Water radiolysis under NET conditions

Fusion Engineering and Design 17 (1991) 265-270 North-Holland

265

Water radiolysis under NET conditions P. L o r e n z e t t o a, E. B j e r g b a k k e b and B. H i c k e l c a The N E T Team c / o IPP. Boltzmannstrasse 2, D-8046 Garching bei Miinchen, Germany b RISO National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark c CEA-CEN Saclay, B.P.2, F-91191 Gifsur Yt~ette, France

Radiolytic decomposition of the NET-plasma Facing Component and Blanket water coolant is expected due to the intense radiation fields present inside the vacuum vessel. A sensitivity analysis has been performed by computer simulation to study the formation and the suppression of the radiolytic products under NET conditions. The formation of the stable radiolytic proddcts (oxygen, hydrogen and hydrogen peroxide) formed within the water due to the decomposition induced by mixed neutron and gamma irradiation is given for several cases, which are relevant to the First Wall water coolant. The influence of coolant temperature (20°C, 70°C, 130°C, 200°C), heterogeneous decomposition of H202, added solutes such as hydrogen, and ferrous ions simulating possible corrosion products has been studied. There are still large discrepancies in the literature on the radiation chemical yields (G-values) in the case of the neutron irradiation. Therefore three sets of G-values have been considered, and the sensitivity of the applied G-values for mixed neutron and gamma irradiation has been investigated as well. For all these cases, the hydrogen concentration required to suppress the water radiolysis is given, and some critical issues with respect to the specific contribution of neutrons with high energy typical for the fusion spectrum (10-14 MeV) on the radiolytic water decomposition have been discussed.

1. Introduction

2. First wall features

Radiolytic water decomposition is expected for the N ET In-Vessel component water coolant due to the intense radiation fields present inside the vacuum vessel. The experience gained in nuclear power reactors shows the necessity to suppress the water radiolysis in order to prevent the formation of pockets of a potentially explosive H 2 and 0 2 gas mixture in case of a closed cooling circuit, and a high concentration of oxidizing species which is detrimental for the aqueous corrosion behaviour of the austenitic stainless steels. The irradiation and thermal hydraulic conditions of the N E T In-Vessel component water coolant are quite different from those of fission reactors. Therefore, an R & D programme, including in-pile experiments and computer simulations, is being undertaken to assess the formation and the suppression of the radiolytic products under NET conditions. This paper presents the main results of a sensitivity analysis performed by computer simulation of the concentrations of H2, 0 2 and H 2 0 2 produced by radiolysis under NE T First Wall irradiation and thermal hydraulic conditions.

2.1. Thermal hydraulic parameters

The first wall and the back plate have the same cooling circuit, both being poloidally cooled. The inlet of the water coolant is on the back plate, while the outlet is on the top of the first wall. In a conservative way, a tubing length of 10 m has been considered for the calculation, with a coolant velocity of 5 m / s . That gives a 2 s time period to the coolant to flow through the first wall irradiation zone. The inlet temperature is 6 0 ° C and the outlet temperature has been taken as 100 °C. The total pressure of about 1 MPa prevents any water coolant boiling inside the cooling tubes. 2.2. Nuclear heating

A one-dimensional neutronic analysis has been performed to assess the nuclear heat deposition into the water coolant due to the neutron and gamma interactions. The maximum power density in the water at the equatorial plane is about 11 W / c m 3. About 89% of the total heating is due to neutrons and about 11% to

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266

P. Lorenzetto et al. / Water radiolysis under N E T conditions

gammas. T h e fraction of h e a t deposition versus neutron energy is r e p r e s e n t e d in fig. 1. A b o u t 60% is due to n e u t r o n s of energy b e t w e e n 9 a n d 14 MeV, which is typical of fusion conditions. In t h e fusion spectrum, the fraction of n e u t r o n s with energy above 9 M e V is a b o u t 23%. T h e h e a t deposition within t h e b a c k plate w a t e r coolant has b e e n neglected for the c o m p u t a t i o n s , as well as the poloidal variation. T h e irradiation dose within t h e w a t e r coolant t a k e n for the calculations is t h e n 2.3 × 10 4 Gy (2.3 M r a d ) p e r pass of 2 s.

and Burns' e x p e r i m e n t s were in neutral water while K a t s u m u r a et al. d e t e r m i n e d G-values for an aqueous 0.8 N H 2 S O 4 solution. They only give thc sum of ( ; ( t D a n d G(%q); the distribution in table 1 is an cstimatc. F u r t h e r m o r e , they d e t e r m i n e d all thc G-values for n e u t r o n irradiation up to 1 0 0 ° C . They also determ i n e d G( - H z O ) , G ( H z ) and G ( H + e,~) up to 150 ') ('. a n d e s t i m a t e d these G-values up to 275 ° C. T h e g a m m a G-values comc from S c h e s t c d ct al. [5] and arc ass u m e d to be c o n s t a n t with t e m p e r a t u r e . T h e reaction m e c h a n i s m with the best known rate constants and activation energies is taken from Bjergbakke et al. [6], and is given in table 2 fl)r pure water and in table 3 liar the f e r r o u s / f e r r i c ion reactions. T h c irradiation time of the first wall water coolant has b e e n t a k e n as 2 s pcr pass. T h e time intcrval b e t w e e n passes c a n n o t be defined precisely sincc the cooling loop is not completely designed yet. In the computations, an interval of 8 s has b e e n used which is more t h a n e n o u g h to allow total radical decay. T h e c o m p u t a t i o n s have b e e n in all cascs r c p e a t e d using the c o n c e n t r a t i o n result as input data for the next pass until a steady state is r e a c h e d or a steady c o n c e n t r a tion increase is d e m o n s t r a t e d .

3. Calculation method

4. Results

T h e c o m p u t e r p r o g r a m m e used for the calculations is C H E M S I M U L [1]. T h e r e are large discrepancies in the literature on the radiochemical yields (G-values), defined as the n u m b e r of molecules or radicals prod u c e d p e r 100 eV a b s o r b e d energy, for fast n e u t r o n irradiation. T h e r e f o r e , t h r e e sets of G-values have b e e n c o n s i d e r e d in this analysis. They are from Kats u m u r a et al. [2], J e n k s [3] a n d B u r n s [4], respectively, and are s u m m a r i z e d in table 1. In particular, J e n k s '

4.1. R o o m temperature

Total neutron heatin9 : 10.1 W/cm3 1,0

0.8 ggo.e

0.4 2:.~ -~ 0.2

0.0

1 10 100 Neutron energy (MeV)

Fig. 1. Neutron heating in First Wall water coolant.

At room t e m p e r a t u r e and w h a t e v e r thc set of Gvalues used, the oxygen a n d hydrogen peroxide ( H , O 2) c o n c e n t r a t i o n s are very high a n d still increasing after 6 passes. Figure 2 r e p r e s e n t s the molecular p r o d u c t conc e n t r a t i o n s versus the n u m b e r of passes o b t a i n e d with Jenks' G-values, which are t h e most severe in the case of p u r e w a t e r at 20 ° C.

Table 1 Radiation chemical yields (G-values) Radicals or Molecules

Gamma (72--values

e,q H

eaq + H OH H2 H~O 2 HO 2 - H 2O " Estimated values.

Neutron G-values Katsumura

Jenks

Burns

RT

100 ° C

200 ° C

itT

RT

2.66 0.55 3.21 2.67 0.45 0.72

0.75 1/.50 1.25 0.68 //.99 1.27

1.20 0.80 2.00 1.36 0.58 0.96

1.48 0.98 2.46 1.84 ~ 0.24 0.57 ~'

4.11

3.21

3.21

2.98

0.36 0.36 0.72 0.47 l.l 2 1.00 0.17 2.80

0.93 0.50 1.43 1.09 0.88 0.99 0.04 3.15

P. Lorenzetto et al. / Water radiolysis under NET conditions

267

Table 2 Reaction mechanism for pure water. Rate C st (l/mol s) = A e x p ( - E A / R T ) , EA = Activation Energy (kcal/mol) No. 2 3 4 5 6 7 8 9 10

OH OH OH OH OH OH OH 2 solv e solv e solve

11

solve

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

solv e solve solv e solv e H H H H H H HO2 HO 2 02

1

A

Reactions

HO 2

02 H202

HO2 H20 2 O H+ H20

+ OH = H202 + solve = OH+H =H20 +HO 2 = 02 + 02 = 02 +H20 2 = H20 +H2 =H +2H20 =2OH +H20 +H = OH +H20 + H O 2 = H20 2 +02 +H20=HO; +H20 2 = OH +H + =H +H20 =H + 02 =02 +H =H 2 +HO 2 = H202 + O~ = HO2 +H202 = OH +OH=solve+ 02 = HO 2 + HO 2 = 02 +02 = 02 +0 2 +H20=O 2 =02 +H + = HO 2 +OH= HO 2 +H20 = H202 =H20 +O = 02 + OH= H20 = H÷

+H20 -I-O H +HO 2 +H20 +H2 +H 2 +OH +OH+OH +OH-

+H20 +H20 +H202 + HO 2 +HO2 +OH+H + +H20 +OH+O

+OH-

D i f f e r e n t initial hydrogen c o n c e n t r a t i o n s have b e e n used to illustrate eventual lower limits for inhibition of water d e c o m p o s i t i o n at 20 ° C. In the worst case (Jenks' G-values), a H e c o n c e n t r a t i o n of 10 -4 M is not enough to get a steady state with a low 0 2 concentration. With 10 -3 M H 2, the radiolysis is s u p p r e s s e d and the residual 0 2 c o n c e n t r a t i o n is very low (10 -9 M). However the H 2 0 2 c o n c e n t r a t i o n is still quite high, about 3 × 10 -5 M. The effect of corrosion products, which can be p r e s e n t within the w a t e r coolant, has b e e n simulated using ferrous ion addition at a c o n c e n t r a t i o n of 2 × 10 -6 M, which is a realistic solubility at neutral pH. The results of the simulation have shown that this ion has a catalytic effect on the w a t e r r e c o m b i n a t i o n in absence of dissolved H 2 since the 0 2 and H 2 0 2 concentrations are lower by a factor of two approximately.

EA

1.4× 1011 4.25 x 1012 4.25 x 1012 1.34 x 1012 1.33 x 1013 9.1 × 109 1.05 x 1011 5.23 × 1013 3.39 X 1012 3.28 × 1012 2.66 X 1013 2.72 × 1012 3.74 x 1012 3.28 × 103 1.71 × 1012 2.00× 1012 3.4 X 1012 3.4× 1012 4.76× 101° 2.55 × 109 3.4 × 1012 3.3 × 109 2.18 × 10 9 2.77× 1017 1.31 × 108 8.21 × 1012 1.05 × 1012 9.42 × 106 3.78 × 107 1.64× 1012 2.35 × 1013 4.27× 10 3

1.84 3.0 3.0 3.0 4.2 3.4 4.6 5.3 3.0 3.0 4.5 3.0 3.0 3.0 2.6 3.5 3.0 3.0 3.9 3.0 3.0 4.9 1.8 19.0 3.0 3.0 4.5 3.0 17.0 3.0 3.0 3.0

However, this ion has no effect on the w a t e r recombination with an initial 10 4 M H 2 concentration, whatever the set of G-values used.

4.2. Effect o f eleuated temperatures The effect of elevated t e m p e r a t u r e s has b e e n studied for 70 ° C, 130 ° C and 200 o C. Increasing the temp e r a t u r e decreases the w a t e r decomposition as it is shown in fig. 3, but the 0 2 and H e O e c o n c e n t r a t i o n s are still relatively high (above 10 -5 M) below 100 ° C. Figure 4 shows the influence of H 2 addition on the c o n c e n t r a t i o n of the oxidizing species with the t e m p e r ature. It can be seen that in the p r e s e n t conditions, a H e c o n c e n t r a t i o n of 10 -3 M is e n o u g h to keep the O 2 and H 2 0 : c o n c e n t r a t i o n s below 3 × 10 6 M (0.100 w p p m ) in the t e m p e r a t u r e range of interest for NET.

P. Lorenzetto et al. / Water radiolysis under NET conditions

268

(Jenks' G-values)

(Katsumura's G-values) 10 ~

6

,

~5-

!H2

xl l O y ,

/

g

o

10 ~ 10 ~

'~

10 ~,

o

10' 10 ~

,,,. e-

4 ~

0

3c

o o

1

10 <~ 10 ~°

t

0

2

4 6 No of passes

L

10:~

8

50

100

150

Temperature

Fig. 2. M o l e c u l a r c o n c e n t r a t i o n s versus n u m b e r of passes in the i r r a d i a t i o n zone. (Katsumura's G-Values)

200

(°C)

Fig. 4. M o l e c u l a r c o n c e n t r a t i o n s at e l e v a t e d t e m p e r a t u r e s with 1(I 3 M t I ~ .

~.

10:

0.1 w p p m

specified for the PWR

,.-

10 ~ 10 4

•-

10 ~

4.3. Heterogeneous decomposition o f H 2 0 2

o* ¢-

10 ~ ................ i ¸ r ........... ~ ~2 ;"~ 1 0 ~ .... e o n c e n t r a t i o o s after 6 passes H202 10 ~ 10 ~ . . . . . . . . : t

0

o

0

10 ,0

0

The

is t h e u p p e r

initial 02

limit of the 02

on

the

02

and

radiolysis. On

Fig. 3. M o l e c u l a r c o n c e n t r a t i o n s at e l e v a t e d t e m p e r a t u r e s .

concentration H,O,

the other

at the

concentrations hand,

the

105

Fe 2+

+OH

107

Fe 3 +

+ solv e

108 109 110 111 112 113 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130

Fe ~+ Fe ~ + Fe z + Fe 2 + Fe 3+ Fe 2 H Fe ~ + F e O H 2+ F e O H 2+ FeO2H~ FeO2H; F e O H 2+ F e O H 2+ FeO2H f FeO2H ~ FeOOH FeOOH F e O H 2+ FeO2H4 FeOOH 02

+ H + t1 + HO, + 02 +O, + H,O. +H20 + H20 +H" + H,O +If+ +FeO2H2 + +son e +1f + solve +H +sotv e +H20 +H +O2 +O, +0~ + I t 20

= Fe 3*

produced

by of

H 2 0 2 in H 2 0 a n d ?-02 is p r o b a b l y f a v o u r e d b y t h e catalytic effect of the tubing surface, and therefore

+OH

Fe 2 +

= = = = = = =

Fe :~* Fc 2 + Fe 3 Fe~ F e z+ Fe 3~ H. FeOH 2+ -Fe3+ = FeO2 H+ = F e O t t 2+ = FeOOH - Fe 2+ = Fe 2~ = Fe 2+ = Fe 2~ = Fe 2+ = Fe 2+ = Fe2+ - F e 2~ - Fe 2~ + 0 ~ = 2Ott

of

decomposition

RC

Reactions

beginning

zone has a great influence

Table 3 R e a c t i o n m e c h a n i s m for f e r r o u s / f e r r i c ions. R C = rate c o n s t a n t at r o o m t e m p e r a t u r e No.

concentration

primary circuit water coolant.

e a c h p a s s in t h e i r r a d i a t i o n

50 100 150 200 T e m p e r a t u r e (°C)

i

+H + l-t* + HO~ +O~ +O~ + OH + Ot1 +H+ +tt20 +H ~ +tt20 + F e O H 2+OH +H~O + 201t +OI-f +3OH + 2Ott +OH + 2OH + OH

+ OH

+H20 +H,O

+H20

+02 + O, + O ?-

3.4× 2.0× 1.3 x 111× 1.6x 2.0 × 5.0 x 62 l.tl X 5.4x 4.7 × 1.1X 8.0 x

l0 s 10 m 10 7 l0 s l0 a I0 s 10 '~ 10 ~ 105 1()'~ 10 3

10 'j

1.014104

2 . 0 x I0 I<) 1.0× 1() s 2.0X 10 u) I.O× lO s 2.0 × 10 I° 1 . 0 x l0 s 5.0 × 10 s 5 . 0 × l0 s 5 . 0 × t0 s 1.0 × l0 s

P. Lorenzetto et al. / Water radiolysis under NET conditions

269

may increase the 0 2 concentration before the next pass in the irradiation zone. A computation has been performed at room temperature assuming that all the H 2 0 2 produced during one pass is converted to H 2 0 1 and 302 outside the vacuum vessel, that means before the next pass. In such a case the results of the H202 concentration with 10 -4 M dissolved H 2 is three orders of magnitude higher (about 10 -3 M after three passes with a steady state increase) than the case without H 2 0 : decomposition. With 10 -3 M initial H 2 concentration, steady state is reached with a low 0 2 concentration, but the H 2 0 2 concentration is still quite high, about 3 × 10 -5 M (Jenks' G-Values).

tion. For these computations, the 3' radiation contribution producing free radicals, which are in favour of water recombination, has been kept since it will be present in the actual NET conditions of irradiation. Conversely, it has been assumed that neutrons do not produce radicals, but only H 2 and H202, which is in favour of water decomposition. The results have shown that steady-state concentrations for 02 and H202 can still be reached, provided that the H 2 concentration is large enough as is shown in fig. 5. However, the final H202 concentration is very large, about 10 -4 M, even with 10 -3 M of dissolved H e.

4.4. Critical G-Values

5. Discussion

In order to distinguish between the effect of rate constant change and G-value change with temperature, computations have been done at 70 o C, 130 ° C and 200°C using in all cases Katsumura's G-values at 20 o C. No steady state is reached and after four passes, the concentrations of 0 2 and H 2 0 2 (3.7 × 10 -5 M and 3.0 × 10 -4 M, respectively at 130°C) are more than one order of magnitude higher than the concentrations obtained in section 4.2, i.e. taking into account G-value change with temperature. That points to the importance of the G-value assessment for the evaluation of the radiolytic water decomposition at high temperature. However, as it has been discussed in section 3, there are quite large discrepancies in the literature on the G-values for fast neutron irradiation at room temperature, and very few measurements have been done at high temperature. Furthermore, the NET irradiation conditions are quite different from those of fission reactors as it is discussed in section 5. Therefore, a sensitivity analysis has been done at room temperature to explore if G-values may have values that can prevent the inhibition of water decomposition by H 2 addi-

The results of the parametric analysis presented in the previous sections have shown that with the considered assumptions, the water radiolysis can be suppressed and the 0 2 and H 2 0 2 concentrations can be kept at acceptable level. However, large uncertainties are still present on the assessment of the reaction mechanism at high temperature, but above all on the assessment of the G-values for neutrons at high temperature as well as at room temperature. Furthermore, these G-values have been experimentally assessed for fast neutron irradiation. Those corresponding to fusion neutrons, in particular for neutrons with energy above 9 MeV are unknown. Such a contribution may result in higher O 2 and H202 concentrations, as it is shown in the sensitivity analysis presented in section 4.4. On the other hand, some (n, p) and (n, a) nuclear reactions occur with neutrons with energy between 9 and 14 MeV, for which no data exists in fission reactor experience on their contribution to the water radiolysis. However, it has been demonstrated experimentally that, for the same energy, alpha particles have a higher linear energy transfer (LET) than neutrons. This parameter is defined as the linear rate of loss of energy (locally absorbed) by the ionizing particle traversing the water and is often given in e V / n m . The higher the LET, the higher is the decomposition of the water. Therefore, the alpha particles as well as the particle recoils produced by these high-energy neutron reactions may favour the water decomposition. Due to the significant fraction (about 23%) of neutrons with energy above 9 MeV in the fusion neutron spectrum, and due to the large fraction of the absorbed dose (60%) within the water coolant of these typical fusion neutrons, even with lower cross sections than for the typical (n, p) reactions with fast neutrons, the contribu-

10 .3 ~

10'

~

~. [H21=10

10 s m

'H202

M

1 0 6. . . .

10 ~

[

=-3

[H2]=10

M

jo2--"

--~.,

I 0 B' 1 0 -~

1.0

1.1

1.2

1.3

1.4

1.5

G(H202) , G(H2)

Fig. 5. Effect of G-value variation on the molecular concentrations.

270

P. Lorenzetto et al. / Water radiolysis under N E T conditions

tion of the neutrons with high energy on water radiolysis under N E T conditions may be a critical issue. This issue may be critical not only with respect to the possibility to suppress the water radiolysis by H 2 addition, but also with respect to possible high steady-state 0 2 and H 2 0 2 concentrations produced in such conditions, which may have a detrimental effect on the corrosion behaviour of the 316L stainless steel. The upper limit of the O 2 concentration specified for the water coolant of the P W R primary circuit is 0.1 wppm (about 2 × 10 -6 M at 320° C). The upper limits of the 0 2 and H 2 0 2 concentrations under N E T conditions are still 1o be specified. A N E T R & D program is currently being undertaken to assess these limits.

6. Conclusion The results of this c o m p u t e r simulation have shown that the water decomposition under N E T First Wall irradiation and thermal hydraulic conditions is not acceptable without H 2 addition. Radiolytic decomposition of pure water decreases with increasing temperature but has been suppressed at temperatures of interest for N E T (60 up to around 100 °C), and with the assumptions made for the calculations, by addition of about 10 -3 M H > This concentration corresponds to less than 0.15 MPa H2 partial pressure. The concentrations of O 2 and H 2 0 2 produced by radiolysis are less or around 3 × 10 _6 M, i.e. 0.100 wppm, which is the upper limit for the 0 2 concentration specified for the water coolant of the P W R primary circuit. The upper limit relevant to N E T conditions needs to be specified. However, there are large uncertainties on the reac-

tion mechanism at temperatures above room temperature and on the neutron radiation chemical yields. Furthermore, there is a lack of data in the literature on the possible effect of neutrons with energy above t; MeV, which are present in the fusion spectrum, on the radiolytic water decomposition. This analysis has not considered their contribution, and that may lead to unacceptable high O . and H 2 0 2 concentrations. This issue needs to be settled.

Acknowledgement We are grateful to Dr. W. D a e n n e r for his contribution in doing neutronic calculations.

References [I] O. Lang Rasmussen and E. Bjergbakke, CHEMS1MUL A programme package for numerical simulation of chemical reaction systems, RISO N.L., Rise-R-395 (1984). [2] K. Katsumura, Y. Takeuchi, D. Hiroishi and K. Ishigure, Fast-neutron radiolysis of acid water at elevated temperatures, Radiat. Phys. Chem. 33 229-306 (1989). [3] G.H. Jenks, Effect of reactor operation on HFIR coolant, ORNL-3848 (1965). [4] W.G. Burns, Suppression by dissolved hydrogen of the radiolysis of water by mixed radiation fields, AERE-M2702 (1975). [5] K. Sehested, E. Bjergbakke and H. Fricke, The primal3, species yields in the 6°Co gamma-ray radiolysis of aqueous solution HzSO 4 between pH 7 and 0.46, Radiat. Res. 56 (1973) 385-399. [6] E. Bjergbakke, K. Sehested, O. Lang Rasmussen and H. Christensen, Input Files for computer Simulation of Water Radiolysis, Ris0-M-2430 (1984).