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Evaluation of the Sodium-Water Reaction in Heat Transfer Systems

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: January 1, 1957 | doi: 10.1021/ba-1957-0019.ch010

D. D. ADAMS, G. J . BARENBORG, and W. W. KENDALL Knolls Atomic Power Laboratory, General Electric Co. Schenectady, Ν . Y.

In considering sodium for use as a heat-transfer agent, the chemical reaction which results in the event of leakage between sodium and the sec ondary fluid is an important factor. The atomic energy industry has acquired considerable exper ience in the testing of sodium-heated steam gen erators. No instance of violent reaction resulting from leakage in these tests is known. A review of sodium-water reaction experiments forms a basis for estimating the effects of chemical reac tion resulting from potential leakage. System de sign recommendations to minimize the effects of leakage are also presented.

THE past ten years h a v e seen a g r o w i n g interest i n the use of s o d i u m a n d its alloys w i t h potassium ( N a K ) as heat-transfer fluids. A l t h o u g h the m a i n impetus i n the development of these metals has come f r o m the atomic energy p r o g r a m o n power reactors, the experience gained is also applicable to the use of these metals as h i g h - t e m p e r a t u r e heat-transfer fluids i n other industries. O n e of the problems i n the use of s o d i u m or N a K is the possibility of a v i g o r ous c h e m i c a l reaction r e s u l t i n g f r o m leakage between the l i q u i d m e t a l a n d the fluid w i t h w h i c h it is e x c h a n g i n g heat. T h i s latter fluid is n o r m a l l y water. T h e resulting c h e m i c a l reaction is capable of p r o d u c i n g b o t h m e c h a n i c a l a n d corrosive damage to the system. T h e discussion i n this paper is confined to the m e c h a n i c a l damage aspects (pressure, temperature, a n d flow stoppage effects caused b y the c h e m i c a l reactions). C o r r o s i o n effects m a y nevertheless be important a n d must be considered f o r a n y specific system. F u r t h e r , o n l y s o d i u m - w a t e r reactions are discussed, a l t h o u g h the results are felt to be applicable to other a l k a l i - m e t a l r e actions i n w h i c h the reaction heat a n d amount of gas f o r m e d are s i m i l a r . A n o t h e r p r o b l e m p e c u l i a r to the application of a s o d i u m - t o - w a t e r heat e x changer system i n a n u c l e a r power plant is the reactor safeguards' considerations of a possible leak. I n certain reactors, the i n t r o d u c t i o n of h y d r o g e n o r its c o m pounds c a n cause serious control difficulties. T h e r e f o r e , it is necessary to ensure that a direct leak of w a t e r - t o - r e a c t o r coolant is v i r t u a l l y impossible. T h e a b i l i t y of s o d i u m - c o o l e d reactors to produce steam at c u r r e n t c o m m e r c i a l conditions of superheat a n d pressure has resulted i n considerable design effort to p r e v e n t this direct leakage. A t present, two methods are available to reduce the p r o b a b i l i t y of direct leakage to a safe l e v e l . T h e first of these employs a n intermediate a l k a l i - m e t a l heat-transfer system between the s o d i u m reactor c o o l ant a n d the steam generating system. T h i s m e t h o d is generally applicable o n l y w h e n space limitations are not present. 92

In HANDLING AND USES OF THE ALKALI METALS; Advances in Chemistry; American Chemical Society: Washington, DC, 1957.

ADAMS, BARENBORG A N D KENDALL—SODIUM WATER REACTION IN HEAT TRANSFER

93


T h e second m e t h o d provides a double b a r r i e r w i t h i n the steam generator itself. T h e double b a r r i e r has c o n v e n t i o n a l l y b e e n i n the f o r m of concentric tubes a n d double tube sheets w i t h the intermediate space filled w i t h a static t h i r d fluid to assist i n the transfer of heat. T h e t h i r d - f l u i d system was e q u i p p e d to detect any leaks w h i c h might occur i n either of the single b a r r i e r s . U n d e r the d o u b l e w a l l philosophy as o r i g i n a l l y developed, i f a leak o c c u r r e d i n either b a r r i e r , the heat exchanger was t a k e n out of service for r e p a i r or replacement. F u r t h e r discussion of n u c l e a r reactor c o n t r o l problems r e s u l t i n g f r o m water leakage is b e y o n d the scope of the present paper, a n d the discussion presented hereafter is o n the effects of the c h e m i c a l reaction between s o d i u m a n d water. A s a c o r o l l a r y of this discussion, the necessity for the use of a d o u b l e - b a r r i e r heat exchanger i n systems where n u c l e a r control problems are absent has b e e n e x a m ined. T h e desirability of e l i m i n a t i n g the d o u b l e - b a r r i e r design w h e r e feasible is obvious. T h e d o u b l e - b a r r i e r results i n a m o r e c o m p l e x design w i t h associated fabrication a n d operational problems, requires a d d i t i o n a l heat transfer area due to the increased t h e r m a l resistance of the double b a r r i e r , a n d requires e x t e r n a l e q u i p m e n t to h a n d l e the t h i r d - f l u i d system. A l l these factors increase the size a n d cost of the heat exchanger. I n order to evaluate system b e h a v i o r i n the event of a w a t e r - t o - s o d i u m leak, studies have been conducted b o t h at K A P L ( K n o l l s A t o m i c P o w e r L a b o r a t o r y , operated for the U . S . A t o m i c E n e r g y C o m m i s s i o n b y the G e n e r a l E l e c t r i c C o . , Schenectady, N . Y . ) a n d M S A ( M i n e Safety A p p l i a n c e s C o . , C a l l e r y , P a . ) . A s u m m a r y of data a c q u i r e d i n the K A P L studies a n d i n the earlier w o r k at M S A is i n c l u d e d below. I n addition, three instances of leaks w h i c h actually o c c u r r e d i n operating s o d i u m - w a t e r heat exchangers are presented. T h e i n f o r m a t i o n obtained to date is sufficiently encouraging to justify serious consideration of the use of s o d i u m - w a t e r heat exchangers without d o u b l e - b a r r i e r protection f o r those systems i n w h i c h n u c l e a r c o n t r o l p r o b l e m s are absent. A t least one large project ( D e v e l o p m e n t a l F a s t B r e e d e r P o w e r Reactor, A t o m i c P o w e r D e v e l o p m e n t Associates, Inc.) is p r e p a r i n g to operate a s o d i u m - h e a t e d steam generator of substantial size ( a p p r o x i m a t e l y 1000 k w . ) w i t h a s i n g l e - b a r r i e r design (7). I n the a p p l i c a t i o n of this design to the n u c l e a r power plant, protection f r o m the n u c l e a r h a z a r d is afforded b y a n intermediate a l k a l i m e t a l loop a n d heat exchanger.

Experimental T h e pressure, temperature, a n d p l u g g i n g effects w h i c h result f r o m a s o d i u m water reaction are based o n the f o l l o w i n g c h e m i c a l equations: Na(l)

+ H 0(g) = NaOH(c)

Na(l)

+ N a O H ( c ) = N a * 0 ( c ) + J H ( g ) Δ H ° 291 =

2

+ i H ( g ) Δ H ° 298 = -45.7 k c a l . 2

2

Na(l)

+ è H*(g) = N a H ( c ) Δ Η

β

+1.89 k c a l .

298 = — 1 3 . 7 k c a l .

(1) (2) (3)

R e a c t i o n 1 is e x t r e m e l y r a p i d , generally b e i n g l i m i t e d o n l y b y the speed of m i x i n g . R a p i d m i x i n g a n d reaction are facilitated b y the h i g h s o d i u m flow rates e m p l o y e d i n heat-transfer systems. Less is k n o w n , however, about the rate a n d extent to w h i c h the second a n d t h i r d reactions proceed. U n d e r conditions e n countered i n heat-transfer systems, their rates a r e generally considered to be slow i n c o m p a r i s o n to R e a c t i o n 1. E x a m i n a t i o n of the equations shows that R e a c t i o n 2 w o u l d double the system pressure rise w h i c h w o u l d result w e r e R e a c t i o n 1 to proceed alone. T h e f o r m a t i o n of s o d i u m h y d r i d e i n R e a c t i o n 3 w o u l d reduce the pressure b u i l d - u p . T h e t h e r m a l effects of a s o d i u m - w a t e r reaction are l a r g e l y influenced b y R e action 1 alone. T h e heat p r o d u c e d b y R e a c t i o n 2 is negligible i n comparison, a n d the heat e v o l v e d b y a moderate rate of s o d i u m h y d r i d e f o r m a t i o n w o u l d l a r g e l y be b l a n k e t e d b y the heat exchanger load a n d b y heat losses f r o m the system. P l u g g i n g effects result f r o m the fact that a l l of the solid reaction products f o r m e d are p r a c t i c a l l y insoluble i n s o d i u m . I n considering the pressure effects of a w a t e r - t o - s o d i u m leak, the possibility of shock effects r e s u l t i n g f r o m the c h e m i c a l reaction must be e x a m i n e d . T o this end, experiments w e r e conducted b y N e l s o n (1 ) to measure the shock w a v e e n e r -


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gies obtained f r o m the s o d i u m - w a t e r reaction. T h e conditions of the experiments were chosen to obtain m a x i m u m rate of reaction. L i q u i d N a K was substituted for s o d i u m , a n d nine N a K charges (1 to 5 k g . each) were detonated i n water w i t h a n explosive booster charge. Booster charges were detonated without N a K present to serve as a control.


T h e energy c a r r i e d b y the pressure wave i n this experiment was calculated f r o m pressure measurements a n d f o u n d to be v e r y l o w . I n no case was it h i g h e r than 5 cal. p e r g r a m of N a K or m o r e than 0.33% of the available energy. S i m i l a r experiments w i t h trinitrotoluene showed that 25% of the available energy is c a r r i e d i n the shock wave. These tests indicate that it is impossible to m i x s o d i u m a n d water fast enough t h r o u g h a n y conceivable leak i n a heat-transfer system to obtain a reaction p r o d u c i n g the shock waves characteristic of h i g h explosives. T o evaluate the use of p r e s s u r e - r e l i e f devices, a n u m b e r of tests were c o n ducted b y K i n g (3-6). Water was forced into N a K t h r o u g h a à-inch pipe u n d e r a pressure differential of 200 pounds per square inch. T h e pressure generated was vented t h r o u g h a 1-inch relief v a l v e set at 200 pounds p e r square i n c h . These reaction studies are notable, because they demonstrated that the pressure resulting f r o m the reaction c a n be r e l i e v e d b y conventional relief valves. These studies also showed that sharp pressure pulses resulted w h e n the N a K was at r o o m temperature but were r e d u c e d as the N a K temperature was increased. (In tests w i t h the N a K i n i t i a l l y at r o o m temperature, reaction products plugged the relief valve, resulting i n damage to the reaction vessel w h i c h h a d passed a hydrostatic test at 3000 pounds per square inch. In the runs at higher temperature, the relief v a l v e was protected b y a baffle, a n d no m e c h a n i c a l damage occurred.) W i t h the N a K preheated to 600 °F., the reaction proceeded to completion without pressure surges, a n d the m a x i m u m pressure was essentially l i m i t e d to the relief v a l v e setting. It is not expected, therefore, that severe pressure pulses w o u l d r e sult f r o m leaks i n heat-transfer systems, because temperatures i n these systems n o r m a l l y r u n i n the 600 °F. range or higher. These tests f u r t h e r indicated that a higher initial gas pressure over the N a K r e d u c e d the pressure surges. W h e n the i n i t i a l N a K pressure was increased to 200 pounds p e r square i n c h or more, the pressure p u l s i n g effect was m i n i m i z e d . T h e pressure pulses first observed b y K i n g have appeared to some extent i n n e a r l y e v e r y s o d i u m - w a t e r reaction test. It is the authors' o p i n i o n that these pulses are obtained b y n o n u n i f o r m m i x i n g of the reactants. T h e gas f r o m the i n i t i a l reaction separates the reactants, a l l o w i n g a slug of unreacted water to enter the reaction zone. W h e n the gas b u b b l e f r o m the initial reaction disperses, a s u d den m i x i n g of the reactants produces a h i g h local pressure. T h i s pressure peak lasts u n t i l the i n e r t i a of the l i q u i d can be overcome a n d the local pressure e q u a l ized w i t h the pressure i n the r e m a i n d e r of the system. T h e pressure pulses are different f r o m shock waves i n that, although of longer d u r a t i o n ( i n the order of m i l l i s e c o n d s ) , they are r e l a t i v e l y l o w i n pressure. These pulses are generally a t tenuated as they t r a v e l throughout the l i q u i d p i p i n g system a n d do not appear i n the gas phase. T h e tests discussed thus far, w h i l e i m p o r t a n t i n defining the effects expected f r o m a n a l k a l i m e t a l - w a t e r reaction, have i n v o l v e d large quantities of water a n d c o m p a r a t i v e l y little a l k a l i metal. I n the event of water leakage into s o d i u m i n a heat-transfer system, the s o d i u m w o u l d be i n excess. T o investigate the effects of a s o d i u m - w a t e r leak i n heat transfer systems, a s m a l l test system w a s operated at K A P L . T h e system consisted of a water boiler using one d o u b l e - w a l l e d tube w i t h flowing s o d i u m o n the tube side, boiler water o n the shell side, a n d m e r c u r y as the t h i r d fluid. I n each test r u n a hole was fabricated i n the outside tube. A f t e r the b o i l e r reached e q u i l i b r i u m conditions, steam pressure of 100 pounds per square i n c h a n d 475° to 500 °F. s o d i u m temperature, the inner tube was completely p a r t e d b y a tensile l o a d a p p l i e d to a p r e f o r m e d p e r i p h e r a l notch. T o date two test runs have been completed. I n the first, the leak i n the outside tube was a 0.035-inch h o l e ; i n the second, the leak was r e d u c e d to a 0.009-inch hole. It is p r o b a b l e that i n neither r u n was the s m a l l hole t h e l i m i t i n g factor o n the rate of water injection, since water l e a k e d into the m e r c u r y annulus p r i o r to the i n n e r tube f a i l u r e . T h i s reserve of water t h e n was admitted to the s o d i u m


ADAMS, BARENBORG A N D

KENDALL—SODIUM WATER REACTION IN

95

HEAT TRANSFER

t h r o u g h the larger f a i l u r e i n the inside tube. In b o t h bases the reaction was r e l a t i v e l y moderate. T h e pressure traces obtained for the first a n d second test r u n s are s h o w n i n F i g u r e s 1 a n d 2, respectively.


In the first test r u n the two pressure traces represent measurements at inlet a n d outlet of the boiler. B o t h traces show the characteristic pressure pulses o b tained i n the w o r k of K i n g . T h e first, second, a n d f o u r t h pulses detected at the s o d i u m inlet appear the strongest, thereby i n d i c a t i n g the reaction p r o d u c i n g these pulses was nearest to this pressure detector. T h e t h i r d pulse is due to a reaction taking place nearer the s o d i u m outlet. These pressure pulses were p r o d u c e d b y n o n u n i f o r m reaction effects s i m i l a r to those p r e v i o u s l y discussed. T h e pressures obtained at the inlet a n d outlet are different, i n d i c a t i n g that appreciable p l u g g i n g of the tube took place. W h e n the unit was disassembled, a m i x t u r e of sodium, m e r c u r y , a n d s o d i u m - w a t e r reaction products was f o u n d i n the tube. In the second, test pressure measurements were also obtained f r o m the system expansion tank. T h i s pressure rose smoothly to a l e v e l corresponding to the p r e s sure relief setting. A g a i n the pressure at the s o d i u m inlet a n d outlet b e h a v e d as though they were independent. T h e pressure at the outlet was moderate a n d is not s h o w n i n F i g u r e 2. T h e negative pressure r e c o r d e d at the inlet is a n absolute

-J0

TIME

IN

i 2

1 4 6 θ SECONDS AFTER LEAK 1

1

1 10

1 12

I 14

I 16

I 18

I 20

I 22

1 24

_J 26

1 28

Figure 1. Pressure traces for a 30-mil leak

pressure of 7 pounds per square i n c h , i n d i c a t i n g that t e m p o r a r y p l u g g i n g of the p i p i n g a n d absorption of the h y d r o g e n b y s o d i u m h y d r i d e f o r m a t i o n m a y have t a k e n place. W h e n the u n i t was disassembled, the tube was f o u n d to be c o m pletely empty. T h i s was caused b y the fact that the leak was a l l o w e d to continue long enough to w a s h out the s o d i u m a n d the reaction products completely. I n b o t h test runs the s o d i u m flow completely stopped i m m e d i a t e l y after the f a i l u r e i n the inside tube. T h e results of the above tests are indicative of a f a i r l y large leak between s o d i u m a n d water. A n actual leak i n a system u s u a l l y takes some time to develop, b e g i n n i n g as a s m a l l crack a n d g r o w i n g larger. In such a system the r a p i d h y d r a u l i c transients, p r o d u c i n g the r a p i d reaction, a n d pressure pulse w o u l d not be expected. It is notable that even i n the test system used here the pressure b u i l d - u p at the expansion tank was u n i f o r m . T o investigate f u r t h e r the effects of water leakage into excess sodium, tests were made at K A P L u s i n g a 1-inch p u m p e d s o d i u m loop. K n o w n amounts of water (6 to 12 grams) were injected into the loop w h i c h contained 20 pounds of s o d i u m . F i g u r e 3 shows a t y p i c a l pressure a n d flow trace obtained after saturated water h a d been injected into flowing s o d i u m at 500°F.(2). T h e gas v o l u m e i n the e x p a n sion t a n k was sized so that the pressure increase w o u l d be 100 pounds per square i n c h if a l l the water were to r e m a i n as steam, or 50 pounds per square i n c h i f a l l the water reacted a c c o r d i n g to R e a c t i o n 1. A s the w a t e r w a s i n j e c t e d s u d d e n l y as saturated water at about 500 °F., the i n i t i a l pressure rise was sharp. Pressure oscillations were noted b y the pressure-sensing element located a p p r o x i m a t e l y 6 feet f r o m the point of injection. T h i s oscillation was p r o b a b l y due to




96

expansion a n d contraction of the gas b u b b l e f o r m e d , before the b u b b l e pressure equalized w i t h the expansion tank pressure. T h e pressure at the expansion tank d i d not show a n y oscillation. T h e system pressure increased b y 20 pounds p e r square i n c h i m m e d i a t e l y a n d thereafter d r o p p e d b y about 8 pounds per square i n c h i n 20 seconds. If no s o d i u m h y d r i d e were f o r m e d , the m i n i m u m pressure increase at the expansion tank should have been 50 pounds per square i n c h . A s the m a x i m u m increase was o n l y 20 pounds per square i n c h , it appears that a considerable fraction of the h y d r o g e n e v o l v e d b y R e a c t i o n 1 must have reacted f a i r l y r a p i d l y b y R e a c t i o n 3. T h e pressure d e crease i n the e x p a n s i o n tank is f u r t h e r i n d i c a t i o n of h y d r o g e n absorption b y s o d i u m formation. T h e trace of the s o d i u m flow shows a response p e c u l i a r to this system. T h e s u d d e n flow d r o p after injection was caused b y v a p o r i z a t i o n of the water w h i c h pushed s o d i u m t h r o u g h the flowmeter i n the reverse d i r e c t i o n to the expansion tank. A f t e r this transient, the s o d i u m flow r e t u r n e d to n o r m a l , u n t i l the reaction products r e a c h e d a v e r t i c a l l e g . T h e gaseous reaction products t h e n r e d u c e d the h e a d o n the p u m p discharge, thereby increasing the flow. T h e final decrease i n flow w a s p r o b a b l y caused b y the reaction products' p a r t i a l l y p l u g g i n g the system a n d thereby increasing the system pressure d r o p .

-J SODIUM

SURGE TANK PRESSURE

SODIUM

PRESSURE

I

1-

I S £ί 5 1

$

25 Ο 125

NEAR INJECTION

POINT

Fi Β

25 0

2 TIME

IN

4

6

10

12

14

20

22

24

SECONDS

Figure 3. Pressure a n d sodium flow traces after water injection


26


KENDALL—SODIUM WATER REACTION IN

HEAT TRANSFER

97


F u r t h e r runs were m a d e o n the 1-inch system i n order to determine the p l u g p i n g effects of the reaction products on restricted flow passages, i n this case s i m u lating the close h y d r a u l i c clearances of a n u c l e a r reactor. F o r this purpose, a n orifice plate h a v i n g f o r t y - f o u r 5 0 - m i l holes was inserted i n the loop, and r u n s were made i n w h i c h the s o d i u m temperature was v a r i e d f r o m 400° to 625°F. A t the lowest temperature, complete p l u g g i n g of the orifice plate o c c u r r e d , w h i l e at the highest temperature there was no immediate p l u g g i n g effect. A t intermediate temperatures, p a r t i a l p l u g g i n g was noticed (2). T h e lack of p l u g g i n g at 625°F. is attributed to s o d i u m h y d r o x i d e h a v i n g a m e l t i n g point of 605 °F. T h e results of this test show that the presence of excess s o d i u m significantly reduces the pressure rise obtained because of the r a p i d f o r m a t i o n of N a H ( R e a c tion 3). A l t h o u g h the test was p e r f o r m e d on a b a t c h - i n j e c t i o n basis, the data indicate that w i t h a continuous leak o n l y m i n i m u m - p r e s s u r e oscillations w o u l d occur, a n d the o v e r - a l l pressure w o u l d be less t h a n predicted f r o m Reaction 1. T o explore f u r t h e r the system effects w h i c h result f r o m the continous l e a k age of water into a c i r c u l a t i n g s o d i u m stream, K A P L is c u r r e n t l y m o d i f y i n g a n 8i n c h pipe diameter system for l a r g e - s c a l e water injection studies. T h e system to be used for the large-scale tests w i l l contain a p p r o x i m a t e l y 4000 pounds of s o d i u m a n d have a surge v o l u m e of 25 cubic feet. W a t e r - l e a k rates i n a range f r o m 0.1 to 100 pounds per m i n u t e are to be tested, a n d pressure, temperature, a n d p l u g g i n g p h e n o m e n a w i l l be studied. T h e results of these tests, complemented b y the tests discussed above a n d b y experience gained f r o m operating test-heat exchangers, w i l l help f o r m a sound e x p e r i m e n t a l foundation for any future decision to e l i m inate the d o u b l e - b a r r i e r philosophy.

Experience from Operating Systems D u r i n g the p e r i o d K A P L has been testing steam generators, instances of l e a k age between s o d i u m a n d water have been r a r e . T h e r e have been, however, three h e a t - e x c h a n g e r failures that resulted i n the m i x i n g of water a n d s o d i u m . T h e data f r o m these failures give a n i n d i c a t i o n of what m a y be expected f r o m a s o d i u m water leak. O f the failures w h i c h definitely resulted i n s o d i u m - w a t e r reactions, two of these failures o c c u r r e d i n one steam generator b e i n g used i n n a t u r a l c i r culation heat-transfer studies at K A P L . T h i s unit, s h o w n i n F i g u r e 4, was a s h e l l and tube evaporator w i t h s o d i u m on the tube side a n d water on the s h e l l side. Because it was an e x p e r i m e n t a l unit, o n l y a single b a r r i e r was used i n separating the s o d i u m a n d the water.

Figure 4.

Natural circulation evaporator


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98

CHEMISTRY SERIES


T h e i n i t i a l f a i l u r e i n this unit was first indicated b y a drop i n the s o d i u m flow to zero d u r i n g a n a t u r a l c i r c u l a t i o n heat transfer r u n . T h e u n i t at this time was operating w i t h a s o d i u m - i n l e t temperature of about 700°F., a s o d i u m pressure of about 5 pounds per square i n c h gage, and a b o i l i n g water pressure of 485 pounds. F l o w was restored i n about 0.5 h o u r b y a p p l y i n g a d d i t i o n a l heat to the s o d i u m a n d b y activating the loop electromagnetic p u m p . O p e r a t i o n of the loop was then continued for about 9 hours i n order to p r o v e conclusively the existence of a s o d i u m - w a t e r leak. T h e second f a i l u r e i n this u n i t o c c u r r e d sometime after the unit h a d been put b a c k into operation. A g a i n the first i n d i c a t i o n of a leak was a l o w s o d i u m flow rate d u r i n g a n a t u r a l c i r c u l a t i o n heat-transfer r u n . F r o m experience gained w i t h the first leak, the flow trend was noticed a n d p u m p power applied early enough to prevent complete loss of flow. T h e gas i n the s o d i u m surge tank was sampled a n d f o u n d to contain h y d r o g e n , thus p r o v i d i n g further evidence that a leak existed. A f t e r the discovery of the leak, the system was continued i n operation for about 12 days. D u r i n g this time three more n a t u r a l c i r c u l a t i o n heat-transfer runs were made. L e a k a g e a p p a r e n t l y continued d u r i n g these runs, as the i m p u r i t y content of the s o d i u m increased d u r i n g the 12-day p e r i o d . [ I m p u r i t y content was measured b y use of a " p l u g g e d i n d i c a t o r " (1).] W h i l e the t h i r d r u n was u n d e r w a y on the twelfth day, a sudden pressure rise was experienced i n the s o d i u m surge tank f r o m 5 to 30 pounds per square i n c h gage i n less t h a n 2 minutes. A t this time the r u n was t e r m i n a t e d a n d no f u r t h e r heat-transfer runs were made on the loop. D a t a available on the first f a i l u r e d i d not p e r m i t an estimate of the water l e a k rate. F o r the second leak, however, sufficient data were available to p e r m i t an estimate of the leakage rate d u r i n g the final heat-transfer r u n . Estimates of this leakage rate were made f r o m data both on the pressure rise and o n the increase i n s o d i u m i m p u r i t y content. These estimates indicated an average leak rate of a p p r o x i m a t e l y 0.01 p o u n d per m i n u t e . M e t a l l u r g i c a l e x a m i n a t i o n of the failure showed that the leak resulted f r o m a crack i n a t u b e - t o - t u b e sheet w e l d . A section t h r o u g h this crack, w h i c h e x t e n d ed a r o u n d about two thirds of the tube p e r i p h e r y , is s h o w n i n F i g u r e 5. A c a l c u lation was made of the water leak rate t h r o u g h the failure. T h i s calculated result was a p p r o x i m a t e l y 10,000 times h i g h e r than the 0.01 p o u n d per m i n u t e noted above, a leak rate too large for the effects observed. A l t h o u g h appreciable error is expected i n such a calculation, other factors aside f r o m calculation limitations

Figure 5.

Failure in natural circulation evaporator



KENDALL—SODIUM WATER REACTION IN HEAT

TRANSFER

99

caused this d i s c r e p a n c y : p a r t i a l p l u g g i n g of the c r a c k b y the reaction products of s o d i u m a n d water, a n d the presence of steam w i t h the water. T h i s steam c o u l d have come either f r o m flashing of the saturated water or f r o m the steam n o r m a l l y present i n a b o i l i n g evaporator. T h e presence of these two factors w o u l d a p p r e c i ably reduce the water leak rate for a crack of g i v e n size. T h e second unit i n w h i c h a f a i l u r e resulted i n the m i x i n g of water a n d s o d i u m o c c u r r e d i n a superheater unit w h i c h was part of a steam generator w i t h a p e r h o u r capacity of about 10 χ 10 B . t . u . b e i n g tested at the M S A (8). T h i s s u p e r heater, s h o w n i n F i g u r e 6, was a tube a n d shell heat exchanger w i t h s o d i u m o n the s h e l l side a n d steam o n the tube side. T h e unit was of d o u b l e - b a r r i e r design w i t h m e r c u r y as the t h i r d fluid. T h e t h i r d fluid was n o r m a l l y m a i n t a i n e d at a pressure intermediate between the s o d i u m a n d steam system pressures.


e

T h e first evidence of the s o d i u m - t o - w a t e r leak was a drop i n s o d i u m flow f r o m 40 to 10% i n 30 seconds w i t h a subsequent decrease to zero i n the next 15 minutes. T h e plugs w h i c h caused the drop i n s o d i u m flow were p r i n c i p a l l y i n the s o d i u m tubes of the evaporator unit of the generator w h i c h was d o w n s t r e a m of, and i n series w i t h , the superheater. A b o u t 75% of the tubes i n this u n i t were cleared without cutting open the evaporator. T h e r e m a i n i n g 2 5 % , however, h a d to be opened b y treating each tube i n d i v i d u a l l y w i t h a steam lance. E x a m i n a t i o n p r i o r to cleaning of the plugs i n the latter tubes showed the plugs to be p r i m a r i l y composed of s o d i u m h y d r o x i d e a n d / o r s o d i u m oxide. A s the steam generator was b e i n g operated i s o t h e r m a l l y at 700 °F. without water w h e n the flow stoppage occurred, the s t e a m - t o - m e r c u r y b a r r i e r must h a v e failed i n i t i a l l y without detection, d u r i n g previous steaming operations. T h i s leak w o u l d then have a l l o w e d steam to condense i n the m e r c u r y system a n d f o r m a n undetected water l a y e r i n the m e r c u r y - s y s t e m expansion tank. T h e i n i t i a l f a i l u r e was p r o b a b l y not detected because of a fault i n the l e a k - d e t e c t i o n a l a r m system. T h e subsequent leak i n the s o d i u m - m e r c u r y b a r r i e r then a l l o w e d the water c o l lected i n the m e r c u r y system to enter the s o d i u m . T h e only m e t h o d available to estimate the rate of leakage for this f a i l u r e is based on the size of the m e r c u r y - t o - s o d i u m crack s h o w n i n F i g u r e 7. A s i n the

STEAM

Figure 6. Superheater unit tested at MSA


A D V A N C E S IN

100

CHEMISTRY SERIES


calculation for the n a t u r a l c i r c u l a t i o n evaporator, it was assumed that the crack acted as an orifice a n d that the fluid flowing t h r o u g h the crack was l i q u i d water. T h e m a x i m u m leak rate was calculated i n this m a n n e r to be about 200 pounds per m i n u t e . T h e average leak rate w o u l d be expected to be less t h a n this v a l u e . In the absence of definite t h i r d - f l u i d system pressure data, it was assumed i n the calculation that the t h i r d - f l u i d system was at its n o r m a l pressure of 300 pounds per square i n c h gage. T h e authors estimated that the total amount of water w h i c h entered the s o d i u m system was about 6 pounds. A g a i n the value of m a x i m u m leakage rate is p r o b a b l y h i g h , a l t h o u g h this calculation is felt to be more accurate t h a n the orifice flow c a l c u l a t i o n for the n a t u r a l c i r c u l a t i o n evaporator, since g e o m etry of the crack was simpler, thus p e r m i t t i n g a more accurate estimate of the flow area. T o s u m m a r i z e the experience gained, it was f o u n d in the case of the n a t u r a l c i r c u l a t i o n evaporator that a s m a l l leak d i d not cause the system to p l u g w h e n

Figure 7.

Failure in superheater unit


101


p u m p i n g p o w e r was available a n d that c o n t i n u e d operation was possible i n the presence of the leak. A l s o , the leak rate was a p p a r e n t l y r e d u c e d over that possible for l i q u i d water either b y the presence of steam w i t h the w a t e r o r b y p a r t i a l p l u g ging of the c r a c k . F o r a leak of moderate size, as i n the superheater u n i t , loss of s o d i u m flow d i d occur a n d c o n t i n u e d operation i n the presence of the leak was not possible. H o w e v e r , i n none of the leaks observed i n operating e q u i p m e n t to date has there b e e n evidence of a c h e m i c a l reaction of sufficient violence to p r e sent a danger to operating p e r s o n n e l or adjacent equipment. I n the case of a l l three leaks, the s o d i u m - w a t e r reaction of itself was not vigorous enough to i n dicate the leak, b u t r a t h e r i n each case the l e a k was detected i n d i r e c t l y b y the p l u g g i n g effects of the insoluble reaction products.


Proposed Design Concepts O n the basis of the reaction studies completed a n d the operating experience a c q u i r e d to date, it is n o w possible to develop a n e w philosophy i n the design a n d operation of s o d i u m - w a t e r heat-transfer systems. R e a c t i o n studies have i n d i c a t e d that pressures i n a c i r c u l a t i n g system for a p a r t i c u l a r leak rate do not n o r m a l l y exceed those p r e d i c t e d b y R e a c t i o n 1. W h e r e pressure peaks have o c c u r r e d i n c i r c u l a t i n g systems, they have been of v e r y short d u r a t i o n a n d not of serious c o n sequence. N e i t h e r pressure rises n o r temperature rises have caused m e c h a n i c a l damage i n those cases w h e r e leaks have o c c u r r e d i n operating heat exchangers. A l t h o u g h substantiation i n a large scale demonstration is still r e q u i r e d , it w o u l d n o w appear that the designer c a n , w i t h reasonable assurance, e m p l o y elementary tools i n designing a s o d i u m - w a t e r heat-transfer system to p r e v e n t serious m e c h a n i c a l damage s h o u l d leakage occur. A s noted above, the amount of gas e v o l v e d f r o m a t y p i c a l s o d i u m - w a t e r leak w i l l n o r m a l l y be somewhat less t h a n that i n d i c a t e d b y R e a c t i o n 1. B y c o m b i n i n g the stoichiometry of this reaction w i t h the i d e a l gas l a w , a conservative v a l u e of the r e s u l t i n g pressure rise c a n be a p p r o x i m a t e d b y Δ Ρ =

kl

(

) RT

2 χ 18 χ 60 where

(4)

~y

Δ Ρ = system pressure rise, pounds p e r square i n c h L = average leak rate, pounds p e r m i n u t e (V

2

t = time,

mole H / m o l e H 0 ) 2

8

seconds

R = gas constant, 10.73 ( l b . / s q . inch)

(cu. f t . ) / l b . m o l e ° R .

Τ = temperature, ° R . V = surge v o l u m e , c u . feet A s e m p l o y e d i n these derivations, L represents a n average constant leak rate. I n actual practice, the leak rate w i l l v a r y w i t h the differential pressure across the leak. N o r m a l l y the use of a constant v a l u e of L s h o u l d p r o v e adequate. I n a p a r t i c u l a r application, however, the designer m a y w i s h to consider the decrease i n L w i t h rise i n s o d i u m pressure. E q u a t i o n 4 assumes a constant gas v o l u m e i n the s o d i u m system a n d the instantaneous transmission of pressure throughout the system. A s s u m i n g a n average system temperature of 700°F., E q u a t i o n 4 reduces to: L A Ρ — 5.76 — t (5) V E q u a t i o n 5 is plotted for several values of L/V i n F i g u r e 8. I n system design, a plot of this type c a n serve to determine the surge v o l u m e r e q u i r e d to protect the s o d i u m system f r o m exceeding design pressures. T o use the plot, first select the m a x i m u m allowable system pressure a n d the system pressure w h i c h w o u l d




102

0

10

20

30

40

50

60

70

80

90

WO HO

120 130 140 150

TIME * SEC.

Figure 8. Rate of system pressure rising resulting from leakage of water into sodium

c l e a r l y indicate a n i n - l e a k a g e of water. T h e latter pressure, c a l l e d the a l a r m p r e s sure, s h o u l d be safely above a n y transient pressures w h i c h m i g h t occur i n n o r m a l system operation. T h e difference between these two pressures determines the total time available for relief of the l e a k - i n d u c e d pressure b u i l d - u p . W h i l e relief c a n be accomplished i n a n u m b e r of ways, perhaps the most s t r a i g h t f o r w a r d is the r e l i e f of pressure f r o m the water side ( d o w n to the s o d i u m side p r e s s u r e ) , essentially stopping f u r t h e r leakage. A n alternative method w i t h some advantage involves direct r e l i e f of the s o d i u m system. U s i n g the difference i n the two pressures d e t e r m i n e d above, as w e l l as the total time r e q u i r e d for the control system to f u n c t i o n a n d for the water side p r e s sures to be r e l i e v e d , the designer c a n refer to a plot of the type of F i g u r e 8 a n d determine the m a x i m u m v a l u e of L/V to be designed into his system. T h e v a l u e of L selected w i l l d e p e n d p r i n c i p a l l y o n the design of the heat exchanger, a n t i c i pated operating conditions, the f r e q u e n c y of inspection, a n d , if available, e x p e r i ence w i t h s i m i l a r equipment. O n c e a conservative v a l u e of L is chosen, the n e c essary surge v o l u m e , V , is selected to give the r e q u i r e d L/V ratio. T h e above process c a n be reversed, first selecting Δ Ρ a n d L/V a n d then designing the control system a n d relief system to f u n c t i o n w i t h i n the r e q u i r e d time. T h e significance to be attached to use of the above type of analysis is twofold. F i r s t , suffiicient data have n o w been collected to indicate that explosive reactions w i t h energetic shock waves do not occur i n t y p i c a l heat exchanger failures. S e c o n d l y , experience a c q u i r e d i n recent years n o w provides a basis for estimating the rate of leakage w h i c h m i g h t be expected i n a p a r t i c u l a r design of s o d i u m - w a ter heat exchanger. It is expected this basis w i l l be extended b y future e x p e r i ence. T h e temperature rise r e s u l t i n g f r o m water leakage into a c i r c u l a t i n g s o d i u m system m a y be considered b y c o m p a r i n g the temperature effect w i t h the pressure effect. A s s u m i n g that a l l heat generated b y leakage f r o m water into s o d i u m is that due to R e a c t i o n 1, the rise i n temperature of the s o d i u m system is a p p r o x i m a t e d b y A

T

=

QL/60t Wc

where

Δ Τ = Q = L = t = W = c =

(6)

s o d i u m temperature rise, ° F . heat of reaction, 4,550 B . t . u . / l b . P L O leak rate, pounds p e r m i n u t e time, seconds weight of s o d i u m charge, pounds specific heat of s o d i u m , 0.3 B . t . u . / l b . - ° F .

E q u a t i o n 6 reduces to: (7)



103

T h i s a p p r o x i m a t i o n is based o n the assumption that s o d i u m c i r c u l a t i o n rates are sufficiently h i g h to distribute generated heat u n i f o r m l y throughout the s y s tem. E x c e p t i n the case of n o n c i r c u l a t i n g systems, this assumption is m o r e t h a n counterbalanced b y the fact that system heat losses a n d the heat capacity of sys tem p i p i n g a n d components have not been considered. W h i l e s h o r t - l i v e d , localized temperature excursions i n excess of that calculated m a y occur a n d m a y be d e t r i m e n t a l to the extended operation of a system, immediate failure w o u l d not be expected. C o m b i n i n g E q u a t i o n s 5 a n d 7, w e find that V = 43.1— Ρ W

ΔΤ


A

(8)

A n average system operating temperature of 700 °F. was assumed i n f o r m u l a t i n g E q u a t i o n 5. R i g o r o u s l y , E q u a t i o n 8 takes the f o r m , Δ Τ / Δ Ρ = 49,900/Tav. W W , w h e r e T . = operating system plus one h a l f the temperature rise d u e to leakage. A s V/W w i l l generally range f r o m a v a l u e of 0.002 to 0.01 cubic foot p e r p o u n d (10 to 5 0 % surge v o l u m e ) , the ratio of Δ Τ / Δ Ρ w i l l range between 0.0862 a n d 0.431. A relationship such as E q u a t i o n 8 c a n be used to estimate the temperature effects w h i c h w i l l result w i t h a corresponding pressure incident. I n most cases a system adequately protected f r o m overpressure w i l l also be protected f r o m o v e r t e m p e r a ture. a v

W h i l e temperature a n d pressure effects have generally been considered to be the most severe results of a w a t e r - t o - s o d i u m leak, recent experience indicates that the most substantial problems w h i c h result are i n v o l v e d w i t h system p l u g g i n g . Because of the difficulty i n differentiating between the various reaction products, it is not k n o w n whether p l u g g i n g is caused b y one of the reaction products or b y some combination of them. C o n s i d e r a b l e i n f o r m a t i o n is available on the p l u g g i n g characteristics of s o d i u m oxide i n sodium. Less is k n o w n , however, about the p l u g g i n g characteristics of s o d i u m h y d r o x i d e a n d s o d i u m h y d r i d e . F o r this reason a n d also because the solubilities of these three reaction products i n s o d i u m are s i m i l a r f r o m 400° to 800 °F., p l u g g i n g effects are discussed i n terms of s o d i u m oxide concentration. T h e concentration of o x y g e n - b e a r i n g components i n s o d i u m is n o r m a l l y e x pressed as weight per cent o x y g e n . T h e concentration of o x y g e n i n s o d i u m r e s u l t i n g f r o m water leakage c a n be expressed b y

C = where

C L t W

= = = =

-

8

J ° - 100% w

(9)

weight % o x y g e n leak rate, pounds p e r m i n u t e time, seconds weight of s o d i u m charge, pounds

Setting C = 0.016%, the s o l u b i l i t y of s o d i u m oxide at 600°F., w e get W L = 0.0108 — t

(10)

E q u a t i o n 10 relates water leak rate to the time to reach s o d i u m oxide saturation at 600°F. T h i s relationship is plotted i n F i g u r e 9 for two values of W . It is signifi cant to note that a n y large leak w i l l cause p l u g g i n g conditions i n just a matter of seconds, e v e n i n a large system. C o m b i n i n g E q u a t i o n s 4 a n d 10, a n d assuming a n average system temperature of 700 °F., we find W A P = 0.0622 — (11) V Inspection of this relationship, w h i c h holds for a n y size leak, indicates that a system w i t h 10% surge v o l u m e w o u l d become saturated w i t h o x y g e n w h e n the pressure rise r e s u l t i n g f r o m a water leak reached a little over 30 pounds p e r square



104 'ΟΟ-ϋ


90-fl

; 0

ι 20

1 40

1 60

1 80

1 100

1 120

1 140

TIME - SEC.

Figure 9. Time required to reach oxygen saturation vs. leakage of water into sodium

inch. A c o r r e s p o n d i n g l y smaller pressure rise w o u l d occur i n systems w i t h l a r g e r surge volumes. T h e equation is suited to use f o r systems w i t h a n average t e m p e r a ture of 700°F. a n d a c o l d l e g temperature of 600°F. S i m i l a r expressions c a n be developed f o r a n y other temperature conditions. T h e r a p i d rise i n o x y g e n concentration w h i c h results f r o m water leakage i m mediately suggests the use of a p l u g g i n g indicator as a means of leak detection i n h i g h - t e m p e r a t u r e systems. A c t u a l l y , most of the leaks encountered i n the past were first signaled b y flow restrictions. I n designing specifically f o r leak detection, however, a special p l u g g i n g indicator should be installed i n a s m a l l , continuously operating bypass d o w n s t r e a m f r o m the heat exchanger. T h e p l u g g i n g disk s h o u l d be m a i n t a i n e d at system temperature where it w i l l not be affected b y n o r m a l c o n centrations of s o d i u m oxide, p r o v i d i n g the latter is generally c o n t r o l l e d w e l l b e l o w saturation. B a s e d o n present knowledge, a n operating temperature for this type indicator above the m e l t i n g point of s o d i u m h y d r o x i d e c o u l d not be r e c o m m e n d e d . D a t a presented above f o r the 1-inch p u m p e d loop test p r o v e the effectiveness of an indicator of this type at temperatures of 500 °F. a n d below.

Conclusions T h e reaction between s o d i u m a n d water i n p r a c t i c a l heat-transfer systems w i l l not generally result i n m e c h a n i c a l damage to the system. T h e l a c k of damage is p r o b a b l y due i n large part to two things: the absence of shock waves f r o m the s o d i u m - w a t e r reaction p e r se, a n d the relative smoothness of the reaction at n o r m a l h e a t - e x c h a n g e r temperatures. T h e m a x i m u m pressure e x p e r i e n c e d f o r a n y significant d u r a t i o n of time w o u l d n o r m a l l y be no greater t h a n the water p r e s sure a n d the temperature effects w o u l d be r e l a t i v e l y m i n o r . S y s t e m p l u g g i n g a n d subsequent problems i n the r e m o v a l of the reaction products m a y occur, however, if the leak rate is sufficiently large. These conclusions are supported b y the r e sults of test w o r k at K A P L a n d other laboratories o n the injection of water into s o d i u m a n d b y experience gained f r o m failures i n operating heat transfer systems. C r i t e r i a presented i n E q u a t i o n s 4 a n d 8 c a n be used to design the system to w i t h s t a n d the pressure a n d temperature effects of the s o d i u m - w a t e r reaction. T h e use of these criteria requires a reasonable estimate of the m a x i m u m leak rate expected. T h e experience gained w i t h the s o d i u m - w a t e r reaction has also indicated the desirability of some design considerations i n order to m i n i m i z e the effects of l e a k age i n a s o d i u m h e a t - t r a n s f e r system. T h e considerations a r e : P r o v i d e the system w i t h adequate v e n t i n g capacity. P r e f e r a b l y this v e n t i n g capacity s h o u l d be p r o v i d e d o n the h i g h - p r e s s u r e side ( n o r m a l l y the water side) to p r o v i d e for r a p i d equalization of the pressure across the leak, thus essen t i a l l y stopping the leakage.



ADAMS, BARENBORG A N D KENDALL—SODIUM

WATER REACTION

IN HEAT TRANSFER

105

W h e r e feasible, design the low-pressure s i d e (usually t h e s o d i u m s i d e ) of the heat exchange system for n o n r u p t u r e at the m a x i m u m pressure of the h i g h pressure side. N o r m a l l y this n o n r u p t u r e design should not be based o n the same factor of safety used i n designing for n o r m a l operating conditions. O f t e n this n o n r u p t u r e condition w i l l be automatically achieved t h r o u g h code design of the s y s t e m for n o r m a l operating conditions or b y the use of standard components w h i c h are overdesigned for n o r m a l operating conditions. T h e i n f o r m a t i o n discussed above warrants serious consideration of the use of s i n g l e - w a l l heat exchangers i n s o d i u m heat-transfer systems. C o n s i d e r a t i o n of the use of s i n g l e - w a l l exchangers should include a t h o r o u g h e x a m i n a t i o n of a l l possible consequences to the system a n d its components i n the event of a h e a t exchanger leak. It m a y be desirable to use several s m a l l heat exchangers r a t h e r t h a n a single unit, so that a leak i n one u n i t w o u l d not r e q u i r e complete s h u t d o w n of the entire heat-transfer system. If it is d e t e r m i n e d that s i n g l e - w a l l heat e x changers are permissible, their use w i l l result i n a r e l a t i v e l y simple, s m a l l , a n d l o w - c o s t unit. T h i s simplified design m a y result i n a h i g h l y r e l i a b l e piece of e q u i p ment.

Literature Cited (1) (2) (3) (4) (5) (6) (7) (8)

J a c k s o n , C . B . , "Liquid M e t a l H a n d b o o k , S o d i u m - N a K S u p p l e m e n t , " 3rd ed., U. S. G o v ernment P r i n t i n g Office, Washington 25, D . C . , 1955. K a r n e s , H. F., private c o m m u n i c a t i o n . K i n g , E. C., " R e a c t i o n of N a K a n d H2O," M i n e Safety A p p l i a n c e s C o . , TR-XI (Sept. 7, 1951). Ibid., T R - X I I ( J a n . 30, 1952). K i n g , E. C., Wedge, C . Α . , Jr., " R e a c t i o n of NaK a n d H2O," M i n e Safety A p p l i a n c e s C o . , T R - I I I (Feb. 1, 1950). Ibid., T R - V I I ( N o v . 1, 1950). M o r a b i t o , J. J., S h a n n o n , R. H., P a p e r 55-A-189, ASME A n n u a l M e e t i n g , N o v . 13-18, 1955. T i d b a l l , R . Α . , Progress Reports 25 a n d 26, M i n e Safety A p p l i a n c e s C o . , October 1954 to J a n u a r y 1955.


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