Total reactivity of reactor

4 Other Beactiwity Losses. Experimental setups around and in the reactor lattice will further reduce the reactivity of the machine.t An additional 5% in Ah/h was arbitrarily assigned to handle.this. It is estimated that this margin in Ah/h represents an absorber of total crosa-section 330 cm placed uniformly in a 2-kg reactor. It may be pointed out that in the present ORNL reactor. the total poison cross-section which will take up the excess available for experiments, i.e.. about O.S%, is 4000 cm if spread uniformly or 500 cm if concentrated at the center.  

9 Total Besetlvity of Beseter. The overall minimum excess reactivity built into the reactor is summarized in Table 4.3.B. 

Bisimsm Bosetlvity Losses Bhleh Bast Bo Allosod for is Bosetor 

From the curve -o.f critical mass versus fc in Fig. 4. 3.D it may be esti-aated that about 2.6 kg is needed to ensure.an excess Afe/h of 18.9%. 

A proper analysis of the tine dependent behavior of.a reactor operating on thermal neutrons must take into account the important effects on its criticality, reactivity, and stability which arise from such factors as fission i products of high thermal-neutron capture cross-section, depletion, temperature, average neutron lifetime in the reactor, flux level, and reactor period. As has been seen in the requirements placed on the.reactor, considerable excess reactivity must be built into the active core before start-up. The control rods must keep the reactivity below the critical value before and during start-up. 

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