Reactive void formation

The subsequent events led to the generation of an increasing number of steam voids in the reactor core, which enhanced the positive reactivity. The beginning of an increasingly rapid rise in power was detected, and a manual attempt was made to stop the chain reaction (the automatic trip, which the test would have triggered earlier, had been blocked). However, there was little possibility of rapidly shutting down the reactor as almost all the control rods had been completely withdrawn from the core. The continuous reactivity addition by void formation led to a prompt critical excursion. It was calculated that the first power peak reached 1(X) times the nominal power within four seconds. Energy released in the fuel by the power excursion suddenly ruptured part of the fuel into minute pieces. Small, hot fuel particles (possibly also evaporated fuel) caused a steam explosion. 

An example of a situation in which the condition (A. 18) is approximately valid is a reactor in which the temperature of the moderator can be linearly related to the power history but in which the reactivity at time t is a, nonlinear function of temperature at time t. This situation might occur with non-linear thermal expansion, for example. It might also approximately represent the situation for void formation in a water reactor, where the heat content of the moderator could be expressed as a linear function of powt history but the void volume, and hence reactivity, would be a non-linear function of moderator heat content. Such an assumption would, however, ignore dynamical effects on the voids. 

However, power reactors require significant amounts of reactivity (i.e., well above the amount needed to go prompt critical if added suddenly) that must be provided by movable control absorber devices (or removable poison dissolved in the piimaiy coolant) under the direction of a licensed operator and following jq>proved procedures during reactor start-up and the transition to equilibrium full-power operation. This positive reactivity is needed to compensate for losses associated widi increased core temperature, reduced coolant density including bubble void formation, and equilibrium fission product poison loads, especially Xe. Consequently, it is only possible to limit the amount of reactivity that could theoretically be inserted to small, intrinsically safe values when the reactor is already in the normal full-power operating mode with all movable control devices very near their maximum withdrawal positions (and when the dissolved poison concentration is close to zero). 

One effect of an increase in temperature which is important enough to merit discussion as a separate topic is the reactivity change due to formation of voids in the coolant, either as part of the normal operating condition, as in the boiling water reactor, or under accident conditions in reactors such as the pressurized water reactor or the fast breeder reactor. The effects of void formation are similar to those of reduction in coolant density already considered, e.g., reduced moderation, giving increased leakage and resonance capture, and reduced absorption, leading to an increase in the thermal utilization, but the effects can be more dramatic on account of the more rapid possible variation of coolant density. 

We first discuss the overall chemical process predicted, followed by a discussion of reaction mechanisms. Under the simulation conditions, the HMX was in a highly reactive dense fluid phase. There are important differences between the dense fluid (supercritical) phase and the solid phase, which is stable at standard conditions. Namely, the dense fluid phase cannot accommodate long-lived voids, bubbles, or other static defects, since it has no surface tension. Instead numerous fluctuations in the local environment occur within a timescale of 10s of femtoseconds. The fast reactivity of the dense fluid phase and the short spatial coherence length make it well suited for molecular dynamics study with a finite system for a limited period of time. Under the simulation conditions chemical reactions occurred within 50 fs. Stable molecular species were formed in less than a picosecond. We report the results of the simulation for up to 55 picoseconds. Figs. 11 (a-d) display the product formation of H2O, N2, CO2 and CO, respectively. The concentration, C(t), is represented by the actual number of product molecules formed at the corresponding time (. Each point on the graphs (open circles) represents a 250 fs averaged interval. The number of the molecules in the simulation was sufficient to capture clear trends in the chemical composition of the species studied. These concentrations were in turn fit to an expression of the form C(/) = C(l- e ), where C is the equilibrium concentration and b is the effective rate constant. From this fit to the data, we estimate effective reaction rates for the formation of H2O, N2, CO2, and CO to be 0.48, 0.08,0.05, and 0.11 ps, respectively. 

The areas concerning monolithic intermetallics which have been studied in recent years are (i) the formation of mctastable aluminas, and their transformation to stable a-alumina, (ii) the formation of interfacial voids and scale adherence and how these are influenced by reactive elements and sulfur, and (iii) accelerated oxidation at intermediate temperatures. Additionally the applications oriented areas of (iv) coatings, (v) oxidation of composites, and (vi) life predictions have received attention. 

The Formation of Interfacial Voids and Scale Adherence and how these are Influenced by Reactive Elements and Sulfur... 

An important result of the present analytical study is the detection of S in Y-rich precipitates within the alloy. This observation supports the sulfur effect model", which proposes that reactive elements getter the S [16,17]. The presence of S reduces the surface energy of the metal and thereby promotes the formation of interfacial voids, as has been shown by Grabke ct al. [18]. Although no S-rich particles have been found in NiAl - 0.2 wt% Zr. it is possible that the Zr can lower the activity of S by solute - solute interaction. Further work is necessary in order to study the interaction between reactive elements and S. 

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