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(The negative temperature coefficient of the graphite i s of l i t t l e value during a rapid excursion because of i t s long reaction time.) However, t h e action of t h e safety system i s required t o prevent equipment damage i n t h e most severe cases considered. One of the greatest p o s s i b i l i t i e s f o r equipnent damage i n this system i s collapse of t h e control rod thimbles. However, t h e pressure excursions calculated f o r t h e core were very small, and they always occurred before the temperature excursions reached the point where they might reduce t h e allowable s t r e s s on the thimbles. The r e a c t i v i t y incidents considered, along with a b r i e f description of how each might occur, are l i s t e d below. No attempt was made t o evalua t e the c r e d i b i l i t y of the occurrences. 1. Uncontrolled rod withdrawal- Simultaneous withdrawal of a l l three control rods, with c r i t i c a l i t y being a t t a i n e d while t h e rods are moving i n the region of maximum d i f f e r e n t i a l worth, i s postulated. 2.

Cold slug accident - The f u e l c i r c u l a t i n g pump i s s t a r t e d with t h e core j u s t c r i t i c a l a t 1200'F while the f u e l i n t h e external loop i s a t 900 OF.

3.

Abnormal concentration of uranium during fuel addition - A slug of 120 g of 235U, added i n the pump bowl, i s assumed t o go around t h e loop without d i l u t i o n and enter t h e core as a "front" uniformly distributed t o a l l f u e l channels.

4.

Displacement of graphite by f u e l s a l t - Loss of an e n t i r e graphite s t r i n g e r (62 i n . long) from the center of t h e core and replacement by f u e l salt i s postulated.

5.

Premature c r i t i c a l i t y during f i l l i n g - The core i s f i l l e d a t t h e maximum possible r a t e with f u e l s a l t i n which the uranium concentration has been increased 60%by selective freezing i n t h e drain tank. C r i t i c a l i t y i s achieved with t h e core 60% f u l l .

6.

Loss of f u e l circulation - Failure of the power supply t o the f u e l c i r c u l a t i n g pump and t h e onset of natural-convection c i r c u l a t i o n are considered.

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.

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7. Loss of load - Instantaneous loss of a l l heat-removal capability a t t h e radiator i s assumed. I n all the events considered, except t h e f i l l i n g accident, t h e reduction i n r e a c t i v i t y obtained by dropping any two of t h e three control rods (with a 0.1-sec time delay and an acceleration of 5 ' f t / s e c 2 ) limited t h e power, temperature, and pressure excursions t o e a s i l y tolerable values. I n t h i s context, tolerable values a r e those a t which no damage t o equipment i n t h e reactor c e l l occurs. I n the case of the fuel-pump power f a i l u r e , an additional action, closure of the radiator doors, i s required t o avoid freezing t h e coolant salt i n t h e radiator. Aside from the f i l l i n g accident, t h e incident with t h e greatest damage p o t e n t i a l i s the uncontrolled withdrawal of t h e control rods.

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ORNL-3708  

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

ORNL-3708  

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

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