Avian’s Blog

Today I went on a tour of the power plant Krško, the only Slovenian nuclear power plant (the trip was organized by EESTEC).

We weren't allowed to bring anything with us on the tour (hence a lack of pictures of the most interesting stuff in this post). They also performed some kind of a biometric scan of my right hand, gave me a temporary RFID tag and assigned an armed guard to each group of visitors. No dosimeters were given though (like when I visited the small research reactor in Podgorica a few years ago). But then we also didn't visit any of the areas where they handle radioactive stuff.

A model of the fuel bundle that contains pellets of enriched uranium oxide.

They gave us a complimentary nuclear power plant starter kit containing three (fake) fuel pellets. Now I only need some 7 million more of them to get enough for the whole bundle :)

Our guides really took their time and answered all of our questions, even some that probably weren't very pleasant from their point of view. It seemed to me that they are working hard to present the power plant to the public in the friendliest way possible. They explained the basic principles (Krško uses a PWR-type reactor) and also told us quite a few things that were surprising to me.

• Every physics book I read says that the power output of the reactor is controlled by moving the control rods in and out of the reactor. I learned that this isn't so, because having control rods only partially inserted would burn the uranium fuel at the bottom of the reactor faster than at the top. This is a bad thing (they try very hard to keep the reaction rate constant throughout the volume of the reactor). So a better way to do this is to vary the concentration of a neutron absorber in the coolant. Rods are only used for fast changes (like in an emergency). Interestingly the Wikipedia article linked above gets this right.
• The plant was designed by the Westinghouse company, while the actual work was done by workers and specialists from local (then Yugoslavian) companies. The design specifications were so strict that all markings on the equipment, pipelines, etc. are in English. For example even the color codes of the wires used in the buildings are according to American standards instead of European. I'm surprised they managed to adapt the plans to the 50 Hz frequency we use here :)
• Hruske writes how nuclear power plants run on QNX. Krško actually doesn't run on anything. All control loops are still implemented with analog electronics (which is considered a Good Thing by the maintenance engineer I talked to). They recently installed a new digital monitoring system though. It runs on some software that nobody there actually knew how it was called (but they all agreed it wasn't Windows). The main control room is full of switches, analog gauges and warning lights.
• New fuel elements are transported to the plant from USA first by a ship and then by trucks. A couple of times they also flew them in on an airplane. It's interesting that usually our newspapers are full of reports how a convoy of trucks carrying nuclear fuel will be traveling through this-or-that distant country but I never seen an article about how they transported fuel for Krško. On the second though this may be a good thing or there would be people chaining themselves to trucks and things like that.