Characterisation – Fissile Material Assessment

Plant Characterisation aims to deliver robust information on the radiological, chemical and physical status of a facility, materials and wastes from all areas of the Sellafield site in order to inform decisions.

Innovative characterisation tools and techniques will seek to complement the existing ‘baseline methodologies’ so that teams are better placed to perform characterisation activities and collect qualitative and quantitative data.


The AFNA approach is based on the use of a long-lived isotopic neutron source with which to interrogate nuclear materials combined with organic scintillators and our novel pulse-shape discrimination (PSD) technology.

This method provides several advantages over standard methods such as passive neutron coincidence counting (PNCC) with thermal neutron counters (such as 3He or boron trifluoride detectors) in that it has the capability to measure fissile nuclides on the basis of fast neutrons directly, enables a reduced count time and has the capability to perform accurate measurements on difficult-to-measure waste forms such as plutonium fluoride and aged materials.


The status of the technology:

  • Neutron assay can be configured in one of two ways: the neutrons are either evolved naturally from a material under scrutiny without a stimulus, or they are stimulated by thermal neutrons; photons can also be used to provoke the emission.
  • The former, passive assessment is relevant to the assay of plutonium via spontaneous fission of 240Pu and neighbouring even-numbered isotopes.  Although, only constituting a relatively small proportion of the total elemental plutonium content (say 6% wt.) the passive rate of emission is often sufficient for assay.
  • The latter approach, that is stimulated emission, is used for the assessment of uranium, typically via fission (and therefore neutron emission) induced with neutrons from an americium-lithium source. As with plutonium, it is the minor isotope 235U that fissions, < 3% wt., but there is usually sufficient for an assay to be made, especially given that it is the assessment of 235U enrichment that is usually of interest.
  • A variety of assessments have been reported with relatively large numbers of detectors, both thermal devices (BF3 and 3He) and, less widely, fast neutron systems comprising organic scintillators, as used here.
  • The distinction of the current approach is that a small number of detectors have been used; the events detected with them are processed in real-time and simultaneously to yield pseudo-instantaneous assessment readings.


The challenge addressed was the assay of fissile nuclides with particular focus on difficult-to-measure waste forms i.e. high (alpha,n) emitters such as plutonium fluoride and aged materials to provide an effective method for accurate measurement of Sellafield’s fissile material contaminated orphan wastes.

Ideally the method would have a reduced count time compared to traditional methods (greatly improved precision) and the assay system would have a compact/rugged geometry. Technical challenges identified were:

  • To apply an ultrafast method of recording the neutron radiation emissions from fissile materials before contaminant sources perturb assessments.
  • To minimise the perturbation that the detectors exert on the radiation field that is evolved by the material.
  • To confine as many of the neutrons within the measurement environment as possible.
  • To configure such an environment to render the measurement system portable.


The fundamental novelty is that our approach to neutron coincidence counting operates on the nanosecond timescale, whereas prior methods are 1000 times slower.  This is because prior methods rely on the detection of neutrons after thermalization whereas our approach is based on the digitisation of the fast neutrons as they are emitted in fission.

This enables the separation of contaminant sources of radiation that are not associated with fissile material, improved accuracy and the potential to extend safeguards assay into application areas where it is not currently possible, such as in environments where there are contaminant sources of radioactivity or for mixtures of nuclear materials.

Trials of an AFNA prototype in novel configurations with a wide variety of fissile standards were performed previously under a collaborative R&D project between Pajarito Scientific Corp. and Los Alamos National Laboratory (LANL) in 2014.

This demonstrated that the hardware can be arranged into a compact system which requires no polyethylene moderator covering the detectors. With significantly reduced shielding, a complete portable / rugged system can be manufactured which would greatly enhance operational field capabilities for decommissioning and decontamination applications.

The objective of this study was to undertake modelling and benchmarking of the AFNA concept (to include comparison and summary of the 2014 results) with respect to the application areas discussed in the Sellafield Game Changers Business Case (GC-045).

The study would evaluate the performance capabilities for target waste streams and evaluate optimum design characteristics.


The target market is any company or organisation with the responsibility to manage nuclear materials i.e. uranium and plutonium, and facilities that have dealt with these materials.  For example: Sellafield Sites Ltd., Cavendish, AWE, IAEA, DSRL, NNL, Los Alamos National Laboratory, Oak Ridge National Laboratory.

  • Market size: Estimated £5M
  • Territories: UK, US, Europe.
  • Proportion attainable: ~20%


The background IP has been gathered and filed in key markets.

Modelling of the concept in its present form has been completed so that the expected performance is established.

A UK based prototype has been constructed and measurement performance validated at a reference site (NPL)

Contact has been made with key end-users and specific requirements discussed. Facilities within SL which might be accessed were identified.

This has led to thinking around an alternative form of prototype, capable of handling 200 l drums, to be explored to produce a Route Map for a further Development Phase

Advances made – ‘deltas’

  • No interference from ‘accidental’ neutrons
  • Access to higher orders of multiplicity – triples and quads were measured
  • Both temporal and spectroscopic information available
  • Less hazardous from criticality consideration
  • Very little moderator material needed
  • Various embodiments (and number of detectors) possible
  • Hence ease of deployment and possible field use (portable/transportable)
  • Potentially cheaper than He3 tubes


In the first instance, please contact the Game Changers team for any enquiries or requests for further information regarding this technology and the challenge it addresses:

Paul Knight, NNL / 07900 906244

Frank Allison, FIS360 / 07714 980164


Dr Frank Cave

Hybrid Instruments Limited
The Gordon Manley Building
Lancaster Environment Centre
Lancaster University LA1 4YW

Telephone: +44 (0)1524 595048




Hybrid Active Fast Neutron Analysis

Figure 1. AFNA unit is shown open such that three of the four organic scintillation detectors are visible with cabling etc.

Hybrid Active Fast Neutron Analysis

Figure 2. AFNA unit is shown closed as would be the case when the system is in use.

For both images, the system is shown under test at the National Physical Laboratory, Teddington.