Using the concepts of neutron activation analysis, the variation in neutron flux (and hence the power density) in an operating reactor is easily determined.
Let us recall that for neutron absorption, the counts recorded during a count time tc, following an irradiation time ti, and wait time tw, is:
where:
The expression clearly indicates the direct response of the irradiated material to the thermal neutron flux at the location of the irradiation. It then follows that if a wire, or ribbon, of properly chosen neutron-absorbing material were placed in the reactor core during operation, the resulting distribution of activation product(s) along the wire (ribbon) would reflect the neutron population during the exposure.
The Reactor Facility has dysprosium/aluminum (Dy/Al) wires available to use as activation wires in the core. These are well-suited for mapping the vertical variation in the neutron flux in a given element, as the wire is easily (relatively) inserted between the fuel plates of an element, and easily removed at the end of the irradiation.
It follows that a three-dimensional map of the neutron flux may be accomplished by conducting a series of flux-wire measurements across the entire core. Because of the impact of the wire with its relatively high cross-section, it is not possible to irradiate wires in all locations simultaneously.
In preparation for placement of the wire, it must be checked for straightness, mechanical flaws, and residual radioactivity, if any. In addition the length should be checked to ensure measurements well into the reflector region. The upper end of the wire requires a stop that will assure positioning of the wire, and a light string of sufficient length should be tied to the wire.
Determine the fuel element in which the vertical flux variation is to be sampled. Then, following established reactor operating procedures, pull that fuel element to the surface of the pool, while carefully monitoring the radiation fields in the area. Remember, fuel moves must be conducted under the direct control of a licensed SRO, and adherence to procedures is mandatory.
Keeping the entire fuel assembly under water, just below the surface, insert the wire carefully, seating the stop against the top edge of the fuel or the lifting bail.
Replace the fuel assembly in the core, carefully paying out the string to prevent the wire from moving.
A pre-experiment calculation shall have been made to determine the reactor power level and length of run for this particular wire.
Place a gold foil into the sample holder for normalization of data purposes.
The reactor operators will give the desired operating profile the shut down. Log in all clock times.
Pull the wire from the core and let it hang in the pool until the aluminum activity becomes negligible.
When the Al activity has died away, remove the wire from the pool, using proper handling and radiation monitoring techniques.
Bring the wire to the counting set-up which includes a collimator detector, and shielding. In addition the set-up must have a means of measuring the distance from the collimator window to the wire stop for each measurement.
Record counts, count time, clock time, and distance from the reference for points along the wire. Maximum data spacing should be one inch.
The significance of the flux shape and the actual values should be discussed in detail.
