S. NIEMEYER, I. HUTCHEON
Lawrence Livermore National Laboratory, Livermore, California,
United States of America Presented by N.G. Wimer
A sample of high enriched uranium (HEU) was seized in Rousse, Bulgaria, on 29 May 1999, at a border crossing into Romania. Search of a suspect’s vehicle uncovered a lead canister in the trunk. The initial examination of the contents by Bulgarian scientists indicated that the sample was, indeed, HEU, and arrange- ments were made for a US team of nuclear forensic scientists from several national laboratories to conduct a thorough examination. This report provides a summary of the results.
The HEU sample was contained in a glass ampoule embedded in yellow wax that filled a cylindrical lead container. A broad set of techniques was used to examine both the nuclear and the non-nuclear materials. Our general experi- mental approach has been previously described at meetings of the Nuclear Smuggling International Technical Working Group (ITWG), but this case repre- sents the application of the most diverse set of nuclear forensic measurements for an actual seized sample.
Analysis of the HEU itself included particle characterization, stoichiometry, impurity elements, residual nuclides, age dating, and uranium and plutonium isotopics. Measurements by XRD, SEM and TEM showed that the sample was mostly U3O8, with minor amounts of two other phases.The powder was extremely fine grained (160 nm mean) and quite uniform in size. Most grains (95%) are equidimensional, with the remainder rod or plate shaped.The uranium was 72.7% 235U with a high 236U abundance of 12.1%. The sample was reprocessed, reactor irradiated material.The original uranium enrichment was 90% and the irradiation burned up about 50% of the initial 235U. Plutonium was present at a very low level (3 ppb); the 239Pu abundance was 82% with 240/239 = 0.12. Three fission products were detected at low levels, giving unambiguous evidence of fuel recycling. The total impurity content was about 600 ppmw (mostly S, Cl, Fe and Br), which we interpreted as indicating a batch processing operation because the impurities were too high for a laboratory scale operation.
We determined the age of the chemical reprocessing by using seven nuclide systems.The multiple clocks enabled us to evaluate the validity of the assumptions
in the dating schemes. Slightly higher ages for Am and Ac chronometers are consistent with a Purex process. The mean age of the remaining clocks is 30 October 1993, with an uncertainty of only 25 days. The similar age for the Pu/Am clock suggests that the plutonium was not from post-processing contamination.
The non-nuclear materials provide a number of forensic clues. The yellow waxy liner material was an uncommon type of paraffin wax that had unusual organic compounds (naphthalene derivatives). The yellow colourant was barium chromate, which is rare in Europe and the United States but common in Brazil, China, India and Eastern Europe. The ampoule was common borosilicate glass. We noted that ampoules have been used as containers for archiving nuclear samples, but a glass ampoule had not been previously reported as a container for smuggled nuclear material. Two pieces of paper were analysed. One was attached to the exterior of the lead container and the other was wrapped around the ampoule. Both contained softwood and hardwood species, and the fibres were produced by the Kraft pulping process. The species of wood were not from North America or Western Europe, but were consistent with Eastern Europe. Finally, the lead container was hardened with 5 wt% antimony. The lead isotope composition excluded US ores, but was consistent with lead from several European and Asian mines. Taken altogether, the non-nuclear evidence was most consistent with assembly of the package in Eastern Europe from local sources.
Nuclear isotopics can be used to constrain the type of reactor in which the HEU was irradiated. Detailed reactor calculations, using WIMS and GLASS codes for a pin fuel cell geometry, pointed to a light-water reactor with a very thermal spectrum. We calculated that a burn-up exposure of more than 300 000 MW·d/MT was required to match the uranium isotopics. However, we found it quite difficult to simultaneously match the uranium and plutonium isotopics using these codes and the assumed geometry. Our preferred interpreta- tion is that it was a research reactor.
Although we are not able to pinpoint the type of reactor, we emphasize that we identified a number of distinguishing characteristics of this HEU sample that would enable its linkage to other samples from the same reactor or reprocessing environment. The very low abundances of the minor uranium isotopes,232U and 233U, coupled with the high abundance of 236U, are one such feature. Coupling the plutonium isotopics to the uranium composition provides an especially distinctive signature. The particle size distribution was also highly unusual for a reactor fuel. The accuracy with which we determined the time the material was last chemically processed also offers a potential pathway to attribution. If nuclear regulatory agencies maintain chronological records of activities at known reprocessing facil- ities, the physical and chemical characteristics discussed here may be sufficient either to identify a specific facility or, at least, to eliminate a large number of potential reprocessing facilities.
(Session 3)
Chairperson I.L.F. RAY
Institute for Transuranium Elements, European Commission
Co-Chairperson R.C. HANLEN