Operational Reactivity Transients

Test scale fuel loads> whlch by reason of loading errors or Inherent differences In operational reactivity transient behavior may result In or have the potential for providing significant changes In reactivity shall be limited In size such that the additional reactivity of the test lo id could not cause an increase of more than 1 per cent In the gross pile reactivity in the event of a loading error such as omission of poison pieces. 

Adjustment at rates fast enou to override all normal operational reactivity transients and to minimize delays at startup. However, the system must be limited to its withdrawal rate to that rate which, In coincidence with simultaneous failure of the Primary Safety System, would not result In fuel melting. 

Control of the core is affected by movable control rods which contain neutron absorbers soluble neutron absorbers ia the coolant, called chemical shim fixed burnable neutron absorbers and the intrinsic feature of negative reactivity coefficients. Gross changes ia fission reaction rates, as well as start-up and shutdown of the fission reactions, are effected by the control rods. In a typical PWR, ca 90 control rods are used. These, iaserted from the top of the core, contain strong neutron absorbers such as boron, cadmium, or hafnium, and are made up of a cadmium—iadium—silver alloy, clad ia stainless steel. The movement of the control rods is governed remotely by an operator ia the control room. Safety circuitry automatically iaserts the rods ia the event of an abnormal power or reactivity transient. 

The fifth question focuses on a particular fixed volume element in the reactor and whether it changes as a function of time. If it does not, then the reactor is said to operate at a stationary state. If there are time variations, then the reactor is operating under transient conditions. A nontrivial example of the transient situation is designed on purpose to observe how a chemically reactive system at equilibrium relaxes back to the equilibrium state after a small perturbation. This type of relaxation experiment can often yield informative kinetic behavior. 

A depressurization accident with a subsequent water ingress was found to not represent an intolerable load upon the pressure-keeping containment in terms of peak pressure and pressure transient. Reactivity transients including false operation of absorber rods, water ingress, or a decrease of core inlet temperature were also analyzed to cause no serious damage to the heat exchanging components, unless both reactor shutdown and helium circulators fail at a time. 

If the operational triplet energy level for excited states of humic substances Is Indeed around 57 kcal/mole, then pollutants which have triplet energy levels below this value are likely to photoreact by an energy transfer mechanism mediated by humic substances or DOM. If a redox potential of an environmentally reactive transient produced by humic substances Is higher than that of a xenoblotlc, then the xenoblotlc may be oxidized, etc. If the reaction Is also klnetlcally favorable. 

A reversible reactivity transient, which sets in at a specific core AT was observed during low power operation. The magnitude of the transient reduces and the AT at which the reactivity transient sets in increases with respect to increase in core flow. The reactor operation was continued above a primary flow of 450 mVh, as the phenomenon does not occur above this value. 

In 1998, when reactor was being operated at 8 MWt for irradiation of zirconium-niobium (Zr-Nb) alloy, reactivity transient, that is repeatable in nature, was encountered. Following are the observations ... 

Finally, if attention is focused on a fixed volume element in the reactor, all properties in that volume element may or may not change with time. If they do not, the reactor is said to operate at the stationary state. If they do, the system will operate under transient conditions. A nontrivial example of the latter situation obtains when a chemically reactive system at equilibrium is submitted to a perturbation and its kinetic behavior is deduced from the fact that the perturbed system returns or relaxes to equilibrium. The perturbation may be periodic. In any event, this is a typical relaxation experiment. 

No reactivity transient incident recurred during the reactor operation in the year... 

The Initial 21 6 Ib/ft fuel element for N Reactor Is not the only fuel choice possible It is a conserva.tive choice one desi ied so that the reactl-y ty transients should be within the capacity. of the horizontal rod system. A heavier fuel element Is not impossible sad may well be charged after the reactor has been operated and the actui reactivity transients and effective control rod strengths determined ... 

Answer (1) To control the approach to critical, (2) to adjust and control rate of rise, (3) for minute-to-minute adjustment of multiplication. factor during equilibrium operation, (4) compensates for reactivity transients that is, xenon transients, tesqperature effects, turnaround, etc., (5) to control heat distribution, and (6) to shut down the reactor quickly under controlled conditions. 

Answer An approach-to-limit seem control feature would be particularly effective In monitoring heat Shifts that may occur during scram recoveries and during periods of rapid rod motion required to control the reactivity transients following reactor start-up. Uhder equilibrium operating conditions, a flexible approach-to-limit monitor of this type would permit a safe and balanced approach to higher process tube limits. 

Does the turnaround reactivity transient impose any limit on reactor operation ... 

Answer The magnitude of the reactivity transient from startup to turnaround will depend primarily on the power level, the amounts of iodine and xenon present, and the metal and graphite coefficients. Thus, the startup level and operating efficiency will he limited by the amoxmt of rod available for controlling the reactivity transient following startup. A particular i>ower level is determined through calculational methods such that the resulting transient will barely not exceed the capacity of the control rod system. 

Reactivity Transient - The variation of available excess reactivity with time. Since the iiactor is normally operated with a nearly constant power level (tha.t is, Kex equal to 1.000) the reactivity transient is the variation with time of the amount of control red, plus any supplementary control in the reactor. 

The following example apphcable to pressurized water reactors (PWRs) may further illustrate the approach described. One of the SFs relevant for Levels 1-3 of defence in depth is prevention of unacceptable reactivity transients. This SF can be challenged by insertion of positive reactivity. Several mechanisms lead to such a challenge, including control rod ejection, control rod withdrawal, control rod drop or misalignment, erroneous startup of a circulation loop, release of absorber deposits in the reactor core, incorrect refuelling operations or inadvertent boron dilution. For each of these mechanisms there are a number of provisions to prevent its occurrence. For example, control rod withdrawal can be prevented or its consequences mitigated by ... 

There are similarities between BN-350 and the Phenix plant in France, which is of a similar age. Phenix also has been operating for more than 20 years, and has been subject to a review of the requirements for future operation which has revealed the need to improve the ability of parts of the auxiliary systems to withstand seismic damage. There have been leaks on the secondary side of some of the intermediate heat exchangers and as a result they are all to be replaced. Inspection in the period 1991 -1993 revealed cracks in some of the secondary circuit components made of 321 stainless steel, and repairs have been made. In 1989 and 1990 a series of fleeting negative reactivity transients were experienced, but in spite of intense investigation the cause has not been identified. 

To facilitate modeling of the metal fuel used in KALIMER, several reactivity models are modified in SSC-K code. For neutronic calculations, SSC-K uses point kinetic equations with detailed reactivity feedback from each channel. Reactivity effects are required both for transient safety analysis and for control requirements during normal operation. Reactivity changes are calculated for control rod scram, the Doppler effect in the fuel, sodium voiding or density changes, fuel thermal expansion, core radial expansion, thermal expansion of control rod drives, and vessel wall thermal expansion. Figure 5 shows the components of reactivity feedback considered in the KALIMER core. The effect of fuel expansion becomes more significant when metallic fuel is used. 

Reactive control is also possible through synchronous condensers. As they rotate, the rotor stores kinetic energy which tends to absorb sudden Huctuations in the supply system, such as sudden loadings. They are. however, sluggish in operation and very expensive compared to thyristor controls. Their rotating masses add inertia, contribute to the transient oscillations and add to the fault level of the system. All these factors render them less suitable for such applications. Their application is therefore gradually disappearing. 

An important requirement of kinetic studies for automotive aftertreatment devices is the capability of performing dynamic reactive experiments. Steady-state tests provide useful information for identification of reaction pathway and stoichiometry, but cannot capture the real operating behavior of catalytic converters for vehicles, which is transient in nature. Indeed, this is so not only because of the continuously changing conditions (temperature, composition, flow rate) of the engine exhausts as extensively addressed in the following sections, the principles of NSRC and SCR applications largely rely on the storage/reaction/release dynamics of NOx and of NH3, respectively. 

This represents the typical feed mixture to SCR converters when no oxidation precatalyst is applied, as for instance in the case of NOx abatement from stationary sources. With such a feed mixture the main deNOx reaction occurring over V-based catalysts is the so-called standard SCR (R6 in Table V). Transient experiments in a wide range of temperatures (50-550°Q were performed in order to develop a suitable kinetic model of the NH3-N0/02 reacting system. The study of the standard SCR kinetics was particularly focused on the characteristics that are critical for mobile applications, namely the behavior during transient operation and the reactivity in the low temperature region. 

In all four cases, tlie initial reaction rates at the start of illumination in the continuous-feed photoreactor were higher than the pseudo-steady-state reaction rates the reaction rates declined over time until pseudo-steady-state operation was achieved. Tliis apparent deactivation phenomenon, often observed with aromatic contaminants, is discussed in Sec. III.E. In a transient reaction system, the time required to reach pseudo-steady-state operation also appears to increase in the same order as the reaction rates. For example, for the continuous photocatalytic oxidation of aromatic contaminants at 50 mg/m in a powder-layer photoreactor, the time required for pseudo-steady-state operation to be achieved was reported to be approximately 90 min for benzene, 120 min for toluene, and as long as 6 hr for wz-xylene [50,51]. Under such conditions, the difference in reaction rates between the aromatic contaminants is magnified by the fact that the more reactive aromatics retain their higher transient reaction rates for longer periods (Fig. 7). 

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