In a uranium-fueled thermal reactor, the reactivity changes with burnup are asso­ciated with the following factors:

(i) Depletion of 235U;

(ii) Buildup of 239Pu;

Natural Uranium-Fueled and

Graphite-Moderated Reactors

Burnup

LWRs

Fig. 1.3 Reactivity change due to 235U depletion and 239Pu buildup. (a) Reactivity decrease due to

235 239 235 239

U depletion, (b) reactivity increase due to Pu buildup, (c) reactivity change of U and Pu

(iii) Buildup of 241Pu;

(iv) Buildup of non-fissile nuclides, 240Pu, 236U, and 242Pu;

(v) Buildup of highly thermal neutron-absorbing FPs (135Xe and 149Sm); and

(vi) Buildup of other FPs.

Items (i) and (ii) mainly lead to reactivity changes by increase or decrease in amounts of fissile nuclides. Reactivity drops due to exponential depletion of 235U and reactivity rises due to buildup of 239Pu as shown in Fig. 1.3a, b, respectively. The combined reactivity change of U and of Pu is presented in Fig. 1.3c. In an enriched-uranium fueled LWR, the effect of 235U is generally larger than that of 239Pu and it results in a reactivity decrease with burnup. In the case of a fuel type of natural uranium and high conversion ratio for graphite-moderated reactors, there is

an early reactivity increase and then a decrease with burnup because 235U and 239Pu

235 239

produce comparable reactivity effects and the fissile replacement of U by Pu proceeds.

240Pu, 241Pu, and 242Pu are produced with burnup by neutron capture. 240Pu has a large thermal-neutron absorption cross section and has a negative reactivity effect. 241Pu produced by the subsequent neutron capture is a fissile nuclide and has a positive reactivity effect. On the other hand, 242Pu produces a small negative reactivity because its production amount and cross section are small.

236 235

U produced from U by neutron capture also gives a small negative reac­tivity effect.

It is also necessary to consider the negative reactivity effect by buildup of FPs, which can be classified as two significant FPs and other FPs. The former are 135Xe and 149Sm which have very large absorption cross sections. Within a short time after burnup (usually within several hours to several days), they reach an individual equilibrium concentration by the balance between production and destruction because of the strong neutron absorption, and they have a constant negative reactivity effect. The other FPs continue to be accumulated with burnup because of low absorption cross sections and to increase the negative reactivity.

Typical reactivity changes for such individual reactivity effects are shown in Fig. 1.4 for natural uranium-fueled, graphite-moderated reactors. After an initial increase in reactivity due to the buildup of 239Pu, there is a gradual reduction in reactivity due to the depletion of 235U and the buildup of neutron absorbing materials. These analyses of reactivity changes with fuel burnup are used to determine the fissile amount for criticality during reactor operation and to deter­mine the reactivity worth of control rods.