The effective multiplying coefficient is: the ratio of generated neutrons in every generation to generated neutrons in last generation. So to operate the nuclear reactor in steady state, this parameter should be: 1 means the generated neutrons in every generation are equal with neutrons which have absorbed or leaked in last generation that means: critical state. The minimum value of K, ff is: 0 and maximum of it is: u namely: 2.43.

The effective multiplying coefficient is [5]:

K =n. f. V£.P .P =n. f. V£.P (16)

ef FNL THNL J r TNL

As the thermal fission coefficient (n) is [5, 6]:

Where:

and:

NO = 2NU = 2 = 2——— m-^A——

MU rM235 +(1-r )M 2

Also the thermal absorption coefficient (f) is [5]:

The resonance escape probability for fast neutrons (p) also is calculated as following [5]:

238

(22) (23) (24) (25)

P = ‘

Where:

and:

E. — N235 .o235 + N238.a?8 + N°.o° + NH. oh + N°M. o° + NZr. oZr

If the enrichment of fuel is 100% then the resonance escape probability for fast neutrons (p) will be maximum value.

According to enrichment of applied fuel and its mass can write [7]:

r-mjL

(26) (27) (28) (29) (30) (31)

m

Where [8]:

NLm. Mfm V mU°2 — mfm = A = Pfm^fm

A

and:

dpn dt

and:

G (p) = VGT (p).p (32)

Firstly the transfer function is supposed. Secondly due to mostly reactor’s cores are twin therefore once a signal with delay time (td) from first part to second part is transmitted then that signal with a same delay time from second part to first part will be transmitted.