Xenon poisoning

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 ... 

In reactor control problems and in reactor neutron balances, the quantity of interest is the poisoning ratio r, which is the ratio of neutrons absorbed by the poison to neutrons absorbed in fission. Assuming for simplicity that the neutron flux is constant throughout the reactor, the xenon poisoning ratio at steady state is... 

Figure 2.16 shows the growth of xenon to its steady-state value during reactor operation and its subsequent decay after the reactor is shut down. The quantity plotted is the xenon poison ratio, which is the ratio of the rate of absorption of neutrons by xenon to the rate of absorption of neutrons in fission of NxtOxJNfOf. Curves are given for fluxes of... 

Figure 2.16 Xenon poison ratio during reactor operation at constant flux and after shutdown.

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. 

Reactor stabilised at 200 MW(th)with difficulty—no spare reactivity (mainly due to xenon poisoning) to increase power further. Even so, to achieve this required withdrawing more rods than was allowed in the station rules "... 

One danger in a brief review article is that the examples given may be so slight in content that significant aspects of the theory may not be adequately illustrated. We have chosen, therefore, to confine the examples to a single model or problem, namely, the shutdown of a nuclear reactor subject to xenon poisoning, in order to save having to develop the details of the... 

To clarify some of these ideas, we select an example from the problem of shutting down a reactor in a fashion that overcomes the xenon poisoning transient. This example is of some practical value, although it has perhaps the bigger attraction of illustrating several points about the optimum theorem. 

After reactor shutdown, the xenon poisoning Initially increases. 

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. 

A. G. Ward, Universal Curve for the Prediction of Xenon Poison after a Reactor Shutdown, Atomic Energy of Canada Limited, CRRP-685, Jan. 25, 1957. 

Thermal power was to be reduced in stages from 3200 MW to 700 MW (MW). Operator error caused the power to undershoot to 30 MW, and the consequent increase in xenon poisoning meant that the operators could only manage to raise the power back to 200 MW. This power level was lower than stipulated in the test instructions. The operators should have abandoned the test at this point. ( Xenon poisoning is a transient behavior of nuclear reactors during power reductions. Xenon-135 is a fission product that absorbs neutrons, and which decays with a half-life of about 9 h. When power is reduced, xenon-135 levels increase, which makes it difficult to increase power until the xenon-135 has decayed.)... 

Graphite temperature Metal temperature Xenon poisoning... 

Answer Although a potent absorber of neutrons, samarium is a minor problem compared with that of xenon poisoning. Promethium-1 9 is one of the fission products of lj235 and decays on a two-day half-life to samarium-1 9 which is a stable nuclide with a large absorption cross-section (66,000 barns) for thermal neutrons. As the samarium poison accumulates, the chance for burnout by neutron capture increases, so the pile reactivity absorbed by samarium reaches a maximum or saturated value of around 600 c-mk under steady operating conditions. 

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. 

Answer Yes, the approach to maximum operating level must be made cautiously because xenon poisoning will continue to increase as a result of the final raise in power level. This will require rod withdrawal to compensate for the xenon increase if the power level is too high, extensive rod withdrawal may cause tube outlet or graphite temperatures to be exceeded. By slowing the power raises as limits are approached, the equilibrium power level is reached at approximately the same time as the equilibrium reactivity. Final adjustment of control rods, power level, flux and temperature distribution can then be made to approach a maximum power level without forced changes in rod configuration. 

The amount of Xenon poisoning varies with flux, but is not directly proportional to flux (since it decays at a rate proportional to number of atoms of Xe-135 and is burned out at a rate proportional to flux). 

Figure 4 shows the buildup of Xenon poisoning as a function of time after reaching full power for various neutron flux levels, In all cases, the reactivity loss (from a clean start) due to Xenon, will reach 90% of equilibrium value in about 24 hours and full equilibrium level in 40 to 50 hours. 

Two equations used to determine steady state xenon poison levels in a reactor core are the equilibrium xenon balance and the equilibrium iodine balance. 

After a reactor startup, xenon poisoning reaches its equilibrium value at 30 HWD/T (2.5 days) and has a value of 2.8 per cent A k/k for an average pile exposure of 700 After reactor shut-... 

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. 

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