TRIGA Reactor Calorimetric Calibration

| May 18, 2015



  • To observe and understand the OSTR calorimetric calibration procedure.
  • To perform calculations to accurately determine reactor thermal power.
  • To become familiar with how OSTR Nuclear Instrument (NI) channels are adjusted.
  • To understand how changes in core configuration can affect NI calibration.




One requirement for safe and efficient operation of a reactor is the ability to accurately and reliably measure reactor power. As part of the licensing process, the Nuclear Regulatory Commission (NRC) will approve license applications or amendments containing a Technical Specification chapter. The purpose of the Tech Specs is to define an operating envelope within which the integrity of the fission product barriers is guaranteed. The Tech Specs set operating limits for such things as maximum allowed power, temperature and pressure.


To maximize income, power plants will operate as close to their maximum allowed power as possible. In order to demonstrate license compliance, power plants must be able to measure power using methods tied directly to prime standards. Prime standards are measurement standards or techniques established by the government which can be difficult to implement, but give highly accurate results. In a power plant, power can be calculated using the relationship:




Flow and temperature are typically measured using a venturi flowmeter and thermocouple type temperature sensors. Power can thus be measured continuously while the reactor is online, but venturis and thermocouples are not the most accurate of measuring devices.


At the OSTR, thermal power is determined using a slightly modified form of eq (1):




Rather than using a coolant flow rate, the product MCp is calculated for a reactor tank containing a certain amount of water, fuel, graphite and aluminum. Time (Δt) and temperature rise (Tfinal-Tinitial) can be determined by processing reactor tank temperature data. Although Resistive Temperature Devices (RTDs) are considered to be more accurate, the OSTR makes use of a bank of four stalk-mounted thermocouples. Increased accuracy is provided by using the average temperature of the four thermocouples. This method eliminates the need to perform accurate flow measurements and can be used at the OSTR due to the large available coolant reserve present in the reactor tank. Power plants cannot be operated for any length of time at high power without cooling flow.




The staff will configure the reactor for a calorimetric calibration. Preparations include cooling the tank to ~25°C and securing all flow paths (cooling and purification letdown) to and from the tank. A small mixer will be installed in the tank. This mixer helps maintain the tank close to isothermal conditions. An array of four thermocouples will be installed to measure temperature at a 30 second interval. Baseline temperature data collection will be collected for at least 20 minutes. The reactor may be taken critical during this time as long as power is maintained below 100 W.


When ready to commence power calibration, perform the following:


  • Perform a square wave to raise power to ≤ 1.0 MW.
  • Operate at constant power as long as possible. The length of the calorimetric will be determined by tank temperature. The tank high temperature alarm setpoint is 42°C.
  • Observe and record reactor power indicated on the Safety and the Percent channel at the beginning, and end of the power calibration.
  • Manually SCRAM the reactor.
  • Continue collection of temperature data for at least 20 more minutes.
  • Export temperature and Linear channel data to an Excel spreadsheet.


After the calorimetric calibration is complete, the staff will restore the plant to its normal operating configuration.




  1. Construct three lines from the temperature data. Line A is constructed from temperature data before the power increase. Line B is constructed from the temperature data while at power. Line C is constructed from temperature data collected after the SCRAM. Then calculate the coordinates of the two points specified by the intersection of lines A and B, and lines B and C. Finally, determine the slope of the line between the two points.


  1. Given the following information, calculate MCp, the sum of the products of mass and specific heat for each material contained in the reactor tank (cite references for density and Cp!):


  • Tank H2O volume: 4600 gallons (17413 liter)
  • Aluminum mass:             450 lbs (204 kg)
  • Graphite mass:      60 lbs (27 kg)
  • Number of FE’s:             90
  • U ZrH6 mass per FE: 4.77 lbs (2167 grams)

NOTE: Cp for TRIGA fuel is 595.988 J/kg-C.

  • Stainless mass per FE: 0 lbs (454 grams)
  • Neglect other materials (B4C, Erbium, detectors, etc.)


  1. From the results in parts 1 and 2, calculate average reactor thermal power. From the Linear channel data, calculate average linear power during the calibration.



  1. The procedure asks you to observe the safety and percent power readings at the beginning and end of the calibration run. If these values are different at the beginning and end, explain why. If they are not reading the same as the expected power level (i.e. the power you are performing the calibration at), explain why.


  1. Assuming your calculated thermal power is correct, how should each of the NI channels (Safety, Percent, Linear) be adjusted? Explain how you would actually perform these adjustments, if needed.


  1. When the reactor is shutdown for an extended period, tank temperature can be raised by running the primary pump with secondary cooling secured. The primary pump is rated at 28 Hp. How much will the primary temperature rise if the primary pump is run for nine hours (assume zero heat loss from the tank)? The mixing pump is rated at 0.95 Hp. How much of the temperature rise during a 15 minute calorimetric is due to the mixing pump?


  1. The OSTR will be run continuously for six days at full power in order to produce Molybdenum for Tc-99 production. Suppose the OSTR has run for 5 days when a pump malfunction forces a shutdown. The reactor shuts down at 6 am on July 30 and is restarted to full power at 12 noon following repairs. Following restart, pool temperature slowly rises from 30°C to 40°C over the next 8 hours. With the reactor control system in automatic mode, describe how the regulating rod responds over this period due to changing temperature and changing Xenon conditions.


  1. Calorimetric calibrations are performed with the core in the NORMAL configuration, with a fuel element installed in the B-1 gridplate position. The rest of the time, the reactor is usually operated in the ICIT or CLICIT configuration with an experiment facility installed in the B-1 gridplate position. Explain why it is conservative to calibrate the NIs in the NORMAL mode when the core is almost always operated in the ICIT or CLICIT mode.


  1. During normal operation, although there is no forced circulation through the core, tank water is circulated from the reactor tank to a heat exchanger and back to the tank. You observe that heat exchanger flow is 500 gpm, heat exchanger inlet temperature is steady (constant over time) at 35°C and the reactor is operating at 1 MW. What is the temperature of the water leaving the heat exchanger and returning to the reactor tank? The specific heat of water in this temperature range may be assumed constant and equal to 4.18 J/gmºC. Density of water in this temperature range may be assumed constant and equal to 1.0 gm/cm3.


  1. (Optional extra credit for undergrads, mandatory for grads) There are many ways to monitor reactor power. Propose a new method, other than a calorimetric technique, to measure OSTR power. Describe the proposed method in detail, including an explanation of how the method could be periodically calibrated and what range of power the method is suited for.


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