The reactivity controlled by soluble boron concentration adjustment is relatively slow temperature change from cold to hot, excess reactivity decline during an operating cycle, and FP concentration variation. The reactivity variations can be approximated as the following three items.

• Reactivity change from cold to hot temperature: about 6 %Ak/k

• Reactivity decline during operating cycle: about 10 %Ak/k (depends on cycle length)

• Xe reactivity: about 3 % Ak/k at equilibrium full power and maximum about 6 % Ak/k after shutdown.

Figure 3.44 shows dependence of critical boron concentration on cycle burnup at hot full power operation, which is monotonous, and therefore it is easy to manage the excess reactivity.

The Xe reactivity is important in core management. A reactivity varia­tion from the equilibrium Xe condition to after reactor shutdown is

described in the fourth graph of Fig. 3.38. 135Xe, which has a very large neutron absorption cross section, is produced mainly through fission!135I (half-life 6.7 h) ! 135Xe (half-life 9.2 h) ! 135Cs (half-life

2.6 x 106 years). The Xe reactivity temporarily increases by decay of 135I after reactor shutdown, and monotonously decreases after a peak at about 8 h and becomes almost zero after about 3 days. When the reactor re-starts up within 3 days after shutdown, it is necessary to evaluate such Xe reactivity and to predict a critical point, and then to adjust the soluble boron concentration.

The differential boron worth is shown in Fig. 3.45.