Fission product reactivity

The principal natural phenomena that influence transient operation are the temperature coefficients of the moderator and fuel and the buildup or depletion of certain fission products. Reactivity balancing may occur through the effects of natural phenomena or the operation of the reactor control system using the RCCs or chemical "shim." A change in the temperature of moderator or fuel (e.g., due to an increase or decrease in steam demand) will add or remove reactivity (respectively) and cause the power level to change (increase or decrease, respectively) xmtil the reactivity change is balanced out. RCC assemblies are used to follow fairly large load transients, such as load-follow operation, and for startup and shutdown. 

As xenon-135 and samarium-149 are formed in a reactor, they reduce the multiplication factor by decreasing the thermal utilization factor, f, Since the formation of fission product poisons is a direct function of the fission rate, as power level changes the amount of poison present in the reactor also changes. Control system reactivity insertions such as rod motion and chemical shim must be made to compensate for fission product reactivity. 

Concerning the nuclear data in the JEF-2.2 library, a general conclusion was that the inelastic scattering cross-sections for many even mass nuclides were about a factor of 2 low because of the neglect of direct interaction effects in the nuclear models used to calculate these cross-sections. The contribution of inelastic scattering is of the order of 15% of the total fission product reactivity effect. The most important even mass nuclei are Mo-98, - 100, Ru-102, - 104, - 106, Pd-106, - 108, Xe-132, - 134, Nd-146, - 148 and Sm-152. Improved theoretical models have now been developed and validated by direct measurements of the differential cross-sections of Pd isotopes. As a consequence of the analyses of the integral measurements, it has been concluded that the contribution of fission products to the variation of reactivity with bum-up can be predicted to within 5% (Is) for a conventional fast reactor. 

The Model 412 PWR uses several control mechanisms. The first is the control cluster, consisting of a set of 25 hafnium metal rods coimected by a spider and inserted in the vacant spaces of 53 of the fuel assembhes (see Fig. 6). The clusters can be moved up and down, or released to shut down the reactor quickly. The rods are also used to (/) provide positive reactivity for the startup of the reactor from cold conditions, (2) make adjustments in power that fit the load demand on the system, (J) help shape the core power distribution to assure favorable fuel consumption and avoid hot spots on fuel cladding, and (4) compensate for the production and consumption of the strongly neutron-absorbing fission product xenon-135. Other PWRs use an alloy of cadmium, indium, and silver, all strong neutron absorbers, as control material. 

Studies of the influence of irradiation on the kinetics of oxidation have been confined to post-irradiation work. In general, prior irradiation increases reactivity, although there are considerable inconsistencies in the enhancements obtained The effects can be derived from an increased surface area associated with the swelling voids produced in the metal by the irradiation, and can also probably arise to a lesser extent from chemical effects of the fission products. 

There is also a third type of reactive species that we shall discuss in detail in Chapter 9, namely radicals. Briefly, radicals are uncharged entities that carry an unpaired electron. A methyl radical CH3 results from the fission of a C-H bond in methane so that each atom retains one of the electrons. In the methyl radical, carbon is sp hybridized and forms three CT C-H bonds, whilst a single unpaired electron is held in a 2/ orbital oriented at right angles to the plane containing the ct bonds. The unpaired electron is always shown as a dot. The simplest of the radical species is the other fission product, a hydrogen atom. 

We report on a number of on-line chemical procedures which were developed for the study of short-lived fission products and products from heavy-ion interactions. These techniques combine gas-jet recoil-transport systems with I) multistage solvent extraction methods using high-speed centrifuges for rapid phase separation and II) thermochromatographic columns. The formation of volatile species between recoil atoms and reactive gases is another alternative. We have also coupled a gas-jet transport system to a mass separator equipped with a hollow cathode- or a high temperature ion source. Typical applications of these methods for studies of short-lived nuclides are described. 

The term eff — 1 is called excess reactivity, and /eff — l)/ eff is called reactivity. Because the fissile material is continuously used up by fission and because the fission products absorb neutrons, a certain excess reactivity is necessary to operate a nuclear reactor. This excess reactivity is compensated by control rods that absorb the excess neutrons. These control rods contain materials of high neutron absorption cross section, such as boron, cadmium or rare-earth elements. The excess reactivity can also be balanced by addition to the coolant of neutron-absorbing substances such as boric acid. 

In some nuclear reactors, U02 fuel elements in stainless steel cans are subjected to high rates of burning that is, substantial quantities of fission products are formed. As a result, appreciable amounts of 02 are liberated (from U02) which will then react either with the fission products (zirconium, molybdenum, cerium, etc.) or with the stainless steel of the can. Decide, with the help of Fig. 10.2 whether zirconium and molybdenum are oxidized in preference to the steel (T=750°C). (In fact sufficient reactive elements are formed to take up the oxygen.)... 

Another important objective is to follow the changes in reactivity that take place as fissile nuclides are depleted or formed from fertile nuclides, and as neutron poisons are formed through buildup of fission products or burned out through reaction with neutrons. 

Boron may also be used as a burnable poison to compensate for the change in reactivity with lifetime. In this scheme, a small amount of boron is incorporated into the fuel or special burnable poison rods to reduce the beginning-of-life reactivity. Bumup of the poison causes a reactivity increase that partially compensates for the decrease in reactivity due to fuel burnup and accumulation of fission products. Difficulties have generally been encountered when boron is incorporated directly with the fuel, and most applications have used separate burnable poison rods. 

---