Engineering topics

Rombough, Charles

Publications and source records attributed to Rombough, Charles.

4 records

DOE OSTI2016

Solving the Interface Problem in Criticality Analysis

In spent fuel pools, there are usually two or more disparate regions. One region may be dedicated to storing fresh fuel and the cells are further apart while another region is designed for burned fuel where the cells are relatively close together. The interaction across the interface between the two regions presents a criticality challenge. If the reactivity of a region increases due to the presence of the other region, then measures would have to be taken to mitigate the effect (such as removing fuel or inserting neutron absorbers at the interface). The interface between two disparate regions in the spent fuel pool must be examined to ensure that the interface does not increase k{sub eff} over the k{sub eff} of the individual regions. This can be done 1) by inspection, 2) by region separation, or 3) by direct calculation. When doing a direct calculation, the nominal k{sub eff} of each region by itself (before adding bias and uncertainty) needs to be the same so that the interface effect can be known. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2016

New Approach to Eccentric Positioning of Fuel Assemblies in a Spent Fuel Pool

Criticality analyses of a spent fuel pool (SFP) typically assume the fuel assemblies are centered in the cells. In reality fuel assemblies are not constrained to be centered but can be placed anywhere within the available space. For SFPs with absorber panels it has been shown that the most reactive condition is with the fuel assemblies centered. However, for SFPs without absorber panels eccentric positioning of assemblies increases reactivity. Historically, it was common practice to analyze eccentric positioning by modeling a 2x2 arrangement of the fuel where each assembly is positioned toward the center (with reflective or periodic bounding conditions). The delta k between the eccentric model and the center model was taken as an uncertainty which was then statistically combined with other uncertainties. This historical approach had two problems. First, arrangement of more than 4 assemblies placed toward the center of the model produced higher k's. Second, in a large pool it was likely that 4 assemblies collocated toward each other would exist so asymmetry should be treated as a bias rather than an uncertainty. The number of collocated assemblies is important to the magnitude of the effect. Since spent fuel pools are large it is expected that some collocated eccentrically placed assemblies will occur so the eccentric placement reactivity should be accounted for by a bias. The magnitude of this bias depends greatly on the number of eccentrically placed assemblies that are collocated. A simple probability model assuming random placement can be used to establish the number of collated eccentric assemblies. It has been shown that for a SFP with 800 cells there is less than a 5% probability that more than 16 eccentric assemblies will be collocated. The eccentricity bias using 16 eccentric assemblies collocated is small (on the order of 0.001 in delta k or less) and should be easy to add as a bias in the SFP criticality analysis.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2016

Fission Gases in Spent Fuel Pool Criticality Analysis

Historically all fission products, including fission gases have been included in the criticality analysis of many spent fuel pools (SFP). For transport casks ISG-8 Rev. 3 states, 'Credit for neutron absorbing isotopes other than those identified in Tables 1 and 2 should be accompanied by assurance that such isotopes are non-volatile, and non-gaseous,..' Recent questions from the NRC have challenged using fission gases in spent fuel pool criticality analyses, noting that the time the fuel is contained in the spent fuel pool can be long. This paper establishes a conservative approach for using some of the fission gases in the criticality analysis. Calculations have been performed for Indian Point Unit 2 to determine the impact of the fission gas release. The calculations have been run with the noble gases reduced by 32%, the halogens reduced by 10% and the Rb reduced by 44%. At 43 GWd/MTU, the impact of reducing the isotopes described here is only 0.0014 in k. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2016

Impact of Concrete Composition on Criticality at Nuclear Power Plants

There is no standard concrete composition for criticality analysis. Concrete is a composite material composed of aggregate bonded together with cement which hardens over time. Aggregate consists of large chunks of material in a concrete mix, generally coarse gravel or crushed rocks such as limestone or granite, along with finer materials such as sand. Water is mixed with the dry powder cement/aggregate blend. The concrete solidifies and hardens. After the concrete is hardened there can be water loss with time. The main components in concrete generally have small neutron cross sections so the absorption cross section is often dominated by the impurities. The content of the impurities are rarely documented in a manner that is generally needed for crediting in a criticality analysis. This paper assesses the impact of conservative concrete on a sample dry storage rack and a spent fuel pool. The SCALE computer code has four types of concrete in the Standard Composition Library.

36 MATERIALS SCIENCE↗