.

m

I 25

-

i

rn 2O

I

15

i

0/

/

10

5

"-1

-4

o 0

10 mm 20

deflection Key +

section

90 °

×

section

210 °

a

section

330 °

--

calculated

61

calculated measured

(d) Fig. 6. (continued).

from hoop

strain

62

K. Schimmelpfennig / Design criteria .for PVRCs Jbr HTRs

7. In-service surveillance

References

Concerning the question of P C R V surveillance, published experiences with foreign vessels (Great Britain) and with the T H T R vessel have been gathered. It turned out that it is no problem to monitor the proper vessel behaviour during the pressure test. Here, a combination of vibrating wire gauges e m b e d d e d in undisturbed regions, which can easily be modelled by calculation, with suitable deflection measurement devices is fully satisfactory. Fig. 6 shows some results like that from the T H T R pressure test, published in more detail by the author in [9]. Still to be discussed is the question in which way the in-service surveillance should be best carried out. In case of non-grouted prestressing tendons, direct and complete checking of prestressing forces is possible by using the tendon-lift-off technique. This is undoubtedly the best method to convince oneself of the proper state of a PCS. (This method is used in Great Britain.) Regarding corrosion protection most experiences exist with mortar grouting, although in a few cases not without problems. This excludes, however, direct measurement of prestressing forces. Use of organic grease, which has been the alternative for P C R V s up to now, is a rather young method. Experience for time periods of 40 years is not existing, and also here some problems are known. The safer a corrosion protection system, the less prestressing force control is necessary and the more substitution by other monitoring methods becomes possible. It is, therefore, necessary to focus further research activities on the question which combination of corrosion protection system and monitoring is the safest and most economic - even including a combination of pure wires or strands and suitable inspection methods.

[1] Endbericht zum Verbund-Forschungsvorhaben des BMFF "AusIegungskriterien ffir hochtemperaturbelastete mctallische und keramische Komponenten sowie des Spannbeton-Reaktordruckbeh~ilters zukfinftiger HTR-Anlagen", Band III, Teil C: Spannbeton-Reaktordruckbeh~ilter, Kernforschungsanlage Jiilich GmbH, (August 1988). [2] U. Schneider, U. Diederichs and A. Weber, Behaviour of HTR-concrete at elevated temperatures, State of the art, Proc. IAEA Specialists' Meeting on Design, Criteria and Experience with Prestressed Reactor Pressure Vessels for Gas Cooled Reactors, Lausanne (1984). [3] A. Weber, G. Becker and U. Diederichs, Effects of longterm thermal exposure on the behaviour of HTR-concrete, Proc. 8th Int. Conf. on SMiRT, Vol. H 5/3, Brussels (1985). [4] A. Weber, G. Becker and U. Schneider, Creep strength of sealed concrete at elevated temperatures, Proc. 8th Int. Conf. on SMiRT, Vol. H 5/10, Brussels (1985). [5] U. Diederichs, C. Ehm, A. Weber and G. Becker, Deformation behaviour of HTR-concrete under biaxial stresses and elevated temperatures, Proc. 9th Internat. Confer. on SMiRT, Vol. H 109-114, Lausanne (1987). [6] H.M.S. Abdul-Wahab and J. Horrop, The rigidity of perforated plates with reinforced holes, Nucl. Engrg. & Des. 5 (1967) 134-141. [7] W. Zerna, THTR-SpannbetonbehS_lter, Bericht Nr. 1: Nachweise ffir den Gebrauchszustand, Bochum (Dezembet 1970, erg~inzt April 1971 und Miirz 1972) not published. [8] W. Zerna, THTR-Spannbetonbehiilter, Bericht Nr. 2: Nachweis der Grenzzusfiinde, Bochum (Oktober 1970, erg~inzt November 1971 und Miirz 1972) not published. [9] K. Schimmelpfennig, Evaluation of measurements during pressure test and previous history of the THTR 30/) PCPV, Proc. IAEA Specialists' Meeting on Design, Criteria and Experience with Prestressed Reactor Pressure Vessels for Gas Cooled Reactors, Lausanne (1984).