Design study of fusion material irradiation test facility in Japan

Section

13

TEST FACILITIES

Journal of Nuclear Materials North-Holland, Amsterdam

141-143

(1986) 1003-1010 1003

DESIGN STUDY OF FUSION MATERIAL A. MIYAHARA’, NISHIZAWA4

K. MIYAHARA~,

IRRADIATION

S. KAWASAK13,

TEST FACILITY

K. YAMAMOT04

IN JAPAN

and H.

‘Institute of Plasma Physics, Nagoya University, 464 Nagoya, Japan ‘Faculty of Engineering, Nagoya University, 464 Nagoya, Japan ‘Faculty of Science, Kanazawa University, 920 Kanazawa, Japan ‘Tsurumi Works, Toshiba Cooperation, 230 Yokohama. Japan

The design studies of the testing facility for fusion material irradiation following the concept of FhUT have been carried out based on current technology for accelerators, fusion reactors and materials. The facility consists of three sub-facilities: accelerator, target assembly and service cell. Special consideration has been given to fulfill the specifications of high current accelerator and to perform highly reliable operation of two to three years without maintenance. Balance between remote maintenance and adoption of low activation materials is the key issue of this facility to raise the operational performance.

flux. It is very unfortunate that the efforts to construct FMIT in the framework of international collaboration have been interrupted. In the following , the work performed recently to improve FMIT facility design in Japan is described. The emphasis is on proper definition of critical problems and construction of an FMIT data base by using the results of fusion, accelerator and nuclear technologies.

1. Introduction In fusion reactors, 14 MeV neutrons, which are produced in deuterium/tritium core plasma, penetrate freely the vacuum wall and deposit their energy in the energy conversion area, namely the blanket. However, during their excursion into ihe blanket, they introduce serious problems of activation, changes of physical properties and radiation damage of the wall materials. For this reason, the fundamental problem for material studies is how to optimize the material selection from the stand point of energy pay back in parallel with the high /3 or high density approach of plasma physics studies. It is also necessary to set up a near term goal because a stepwise approach is essential for realistic fusion research programs. At this moment, materials which can withstand the fluence of 300 MW yr/m* (10 MW/m* x 30 yr) are postulated as a long term goal, while 3-10 MW yr/m* (0.3-l MW/m* x 10 yr) as a near term one. In order to investigate radiation effects of 14 MeV neutrons for candidate materials, an intense neutron source plays a very important role. Of course a material-test fusion reactor is an ideal facility [l-3], because it can irradiate large area samples. However, but it is still far beyond the present states of fusion technology. As the first step in this direction, a rotating-target neutron source such as RTNS-II [4] has been constructed and used to investigate material behavior with the flux range of around lo’* n/s cm*. Although a fluence of 10” n/cm2 has been achieved for one year irradiation, the fIuence is still too small to perform laboratory tests even for materials of BCX (Burning Core Experiment) class machine. FMIT (Fusion Material Irradiation Testing) facilities, based on combination of 35 MeV Dt linac to liquid Li target using stripping reaction, were proposedl and intensive design studies, research and development efforts were carried out [S-IO]. The facilities are expected to produce a neutron flux of 10’” n/s cm2 for irradiation of miniature samples. The key technologies are stable operations of high current CW linac and liquid Li target which can accept very high D’

~22-3115/86/$3.5~ 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

2. Choices of accelerator parameters Linac components envisaged in this article are the standard version, similar to the ones which have been proposed in recent years to supply the deuterium beam of the required intensity and quality for the stripping reaction considered. The layout of the whole assembly from the ion source to the Alvarez linac is shown in fig. 1. The precise information of the actual pe~ormance of these elements, however, is still beyond the reach of our experimental knowledge. Especially the considerations on the long term continuous operation should be employed, because various difficulties in machine maintenance and manipulation caused by the inevitable beam spills in the successive stages of beam handling and neutron backstreaming from the target area are anticipated. The problem of the scaling up of the accelerator assembly stage by stage is a subject of particular interest. The principal parameters of the device are summarized below. 2.1. ion source Source of initial deuterium beam which has to work continuously for a few years at least as requested in any FMIT project, is one of the most important issues for getting reliable operation results for the whole equipment. Because there is hardly any possibility for finding some approach that results in a remarkable improvement in the limited life of the cathode filament of the ion source of the duopigatron [ll], an alternative way is proposed in a filament-free system as ECR source, B.V.

1004

A. Miyahura

et al. I Design study of FMIT futility in Japan

Target

A fllvarez

/ RFQ ,’

r 4.5rnt

Fig. 1. The layout

of the whole

which is being prepared by the RIKEN group [ 121. The source plasma is created in a conductive cavity with the several kW RF power with a frequency of 2.45 GHz, being driven in a whistler mode so as to obtain a high plasma density up to 10’3/cm3. Tentative parameters of the extraction voltage and the beam intensity through the anode aperture would be assigned to be 80-100 kV and 120 mA respectively. The extraction systems have to fit the ECR source specifically and the tradeoffs have to be involved between the beam emittance and the following RFQ’s acceptance. The source could be highly promising if well established. Multiple sources and remote maintenance systems will be one of the benefits for the system.

accelerator

/Injecton

system

for intense beams more than several tens in mA. For the design procedure of the structure, no complete theory is available to track the beam for minimizing the beam spill in the three dimensional space. We have simply followed the technique of “Generalized Approach” proposed by LASL [14] several years ago. Assuming 50 keV injection and 2 MeV output energies, examples of the results obtained numerically in our design study of the RFQ are shown in fig. 2 and fig. 3. Fig. 2 gives the transmission efficiency with parameter of beam intensity. Basic characteristics of the RFQ as the radial defocussing parameter, /3, the equilibrium

‘00------7 -0

2.2. RFQ

80

Since RFQ was proposed by the USSR physicists [13] as a concept of revolutionarily low energy accelerating and adiabatic bunching, it has proved to solve the fundamental design problems of high capture efficiency and low beam spill. It has rather simple mechanical structures which work as to accelerate and focus the beam simultaneously and so reduces the engineering problems which otherwise are very serious for the design of the low energy sections of the linac. The vane structure of RFQ described below is separated into four types corresponding to the four sections including (1) radial matching section, (2) shaper, (3) gentle buncher and (4) accelerator. The beam dynamics in the vane space are mostly dominated with the spacecharge self field as well as with the external RF fields

particle

“\

/Good I

0

40 Current

80

120 (mA

Fig. 2. The effect of space charges

160

200

)

for transmission

efficiency.

A. Miyahara et al. I Design study of FMIT facility in Japan

phase, @, and the modulation, m, vary with longitudinal distance are given in fig. 4, where the applied voltage between the vanes is specified to be 189 kV and the whole chamber length to be 435.5 cm. The transmission is improved by going up to higher injection energy as shown in fig. 5, close to 90% for the initial particle energy of 100 keV. Construction and testing of a prototype model with lower duty and/or continuous operation are of absolute necessity. The problems related to CW operation such as the choice of materials for vane and surrounding walls, cooling and feeding of high power RF should be solved. These items relate closely to the fusion technology area, such as TiN coating to reduce secondary electron emissions and special purpose material investigations for high power RF insulators.

-

3 o\

(0)

25mA

(b)

50mA

(Cl

75mA -30

if v -60

Cel I Fig. 3. Change

B

2.3. Alvarez linac Design of Alvarez structure for accelerating the deutron particles from the injection energy of 2 MeV to 35 MeV at the exit, does not present any problem in the sense of the basic linac theory. Design tradeoffs exist between the choices for the operating frequency, the commercially available RF power system, the machining problems and the beam dynamics. In this design, important decisions involve the choice of focusing Qs and CW RF sources for the maximum power of 7 MW working at the frequency of 80 MHz. The main parameters of the Alvarez linac and the beam evolution during the acceleration are calculated and listed in table 1. Preliminary assumptions include an average field of 1.5 MV/m, a 2 cm bore radius, for the focussing, and the lattice of FODO are used. The beam injected from the proceeding RFQ is to be bunched within f 20 degrees of the RF phase. The final transmission with the parameters of table 1, reaches around 93%. The 63 cells are located along the total path of 28.39 m. As shown in fig. 6, the beam is well stabilized even to levels as high as 100 mA. As with RFQ, even a very small fraction of the beam spill can cause unfavorable radioactive contamination in the accelerating structures, chamber walls and the immediate surroundings, which would make the situation for maintenance very difficult. It should be pointed out that it is extremely important to clarify how and where the beam particles lose their stability in the course of the acceleration and hit the surroundings. In the numerical calculation mentioned above, the normalized emittance of the beam is supposed to be 7.5 nmm.mrad. The emittance of the output beam might be critically subject to the regime of the perfor-

number

of transmission

efficiency

/

1005

100

vs cell number.

1

I

I

aTa 80 4

2 -

-30

60,

-60

+i a

/ /O

60-

.-5, ?I ‘E 40* k c‘

20-

0

I 25

1 50

I njection Cell Fig. 4. Basic parameters

of RFQ.

No.

I

I

e-

co 4-

I

Fig. 5. Transmission

efficiency energy.

1 100

I 75

Energy

( keV)

as a function

of injection

1006

A. Miyuhara

Table 1 Main parameters NC

of Alvarez

el 01. I Design

At&y

of FMIT facility

linac

ws

BETA

dBldX

L-QUAD

TTF

10 20 30 40 50 60 63

2.0000 4.6454 8.3593 13.1031 18.8238 25.4446 32.8806 35.2563

0.0461 0.0702 0.094 I 0.1176 0.1406 0.1630 0.1848 0.1912

30.00 28.02 29.16 31.54 29.97 27.17 24.49 23.64

X.00 8.02 8.34 8.73 9.18 10.96 11.55

0.9323 0.925 I 0.9169 0.9044 0. X883 0.8696 0.8633

NC: WS: BETA:

cell number beam energy (MeV) &beta) value

dBi&:

field gradient of Q magnet (T/m) quadrupole magnet core length (cm) transit time factor gap length/cell length cell length (cm) total length (cm)

1

L-QUAD: -I-I-F: GIL: CL: TL:

in Jupun

mance of the ion source and the possible non-linear effects involved in the beam dynamics. The value of the normalized emittance, however, is probably overestimated. The use of permanent magnets (SmCo, or equivalents) is preferred for simplifying the accelerator structure and getting rid of many sources of trouble resulting in the unreliable operation of the system. The assumed gradient of the magnetic fields, a!?/&, 50 (T/m) is within the reach of the present magnet technology. With regards to the RF power source, the application of the power system designed originally for ICRH in the large scale Tokamak JT-60 is expected. It has been normally operated with the frequency of 110 MHz in the power level of 750 kW for the period of 10 s. The EIMAC 8973 tube used in the oscillator could be CW operated with the frequency of 80 MHz to deliver the RF power of 1 MW.

GIL

CL

TL.

0.2050

25.X69 34.810 43.618 52.256 60.677 6X.843 71.239

21x.090 526.055 922.715 1406.566 1975.637 2627.540 2838.866

0.215x 0.2277 0.2446 0.265 1 0.2873 0.2945

through the beam duct to the accelerator space may cause fatal radioactive contaminations if it is not reduced by bending the beam path as shown in fig. 1. Sufficient assessment of the radioactive environment resulting in these effects must be made. The subsidiary effects of the radioactivity on the accelerator components must also be carefully considered.

I

-

:

OmA

E -5 X

Beam

20

transport

Cel I

The deuterium beam having the particle energy of 35 MeV has to be transported into the target area, maintaining the intensity of 100 mA and the distribution over the limited cross section, with use of a series of deflecting dipoles and quadrupoles for focussing. The maximum beam current reaches about 2 A at the peak value, and at this high value the effect of the self fields including space charge and its neutralization factor due to the residual gases cannot be neglected in any estimation of the beam behaviour. The target area becomes highly activated with the radiation of neutrons and gamma rays. The backstreaming of these radiations

lOOmA

IO

L.-

2.4.

:

---~-

20

!

40

60

40

60

No.

I

0

I

20

Cel Fig. 6. Envelopes

of beams

I

No.

(x-direction

and y-direction).

A. Miyahara

et al. I Design study of FMIT facility

3. Target and irradiation cell 3.1. Target As candidate materials for the deuterium stripping reaction target, lithium and beryllium are appropriate because of their spectra and high neutron yields. Considering that the intense deuterium beam up to approximately 3.5 MW is focused onto the target with the vertical dimension of 1 cm and the horizontal dimension of 3 cm, a fast-flowing jet of lithium is proposed for the FMIT [15]. The flowing jet seems very suitable not only for minimizing the temperature rise of the target but also for generating a vacuum around the jet nozzle where the deuterium beam vacuum duct joins. However, it seems necessary to pay attention to such probable uncertainties as the disturbance of the steady state lithium flow caused by an abrupt abnormal behavior of the deuterium beam or by the swelling of the jet nozzle backwall, which might result in lithium evaporation or tritium discharge. On the other hand, a rotating target seems favorable, assuming a stepwise increase of the deuterium beam. It can be utilized, if the beam duty is onethousandth or even one-hundredth of the full beam power. It will also be useful for the alignment of the beam or for the test run of the facility. As a material for the rotating target, beryllium is one of the options, and adaptable discs made of beryllium are available on the market. The selection of coolant compatible with beryllium is one of the R & D items. It seems encouraging that the study of beryllium for fusion application is in progress between JET (Joint European Torus), Sandia National Laboratories and Oak Ridge National Laboratory. The rotating beryllium wheel could produce neutron yields as high as RTNS-II, and would be easier to maintain and have a lower price compared to the lithium loop which will be highly activated, if NaK can be utilized as a coolant for beryllium discs. Fig. 7 illustrates both a nozzle target and a rotating target.

1007

in Japan

may not be appropriate. Moreover, the coupling and decoupling of coolant tubes to the container by remote handling requires special effort. The hydraulic rabbit containing the test specimens and utilizing NaK as a fluid provides easy positioning and handling. The positioning accuracy is inferior to other types and coaxial tubing results poor neutron economy, however. The temperature of test specimens is determined by the inlet and outlet temperatures of the coolant (NaK). The measurement of the temperature distribution of the container will require a considerable effort. The neutron dose can be measured as the accumulated dose. 3.3. Test cell The test cell which accommodates the intense neutron source will require thick shielding walls and should Nozzle

Rotating

Disc

n

Li+Be

Fig. 7. Types of target. Stalk

X.Y

Table

Hydraulic

Rabbit

3.2. Test assembly In order to completely utilize the small region of the highest neutron flux in the FMIT, the design of a test assembly which contains materials test specimens should be emphasized. Considering that the specimens require accurate positioning or alignment, temperature control and ease of handling, three types of test assemblies, as shown in fig. 8, are to be discussed. The stalk type is proposed as VTA-1 in the FMIT; however, its positioning mechanism seems to be rather sophisticated. Therefore, its reliability should be verified in the actual neutron environment. The combination of a vertical slide and an X-Y table seems to be the most simple for accurate positioning; however, the actuators are located in a high irradiation environment and it

/,‘,1

,1’,,

f

/ J ,,,,,,

Fig. 8. Several methods for positioning test piece on the beam line.

A. Miyahara

1008

et al. I Design study of FMIT facility

be built underground. According to the Thomas’ equation, the ceiling plug of the test cell needs concrete thickness of more than 5 m for restricting the skyshine dose in the vicinity of the facility to 5 mR/yr. The concrete shield plug of the test cell will be divided to two sections, namely upper and lower shieldings, in order to achieve easy fabrication and handling. During irradiation tests, such remote maintenance equipment as manipulators and monitoring devices should be retracted and shielded as shown in fig. 9. 4. Test matrices 4.1.

Material

test in high flux

irradiation

Main roles of FMIT for the development and testing of fusion materials are considered to be as follows: (a) Fundamental research of high energy neutron irradiation effects on the microstructure and mechanical properties of materials. (b) Irradiation effect research for the development of the first wall structural materials of FERs (Fusion Experimental Reactors). (c) Irradiation effect research for the development

‘I

I’

I

‘m

r

.

;. /Spare

Room

Door-

/

Fl’oor-Mobile

Fig. 9. Manipulators

pull-out

Over- head Crane Manipulator

Manipulator

plan during

neutron

irradiation.

in Japan

of the first wall structural materials of power reactors. At the present time, the Japan-US joint research project of heavy irradiation effect on the fusion materials is being planned to start in 1987. In this project, however, heavy irradiation must be done by using fission reactors, such as FFTF/MOTA or HFIR of HEDL or ORNL, respectively. The analysis and clarification of helium effects on the microstructure evolution can be expected only for isotope tailored stainless steels. The effects of cascade and/or subcascades formed under high energy neutron (fusion neutron) irradiation on the microstructure of materials is not necessarily clarified in this research. The effect of helium bubbles on the various kinds of mechanical properties of heavily irradiated stainless steels is also difficult to investigate. Therefore, FMIT is an essential facility for the fundamental study of heavy irradiation effects of high energy neutrons (including cascade effect and helium effect) on the microstructure and mechanical properties of materials. It is recognized that the integrated first wall load over the lifetime of FERs is 3 MW yr/m2 (30 dpa). PCA (Primary Candidate Alloy: a Ti modified 316 austenitic stainless steel) and ferritic stainless steels (9CrlMo, 9Cr-2Mo steels, and particle dispersion steels) are now being investigated and developed for the first wall structural materials of FERs. The mechanical properties and microstructural evolution of these alloys under heavy irradiation of high energy neutron can be clarified after the irradiation at FMIT. On the other hand, considering the necessity of personnel access to the inside of the system, the development of low activation structural materials becomes very important. Recently. Fe-Cr-Mn and Fe-Cr-W steels are investigated as low activation materials. The properties of these steels also can be checked after the irradiation at FMIT. The integrated first wall loading over lifetime of future power reactors could be as high as 300 MW yr/m’. This loading will introduce the displacement damage of around 3000 dpa on first wall material. At the same time, such extremely high fluence of fusion neutron generates several percent of helium and hydrogen in the material. These effects cause the severe problem of the material property degradation. It is considered that this problem must be solved from both the’side of the design of reactor and the material development. Furthermore, the first wall structural materials of the power reactors should be low activation. Because material reinassembling is desirable within 30 to 100 years after the shutdown of power reactors, C, Si and V will be the candidate elements which compose such very low activation structural materials. Although in Opperman’s report [15] the high flux test matrix and various test specimens are described in detail, the following tests can be added to the test matrix.

A. Miyahara et al. / Design study of FMIT facility in Japan

(1) Charpy impact tests of subsize or small size specimens. (2) High speed dynamic tensile or bending tests of small specimens. (3) Compact tension tests. (4) Shear punch tests of TEM discs. (5) Fatigue tests of small and thin specimens (postirradiation test). (6) Field ion microscope imaging atom probe analysis. Test items of high flux irradiated materials is shown in table 2. 4.2. Material test in low flux irradiation The need for low flux irradiation testing of electrical and thermal insulators, superconducting magnet and stabilizer materials, fusion reactor diagnostics, breeding materials, components, etc., has been described in several papers [15,17,18]. Each of these materials has special testing requirements, and many of these tests need long setup times, i.e., magnet components or blanket tests with extensive instrumentation at cryogenic conditions. Therefore, handling considerations become very important and the establishment of remote handling and setting techniques for the test assembly and other instruments are required. 5. Conclusions In conclusion, we summarize the items which are necessary to considerably shorten the time for construction and testing before full operation. (1) Critical inspection of the system whether it can operate at a given site or not, is necessary, namely

Table 2 Test items of high flux irradiated materials and specimen size (unit: mm; 0 means diameter) TEM observation Tensile test Wire Flat specimens

3.00 x 0.3 0.50 x 1.3 to2 0.4 x 2.5 x 12.5 to 3.2 Half size ; subsize specimen

High speed dynamic tensile and bending test Compact tension test (small size specimens) Shear punch test of TEM discs Pressurized tube creep test Post irradiation creep and rupture test Post irradiation fatigue test Field ion microscope imaging atom probe analysis (thin needle specimens)

1.5 x 1.5 x 25 16.00 x 1.5 to 3 3.00 x 0.3 3.00 x 13 0.5 x 2.5 x 20 0.3 x 15 x 10

0.20 x 10

1009

requirements of electric power, cooling water and compatibility with environment. (2) Construction of high current CW D+ linac is difficult but necessary. Detailed investigations and input from the experiences of fusion technology, such as coating techniques and high power RF technology, are necessary. Of course the problem to be solved must be defined very clearly, and international collaboration with task sharing is extremely useful for this area. (3) CW Operation of l-2 years without maintenance is a technological challenge. Key issues to achieve such performance must be addressed and development should start now. An example of needed development is an ion source without filaments. of low activation materials for ac(4) Adoption celerator and irradiation cell components is necessary although remote maintenance techniques will be used. (5) Disposals of irradiated materials are an important item to be considered. Besides above mentioned items, the upgrading of the specifications for the facilities by using negative ion source and optimizing machine operations should be considered in the future. This consideration is necessary because the economic aspects of fusion reactors become stronger for the next step investigations. This work was supported by a special Grant-in-Aid of Fusion Research of the Ministry of Education, Science and Culture. We also appreciate many published works which have been done by FMIT research and development teams in US and beneficial data base established by them for future construction.

References PI T. Kawabe and H. Nariai, J. Fusion Energy 3 (1983). PI J.N. Doggett, B.G. Logan, J.E. Osher, K.I. Thomassen and W.D. Nelson, Nucl. Engrg. Des/Fusion 2 (1985) 223. 131 G.A. Emmert et al., ibid. ref. [2], 2 (1985) 239. [41 D.W. Heikkiman and C.M. Logan, IEEE Trans. Nucl. Sci. 28 (1981) 1490. for an Accelerator-base and Neutron PI Proposal Generator, BNL 20159. July 1975. PI P. Grand and A.N. Goland, Nucl. Instr. and Meth. 145 (1977) 49. [71 R.A. Jameson, IEEE Trans. Nucl. Sci. 26 (1979) 2486. @I J.W. Hagen, E.K. Opperman and A.L. Trego, J. Nucl. Mater. 123-124 (1980) 958. [91 W.D. Cornelius, IEEE Trans. Nucl. Sci. 32 (1985) 3139. WI R.J. Furke and J.J. Holmes, J. Nucl. Mater. 133-134 (1985) 869. Ull E.A. Mayer, D.D. Armstrong and J.O. Schneider, IEEE Trans. Nucl. Sci. 28 (1981) 2687. WI S. Ishii, H. Ameniya and M. Yanokura, to be published in Jpn. J. Appl. Phys.

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[13] J.M. Kapchinskii and V.A. Taplyakov. Tekhnika Eksperimenta 119 (1970) 19. [14] K.R. Crandall, R.H. Stokes and T.P. Wangler. in: Proc. 10th Linear Accelerator Conf., September 10-14. 1979. BNL-51134 (1980). p. 205. [15] E.K. Opperman, Fusion Materials Irradiation Test Facility-Experimental Capabilities and Test Matrix. Jan. 1982. HEDL-TME X1-45. UC-20.

o,f FMIT fuciliry in Japun

]lh]

H. Katsuta, T. Kondo and M. Odera, J. At. Energy Sot. Jpn. 27 (1985) 1102. [17] R.E. Gold. E.E. Bloom, F.W. Clinard, D.L. Smith, R.D. Stevenson and W.G. Wolfer. Nucl. Technol./Fusion 1 (1981) 169. [IX] W.E. Stacey. Jr. et al.. US INTOR Report. INTOR 1X11, Georgia Institute of Technology, Atlanta. GA (19X1).