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Uranium Downblending Calculator - HELP

(last updated 12 Dec 2009)

Contents:


Introduction

This calculator performs calculations of the material balance for downblending of Highly Enriched Uranium (HEU). This process is being used to make excess weapons-grade uranium suitable for use as fuel in nuclear power plants.

The calculator is independent of the actual downblending technique used, such as mixing of liquids (uranium in the form of uranyl nitrate), or mixing of gases (uranium in the form of UF6). Any effects or losses of the necessary conversion steps are disregarded.

The material balance is presented in a flow chart. Upon entry of one value into any of the flow chart's input fields, all other fields are calculated accordingly. So, it is possible to calculate the balance per tonne of HEU feed UO3, as well as per tonne of U in UO2 of the Low Enriched Uranium (LEU) product, for example.
Note: if U-236 is present, the actual concentration of U-235 must be higher to compensate for the neutron toxicity of U-236.

The "SWU contained" shown for HEU feed and Blendstock are the Separative Work Units (SWU) actually spent for the enrichment of the HEU, and, if applicable, the Blendstock. The "SWU equivalent" shown for the LEU product is the SWU that would have been spent, if the LEU product would have been obtained from straight enrichment rather than downblending; this value is shown for comparison.

The parameters used for the calculation can be set in the Process Parameters table. These parameters show reasonable initial values which can be modified as needed. There are no other hidden parameters used in the calculation.

Upon entry of any parameter, some Product (LEU) properties are calculated: the actual concentrations of the isotopes U-235, U-234 and U-236 in the product, and the SWU Loss from Blending (and Blendstock enrichment, if applicable).
The concentrations of U-234 and U-236 are also shown as microgram per gram U-235 (g/gU-235). These figures are useful for comparison with the ASTM specifications (C 996-90) in LEU:

Exceedence of the ASTM specifications is indicated by yellow and/or red color flags.
The SWU loss is caused by the decreasing efficiency of the enrichment process with higher product assays; it moreover comprises any Blendstock enrichment, if applicable. It is expressed as percent of the SWU spent for the HEU enrichment.

For background information, see also Uranium Downblending.

Note: The natural uranium and SWU equivalent for the blended material can be determined by entering the LEU UO2 figure in the appropriate field of the Nuclear Fuel Material Balance Calculator, or by using the Material Balance of the Recycled Nuclear Fuel Cost Calculator.

 

Process Parameters

HEU Feed
Assay [wt-% U-235] · [wt-% U-234] · [wt-% U-236]
Weight-percent of the fissile isotope uranium-235 and of the byproducts uranium-234 and uranium-236 in the uranium contained in the HEU feed (highly enriched uranium).
U-235 assays are > 20% for HEU, by definition. The assays of HEU currently being downblended are in the 40% to 90% range.
U-234 is a minor isotope contained in natural uranium; during the enrichment process, its concentration increases even more than that of U-235. Caution: Upon entry of a value for the U-235 assay, or the HEU Origin or Tails Assay (see below), the calculator automatically determines a value for the U-234 assay. In case the estimated value is inappropriate, it can be overwritten.
U-236 is a byproduct from irradiation in a reactor and may be contained in the HEU, depending on its manufacturing history. HEU reprocessed from nuclear weapons material production reactors may contain U-236 concentrations as high as 25%.
Origin Assay · Tails Assay [wt-% U-235]
Weight-percent of uranium-235 in the uranium originally used as feed resp. generated as tails in the enrichment process that supplied the HEU

Blendstock
Sample
Selection of a sample parameter set initializes the Blendstock parameters. After a selection is made, single parameters may be modified as required.
Assay [wt-% U-235] · [wt-% U-234] · [wt-% U-236]
Weight-percent of the isotope uranium-235 and of uranium-234 and uranium-236 in the uranium contained in the blendstock used for downblending.
U-235 is contained in natural uranium at 0.711 wt-%, and in depleted uranium typically at 0.2 - 0.35 wt-%. Often however, slightly enriched uranium at typically 1.5 wt-% U-235 is used as a blendstock to dilute byproducts contained in the HEU feed.
U-234 is a minor isotope contained in natural uranium. Caution: Upon entry of a value for the U-235 assay, or the Blendstock Origin or Tails Assay (see below), the calculator automatically determines a value for the U-234 assay. In case the estimated value is inappropriate, it can be overwritten.
Note: For depleted uranium blendstock it is assumed that it is a by-product from enrichment of natural uranium to 4% U-235, if no other product assay is entered below. Enrichment is assumed for blendstock-production, if the blendstock U-235 assay is higher than the Blendstock Origin Assay; if, in this case, the Blendstock Origin Assay is less than 0.711 wt-%, then re-enrichment of depleted uranium is assumed for blendstock production; for the DU fed into the re-enrichment process it is assumed that it was generated from enrichment of natural uranium to 4%.
U-236 is not found in natural uranium, but any processed uranium may contain traces of U-236, depending on its manufacturing history.
Origin Assay · Tails Assay [wt-% U-235]   (Only required if Blendstock is obtained by enrichment of Blendstock Origin)
Weight-percent of uranium-235 in the uranium originally used as feed resp. generated as tails in the enrichment process that supplied the Blendstock.
Note: In case the Blendstock Assay is lower than the Blendstock Origin Assay - that is DU is used for blendstock, the product assay for the enrichment process (in which the DU was generated) can be entered instead of the tails assay. Otherwise a product assay of 4% is assumed.

LEU Product
Assay [wt-% U-235 equiv.]
Weight-percent equivalent of the fissile isotope uranium-235 in the uranium contained in the LEU product (low-enriched uranium).
For LEU that is free of the neutron poison U-236, this value is identical to the actual U-235 concentration, otherwise the actual U-235 concentration must be higher to compensate for the effect of U-236.
Origin Assay · Tails Assay [wt-% U-235]
Weight-percent of uranium-235 in the uranium used as feed resp. produced as tails in a hypothetical enrichment process that would supply the same amount of LEU product as the actual downblending process does.

General
Factor for U-236 effect [excess wt-% U-235 per wt-% U-236]
Excess concentration of U-235 required to offset for the neutron-absorbing effect of U-236 present in the LEU product.
Typical values are in the 0.2 - 0.3 range, depending on reactor and fuel type.

 

Isotope concentrations in U.S. DOE uranium [wt-%]
Sources: Moody 1994; Y/ES-063/R2, DOE 1995
EnrichmentU-235U-234U-236
93%93%1.0%0.445%
50%50%0.425%0.231%
4%4%0.0334%0.0154%
0.9%0.9%0.00948%0.00325%
natural0.711%0.00541%0
depleted to 0.2%0.2%0.00356%0

 

Calculation Details

The formulae used by the calculator for the enrichment process can be found under Separative Work Unit (SWU) in the "Enriched uranium" article of Wikipedia.

 

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