WP 5: Nuclear decay data

Background

New generation nuclear reactors are being developed with the aim of providing safer and more efficient energy production. Closed and open fuel cycles are being studied based on uranium and thorium. Accurate decay data for actinide nuclides and their decay chains are important in the nuclear fuel cycles of both thermal and fast reactors. These data have also found increasing application in many other power-related fields such as fuel manufacture and reprocessing, waste storage and management, nuclear facility design, safety assessments and safeguards/proliferation issues.


State of the art

An IAEA Coordinated Research Project (CRP) was initiated to improve the actinide decay data library of the International Atomic Energy agency [1]. The heavier actinides (and their decay products) have generated interest in recent years because of their importance in the foreseen development and adoption of nuclear power plants in which the fuel will contain consistent and controlled amounts of these nuclides. Reliable and accurate decay data and neutron-induced cross sections are required for the implementation of sound operational and high burn-up strategies. The efforts to provide such data are ongoing.

A list of specific needs for improved actinide decay data have been outlined in a recent review at IAEA [2]. Additional inadequacies in actinide decay data can be identified. The actinide decay data of importance include half-lives, α-particle and γ-ray energies and emission probabilities. Their definition to good accuracy provides the means of monitoring the presence and transport of these actinides in nuclear facilities, as well as assisting in the detection of any clandestine activities [1].

238U disintegrates by alpha emission to two excited levels and to the ground state of 234Th. Only one direct measurement of alpha-particle emission probabilities of 238U has been reported since 1961, i.e. in 2000 by García-Toraño [7]. The measurements were done on sources prepared from natural uranium with activities between 2 Bq and 4 Bq. The energy resolution obtained was about 20 keV. The currently recommended values [8] for the emission probabilities for the two major peaks carry an absolute uncertainty of 0.5 % (which corresponds to a relative uncertainty of more than 2.2 % on the second peak), yet the absolute difference with the values in the 8th edition by Firestone amounts to 2 % [9].

This JRP will produce sources of highly enriched 238U in order to minimise the influence of other alpha emitting isotopes. The material will also be chemically separated from its decay products. Sources will be prepared by electrodeposition and the conditions will be optimised (i.e. thinner, less active sources) such that the energy resolution will be significantly better than the 20 keV reached in the past, say around 15 keV. Off line gain stabilisation techniques will be used to reduce degradation of the resolution due to gain changes over the long measurement campaign envisaged (of the order of one year, including background measurements). Measures will be taken to reduce background count rates to a minimum. The measured peak ratios will be carefully corrected for conversion electrons produced in the subsequent electromagnetic transitions and creating coincidence summing effects with some of the detected alpha-particles.

The final aim is to verify the validity of the recommended alpha emission probability values with new data having at least comparable or – preferably - lower uncertainties, i.e. <0.5 % absolute uncertainty on the main peaks.

Another topic related to decay data, i.e. the shape of beta spectra, is also addressed in this work package. The decay heat produced by fission products, amounting to 8 % of the energy generated during the fission process, needs to be well known for safe operations (reactor shutdown, post irradiation handling of nuclear fuels). This decay heat corresponds to the radiant energy (beta, gamma) emitted by the natural decay of those nuclides, which is imparted to surrounding medium. When the reactor is shutdown, this energy remains the main source of heating and decreases according to cooling time. It has to be accounted for in the cooling process. In particular, this requires a good knowledge of the mean beta energy produced by beta-decaying nuclides.

This decay heat is an important parameter for designing facilities and post-irradiation handling of nuclear fuel, and has been addressed for many years through theoretical calculation and specific experiments. In the 1990s, the so-called method Total Absorption Gamma Spectrometry (TAGS) was developed in order to determine experimentally the beta branching ratios and mean beta energy per disintegration for short-lived ill-defined fission products. The replacement of previous mean energies obtained from theory (gross theory of beta decay) with some TAGS values resulted in some inconsistencies. Consequently the IAEA held consultants' meetings (2005, 2006) to address that question and recommend new TAGS measurements.

The proposed research topic deals with a more fundamental question, which cannot be solved directly by the TAGS method. Indeed that method gives only access to the energies of the beta transitions, which correspond to the maximum energies of beta spectra. The quantity of interest, i.e. the mean beta particle energy, is derived from this maximum value and from the theoretical shape of the beta spectrum. The knowledge of the theoretical shape is quite satisfactory for allowed spectra, but this is not the case for spectra of the forbidden type. This could lead to uncertainties up to 10 % or more in the estimation of the mean energy. There is a need to improve that knowledge by reviewing theoretical data and developing dedicated experimental devices.

The review of current knowledge of theoretical beta spectra (specifically of forbidden type) and experimental determinations of beta energy spectra for some nuclides using innovative instrumentation based on cryogenic detectors [5] could lead to a better prediction of the shape of forbidden beta spectra and of their mean energy. This is fundamental work of metrology, leading under application to the improved calculation of decay heat induced by nuclear fission.


Aims of the work package

This work package has thus two major aims.

This work package aims at improving knowledge of decay data for radionuclides playing a major role in power-related fields. The JRP will measure some nuclear data that have been identified as incomplete or inconsistent, and in which National Metrology Institutes (NMIs) could provide a major impact with their dedicated facilities for primary standardisation of activity. NMIs have a proven record of providing accurate decay data such as half-lives [3] and alpha-particle emission probabilities [4]. One of the important nuclides for which an explicit demand for better decay data has been expressed (cf. IAEA priority list) is 238U. New measurements are needed on its half-life and alpha-particle emission probabilities.

This work package aims at improving knowledge of beta spectra, through experimental determination using the technology of cryogenic detectors which is innovative in that field of application, and offers exceptional characteristics in terms of energy resolution and detection efficiency over a wide energy range. Typically, for low activity sources, a detection efficiency greater than 99 %, a detection threshold at ~1 % of the maximum energy and an energy resolution of ~0.2 % of the maximum energy can be achieved. The development of such beta spectrometry systems could lead to significant improvements in the experimental determination of beta spectra and to the verification of theoretical models, especially for beta spectra of the forbidden type. This task relies on the experience already acquired by CEA [5,6] in activity and spectrometry measurements using cryogenic detectors and on a study of beta spectrometry using that technique, with positive results. This improvement could have impact in several fields of application, including decay heat calculation.


Scientific tasks

Task 5.1 Measurement of the alpha-particle emission probabilities of 238U (JRC (IRMM), CIEMAT)

Task 5.2 Development of beta spectrometry using cryogenic detectors (CEA)


Selected references

[1] M.A. Kellett, F.G. Kondev, A.L. Nichols, IAEA Coordinated Research Project: Updated decay data library for actinides, Applied Radiation and Isotopes 66 (2008) 694–700

[2] A.L. Nichols, IAEA, Actinide Decay Data: Measurement requirements identified to date (IAEA – October 2008)

[3] S. Pommé, T. Altzitzoglou, R. Van Ammel, G. Sibbens, R. Eykens, S. Richter, J. Camps, K. Kossert, H. Janßen, E. García-Toraño, T. Durán and F. Jaubert, Experimental determination of the 233U half-life, Metrologia 46 (2009) 439-449

[4] E. García-Toraño, M. Teresa Crespo, M. Roteta, G. Sibbens, S. Pommé, A. Martín Sánchez, M. P. Rubio Montero, Simon Woods, Andy Pearce, α-particle emission probabilities in the decay of 235U, Nuclear Instruments and Methods in Physics Research A 550 (2005) 581–592

[5] M. Loidl, M. Rodrigues, B. Censier, P. Cassette, T. Branger, D. Lacour, "First measurement of the beta spectrum of Pu-241 with a cryogenic detector", to be published in Applied radiation and Isotopes

[6] M. Rodrigues, E. Leblanc, M. Loidl, J. Bouchard, B. Censier, A. Fleischmann, A. Burck, H. Rotzinger and C. Enss, "A metallic magnetic calorimeter for hard X-ray and gamma-ray spectrometry", Journal of Low Temperature Physics 151 (2008) 1080-1086.

[7] E. García-Toraño, Appl. Radiat. Isot. 52 (2000) 591-594

[8] V. Chisté, M-M. Bé Table de Radionuclides (2006)

[9] R.B. Firestone, Table of Isotopes (1996)