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107 The source requirement could be met with an antimony-beryllium source, but the 60-day h a l f - l i f e of 324Sb d i c t a t e s a substantially greater source t o avoid frequent replacement. (The neutron f l u x i n t h e thermal shield a t t h e source tube i s too low t o maintain the 124Sb a c t i v i t y , even a t f u l l power.)

Because of uncertainties i n the calculations of the source requirement, t h e f i n a l specification of the external source w i l l be based on measurements t o be made with the reactor vessel i n s t a l l e d . The cons t r u c t i o n and s t a r t u p schedule i s such t h a t there i s t i m e f o r procurement of a source a f t e r these measurements and before the source i s needed f o r nuclear operation. Kinetics A number of studies, using both d i g i t a l and analog computer techniques, have been made t o explore t h e kinetic behavior of the reactor. These calculations can be divided i n t o two categories: those dealing with small disturbances such a s random "noise" and changes i n load demand t h a t would be encountered i n normal operation, and those dealing with major disturbances t h a t a r i s e from abnormal s i t u a t i o n s .

Normal O-oeration The reactor w i l l be self-regulating during normal operation because of t h e negative temperature coefficients of both the f u e l and graphite. The degree of self-regulation depends on t h e power l e v e l and i s r a t h e r sluggish even a t full power. A number of f a c t o r s contribute t o the slow response of t h e system. These include: 1. low power density i n t h e core,


high heat capacities of the f u e l and graphite,


low heat-transfer rate between f i e 1 and graphite,

4. low heat production i n t h e graphite, 5.

long loop delay times between t h e heat sink a t the r a d i a t o r and t h e heat source i n t h e core.

Figures I1 and 1 2 show t h e r e s u l t s of analog calculations of t h e system response t o changes i n load demand a t the radiator, i n the absence of external r e a c t i v i t y control. I n both cases, slow changes i n demand were s t a r t e d a t zero time, and the temperature and power response of t h e reactor were recorded. The slow decrease i n f u e l temperatures a f t e r t h e increase i n power and the increase a f t e r t h e power reduction r e f l e c t t h e attainment of t h e steady-state temperature ,distribution i n t h e graphite. The slow power o s c i l l a t i o n a t low power w a s observed i n a l l simulations and appears t o be an inherent c h a r a c t e r i s t i c of t h e system. During routine operation, some external r e a c t i v i t y control w i l l be imposed through t h e use of a servo-operated regulating rod. This i s ex-