Reactivity Losses

For various reasons, e-g. Xe poisoning, depletion of II, etc., the reactivity of the reactor is.lower during high-power operation than during zero-power operation. The reactor therefore must be built considerably larger than is necessary to be critical in the cold, clean state. Before summarizing the reactivity losses which are entailed by Xe + Sm, depletion of U, and temperature rise, the conventions for expressing reactivity loss will be reviewed briefly  

If a poison is placed in or removed from a reactor which is just critical, the neutron density will decay or rise with a period T. This period is directly proportional to the change in reactivity, which we denote by 

It is possible to calculate the reactivity change therefore the period in terms of the abso rption cross-section of the poison and its position in the. reactor. More generally, if any perturbation is imposed on the reactor, e.g., if the density of the aioderator is changed, then the reactivity will change in a aianner which is calculable in terms of the magnitude of the perturbation and its position in the reactor. 

7) number of neutrons produced for each thermal neutron absorbed in uranium. 

However, in a small enriched reactor such as the MTR, both p and are sufficiently close to unity that, to a very good approximation. 

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The reaction rate data developed for each chemical in the tables are used to select a reactivity class as described earlier, and hence a first-order rate constant for each medium. Often these rates are in considerable doubt thus the quantities selected should be used with extreme caution because they may not be widely applicable. The rate constants kj h 1 are used to calculate reaction D values for each medium DK as V/ k,. The rate of reactive loss is then DRif mol/h. 

The experimental quantity used to characterize heterogeneous reaction rates is the "reaction probablity", y, which is defined as the fractional collision frequency that leads to reactive loss. Kinetic data for the generally irreversible reactive uptake of trace gas species on condensed surfaces are expressed in terms of uptake experiments, where the disappearance of the species under consideration and/or the appearance of one or more reaction products has been observed. Such processes may not be rate limited by Henry s law constraints, however the fate of the uptake reaction products may be subject to saturation limitations. 

Reactive Loss (adverse life events). Physical illness (myocardial infarct, cancer). Drugs (antihypertensives, alcohol, hormones). Other psychiatric disorders (senility). More than 60% of all depressions. Core depressive syndrome depression, anxiety, bodily complaints, tension, guilt. May respond spontaneously or to a variety of ministrations. 

The microcanonical ensemble may be depleted in the vicinity of the transition state by the absence of trajectories in the reverse direction. This assumption is often referred to as the ergodic approximation, that the microcanonical ensemble is rapidly randomized behind the reaction bottleneck faster that reactive loss can perturb the distribution. 

One possible cause of molecular weight limitations is the reactivity loss of the organometallic center due to increased steric hindrance from... 

Reactions taking place on the surface of solid or liquid particles and inside liquid droplets play an important role in the middle atmosphere, especially in the lower stratosphere where sulfate aerosol particles and polar stratospheric clouds (PSCs) are observed. The nature, properties and chemical composition of these particles are described in Chapters 5 and 6. Several parameters are commonly used to describe the uptake of gas-phase molecules into these particles (1) the sticking coefficient s which is the fraction of collisions of a gaseous molecule with a solid or liquid particle that results in the uptake of this molecule on the surface of the particle (2) the accommodation coefficient a which is the fraction of collisions that leads to incorporation into the bulk condensed phase, and (3) the reaction probability 7 (also called the reactive uptake coefficient) which is the fraction of collisions that results in reactive loss of the molecule (chemical reaction). Thus, the accommodation coefficient a represents the probability of reversible physical uptake of a gaseous species colliding with a surface, while the reaction probability 7 accounts for reactive (irreversible) uptake of trace gas species on condensed surfaces. This latter coefficient represents the transfer of a gas into the condensed phase and takes into account processes such as liquid phase solubility, interfacial transport or aqueous phase diffusion, chemical reaction on the surface or inside the condensed phase, etc. 

The major perturbations which occur in the reactor are Xe Sm poisoning, depletion, and temperature rise. The reactivity loss, k/k, for each of these has been calculated and is discussed below. 

After shutdown, the Xe produced by decay builds up and goes through a maximum a H br at this time the additional reactivity loss dne to Xe growth 40.6%. The total extra Xe loss is somewhat less than this because any Xe which is already present at shutdown simply decays without going through a maximum. The time course of the Xe which grows from I is shown in Fig. 4. 3.C (curve labeled I —> Xe — Cs ), and the time course of. the Xe -which deca.ys directly is given in the curve labeled Xe - Cs . 

The overall time behavior of the extra reactivity loss is also given in Fig. 4.3.C. It is. seen that, in order to override the Xe Sm loss at... 

It was not regarded as. a practical necessity that so much reactirity be built unto the reactor that it could override the Xe at all times.after shutdown. However, there should be sufficient extra reactivity so that every accidental scram does not keep the resctbr shut down for several days. It was felt that if the reactor can override X hr of accumulated Xe + Sm , it will ordinarily have sufficient operational flexibility. To achieve this it is necessary, according to Fig 4.3.C, to build into the reactor an excess reactivity of 9.6% (4.7% for reactivity loss in 30 min after shutdown plus 4.9% equilibrium loss). With this available Ak/k the machine can be started up within the first half hour after a scram, or after two days if not started during- the "grace" period. 

Hexagonal geometry power distribution has been calculated for FBTR core to estimate precisely heat generation rates. The variation of power coefficient and bumup reactivity loss rate as a function of bumup in FBTR has been measured. 

Highly enriched mixed oxide (MOX) fuels and Pu fuels without uranium were considered for Pu burning enhancement. It was found that Pu consumption rates essentially depend on Pu enrichment. Both bumup reactivity loss and Doppler coefficient are important criteria for highly enriched MOX fuel cores. Cores without uranium were found to consume the Pu at a very large bumup rate close to the theoretically maximum value of 110-120 kg/TWhe. The introduction of UO2 in an internal blanket is effective in enhancing the Doppler coefficient, it causes a minor increase in the sodium void reactivity in non-uranium cores. 

A parametric study carried out in order to establish the design orientations of burner cores taught us first that a considerable reduction of the fuel inventory (or dilution ) is always necessary to be able to operate a large core with a high plutonium content and therefore with an attractive plutonium burning performance. This dilution results in a decrease in in-pile fuel residence time as well as in a reduction (favourable) of the sodium void reactivity, whereas a decrease of the uranium content of the fuel brings about a reduction of the Doppler effect, a decrease of the conversion ratio which causes a daily reactivity loss ttiat makes it difficult to achieve long irradiation cycles, as well as a reduction of tiie delayed neutron fraction. 

The management of the marked reactivity loss and the optimization of the relative values of the Doppler and sodium void effects appear to be the two major issues to be dealt with so as to ensure the feasibility of Pu burner cores. In this respect, heterogeneous core designs, in which a part of the inert material replacing the fuel in the dilution process is gathered in specific subassemblies, are of particular interest. 

For FBR cores, it is substantial to improve the prediction accuracy of the bumup characteristics such as bumup reactivity loss, number density change of nuclides and... 

U ence cores showed a reactivity loss of about 20 Ih (2 X 10 kA)> At the time of these measurements, this reactivity change was attributed to temperature changes in both the reactor core and structural components. 

Subsequent measurements on ZPPR also showed a systematic reactivity loss, with time, which could not be attributed to temperature effects or other known experimental uncertainties. 

Comparison of Measured and Calculated Reactivity Losses in ZPR-3 and ZPPR... 

The problem of fissile isotope decay in a critical facility may not be significant in relating measurements made within several hours of each other, since, the reactivity loss ls <0.5 Ih/day. However, in a sequence of measurements performed over several days or longer, or in experiments where very high precision is otherwise obtainable, some correction for this effect may well be necessary. The rate of reactivity loss would depend on the type of fuel, higher concentrations of Pu leading to larger reactivity effects. 

Such a failure would result in a rapid depressurization of the system. Any lowering of water density within the process tubes will result in a reactivity loss. Thus. the immediate effect would be a reduction in reactivity. The later addition of cold water to the lattice will result in a reactivity Increase from 3.1 to 3 6 per cent k depending on reactor conditions. In all cases I the incremental control held in safety rods and the ball 3X system are sufficient to control the excess reactivity. See Table. ... 

The NPR has an undermoderated lattice, a water loss results in a reactivity loss, and therefore the addition of more moderator... 

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