94 of delayed neutrons from the shortest-lived group of precursors has essentially the same distribution as the prompt-neutron flux. At the other extreme the distribution from the longest-lived group is nearly flat. The effects of the distorted source distributions and the lower average energies of the delayed neutrons on their nonleakage probabilities were included in the evaluation of the effective delayed-neutron fraction. The total effective fraction of delayed neutrons is 0.00362 at a 1200-gpm circulation rate and is 0.00666 in the static core. The total yield of precursors is 0.00641. The effective delayed-neutron fraction was used in subsequent kinetics calculations instead of trying to account for precursor transport during transient conditions. Gas Bubbles. The use of the xenon-stripping device in the fuel-pump bowl results in the introduction of helium gas bubbles into the circulating fuel stream. At operating conditions the fuel in the core may contain about 1 vol of helium bubbles. The presence of these bubbles makes the fuel compressible, causing a pressure coefficient of reactivity. In addition, the presence of the gas modifies the fuel temperature coefficient of reactivity because the density of the salt-gas mixture changes with temperature at a rate different from the density of the salt alone. For rapid changes in pressure the mass of helium in the core remains essentially constant, and the reactor exhibits a positive pressure coefficient of reactivity. However, for very slow changes, the gas volume in the core is inversely proportional to the compression ratio between the pump bowl (where the gas is introduced) and the core. "his leads to a negative pressure coefficient. The presence of gas increases the temperature coefficient of the mixture density; this leads to an increase in the size of the negative temperature coefficient of reactivity.
The magnitudes of the pressure and temperature coefficients of reactivity with entrained gas in the core are listed in Table 6 f o r a core temperature of 1200째F and a pump-bowl pressure of 5 psig. During normal operation, the presence of entrained gas introduces additional power "noise" as fluctuations in core outlet pressure are converted to reactivity perturbations. In power excursions the gas enhances the negative temperature coefficient of reactivity of the fuel. At the same time, the pressure coefficient makes a positive contribution to reactivity during at least part of the power excursion. However, the relation between pressure rise and temperature rise in any credible excursion is such that the net reactivity feedback from the combined pressure and temperature effects is negative.
Control Rods The MSRE contains three flexible control rods whose centers are located at three corners of a &in. square around the axis of the core. The rods operate in re-entrant INOR-8 thimbles, 2 in. in diameter with 0.065-in. walls. The poison elements are hollow cylinders, 1.08 in. OD by 0.84 in. ID and 1.3 in.long, containing 30 w t $ and 70 w t $