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By Charles Rhodes, P.Eng., Ph.D.

The pancake shaped core zone of a FNR can be divided into smaller zones, each about one neutron diffusion length in diameter. In the FNR discussed herein each zone is the width of one movable fuel bundle plus the width of one fixed fuel bundle. Each such zone is centered at the middle of the movable fuel bundle and includes half the width of each adjacent fixed fuel bundle.

If all the fuel bundles were in every way identical the reactor would be characterized by a reactivity versus temperature graph where the reactivity gradualy decreases with increasing reactor temperature.

The reactor would seek an operating point where the reactivity is exactly zero, and with no thermal load the average fuel temperature would be the fuel temperature at:
reactivity = zero.

In a real reactor due to fuel non-uniformity, without reactivity correction each zone has a different reactivity than the adjacent zones. Hence each zone has a different average temperature.

These zone to zone temperature differences are reduced by horizontal diffusion of excess neutrons from more reactive zones to less reactive zones.

During reactor setup the insertion of the movable fuel bundles into the matrix of fixed fuel bundles is slowly changed to try to make the reactivity of all the zones identical. Under thermal load when the zone reactivities are all equal the zone temperatures should all be equal.This procedure levels the operating temperature across the reactor.

Absent reactivity correction some zones will be more reactive than others which leads to hot spots across the reactor. Since the hot spots approach fixed high temperature limits the presence of hot spots reduces average reactor power.

Within a single fuel bundle the neutron flux is almost constant. However, if the various fuel rods within the bundle have different fissil atom concentrations the temperatures of the individual fuel rods will be different.

The reactor has high temperature material limits. These limits apply to the hotest fuel rod. Hence the average fuel rod temperature of the fuel assembly must be significantly less than the average fuel rod temperature of the hotest fuel rod. For example, if the average temperature of the hotest fuel rod is 500 degrees C then the average temperature of the entire fuel rod assembly may be only 460 degrees C.

Each zone exhibits highly stable control only over a small reactivity range which range is determined by the fraction of delayed neutrons. This range for Pu-239 is about one third of the range for U-235. Thus for each zone there is a corresponding temperature range of highly stable control. We must be concerned that the coldest coolant feeding the most reactive zone does not take that zone out of its highly stable range. Even if the zone goes out of the highly stable range we must ensure that the zone will remain stable with prompt neutrons so that the zone cannot become prompt critical. We must ensure that if the system becomes prompt critical that the number of neutrons cannot grow faster than fuel thermal expansion can suppress the prompt critical state.

In this matter we must be concerned about the delayed increase in reactivity caused by sodium and steel thermal expansion as compared to an almost instantaneous reduction in reactivity due to fuel thermal expansion.

As long as fuel geometry and coolant temperature changes are made very slowly prompt neutron criticality should be avoidable. However, we need to have a good understanding of the upper limits on those maximum rates of change. We also need to be concerned about the maximum allowable temperature differences between adjacent reactor zones.

This web page last updated June 15, 2021

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