Passive neutron albedo reactivity measurements of spent nuclear fuel

The upcoming disposal of spent nuclear fuel in Finland creates new challenges for nuclear safeguards. Part of the national safeguards concept for geological repositories, developed by STUK — Radiation and Nuclear Safety Authority, is non-destructive assay (NDA) verification of all fuel items before disposal. The proposed verification system is a combination of PGET (Passive Gamma Emission Tomography), PNAR (Passive Neutron Albedo Reactivity) and weight measuring NDA-instruments. PGET takes a pin-level image of the fission products inside of a fuel assembly and PNAR verifies the multiplication of the assembly, a quantity that correlates with the fissile content. PGET is approved by IAEA (International Atomic Energy Agency) for safeguards measurements, but the feasibility of PNAR has not yet been established. A first of its kind PNAR prototype instrument was built in a collaboration coordinated by STUK. This paper concludes the results of the first measurements of spent BWR (Boiling Water Reactor) nuclear fuel with the prototype in July 2019. Based on the measurements, the ability of the PNAR instrument to detect the presence of fissile material in a repeatable manner in a reasonable amount of time was demonstrated. Furthermore, the instrument was able to detect differences in multiplication between partially and fully spent fuel assemblies, and axial differences in multiplication within a single assembly.Peer reviewe

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Passive Neutron Albedo Reactivity (PNAR) Prototype for Spent Nuclear Fuel Verification

The Finnish safeguards concept for encapsulation plant and geological repository includes verification of all fuel items before encapsulation and disposal. The verification process is carried out according to IAEA Group of Experts ASTOR (Application of Safeguards TO Repositories) recommendations. One of the non-destructive assay (NDA) instruments featured in the concept is Passive Neutron Albedo Reactivity (PNAR). The PNAR device will be used to verify the presence of fissile material in spent nuclear fuel assemblies. The PNAR instrument makes two relative neutron flux measurements. One in a neutron albedo maximizing an one in a neutron albedo minimizing configuration. From the ratio of the two measurements, a measure of the presence of fissile material can be calculated. The albedo minimizing configuration is achieved by surrounding the fuel assembly with cadmium. A prototype of the PNAR device is being built by STUK. The prototype is designed for underwater measurements of BWR fuel used in Olkiluoto nuclear power plants. This article presents the final design of the prototype device and presents the upcoming measurement plans with the detector. A mock-up PNAR detector pod was assembled and tested under laboratory conditions to quantify the detector signals and find potential sources of errors in the design

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"HE 180/2004 vp laiksi oikeusapulain muuttamisesta

Spent BWR fuel characterisation combining a fork detector with gamma spectrometry:Report on Task JNT A 1071 FIN of the Finnish Support Programme to IAEA Safeguards

The LWR spent fuel assemblies have to be verified at the partial defect level before they become difficult to access. According to the IAEA's criteria the partial defect test for spent fuel should be able to detect if half or more of the fuel pins have been removed from an assembly and possibly replaced by dummies. Euratom applies similar criteria. Therefore a standard verification procedure needs to be developed using an appropriate combination of measurements and theoretical calculations. Two experiments with an "upgraded" fork detector were performed at the TVO KPA Store in September and in December 1999. On the whole, 26 assemblies were measured. In the "upgraded" fork detector the total neutron count and the gross gamma measurements are complemented with gamma spectroscopic measurement using an integrated measurement head. A cadmium-zinc-telluride (CZT) detector is placed on the same vertical level as the fission and ionisation chambers. This enables simultaneous gamma and neutron measurements at one location. In the upgraded fork model the fork prongs can also be removed and gamma spectrometric measurements can be done using only the CZT detector. This allows more versatile placement of the target fuel assembly allowing various kind of gamma spectroscopic scanning measurements. In this report a gamma spectroscopy based correction to the gross gamma data is introduced. This corrected gross gamma signal seems to describe more consistently the burnup of the assembly than the 137Cs intensity obtained by direct gamma spectrometry. Concerning the measured neutron data of assemblies with different enrichments, an enrichment correction method based on calculations made with the ORIGEN-S program is introduced in this report. In addition, the share of 244Cm neutrons of the total neutron source is derived from the results calculated with the PYVO program. These corrections to the neutron signal seem to improve the correlation of the neutron signal to the burnup and to the gross gamma signal. The PYVO program can be considered as an essential tool in the analysis. With help of the PYVO the 244Cm share of total neutron counts, the 244Cm neutron source term and the 137Cs activity of measured assemblies can be calculated. In addition, the axial activity profiles of one assembly are compared with the calculations made by ORIGEN-S. These comparisons show a remarkable agreement between the measured and calculated results

Spent BWR fuel characterisation combining a fork detector with gamma spectrometry:Report on Task JNT A 1071 FIN of the Finnish Support Programme to IAEA Safeguards

The LWR spent fuel assemblies have to be verified at the partial defect level before they become difficult to access. According to the IAEA's criteria the partial defect test for spent fuel should be able to detect if half or more of the fuel pins have been removed from an assembly and possibly replaced by dummies. Euratom applies similar criteria. Therefore a standard verification procedure needs to be developed using an appropriate combination of measurements and theoretical calculations. Two experiments with an "upgraded" fork detector were performed at the TVO KPA Store in September and in December 1999. On the whole, 26 assemblies were measured. In the "upgraded" fork detector the total neutron count and the gross gamma measurements are complemented with gamma spectroscopic measurement using an integrated measurement head. A cadmium-zinc-telluride (CZT) detector is placed on the same vertical level as the fission and ionisation chambers. This enables simultaneous gamma and neutron measurements at one location. In the upgraded fork model the fork prongs can also be removed and gamma spectrometric measurements can be done using only the CZT detector. This allows more versatile placement of the target fuel assembly allowing various kind of gamma spectroscopic scanning measurements. In this report a gamma spectroscopy based correction to the gross gamma data is introduced. This corrected gross gamma signal seems to describe more consistently the burnup of the assembly than the 137Cs intensity obtained by direct gamma spectrometry. Concerning the measured neutron data of assemblies with different enrichments, an enrichment correction method based on calculations made with the ORIGEN-S program is introduced in this report. In addition, the share of 244Cm neutrons of the total neutron source is derived from the results calculated with the PYVO program. These corrections to the neutron signal seem to improve the correlation of the neutron signal to the burnup and to the gross gamma signal. The PYVO program can be considered as an essential tool in the analysis. With help of the PYVO the 244Cm share of total neutron counts, the 244Cm neutron source term and the 137Cs activity of measured assemblies can be calculated. In addition, the axial activity profiles of one assembly are compared with the calculations made by ORIGEN-S. These comparisons show a remarkable agreement between the measured and calculated results

Sleep in psychotic disorders: Results from nationwide SUPER Finland study

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