ML19294C079

Preliminary Rept on Radiolysis & Recombination in Rcs

ML19294C079
Person / Time
Issue date:	07/12/1979
From:	Advisory Committee on Reactor Safeguards
To:
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ML19294C077	List: ML19294C076 ML19294C079
References
ACRS-SM-0154, ACRS-SM-154, NUDOCS 8003060649
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Wc PRELIMINARY REPORT ON RADIOLYSIS AND' RECOMBINATION IN REACTOR C I.

Introduction Reactor Coolant System water decomposes and recombines under the influence of reactor tylie radiations. During this process an equilibrium state will be reached initerms of radiolytic products after a given period of time.

The concentrations of these radiolytic products (ions and free radicals) is dependent on the concentration of trace impurities present, the temperature of the coolant, and the overall pH of the coolant water. Measurements of radiolytic decomposition have been made since the late 1920's using X-rays and solutions containing radioactive isotopes. In the early 1950's numerous researchers examined the effects of radiolysis and recombination in light During this period it was noted that reactor data water reactor coolant.

on radiolysis was highly irreproducible because of the sensitivity of radio-lytic yields to trace impurities. The main concern at the time was assuring 2 gas could not be generated and 0 that potentially explosive mixtures of H2 and that the radiolytic products which could accelerate corrosion effects could be suppressed.

The Nature of Imourities Present Impurities present in reactor coolant system water are a result of corrosion effects (and additives used to inhibit corrosion) and Boric Acid used as a reactivity control (along with LiOH to adjust the pH of the water following Boric Acid addition). A common example of corrosion effects are those associated with iron. This corrosion occurs via the following reactions:

2Fe + 3H 0 4 Fe 0

+ 3H 2

23 2

3Fe + 4H 0 -5 Fe 0

+ 4H 2

34 2

Under the presence of sufficient dissolved oxygen in the water a more rapid corrosion occurs via:

2Fe 0 4Fe + 30 2

23 3Fe + 20 Fe 0 2

34 These reactions add ions of Fe, an'd 0 to the water which can then interfere with reactions of radiolytic products attempting to recombine.

The use of Boric Acid (H B0 ) as a reactivity control mechanism and LiOH 3 3 to maintain the coolant slightly alkaline introduces the presence of Lithium and Borate ions.

Additionally impurities. ch as Cloride and Fluoride ions will be present in the water due to natural environmental concentrations found in the water used )y the reactor.

Early researchers found that to assure optimum recombination in the H

presence of these impurities an overpressure of H2 gas was needed.

2 gas will effectively act as an 02 scavenger. Typical reactor coolant chemistry specifications require maintaining the following:

b 3

b b

2_

Maximum 0

< 0.1 ppm 2

Maximur. F-

<, 0.1 ppm i

1 Max #num Cl- : (, 0.1 ppm 3

No nal H 15-40 STD cm /kgH O 2

2 The Physical Processes Involved in Radiolysis RadiolysisoccurswhenawatermoleculeinteractswithanenergeticC(,h,Y or neutron. The net end result is the generation of Compton electrons as energy is transferred and absorbed by the water molecules.

In a normal reactor environment # particles are resultant from the 10 (

) Li B

reaction with dissolved Boric Acid in the coolant. Y-radiat s are a result of radioactive fission product decay in the fuel and activation-roduct decay in the coolant and reactor structural materials.

p$-radiations will be minimal if the cladding surrounding the fuel is intact. Under accident conditions, however, where fission products can escape as a result of fuel damage the contributions from of and f sources should increase significantly.

The actual dissociation of P,30 molecules yields a series of aequous OH, H0 ).

Because the formation electrons and free radicals tH, H, H 022 2

2 of these radiolytic products occurs in a,very localized region in the reactor (near where an individual radiation passes through the water) the recombination back to water molecules proceeds quite rapidly. Remaining free radicals and ions then diffuse away from the region in which they were formed and enter into r^ actions with other ionic species which may be present. TABLE I gives tne yields of various free radicals as oetermined by experimental measures.

It is significant to note that oxygen is not a direct product of the radiolysis. Oxygen, however, can be generated by certain recombination type reactions.

e

TABLE I 6

s Direct Yields from Radiolysis, g (

) in molecules 100eV f

H+e H

H Ho H0 e,

g g

gg g

+.,,.g.,

2.31 2.86 0.55 0.44 0.70 2.34 e 0.00

.02 Y, f mixed 0.55 0.45 0.70 2.60 0.02

.02 8

2.60 R

H 0.36 0.72 0.36 1.12 1.00 0.47 0.17 4.0 eV A

10 (n,,,,)7Li 0.04 0.20 0.16 1.70 1.30 0.10 0.30 24.0 eV g

M.

t e,

II. Radiolysis and Recombination in Pure Water

• II.

1.

Sumary of Reactions Taking Place A survey of available literature indicates that the following reactions are possible radiolysis/ recombination reactions:

+ 10 H ~

(xi) H + -+ O - M Ho

0) 2 e {

t z

(ii) eak + Hs + b y (+p to)

Nx) og + yt S H (+ N o) k 0

t Oli) e.{ + k Hz + oH-(xxi) oH + H 0 k H0z (+ H o) t 2 t

Uv) eq 4 oH b os-(+w o) bxii) OH + Ho h Oz (+ Hz )

o z

2 Ho - (+Hz )

(xxiii) 2.oH H0 (v) ea{ + H0 o

z 2 2 2

b

(+ H,o)

(xxiv) O H + 0' HO -

Oz (vn e eot z

q (vii) e

+0-h o9-(vxv) oH +04-0- (+ Hz )

o q

2

(.xxvi) OH +O - b oH 4 0 eaq + 0 0--

(+Hz )

z 2

o (viii)eq +0-M 0 --

(+ Hz )

(xxvii) H + Hz0I H0++O o

3 2

t 3

(ix) ea{ +H 0 OH + oH- (+Hz0)

(XX"IIO NO H

+0-I z

k.

(xxix) HOz + Hz zo h OH+02 (+ Hz )

2 2 o

(x) H + Oz -- r Ho2 k

k,,

(xxx) 1 Hot -- 4 H z 2 + 0 0

2 lxi) H + Hz0z --* OH (+ Ks0)

~ k

k,z (xxxi) Ho + 0

---' WO '+0 z

t z

2 (xii) H + Ho ~ Hz (xxxii) Zo k% Oz--

o z

(xiii) 2H ~k., Hz k

k, (uidi) O' (+ H o) - n + OH + OH-t (Mv) H + 0 H *, Hz0 k.3 k,

(vxxiv) 0 + Hz to e, 0 - (+ N 0) 2 t

"_ [,

(xW) 2.0 - (+2Hz )b H 0 +zoH +0z

(**) "

"i o

2t 2

20 - (+ Hz )b 0 + OH +o (xvi) H +0' OH-o H

z 3

2 4

o-(xv'ti) H + Oi Hz (xv,,i) H+ + 0-h O H

• Neglecting the effects of impurities

Reaction Kinetics and Influence of Temperature and pH II, 2 The reaction kinetics during radiolysis and recombination are dependent

-1*

ies of rate constants (which are in units of (moles / liter)~I sec upon a s

?

Values ei these rates constants were found in the following Tables from the National Bureau of Standards:

(i) Selected Specific Rates of Reactions of Transients from Water in Aequous Solutions

- Hydrated Electron,'NSRDS-NBS 43 (ii) Selected Specific Rates of Reactions of Transients from Water in Aequous Solutions

- Hydrated Electron Supplemental Data, NSRDS - NBS 43, Supplement (iii) Selected Specific Rates of Reactions of Transients from Water in Aequous Solutions

- Hydrogen Atom, NSRDS - NBS 51 (iv) Selected Specific Rates of Reactions of Transients from Water in Aequous Solutions

- Hydroxyl Radical and Perhydroxyl Radical and their Radical Ions These rate constants (except where noted specifically) were determined at or near room temperature.

In general (and as a first approximation) the temperature dependence of the rate constants follows Arrhenius' equation.

-Ea$/RT k (T)

Ae

=

g O

- where:

k (T) is the rate constant at T Kelvin j

A is a proportionality constant j

Ea is the activation energy for the reaction j

R is the Gas Law Constant T is the solution temperature in K

-6 If k (T ) is known and k (T) is desired, it may be obtained from the following j g relationship:

k; (T) f;(T) = k;(T.) x k; (T.)

I

.)

k;(T)

Ae

~*k

~

e

- E2; / ET.

k.(T.)

Ae T = 623*K {%Z *F)

Example.:

E = 3.okat/~1e., T : 29E*K (2s'c) o 2

NlTl m exp ~,(3 o x to' cal /mle )(.09:311 "'ff'0"") 293 k)3 i

T k tT.)

.o szos I;ter

-t~ ptere, i

mk-g

= cv p [ 15to.9s7 g - g)

= N.07

- E = +.5 kal /mok, T = 623*K 2

b

= exp - 226,s. 6% ( 3 M8f) 2 k;(T.)

= 52.7E5

-E 2 = 5.0 kcal/wole, T.= 623* K

k. (T)

L

= 82.oi7 k;(I) s

- Ea = l 0 kcal/S le, T = 623 K k - (1) = 41s.0%

k.,(1.)

- - for Ea= 7.s keal/mok, T= (,23*K k(

"1L}2."l77

=

ki 7.)

1 The dependence of the reaction rate constant on the pH of the reactor coolant system water has been noted in a number of basic experimental measurements of transient effects. In certain areas it must be noted it is extremely difficult to determine rate constants in certain pH regions due to the effects of competing reactions which tend to confound the experimental process.

Figures II.2 A, B, C, and D show the measured variation of known reaction constants measured within different pH regions. The reaction H + H+ was specifically omitted from these figures as well as fron the previous table showing expected reactions because it proceeds with an extremely slow rate 3

constant (e.g. <<.10 )

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. II. 3 Hydrogen and Oxygen Generation via Radiolysis and Recombination Hydrogen is produced directly via radiolysis and from recombination of I

certainfrheradicals. The production rate via radiolysis is given by:

i d[Hz]

-kh(Hy cat rad.

where: k is a proportionality constant pistheradiationflux g (H ) is the microscopic yield 2

Hydrogen is also generated by the following recombination reactions:

(i) 2.eg5 H z. + 2 0 H -

E ~ 3.0 kcal/ mole

~

a k

(iii) eq~+H Ht + o H-E 3.0 kcal/mle a

b Hz E 7 3.0 kcal/ mole (xiii) 2.H a

The removal of H occurs due to the following reaction:

2 (r;c)

O H + H z.

H (+ Hz.o)

Ea ~ 7.o kcal/ mole return (to solution) effects a net kinetic Neglecting stripping and H2 equation may be formulated as:

MEN 3 - kgg(HD + k [e.3-3*4 k lead [W] + k 3[H]2_ gn,[ogyg,,)

L i

i dt Generally quoted values (from the National Bureau of Standards Data Base) for the reaction rate constants at room temperature are:

( ;("5.5 x10 g'~ ("3.0 xio

9 ICS(([tec)5"-

I"Ylife c) SE'-

4.5 xio' 1.3 x to '*

g g

(**Ic5/g;{,,) ste.

(*'l*S/g,ge,)sec.

O

~

d bz3 1

30 - Kf g(Hy + k [a il, + k [e.(],,,[H]q,+ k,3 [H],*

At equilibrium:

i c

3

~kz.[8N3cg[Hz3cq.

qH@

x4pHtuk,Ce.i31.4 ks Ee.iL.00.+ kn EHP.

yields:

Solving for 2

3 3

3 q

(4 h

^

t t

k. [OH3eg, 2

At lower temperatures (near room temperature) it should be noted k; ) (accounting for H2 removal is roughly 10-3 of the rates of production of H. But as 2

temperature increases we note the following effect shown below (assuming boiling does not occur).

REACTION RATE CONSTANTS IN (wo\\es/\\;terf'sec."

k k

k k

1 3

13 20 9

10 10 7

ROOM TEMPERATURE:.

4.5 x 10 53.0 x 10

~1.3 x 10 24.5 x 10 U

10 10 10 8

200 F 93.3 C 366 K

=1.4 x 10

=7.7 x 10

= 3.3 x 10 c4.0 x 10 U

10 11 10 9

300 F 148.8 C 421.8 K = 2.4 x 10 21.3 x 10

= 5.7 x 10 2 1.4 x 10 U

U U

10 11 10 9

400 F 204.4 C 477 K m3.7 x 10 22.0 x 10 28.7 x 10 e 3.8 x 10 10 11 11 9

500 F 260 C 533 K

=5.1 x 10 c2.8 x 10

< 1.2 x 10 e 8.3 x 10 10 11 11 10 600 F 315.5 C 588.5 K g6.7 x 10

=3.7 x 10 2 1.6 x 10

% 1.5 x 10 Thus at near cold shutdown temperatures'we would expect a greater buildup of H2 gas in solution, while at higher temperatures (characteristic of power operation) because of the rate at which the reaction:

OH+Hz H (+ H 2.0)

- proceeds, we would expect less.

0xygen, as noted previously, is not directly produced by radiolysis. Dissolved oxygen gas (0 ) cg be formed via certain reactions taking place in the 1

2 recombination phase from the free radicals and ions present. Noting the is generated by the following reactions:

previously defined reaction mechanisms, 02 k

Os (+ Hz )

Ea 7 3.0 kcal/ mole o

(xxii)

OH+H02 (xxvi) OH+Of OH' + Oz Ea.7 (xxvii) OH 4 HzOz H0++O.

Ea T 3

z HO+Hzgo b o H + o

(+H o)

Ea ; 2.0.0 kcal/ mole (xxi x) t n

t

5.0 kcal[mok (xxx) 2. H0 z b H20z + Oz Ea HOz + O - b Ho

+ 0 Ea T (xxxi) t z

2 go - (4 gg o]h HzO +20H 40 g

t 1 Ea ? 5.0 kcal/ mole (xxxv) 2 0 - (+ H z0)

HO[+0H +0 Ea E g

7 Dissolved oxygen is consumed via the following reactions:

b 0 - (+ Hz )

E t 3.0 kcal/vnole

(.vi) eag + 0 o

a 2

2 k

(x) H + Oz 4 H%

Eat 3.0 kcal/ mole.

Neglecting for the time being stripping and return effects, a net kinetic equation for 0 may be written as follows:

2 d [ 23 = kz2[ H3CHo ] + kzu[0H][,0 -3 + kz r [oH3[Hz z3 4 kgq [H0 ][Hz g]

o o

2 z

2 cat t k, [ H0 ][0 -] + ks3,[0 T 4 k sh E0'3

+ k, [H0 ]*

2 2

t 2

2 3

2

- k [eq1.o ) - kio EW3Chl z

- Generally quoted values for the reaction rate constants from the National Bureau of Standards Data Base are as follows:

1 k E k * ~ (~2.0 = 10 _

(~fn/e,.) 5"-

'%,,) su.

k"E 3*

k2 E I^

(~'%e,.) sec-(~sha,.)ac-

s. s x io

i. 3 x io 'a k n ~ (wies / lite,.) Se c.

~

k~47 (moies/g;4,,) sec.

= --

6 g,o x io7 kso- (~2.7 x t0 ks' m u

~

tes/ia,,)u.c.

(*/iae,Mc.

1.7 x 10' Se x 10 '

~

(3 M ( w ks/gih,.)Sec.

(moles [j y,,) sec.

~

~

A major mechanism for preventing net 02 generation via radiolysis is to maintain an overpressure of H2 gas on the Reactor Coolant System. In theory this is because 02 generation is a result of reactions involving OH, H0, and 2

H0 radicals. All of these we note depend on the availability of OH. By 22 providing excess H2 gas in solution the Reaction:

oH + H g H

(+ H o) t is f avored and significant quantities of OH radical are consumed. A number of researchers as well as private industry experience indicates that maintaining 3

as little as 5 STD cm /kgH O will suppress virtually all 02 generation under 2

normal operation and maintain the equilibrium level at <.02 ppm.

o O

t II.

4.

The Effects of Gas Strippino and Return to Solution If reactor coolant pressures and temperatures exceed the saturation line bulk boiling will ensue in the core the effects of this on the Hydrogen and Oxygen concentrations is as follows:

Insoluthn:

d [HO ( k4 (y,) + k,[g -]

+ ks beaklbN) + kis[N) ~ kta[#bbd 5

dt

- R (Hz) + Rg (Wz) 3 dE0J kuloHXHoO + ku [0Hl0i] + kn [oHKHz d + kg,[Ho ][Hgo2]

o g

=

dr

+ k.s. [Ho ]* 4 kg, [Ho ][0 -3 + km[o -] + k s6 [0i]2 z

s a

3 z

k. (e.{3[o ) - k,.[w]fod - e,(o ) + R,(o,)

z z

'.)) is the rate 4 ys sfe:pping, with

-whe re : Rs(Hz) s R3(Hz) = k [H ] Lg x sTramuc, ante 3

a Le Leastg of the boiling ckannel ks = proporbaldy constant

-W wkere-R (Hz) (or: Re(o )) is tkt rate of rehen 62 seMien a

z R, ( Hz) = k, [H "f-] K,x SUUACF A264 oF STE44[ QUID 8MTERFACE z

g k, = proporbalify cons + ant Ka = Henry's Law cons 4 ant as n &nch of Tempe,ahre In the vapor space an additional recombination effect occurs which has been experimentally measured by Isbin, this is noted as follows:

d [H *d _ R ( 91) - R,( H ) - kop (2 [Hz'ar-][0 "*P-]"2 2

t 2

3 at d E0 N = R (o ) - R (0 ) - 2 k,,g.k"* [Hsv r-][0

• f '

t 2

3 z

2 At

hy is the gama ray flux

- where

,k

,is the vapor space recombination rate b

III. The Effects?of Trace Impurities on Radiolysis and Recombination s

III. 1 Types of Impurities Present and Their Concentrations A review of Industry Data for operating PWRs indicates the following levels of metallic impurities exist in reactor coolant water under normal plant operation:

Species Minimum Maximum 54Mn 0.04. ppb 2.2 ppb 58Co,60C5 0.04 ppb 0.3 ppb Fe 0.5 ppb 90.0 ppb Ni 0.4 ppb 15.0 ppb Cr 0.04 ppb 7.0 ppb As a result of Boric (H B0 ) addition and LiOH pH buffering the 3 3 following species will also be prc.nt:

Species Minimum Maximum 2

B0 13 ppm 6.0 x 10 ppm 3

Li*

0.1 ppm 2.1 ppm As a result of impurities in the makeup system supplying reactor coolant system water:

Species Maximum Cl-O.1 ppm F-0.1 ppm will be addressed later.) 2 gas have been ignored for the moment and (The effects of dissolved N

_ 18 These values can be converted to chemical concentrations as follows For Manganese:

I moles Mn/ gram Mn

.04 grams Mn

/54.94

)

.04 pp Mn =

9 10~3 liters 2d u / gram H O 10 grams H 0 2

p e

10-10 mles/ liter 7.28 x

=

2.2 grams MN

/54.94 "I"5 ""/ gram Mn 1

2.2 ppb Mn =

HU 9

10-d liters 2 / gram H O 10 grams H O 2

2 10-8 moles / liter 4.0 x

=

For Ccbalt:

.04 grams Co

/58.93 m les Cc/ gram Co 1

.04 ppb Co =

9 10~3 liters H O gram H O 10 grams H O 2

2 2

6.8 x 10-10 moles / liter

=

0.3 grams Co

/58.93 moles Co/ gram Co 1

0.3 ppb Co =

h0 10 grams H O 10~3 liters 2 / gram H O 9

2 2

5.09 x 10-9 moles / liter

=

For Iron:

0.5 grams Fe

/55.85 molas Fe/ gram Fe 1

0.5 ppb Fe =

N0 9

10-3 liters 2 / gram H O 10 grams H O 2

2 8.95 x 10-9 *0I'S/ liter

=

1 N

90 grams Fe

/55.85

/ gram Fe 90.0 ppb Fe =

h0 9

10-3 liters 2 / gram H O 10 grams H 0 2

2

~

1.61 x 10-6 moles / liter

=

19 For Nickel:

1 moles Ni 0.4 grams Ni

/58.71

/ gram Ni 0.4 ppb Ni

=

10 grams H O 10-3 liters H O / gram H 0 9

2 2

g 6.8 x 10~9 moles / liter

=

1 m les Ni 15 grams Ni

/58.71

/ gram Ni 15.0 ppb Ni

=

9 10-3 liters H 0 / gram H O 10 grams H O 2

2 2

2.55 x 10-7 moles / liter

=

For Chromium:

0.4 grams Cr

/52.0 m les Cr/ gram Cr 1

0.4 ppb Cr

=

10 grams H O 10-3 liters H 0 / gram H O 9

2 2

2 7.7 x 10~9 moles / liter

=

I moles CR 7.0 grams Cr

/52.0

/ gram Cr 7.0 ppb Cr

=

9 10-3 liters H O / gram H O 10 grams H 0 2

2 2

1.3 x 10-7 moles / liter

=

For Boron:

/61.83 3 3/ gram H B03 3.0 grams H B03 3

3 10 ppm

=

6 10-3 liters H O / gram H O 10 grams H O 2

2 2

4.8 x 10-5 moles / liter

=

H%

2

/61.83 3 3/ gram H B03 3

6.0 x 10 grams H 803 3

3 1.5 x 10 ppm =

6 10-3 liters H 0/ gram H O 10 grams H O 2

2 2

9.7 x 10-3 moles / liter

=

For Lithium:

I/6.939 moles Li+/ gram Li+

=

0.1 grams Li+

0.1 ppm h

10 grams H O 10-3 liters H 0/ gram H O 6

2 2

2 i

1.4 x 10-5 moles / liter

=

1/6.939 moles Li+/ gram Li*

2.1 grams Li+

2.1 ppm

=

10 grams H O 10-7 liters H 0/ gram H 0 6

2 2

2 3.3 x 10-4 moles / liter

=

For Chlorine:

1 moles C1-0.1 grams Cl-

/35.45 gram Cl-0.1 ppm C1

=

6 10 grams H O 10-3 liters H 0/ grams H O 2

2 2

2.82 x 10-6 moles

=

liter For Fluoride:

, p-1 0.1 grams F-

/19.0 gram F-0.1 ppm F

=

10 grams H O 10-3 liters H 0/ grams H O 6

2 2

2 5.26 x 10-6 moles / liter

=

21 -

Sumarizing these calculations we note:

Speciesh Maximum Concentration Minimum Concentration (moles / liter)

(moles / liter) 7.28 x 10-10 Mn 4.0 x 10-5.09 x 10-9 6.8 x 10-10 Co 1.61 x 10-6 8.95 x 10-9 Fe 2.55 x 10-7 6.8 x 10-9 Ni 1.3 x 10-7 7.7 x 10-9 Cr B03 9.7 x 10-3 4.8 x 10-5 Li+

3.3 x 10-4 1.4 x 10-5 Cl-2.82 x 10-6 F-5.26 x 10-6 In viewing the magnitudes of these anticipated concentrations of impurities, we

, Li+, C1, F to play a significant effect would expect the effects of Fe, B03 on recombination with Ni and Cr playing minor roles.

• ~*

III. 2.

Chemical Reactions Taking Place During Recombination The impurities present take part in a number of oxidation / reduction reactions. These are partially noted below.

Fe Reactiams OxidhtionReactions Reduction Reactions H + H+ + Fe+2 % Fe+3, g H + Fe+3 --* Fe+2 g+

2 OH + Fe+2

'> Fe+3 + OH-eh+Fe+3*Fe+2(g0) a 2

2 2 + Fe+2 4 Fe+3 + OH + OH~

0~ + Fe+3M Fe+2 + 20H" H0 0

+ Fe+3___,p,+2 + 0 H+ + H02 + Fe+2-

'm Fe+3 + H 0 2

22 2

H0

+ Fe*2

'* Fe+3 + H0 2

2 0" + Fe+2 (+H O) 9 Fe+3 + 20H" 2

B0 Reactions 3

0xidation Reactions Reduction Reactions 9

e

- 23 Li+ Reactions Oxi tion Reactions Reduction Reactions 6

C1~ Reactions 0xidation Reactions Reduction Reactions H* Cl~ + OH 4 C1 (+H 0)

H + C1 % Cl~ + H+

2 Cl~ + OH

• 0H + Cl H + Cl-(H O)

• hcl + e 2

~

ag F~ Reactions Oxidation Reactions Reduction Reactions

+ HF --* HF~ (+H O) ---> H + F ~

~

H + F~ (+H O) e HF + e,q e,

2 2

The main effect of impurities in solution is that they compete with many of the recombination reactions going on and thus hindering net recombina-overpressure will tion. Even so, under normal operation a sufficient H2 suppress net 02 generation.