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Resource performance management of graphite stacks is underway at Russia’s Smolensk NPP. In October, the station completed a planned repair at unit 1, during which the straightness of 137 graphite columns was restored – 8% of the total volume. The next stage will take place at the station next year. “Specialists continuously monitor the graphite stack, an irreplaceable part of the RBMK reactor, and have long known that over time it begins to change shape: to swell and crack, which causes the process channels to deform,” explained Alexey Leshchenko, Chief Engineer at Smolensk NPP. “Their deformation, or the so-called deflection arrow, is the most important operational parameter that determines the further operation of the reactor plant. The maximum value is 110 mm, more than that is unsafe. At unit 1, the largest deflection is 99 mm.”

The essence of resource performance management is to restore the straightness of graphite columns. According to the deputy head of the reactor workshop, Sergei Orlov, described the work. After the reactor cooled down, the fuel was unloaded, the technological channels were removed, and the graphite blocks from which the columns were assembled were cut longitudinally along the entire height.

“We used a special device with a cutting unit and a video surveillance system (the image from the camera was displayed on the operator’s monitor). Next, the holes were calibrated, the cuts were inspected for the presence of fragments that would interfere with the closure of the blocks, and they were removed,” he said. “Then the graphite stack was ‘shaken’ – the main circulation pumps were turned on to dynamically influence the graphite, and further inspections took place. Some cells were re-cut along the closed cuts, the deflection arrows were measured, new technological channels were installed and fuel was loaded.”

“The technology that makes it possible to control the resource characteristics of the reactor core was developed by the innovators of the Leningrad NPP more than 10 years ago,” noted First Deputy Chief Engineer Vadim Skirda. “Later, the procedure was significantly improved. Now the procedure at the Leningrad and Kursk stations is already an integral part of their operation. Having studied their experience and applied it in practice, we now have an adequate response to the process of masonry shaping.”

Alexey Leonov, chief specialist of the nuclear safety and reliability department at Smolensk NPP said: “The geometry of the channels and the deflection of the graphite columns were measured using high-precision systems produced by the Prolog company. The results confirmed previous forecasts and the need to proceed with the first full-scale cutting. The volume and sequence of restoration work, the coordinates of 137 cells for cutting (8% of the total volume of columns with technological channels) were determined by specialists from the NA Dollezhal Scientific Research & Design Institute of Power Engineering (JSC Nikiet) in accordance with programme calculations.”

“The ambitiousness of the task lay not only in the fundamental novelty for us, but also in the fact that it had to be solved in an extremely short time,” added Deputy Chief Engineer Andrey Piskov. “It took a lot of staff effort to make everything work. We the resource management project, the roadmap of which was updated more than once to meet changing requirements. As a result, the optimisation took 16 days, the planned repairs were completed in 109 days instead of 125, the emergency control itself took 58.9 days, which became one of the key events both for the nuclear power plant and for [nuclear utility] Rosenergoatom.”

He added that the use of an automated process control system (developed by Smolensk nuclear scientists) helped to better plan and adjust volumes, and allocate resources more optimally. The current situation was always visualised on a cartogram. “This allowed us to predict potential problems and take compensatory measures in advance.”

Smolenskatomenergoremont (SAER) also launched its own project to maintain the specified pace of replacement of technological channels. “Our personnel were engaged in dismantling the technological channels at the bottom of the apparatus and collecting waste from cutting graphite,” said SAER Chief Engineer Maxim Marochkin. “We had experience in performing such work, but in previous years the volumes were significantly smaller. In order to replace 137 channels on time, we worked in three shifts, around the clock, seven days a week, together with specialists from the centralised repair shop of the Smolensk NPP and the Prolog company. Any difficulties that occurred were resolved through personnel rotation, hourly planning with the issuance of shift assignments, and constant monitoring by the management of the nuclear power plant and our branch.”

The main result is that it was possible to reduce the curvature of the columns throughout the entire volume of the graphite stack to values ??that allow the safe operation of the reactor installation for 300 effective days. An effective day means continuous 24-hour operation at rated power.

“In 2024, unit 1 will undergo the next stage of its development, and preparations for it are in the active phase,” noted Andrey Piskov. “We will introduce new technical methods: for calibrating graphite columns, collecting and removing graphite. Prolog has begun developing equipment; the engineers of this company say they do not expect revolution, but evolution is possible. In addition, we plan to increase the number of cutting jobs from four to five and take corrective measures to reduce defects in welding operations.

Image courtesy of Smolensk NPP

Date: Thursday, 23 November 2023
Original article: neimagazine.com/news/newswork-begins-to-straighten-graphite-stacks-of-rbmk-reactors-at-smolensk-npp-11317845