The hydrogen- and tritium permeation through helium heated tube walls

•4 m m h o f Nuclear Energy. Voi. 3. pp. 343 to 349. ~

Prem 1976. ~

In Nm'the~ Ireland

THE HYDROGEN- A N D TRITIUM PERMEATION THROUGH HELIUM HEATED TUBE WALLS H. D. ROmuo and J. BLUMENSAAT Kernforschungsanlage Jttlich, Institut fiir Reaktorentwicklung Summary--When discussing the application of high temperature heat from gas-cooled nuclear reactors for chemical processes some interchange of hydrogen and tritium through the walls of the heat-exchanging system by permeation must be included in the economic and safety considerations. In spite of promising laboratory results on the effectiveness of natural permeation barriers, a reliable statement on the extent of permeation during operation seems questionable. Therefore we took up further investigations in our institute, which will be described here.

1. I N T R O D U C T I O N

Gas-cooled high temperature reactors (HTRs) open the possibility of a far better conversion of nuclear energy into usable energy than water-cooled reactors. As two-cycle systems with steam turbines, HTRs are now being introduced into the market. Moreover, they comprise an important development potential for higher core temperatures. The successful operation of the nuclear demonstration plant AVR at a medium gas exit temperature of 950°C justifies the extensive investigation program in the Federal Republic of Germany, devoted to the use of HTRs for basic chemical processes. A common feature of all processes in question is the presence of hydrogen in any chemical composition in the secondary circuit. By that, a problem arises, that was already known in the conventional power generation and process technique, to be sure, but did not have great significance. It is the permeation of hydrogen through the metal walls of the heat exchanger system into the primary circuit. This hydrogen enters the core, where it can react with the graphitic structural components or the fuel elements, thus generating methane and giving rise to an increased core corrosion. Besides this mechanism, one has to consider the permeation of tritium which is produced by several nuclear reactions in the core; tritium permeates in the opposite direction into the secondary circuit, thus causing a radioactive contamination of the chemical products. In principle both permeation problems could be governed by a proper design of the gas purification system which is necessary in any case, or eventually by the insertion of an intermediate heat exchanger. But all these measures would affect the economy of a nuclear process heat system. This situation

suggests to investigate the extent of permeation and the effectiveness of natural barriers in more detail. 2. D E F I N I T I O N O F T H E P R O B L E M AND CURRENT KNOWLEDGE

The following statements are related to the steam-methane reforming process--for the time being the most promising chemical process for the coupling of nuclear heat--and to a pebble bed reactor with the highest temperature potential at present (Schulten et al., 1974). By there forming process, the initial gas mixture consisting of methane and steam is converted mainly into carbon monoxide and hydrogen. A surplus of steam prevents the poisoning of the catalyst by methane cracking. Along the tube length, the hydrogen content increases with rising temperature to some 40 vol.- Yo, thus giving rise to an increased permeation through the tube walls. For metallic bright surfaces the permeation rate is described by the following simplified relation: Y =-

K

• ~/,o

(1)

X

J: permeation flux, normally expressed in standardized volume per area and time [cmS(STP).cm-~.sec -1] x: wall thickness [cm] p: hydrogen partial pressure [bar] K: permeation coefficient, the dimension of which follows from the other factors: [cma(STP).cm-X.sec-l.bar--I/2] Equation (I) can be derived from Fick's first diffusion law and Sieverts' distribution law, ~/p---dependence of which is indicating, that the hydrogen passes the metal latticein the atomic form.

343

344

H. D. R6nRm and J. BLUMEN,~,AT

The permeation coefficient includes not only material properties but also the temperature dependence, which can be characterized by a frequency factor Ko and an activation energy Q (in kcal/mole) in the well-known Arrhenius-relation: K = K0 "exp ( - - Q / R T )

(2)

R is the gas constant and T the Kelvin-temperature. Besides that, K may contain isotopic differences in the mobility of protons and tritons in the metal matrix. As far as measurements have been carried out with high temperature alloys, you will find within the limits of accuracy we are dealing with, that the results are sufficiently conform. Thus the following data will permit a useful estimation: K o ~ 2 • 10-2 cmZ(STP).cm-l.sec-X.bar-t/2;

Q ~ 16 kcal.mole -1

(3)

Supposing a linear increase of wall temperature along the gas stream in the heat exchanging tubes from 550 to 900°C, you can calculate with a weighted mean value for the permeation constant K ~ 1 • 10-s cmZ(STP).cm-t.sec-l.bar -1/2 corresponding to an effective wall temperature of about 800°C. Neglecting other effects, leakage for example, this alone would lead to a hydrogen content in the primary circuit, that would make necessary a substantially higher gas purification capacity than is provided for in the present design (R~Shrig et al., 1975). The alternative were an increased production and decay rate of methane with all risks of carbon transport to the hottest spots in the core and, on the other hand, the possibility of a catalyzed carbon deposition on metal components and perhaps in its consequence a dangerous carburization. Moreover, an abrasion of graphite from fuel elements could cause an increased solid fission product release. Fortunately, recent investigations have shown, that oxidic or carbidic surface layers can effect a drastic reduction in the extent of hydrogen permeation. Such protective films seem to be stable on many metals and alloys even under highly reducing gas atmospheres. Their characteristic hydrogen permeation shows a linear pressure dependence indicating that in this case hydrogen diffuses in its molecular state. As for the system metal + oxide film, there results a pressure dependence of hydrogen permeation where diffusion through the metal is rate controlling at high partial pressures, and diffusion through the oxide is dominant at low partial pressures. It is true that deviations from Sieverts'-Cp-law at low pressures (as well as at

very high pressures, but they are not of interest here) have been observed tens of years ago, but only recently they have been interpreted by Strehlow and Savage in the manner outlined above (Strehlow et aL, 1974). Taking into account possible defects in the oxide layer, which result in a direct access of some hydrogen to the metal surface, a complex pressure dependence was predicted, which we have found indeed with an Incoloy-800 sample (R6hrig et al., 1974). The results are given in Fig. 1 Here the permeation rate through a tubular sample was measured. The atmosphere on the inside was hydrogen and argon and on the outside a stream of pure argon. The curves are drawn in a double logarithmic scale. The parameter is the temperature. Only above some hundred mbars the Cp-law becomes applicable. The measured data can be described quantitatively by means of a simple model, that is illustrated by Fig. 2. One part of the permeation flux which has apparently to pass at first an oxide film and afterwards the metal, is exposed to the series connection of a linear and a non-linear permeation resistance, whilst the other part with direct access to the metal passes a resistance connected in parallel. The following mathematical relations were derived from this model in complete analogy to Kirchhoff's laws for the electric current: (4)

•It = Lox(F' -- PAt)

Lox: permeation conductivity of the oxide layer [cm3(STP).cm-2.sec-l.bar -x ] PM: effective hydrogen pressure at the metal]oxide interface [bar] On the other hand: ./i =

Lm" ~,:p~

•I2 = L ~ .

?'p

(~a) (Sb)

LM1,2: shared permeation conductivities of the metal [cm3(STP).cm-~.sec-t.bar -tJ~] From J = & + &

(6)

follows then

J

=

LM1

\2~ox/

LMxI + p + LM-'~2L1MC p -- 2Lox j (7)

When LM2 ~ LMX, one gets:

J = Lgk/-1 [~.' k2--~'-OX/ + P - Z L o , J

(8)

The hydrogen'

'

'

and tritium permeation

I ~";I

'

'

'

I ''"I

r

,

,

I'T,,,I

J m2h

l

-P /

-.-950c

'

-o-

85o c 800C

'

I''''-

, ~ ; , ~ O ~ . -

-

_~-~" ~

j

-

-

..---"

.

- * - 700 C - - -

~.

/--~'-.o-"~/j"" .../-//~/~..

-o-9ooc E

'

~ Incoioy 800

103

345

through helium heated tube walls

~

630C

i

/ ~ ~

÷

102

10' 1

1

I

I iJl~l

10"1

J

100

I

J

I IJ,Jl 101

H2 - p o r t i a l pressure J , l I ,itJl 102

[mborl J

t

I

I JJ

103

Fig. ]. H~-Permeation through Incoloy 800 vs. pressure and temp.

P

j

Fig. 2. Permeation resistances of metal + defective oxide layer. Approximation for high p: K

J ~ L ~ t l " v'p = - " ~/p X

(9)

Approximation for efficient oxide layers and/or low p: J ~ Lox "p

(10)

In the case of a defect-free oxide film the pressure dependence of the permeation flux is described by (8). It applies very well for data, which were measured previously with a ferritic steel sample in our institute (Serpekian, 1974). The results are shown in Fig. 3. They indicate that even with the highest partial pressures used here, the measured permeation rates

remain markedly under the Cp-asymptote. Only the arithmetic analysis therefore yields the true permeation coefficient. Even when a thin oxide film represents an effective permeation barrier in the range of low partial pressures, which would be of greatest importance for the tritium permeation, you must realize for high partial pressures, that an effect which was measured at normal pressure, cannot readily be transferred to operational conditions. This does not only depend on the physical mechanisms outlined above, but also on the fact that structure, chemical composition, and stability of thick oxide scales are also dependent on many parameters, so that predictions are largely imp6ssible. As an example, Strehlow and Savage h a v e published, that an Incoloy 800-specimen, which w a s pretreated for 1000h in a steam atmosphere of 25 bars at about 540°C, yielded only a slight decrease in hydrogen permeation. On the other hand our own investigations with the same material under steam-hydrogen atmospheres (vol. ratio ca. 1 : 1) revealed a reduction in permeation by more than two orders of magnitude. This is shown in the Fig. 4 together with the effect of a significant change in the activation energy. These curves are discussed in more detail in

346

H. D. ROHRIG and J. BLUMENSAAT u

u

J

/ e.rn~ m2h

I

13 Cr 1,4o t,t,

500 C

o m e o s u l i n g po0nt s x colcutated with

:2oo

p/bar

OJ

-0,1

Fig, 3. H~-Permeation through a ferritic steel at low pressures. 900

, I 10'

950

I '

700

800

6OO

500

+ I I ' 850

/ ?50

6$N

550 TIoC

\ e\ A

Om 17.7 kCOI tool

N 10 ~

\o

\ x

Incolo~ 800 P•1 = 0.27 bor

reduclncJ otto.

well = 3.3 mm

10

0 =

0.7 ~k c o I

~

et

~ s h u t dowr

10t K I T

I

I

I

J

I

I

I

!

7

8

9

10

11

12

13

lt,

Fig. 4. Arrhenius-plots of permeation rates. another paper (Riihrig et aL, 1974). Further investigations yielded that the expected pressure dependence proportional to pt could be verified to some extent, For an estimate let us make the conservative assumption that the factor of decrease only amounts

to 102 and that an extrapolation on the partial pressure in the process gas of nearly 20 bars has to be done according to the pl-law. The result is that the hydrogen level in the primary circuit of a process reactor could be controlled in a plant of the present design. But there is no doubt that more data are necessary. Therefore, we have started an investigation program in our institute. Before describing its concept and the construction of the experimental facilities, some remarks shall be made on the tritium problem. Tritium is produced in HTRs mainly by the following three processes, namely (a) uranium fission, (b) burn-up of lithium impurities in graphite, and (c) by an (n, p)-reaction of the HeS-isotope in the cooling-gas. While fission-borne tritium does practically not take part in the cycle, tritium from lithium burn-up will play a role especially in the burn-in phase of the recator. A steady source for tritium is caused by a certain inavoidable cooling-gas leakage, that makes subsequent helium supplies necessary. The resulting content in the cooling- gas depends on the sinks. Especially the gas purification and the permeation are to be mentioned here, leakage should be avoided as far as possible. Besides that, the graphite could play an important role because of its buffer properties. The influencing mechanisms have been investigated experimentally as a thesis in our institute (Fischer, 1975). From that, we have obtained an understanding that enables us to suppose an equilibrium in tritium distribution between gas volume and reactor graphite to establish during reactor operation. The influence of the processes of tritium-production, absorption, and release slowly

The hydrogen- and tritium permeation through helium heated tube walls

347

3. DESCRIPTION OF EXPERIMENTAL occurring in cooler regions of the core seems to be FACILITIES AND TESTING PROGRAM negligible. The more or less continuous discharging of burnt-up fuel elements with their content of The basic concept of the planned experiments on tritium absorbed in the graphite matrix causes an hydrogen permeation involved with the steamadditional sink, it's true; but compared with the methane reforming process is to establish definite total inventory, this quantity is negligible. In the statements on the influence of gas composition and stationary state, the additionally produced tritium pressure, heat exchanger materials and wall temperacan only permeate or be removed by gas purification. tures. It is however not our objective to vary all In the case of metallic bright heat exchanger walls parameters independently. They rather result the permeation data given above, would lead to a largely from the conventional reformer technique practically complete delivery of this tritium to the or the special demands of nuclear heating, which process gas. On the other hand, in the presence of were investigated with great success in the single oxide scales, an extrapolation of our experimental reforming tube test facility, called EVA (Harth et al., data down to the low partial pressure of tritium in 1974). the cooling gas would give a negligibly small The intended measurements therefore concentrate permeation rate. Concerning the stability of such on the question, what permeation rates have to be efficient protective layers under the reducing assumed in what temperature sections of reforming conditions of reactor helium, we cannot make tuoes, and which of the discussed materials will definite statements at present. Besides that it is behave best under this point of view. The testing discussed whether the permeation of tritium is procedure, resulting from these and other affected by hydrogen, being available in excess considerations, is represented in the Fig. 5. even in the primary circuit. Because of these In a mixing and preheating unit a mixture of 1 unsolved questions, we are preparing laboratory volume part methane and 3 volume parts steam is investigations also on this subject. Our objective prepared with a pressure of 30 bars and a temperature is to measure the permeation rates of both hydrogen of 550°C. This mixture is fed into the first test unit, isotopes while they move simultaneously in opposite which is designed as reforming unit. During each directions. run the temperature will be adjusted on one of the

CHi, 30 BAR 550*C

PREHEATING

XIV

~

H2O : CH/, ~ 3:1

UNIT

XIII

XI

XII

30 BAR 650°C OR 800°C OR 950°U

~

V

CO.E02.CHI*H2*H20

IV

II1

11

CELLS. I TOVCONTA!N CENTRIFUGALLY-CAST TUBE SAMPLES,SAMPLE I IS FILLED WITH A CATALYST. CELLS X1 TOXIY CONTAIN EXTRUDED TUBE SAMPLES COMPOSITION OF PARTIALLY REFORMEDSTEAM-GAS MIXTURES AT THE DIFFERENT STATES:

650*C II 800at XII 9590C l

co

co 2

CH4

H2

H2 o

1.4 6.C ~1,5

5.5 5.9 t..~

lt,.7 6.5 1:2

26.2 42,8 52,1

52,3 38.3 30,8

Fig. 5. Scheme of gas flow in the tests.

348

H. D. R6HRtG and J. BLUMENSAAT

three values given in the figure, so that the reforming process will only proceed to an equilibrium corresponding to the temperature. The expected process gas mixtures are listed in the lower part of the figure. They are led through the test units II-V and subsequently the units XI-XIV. (The missing numbers belong to a second branch not shown here.) The first group is provided for samples of different centrifugally cast materials, whilst the second is reserved for drawn alloys. Centrifugally cast tubes are being employed in great numbers in conventional reformers. They exhibit high strengths. Wrought alloys can be manufactured in nearly any length with good homogeneity. Partly they are well specffied by the German TUV-acceptance conditions or the ASME Boiler and Pressure Vessel Code. They are used as so-called pigtails for the forwarding of the hot process gases and for the compensation of thermal expansions of the reformer tubes. They are also being taken into account for reformer tube materials itself. In the following table appropriate materials are compiled together with the temperature parameters and the test periods. As indicated by (a), the permeation from pure h y d r o g e n atmosphere will be measured for each sample, before being exposed to the process gas mixture. Thus, leaks will be recognized in time by means of an anomaly high permeation rate. The three runs of 2500 hours at each temperature (see b) shall enable us to make a prior decision on the optimum materials on the base of their permeation behaviour. So the following test procedures (c--e) can be limited to these materials. A more detailed description of the test facility, their components and a discussion of special Table 1. Survey on testing program

Materials IN 519 IN 638 IN 643 HK 40

Incoloy 800 Incoloy 800 H Incoloy 802 (lncoloy 807)

Temperature steps 650°C: range of maximum equilibrium shift in process gas composition 800°C: usual process gas outlet temperature 950°C: for studying extreme stresses

Testingprocedures: at each temperature step, usually 30 bars: (a) some hours with pure hydrogen atmosphere (b) up to 2500 h with a process gas mixture (c) long-term tests with selected samples (d) variation of primary gas impurities (e) influence of pressure- and temperature deviations

problems cannot be given here. But because of a special interest in measuring methods and data handling some remarks shall be made. The hydrogen leaving one of the tube sections under examination, enters a carrier gas flow consisting of high-purity argon or in later runs of a noble gas with cooling-gas specific impurities. The various gas streams, now charged with permeated hydrogen, are conducted through a multiswitch subsequently in a continuous cycle to a gaschromatograph for analysis. The gaschromatograph contains two detector systems in parallel. One of them is a thermal conductivity detector suitable for hydrogen contents from 100K down to 500 vpm, whilst the other, being a gas discharge detector, is used for the low concentration range 500 vpm + < vpm. The decision, which detector is working in the allowable range, is made by an on-line process computer connected to the analyzing system. The detector reading is converted into permeation rates by means of a stored calibration function. The other detector signal is rejected. Besides these evaluation functions the computer is used for recording all important operational data, e.g. mass flows, pressures, temperatures etc., and for signalizing dangerous situations. The problems arising with the multiple measurement technique were already solved successfully during our preliminary tests (Diehl, 1973). Finally some remarks concerning the experimental facility for the combined investigation of the hydrogen- and tritium permeation shall be made. This facility is now under construction. It will only consist of one test unit as--in this case and at the time being--the elucidation of fundamental problems is of greater interest than material investigations. The test conditions on the other hand shall be as variable as possible. In addition there are specific problems of measurement technique and radiation protection. The concept of the facility shall shortly be explained according to the flow scheme in the Fig. 6. The test unit--identical to those in the test field for the hydrogen permeation--is shown in the centre. The secondary circuit is to be fed by choice with water steam or--closer to reforming conditions--with steam/hydrogen-mixtures. A relatively low mass rate of flow is planned thus allowing to adjust the volume ratios of steam/gas mixtures at a constant total pressure by the partial pressure of saturated steam, that is by the temperature of the autoclave containing water, saturated steam, and gas. The primary gas (the atmosphere outside the

The hydrogen- and tritium permeation through helium heated tube walls

Exhaust

T- CountingDevices

349

Furnacewith TubeSpecimen

~ A r MixingUnit

* T2

Trihated Wateri Sampling 6 Fig. 6. Device for tritium plus hydrogen permeation studies. sample) shall consist of argon with an addition of tritium. In later experiments the adjustment of an oxidation potential corresponding to H T R conditions is planned here as well. The amount of tritium which permeates through the walls of the sample is desorbed into the steam] gas-mixture of the secondary side and subsequently detected by liquid scintillation. The quantity of hydrogen in the mixture is separated by the condensator, oxidized, and also condensed to the liquid phase. The tritium here is detected as well. Hydrogen is oxidized catalyticly in multistage CuO-beds with interposition of water absorbers. Only thus the severe safety requirements concerning the emission of tritium to the atmosphere can be satisfied. On the primary side of the sample the gas is charged by the hydrogen permeating from the secondary side. After cooling down of the hot gas the hydrogen content is measured by the gaschromatograph. Parallel to this, tritium is measured by gas flow proportional detectors. Subsequently the gaseous tritium is also removed by catalytic oxidation from the gaseous phase. As an additional safety precaution all components of the test facility which contain tritium are situated in a glove box. The glove box is connected to a vacuum system. We hope the described test facilities will help to come to a better understanding and finally to a solution of the problems discussed in this paper. Acknowledgements--The authors would like to thank Mr. Schaefer and Mr. Diehl from our institute and Mr.

Lambrecht from the Gesellschaft fiir Hochtemperaturtechnik, Bensberg, for their valuable help in the planning and assembly of the experimental facilities. 4. REFERENCES

R. Schulten et al., The pebble bed high temperature reactor as a source of nuclear process heat; Vol. 1, Conceptual Design, KFA Jtilich, Jiil-I 113-RG (1974). H. D. ROhrig, R. Hecker, J. Blumensaat and J. Schaefer (1975) Investigations on the problem of Hydrogenand Tritium Permeation in Nuclear Heated Process Plants. Nucl. Engng. Design. 34, 157-167. R. A. Strehlow and H. C. Savage (1974) The Permeation of hydrogen isotopes through structural metals at low pressures and through metals with oxide film barriers. NucL Technol. 22, 127-137. H. D. R6hrig, J. Blumensaat and J. Schaefer 1974 Experimental facilities for the investigation of hydrogen and tritium permeation problems involved with steam methane reforming by nuclear process heat. BNESConf. on the HTR and Process Applications, London, 26-28 Nov. Paper 50. T. Serpekian (1974) Untersuchungen an Dampferzeugerbzw. W~irmetauschermaterialien fiir HTR's. Report Jiil-1111-RG. P. G. Fischer (1975) Verhalten yon Tritium in Reaktorgraphiten. Dissertation TH Aachen and Jiil-1238-RG (1975). R. Harth, C. B. yon der Decken and K. Fehlhaber (1974) Large scale experimental tests of the heat linkage between HTR and steam methane reforming process. BNES-Conf. the HTR and Process Applications, London, 26-28 Nov. Paper 48. W. Diehl (1973) VielstellenmeBtechnik mit Prozel3rechner; Steuerung und Datenerfassung an zwei Priiffeldern zur Untersuchung der Wasserstoffpermeation durch Wiirmetauscbermaterialien. Report JiiI-1020-RG