Crystal clean

Ultrasonic jewellery cleaners use an electromagnetic transducer, essentially a kind of speaker, to agitate a solvent in a bath. The compression wave created by the transducer create tiny bubbles that pop, creating forces that scour surfaces. A variation of this system has been scaled up to clean crud off fuel assemblies by Dominion Engineering, a Virginia, USA-based company (unaffiliated with the utility of the same name).

Hot, active, high-pressure coolant gushing through the primary circuit of PWRs wears the Inconel tube walls in the steam generator, depositing nickel molecules to the primary water; it also erodes the surfaces of reactor pressure vessel and other steel tubing, adding iron. These molecules, which interact with compounds in the water, particularly the boric acid added as a neutron poison, end up stuck to fuel assemblies as hard deposits, particularly in the upper portions of the fuel assemblies where the water is hottest and local boiling may occur.

Because the boron in the crud tends to poison the fission reaction at the top of the fuel assemblies, it interferes with reactor neutronics, so the reactor comes off its design point. Westinghouse has named this phenomenon ‘crud-induced power shift’. Removing crud improves a PWR’s performance by improving the efficiency of the fission reaction, and can prevent power reductions that harm the plant’s economics.

Dominion Engineering supplied its first ultrasonic cleaner (with financial support from the Electric Power Research Institute) to the Callaway Nuclear Plant in 1999, when it was facing just this sort of problem. That system was the first of what have become three successive models. The machines, of whatever vintage, operate underwater. Their top interface resembles a cell in a fuel rack; the fuel crane lowers the fuel assembly into the cleaner. Although the cleaning operation only takes three minutes, limitations of fuel handling slow the entire operation to a maximum of four assemblies per hour. According to David Arguelles, Dominion senior engineer, some utilities clean fuel as it is loaded out of the reactor during an outage; if there is more time, they clean it in the spent fuel pool. Since typically a third of a PWR core inventory is added as fresh fuel every outage, about two-thirds of a core inventory is cleaned per project.

During cleaning, electromagnetic transducers in the device loosen crud. A vacuum pump on the other side of a filter sucks water containing the loosened debris through the filter. Operators measure the gamma radiation of the water travelling to the filter to track the presence of radioactive crud. Since the radiation level depends on the amount of crud particles in the water, operators know that when it tails off, the cleaning process has ended. A higher-efficiency version of the system was launched in 2010. A system is also available for BWRs.

Dominion has sold the device to utilities, and service providers such as AREVA and Westinghouse, who undertake cleaning campaigns for utilities.

Sweden’s Ringhals 4 nuclear power plant, which is owned and operated by electrical utility Vattenfall, has taken fuel cleaning to a new level by using a device to take samples of crud. More accurate sampling data improves the plant’s ability to estimate the extent of crud in the core, and therefore better predict reactor performance using computer models, Arguelles says. “Those models look at the correlation of release rates of primary system components, and chemical transients, how much material is deposited on the fuel. Using those codes effectively requires certain inputs to be assumed; how much crud is there, how much is taken off during cleaning, what is the distribution of crud from one place to the next, and from one elevation to the next. Having the ability to collect field data, how much crud is where, and how much has been removed, allows us to have much more reliable inputs for the codes, and predict a more accurate answer about whether or not they will have problems.”

At Ringhals 4, service provider AREVA sampled 44 assemblies out of 115 that were cleaned and returned to the core in a summer 2012 outage (among other tests). The cleaning system records the total dose rate accrued as each fuel assembly is cleaned, which indicates the amount of crud present. When plotted per fuel assembly in the core, this data can create a kind of indicative core crud map. In addition, when the system takes samples, it records the sampling interval, and the total flow for cleaning that fuel assembly; 1 in 5000, for example. Therefore, by multiplying the mass of the sample by the sampling ratio, it is possible to extrapolate the total weight of crud removed from the entire assembly.

Ringhals aims to compare the efficiency of ultrasonic cleaning with manual scraping, and understand the impact of last year’s Ringhals 4 steam generator replacement on crud. It also wants to more about crud deposition, and understand what kinds of crud are, and are not, removed by the cleaning process. Chemical and radiochemical analysis is being carried out on site. Final results, including nuclide gamma measurements, analysis of crud by element and crud structure analysis using transmission electron microscopy will not be completed until 2013. Ringhals and EPRI are currently discussing a joint project to evaluate and publish the data.

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This article was first published in the December 2012 issue of Nuclear Engineering International

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