XYLENE POWER LTD.

FNR MONITORING SYSTEM

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

INTRODUCTION:
This web page focuses on the design of the FNR's monitoring system. The monitoring system performs an important role in initial setting and later adjustment of the actuator position settings.

For each movable fuel bundle the monitoring system continuously reports the actual insertion depth, the sodium discharge temperature and the gamma ray emission. Ideally the thermal load, as indicated by the gamma ray emision, should be equally shared by all the movable fuel bundles.

For each movable fuel bundle the FNR Monitoring system continuously compares the actual movable fuel bundle insertion depth to the programmed insertion depth setpoint, and issues pulse width commands to the appropriate hydraulic system control valves to make the actual insertion depth smoothly converge to the stored insertion depth setpoint.

The insertion depth setpoints are chosen to achieve the desired FNR setpoint temperature(s).

The gamma emission of each movable fuel bundle is monitored with the long term aim of making the gama emissions of the various movable fuel bundles almost equal to one another.

The sodium discharge temperature of each movable fuel bundle is slow to respond. However, over time it can be used to optimize the fuel bundle's insertion setpoint.
 

MONITORING SYSTEM OVERVIEW:
The monitoring system is mounted in temperature controlled ceiling space above the sodium pool and looks down at a slight angle toward the tops of the indicator tubes, so as to minimize shadowing by the polar gantry crane transverse beam at its rest position.

a) A raster scanning laser in the monitoring system looks down via a temperature controlled quartz window and scans the tops of the indicator tubes. This laser uses time-of-flight to measure the distance to the top of each indicator tube. That distance is geometrically converted into the movable fuel bundle insertion depth for each of the 464 movable fuel bundles. The related monitoring system is in two packages that mount on either side of the transverse beam.

b) A similar near infra-red scanner calculates the temperature of each indicator tube.

c) A grid of 464 gamma sensors is located directly above the indicator tubes.

d) The indicator tubes cause the gamma rays to occur in 464 ceiling spots as projected by the indicator tubes.
 

APPLICATION:
a) The main purpose of the monitoring sytem is to sense the actual positions of the actuators, compare these positions to the corresponding setpoints and and to send appropriate control signals to the actuator hydraulic valves.

b) A secondary purpose of the monitoring system is automatic periodic testing of the actuator hardware to ensure that in an emergency it will follow FNR shutdown commands. For this purpose the actuators are divided into two independant groups such that either group can effect an emergency shutdown.

c)A third purpose of the monitoring system is gamma and temperature monitiring to try to uniformly distribute the thermal load across the active fuel bundles.

d) A fourth purpose of the monitoring system is reporting on FNR status after an automatic shutdown.

e) A fifth purpose of the monitoring system is characterization and compensation for long term reactor degradation mechanisms such as fuel burnup.

f) A sixth purpose of the monitoring system is monitoring FNR total thermal power output.

g) A seventh purpose of the monitoring system is safe automatic progressive realization of changes in FNR setpoint to achieve setpoints of 470 degrees C, 120 degrees C and less than 20 degrees C.

h) A eighth purpose of the monitoring system is to record gamma ray surges for system diagnostic purposes.

Note that if the setpoint is under 99 degrees C the sodium will freeze and the FNR will need externally sourced heat to melt the sodium to permit fission restart.
 

TEMPERATURE SUPERVISION:
A local temperature significantly above the reactor setpoint must cause a slight withdrawal of the corresponding movable fuel bundle.
 

REQUIREMENT:
Any severe event, such as an attack on the FNR enclosure, should trigger a cold shutdown without reliance on the monitoring system. However, full operation of the monitoring system is required to restart the FNR.
 

This web page last updated December 27, 2025.