Page 111

105 Table 11. Reactivity Effects of 135Xe ~~

Fuel A o r C

%

Fuel B

25

50

Ekpect e d

-1.2

-0.7

-0.5

-0.9

Minimum

-1.0

-0.5

-0.3

-0.5

Maximum

-1.7

-1.2

-0.9

-1.5

Pump bowl stripping efficiency,

100

50

Reactivity e f f e c t , $I 6k/k

.,

shown t h a t the presence of c i r c u l a t i n g helium bubbles, introduced by the s t r i p p e r flow i n t h e pump bowl, has a pronounced e f f e c t on the xenon behavior. Because of t h e extreme # i n s o l u b i l i t yof xenon i n molten s a l t , the addition of 1 vol % of c i r c u l a t i n g helium bubbles reduces the equilibrium xenon p a r t i a l pressure ( i n the s a l t and bubbles) by a factor of about 40, f o r a given t o t a l amount of xenon i n circulation. That i s , about 98% of the c i r c u l a t i n g xenon will be i n t h e bubbles. Since t h e xenon p a r t i a l pressure provides the driving force f o r migration t o t h e graphite, the preliminary calculations may overestimate the contribution t o the poisoning by xenon i n t h e graphite.

v

.

Samarium

.

Assuming t h a t a l l t h e samarium c i r c u l a t e s with the fuel, the buildup of t h i s f i s s i o n product w i l l ultimately poison about 0.9% 6k/k a t 10 Mw. T h i s l e v e l w i l l be approached a f t e r about 100 days' operation a t f u l l power. This e f f e c t can be Compensated by uranium addition t o the f u e l . However, each time t h e power i s reduced t o zero, the samarium poisoning will increase an additional O.l$ over a period of 7 days. T h i s increase i s burned out i n about 7 days by operation a t full power. Thus only the 0.1% 6k/k samarium t r a n s i e n t need be compensated by the control rods.

.

Low-Cross-Section Poisons

I

The large majority of the f i s s i o n products may be regarded as an aggregate of low-cross-section nuclides. The poisoning e f f e c t of t h i s group increases i n i t i a l l y a t a r a t e of about 0.003% 6k/k per day a t f u l l power. Neutron Sources

LJ

'.

The presence of a neutron source t h a t i s independent of the f i s s i o n chain reaction i s e s s e n t i a l t o the safe and orderly operation of t h e reactor. Three classes of source neutrons w i l l be available during the

ORNL-3708  

http://www.energyfromthorium.com/pdf/ORNL-3708.pdf

ORNL-3708  

http://www.energyfromthorium.com/pdf/ORNL-3708.pdf

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