Engineering Design of Cryoplant for ITER

Kazuya Hamada, Takashi Kato, Tadaaki Honda, Kunihiro Matsui, Kazuhiko Nishida, Hiroshi Tsuji, Susumu Shimamoto, Kiyoshi Yoshida*, Michael E.P Wykes*, Vladimir Kalinin*, Mikirou Mori** and Akihiro Miyake**

Japan Atomic Energy Research Institute, Naka Fusion Research Establishment, 801-l, Muko-yama, Naka- machi, Naka-gun, Ibaraki, 311-01, Japan, *ITER Joint Central Team, Naka Joint Work Site, 801-1, Muko-yama, Naka-machi, Naka-gun, Ibaraki, 311-01, Japan, **Ishikawajima Harima Heavy Industry Co., Ltd., 3-2-16 Toyosu, Koto-ku, Tokyo, 135, Japan

Japan Atomic Energy Research Institute is studying a cryoplant for International Thermonuclear Experimental Reactor (ITER). ITER consists of twenty Toroidal field coils, eight Poloidal field coils and the total weight is more than 25,000 tons. The cryoplant provides liquid helium, supercritical helium and 80-K helium gas for cooling the coils, structure, cryopump and thermal shield. The required capacity of cryoplant to cool the superconducting coil and the thermal shield is 110 kW + 6,618 liters/hour at 4.5K and 350-kW at 80K. Cryogenic pump is applied to circulation system of supercritical helium. The cryoplant is designed on the basis of Demo Poloidal test facility and CS model coil test facility.

INTRODUCTION

Japan Atomic Energy Research Institute (JAERI) participates in design activities for the International Thermonuclear Experimental Reactor (ITER) in collaboration with European Union, Russian federation and United states of America. From 1993, ITER engineering design activity (EDA) has started and ITER is designed so as to realize construction, based on the latest technology at the end of EDA. ITER has twenty Toroidal field (TF) coils and eight Poloidal field (PF) coils and Total cold mass is 25,000 tons. The function of the ITER cryoplant is to cool all cold elements. The cryoplant design requirements are summarized as follows;

@to operate continuously up to three year @to cool down the coils and the structure from room temperature to 4K or 80K within 30 days. @to allow a 24 hours plasma operation of ITER with sequential plasma pulsing @to operate stably in response to all transient heat loads arising in the various cold mass elements @to recover the cold helium gas at the quench @in the events of failure of one helium refrigerator cold box, continued plasma operation must be

possible, albiet at reduced availability. @to achieve a high energy efficiency and low running cost for minimal capital cost

To accomplish above requirements, the cryoplant components such as turbine, helium compressor and cryogenic pump are designed based on the existing components and parallel system.

CRYOPLANT SYSTEM

The block diagram of the cryoplant is shown in Figure 1. Cryoplant mainly consists of five cold boxes and thirteen cryogenic pump systems. The system is based on the Demo Poloidal Test Facility[2] and CS model coil test facility[3]. The coils and structures are cooled by supercritical helium at 4.5K,

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0.6 MPa. Heat load conditions are listed in Table 1. It is assumed that the adiabatic efficiency of cryogenic pump is 0.7 which is based on the JAERI's R&D and pressure drop of TF coil and PF coil are 0.15 and 0.02~0.075 MPa. Current lead cooling is assumed to be by helium vapor.

Buffer Tank (13unit3s X [ ~ Helium Compressor(Total Helium 3000m at \ f--" Mass Flow Rate:12,250g/s) Compressor 1.8MPa) ~ [ :3,359g/s

l I Cold Box ~ , Recovery ~ Gas Purifier ] (5 units: 22kW Liquid Nitrogen ] I C~176 l ~ ~ ~ ' ~ 2 3 1 i t e r s / h ~ [--- Supply System H 80 K Helium

i - - .eat Low Pressure Gas ~ ' ~ I - - ] / Exchanger Holder(3000m3) I I [ ~ [ , I ,[

�9 ~ I I ['--='q ] ILiquid Helium Tank[ I [ ~ [ [ 1~ 1(250,000liters) } ]

[ Rec~ L [ " " - . . ~ ' t - ~ ' - - - ~ ~ ' [ I ] Heater } ",,.. ~ ~ = ~ ~ ~ ~ _ _ _ _ ~ ] 1 1

f ,I ....... l [ ' - . . ~ [ ITERHeat Load Cryogenic Pump System-t ..... , [ ~ [ 110kW+6618 liters/hour 13 units (26.07kg/s at [ [ [ .~ [ at 4.5K 4.5K,0.6MPa) "-I 350 kW at 80K

I

Figure 1 Block diagram of lTER cryoplant

Table 1 Heat load condition at plasma operation

4-K Heat Load 80-K Heat Load Coil and structure 52.8 kW Plasma Vacuum vessel 220 kW Cryogenic pump 24.7 kW (including 10% margin) Cryostat 45 kW Cold compressor 6 kW Cryogenic piping 25 kW

Cryopump 20 kW Gravity support 10 kW Heat leak of cryoplant 5 kW Contingency 50 kW

Total 108.5 kW Total 350 kW Current lead 6,618 liters/hour

Total refrigeration and liquefaction capacity is 110 kW + 6,618 liters/hour and 350 kW at 80K, for the superconducting coil and thermal shields. To achieve a cooling power for 80-K operation of coils and structure, a liquid nitrogen refrigerator with air separator is applied. The capacity of the gas storage tank is designed to recover all helium gas in the ITER. Total helium inventory of the coil and cryoplant is 27,993.1m 3 and the helium gas weight is 93.7 tons. Accordingly thirteen units of gas storage tanks, which the capacity per one unit is 1,800m 3 at 1.8MPa, are required. The buffer tank acts as a quench buffer tank for coil quenches. The capacity of liquid helium tank is 250,000 liters and determined from operation scenario such as fault mode of one cold box. One train of cryoplant will be provided early in the construction phase to supply the magnet cold test facility at the ITER site. This will allow some early validation of cryoplant components.

COLD BOX DESIGN

The thermal process of the cold box is a Claude cycle with two series of turbines and a cryogenic turbine as shown in Figure 2. The capacity of one cold box is determined as less than 30 kW because the size of 30-kW cold box defines an upper limitation of transportation and installation at the ITER site. The nominal capacity per one cold box is 22 kW + 1,324 liters/hour. The cold box also generates the 30-kW maximum refrigeration power or 5,000-liters/ hour liquefaction rate. The mass and heat balance calculation have been performed and a Figure of Merit, which is defined as the fraction of 1-W refrigeration power to electrical input power such as compressor motor and liquid nitrogen consumption,

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is larger than 1/300. It is assumed that adiabatic efficiency of the turbine and cryogenic turbine are 0.7 and 0.6, respectively, and isothermal efficiency of the compressor is 0.6. The liquid nitrogen consumption is 272 g/s. The total mass flow rate and turbine mass flow rate are 2,450g/s and 580g/s, respectively. Although such a large scale turbine is not conventional, the turbine is feasible using current technology data base.

27 .67kW 33 .33K 23 .61K LN2:272g/s 0 . 6 6 M P a 0 . 6 6 M P a

3.Pa i ,

245%, ~ j 1 . 8 M P a :

I

23 .38kW

15.57K i 6"-- " ~ " 0 ' 1 3 M P a

580.Sg/s ! i

12.83kW

I i 1869g/s '.

I

/ i |

|

8.298K - . 6 .182K . . . . . . 1 . 63MPa 0 . 4 0 7 M P a

1,324 l i t e r s / h o u r

Figure 2 Flow diagram of cold box

H E L I U M C O M P R E S S O R

The helium compressor is one of the most important component of the cryoplant. The total mass flow rate of the helium compressor is 12,250 g/s at the ITER plasma experiment. For a large mass flow rate and down sizing, the turbo-compressor is ideal. Research and development of the turbo-compressor, however, is needed. Thus, two types of conventional compressors are estimated and the results are listed in Table 2.

Table 2 Comparison with Screw and Reciprocating Compressor

Mass flow rate per one set Configuration

Motor Power per one set Total unit number for ITER Installation Area per one set

Actual results for helium refrigerator

Feature

Screw Type Compressor 3,300 g/s

Low press, stage 9 units + High press, stage 9 units

11,700 kW 72 units 450m 2 leading

conventional

Reciprocating Compressor 3,150 g/s

3 units

9,600 kW 15 units 450m 2

not leading large size is used for LNG plant

conventional

The total installation area is the same for each type and there is not a large difference for the initial cost between the two types. A reliability and running cost, such as maintenance and utility consumption are a key parameter and a more detailed study is needed for decision.

C R Y O G E N I C PUMP SYSTEM

A cryogenic pump system has an advantage for down sizing itself because of 4-K operation. JAERI has developed a reciprocating and centrifugal cryogenic pump and applied them to large scale forced flow cooling pulse coil operation[4]. In ITER, centrifugal type cryogenic pump is selected to achieve a high pump head (0.3 MPa), high adiabatic efficiency (more than 0.7) and controllability. Required mass flow rate for each coil is listed in Table 3. Such large cryogenic pump technology is not as conventional as that of turbine. It is estimated that a 3-kg/s cryogenic pump can be developed on the basis of the latest

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technique. However, considering existing cryogenic pump technology (Pump head: 0.1 MPa at 720g/s), an improved of bearing system is needed in order to accomplish a higher mass flow rate and pump head.

Table 3 Mass flow rate of coils

TF winding and PF-1 PF-2& PF-3, PF-4, PF-5, Support and the case (central PF-7 PF-6 and PF-8 structure

solenoid coil) Mass flow Rate TF winding: 3.1kg/s 1.4 kg/s Total : Total: 4.8 kg/s Total: 6 kg/s

TF case: 13.1 kg/s 0.7kg/s Total: 16.2 kg/s

Pump unit TF winding: 1 unit l unit 2.8kg/s • 3kg/s • 1.55kg/s • 2 units 2 units 2 units

TF case: 2.62 kg/s • 5 units

The system has a gas separator and supercritical helium heat exchanger. The piping line between coils and cryogenic pump system is separated from the refrigerator main line to reduce the influence on heat balance of cold box from transient heat load caused by pulse operation or quench.

The inlet temperature of coil is required as 4.5K. Taking account of heat leak (2W/m) at cryogenic piping between coil cryostat and cryogenic pump system, the helium bath which contains a heat exchanger, is evacuated by using cold compressor and around 4.3K is achieved. The required mass flow rate of cold compressor is in the range from 200 g/s to 1,000 g/s. As with the cryogenic pump, improvement of the bearing system is a key factor for achieving a larger capacity.

CONCLUSION

The design of the ITER cryoplant is being performed and it is estimated that ITER requirement of 110 kW + 6,618 liters/hour for the superconducting coils and thermal shield can be realized. The large scale mass flow rate cryogenic components, such as turbine, helium compressor, cryogenic pump and cold compressor, is required. Basic technology is existing and the some research and development for these high speed turbo machine, such as bearing, are needed.

ACKNOWLEGMENT

The authors would like to thank Drs. M. Ohta, T. Nagashima and S. Matsuda for their continuous encouragement and support on this work. This report is an account of work for undertaken within the frame work of the ITER EDA agreement. The view of authors do not necessarily reflect those of the ITER Director, the parties to the ITER EDA agreement, or the International Atomic Energy Agency.

REFERENCE 1 Yoshida, K and Kalinin, V., Requirements and Interface to Cryogenic and Power Supply Plants for ITER Magnet System, Proceeding of Cryogenic Engineering Conference (1995) 2 Kato, T. et al., Operation Performance of DPCF in The Test of The Nb-Ti Demo Poloidal Coil (DPC- U1,U2], Proceeding of 1 lth International Conference of Magnet Technology Vol. 2, 1989, 1350-1354 3 Shimamoto, S and Hamada, K. et al., Construction of ITER Common Test Facility for CS model coil, Proceeding of 14th International Conference of Magnet Technology 1995 4 Kato, T et al, "Cryogenic Helium Pump System for the Development of Superconducting Magnets in a Fusion Experimental Reactor, Proceeding of 17th Symposium on Fusion Technology Vol. 1 1992 887-891