A high-temperature isotopic exchange process for recovering tritium from fusion fuel impurities

Fusion Engineering and Design 18 (1991) 49-52 North-Holland

49

A high-temperature isotopic exchange process for recovering tritium from fusion fuel impurities A. Busigin 1, K.B. W o o d a l l 2, S.K. S o o d 3 a n d K.M. K a l y a n a m Ontario Hydro, 700 University Avenue, Toronto, Ont. Canada M5G 1X6

A High-Temperature Isotopic Exchange (HITEX) process has been developed for processing of fusion fuel streams. The design has advantages over previous systems which have been based on catalytic oxidation or decomposition of impurities, since it eliminates the need for impurity oxidation and electrolysis of DTO, and does not rely on complicated catalytic decomposition reactions. A conceptual flowsheet of a complete HITEX-based fuel processing system is described. Testing of the HITEX reactor is under way with deuterated impurities. Details of the experimental HITEX test loop are reported in this paper, and the test loop is compared to a HITEX design recently proposed for ITER.

1. Introduction

In a fusion reactor, the Fuel Processing Loop (FPL) receives tritiated hydrogen (Q2 where Q = H, D or T) streams containing small amounts of impurities such as CnQ m, CO2, Ar, CO2, N2, NQ3, 02, and Q20. (It is expected that CnQ m shall be primarily CQ4, but minor quantities of other d e u t e r a t e d / t r i t i a t e d hydrocarbons may also be present.) The elemental hydrogen isotopes are separated from the impurities by a cryogenic molecular sieve bed or permeator and sent to the isotope separation system (ISS). The residual elemental hydrogen isotopes and impurities must be processed to recover the tritium bound in the impurities, and then send it in the form Q2 to the ISS. The remaining tritium-depleted impurities are discarded as waste gas. A number of different FPL processes have been proposed: P d / A g permeation with catalytic impurity decomposition [1]; catalytic oxidation with hot U-bed water decomposition [2]; P d / A g permeation with catalytic oxidation and electrolysis [3]; hot U-bed impurity 1 Current address: NITEK corporation, 939 Griffith Street, London Ont. Canada N6K 3S2, Tel: (519) 657-4914, Fax: (519) 657-0283. 2 Mailing address: Ontario Hydro Research Division, 800 Kipling Avenue, Toronto Ont. Canada M8Z 5S4, Tel: (416) 321-4111 Ext. 6833, Fax: (416) 231-5799. 3 To whom correspondence should be addressed. Tel: (416) 592-5501, Fax: (416) 592-4483.

decomposition and cryogenic adsorption [4]; and cryosorption, catalytic oxidation and electrolysis [5,6]. In all of these processes, it has been assumed that the FPL must decompose all the CnQm, Q 2 0 and NQ 3 impurities to liberate the hydrogen isotopes into a Q2 stream, which is then sent to the ISS. In the HITEX process [7], the impurity stream is swamped with HE, equilibrated on the surface of a hot Pt catalyst, and the tritium bound in the impurity compounds is replaced by protium (H). Tritium in the molecular form of HT is then separated from the impurities by a P d / A g permeator. The H 2 swamping required for the HITEX process increases the H / T separative duty of the ISS. However, for the ITER reactor, recent ISS design studies [9] show that the sizing of the ISS for H / T separation is determined mainly by requirements for waste water detritiation and pellet injector propellant cleanup. The addition of a small H 2 / H D / H T stream from the FPL has very little impact on the ISS design. The advantage of the HITEX process is that it avoids the oxidation of the impurity compounds to form Q 2 0 and hence avoids subsequent water handling and reduction. This approach, in contrast to prior approaches, also does not rely on complex chemical decomposition reactions with their consequent unpredictability. Typical design requirements for a fuel processing system plasma cleanup loop are given in [8] for the ITER reactor. Based on 1000 MW fusion power derived from ignition conditions, the torus vacuum ex-

0 9 2 0 - 3 7 9 6 / 9 1 / $ 0 3 . 5 0 © 1991 - E l s e v i e r Science P u b l i s h e r s B.V. A l l rights r e s e r v e d

50

A. Busigin et al. / H I T E X process ,fi)r recot'ering tritiun7

Table 1 ITER design flows and HITEX buffer tank component partial pressures. The second and third columns of this table give the composition of the plasma exhaust gas stream which is received by the ITER fuel processing loop. The fourth and fifth columns give the partial pressures in tank TK-1 after four hours of impurity accumulation, prior to and after I-t 2 swamping Component

Mole fraction

Flow rate (mol/h)

Partial pressure (kPa) Initial After H swamping

Q2 He C,,Q m NQ 3

0.9470 0.0330 0.0112 0.0008 0.0016 0.0016 0.0008 0.0008 0.0016 0.0016

71.025 2.475 0.840 0.060 0.120 0.120 0.060 0.060 0.120 0.120

6.25 12.13 4.12 0.29 (I.44 0.59 0.29 0.29 0.59

Q20 CO Ar CO~ N, O2

81.25 12.13 4.12 0.29 0.44 0.59 0.29 0.29 0.59

haust stream for I T E R has a nominal flowrate of 75 m o l / h and composition (mole fraction) of DT: 0.937; H: 0.010; He: 0.033; and impurities: 0.020. The impurities will include CO, C O 2, N2, CnQ .... NQ3, 02, Q2 O, and Ar, where Q = H, D or T. The plasma exhaust gas stream composition is given in table 1 for the purpose of component design.

pressure of 100 kPa in TK-I, until the quantity of hydrogen added is equivalent to 20 volume changes of TK-1, which should reduce the initial tritium content of the tank by a factor of c > or 10 v, making it possible to discharge the remaining impurities directly to the environment. Circulation by PI is continued even after hydrogen addition is stopped, until most of the H , in TK-I has permeated through SI to the ISS. PI is then stopped, and the system is evacuated. The detritiated impurities from TK-1 are sent to waste gas treatment. The H I T E X process does not contain any components which require a king cycle time for operation, such as molecular sieve beds which require time fl)r regeneration. Therefore, to minimize tritium inventory, a short cycle time of four hours is used. An important feature of the H I T E X process is that the P d / A g p e r m e a t o r S1 need not be designed for a clean separation between impurities and the Q2 species, since the process does not rely on the eft'iciency of this step for a high degree of impurity detritiation. This feature of the H I T E X process significantly reduces the size of the permcator as compared to other P d / A g designs [1,3]. At the end of four hours of accumulation of impurities plus residual Q2 in tank TK-1, the contents before and after swamping are as shown in table 1. In table l it has been assumed that the O . in the original plasma exhaust gas stream has reacted to form Q 2 0 upstream of tank TK-1. Design calculations for sizing of H I T E X process components for I T E R are reported in ref. [7].

2. Process description Figure 1 is a process flow schematic for the H I T E X FPL process. Operation of the H I T E X F P L is batchwise. At the beginning of a batch, tank TK-1 is in an evacuated state. Impurities with some D T carry-over from an upstream p e r m e a t o r or cryoadsorber are sent to TK-1. When the pressure in TK-1 reaches 25 kPa, the impurity flow is stopped and redirected to the second tank TK-2, which is now ready for a new batch. At this time, H 2 is added to TK-I to raise the pressure to 100 kPa, and the circulating compressor P1 is started. P1 draws the mixture of impurities and elemental hydrogen isotopes through reactor R1, where the mixture is isotopically equilibrated. Downstream of reactor R1, the elemental hydrogen isotopes are removed via permeator S1, and the impurities with residual elemental hydrogen are recycled back to TK-I. Hydrogen addition continues in order to maintain a

EXHAUST TO WASTE GAS TREATMENT

~EED STREAM

I

H? ADDIT[Oh VACUUM

HOLDING !

%.....

I:

'

t CmCt;~TlON

S1

gO~APPESSOR P[RMEATO~

OP

T(

SS

Fig. I. Simplified process flow schematic for the HITEX FPL

process. Impurity feed is from an upstream permeator or cryo-adsorber (not shown). S1 is a Pd/Ag permeator, and RI is a high-temperature catalytic exchange reactor.

51

A. Busigin et aL / HITEX process for recovering tritium

VARIATION OF HD/H2 ISOTOPIC MOLE RATIO WITH TIME (WITH 100 ppm HD IMPURITY IN H2)

3. HITEX test loop A test loop has been constructed at Ontario Hydro for demonstration of the HITEX process. The main purpose of the test loop is to obtain performance data for the HITEX reactor. The performance characteristics of all the other components are well known. A comparison of the test loop component sizing with the HITEX design proposed for ITER is given in table 2. The major difference between the test loop and the ITER design is throughput and reactor volume. However, the results of the tests may be scaled-up in a straightforward manner by taking into account the residence time in the reactor (which is directly related to reactor volume and throughput). The initial experiments that will be conducted with the HITEX test loop will use deuterated methane, and will measure the disappearance of deuterated methane (by displacement of the deuterium with protium) and the appearance and disappearance of HD as a function of time. At a later stage, tritium experiments may be conducted. However, at high temperature, it is not expected that there will be much difference in equilibration rate between deuterated methane and tritiated methane. The experiments will also evaluate the effect of the reactor hot-wire temperature on reactor efficiency and power consumption. Figure 2 shows the expected variation of H D / H 2 mole ratio with time, taking into account the 100 ppm HD impurity in normal H 2 used for swamping. The presence of the HD impurity in the added H 2 limits the extent to which the contents of the tank can be

1 REACTOR TANK EXIT ENTRANCE 0

INSIDE TANK

01

=_,

0.01

~)

o(:01

2

0oooI

t 20

I 4o

,

I 60

=

/ 80

,

Reactor

Pt wire

Tank TK-1 H 2 addition Permeator S1 Permeator S1 Reactor CQ 4

diameter (m) length (m) volume (m3) diameter (m) length (m) surface area (m2) volume (m3) flowrate (mol/h) product flow (mol/h) recycle flow (mol/h) total flow (mol/h) residence time (s) total flow (mol/s) 1073 K exchange (mol/s) 773 K exchange (mol/s)

, 120

TIME (min)

Fig. 2. The expected variation of HD/H 2 mole ratio with time in the HITEX test loop, taking into account the 100 ppm HD impurity in normal H 2 used for swamping.

de-deuterated. Note that the HD mole fraction asymptotically approaches 0.0001. The total time for the experiment will be about 100 min. Figure 3 shows the corresponding variation of CH 4 and CD 4 mole fraction with time. In modeling this experiment, it is assumed that the reactor achieves complete equilibration of the feed stream passing through the reactor. Incomplete equilibration would result in a slower decline in the CD 4 concentration. In addition to testing for reactor efficiency, the HITEX test loop will provide information about the

Table 2 HITEX test loop design versus HITEX ITER design Parameter

I 100

ITER design

Test loop

% of ITER

0.150 2.000 0.035 0.001 10 0.0314 2.0 457 457 153 610 5 0.0068 2.19 0.0084

0.050 0.328 0.000644 0.00038 7 0.00836 0.015 4.02 4.02 1.34 5.36 10 5.96x 10 5 0.574 0.0022

33 16 1.8 38 70 27 0.76 0.88 0.88 0.88 0.88 200 0.88 26 26

52

A. Busigin et al. / H l T E X process /br recot,ering tritium

VARIATION OF CH4 AND CD4 SPECIES MOLE FRACTIONS IN TANK WITH TIME 01

T 00~

~ oool ~ o0001 ~

1E05

O

1EO6

g

IE07 O

should not be a problem in the H I T E X hot-wire reactor. The H I T E X design has advantages over previous systems which have been based on catalytic oxidation or decomposition of impurities, since it eliminates thc need for impurity oxidation and electrolysis of DTO, and does not rely on complicated catalytic decomposition reactions. The major disadvantage of the design is the need for H 2 swamping which increases the load on the ISS. However, provided thai this increased load on lhe ISS is small in comparison 1o other ISS loads such as water detritiation and H~ propellant detritiation, the effect on ISS design may be minor.

%

T I M E (rain)

Fig. 3. The expected variation of C H 4 and C D 4 mole fraction with time in the HITEX test loop.

carbon formation temperature on the reactor hot-wire. The equilibrium constant of formation of methane from graphite and hydrogen C+2H2~CH4,

(1)

is given by the relation

Kp

aaCH4 Z_ac

[ 9703 ) 7.889 × 10 6 e x p l - - ~

(2)

where a i is the activity of species i. This equation is obtained by a regression fit to the data in ref. [10]. It follows therefore that the carbon formation temperature for the composition in tank TK-1 (see table 1) is 1079 K. The temperature of the Pt hot-wire can be controlled to avoid carbon deposition since the hot-wire temperature can be determined accurately from its resistivity. The experiments are expected to confirm that carbon deposition can be avoided.

4, Discussion and conclusion A new high-temperature isotopic exchange F P L design is described. The sizes of major components for an I T E R scale system are compared to an experimental test loop which has been constructed. The expected performance of the test loop is discussed. Calculations are also presented to show that carbon deposition

References [1] R.D. Penzhorn, R. Rodrignes, M. Gugla, K. Giinther, tl. Yoshida and S. Konishi, A catalytic plasma exhaust purification system, Fusion Technology 14 (1988) 450-455. [2] J.L Hemmerich, A. Dombra, C. Gordon, E. Groskopfs and A. Konstantellos, The impurity processing loop for the JET active gas handling plant, Fusion Technology 14 (1988) 557-561. [3] S. Konishi, M. Inoue, H. Yoshida, Y. Naruse, H. Sato, K. Muta and Y. Imamura, Experimental apparatus for the fuel cleanup process in the tritium processing laboratory, Fusion Technology 14 (1988) 596-601. [4] P. Schira and E. Hutter, Tritium cleanup on hot uranium powder, Fusion Technology 14 (1988) 608-613. [5] A. Ohara, K. Ashibe and S. Kobayashi, Fuel purification system for a tokamak type fusion reactor, Proc. 12th Symposium on Fusion Engineering, Monterey, California, 12 16 October (1987) 743-749. [6] E.C. Kerr, J.R. Bartlit and R.tt. Sherman, Fuel cleanup system for the tritium systems test assembly, Proc. of ANS Topical Meeting, Tritium Technology in Fission. Fusion and Isotopic Application (1980) 115-121. [7] A. Busigin, S.K. Sood and K.M. Kalyanam, New high temperature istopic exchange fuel processing loop design for ITER, Fusion Technology (1991), to appear. [8] D.K. Murdoch, Fuel processing system (plasma clean-up loop) draft design requirements for NET II/ITER, NET Document N 2 / P / 4 4 1 0 / 1 / A (July 27, 1989). [9] A. Busigin, S.K. Sood. O.K. Kveton, P.J. Dinner, D.K. Murdoch and D. Leger, ITER hydrogen isotope separation system conceptual design description, Fusion Engineering and Design 13 (1990) 77-89. [10] D.R. Stull, E.F. Westrum and G.C. Sinke, The chemical thermodynamics of organic compound, (Wiley, 1969).