Effect of xenon poison

For the first-of-a-kind BN GT unit, base load mode of operation was selected further BN GT units could be operated in load follow modes within electric output variation between 90 and 300 MW(e). The prerequisites for this are fast neutron spectrum (no effect of xenon poisoning in reactivity) and the use of a special gas turbine. To realize load follow operation modes, a demonstration of fuel element reliable performance under multiple power ramps and associated thermo-cycling would be needed, which could be accomplished during the operation of a first-of-a-kind plant. 

Answer The most important fission product to reactor operation is xenon-135 because of its large absorption cross-section - about 3.2 x 10 barns - for thermal neutrons. The effects of the xenon poison transients influence reactor operation in many ways. For example, although the saturated poison effect at equilibrium operation may cause a reactivity loss of 2 to 2-l/2 k (2000-2500 c-mk) in the usual power ranges, the decay of this amount of poison in the shutdown pile will represent a total swing of 4000 to 5000 c-mk from the initial xenon-free pile at startup. This delayed action effect in xenon formation and decay is the major cause for the scram transient, minimum downtime, and turnaround problems encountered in the operation of a hi power reactor. A detailed discussion on the effects of xenon poisoning on pile reactivity may be found in Chapter IV of this series. 

The effects of Xenon poisoning phenomena exacerbated any A Reactor shutdown, and nearly guaranteed that every time the reactor was shut down the operators would lose at least one day s worth of Pu production. The Soviets expected to be able to produce 60-80 g of plutonium per day from the A Reactor. A critical mass of about 5 kg (Rhodes, 1986) was needed for a nuclear weapon, so about 3 months of Continuous full power operation should have been sufficient to generate the required plutonium. 

Effect of Xenon Poison Fraction on Fuel Costs... 

For the reactor described above, it is calculated that 900 atomic parts of per million parts of bismuth are necessary for criticality, if there is no poisoning of the reactor. However, if the effect of xenon and samarium... 

The xenon poisoning effect is well known in the field of nuclear engineering as the effect that prevented early reactors from rapid startup after shutdown. In addition to Xe being created in high relative yields as an independent yield fission product in the fission of U and Pu, it is also created in high amounts by a chain-yield fission product from decay of Te and I via ... 

We continue with a description of major design features and then a more detailed comparison of cooling circuits and reactor stability. We finish with accounts of resonance, Doppler and xenon poisoning effects and some aspects of reactor physics at Chernobyl as more technical appendices. 

These longer-term effects—the rise in temperature, the burnup of fuel, production of neutron-absorbing fission products, etc.—tend to lower the reactivity. Thus considerable positive reactivity must be built in to the fresh, clean and cold reactor. This in turn is contained by absorber rods, to be withdrawn as the effect develops (and perhaps burnable poisons ). One of these effects, the production of xenon, certainly played a part in the Chernobyl accident and is detailed in Appendix 6.1 to this section. 

Information must be available to determifie the individual reactivity contribution of xenon, graphite heating and cooling, metal heating and cooling, excess at shutdown in horizontal rods, and the effects of metal and poison discharge. The s um. of these separate reactivity items can be used to calculate the pile reactivity at. any time. 

The first two chapters serve as an introduction to the basic physics of the atom and the nucleus and to nuclear fission and the nuclear chain reaction. Chapter 3 deals with the fundamentals of nuclear reactor theory, covering neutron slowing down and the spatial dependence of the neutron flux in the reactor, based on the solution of the diffusion equations. The chapter includes a major section on reactor kinetics and control, including temperature and void coefficients and xenon poisoning effects in power reactors. Chapter 4 describes various aspects of fuel management and fuel cycles, while Chapter 5 considers materials problems for fuel and other constituents of the reactor. The processes of heat generation and removal are covered in Chapter 6. 

By 1 (X) am, on April 26, the operators were able to stabilize the power back at 2(K) MWt, but this was as high as they could get it due to the xenon poison buildup that had started during the excursion to lower power and was still continuing. To drag the reactor up to 200 MWt, the operators had pulled far too many of the manual control rods out of the reactor, and the neutron flux distribution in the core was such that the reactivity worth of those rods that would be effective in the first few centimeters of travel back into the core was limited to the equivalent of six to eight fully inserted rods. 

Just after midnight on April 26, the plant operators, who were anxious to complete the turbine spin down test, withdrew the vast majority of the control rods to counteract the effects of the xenon poisoning and keep the reactor critical. The reactor responded by creeping back up to 200 MWt, by about 12 30 am, and the test proceeded (US NRC, 1987). 

Firstly, it is the operational effects arising because of the reactor thermal power itself as also the temperature rise in the various regions of the core. Operational reactivity effects also include poisoning due to fission products like Xenon and Samarium. Secondly, it is the reactivity effect due to the various experimental/irradiation assemblies present in the reactor core. 

Studies have been carried out at BNL on control requirements for an LMFR experiment. The control requirements depend not only on the choice of operating temperatures, the possible xenon and fission-product poisoning, etc., but also on conceivable emergency situations. such as errors in fuel concentration control. In a reactor with a full breeding blanket, the control requirements may have to include the effect of complete loss of the breeder fluid. 

Reactivity Control. The movable boron-carbide control rods are sufficient to provide reactivity control from the cold shutdown condition to the full-load condition. Supplementary reactivity control in the form of solid burnable poison is used only to provide reactivity compensation for fuel burnup or depletion effects. The movable control rod system is capable of bringing the reactor to the subcritieal when the reactor is an ambient temperature (cold), zero power, zero xenon, and with the strongest control rod fully withdrawn from the core. In order to provide greater assurance that this condition can be met in the operating reactor, the core is designed to obtain a reactivity of less than 0.99, or a 1% margin on the stuck rod condition. See Fig. 7. 

If the xenon processing is to have any appreciable effect on the steady-state poisoning ratio, then the processing rate /x must be sufficiently large to increase the value of the group ( xe + 0< xe+/xe) E<1- (2.119). For example, for a flux of 3.5X10 /(cm -s), the... 

To maintain the specified degree of snbcritioality for an indefinite period of time after shutdown, additional means as provided in the design may be used, such as the use of boronated water or other poisons if the temperature, xenon concentration or other transient reactivity effects cannot be compensated for by normal reactivity control devices. 

Answer Adding or subtracting an increment of poison by control rod adjustment results in absorbing more or less neutrons and a decrease cr increase In the local power or fl ux In one of the small pile" regions. Xenon transients resulting from an Increased rate of b ornout when flux is increased, or Increased rate of buildup when flux is decreased, v lll Induce f urther local fl ox changes unless the transient effect is controlled locally. 

Capture of neutrons in other than fuel material derivation of K effective (six factor formula) control elements reflector poisons (xenon and samarium buildup and decay). 

The cluster system of reactivity compensation is used to compensate for temperature and power reactivity effects, reactivity margins for core poisoning by xenon-135 and samarium-149, operating margins to change reactivity during reactor power changes, and to provide core sub criticality under reactor shutdown. 

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