Inspection innovation

The Joint Research Centre (JRC) at Geel in Belgium recently published a technical report that will help to validate the innovative Cristallini method for sampling UF6 at nuclear sites undergoing safeguards inspections.

Safeguards inspections aim to ensure the peaceful use of nuclear material and are part of the non-proliferation of nuclear weapons system. Conversion of uranium ore to UF6 is a preliminary stage in uranium enrichment.

International Atomic Energy Agency (IAEA) safeguards deter the spread of nuclear weapons by early detection of the misuse of nuclear material or technology. This aims to give credible assurances that countries are honouring their legal obligations to ensure nuclear material is being used only for peaceful purposes.

Safeguards consist of a set of technical measures applied by IAEA to nuclear material and activities. The measures are used to verify independently that nuclear facilities are not misused and nuclear material is not diverted from a peaceful use. States accept these measures through safeguards agreements.

IAEA safeguards are an important part of the international security system, in which the Treaty on the Non-Proliferation of Nuclear Weapons is a component of worldwide efforts to prevent the spread of nuclear weapons. Under Article 3 of the treaty, each non-nuclear weapon state has to have a safeguards agreement with the IAEA.

The number of nuclear facilities that require IAEA inspection is increasing. Since 2011, seven new safeguards agreements and 23 new additional protocols have entered into force. The amount of nuclear material under safeguards has increased by 17% and the number of nuclear facilities under safeguards has risen by 5%.

Work is also increasing as more facilities are decommissioned, because inspectors have to verify nuclear materials packaging, movement and disposition.

These increased demands make it more important than ever that IAEA improves its inspection techniques and keeps up with scientific innovation.

In 2015, safeguards were applied in over 180 states: more than 170 of these have comprehensive safeguards agreements; five have voluntary offer agreements; and three have item-specific safeguards agreements. At the end of 2015, additional protocols were in force in over 120 states.

Some 200,110 “significant quantities” of nuclear material were under IAEA safeguards. A “significant quantity” is the amount of nuclear material that needed to make a nuclear bomb.

Increased demands

Scientists at laboratories in Europe and the Americas worked on a rigorous validation programme for the Cristallini UF6 sampling method, which was originally developed by the Brazilian Argentine Agency for Accounting and Control of Nuclear Materials (ABACC) several years ago.

At enrichment plants in Brazil that use centrifuges, routine and unannounced inspections are carried out and UF6 samples taken from process lines and cylinders to verify that the uranium enrichment conforms with design and operator declarations.

The method samples UF6 by adsorption and hydrolysis in alumina pellets inside a fluorothene P-10 tube. It does not require liquid nitrogen. The alumina pellets retain up to few hundreds milligrammes of uranium as a solid uranyl fluoride (UO2F2).

According to the JRC technical report, the new method has several advantages compared with the current sampling technique. Currently UF6 is transferred into a stainless steel tube and transported in the gas phase, with hydrolysis and isotopic analysis being carried out after transport to a laboratory.

In the Cristallini method, the sampling device is cheaper. The UF6 sample content kept at the sampling site is smaller and less reactive, and so is the residual uranium kept at the analytical lab. Since the sample form is solid, non-volatile and chemically less reactive, transport is cheaper and safer in terms of radiological protection. For the IAEA, these advantages could be a significant logistical improvement.

The method was developed at the CNEA lab in Argentina and the results were confirmed by an enrichment facility in
Brazil. If the method is to be used globally, wider validation is required, so a network of scientists from several countries collaborated in 2016 to validate the method.

Labs in Argentina and Brazil, as well as JRC in Germany and Belgium collaborated on the validation work with IAEA safeguards experts in Austria, together with Oak Ridge National Lab and the NBL Program office, USA, and the American Society for Testing and Materials (ASTM).

This involved taking isotope ratio measurements of four reference uranium materials sampled as UF6, using the two different sampling methods: the Cristallini adsorption on alumina method and the standard direct hydrolysis method. The measurements aimed to determine whether using the Cristallini method had any effect on the isotopic composition of the UF6.

For each of the four reference materials, every lab had two subsamples obtained from direct hydrolysis and two taken using the Cristallini method, so each lab analysed a total of 16 subsamples contained in P-10 tubes. The P-10 tubes were randomly assigned to the different labs to minimise any sampling bias.

The isotopic analysis was carried out using the “double-spike” technique by thermal ionisation mass spectrometry because it is much more precise than other methods, such as total evaporation or modified total evaporation.

According to the report, the “double spike” technique is already being used widely for scientific applications because it is more precise. It is a powerful tool for investigating ongoing conversion processes in nuclear facilities and labs, by comparing U-235 to U-238 ratios before and after sample processing.

Results

Results obtained at IAEA Safeguards Analytical Services (SGAS) and JRC were in good mutual agreement, but the Cristallini method showed slight differences for the U-235/U-238 isotope ratios, compared to samples processed by the direct hydrolysis sampling method.

For test samples prepared by ABACC using some certified UF6 reference materials, significant differences of about 0.01% to 0.02% were observed. For other test samples the differences were below 0.005% and insignificant.

The difference in the U-235 enrichment had already been indicated by modified total evaporation measurements performed only at SGAS, although the differences were not significant. “The reason for the observed differences between the sampling methods is not yet known, they can be due to mass fractionation, contamination, memory or other still unknown effects occurred during the sampling or subsequent chemical processing,” according to the report.

However, the report noted that results from JRC and SGAS using the double spike method are very precise. It said: “The observed differences should not prevent the Cristallini sampling method from being implemented and further-on used for its intended purpose, as a new sampling method for nuclear safeguards measurements, in most cases mainly for confirming the declared enrichment of uranium materials within the nuclear fuel cycle.”

The slight differences observed at JRC and SGAS should not be neglected, the report’s authors noted, but included for the intended standardisation of the Cristallini method with ASTM. The benefits of the sampling method are “significant”, including a reduction of the sample amounts to be transported and to be kept at the facilities, and an improvement of the safety and cost of nuclear transports.

According to ABACC, where the sampling was performed, a different sampling manifold was used for each reference material with a different U-235 enrichment in order to eliminate the risk of cross-contamination between materials flowing into the same sampling system. The results from other participating laboratories have to be carefully evaluated, because the measurements might have been performed using different instrumentation and methods, and on different sample sets obtained from ABACC.

The report said the “double spike” and modified total evaporation measurements performed at SGAS and JRC were all performed on the same sample set and show very good mutual agreement. In terms of slight deviations in sample sets, the report noted:

“It would be interesting to observe if similar slight deviations are obtained for other sample sets analysed at other laboratories.”

JRC scientists at Geel are offering support to all participating labs to help with the research work and implementation of the “double spike” method.

The report concluded: “The results from JRC Geel and SGAS using the double spike method play a special role within the validation programme due to the high precision of this method.

“The results are proposed to be included for the intended standardisation of the Cristallini sampling method through ASTM, in particular for defining an additional uncertainty component to account for the sampling process and the subsequent sample preparation, which would have to be attributed in the future to all isotopic measurements on samples taken by the Cristallini sampling method.”