analysis of mechanical effects caused by plasma disruptions in the european breeder out of tube...

E L S E V I E R Fusion Engineering and Design 27 (1995) 407-414

Fusion Engineering and Design

Analysis of mechanical effects caused by plasma disruptions in the European breeder out of tube solid breeder blanket design

with MANET as structural material

L.V. Boccaccini a p. Norajitra b p. Ruatto a

a Association KfK-EURATOM, KfK Kernforschungszentrum Karlsruhe, Institute fiir Neutronenphysik und Reaktortechnik, Postfach 3640, D-76021 Karlsruhe, Germany

b Association KfK-EURATOM, KfK Kernforschungszentrum Karlsruhe, Institutfiir Materialforschung, Postfach 3640, D-76021 Karlsruhe, Germany

Abstract

In this paper we deal with some aspects related to the mechanical behaviour of the European breeder out of tube solid breeder blanket for the DEMO reactor during plasma disruptions. The first aspect regards the properties of the martensitic steel MANET which has been chosen as structural material. MANET is a magnetic material and its fracture toughness properties degrade considerably under irradiation. These two features have been taken into account in the calculation of magnetic forces and in the assessment of conditions of unstable crack propagation respectively. As second aspect, a comparison between an electrically segmented and a continuous blanket design has been performed. The analysis reveals lower mechanical stresses for the second design during the DEMO reference disruption and in case of faster disruptions.

1. Introduction

The European Community has been engaged since 1989 in a test blanket development programme. One purpose of it is to perform, through design and experimental work, a comparative assessment of the most promising blanket concepts for a DEMO application, with a view to selecting by mid-1995 the two best concepts for further development. These blanket concepts are all designed to meet a set of DEMO specifica- tions selected in the framework of the European DEMO Blanket Development Program [ 1]; DEMO is considered here as an upgraded version of the next step

machine. The Karlsruhe Nuclear Centre is performing design and experimental work for the European breeder out of tube helium cooled solid breeder blanket [2].

One of the crucial problems in the blanket design is to demonstrate the capability of the structure to withstand the mechanical effects of a major plasma disruption as extrapolated to DEMO from the experience of present machines. A detailed discussion about methods and modelling used in the calculations of mechanical stresses caused by plasma disruptions has been already presented in some previous work [3,4]. In this paper some aspects which are related to the use of the martensitic steel MANET are discussed. Furthermore, a c o m -

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L.V. Boccaccini et al. / Fusion Engineering and Design 27 (1995) 407 414 409

parison between a segmented and a continuous design is presented to demonstrate the advantages of the second design.

The steel MANET has been chosen as structural material, because it is able to withstand the high neutron fluence in DEMO (70 dpa) without appreciably swelling and has good thermal-mechanical properties, lower thermal expansion and higher strength, in comparison with AISI 316L steel. As far as the mechanical effects of plasma disruptions are concerned, MANET presents two important features which must be carefully investigated in the assessment: the magnetic properties of the material and the degradation of the fracture toughness behaviour under irradiation.

2. Magnetic properties of MANET

MANET is a ferromagnetic material with a non-linear magnetic behaviour followed by saturation of the magnetization. Because it is used as structural material

in the region surrounding the plasma, the magnetic flux distribution outside and inside the blanket structure can be significantly modified. When a plasma disruption occurs, as eddy currents are induced in the structure, electromagnetic forces rise whose magnitude can be greater than in the case without ferromagnetic structural material, resulting in damage to the blanket.

A computer code that allows electromagnetic calculations in the presence of saturated magnetic materials is being developed. In this code the magnetization is discretized by using constant elements. This implies that the magnetization is assumed uniform in the interior of the finite element in which the structure is divided. Further details of the code and results for the outboard blanket are in Ref. [5].

The pattern of the eddy currents due to a plasma disruption (from 19.8 MA to zero in 20 ms) is approxi- mately the same with or without ferromagnetic structural material. In fact, MANET is saturated for low applied fields and the magnetization density vector is determined practically only by the main stationary field

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Force (F) in MN

Torque (T) in M N m

Fig. 2. Resultant forces on the structure for different horizontal sections. Forces and torques are calculated on the geometrical centre of each section and refer to the lower half of the section.

410 L.V. Boccaccini et al. / Fusion Engineering and Design 27 (1995) 4 0 ~ 4 1 4

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(toroidal field). The influence of the saturated magnetic material on the poloidal field is slight, and when a plasma disruption occurs the varying plasma field finds the same situation as a structure that is non-magnetic. Fig. 1 shows the poloidal (on a radial-vertical section) and the toroidal (on the equatorial section) components of the magnetic flux density, comparing MANET with a non-magnetic structure material. The directions of the arrows give the pattern of the flux lines. The modulus values are indicated by means of isolines.

The increase in magnetic forces in the box is then caused only by the thickening of the toroidal component of the total magnetic flux density in the structure due to the presence of the magnetic material. Fig. 2 gives the resultant forces and torques on the structure for different horizontal sections, with comparison between the case of MANET and a non-magnetic steel. These quantities are calculated on the geometrical centre of each section and refer to the lower half of the section. Forces and torques in the presence of a magnetic structural material change mostly only in their modulus values but not their direction. The largest increase in modulus values occurs for the box back wall and the vertical shield. However, this increase is limited to 20% in the first wall of the box, where the largest forces are acting and the highest stresses are expected.

3. Fracture mechanics

Recent investigations performed at the Karlsruhe Nuclear Centre [6] reveal a dramatical degradation of the fracture toughness properties of the martensitic steel MANET-I at irradiation temperatures less than 400 °C. As the minimum operating temperatures of the blanket structure are just in the range beween 300 °C and 400 °C, a large amount of structural material can be damaged by neutron irradiation. During the lifetime of the blanket, pre-existent cracks can grow because of cyclic loads such as thermal loads (fatigue crack propagation). Hence, the crack dimension can reach a critical value at which an unstable crack propagation is possi- ble. In particular, loads caused by plasma disruptions, in which a large amount of mechanical energy is de- posited in few milliseconds into the structure, must be carefully considered in the calculation of the fast fracture limits.

Fig. 3 shows the temperature distribution in the outboard blanket first wall at the torus equatorial plane in which the power densities are the highest. The wall is cooled by helium flowing in the toroidal direction. As the inlet helium temperature is 250 °C and the outlet temperature is 305 °C, the temperature of the structural material varies between 300 °C and 550 °C. The shaded region shows the temperature range in which a degra-

L.V. Boccaccini et al. / Fusion Engineering and Design 27 (1995) 407-414 411

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dation of the toughness properties occurs. About 70% of the structural material has temperatures between 300 °C and 400 °C. As the maximum temperatures in the remaining structure are considerably lower than in the equatorial region, nearly all the structural material of the outboard box will be damaged by neutron irradiation during the lifetime of the blanket.

A calculation of unstable crack propagation conditions has been performed for the region shown in Fig. 3. The section A A has been chosen because the thermal and electromagnetic stresses are there relatively high with temperatures below 400 °C. A semielliptical surface crack is supposed to be at this position. Meth- ods and modelling adopted in the calculation are de- scribed in detail in Ref. [7]. Fig. 4(a) shows the

calculated normal stress (normal to the crack plane orientation) acting along the section A A in the toroidal direction. Fig. 4(b) shows the calculated stress intensity factor as function of the crack dimension (crack depth a). The calculation shows that the condi- tion of unstable crack propagation can be reached for a crack dimension greater than 3 ram. These results have been obtained according to Ref. [8] for load conditions of category 1 ("normal conditions and anticipated faults"). If the disruption could be classified into category 2 ("unlike and design faults") a safety factor of 1/2 instead of 1/3 may be applied, which produces in this case a critical crack dimension of 4 mm.

A comparison of the critical dimension with the dimension which a pre-existent crack can reach at end of component life shows that the requirements of Ref. [8] cannot be achieved. However, advanced martensitic alloys with lower contents of Cr which present a better behaviour of the fracture properties under irradiation are under development. Their use in DEMO applica- tions will contribute to improving the performances of the first wall against fast fracture.

4. S e g m e n t e d a n d c o n t i n u o u s d e s i g n

Two different designs have been analysed and compared, a segmented design (SD) in which the 48 mod- ules of the outboard blanket are electrically insulated from each other, and a continuous design (CD) in which a first wall connection among neighbouring mod- ules allows eddy current to flow in toroidal direction. Another important difference between the two designs is the presence of a passive plasma stabilizer ("saddle loop") in the SD; in the CD this stabilizing function is performed by the continuous first wall.

Fig. 5 presents the currents which flow in the first wall for both designs. The term box current denotes the current that flows within each box closing a circuit among first wall, side walls and back plate. The term toroidal current denotes the current that flows in the first wall closing a circuit around the plasma. The toroidal current is obviously null in the segmented design. Three different plasma disruptions have been analysed: the DEMO reference disruption and two faster disruptions. The reference disruption consists of a linear current quench from an initial value of 20 Ma to null in 20 ms; the two faster disruptions have quench times of 5 and 2 ms.

In the CD a large toroidal current is induced in the first wall; on the contrary, the box current remains relatively low owing to the shielding effect of the

412 L.V. Boccaccini et al. / Fusion Engineering and Design 27 (1995) 407 414

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L.V. Boceaccini et al. / Fusion Engineering and Design 27 (1995) 407 414 413

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toroidal current. In the case of the reference disruption, the box current is reduced by a factor of 4 in comparison with the cor responding current in the SD. Fo r faster disrupt ions the increase in the current is limited

by the greater t ime cons tan t associated with the con- nected structure. On the contrary, in the SD the current in the 2 ms quench t ime is 2 t imes greater than in the reference case.

Fig. 6 shows the load dis t r ibut ion on the first wall for the reference disruption. In b o t h designs the main cont r ibu t ion of the forces acts normal ly to the plates; this con t r ibu t ion is represented in the figure in terms of a pressure. In the SD this pressure is strongly dependent on the toroidal field. Hence, it is realized as internal or external pressure for the different regions of the first wall. In the CD the magnet ic force depends only on the poloidal field; in this case it acts only as internal pressure. On the side walls the ma in componen t of the magnet ic force has a direct ion parallel to the plate surface. This componen t results f rom the interact ion of the box current and the toroidal magnet ic field. Fig. 7 shows this componen t as force per uni t of plate surface. The decrease in the shear component in the CD is caused by the decrease in the box current.

Table 1 shows the resul tant forces and torques for different hor izonta l sections of the segment box. The C D presents lower values; in part icular , the torque T~ with respect to the z axis and the bending T x in the toroidal direct ion are considerably reduced (see the values for the equator ia l section C - C ) . T~ and T x are responsible for the large stresses calculated in the SD (see Ref. [3]).

Finally, the stress analysis shows the advantages of the cont inuous solution. Dur ing the reference disrup-

Table 1 Resultant forces and torques in the structure for different horizontal sections (see also Fig. 2; the results of Fig. 2 and this table cannot be compared, because they are obtained on the basis of different electromagnetic models)

SD CD

Section A F x = -6 .44 T x = 39.70 F x = -5.37 Tx = -4.49 Fy = --0.60 Ty = 63.87 Fy = -0.11 Ty = 54.21 F~ = 0.09 T z = 4.95 F~ = --0.03 T~ = 0.91

Section B F x = -6 .32 Tx = 40.37 Fx = -5.45 T x = -2.87 Fy = --0.36 Ty = 48.22 Fy = -0.09 Ty = 40.98 F~ = 0.04 T z = 3.02 Fz = 0.00 T z = 0.72

Section C F x = --3.33 T x = 18.29 F. = --2.83 T~ = --0.89 Fy = 1.97 Ty = 1.46 Fy = --0.01 Ty = 1.60 F~ = --0.79 T~ = --18.16 F~ =0.22 T~ =0.13

Forces F are in meganewtons; torques T are in meganewton metres.

414 L. V. Boccaecini et al. / Fusion Engineering and Design 27 (1995) 40 ~ 414

tion the calculated stresses are generally lower. Only in the first wall do the stresses increase locally as a result of the larger electromagnetic pressure (1 MPa). As the box is designed to withstand an internal pressure of 8MPa (break of a helium cool- ant tube) without interruption of the operation, such an additional electromagnetic load cannot damage the structure. Furthermore, the stress analysis of faster disruptions shows that only the CD can ensure the structural integrity of the blanket segment.

Acknowledgements

This work has been performed in the framework of the Nuclear Fusion Project of the Kernforschungs- zentrum Karlsruhe and is supported by the Euro- pean Union within the European Fusion Technology Programme.

The authors would like to express their thanks toDr. T. Fett (Karlsruhe Nuclear Centre) for valuable discussions.

References

5. Conclusions

The presence of a magnetic structural material has been analysed by means of a new computer program. The results show that the resultant forces over the different parts of the structure are acting mostly in the same direction as in the case without magnetic material and that their modulus, in the part of the structure where the highest stresses are expected, increases by up to 20%.

As far as the unstable crack propagation conditions are concerned, nearly all the structural material of the outboard presents temperatures at which degradation of the toughness properties of MANET-I under irradiation can occur. The performed assessment shows that conditions of unstable crack propagation can occur in the first wall, where electromagnetic stresses caused by a disruption are added to high thermal stresses already present in the structure.

Finally, the electrical toroidal connection of the first wall (CD) has been analysed and the effects of plasma disruptions have been compared with those in the case of a toroidal insulation of the blanket segments (SD). Since the time constant associated with the toroidal current which flows in the continuous first wall is much longer than the disruption time, the penetration of the varying magnetic field into the blanket structure is de- layed. This results in two advantages, namely it prevents large eddy currents and related magnetic forces in the side walls, which are responsible for the more dangerous load conditions and, secondly, it limits the increase in the forces in case of faster disruptions.

[1] E. Proust et al., Breeding blanket for DEMO, Proc. 17th Syrup. on Fusion Technology, Vol. 1, 1992, pp. 19-33.

[2] M. Dalle Donne et al., DEMO-relevant test blanket for NET/ITER. BOT helium cooled solid breeder blanket, Kernforschungszentrum Karlsruhe, Rep. KfK 4928 and 4929, October 1991.

[3] L.V. Boceaccini, Calculation of electromagnetic forces and stresses caused by a major plasma disruption in the Karls- ruhe solid breeder blanket design for the DEMO reactor, Proc. 17th Symp. on Fusion Technology, Vol. 2, 1992, pp. 1291-1295.

[4] L.V. Boccaccini, Electromagnetic forces and stresses caused by a plasma disruption in the Karlsruhe solid breeder blanket, Workshop on Electromagnetic Forces and Related Effects on Blankets and Other Structure Surrounding the Plasma Torus, EUR 14820 EN, Karlsruhe, October 1992, pp. 158 173.

[5] P. Ruatto, Electromagnetic force computation for a ferromagnetic blanket structure during a plasma disruption, Proc. 2nd Int. Workshop on Electromagnetic Forces and Related Effects on Blankets and Other Structures Surrounding the Fusion Plasma Torus, UTNL-R-0302, Tokai, September 1993, pp. I69-177.

[6] M. Rieth, B. Dafferner and C. Wassilew. Der Einflug von Wfirmebehandlung und Neutronenbestrahlung auf die Kerbschlageigenschaften des martensitischen 10,6% Cr.- Stahls MANET-I, Kernforschungszentrum Karlsruhe, Rep. KfK 5243, September 1993.

[7] L.V. Boccaccini, Fracture mechanics assessment of the Karlsruhe demo blanket during plasma disruptions, Proc. 2nd Int. Workshop on Electromagnetic Forces and Related effects on Blankets and Other Structures Surrounding the Fusion Plasma Torus, UTNL-R-0302, Tokai, September 1993, pp. 189-200.

[8] E. Zolti, D. Munz, R. Matera, V. Renda and D. Acker, Inter- im structural design criteria for predesign of the NET plasma facing components, The NET Team, NET/IN/86 14, 1986.

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