calor decaimiento kastenberg

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Nu c le a r En g in e e rin g a n d De s ig n /F u s io n 1 ( 1 9 8 4) 5 1 - 6 0 5 1

No r th - Ho l l a n d , Ams te r d a m

O N D E S I G N C R I T E R I A F O R A F T E R H E A T A N D D E C A Y H E A T R E M O V A L I N F U S I O N A N D

F U S I O N - F I S S I O N P O W E R P L A N T S

W i l l i a m E . K A S T E N B E R G

School of Engineering and Ap plied Science Universi ty of Cali fornia Los Angeles Cali fornia 90024 USA

Received May 1983

Af te rhea t and decay hea t r emo val sys tems wil l be imp or tan t sa fe ty fea tures in fusion and fus ion- f is s ion hybr id power p lan ts.

Decay heat removal considerations for f ission power reactors are reviewed and design cr iter ia discussed. Aspects of fusion and

fus ion- f is s ion a f te rhea t and decay hea t r emoval a re a lso rev iewed. I t was found tha t a f te rhea t thermal loads in fus ion power

reactors are a factor of 5 to 10 less than decay heat loads in f ission power plants . However, they remain relatively constant over

time periods of interest (out to a year in designs employing stainless s teel) , as compared to f ission plants , whose decay heat

loads drop an order of m agni tude dur ing the f i rs t day .

Dete rm inis t ic c r i te r ia for a f te rhea t r emo val a re presented . Altho ugh based upon f is s ion reac tor experience , they a re modif ied

to account for these d if fe rences. A prob abi l is t ic c ri te r ion was deve loped which is based o n pub l ic hea l th and econom ic

cons idera t ions . A goa l of 15 10 -6 per r eac tor y ear was es tab l ished for a f te rhea t r emo val in fus ion .

For fus ion- f is s io n hybr id reac tor s, a goa l r anging be tw een 6 x 10 -6 and 30 10 -6 per r eac tor year was es tab lished . The

upper l imit cor responds to the recent NR C safe ty goa l qu ide l ine for core mel t f r equency (1 x 10-4 /yea r ) , which has been

suggested for the decay heat removal function in Pressurized Water Reactors. These goals are to be applied to a reliability

analysis where m ajor uncer ta in t ies can be quant i f ied .

1 . I n t r o d u c t i o n

A n i m p o r t a n t p h e n o m e n o n o f p o w e r p l a nt s u s in g

n u c l e a r f u e l i s t h a t t h e y c o n t i n u e t o p r o d u c e e n e r g y a t

l o w l e v e l s a f t e r t h e n u c l e a r r e a c t i o n s c e a s e . B e c a u s e o f

t h is p h e n o m e n o n , s y st e m s a n d / o r f e at u r es m u s t b e

i n c o r p o r a t e d i n t o t h e d e si g n o f t h e s e p l a n t s t o r e m o v e

t h i s c o n t i n u e d p r o d u c t i o n o f e n e rg y , c a l le d d e c a y h e a t

i n f i s s i o n r e a c t o r s a n d a f t e r h e a t i n f u s i o n r e a c t o r s .

A l t h o u g h f i s s io n p o w e r r e a c t o r s h a v e b e e n b u i l t a n d

o p e r a t e d f o r o v e r 25 y e a r s, t h e s u b j e c t o f d e c a y h e a t

r e m o v a l i s a v e r y a c t i v e d e s i g n a n d r e g u l a t o r y i s s u e [ 1 ] .

I n a f i s s i o n p l a n t , d e c a y h e a t i s p r i m a r i l y g e n e r a t e d

b y t h e r a d i o a c t i v e d e c a y o f t h e f i s s i o n p r o d u c t s a n d

s e c o n d a r i l y b y t h e d e c a y o f a c t i n i d e s a n d t h e i n d u c e d

a c t i v i t y i n t h e s t r u c t u r a l m a t e r i a l . I n a f u s i o n p l a n t ,

a f t e r h e a t i s p r i m a r i l y g e n e r a t e d b y t h e i n d u c e d a c t i v i t y

i n t h e b l a n k e t , e . g . t h e f i r s t w a l l , t h e m a g n e t s h i e l d s ,

r e f l e c to r s a n d t h e o t h e r s t r u c t u r a l m a t e r i a l . I n d u c e d

r a d i o a c t i v i t y i n e i t h e r r e a c t o r i s p r i m a r i l y d u e t o n e u -

t r o n i n t e r a c t i o n w i t h r e a c t o r c o m p o n e n t s . T h e i m p o r -

* Work per formed while the au thor was a consul tan t to EPRI .

t a n c e o f d e c a y h e a t o r a f t e r h e a t r e m o v a l f o l l o w i n g

n o r m a l a n d o f f - n o r m a l o p e r a t i o n o f a p l a n t is t w o f o l d :

t o p r o t e c t t h e e c o n o m i c i n v e s t m e n t b y i n s u r i n g t h e

s t r u c t u r a l i n t e g r i t y o f t h e r e a c t o r , a n d t o p r o t e c t t h e

h e a l t h a n d s a f e ty o f t h e p u b l i c b y p r e v e n t i n g t h e r e l e a se

o f r a d i o a c t i v e m a t e r i a l .

T h e o b j e c t i v e s o f t h i s p a p e r a r e a s f o l l o w s : ( 1 ) t o

r e v i e w t h e s t a t u s o f d e c a y h e a t r e m o v a l f o r f i s s i o n

p o w e r r e a c t o r s , ( 2 ) t o d i s c u s s t h e a f t e r h e a t a n d d e c a y

h e a t r e m o v a l q u e s t i o n f o r fu s i o n a n d f u s i o n - f i s s i o n

h y b r i d p o w e r r e a c t o rs , a n d ( 3) t o d e v e l o p s o m e g u i -

d a n c e a n d c r i t e r i a f o r d e a l i n g w i t h t h e s e i s s u e s f o r

f u s i o n a n d f u s i o n - f i s s i o n h y b r i d p o w e r re a c t o r s .

A l t h o u g h g e n e r a l d e s i g n c r i t e r i a e x i s t f o r f i s s i o n r e -

a c t o r s [ 2 ] , t h e r e s u l t s o f s e v e r a l s t u d i e s s h o w t h a t i m -

p r o v e m e n t s i n d e c a y h e a t r e m o v a l c a p a b i l i t y c a n s i g n i fi -

c a n t l y r e d u c e p u b l i c r i s k [ 3 - 5 ] . M o r e o v e r , w i t h t h e

a d v e n t o f q u a n t i t a t i v e s a f e ty g o a l s f o r f i s si o n p l a n t s

[ 6 , 7 ] , t h e r e i s a l s o a d e s i r e t o a u g m e n t t h e s e d e t e r m i n i s -

t i c c r i t e r i a w i t h p r o b a b i l i s t i c c r i t e r i a [ 8 - 1 0 ] .

F o r f u s i o n a n d f u s i o n - f i s s i o n h y b r i d p o w e r p la n t s , a

n u m b e r o f c o n c e p t u a l d e s i g n st u d i e s h a v e b e e n e m -

p l o y e d t o e x p l o r e t h e r e l a t iv e m e r i t s o f c a n d i d a t e b l a n -

0 0 2 9 - 5 4 9 3 / 8 4 / 0 3 . 0 0 E l s e v i e r S c i e n c e P u b l i s h e r s B . V .

( N o r t h - H o l l a n d P h y s i c s P u b l i s h i n g D i v i s i o n )


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W E Kastenberg / Design criteria for afterheat and decay heat removal

k e t m a t e r i a l s w i t h r e s p e c t t o i n d u c e d a c t i v i t y , b i o l o g i c a l

h a z a r d p o t e n t i a l a n d a f t e r h e a t [ 1 1 - 1 5 ] . I t i s a g o a l o f

t h i s p a p e r t o i n i t i a t e f u r t h e r c o n s i d e r a t i o n o f t h i s e s s e n -

t i a l i s s u e a s f u s i o n a n d f u s i o n - f i s s i o n h y b r i d s m o v e

f r o m t h e c o n c e p t u a l s t a g e t o t h e e n g i n e e r in g d e s i g n a n d

d e v e l o p m e n t s t a g e .

2 De c a y h e a t r e mo v a l in f i s s io n r e a c to r s

U n d e r n o r m a l o p e r a t i o n , t h e e n e r g y p r o d u c e d i n a

l i g h t w a t e r r e a c to r L W R ) i s re m o v e d a s p r e s s u r iz e d

w a t e r o r s t e a m t o p r o d u c e e l e c t ri c i ty v i a t h e t u r b i n e

g e n e r a t o r . F o l l o w i n g r e a c t o r s h u td o w n , t h e r e a c t o r p r o -

d u c e s i n s u f f i c i e n t p o w e r t o o p e r a t e t h e t u r b i n e . T h e r e -

f o r e o t h e r m e a s u r e s m u s t b e a v a i l a b l e t o r e m o v e d e c a y

h e a t t o e n s u r e t h a t h i g h t e m p e r a t u r e s a n d p r e s s u r e s d o

n o t d e v e l o p w h i c h c o u l d p o s e a t h r e a t t o t h e r e a c t o r .

T h e s e m e a s u r e s h a v e a s t h e i r f u n c t i o n a l r e q u i r e m e n t s :

1 ) p r o v i d i n g a m e a n s o f t r a n s f e r r i n g d e c a y h e a t f r o m

t h e r e a c t o r c o o l a n t s y s te m t o a n u l t i m a t e h e a t s i n k a n d

2 ) m a i n t a i n i n g s u f f i c i e n t w a t e r i n v e n t o r y i n s i d e t h e

r e a c t o r t o e n s u r e t h a t t h e r e a c t o r c o o l a n t a d e q u a t e l y

c o o l s t h e r e a c t o r f u e l.

P r e s s u ri z e d w a t e r r e a c t o rs P W R s ) g e n e r a l l y h a v e

t h r e e m e a n s o f r e m o v i n g d e c a y h e a t : t h e a u x i l i a r y

f e e d w a t e r s y s t e m , t h e r e s i d u a l h e a t r e m o v a l s y s t e m a n d

t h e t u r b i n e b y p a s s s y s t e m . R e a c t o r w a t e r i n v e n t o r y i s

m a i n t a i n e d b y t h e h i g h p r e s s u r e s a fe t y i n j e c ti o n s y s t e m

o r t h e c h e m i c a l a n d v o l u m e c o n t r o l s y s t e m [ 5 ] .

B o i l i n g w a t e r r e a c to r s B W R s ) u s u a l l y c o m b i n e t h e s e

t w o f u n c t i o n s w i t h t h e f o l l o w i n g s y s t e m s : t h e r e s i d u a l

h e a t r e m o v a l s y s t e m , t h e r e a c t o r c o r e i s o l a t io n c o o l i n g

s y s t e m R C I C ) a n d t h e t u r b i n e b y p a s s sy s t e m . T h e

R C I C s y s t em o p e r a t e s a s a h i gh p r e s s u r e s y st e m , a n d i t

i s b a c k e d u p b y a h i g h p r e s s u r e c o r e s p r a y s y st e m . I n a

B W R , t h e r e si d u a l h e a t r e m o v a l sy s t e m c a n o p e r a t e i n a

n u m b e r o f d i f fe r e n t m o d e s f o r b o t h l o w p r e s s u re i n j ec -

t i o n a n d d e c a y h e a t r e m o v a l [ 5 ] .

A r e c e n t s t u d y [ 1 6 ] d e s c r i b e s m a n y o f t h e d e s i g n

v a r i a t i o n s t h a t h a v e e v o l v e d i n t h e U . S . a n d a b r o a d f o r

P W R a n d B W R d e c a y h e a t r e m o v a l sy s t em s . In g e n e r a l ,

t h e s e v a r ia t i o n s i n v o l v e t h e n u m b e r o f c o m p o n e n t s o r

e q u i p m e n t t r a i n s e m p l o y e d t o m a k e u p t h e d e c a y h e a t

r e m o v a l s y s t e m s . I n t h e U n i t e d S t a t e s a n d a b r o a d , t h e

f o l l o w i n g d e t e r m i n i s t i c d e s i g n c r i t e r i a a r e u s e d f o r d e -

c a y h e a t r e m o v a l s y s t e m s [ 2 ]:

1 ) E n s u r e t h a t f u e l i n t e g r i ty a n d p r e s s u r e

b o u n d a r i e s a r e m a i n t a i n e d .

2 ) W i t h s t a n d f ir e , sa b o t a g e , n a t u r a l p h e n o m e n a ,

a n d o t h e r e x t r e m e c o n d i t i o n s .

3 ) O p e r a t e u n d e r n o r m a l a n d e m e r g e n c y p o w e r

c o n d i t i o n s .

4 ) P r o v i d e m a n u a l b a c k u p c o n t r o l c a p a b i l i t y fo r

a u t o m a t i c s y s t e m s .

5 ) M o n i t o r a n d m a i n t a i n r e a c t o r c o o l a n t p r e s s u r e

b o u n d a r y t h r o u g h i n s p e c t i o n , l e a k d e te c t i o n , a n d

i s o l a t i o n v a l v i n g .

6 ) F u n c t i o n d e s p i t e t h e s i n g l e f a i l u r e o f a n a c t i v e

c o m p o n e n t o r t h e o c c u r r e n c e o f s m a l l p i p e

b r e a k s .

7 ) P r e v e n t sh a r e d n o r m a l o r e m e r g e n c y e q u i p m e n t

f r o m j e o p a r d i z i n g r e l ia b l e s a f e ty o p e r a t i o n s .

8 ) P r o v i d e u n i n t e r r u p t e d c o o l i n g fo r t h i r ty d a y s .

S e v e r a l n o n - U . S , c o u n t r i e s h a v e a d d i t i o n a l c r i t e r i a :

9 ) I n i t i a t e a n d o p e r a t e a u t o m a t i c a l l y f o r a p e r i o d

r a n g i n g f r o m 1 0 m i n u t e s t o 1 0 h o u r s .

1 0 ) F u n c t i o n d e s p i t e t h e s i n g l e f a i l u r e o f a n a c t i v e

o r p a s s iv e c o m p o n e n t i n c o m b i n a t i o n w i t h a

m a i n t e n a n c e o u t a g e i n v o l v i n g a r e d u n d a n t s y s-

tem.

1 1 ) O p e r a t e w i t h o u t o n - s i t e r e p a i r a c t i o n f o r a t le a s t

1 2 h o u r s a n d w i t h o u t o f f s i t e r e p a i r a c t i o n f o r a t

l e a s t 4 8 h o u r s .

M o r e d e t a i l e d s y s t e m i n f o r m a t i o n s u c h a s f l ow r a t e s

a n d d e c a y h e a t l e v el s , n u m b e r o f t r a in s a n d p o w e r

s u p p l i e s a r e g i v e n i n p l a n t s a f e t y a n a l y s i s r e p o r t s S A R s ) .

H o w e v e r , m u c h o f t h i s in f o r m a t i o n p r e s e n t s d e s ig n d e -

s c r i p t i o n s r a t h e r t h a n c r i t e r i a f o r a d e s i g n .

T h e c u r r e n t r e g u l a t o r y i n t e re s t i n d e c a y h e a t r e m o v a l

c o m e s a s a r e s u l t o f t h e R e a c t o r S a f e t y S t u d y [ 1 7 ] w h i c h

f o cu s e d o n t h e S u r ry P W R a n d P e a c h b o t t o m B W R

w h o s e d e c a y h e a t r e m o v a l s y s t e m s w e r e d e s i g n e d i n

a c c o r d a n c e w i t h t h e g e n e r a l d e s i g n c r i t e r i a f o r U . S .

p l a n t s l i s t e d a b o v e . A s n o t e d b y B e r r y a n d S a n d e r s [ 5],

t h e f o l l o w in g o b s e r v a t i o n s c an b e m a d e f o r S u r r y a

P W R ) :

1 ) T r a n s i e n t s a n d c e r t a i n s m a l l l o s s o f c o o l a n t a c c i -

d e n t s L O C A s ) p o s e t h e h i g h e s t p r o b a b i l i t y f o r

c o r e m e l t d o w n .

2 ) O f a l l t r a n s i e n t s a n d s m a l l L O C A s , t h o s e i n v o l v -

i n g t h e f a i l u r e o f h i g h p r e s s u r e i n j e c t i o n a n d

a u x i l i a r y f e e d w a t e r s y s t e m s f o r d e c a y h e a t r e -

m o v a l ) p o s e t h e h i g h e s t p r o b a b i l i t y fo r c o r e

m e l t d o w n .

S i m i l a r l y t h e f o l lo w i n g o b s e r v a t io n s c a n b e m a d e f o r

P e a c h b o t t o m a B W R ) :

1 ) T r a n s i e n t s , t o g e t h e r w i t h f a i l u r e o f t h e r e s i d u a l

h e a t r e m o v a l , s y s te m , o r t h e r e a c t o r p r o t e c t i o n

s y s te m , p o s e t h e h i g h e s t p r o b a b i l i t y f o r c o r e

m e l t d o w n .

R e c o g n i z i n g t h e i m p o r t a n c e o f d e c a y h e a t r e m o v a l in

p r e v e n t i n g c o r e m e l t d o w n , E b e r s o l e a n d O k r e n t [3 ] w e r e

t h e f i r s t t o p r o p o s e a n a l t e r n a t i v e a p p r o a c h w h i c h i n -

c l u d e d d e d i c a t e d s y s te m s t h e s y st e m s d e s c r i b e d a b o v e


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53

have other functio nal requirements as well as decay heat

removal), dedicated rather than shared power supplies

and was bunke red (i.e., especially protected from severe

internal and external hazards such as a turbine blade

missile, tornadoes, floods and sabotage).

Quantitative estimates of potential risk reduction

with improved decay heat removal has been the subject

of inves tigation at Sandia Nat iona l Laborato ry [5,16].

Thei r work can be summarized as follows:

(1) For the three-train Surry high pressure system

and auxiliary feedwater systems, system reliability im-

provements of as much as a factor of ten will result in

less than a factor of two decrease in overall core melt-

down frequency.

(2) For the two train Peachbottom residual heat

removal system, improvement s of as much as a factor of

ten in the reliability of either the residual heat removal

and high pressure service water systems, or the reactor

protection system will result in only about a factor of

two decrease in overall core meltdown frequency.

(3) In PWR's having two installed decay heat re-

moval trains, estimated reductions in coremelt probabil-

ity of at least a factor of ten were attained with the

addition of an auxiliary feedwater or high pressure

injection train.

(4) For BWRs, an add-on single train suppression

pool cooling/low pressure injection system gains a fac-

tor of about six reduction in coremelt probability.

.As noted in item 10 under design criteria, a philoso-

phy has evolved in several European countries that

requires safety systems to be able to sustain both a

random failure of one train a nd a simultaneous maintai-

nence outage of another train, and still retain 100%

operational capacity. This philosophy was not adopted

for improving decay heat removal reliability nor for

reducing the probability of coremelt. Rather, it was

adopted over a concern for special emergencies (e.g.,

airplane crash and explosive pressure waves, etc.), char-

acterized by low frequency. Hence, effort has been

placed on p roviding three, four and sometimes six trains

for decay heat removal, even though the safety benefits

tend to decrease in a probabi listic sense beyond three

trains.

It is important to recognize that the Reactor Safety

Study, as well as the Sandia studies, are based upon

probabilistically evaluated events which do not include

special emergencies (e.g., plane crashes, severe floods,

tornadoes, etc.). The occurrence frequency and signifi-

cance of special emergencies are not easily predicted,

and hence are difficult to quantify. On the other hand,

recent Probabilistic Risk Assessments (PRAs) for the

Zion [18] and Indi an Point [ 19] power plants have

treated severe external events parametrically and de-

duced that while they do not dominate coremelt proba-

bility, they do contri bute significantly to risk.

Before turning to the question of afterheat in fusion

and fusi on/f issi on hybrid reactors, it is of interest to

ment ion decay heat removal in liquid metal, fast breeder

reactors (LMFBRs) and in gas cooled reactors (GCRs).

The decay heat removal system for the Clinch River

Breeder Reactor (CRBR) is typical of what one might

expect with a low pressure system and liquid metal

coolant.

In addition to the normal decay heat removal path

consisting of the balance of plant steam/feedwater sys-

tem (turbine bypass system) there are three backup

systems [20]. The first are protected air-cooled con-

densers which cool the steam drums, on each of the

three coolant loops. A second heat sink can be made

available by open ing the safety relief valve in the steam

line between the steam drum and the turbine and vent-

ing steam to the atmosphere. A protected water storage

tank is available for supplying make-up (auxiliary) water,

while two of the auxiliary feedwater pumps are electri-

cally driven and one is steam driven. Lastly, a com-

pletely separate direct heat removal service (DHRS),

is provided to remove decay heat directly from the

in-vessel, primary loop, and has as its heat sink, an air

blast heat exchanger. The latter represents diversity

from the water system in the steam loop.

The decay heat removal systems using the primary

and intermediate coolant loops depend on forced circu-

lation by the main coolant pumps driven at approxi-

mately 10% speed by small pony motors which depend

on a source of electric power. Similarly the DHRS air

blast heat exchangers rely on electric power. Although

CRBR is designed with redundant and diverse power

supplies, the capability for decay heat removal by natu-

ral circu lation is a desirable feature. With natural circu-

lation, the inherent safety of the plant is enhanced since

no electric power is required to provide adequate circu-

lation in the heat transport or steam generator systems

following shutdown.

Although CRBR and several other LMFBRs (the

Germa n SNR-300 and the French Phrnix) are designed

for natural convection cooling, and the transition from

forced to natural circulation has been verified at the

Fast Test F lux Facility [21], several questions must still

be answered. These include demonstration under actual

LMFBR normal and off-normal conditions, as well as

the passivity of the ultimate heat sink (e.g., the air

cooled condensers) [22].

Gas cooled fast reactors (GCFRs) and high tempera-

ture gas cooled thermal reactors (HTGRs) have also


4/10

54

W E Kastenberg / Design criteria or afterheat and decay heat removal

b e e n d e s i g n e d w i t h a l t e r n a t e a n d d i v e r se d e c a y h e a t

r e m o v a l s y s t e m s , F o r G C F R s , t h e r e a r e t h r e e s e p a r a t e

sys te ms [ 21] :

( 1 ) S t e a m b y p a s s t o t h e c o n d e n s e r u s i n g t h e n o r m a l

p o w e r c o n v e r s i o n s y s t e m ,

( 2 ) O p e r a t i o n o f t h r e e sh u t d o w n c o o l i n g s y s t e m l o o p s

u s i n g t h e s t e a m g e n e r a t o r s, a n d m a i n h e l i u m c i r -

c u l a t o r s ( p u m p s ) d r i v e n b y p o n y m o t o r s , a n d

( 3 ) T h r e e C o r e A u x i l i a r y C o o l i n g S y s t e m ( C A C S )

l o o p s .

T h e C A C S l o o p s h a v e t h e i r o w n i n d e p e n d e n t a u x -

i l i a r y c i r c u l a to r s a n d c o o l i n g w a t e r l o o p s . A n a i r - b l a s t

t y p e h e a t e x c h a n g e r p r o v i d e s t h e u l t i m a t e h e a t s i n k . T h e

C A C S i s d e si g n e d t o p r o v i d e c o r e c o o l i n g f o l lo w i n g a ll

d e s i g n b a s i s e v e n t s , i n c l u d i n g t h e d e p r e s s u r i z a t i o n o f

t h e p r i m a r y c o o l i n g s y st e m . I t h a s a l so b e e n a r g u e d t h a t

t h e h e a t t r a n s f e r c o m p o n e n t s w i t h i n t h e C A C S a r e

l o c a t e d a t s u f f i c i e n t e l e v a t i o n d i f f e r e n c e s s u c h t h a t n a t -

u r a l c i r c u l a t i o n w i l l t r a n s f e r d e c a y h e a t f r o m t h e c o r e t o

t h e u l t i m a t e h e a t s i n k , p r o v i d e d t h e s y s t e m i s p r e s -

sur ized [ 23] , a l though th i s w as never ve r i f i ed .

F o r H T G R s t h e r e a r e t w o m a i n m o d e s f o r r e m o v i n g

d e c a y h e a t :

( 1 ) T h e M a i n L o o p C o o l i n g S y st e m ( M L C S ) , c o m -

p o s e d o f t h e s t e a m g e n e r a t o r s , t h e m a i n h e l i u m

c i r c u l a t o r s , t h e m a i n l o o p i s o l a t i o n v a l v e s , a n d

t h e a s s o c i a t e d d u c t i n g .

( 2 ) T h e C o r e A u x i l i a r y C o o l i n g S y s t e m ( C A C S ) ,

c o m p o s e d o f a c o r e a u x i l i a r y h e a t e x c h a n g e r , a n

a u x i l i a r y c i r c u l a t o r , a n a u x i l i a r y c i r c u l a t o r s e r v i c e

s y s t e m , a n d a c o r e a u x i l i a r y c o o l i n g w a t e r s y s t e m .

T h e C A C S f o r a n H T G R i s a n e n g i n e e r e d s a f e ty

f e a t u r e f o r d e c a y h e a t r e m o v a l i n t h e e v e n t t h e m a i n

l o o p s a r e u n a v a i l a b l e . A t y p i c a l 11 00 M W c H T G R

d e s i g n c a l l s f o r s ix m a i n l o o p s a n d 3 a u x i l i a r y l o o p s ,

e a c h w i t h i t s o w n c i r c u l a to r s , h e a t e x c h a n g e r s a n d s t e a m

g e n e r a t o r s . A r e c e n t s t u d y b y W a s h b u r n [ 2 4 ] i n d i c a t e d

t h a t t h e i n i t i a t i n g e v e n t s o f g r e a t e s t i m p o r t a n c e t o

c o r e m e l t f r e q u e n c y a r e , ( a ) l o s s o f a d e q u a t e a - c p o w e r ,

( b ) l o s s o f C A C S s h u t d o w n h e a t - r e m o v a l a n d ( c ) t h e

l o s s o f m a i n l o o p s h u t d o w n h e a t r e m o v a l .

d e c a y h e a t i s p r i m a r i l y g e n e ra t e d b y t h e f i ss i o n p r o d -

u c t s , a n d a l t h o u g h t h e r e l a t i v e y i e l d s c h a n g e w i t h n e u -

t r o n s p e c t r u m , t h e d e c a y h e a t c u r v e is s o m e w h a t u n i v e r-

s a l. It is a p p r o x i m a t e l y 7 o f o p e r a t i n g p o w e r a t s h u t -

d o w n , 2 a t 1 h o u r , 1 a t 5 h o u r s , 0 .5 a t 1 d a y a n d

0 .1 a t 10 day s .

3 1 Fusion systems

S i n c e a f t e r h e a t i n f u s i o n p o w e r p l a n t c o n c e p t u a l

d e s i g n s is d u e t o n e u t r o n i n d u c e d a c t i v i ty in t h e b l a n -

k e t , a n d p r i m a r i l y i n t h e f i r s t w a l l , i t is m a t e r i a l d e p e n -

d e n t . V o g e l s a n g e t a l . [ 1 1 ] e x a m i n e d a f t e r h e a t a s a

f u n c t i o n o f b o t h t i m e a f t e r s t a r t u p a n d t i m e a f t e r s h u t -

d o w n f o r t h r e e s t r u c t u r a l m a t e r i a l s m a k i n g u p t h e f i r s t

5 0 c m o f t h e b l a n k e t , i n t h e U W M A K - I d e s i g n ( 5 0 0 0

M W t h ) . A f t e r 1 0 w e e k s o f o p e r a t i o n , t h e t h r e e m a t e r i a l s

( t y p e 3 16 s t a i n le s s s t ee l , v a n a d i u m - 2 0 t i t a n i u m a n d

n i o b i u m - l z i rc o n iu m ) a p p r o a c h e d a n a f te r h e at v a l u e

o f 2 2 M W a t s h u t d o w n , o r 0 .4 o f o p e r a t i n g p o w e r . A t

t e n y e a r s o f o p e r a t i o n t h e t y p e 3 1 6 s t a i n l e s s - s t e e l ( 3 1 6

S S ) a n d t h e n i o b i u m - l z i rc o n i u m ( N b - l Z r ) a ft e rh e a t

r e a c h 0 .6 o f o p e r a t i n g p o w e r a t s h u t d o w n . T h e

v a n a d i u m - 2 0 t i t a n i u m ( V - 2 0 T i ) r e m a i n e d r e l a ti v e l y

c o n s t a n t b e t w e e n 1 0 w e e k s a n d 2 0 y e a r s a t 0 .4 p o w e r .

F o l l o w i n g s h u t d o w n ( a f t e r 10 y e a r s o f o p e r a t i o n ) t h e

N b - l Z r s t r u c tu r e a ft e r h e a t r e m a i n e d f a i rl y c o n s t a n t

( - 0 .5 o p e r a t i n g p o w e r ) o u t to 1 0 d a y s . T h e r e a f t e r , i t

d r o p s d r a s t i c a l l y ( a f a c t o r o f 1 0 r e d u c t i o n a f t e r 1 0

w e e k s ) . T h e T y p e 3 1 6 S S a f t e r h e a t r e m a i n s f a i r l y c o n -

s t a n t b e y o n d 1 d a y (a t 0 .4 o p e r a t i n g p o w e r ), d r o p s t o

0 .2 o p e r a t i n g p o w e r a t 1 y e a r , a n d t o 0 .0 6 ( a n o r d e r

o f m a g n i t u d e ) a t 2 y e a rs . T h e V - 2 0 T i s t r u c t u re a f t e r h e a t

d e c a y e d t h e q u i c k e s t ; f r o m 0.4 o p e r a t i n g p o w e r t o

0 .1 i n 1 h o u r , t o a b o u t 0 .0 5 i n 1 d a y ( a n o r d e r o f

m a g n i t u d e r e d u c t i o n ) . I t a p p e a r s t h e n , t h a t t h e 3 1 6 S S

b l a n k e t s t r u c t u r e m a i n t a i n s , o n t h e a v e r a g e , a r e l a t i v e l y

c o n s t a n t a f t e r h e a t v a l u e f o r s e v e r a l y e a r s a f t e r s h u t -

d o w n c o m p a r e d t o o t h e r m a t e r i a l s .

I n a f u r t h e r s t u d y o f f i r s t w a l l / b l a n k e t m a t e r i a l s ,

C o n n e t a l . [1 2] c o m p a r e d i n d u c e d a c t i v i t y a n d a f t e r h e a t

i n f i v e d e s i g n s t u d i e s a s f o l l o w s :

3 Afterheat considerations in fusion and fu sion /fiss ion

sys tems

B e f o r e a t t e m p t i n g t o p o s t u l a t e d e s i g n c r i t e r i a f o r

f u s i o n a n d f u s i o n / f i s s i o n h y b r i d r e a c t o r s it i s o f i n t e re s t

t o e x a m i n e t h e n a t u r e o f a f t e r h e a t i n t h e s e s y s t e m s .

D e c a y h e a t r e m o v a l i n f i s s i o n p o w e r r e a c t o r s i s b a s e d

u p o n a s t a n d a r d d e c a y h e a t c u r v e w h i c h g i v e s t h e

p e r c e n t o f o p e r a t i n g p o w e r a s a f u n c t i o n o f t i m e . S in c e

Afterheat as power

Designer M aterial Shutdown 1 Week

1 Wisconsin 316 SS 0.4 0.4

2 LL L 316 SS 0.4 0.4

3 OR NL niobium 0.2 0.01

4 BNL SAP a 2.0 1.0

5 Princeton PE-16 b 5.0 0.5

SAP = Sintered Aluminum Product (88~ AI, 12 Al2 03).

b Pc-16 = 43 nickel, 39 iron , 18 chrom e.


5/10

W.E. Kastenberg / Design criteria for afterheat and decay heat removal

55

A detail of the first 100 minutes (1 hours) showed a

fairly constant power rating for all cases but the niobium

which dropped by a factor of 5 within the first six

seconds and then remained constant.

Blankets including the mater ials TZM (0.45

titanium, 0.1 zirconium, remainder molybedinum), and

2024 A1 (aluminum) were also compared i n UW MAK-I

by Vogelsang [13]. The total afterheat, at shu tdown

following two years of operation for TZM and V-2 0 Ti

was 1.4 and 1.5 of operating power respectively. The

2024 AI and 316 SS had afterheat on the order of 1

full power. Except for the V-20 Ti which dropped a

factor of 5 in the first hour, the others remained fairly

constant out to one day. In this study, the entire blanket

was considered, rather than 50 cm.

Recently, Youssef and Corm [15] have examined

induced radioactivity and influence of materials selec-

tion in DD and DT fusion reactors. For the SATYR

design using a deute riu m-deut erium fuel cycle, a SAP

blan ket at shutdown has a power rating that is 13 of

operating power. For a ferritic steel blanket (HT-9), the

afterheat is 0.6 operati ng power. Within 1 hour, the

SAP blanket is down to 1 afterheat, and down to

at one day. In the HT-9 blanket the afterheat is con-

stant to 1-hour, and then drops an order of magnitude.

Thereafter, it remains fairly constant (-0. 02 ) in the

time frame of 1 day to 1 year; after which it drops

drastically (an order of magnitude) due to the decay of

the Fe 55.

These reactors were compared with the WITAMIR-I

design [25] using HT-9 and the STARFIRE design [26],

using PCA (2 molybdenum, 16 nickel, remainder steel).

The afterheat at shutdown in the two reactors are ap-

proximately 0.9 operati ng power, or about 30 and 40

MWth respectively. Both are fairly constant for several

months.

3 .2 . Fu s io n f i s s i o n sys t ems

Afterheat in fus ion-fission (hybrid) power pl ant de-

signs is due to structural activation (first wall/blanket),

fission product decay, and decay of actinide and trans-

urani c elements. Hybrid systems have been proposed as

power producers, fissile fuel producers and actinide

burners.

For a fast fission blanket, Kastenberg et al. [27]

determined that the fission product i nvento ry of a hy-

brid should not differ significantly from that of fission

reactors. In this regard then, a hybrid reactor would

posses both the afterheat requirements of pure fusion

with respect to the first wall and structural materials

and the decay heat removal requirements of a fission

reactor.

Recently there has been interest in fission suppressed

blankets for producing fissile fuel [28-30]. Since the

fission product decay heat would domina te a hybrid, the

suppression of fission for breeding purposes, should

also lower the decay heat removal requirements. This is

shown in table 1, which is reproduced from ref. [28].

Examination of table 1 shows several important trends.

For fast fission blankets the decay heat power density

produced in the fertile zone, approaches the afterheat

produced in the first wall, in about a day. For the

fission suppressed blanket, the first wall afterheat

dominates within an hour of shutdown.

3 . 3 . R e m a r k s

The brief survey presented here indicates that

afterheat removal in fusion systems is materials depen-

dent. However, a general trend seems to be present;

afterheat thermal loads will be about a factor 5-10 less

than fission decay heat thermal loads, but will remain

Table 1

Total volumetric afterheat production rates a (W/cm3) at shutdown, and at 1 h and 6 h after shutdown

Blanket concept a

Hours after shutdown

First wall Fertile zone

0 1 6 0 1 6

Uranium fast-fission 0.9

Thorium fast-fission 0.9

Thorium fission suppressed 0.9

Pure fusion 0.9

0.7 0.5 3.8 1.4 1.0

0.7 0.5 2.6 0.9 0.6

0.7 0.5 0.9 0.3 0.2

0.7 0.5 - - -

a Based on 4000 MWth total power.


6/10

56


r e l a ti v e l y c o n s t a n t o v e r p e r i o d s o f i n te r e s t f r o m s h u t -

d o w n t o 1 d a y , f o r s t a i n l e s s s t e e l o u t t o o n e y e a r ) . T h i s

l a t t e r a t t r i b u t e d i f f e r s f r o m f i s s i o n d e c a y h e a t w h i c h

d r o p s a n o r d e r o f m a g n i t u d e w i t h i n 1 d a y . F o r

f u s i o n - f i s s i o n h y b r i d s , t h e c o m b i n e d h e a t l o a d d e c a y

p l u s a f t e r h e a t ) i s a l s o s e n s i t i v e t o t h e b l a n k e t d e s i g n .

F o r t h e f i r s t d a y , f a s t f i s s i o n b l a n k e t s r e s e m b l e f i s s i o n

r e a c t o r d e c a y h e a t l o a d s . F o r f i s s i o n s u p p r e s s e d b l a n -

k e t s , a f t e r h e a t d o m i n a t e s a f t e r 1 h o u r .

4 Criter ia for fusion and fu sio n/ f i ss ion afterheat and

decay heat removal systems

I t h a s b e e n r e c o g n i z e d t h a t l o s s o f a f t e r h e a t a n d

d e c a y h e a t r e m o v a l c a p a b i l i t y i n f u s io n a n d f u s i o n - f i s -

s i o n h y b r i d p o w e r p l a n t s w i ll b e a m a j o r c o n t r i b u t o r t o

r i s k [ 2 9 - 3 1 ] . I n t h i s c o n t e x t , r i s k c a n b e t a k e n i n t h e

p r o b a b i l i s t i c s e n s e: f r e q u e n c y t i m e s c o n s eq u e n c e . F u r -

t h e r m o r e , c o n s e q u e n c e i n c l u d e s b o t h h e a l t h e f f e ct s a n d

e c o n o m i c l o s s .

A n u m b e r o f c o n c e p t u a l d e s i g n s h a v e b e e n c o m -

p l e t e d f o r v a r i o u s f u s i o n a n d f u s i o n - f i s s i o n s y s t e m s ,

b u t l i t tl e a t t e n t i o n h a s b e e n p a i d t o t h i s s a fe t y q u e s t i o n

a s a d e s i g n i s s u e . B a l a n c e o f p l a n t d e s i g n s u s u a l l y

i n c l u d e s y s t e m s a n d c o m p o n e n t s f o r n o r m a l p o w e r o p -

e r a t i o n e . g ., p u m p s , s t e a m g e n e r a t o r s , l o o p s , t u r b i n e s ,

e t c . ) , b u t n o t f o r a f t e r h e a t a n d d e c a y h e a t r e m o v a l .

F r o m a s a f e t y v ie w p o i n t , c a lc u l a t i o n s a re p e r f o r m e d f o r

a v a r i e t y o f s c e n a r i o s s u c h a s l o s s o f f l o w a n d l o s s o f

c o o l a n t , a n d t i m e - t o - m e l t o r t i m e - t o - s t r u c t u ra l f a il u r e

a r e u s e d a s f i g u r e s o f m e r i t . A l t h o u g h t h e s e c o n s i d e r a -

t i o n s a r e i m p o r t a n t i n d e t e r m i n i n g t h e c o n s e q u e n c e s o f

a c c i d e n t s , a n d i n t h e i n i t i a l c h o i c e o f m a t e r i a l s a n d

c o o l a n t s , t h e y a r e o f l i t t l e u s e t o t h e d e s i g n e r i n l a y i n g

o u t t h e b a l a n c e o f p l a n t . I n t h i s s e c t i o n t w o t y p e s o f

c r i t e r ia a r e p r o p o s e d f o r d e s ig n c o n s i d e r a t io n .

4 1 D e te r m in i s t i c c r i te r ia

I n s e c t i o n 2 , e le v e n g e n e r a l d e s i g n c r i t e r i a w e r e g i v e n

f o r d e c a y h e a t r e m o v a l s y s t e m s i n f i s si o n p o w e r p l a n t s .

B e f o r e d e t e r m i n i n g t h e a p p l i c a b i l i t y o f t h e s e c r i te r i a

a n d / o r m o d i f y i n g t h em , s e v e ra l p o i n t s s h o u l d b e d is -

c u s s e d . A f t e r h e a t g e n e r a t i o n i n f i r s t w a l l / b l a n k e t

m a t e r i a l s r e p r e s e n t a s m a l l e r p e r c e n t a g e o n t h e o r d e r o f

1 ~ o r l e s s o p e r a t i n g p o w e r ) t h a n f i s s i o n r e a c t o r d e c a y

h e a t o n t h e o r d e r o f 7 ~ ) a t s h u td o w n . O n t h e o t h e r

h a n d , a f t e r h e a t g e n e r a ti o n t e n d s t o b e f a i r l y c o n s t a n t

f o r u p t o a y e a r f o r s ta i n l e s s s t e e l ) f o l l o w i n g s h u t d o w n ,

w h i l e d e c a y h e a t g e n e r a t i o n d r o p s a n o r d e r o f m a g n i -

t u d e w i t h i n a d a y . H e n c e , u p t o 5 0 M W t h m a y h a v e t o

b e r e m o v e d f o r p e r i o d s o f 1 w e e k t o 1 y e a r i n a f u s i o n

p l a n t .

W i t h t h e s e c o m m e n t s i n m i n d , t h e f o l l o w in g c ri t e ri a

a p p e a r t o b e a p p r o p r i a t e :

1 . E n s u r e t h a t b l a n k e t a n d f u e l s t r u c t u r a l i n t e g r i t y ,

a n d p r e s s u re b o u n d a r i e s a r e m a i n t a i n e d .

2 . W i t h s t a n d f i r e , s a b o t a g e , n a t u r a l p h e n o m e n a , a n d

o t h e r e x t r e m e c o n d i t i o n s .

3 . O p e r a t e u n d e r n o r m a l a n d e m e r g e n c y p o w e r c o n d i -

t ions .

4 . M o n i t o r a n d m a i n t a i n c o o l a n t b o u n d a r y t h r o u g h

i n s p e c t i o n , l e a k d e t e c t i o n a n d i s o l a t i o n v a l v i n g .

5 . P r e v e n t s h a r e d n o r m a l o r e m e r g e n c y e q u i p m e n t

f r o m j e o p a r d i z i n g r e l i a b l e s a f e ty o p e r a t i o n s .

6 . I n i t i a t e a n d o p e r a t e a u t o m a t i c a l l y f o r a p e r i o d r a n g -

i n g f r o m 1 h o u r t o 1 d a y .

7 . P r o v i d e m a n u a l b a c k u p c o n t r o l c a p a b i l i t y f o r a u t o -

m a t i c s y s t e m s .

8 . P r o v i d e u n i n t e r r u p t e d c o o l i n g fo r u p t o t w o m o n t h s .

9 . F u n c t i o n d e s p i t e t h e s i n g l e f a i l u r e o f a n a c t i v e o r

p a s s i v e c o m p o n e n t i n c o m b i n a t i o n w i t h a m a i n t a i -

n e n c e o u t a g e i n v o l v i n g a r e d u n d a n t s y s t e m .

1 0 . O p e r a t e w i t h o u t o n - s i t e r e p a i r a c t i o n f o r a t l e a s t 5

d a y s a n d w i t h o u t o f f - s i t e r e p a i r a c t i o n f o r a t l e a s t 1

m o n t h .

T h e b a s i c a p p r o a c h i n f o r m u l a t i n g t h e s e c r it e r ia w a s

t o a d o p t f i s s i o n r e a c t o r c r i t e r i a a s a p p r o p r i a t e b u t

a c c o u n t f o r t h e p r o l o n g e d g e n e r a t i o n o f a f t e r h e a t . I t

s h o u l d b e p o i n t e d o u t t h a t t h e v a r y i n g t i m e s c a l e s

r e f l e c t t h e d e p e n d e n c e o f t h e a f t e r h e a t l o a d o n t h e

m a t e r i a l s e m p l o y e d . A m o r e c o n s e r v a ti v e a p p r o a c h w a s

t a k e n t o t h e s in g l e f a i l u r e c r i t e r i o n n u m b e r 9 ) b e c a u s e

o f t h e p o t e n t i a l l y la r g e e c o n o m i c l o s s s h o u l d a f t e r h e a t

r e m o v a l f a i l .

4 2 Prob abi l is t ic c r i te r ia

Q u a n t i t a t i v e o r p r o b a b i l i s t i c s a f e t y c r i te r i a t o p r o -

v i d e g u i d a n c e t o r e a c t o r d e s i g n e r s h a v e b e e n u s e d i n t h e

U K f o r s ev e r a l y e a r s a n d h a v e b e e n f o u n d t o b e a u s e fu l

t o o l i n t h e d e s i g n p r o c e s s [ 3 2 ] . W i t h t h e i n c r e a s e d

e m p h a s i s o n t h e u s e o f a q u a n t i t a t i v e a p p r o a c h i n t h e

U S , t h e r e i s n o w a n i n t e r e st i n d e v e l o p i n g q u a n t i t a t i v e

c r i t e r i a f o r t h e m a i n s a f e t y f u n c t i o n s o f f i s s i o n r e a c t o r s ,

b o t h f o r a ss e s si n g t h e a d e q u a c y o f t h e s a f e t y s y s t e m s in

e x i s t in g p l a n t s a n d f o r p r o v i d i n g g u i d a n c e t o t h e d e s i g -

n e r s o f n e w p l a n t s .

R e c e n t l y C a v e e t a l . [ 1 0 ] d e v e l o p e d a s c r e e n i n g

c r i t e r ia f o r e v a l u a t i o n o f d e c a y h e a t r e m o v a l i n P W R s .

T h e s t a r t in g p o i n t i s a s e t o f s a f e t y g o a ls f o r L W R s t h a t

w a s p u b l i s h e d b y t h e N R C f o r p u b l i c c o m m e n t [7 ]. T h e

p r i n c i p a l g o a l s i n t h i s s e t a r e b a s e d o n p u b l i c h e a l t h


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W E Kastenberg / Design criteria for afterheat and decay heat removal

Table 2

Risks for evaluation of afterheat removal

57

Public health Onsite economic Offsite economic

Early fatalities Cost of energy replacement

Delayed fatalities

Early illness

Delayed illness

Genetic effects

Repair costs

Clean up costs

Decommissioningcosts

Occupational health costs

Capital investment loss

Public damage (loss of gross national product

due to interdiction)

Lost wages

Decontamination costs

Public health costs

Evacuation/rehousing costs

Secondary costs (higher electricity costs affect

industrial production)

risk, and there is a supporting goal relating to the

acceptable pr obabi lity of coremelt, i.e., 10 -4 per reactor

year, median value. An allocation scheme was devel-

oped which appropriated 25 x 10 -6 per reactor year to

the shutdown decay heat removal phase (scram to hot

shutdown) and 5 10 -7 for the residual heat removal

phase (hot shutdown to cold shutdown) for PWRs.

Because fusion power plants will have very large

capital costs, economic as well as public health risks

should be included in developing criteria for afterheat

removal. Examples of the risks that should be consid-

ered are shown in table 2.

The work of Kazimi and Sawdye [33] is particularly

useful in developing criteria for afterheat removal sys-

tems. Beginning with the premise that the potenti al

radiological hazards associated with accidents in fusion

reactors should be less than those of commercial light

water reactors, Kazimi and Sawdye determined maxi-

mum tolerable frequencies for large accidents. These

frequencies were defined so as to assure that releases of

fusion reactor induced radioactivity (not tritium) do not

imply a greater radiological hazard than in either the

WASH 1400 PWR or BWR.

Utilizing the UWMAK-I (316 SS blanket) and the

UWMAK -II I (TZM blanket) designs, it was determined

that maxim um tolerable release frequencies were 10 -5

and 4 x 10 -6 per reactor year, respectively. Noting tha t

the radioactivity inventories used were among the highest

possible in Tokamak reactors, and that no release miti-

gation factors were employed, it was concluded that

these numbers represent lower bounds.

From an economic viewpoint, the work of Stucker et

al. [34] and Strip [35] are part icularly useful. Stucker et

al. focused on the costs of closing the Indian Point

Nuclea r Power Plants (two units) before their useful life

was over. They estimated that the costs would be be-

tween 7.7 billion and 17.4 billion and was composed

of the incremental generating costs (Indian Point pro-

duces the cheapest electricity in the New York City

area), one time costs and savings, business costs and

secondary costs (net costs to the local economy). The

major component was the replacement power cost,

estimated at about 8 billion for the two units (ap-

proximately 2000 MWe).

Strip estimated the financial risks of. nuclear power

accidents as a function of accident severity and location

for several power plant types. Included were onsite and

offsite health costs, and onsite and offsite economic

costs. Strip's results, although site dependent, indicate

that onsite costs (i ncluding replacement power costs)

dominate, with offsite costs a close second. Onsite costs

varied between 1 and 10 billion, with clean-up costs

cont ribu ting 1 and 2 billion. For the most severe

accidents, offsite costs were as high as 10 billion. For

less severe accidents, the replacement costs could be

less, if the plant was repaired, and put back on-line.

For the purposes of the analysis here, the following

can be considered. A large accident at a fusion or

fusion-fiss ion power plant involving loss of the blanket

and part of the primary coolant system due to a loss of

afterheat or decay heat removal would involve loss of

capital investment, replacement power costs and clean

up costs. Nuclear power plants costs such as CRBR and

Shoreham are approaching 3 billion. Fusion plants

have been estimated to cost between 2 and 10 billion.

Replacement power for a large fusion plan t might be on

the order of the two units at Indian Point ( 8 billion)

and clean-up costs might be similar to those estimated

for Three Mile Island ( 2 billion). Although it is recog-

nized that the capital investment cost must account for

depreciation, it can be assumed that loss of a fusion

plant might approach 10 billion dollars. If the blanket

wa repairable or replaceable, the loss might approach 5

billi on dollars.

Taking 10 -5 per year as an upper limit for loss of

the blanket, and 10 billion dollars as the total potential


8/10

58


loss, the expected risk (loss) is l0 s dol lars /year,

( 10 0000 /year ). If the goal were relaxed to 10 -4 per

year, the expected loss would be 106 dollars/year ( 1

million/year). An expected loss of 106 dollars/year

appears to be at the high end of acceptability. Since

10 -5 per year for a loss of the blanket may be conserva-

tive from a public health viewpoint, and 10 -4 per year

intolerable from a financial viewpoint, a value between

them o f 5 10 -5 per year might appear appropriate . If

this value is adopted, two other considerations must be

dealt with:

(a) The provision for adequate margin against the

effects of uncertainties in the estimation of the

risks, and

(b) an appropriate allocation of the goal due to loss

of afterheat removal and to other functions whose

failure could lead to loss of the blanket structure,

and release.

Uncertainties can be divided into three categories:

(1) Uncerta inties due to variations in data and which

can be quantified;

(2) uncertainties due to describing extreme events

such as severe earthquakes, floods, etc., and ex-

treme phenomena (the so-called special emergency

situations described in section 2), and

(3) uncertainties due to human errors, design errors,

and extreme human acts (sabotage).

Category 1 uncertainties are those included in de-

sign. Category 2 and 3 uncertainties are unquantifiable

at present, and can be treated as design margins.

Following Cave et al. [10], adequate margin can be

attained by assigning 40% of the goal to Category 1

uncertainties, with the remainder divided equally be-

tween Category 2 and 3. Hence for system design, the

loss of blanke t frequency goal would be 20 10 -6 per

year.

The allocation between the afterheat removal func-

tion and the others required to prevent loss of blanket

integrity should be arrived at from a plant specific

probabilistic risk assessment. Examination of the rela-

tive demand for these functions would optimize the

suballocation. In PWRs, experience leads to a 75%

allocation to decay heat removal function and 25% to

others, such as large break loss of coolant accidents and

other transients [17].

The most effective mechanism for releasing radioac-

tive blanket material is oxidation following a lithium

fire in the UWMAK systems with lithium coolant fol-

lowing a loss of coolant [33]. Plasma disruption may be

a further cause for release, but appears to be localized in

nature [36]. For gas (helium) cooled systems, depressuri-

zation accidents have also been shown to be less of a

contributor to risk than loss of decay heat removal

capability [24]. In summary, there is little evidence to

believe that the split between the afterheat removal

function and the other safety functions will be much

different than that for fission reactors. Hence a 75/25

split is assumed, and will be varied below.

This final allocation yields a reliability requirement

of 15 10 -6 per year for afterheat removal in a fusion

reactor. This value may not be too far from optimum

for the following reasons. If the Category 2 and 3

uncertainties were reduced by a factor of five (12%

allocation), the allocation for afterheat removal would

double ; i.e., it wou ld be 33 10 -6 per reactor year.

Similarly, if it were found that other safety functions

were equal to afterheat removal; (as in BWRs) the goal

becomes 10 10 -6 per r eactor year. Hence the range

10 x 10 -6 to 33 x 10 -6 , with a goal of 15 x 10 -6 ap-

pears reasonable. This analysis is summarized in table 3.

For fusion-fission hybrid reactors, the combined

afterheat and decay heat removal functions must be

considered. At one extreme, the blanket could be con-

sidered a fission reactor, and the NRC safety goal

applied. Using the arguments above, (40% for Category

1 uncertainties, 75% for decay heat removal function)

one arrives at an allocation of 30 10 -6 per reactor

Table 3

Allocation of afterheat removal reliability goal

Allocation Value Totals

Base case

Afterheat function

Other functions

Category 1 uncertainty (40%)

Category 2 uncertainty (30~)

Category 3 uncertainty (305g)

15 10- 6/year

5 10- 6/year

20 x 10- 6/year

15 10-6/yea r

15 x 10- 6/year

5 x 10- 5/year

Reduced uncertainty

Afterheat function

Other functions




33x10 -6

l l x l 0 - s

4410 -6

310 -6

3X10 -6

5 x 10-5 /year

Reduced function

Afterheat function

Other functions

Category I uncertainty

1010 -6

lOxlO -6

20x10 -6


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W E Kastenberg / Design criteria or afterheat and decay heat removal 59

year as the design goal. This extreme represents public

health. From an economic viewpoint, the loss of a

hybrid also represents the loss of an assured fuel supply

to a number of fission reactors. Hence some multiplier,

in terms of expected loss must be used.

Assuming that the monetary loss of a hybrid is a

factor of 5 greater than the monetary loss of a pure

fus ion power reactor , or 50 bil lion dollars ( 50 109),

and that the upper limit on expected loss is 1 million

dollars per year; the goal would become 20 10 -6 per

reactor year. Allocating for uncertainty and function as

before, the combined afterheat-decay heat function-goal

would be 6 10 -6 per reactor year. Hence a range of

6 10 -6 to 30 10 -6 per reactor year may be ap-

propriate for hybrids.

and can be used in a variety of ways. They can be used

to determine the number of loops, steam generators

and/or heat sinks required in the balance of plant. Or,

on the subsystem level, they can determine the number

of trains, pumps, valves, etc. needed to provide such

things as auxilliary feedwater.

It should be noted that one approach for insuring the

decay heat removal function in liquid metal cooled

systems is by natural circulation. Although some fusion

systems are designed with lithium coolants, the geome-

tries employed might preclude its use as a viable option.

For fusion-f ission hybrids, gas cooling appears to be

the favored approach, which necessitates high pressure.

Since depressurization is a design basis event, natural

circulation might be precluded as well.

5 Sum mary and conclusions

Design criteria for afterheat and decay heat removal

in fusion and f usion-fiss ion power plants were pro-

posed in this paper. Deterministic criteria were derived

by reviewing the general design criteria for fission plants

and modifying them to account for the different fea-

tures of fusion and fusion-fiss ion after- and decay heat.

The fraction of full power for afterheat (fusion)

tends to be an order of magnitude less than that for

decay heat (fission) and it decays rather slowly by

comparison. However, if fusion reactors produce high

thermal power (5000 MWth ) the total heat load will be

comparable over the time of interest (1 day to 1 week).

As a result, it is proposed that the general design

criteria for fission plants be made more conservative for

fusion reactors: in particular it is proposed that the

afterheat removal system function despite

both

a single

failure of an active or passive component in combination

with a maintenanc e outage involving a redunant system.

Moreover, the time for automatic operation, and on-site

and off-site repair action are extended.

Probabilistic criteria were developed from both an

economic and a public health viewpoint. Using a lower

limit for public health risk and an upper limit for

financial loss, an allocation scheme was proposed to

account for uncertainty and function. The following

design criteria are proposed:

Reactor Function Criterion

Fusion Afterheat removal 15 10-6/year

Fusion-fission Afterheat/ 6 10- 6 to

decay heat removal 30 10-6/year

These values are meant to be target values for design

Acknowledgement

This work was supported by the Electric Power

Research Institute. The author wishes to thank Dr. Noel

Amherd for his interest, support, and encouragement.

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calor decaimiento kastenberg

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