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Nuclear Fuel Population Health Risk Calculator - HELP

(last updated 9 Oct 2010)

Contents:


Introduction

This calculator performs collective health risk calculations for the general population from production and use of nuclear fuel. The focus is on the front end of the nuclear chain, while some summary values for reactor operation and reactor waste are included for comparison.
The calculator uses the following assumptions: the uranium is mined in an open pit or underground mine, and it is enriched for use in light water reactors, such as pressurized water reactors (PWR) or boiling water reactors (BWR). The reactor fuel is produced from this natural uranium, no MOX nor recycled uranium is used. The spent fuel is conditioned and disposed of in a final repository, no reprocessing is used.

The material balance used for the risk calculations is presented in the Material Balance table. Upon entry of one value into any of the table's input fields, all other fields are calculated accordingly. So, it is possible to calculate the risk per tonne of uranium mined, as well as per Gigawatt-year (GWae - this is the typical annual production of a 1300MW reactor) of electricity produced in the power plant, for example. (For more detailed material balance calculations, see the Nuclear Fuel Material Balance Calculator).

The results of the risk calculations are presented in the Health Risk Summary table in terms of collective dose in man-Sievert, and expected number of fatal cancers resulting from this dose. Some intermediate results are shown in the Source Term (Radon-222) table, where TBq stands for 1012 Becquerels.
You can also enter a value into any of the Health Risk Summary or Source Term table fields, instead of entering one into the Material Balance table. This facilitates comparison.

The Initial Radon-222 Release Rate from Tailings table shows the initial radon-222 releases per unit area, to allow for a comparison to current standards. These values are for display only and are not for change. For the source term calculations, the calculator takes the decrease of radon release with time into account, as caused by decay within the tailings (see Uranium Radiation Properties).

The parameters used for the calculation can be set in the Health Risk Parameters and Process Parameters tables. These parameters show reasonable initial values which can be modified as needed. There are no other hidden parameters used in the calculation. Any assumptions made for the calculations are described on this page.

For the front end of the nuclear fuel chain, the calculator considers only the major sources of health hazards to the public:

 ConsideredNeglected
MineRadon release during operationRelease of dust
Release of liquids
Releases after shutdown
Releases from waste rock piles
MillRadon release during operationRelease of dust
Release of liquids
Releases from plant decommissioning
Mill TailingsRadon release during operation
Radon release after decommissioning
Release of dust
Seepage
Loss of integrity from erosion etc.
Risk of dam failure
ConversionNilReleases during operation
Releases from wastes
EnrichmentNilReleases during operation
Risk of UF6 transport accidents
Hazards from depleted UF6 cylinder storage
Releases from depleted uranium disposal
Fuel FabricationNilReleases during operation
Releases from plant decommissioning
Releases from waste disposal

See special instructions for offline use of this calculator.

Users who need a more sophisticated analysis than is possible with this calculator should have a look at Dose Modeling Software.

Dose calculations for individuals exposed to radon releases from uranium mines, mills and tailings can be performed with the Uranium Mine and Mill Resident Individual Dose Calculator.

Dose calculations for individuals exposed to known concentrations of various forms of uranium can be performed with the Uranium Radiation Individual Dose Calculator, and individual doses from known concentrations of radon can be calculated with the Radon Individual Dose Calculator.

 

Health Risk Parameters

Mine: Specific Radon Emission [GBq/t U3O8]
Release of Radon-222 from the mine per tonne U3O8 produced in the mill. GBq stands for 109 Becquerels.
The following table shows the extremely high variations for this figure (UNSCEAR1993, all for 1989):
Ranger Mine (Australia)22
former Panel Mine (Canada)110
former Ronneburg Mine (Germany)210
former Denison Mine (Canada)540
Rabbit Lake Mine (Canada)760
former Aue Mine (Germany)2000

Mill: Specific Radon Emission [GBq/t U3O8]
Release of Radon-222 from the mill per tonne U3O8 produced. GBq stands for 109 Becquerels. This value shows small variations around 13 (UNSCEAR1993).

Mill Tailings: Tailings Thickness [m]
Thickness of the tailings in the deposit, typically in the range of 10 to 50 meters (used for calculation of surface area, square shape assumed)

Mill Tailings: Tailings Density [g/cm3]
typically 1.6

Mill Tailings: Specific Radon Exhalation Rate [Bq/m2s per BqRa-226/g]
Release of radon-222 from the bare surface of the tailings deposit, normalized per square meter and per contents of radium-226 in the tailings.
Radium-226 is the precursor of radon-222 in the decay chain. It is assumed that radium-226 in the ore is in secular equilibrium with uranium-238, and that all radium-226 contained in the ore ends up in the tailings. Often, a rule-of-thumb value of 1 is used for unsaturated tailings (EPA1986); this applies for "infinite" (in terms of radon exhalation) thickness of the tailings, that is more than 4 meters. In case of a smaller tailings thickness, the value has to be corrected appropriately.
For a given situation, the Uranium Mill Tailings Cover Calculator can be used to calculate the specific radon exhalation rate ("Specific Bare Source Flux from Layer 1").

Mill Tailings: Time to Cover Installation [years]
Time in years during which the full surface of the bare tailings is exposed, usually during active operation of the mill. If only part of the surface is exposed (as for the presence of ponding water, or for phased disposal, for example), decrease this value accordingly.

Mill Tailings: Cover Radon Retention [%]
Percentage of radon-222 release retained by cover on top of tailings.
Set this value to zero for bare tailings. Engineered covers (whether from clay or synthetic materials) show high radon retention rates immediately after installation, while their long-term performance is a matter of discussion. Water covers of more than 1 meter have high radon retention rates, if undisturbed. The following table shows some examples for earth covers (EPA1986):
Earth TypeMoistureRadon Reduction
Cover Depth
0.5m1m2m3m
sandy soil3.4%29%50%75%88%
soil7.5%37%60%84%94%
soil12.6%50%75%94%98%
compacted moist soil17%68%90%99%>99%
clay21.5%94%>99%>99%>99%
For a given situation, the Cover Radon Retention can be calculated with the Uranium Mill Tailings Cover Calculator.

Common Data (Mine, Mill, Tailings): Population Density [persons per km2]
An uniform population density is assumed. The population density at uranium mine and mill sites varies considerably: 3 - 25 (Western US), 160 (Central Europe)

Common Data (Mine, Mill, Tailings): Truncation Distance for Air Dispersion [km]
Radius of the area around the mine, mill and tailings deposit, beyond which the cumulative dose and risk assessment is to be truncated.

Common Data (Mine, Mill, Tailings): Mixing Layer Height [m]
The troposheric mixing layer height is the maximum height above ground the radon plume can reach. It highly depends on the climatic conditions, day-night cycle etc. The value used for NRC's model mill in NUREG-0706 is 850 meters.

Common Data (Mine, Mill, Tailings): Annual precipitation [cm]
Precipitation washes some of the radon out from the plume. This value is only taken into account in the "precise" mode (see below). The value used for NRC's model mill in NUREG-0706 is 31 cm.

Common Data (Mine, Mill, Tailings): Radon Progeny Equilibrium Fraction (used in fast mode only)
Describes the ratio of the short-lived decay products of radon-222 compared to radon-222 itself in the air. A typical value for larger distances is 0.7. In the "Fast" mode, the calculator uses this single value, while, in "Precise" mode it automatically takes radon progeny ingrowth during travel time (resp. travel distance) into account.

Common Data (Mine, Mill, Tailings): Air Dispersion Calculation Mode
The calculation of radon dispersion in the air is the most time-consuming step in the calculation. Therefore, two options are provided:
In the "Fast" mode, the calculator does not take into account radon progeny ingrowth and plume depletion during its travel time. This mode thus overestimates the risk, in particular for very short and for very long distances (see also Air Dispersion Modeling).
In the "Precise" mode, radon progeny ingrowth during travelling time is considered automatically, and two effects responsible for plume depletion are taken into account: precipitation scavenging and the decay of radon (with its half-life of 3.8 days). On slow machines, it may take 20 seconds or so for the results to appear. For the user's convenience, the air dispersion calculation is only performed during the first run, and otherwise only if any parameters affecting air dispersion have been changed.

Common Data (Mine, Mill, Tailings): Fractional Frequency of Wind Speed Classes
The distribution of wind speed frequencies is the basic input data for the air dispersion calculations. Since the calculator only evaluates cumulative doses and risks for an assumed uniformly distributed population, no wind direction data is needed - considerably decreasing the number of values to be entered.
The table contains the fractional frequencies (in percent) of the wind speed fitting into any of six wind speed classes, for each of the stability classes A to F. The wind speed classes are described by their average wind speed in meters per second. The stability classes are defined according to Pasquill, as follows: A - extremely unstable, B moderately unstable, C slightly unstable, D neutral, E moderately stable, F very stable.
The total of all fractional frequency entries in the table should be 100%. If the total differs from 100% by more than 1%, the values are scaled internally (upon user confirmation) to obtain a total of 100%. If not all wind speed classes are needed, leave the fields for the average speed of the unused classes free (the unused classes must be at the end). The DEFAULT data set is taken from NRC's model mill in NUREG-0706, located at a site with a semiarid climate in the Western U.S.

Power Plant: Normalized collective effective dose (without global Carbon-14) [man Sv per GWae]
Collective dose from reactor operation per unit electricity produced, excluding contributions from globally dispersed Carbon-14. This value depends on a number of parameters and assumptions, which cannot be detailed here. UNSCEAR1993 gives a figure of 1.34 for the local and regional component.

Power Plant: Normalized Carbon-14 Release [TBq per GWae]
UNSCEAR1993 gives a value of 0.12 for pressurized water reactors (PWR), and of 0.45 for boiling water reactors (BWR).

Power Plant: Normalized all time collective effective dose for globally dispersed Carbon-14 [man Sv per TBq]
Dose committment for all time resulting from the release of Carbon-14. UNSCEAR1993 gives a value of 120 manSv/TBq, assuming an equilibrium world population of 1010 people reached in the next century and maintained over the next thousands of years.

Reactor Waste: Normalized collective effective dose [man Sv per GWae]
Collective dose from reactor waste per unit electricity produced. This value depends on a number of parameters and assumptions, which cannot be detailed here. UNSCEAR1993 gives a figure of 0.5 for the low and intermediate level waste, and a figure of 0.05 for the spent fuel, but "These estimates are highly uncertain...".

General Data: Radon Dose Factor [nSv/h per Bq/m3 EEC]
Effective dose coefficient for exposure to equilibrium equivalent concentration (EEC) of radon. UNSCEAR1993 uses a value of 9, ICRP65(1993) uses a value of 6.4. Based on the revised dose factor for the public of 9 mSv/WLM presented in J.W. Marsh et al: Dose conversion factors for radon, in: Health Physics Vol. 99, No. 4 (Oct. 2010), p. 511-516, the resulting value is 14.1 nSv/h per Bq/m3 EEC (with 1 WLM = 6.37·105 Bq·h/m3 of EEC of Rn-222).

General Data: Radiation Risk Factor [1/Sv]
Fatal cancer risk for lifetime exposure of the general population at low doses and low dose rates per unit dose received. ICRP60(1991) uses a value of 0.05

General Data: Truncation Time for Dose Calculations [years]
Time in years after which the dose calculations for Radon releases from tailings and for Carbon-14 releases from nuclear power plants are truncated. For tailings, time starts with cover installation. Numbers can also be entered in scientific notation (for example: 1e6 for 1 x 106 = 1 million).
The radon production rate within the tailings deposit only decreases at the rate given by the half-life of thorium-230, which is 80,000 years (thorium-230 is the precursor of radium-226 in the decay chain). The radon production therefore continues for more than 10,000 years at nearly its initial intensity. After around 500,000 years, the radon production rate has decreased to the level given by the residual contents of uranium left in the tailings - a matter of ore grade and mill extraction losses (see process parameters below). The calculator takes the decay of all radon-precursors into account.
Globally dispersed Carbon-14 from nuclear power plant operation remains available in the environment for long periods of time, taking into account the 5730 year half-life of this nuclide.
Given the long half-lives of the radionuclides of interest, it is obvious that very long periods of time have to be taken into consideration. Nevertheless, calculations are often cut off after 100 or 1000 years, UNSCEAR1993 uses 10,000 years: but the hazard continues for much longer periods of time.
The calculator assumes constant conditions for the whole period considered, as for example for tailings cover performance, population density, and climatic conditions, but these assumptions, of course, are highly speculative.

 

Process Parameters

Ore Deposit: Waste/Ore Ratio
At conventional uranium mines, overburden and waste rock has to be removed to get access to the uranium ore. The waste-to-ore ratio can range between 1 and 5 for underground mines and between 1 and 60 for open pit mines.

Ore Deposit: Ore Grade [% U]
Weight-percent of uranium contained in the ore removed from the ore body for processing in the mill. Other units used are % U3O8, among others (see also Unit Conversion). Ore grades being processed at present cover a wide range of 0.026% U (Rössing, Namibia) to 1.1% U (Key Lake, Canada). New uranium mining projects under development even have ore grades of up to 12.7% U (McArthur River Project, Canada).

Mill: Extraction Losses [%]
Not all of the uranium contained in the ore can be recovered in the milling process. The extraction losses are depending on the grade of the ore processed. Upon entry of an Ore Grade value, the calculator presents an estimated value for the Mill Extraction Losses. If you want to use another value for the losses, you can overwrite it.

Conversion: Losses [%]
Production losses during the conversion process.

Enrichment: Product Assay [% U-235]
Weight-percent of the fissile isotope uranium-235 in the uranium contained in the product stream (enriched uranium hexafluoride) of the enrichment plant. Values for use in pressurized water reactors (PWR) range between 3.6% and 4.1%, and for use in boiling water reactors (BWR) between 3.0% and 3.2%. (Note: Natural uranium contains 0.711 wt-% of uranium-235)

Enrichment: Tails Assay [% U-235]
Weight-percent of the isotope uranium-235 in the uranium contained in the waste stream (depleted uranium hexafluoride) of the enrichment plant. Typical values range between 0.25% and 0.30%. The tails assay can be selected according to economic feasibilty.
> See graphs: Cost balance of uranium enrichment · Optimal tails assay
(Note: feed cost includes uranium price plus conversion cost)
> See also: Uranium Enrichment Cost Optimizer
> View Current Uranium Prices

Fuel Fabrication: Losses [%]
Production losses during the fuel fabrication process.

Power Plant: Fuel Burnup [GWd/t U]
Thermal energy produced in the nuclear power plant from 1 metric tonne of enriched uranium contained in the nuclear fuel. It ranges between 40 and 43.4 GWd/t U for pressurized water reators (PWR), and 33 and 40 GWd/t U for boiling water reactors (BWR). GWd stands for Gigawatt-days, 1 GWd = 24 million kilowatt-hours.

Power Plant: Efficiency [%]
Efficiency of converting thermal energy into net electricity, ranges between 32% and 34.5%.

 

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.

Air Dispersion Modeling

The calculator uses the sector-averaged Gaussian plume algorithm (for details, see NUREG-0706, or Parks1998). This algorithm is valid for flat land only.
This calculator only covers radon releases, not particle releases. The uranium mine, the mill and the tailings pile together are regarded as one point source with a stack height of zero. These simplifications produce some error in the near field, while they practically don't matter for cumulative calculations over larger areas - the main purpose of this calculator.

In the "Fast" mode, an analytical solution is used, which runs very fast, but cannot account for radon progeny ingrowth or plume depletion effects.
In the "Precise" mode, radon progeny ingrowth and plume depletion from precipitation scavenging and radon decay is taken into account. For this case, there exists no analytical solution and time-consuming numerical integration has to be performed. Precipitation is accounted for with a scavenging coefficient of 10-7 a/(cm s).
It is highly recommended to use the "Precise" mode for very short distances (such as 10 km) and very large distances (such as hundreds of kilometers) to be modeled.

 

Bibliography

 

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