Research in NDA Techniques for Waste Characterization at the JRC

INTRODUCTION

Under a new research programme for support to nuclear decommissioning, the Joint Research Centre will intensify research activities in an effort to complement the expected increase of decommissioning activities in Europe in the coming years. Under this heading various initiatives related to nuclear waste characterization will be undertaken.

At the Nuclear Security Unit of the Institute for Transuranium Elements (ITU), research in waste characterization has traditionally focused on NDA methods, as applied in nuclear safeguards, for characterization of alpha active waste. In the coming years this activity will expand to include also free release waste assay methods and instrumentation.

This paper will report on the status of the primary methods and instrumentation currently being investigated in our laboratory including the active and passive neutron interrogation of alpha active waste, as well as an upcoming international project for free release waste measurements. The Nuclear Security Unit has a strong training and education programme mainly in the areas of nuclear security and nuclear safeguards through the ITU EUSECTRA programme. Newly added to this portfolio is the Decommissioning Summer School which is organized jointly with institutions with a considerable radioprotection programme including the IAEA.

JRC Drum Monitor

The JRC Drum Monitor is a custom designed passive neutron counter for verification measurements of plutonium content in standard 210-litre conditioned and unconditioned low-level nuclear waste. The instrument is periodically deployed and operated by Euratom safeguards inspectors in European nuclear fuel cycle facilities for the purpose of verifying operator’s declarations

of Pu content of individual waste items. During 2013 a refurbishment of the mechanical and electrical components took place to achieve CE certification for the handling of 500 kg (cemented) waste drums.

The instrument is currently used for measuring simulated waste in the ITU laboratory while waiting for the next deployment request from Euratom safeguards. The JRC Drum Monitor is depicted in Fig 1a and Fig. 1b below.

In the JRC Drum Monitor, the detection system is composed of a total of 148 3He neutron proportional counters (4 bar pressure) embedded in high density polyethylene (HDPE) modules. The detectors surround the drum in all directions constituting a 4pi geometry. All detector modules are covered in 1mm cadmium on all surfaces (coloured red in Fig. 1b). This is to guarantee an exponential thermal neutron lifetime (die-away time) in the modules independent of energy of the neutrons escaping the drum. To benefit the detection efficiency the detector modules have been placed in close proximity to the drum (see Figure 1b). Note that no mechanical structures are needed in the measurement chamber as the drum (up to 1000 kg) is hanging from the external crane during measurements. This serves the purpose of maintaining a close coupling between drum and detection system, but also to avoid the influence of metal structures on the cosmic ray induced neutron background. Another feature in this respect is the substantial shield against external neutron sources composed of HDPE of about 250 mm thickness. 28 parallel analogue electronics chains composed of custom designed pre-amplifier/amplifier/discriminator circuits process the neutron detection events in the 3He detectors. The 28 digital signal outputs are connected to a digital mixer unit to produce a single signal pulse train for the signal analyzer (Canberra JSR-14 shift register, INCC neutron coincidence measurement software).

Fig. 1a. JRC Drum Monitor in the ITU lab following refurbishment for CE qualification.

Fig 1b. Schematic of the shielding and detection system of the JRC Drum Monitor.

The instrument applies neutron correlation

analysis, or also known as neutron multiplicity counting, for the absolute determination of plutonium mass in waste drums [1, 2]. In this method the quantities singles, doubles and triples are derived form measured frequency distributions of signals representing neutron detections in short observation intervals. Each of the measured quantities (singles, doubles and triples) can be expressed in a simple equation as function of sample and instrument parameters. When solving the three equations the spontaneous fission rate (proportional to the Pu mass) and two other sample parameters are determined. In the case of bare Pu samples or samples of well-defined geometry the detection efficiency can often be considered to be a known instrument parameter. These parameters are chosen among: the neutron detection

efficiency, the sample self-multiplication, and the (alpha, n) reaction rate. In the case of nuclear waste with an unknown source distribution and absorbing waste matrix, the efficiency must be considered unknown. Luckily the self-multiplication can be considered negligible due to the small sample mass. Thus the preferred analysis procedure for dense waste matrices is to solve the three equations to determine the Pu mass, the detection efficiency, and the (alpha, n) reaction rate.

The JRC Drum Monitor is currently undergoing performance tests in the ITU laboratory using simulated waste drums and small plutonium standards. Also the variations in signal background produced by cosmic ray interactions is being studied in detail as this is the main factor influencing the detection limits of the instrument [3].

Research in neutron interrogation of alpha active waste

The Nuclear Security Unit of ITU operates an experimental facility for research in NDA methods and instrumentation with potential applications in nuclear safeguards and nuclear security. The facility called the Pulsed Neutron Interrogation Test Assembly (PUNITA) incorporates a pulsed 14-MeV (D-T) neutron generator. A nuclear safeguards application currently being studied in PUNITA concerns the mass determination of small quantities of fissile material in nuclear waste. For this purpose bursts of thermal neutrons produced by the neutron generator and the large graphite liner induce fission in fissile isotopes of the sample. The fission neutrons are detected and analyzed in the time domain. The analysis method [4] consists of a further development of the standard passive neutron correlation technique.

PUNITA is a versatile experimental tool. The instrument is designed to give the experimenter maximum flexibility with respect to detector and sample arrangements while maintaining a relatively high interrogating pulsed neutron flux. In the closed configuration the instrument forms a cube with a central void called the sample cavity (Fig. 2). The top and bottom sides of the cube are part of the central structure, while the four vertical sides are located on mobile trolleys. The size of the sample cavity is 500 mm by 500 mm cross-section by 800 mm height. The accelerator assembly of the (D-T) generator has a length of only 430 mm and can be placed without constraints anywhere inside the cavity. A thick graphite liner is located on all six sides of the sample cavity. On the vertical sides, the graphite liner is integrated in the vertical doors providing full access to the cavity from all sides when in the open configuration. A total of 1350 kg of reactor grade graphite is used in the liner.

Fig. 2. The JRC Pulsed Neutron Interrogation Test Assembly (PUNITA).

A fast neutron detector module is integrated in each of the six sides immediately behind the graphite liner. The modules, nominated fission neutron counters, include sixteen 3He proportional detectors of 4 bar, 25.4 mm diameter, and 1000 mm length on each side. The single row 3He detectors are embedded in a block of polyethylene with a 1 mm cadmium cladding on all surfaces. The purpose of these neutron counters is to detect neutrons from fissions generated by the thermal neutron flux inside the cavity. The cadmium liner on the fast neutron detector modules prevents the thermalized source neutrons from reaching the detectors while the fast fission neutrons pass. The detection efficiency of fission neutrons is about 15%.

Fig. 3. Cross section of the PUNITA facility with principle components indicated.

A neutron shield of 300-350 mm polyethylene is placed behind the fission neutron counters on all six sides. Fig. 3 shows the positions of the permanently mounted neutron detectors.

The nuclear safeguards application of PUNITA concerns the determination of the mass of fissile material in a sample, e.g. a waste drum, independent of matrix materials and spatial source distribution. The method uses instrumentation similar to the well-known differential die-away technique although PUNITA applies more fission neutron detectors to achieve high detection efficiency and a large graphite liner to prolong thermal neutron lifetime for efficient interrogation. The analysis method is derived from the passive neutron correlation technique for assay of spontaneous fissile materials. Fission neutrons are detected in observation intervals during the thermal neutron interrogation. From frequency distributions of detected groups of neutrons, correlated single and double events can be derived which are used for calculating the induced fission rate together with other sample parameters [4]. The method, including a fast list-mode multi channel signal analyzer, is currently being tested at PUNITA. The method is particularly effective for small amounts of fissile material and is expected to be capable of assaying 235U and plutonium in the milligram range for bare samples and an order of magnitude larger when embedded in matrix materials.

Decommissioning Summer School at the JRC

The Summer School on Nuclear Decommissioning and Radioactive Waste Management is a joint initiative launched in 2009 by the University of Milano, the IAEA , the Italian Society for Radiation Protection (AIRP), and the Nuclear Decommissioning Unit of the JRC [5]. In 2013 the Summer School became part of the European Nuclear Safety and Security School (EN3S) managed by the Institute for Transuranium Elements (ITU). For this reason the Nuclear Security Unit of ITU this year co-organized the 5th event of the Summer School.

The Summer School is aimed first of all at students and professionals from nuclear industry but also at internal JRC professionals, who operate in the field of decommissioning and waste management (D&WM). Since 2011 the venue of the Summer School has been the Ispra site of the JRC which is currently undertaking a substantial decommissioning programme of old nuclear research installations. During the Summer School site visits and hands-on training and demonstrations take place in waste management facilities and research laboratories on the site. This allows a unique opportunity for students and professionals to become familiar, and gain

experience, with the field by receiving both theoretical lectures and direct practical instruction. This is complemented by lectures presented by experts from European industry and academic institutions. These topics include waste characterisation, clearance procedures, decontamination, dismantling, waste storage, etc.

The Summer School events in 2012 and 2013 focused on operational issues in radioactive waste management and nuclear decommissioning. The 5-day school focused on the following six specific topics:

1. Involvement with society and stakeholders

2. Radiological characterization and facility release; regulatory issues

3. Hands-on visits to JRC waste management facilities and research laboratories

4. Radiation protection

5. Operational decommissioning experience in Europe

6. Waste management

In 2014 and beyond, the Summer School is likely to expand further towards training of students to bring the Summer School in alignment with standard academic education. One possibility being considered is to extend the event with an additional 3-day session for students with specific training in radiation detection and instrumentation, and to include case studies to let students apply the newly gained knowledge directly on realistic and practical decommissioning scenarios.

Collaborative research project on characterization of free release waste

In response to the EMRP 2013 call for the European Metrology Research Programme for projects by the European Association of National Metrology Institutes (EURAMET), a consortium of 15 European research institutions submitted a joint research proposal for funding called Metrology for decommissioning nuclear facilities - MetroDecom. In December 2013 the three-year project was accepted and is planned to launch in September 2014. Two scientific institutes from the JRC, IRMM and ITU, are participating to MetroDecom.

The joint research proposal, MetroDecom, is intended to focus on traceable on-site measurement methods of decommissioning nuclear sites and subsequent waste management. The specific objectives was defined as:

1. To develop methods for the radionuclide characterization of different types of solid materials present on decommissioning sites. This should include development of novel measurement techniques that improve the mapping of activation and contamination inside

nuclear facilities, determination of statistically valid sampling methods for representative samples for radiochemical analysis, and development of automated radiochemical analysis procedures.

2. To design a measurement facility for segregation of waste into streams (repository or possible free release), based on high sensitivity detectors (e.g. scintillators) and develop measurement and calibration procedures for the operation of this facility.

3. To develop and implement free release measurement technologies and develop tools and techniques for scanning of wastes with heterogeneous density distribution.

4. To develop measurements in radioactive waste repositories and improve monitoring methods during decommissioning.

5. To develop reference materials and standard sources for calibration, validation and testing of devices, instruments and procedures developed in the above objectives.

The contribution of the Nuclear Security Unit will concentrate on the acceptance measurements of nuclear waste destined for free release. The specific goals of this work includes:

− to develop new tools and techniques for scanning of wastes with heterogeneous density distribution prepared for free release measurement to obtain information about absorption of gamma radiation in measured material and create a method for continuous self-absorption correction

− to develop passive neutron counting and perform study of achievable detection limits of alpha containing waste under field conditions.

Conclusions

This paper has given a brief overview of four research projects concerned with nuclear waste characterization which are ongoing or planned to launch in the near future.

The Nuclear Security Unit of ITU is destined to increase the research activities in nuclear waste characterization in the coming years. A key factor is this development is the JRC research programme “Support to Decommissioning” which will launch in early 2014 on initiative of the Director General of the JRC. Some projects will be a continuation of projects already undertaken under the nuclear safeguards and security programme, while others will be in new areas and mainly on collaboration with external institutions.

REFERENCES

1. D. M. Cifarelli and W. Hage, “Models for a Three Parameter Analysis of Neutron Signal Correlation Measurements for Fissile Material Assay,” Nucl. Instrum. Methods, A 251, 550, 1986.

2. N. Ensslin, W. C. Harker, M. S. Crick, D. G. Langner, M. M. Pickrell and J. E. Stewart, "Application Guide to Neutron Multiplicity Counting", Los Alamos National Laboratory, LA-13422-M Manual, 1998.

3. M. G. Paff, B. Pedersen, J.-M. Crochemore, V. Mayorov, M. Mosconi, E. Roesgen, E. Pirovano, V. Canadell Bofarull, "Characterization of the Cosmic Ray Induced Neutron Multiplicity Background of a He-3 Passive Drum Counter for Plutonium Waste Verification", Proceedings of the Institute of Nuclear Materials Management 54th Annual Meeting in Palm Desert, CA, July 2013.

4. B. Pedersen, W. Hage, A. Favalli, G. Varasano, “Assay of small fissile masses in waste by the active neutron correlation technique”, Proc. of Symposium on International Safeguards: Addressing Verification Challenges, IAEA, October 2006, Vienna, Austria.

5. D. Giuffrida et. al., “The Decommissioning and Waste Management Summer School” at the Joint Research Centre in Ispra Italy”, Conference Proceedings of Nuclear Education and Training NESTET 2013, 17-21 November 2013,Madrid, Spain

Contact: Bent Pedersen

EC - Joint Research Centre Institute for Transuranium Elements Nuclear Security Unit Via E. Fermi 2749 – Ispra (VA) Italy bent.pedersen@jrc.ec.europa.eu