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XYLENE POWER LTD.

FNR CORE ZONE GEOMETRY

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

INTRODUCTION:
The FNR core zone geometry is set by adjusting the amount of insertion of mobile core fuel bundles into the matrix of slightly wider fixed core fuel bundles. The following diagrams show a line of 9 fixed core fuel bundlles separated by 8 slightly narrower mobile core fuel bundles. The mobile core fuel bundles are inserted into the bottom of the matrix of fixed core fuel bundles. On these diagrams the shrouds separating the fixed and mobile fuel bundles appear as thin black lines, core fuel portions of the fuel bundles are shown in red, the blanket portions of the fuel bundles are shown in green, the plenum portions of the fuel bundles are shown in light blue and the fuel tube end caps appear as solid black.

When the reactor is in cold shutdown the lower ends of the mobile core fuel bundles are 1.2 m below the bottom of the matrix of fixed core fuel bundles. As shown on the following diagram when the reactor is in cold shutdown the core fuel portions of the fixed and mobile fuel bundles are well separated and are isolated from each other by neutron absorbing U-238 in the blanket fuel, shown in green, which hold the reactor sub-critical.

When the reactor is operating with new core fuel (average 20% Pu, no fission products) the mobile fuel bundles are partially inserted into the matrix of fixed fuel bundles. That partial insertion causes the mobile core fuel to partially overlap the fixed core fuel which causes the reactor to become critical. Once critical further insertion of the mobile fuel bundles into the matrix of fixed fuel bundles raises the reactor operating temperature because the reactor reactivity decreases as the reactor temperature rises due to thermal expansion of the fuel.

The following diagram shows the relative positions of the fixed and mobile fuel bundles when the reactor is operating and the fuel is new (has recently been reprocessed).

As the fuel ages the average plutonium concentration in the reactor core gradually decreases and the fission product concentration in the reactor core gradually rises. To compensate for this fuel aging mechanism from time to time the mobile fuel bundles are very slowly further inserted into the fixed fuel bundle matrix to maintain the design primary liquid sodium surface temperature. This further insertion reduces the fraction of fission neutrons which diffuse out of the reactor middle core zone and are absorbed in the upper and lower core zones and by the reactor blanket. When the reactor fuel has aged to the point that it is due for reprocessing the average plutonium concentration in the core fuel has dropped to about 12.7% Pu and the core fuel portion of the mobile fuel bundles fully overlaps the core fuel portions of the fixed fuel bundles as shown in the following diagram. In this state the mobile fuel bundles are fully inserted into the matrix of fixed fuel bundles. When the core fuel requires reprocessing the primary liquid sodium surface temperature will fall below its design value in spite of full insertion of the mobile fuel bundles into the matrix of fixed fuel bundles.


 

FUEL MECHANICAL STABILITY:
A FNR ususally operates close to the threshold of criticality. It is essential for the fuel geometry to remain mechanically stable to avoid reactivity surges related to uncontrolled fluctuations in fuel geometry caused by an imperfect mechanical control system which will have some position error due to hysterisis and other mechanical issues. The requirement for fuel geometric stability leads to a fuel bundle arrangement consisting of mixed octagons and squares that provides horizontal as well as vertical fuel assembly mechanical stability.
 

ORDER OF ASSEMBLY:
Note that mobile FNR fuel bundles are always added to the assembly in the fully retracted position BEFORE adjacent fixed fuel bundles are added and are always removed AFTER adjacent fixed fuel bundles are removed so that during fuel bundle addition or removal a mobile fuel bundle is NEVER passed through an assembled fixed fuel bundle matrix.
 

THERMAL POWER CONTROL:
In a liquid sodium cooled FNR the primary means of thermal power control is fuel thermal expansion which changes the local reactivity. As the reactor core zone fuel temperature rises above its setpoint fuel thermal expansion will cause the core zone reactivity to drop below unity which turns off the chain reaction. Note that as the reactor thermal load decreases the core fuel temperature increases which shuts down the chain reaction. In effect the FNR maintains a constant average fuel temperature. At maximum thermal load the core fuel centerline temperature will decrease while the temperature drop between the core fuel center line and the liquid sodium increases. Thus the liquid sodium surface temperature may drop by as much as 50 degrees C ____ below its corresponding minimum load value of 540 degrees C. With natural circulation of liquid sodium this temperature drop reduces primary liquid sodium flow and hence reactor thermal power capacity.
 

MIDDLE CORE ZONE:
The middle core zone is the region where core fuel rods in fixed fuel bundles overlap core fuel rods in mobile fuel bundles.
[Middle core zone thickness]
= [0.7 m - (Mobile fuel bundle withdrawn distance)]

Thus the middle core zone length varies from 0 m to 0.7 m. However, to obtain reactor criticality when the core fuel averge Pu fraction is 20% the minimum overlap length between the core fuel mobile fuel bundles and the core fuel fixed fuel bundles should be about 0.35 m.

In the reactor on state the middle core zone is super critical. The rate of neutron production in the middle core zone precisely equals to the rate of neutron absorption in the middle core zone plus the rate of neutron diffusion out of the middle core zone into the adjacent upper and lower core zones.

In practice during reactor turn-on the middle core zone thickness is gradually increased until the reactor becomes critical. The liquid sodium temperature will then rise until the reactor becomes subcritical. Then the middle core zone thickness is again slightly increased until again the liquid sodium temperature rises to make the reactor subcritical. This procedure is repeated over and over again until the liquid sodium temperature reaches the desired value of 490 degrees C. If the reactor design is correct the desired liquid sodium operating temperature will be reached when the middle core zone thickness is about 0.35 m and the average Pu weight fraction in the core fuel rods is 20%.

As the core fuel ages to maintain the desired liquid sodium operating temperature the middle core zone thickness must be gradually increased to ultimately become 0.7 m when the Pu weight fraction in the core fuel has fallen to about 12.7%.
 

UPPER AND LOWER CORE ZONES:
[Upper and lower core zone thicknesses] = [0.7 m - (middle core zone thickness)]

With new core fuel:
[Average upper and lower core zone average fissile fuel concentration]
= [0.5] [Average middle core zone fertile fuel concentration]

With old core fuel:
[Upper and lower core zone height] = 0
 

In the upper and lower core zones there is not enough neutron gain to sustain a chain reaction.
 

UPPER AND LOWER BLANKET MINIMUM THICKNESS:
[Upper and lower blanket minimum thickness]
= [1.8 m - (Upper or lower core zone thickness)]
= [1.8 m - ([0.7 m - (middle core zone thickness)])]
= 1.1 m + (middle core zone thickness)
 

OPTIMUM REACTOR MODULATION:
The optimum reactor modulation will occur when the average fissile concentration in the upper core zone equals the average fissile concentration in the lower core zone.

As shown on the web page titled: FNR FUEL RODS the initial Pu weight fraction in the mobile core rods should be 24.______% and the average Pu weight fraction in the fixed core rods should be 16.______%.

When the core fuel is due for reprocessing the average Pu weight fraction in the core fuel will be about 12.7%. We must examine the reactor material distribution to determine if the middle core zone will remain critical at this average Pu fraction in the core rods.
 

MULTIPLE INDEPENDENT SHUTDOWN MECHANISMS:
If all of the fuel bundles are initially at their normal operating state and then one half of the mobile core fuel bundles are withdrawn the reactor will reliably shut down. This safety shutdown condition enables two fully independent reactor cold shutdown mechanisms. In the normal operating state the core fuel is in two slightly displaced layers. In the shutdown state the core fuel is in a combination of slightly displaced and widely displaced layers.
 

REACTOR OFF:
When the reactor is off the mobile fuel bundles are 1.2 m withdrawn.

In the reactor off state:
Middle core zone thickness
= 0.7 m - 1.2 m
= - 0.5 m

In the reactor off state there must be absolute certainty that the chain reaction will stop regardless of the liquid sodium temperature.

The fuel bundles are engineered such that with the mobile fuel bundles 1.2 m withdrawn the upper and lower core zones are both sub-critical. Ideally if a single active fuel bundle in an array of active fuel bundles with their mobile portions withdrawn has its mobile portion fully inserted the reactor should remain subcritical. This feature is necessary to prevent problems if a single fuel bundle should jam in its fully inserted position.

In the reactor off state the fissile fuel density in the upper and lower core zones is same as in the reactor on state. Hence this fissile fuel density must cause the reactor to be subcritical.

In the reactor cold shutdown state there is no middle core zone.
 

SUMMARY:
Proper reactor operation is highly dependent on initial correct fuel rod Pu weight fractions.
 

REACTOR MATHEMATICAL MODELLING:
We must solve the diffusion equation to find the diffusion fluxes of neutrons from the middle core region through the adjacent upper and lower core regions and then into the blanket regions in terms of the middle core height. These mathematical solutions are presented at:
FNR CORE and at FNR BLANKET.
 

PLUTONIUM DOUBLING TIME:
An issue of great importance in large scale implementation of FNRs is the FNR run time required for one FNR to breed enough excess Pu-239 to allow startup of another identical FNR. This time may be calculated using the approximation that each plutonium atom fission releases of 3.1 neutrons of which 2.5 neutrons are required for long term sustaining reactor operation leaving 0.6 neutrons for breeding extra Pu-239. Thus fission of one atom of Pu-239 forms 0.6 atoms of extra Pu-239 in addition the the Pu-239 breeding that is required to sustain the nuclear reaction.

In one reactor cycle time 15% of the fuel weight fissions, which is 3 / 4 of the initial plutonium weight.

Thus in one reactor cycle time the fractional increase in Pu is:
0.6 (3 / 4) = 0.45

Thus the plutonium doubling time is:
(1 cycle time) / 0.45
= 2.22 cycle times

If one cycle time = 30 years then:
Pu doubling time = 2.22 (30 years)
= 66.67 years

This is the time required for one FNR to form enough excess Pu to enable starting another similar size FNR. Clearly this doubling time is too long to enable rapid deployment of FNRs using Pu obtained from FNR breeding. In the near term the number of operating FNRs will be limited by the availability of FNR core fuel for reactor startup.

With large scale implementation of FNRs the available supply of plutonium and trans-uranium actinides will soon be exhausted. Hence the issue of the Pu-239 doubling time physically constrains the rate of growth of the FNR fleet.

Thus FNRs are viable for disposing of transuranium actinides but due to the Pu-239 doubling time will not in the near future by themselves provide enough thermal power capacity for complete displacement of fossil fuels.
 

FNR DESIGN CONSISTENCY:
An issue that remains to be demonstrated is whether the present FNR mechanical design will meet the design target of reactor criticality at an average Pu weight fraction in the core fuel of 12.7%.
 

This web page partially updated June 4, 2020

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