Alpha-Neutron Reaction Calculator - HELP

(last updated 23 Nov 2009)

Contents:

• Introduction · Material Input · Output Parameters · Calculation Details · Bibliography

Introduction

The calculator assumes a homogeneous solid compound of uranium and considers neutron production within this compound from the following sources:

• Alpha-Neutron (a,n)-reactions: The alpha radiation emitted from the decay of the uranium (and its decay products) can initiate nuclear reactions in some of the lighter elements of the compound, transforming certain nuclides to others, while emitting a neutron. Such reactions are rather frequent in uranium hexafluoride (UF₆). The reactions covered by this calculator are:

  Table 1
  

  Target Nuclide	Residual Nuclide
  	intermediate	stable
  N-14	F-17 (beta +)	O-17
  N-15	F-18 (beta +)	O-18
  O-17		Ne-20
  O-18		Ne-21
  F-19	Na-22 (beta +)	Ne-22

  > View animation of alpha-neutron reaction with U-238 and F-19 .

• Spontaneous Fission (SF): Some heavy isotopes (in particular U-238) undergo spontaneous fission at a rather low frequency. Each fission produces approx. 2 neutrons.

The calculator determines the neutron production rate from these two sources within the given compound. (see also Calculation Details)

The calculator considers the uranium-isotopes U-238, U-235, and U-234, and all their decay products. In case of uranium recycled from spent fuel, also U-237, U-236, U-233, and U-232, and their decay products are considered.
For an assessment of future exposure situations (waste management!), time delays can be selected, for taking decay and ingrowth of nuclides into account.

The parameters used for the calculation can be set in the Material Input and Output Parameters tables.

The "Query nuclide database" button allows to view the data stored for all elements and nuclides used by the calculator.
The "Show material data" button shows the data stored for the material type selected in the Material Input table.

The calculation results are presented in the Result window. Its contents can be copied and pasted to other applications.

The calculator is suitable for offline use.

Material Input

Input

enter number (mandatory!) and select appropriate unit.

• If the checkbox "U in/as" is checked in combination with a mass unit, then the input means the uranium contents of the material selected, otherwise it means the mass of the material selected.
  
• If the unit "Bq U-238 in" is checked, the input mass is determined from the uranium-238 activity contained. However, the neutron rate is calculated for the complete nuclide mix contained in the material type selected.
  
• If the unit "Bq U_nat in" is checked, the input mass is determined from the combined activities of the natural uranium isotopes U-238, U-234, and U-235. However, the neutron rate is calculated for the complete nuclide mix contained in the material type selected.
  

Type

select appropriate type
The calculator assumes that all compounds are in solid form. For elements other than uranium, natural isotope abundance is assumed.

Parameters: natural uranium

uranium obtained from natural sources. The U-238/U-234 and U-235 decay series are considered.

in equilibrium with progeny (used for natural uranium)

uranium of natural isotope composition, in secular equilibrium with its decay products (U-238/U-234 and U-235 series)

pure (used for natural uranium)

pure uranium of natural isotope composition, without decay products. (0.711 weight-% U-235; U-234 in equilibrium with U-238).

enriched to ... wt% U-235 (used for natural uranium)

pure uranium with U-235 contents enriched to level higher than natural. Values for use in pressurized water reactors (PWR) range between 3.6% and 4.1 wt-% (weight-percent), and for use in boiling water reactors (BWR) between 3.0% and 3.2%.

depleted to ... wt% U-235 (used for natural uranium)

Pure uranium with U-235 contents depleted to lower than natural level. Typical values range between 0.2% and 0.3%. This tails assay can be selected according to economic feasibilty. Since the concentration of U-234 in depleted uranium is depending on the product assay obtained in the enrichment process, an input is also required for this figure.

Parameters: recycled uranium

uranium obtained from recycling of spent fuel. The U-237/U-233, U-236, and U-232 decay series are considered in addition to the U-238/U-234 and U-235 series. Fission products and transuranics are not considered.
The composition of the uranium isotopes is determined according to the selection from the "burnup / initial enrichment" pick list, based on [Neghabian1991]. The burnup unit GWd/tHM stands for Giga-Watt-days per metric tonne heavy metal. A 5 year storage time after reactor unload is assumed.

as is (used for recycled uranium)

uranium as is recycled from spent fuel

re-enriched to initial U-235 equivalent (used for recycled uranium)

recycled uranium re-enriched to initial U-235 equivalent; the actual U-235 concentration is higher to compensate for the presence of U-236.

depleted to 0.2 wt% U-235 (used for recycled uranium)

recycled uranium depleted to 0.2 wt% U-235

Output Parameters

Delay

Delay in years, after which the neutron rate is calculated.
The radiation from uranium and its decay products is not constant over time, although the half-lives of the natural uranium nuclides are extremely long (see uranium radiation properties). In cases, where disequilibrium obtains between uranium and its decay products, considerable increases of the radiation levels can occur over time, due to the ingrowth of decay products. This is of particular concern for the long-term management of uranium-bearing material as a waste, and for uranium recycled from spent fuel.
For Delay = "none" the isotopic composition as given by the parameters entered in the Material Input section is used for the calculation. However, this is not representative for most exposure situations, since short-lived decay products are growing in within a few months. So, a delay of "1 year" will best fit most actual exposure situations for natural uranium; "10 years" is of interest for situations involving recycled uranium, since there is a peak from the ingrowth of U-232 decay products.
For the management of radioactive waste dumps, longer design lives are of interest. For other delays, the time can be entered in the "other" field.
More than one delay can be checked to perform multiple calculations in a single run; this affects calculation time, however.