"We hang the petty thieves, but appoint the great ones to office." - Aesop
Story time again. A long time ago, in a galaxy far, far away... I was a reactor operator on a TRIGA MK 4 swimming-pool reactor. Below is an image of a MK 3 reactor in operation. To the right and left alongside the pool walls underwater, are suspended racks containing spare (or spent) fuel elements. At the upper left (submerged) is a shiny plate containing several holes. This might be a newer style of fuel inspection tool.
The MK 4 operated around the clock, for months on end, at full power - which was a very modest 2 Megawatts thermal. Even thought it only put out 2 Megawatts, it was still burning up fuel, and fission products were constantly accumulating in the fuel elements. The internal operating temperature of these fuel elements was around 300 degrees C (570F).
Fission products build up inside the fuel element cladding, and several of these fission products are gases. This gas is hot when the reactor is in operation, so its pressure increases. The heat and pressure have a tendency to bulge the thin fuel element cladding, and also to elongate the fuel elements.
Below: New TRIGA fuel elements.
From time to time, it was necessary to shut down the reactor and inspect the fuel elements as part of the facility's license requirements. The reactor would be shut down, and a fuel element measuring tool would be mounted off to one side of the reactor pool.
The measuring tool consisted of two devices: A ring gauge to measure the diameter, and a jig for measuring the overall length of each fuel element. These measurements had to be performed with the fuel elements submerged, as they were highly radioactive. Handling and measuring fuel deep underwater is difficult, so the measurements were done at about 10 feet of water.
The ring gauge was a long and thick rectangular block of aluminum plate, suspended underwater at each end by long aluminum rods. The upper ends of the rods were attached to a temporary platform above the water. The aluminum plate had several holes machined in it, each one a few thousandths of an inch larger in diameter than the one next to it. Whichever hole the fuel element would pass through without rubbing would allow a determination of the amount of bulging. Once a certain diameter was exceeded, fuel elements could no longer be used. If memory serves, only two elements ever failed the ring gauge test, and they were quite old.
Below: A ring gauge similar to the one used to test for bulging. This one is not as thick or robust as the actual fuel element measuring tool.
The inspection process went like this: Grab a fuel element from the (shut down) reactor with the fuel handling rod, bring it over to the side of the pool where the fuel measuring tool was, raise it up from the bottom of the pool to the measuring tool, then see which ring it could pass through without any resistance. Record the information, and then set the fuel element in the length measuring jig and release the fuel handling rod from it. Next you would measure the fuel element's length, re-attach the fuel handling rod, and then return it to the reactor.
The length measurement was a bit trickier than the ring gauge test. The fuel element was placed into a submerged jig. This jig had a 10 ft long aluminum dowel that came up out of the water, the bottom of which would rest on top of the fuel element.
The jig would be calibrated with a "standard fuel element blank" - a fuel element with no fuel in it that was used as the length standard. A dial indicator would be set to zero using this standard length. Once the calibration for zero elongation was set, we could begin measuring the fuel elements that had been in use.
After a real fuel element was in place, the dowel was set down onto one of the 3 fins on top of the fuel element and measured. Then the fuel element was rotated, and the next fin was measured. Then the final fin was measured. This would give an indication of both elongation and bowing for each fuel element. The setup of this jig had to be perfectly consistent from year to year in order to get accurate measurements and obtain a growth trend.
It was a tedious process, because the core held about 90 fuel elements, and these measurements had to be done one fuel element at a time. It was a bit tricky to latch onto the fuel element at the end of a 30 foot long aluminum pole that flexed quite a bit. It was also a bit tricky to make sure you had the correct one on the end of the pole. You also had to be suited up in Tyvek, and wear gloves, because there was definitely contamination available, and you were stirring up even more by handling the fuel elements.
We had an issue one time. The reactor sat 30 ft below the surface of the water, but the fuel inspection jig was just 10 ft below the surface, so each element had to be lifted 20 ft, hand over hand, with the fuel handling rod.
The reactor room had a criticality alarm, which was set to sound at a radiation level of 5 Rem/hr. That's a lot of radiation, and the alarm was an incredibly loud klaxon. The criticality alarm also sent an alarm to the guard command station. A criticality alarm was required, because the facility had enough spare fuel on hand to assemble a critical mass, if someone were clueless or malicious enough to do so.
I had to test the criticality alarm system weekly, using a Cs-137 source at the end of a long metal rod. You had to hold the Cs-137 source pretty close to the detector to set off the criticality alarm for testing, but it was *never* supposed to alarm. Its only purpose was to sound if there was an unshielded nuclear chain reaction inside the building.
One year, we had shut down the reactor between Christmas and New Year's Day. After a while we shut down the pool's cooling system so that we could have a clearer view of which fuel elements we were pulling. We were in the process of measuring each fuel element - into the wee hours of the morning. One guy was handling the fuel, one guy was measuring (me), and another guy was recording the measurements.
The guy handling the fuel got a bit talkative with us, as he was reeling the fuel element up to the 10 foot level, and he overshot. Overshot - as in he nearly pulled the glowing fuel element above the surface of the water.
Suddenly he said "Wait, how high is it???" He rapidly jammed the fuel handling pole with the attached fuel element deeper into the water. I quickly looked over my shoulder to check the criticality alarm on the wall, and it was coming down from 3 Rem/Hr. No telling how hot it had been where we were working.
It would have been really difficult to explain a criticality alarm at 3 AM. The night shift guard was really chill dude, but I don't know if he would have been willing to cover something like *that*.
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