Axial Flux Mapping using traveling in-core probe in pulstar fuel assemblies.

DATE PERFORMED: jan 27 DUE DATE: Feb 10

NAME: Jasem Alahmed
PARTNERS:

INSTRUCTOR: Dr. Anistratov
TEACHING ASSISTANT: Pedram Ghassemi

ABSTRACT

Traveling in-core probes are used to calculate the axial flux of fuel assemblies in the Pulstar reactor. The neutron flux is measured as a function of height and distance traveled in the core. This report starts by stating the theory , hypothesis then calculate the results and perform analysis. it then will list the equipment used and the procedure followed when performing the experiment. the following section list the findings and graphs explaining how the results relate. the next section discusses the results and answer some questions. Finally the conclusion and references.
INTRODUCTION AND THEORY

Regular checks are performed to insure the safety of the reactor. Among these methods is in-core flux mapping. in-core flux mapping is done to verify calculations of core performance, making sure safety margins are being met. Such as linear power density and peak to average power ratios. Furthermore, is can assist in finding problems within the reactor such as power oscillations, and local hot spots to be rectified.
Determining the neutron flux in a reactor can help determine parameters (power density) and help estimate fuel use. Increases overall efficiency of the reactor and its performance.

The objective of this lab is to support our hypothesis that TIPs is an effective way to measure neutron flux in the reactor system. There are 3 peaks , the lower reflector peak, the peak in the middle of the flux profile and the upper reflector peak. the lower reflector peak is caused by the neutrons coming from fission reactions and neutrons reflected from the reflector. the upper reflector peak has a lower peak than the lower reflector peak because of its proximity to the control rods. control rods absorb neutrons hence fewer neutrons are detected in that region. the peak to average ratio will be calculated in the results section using numerical integration.
PROCEDURE AND METHODS

Equipment

Pulstar reactor, traveling in-core probe, Keithley electrometer, power supply for the detector and digital acquisition system.

lab procedure:

Reactor starts at 0,5kW. an aluminum tube will be inserted into coolant channel. the TIP is inserted till it reaches the bottom.using electro-mechanical drive , while marking the position of start of the drive simultaneously, the reactor personal will drive the detector out of the core. the digital data acquisition system will generate records or TIP current as a function of time. when the top reflector is reached the drive stops and the measurements are recorded. (measure distance from bottom location using meter stick. 5 channel locations were used. steps are repeated in different coolant channels of different fuel assemblies.

RESULTS

[pic] figure 1

figure one shows the flux profiles for all five channels. All have the same trend except for channel D4, sharp peak before the upper reflector peak.

|channel |average |Fuel peak |Peak-to-average |
|D2 |0.532992 |62.2456 |2.200681 |
|D3 |0.621453 |71.0357 |2.08359 |
|D4 |0.693013 |71.1278 |2.003117 |
|D5 |0.71403 |82.3109 |1.90167 |
|D6 |0.55188 |88.3012 |1.60321 |

[pic] figure 2

figure 2 shows the five radial flux values at a height of 10 in from the bottom at the five channels.
DISCUSSION

The results shows the effectiveness of TIPs for in-core neutron flux mapping, supporting our hypothesis stated in the introduction. All the flux profiles measured in the lab were as predicted except for the flux profile of channel D4 where we had a sharp peak before the upper reflector peak.

Figure 1 shows us where the absolute maximum of the flux profiles are located. (Highest peaks at the bottom) theses peaks are due to the fuel rods and reflectors close to it. Hence neutrons coming from fission reactions and reflecting from reflector affect the TIP.

As the TIP is driven up, new peak will appear in the middle of the reactor. The peak to average ratio is calculated to tell nuclear reactor operators if they are within the safety margins and to make sure any hot spots or problems are fixed. If the peak to average ratio is too high or too low then this would be an indicator that reactor is not operating properly.

There will be another peak as TIP is driven upwards again. The upper reflector is less than lower reflector peak since the control rods absorb the neutrons before TIP detects them. As TIP goes higher flux will go to zero due to control rods absorbing neutrons.

The discrepancy noticed is the sharp peak before the upper reflector peak in channel D4. This might be due to an error while driving the TIP. Another reason might be that the reflector effects are greatest in this channel which might have caused detector to make errors.
CONCLUSIONS
In this lab our purpose is to calculate neutron flux using TIP. This lab explained how the TIP is used and its benefits in ensuring safety margins are followed. It required to use numerical integration to find peak to average flux ratio and why it is important. The shape of the flux could be affected by the reflectors, control rods and other aspects of the nuclear reactor. Overall very informative and helpful.

REFERENCES
Axial Mapping hangout • • • Nuclear Reactor Analysis, J.J. Duderstadt and L.J. Hamilton, John Wiley & Sons, 1976.