Exercise No. 9: In-core neutron flux mapping

Purpose of experiment

The purpose of the experiment is familiarization with the construction and operation of compensated boron-lined ionization chambers, which are frequently used as nuclear instrumentation detectors. Basic principle of chamber compensation are explained and demonstrated practically in a mixed neutron and gamma radiation field. Measurements of compensated ionization chamber response linearity are performed at different degrees of compensation.

Outcome / What you will learn
Students will:

  1. learn about the construction and operation of a gamma-compensated ionization chamber
  2. experimentally determine the compensation voltage
  3. test the linearity of chamber response at different degrees of compensation.
Execution
  1. The construction and operation of a compensated ionization chamber is discussed.
  2. Chamber compensation is demonstrated in the gamma radiation field in the reactor core at zero power.
  3. The current vs. voltage characteristic is measured at different reactor power levels.
  4. The chamber response linearity vs. reactor power is tested at different degrees of chamber compensation.

Exercise No. 8: In-core neutron flux mapping

Purpose of experiment

A nuclear reactor in operation is a source of a mixed neutron and gamma field, a shut-down nuclear reactor is a source of a gamma field. The purpose of the exercise is the measurement of the gamma field intensity in the vicinity of the fuel elements in the reactor core and in locations outside the reactor core, during reactor operation and in shutdown conditions. The fraction of the delayed gamma field magnitude vs. the total (composed of prompt and delayed gamma rays) during reactor operation is estimated by measurements following a rapid reactor shutdown (SCRAM).

Outcome / What you will learn
Students will:

  1. learn about prompt and delayed gamma radiation
  2. become familiar with miniature ionization chambers
  3. characterize gamma field intensity distribution inside the reactor core and in ex-core locations
  4. estimate the fraction of the delayed gamma field magnitude during reactor operation.
Execution
After discussion on the physical background and the experimental setup, students:

  1. measure the ionization chamber leakage current and background
  2. measure the ionization chamber current dependence on the reactor power (0 – 250 kW)
  3. characterize gamma field intensity distribution inside the reactor core and in ex-core locations
  4. measure gamma field intensity after a rapid reactor shutdown (SCRAM) and estimate the delayed gamma field magnitude fraction during reactor operation.

Exercise No. 7: In-core neutron flux mapping

Purpose of experiment

The purpose of the experiment is the measurement of relative axial neutron flux distributions in several experimental locations in the core of the JSI TRIGA reactor. This is accomplished through the use of miniature fission chambers with an external diameter of 3 mm, inserted into specially designed guide tubes and a JSI-developed pneumatic positioning system.

Outcome / What you will learn
Students will:
  1. measure axial neutron flux profiles with miniature fission chambers in several different measuring positions and control rod configurations
  2. visualize the effect of the insertion of a control rod on the neutron flux distribution
  3. analyse the shape of the neutron flux profile and compare it with theoretical predictions.
Execution

Students perform relative axial fission rate profile measurements in the core of the TRIGA reactor in several radial positions, and compare the measured profiles with theoretical predictions. Features in the measured profiles due to heterogeneity of the reactor fuel are discussed. Measurements are performed with two control rod configurations in order to visualize on the spot the effect of the insertion of a control rod on the neutron flux distribution.

Exercise No. 6: Control rod worth calibration

Purpose of experiment

The void coefficient of reactivity is one of the most important coefficients of reactivity for safe reactor operation. The JSI TRIGA reactor is equipped with an electro-pneumatic system with which air can be injected into the reactor core to simulate void formation through boiling. The purpose of the experiment is the measurement of the magnitude of the void coefficient as a function of the radial location in the reactor core.

Outcome / What you will learn
Students will:
  1. observe the change in reactivity caused by the presence of air bubbles in the reactor core
  2. understand the magnitude and sign of the reactivity coefficient, depending on the location in the reactor core where air bubbles are injected
Execution

The task of the exercise is to measure the void coefficient of reactivity by introduction of void into several different positions in the core. The total volume of the voids is estimated on the basis of the flow rate, which is measured by the electro-pneumatic system. The magnitude and sign of the void coefficient of reactivity is determined and visualized on the spot, in dependence on the position on the reactor core.

Exercise No. 5: Control rod worth calibration

Purpose of experiment

The negative reactivity induced by control rod insertion is a parameter of key importance for safe reactor operation. The relation between the control rod position and reactivity is non-linear, therefore the control rod worth calibration curves are of great assistance to the reactor operator, enabling the estimation of the shutdown margin and critical control rod positions. The purpose of the experiment is a demonstration of several methods of control rod worth calibration.

Outcome / What you will learn
Students will:

  1. gain knowledge on the dependence of the negative reactivity on the depth of the control rod insertion (integral worth)
  2. gain knowledge on the control rod worth at different positions (differential worth)
  3. learn how to experimentally calibrate control rods using the rod swap and dynamic rod insertion methods.
Execution

After discussion of the basic concepts, students determine the integral and differential worth curve for a chosen control rod (e.g. the regulating rod of the JSI TRIGA reactor) using the rod swap method and the dynamic rod insertion method. In each step of the rod swap procedure, students predict the magnitude of the changes in reactivity due to insertion / withdrawal of the measured / calibrated control rod, and analyse the results on the spot. In the dynamic rod insertion method, students coordinate and time the measurement sequence and analyse the results.

Exercise No. 3: Fuel temperature coefficient of reactivity

Purpose of experiment

A negative fuel temperature reactivity feedback effect is of key importance in inherently safe reactor design. The purpose of the experiment is to measure the fuel temperature reactivity coefficient of the TRIGA reactor, i.e. the reactivity change due to a change in the fuel temperature.

Outcome / What you will learn
Students will:

  1. discuss the physical principles governing fuel temperature reactivity feedback
  2. observe the response of the reactivity, fuel temperature and reactor power, in a sequence of swift changes in reactivity, caused by the movement of a control rod
  3. understand the feedback effect of the fuel temperature on the reactivity and the power of a reactor – this being a prerequisite for understanding the temperature and power reactivity defects.
Execution

After discussion on the physical principles governing the fuel temperature reactivity effects, students determine the fuel temperature coefficient of reactivity as a function of the fuel temperature, and the power coefficient of reactivity as a function of power.

Exercise No. 2: Reactor response to step reactivity changes

Purpose of experiment

The purpose of the experiment is to demonstrate the principles of reactor kinetics at low power levels and in the operating power level range by inducing step reactivity changes. The reactor power response to reactivity changes is analysed with the aid of a digital reactivity meter and simple theoretical kinetics models.

Outcome / What you will learn
Students will:

  1. observe and understand the reactor power and period response to a sudden reactivity change at zero power and in operating power level range
  2. experimentally determine the point of adding heat (POAH)
  3. experimentally verify the physical models describing the reactor kinetics by observing the reactor response to step reactivity changes.
Execution

After an initial discussion, students observe the reactor power and asymptotic period response to sudden (step) reactivity change in different power ranges: in the low (zero) power range and in the operating power range. The dependence of the reactor period vs the magnitude of the reactivity is measured and compared with theoretical predictions. The point of adding heat (POAH) of the reactor is determined experimentally.

Exercise No. 1: Critical experiment

Purpose of experiment

The critical experiment is one of the fundamental experiments in Reactor Physics. Its main purpose is to determine the critical number of fuel elements and / or control rod positions in a critical assembly. The experiment is regularly performed both at experimental and power reactors after each core modification. The purpose of the experiment is to demonstrate the procedure to reach criticality starting from a deeply subcritical state in a controlled sequence.

Outcome / What you will learn
Students will:

  1. become aware of the importance of the critical experiment
  2. perform the critical experiment by control rod withdrawal and plotting of the M-1 diagram as a function of reactivity insertion
  3. observe the transients present when approaching criticality and examine the validity of neutron kinetics models in subcritical state.
Execution

Students perform the critical experiment via gradual control rod withdrawal, measurement of the neutron signal on the starting channel and plotting the M-1 diagram. At every step, students estimate the critical control rod position by extrapolation. To experimentally confirm if the achieved state is subcritical, critical or supercritical, the neutron source is withdrawn from the reactor core following multiple steps, and the time dependence of the neutron source is monitored on line.

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