Reactor Decay Heat inPu239: Solving theγDiscrepancy in the 4–3000-s Cooling Period

Abstract

The feeding probability of 102;104;105;106;107 Tc, 105 Mo, and 101 Nb nuclei, which are important contributors to the decay heat in nuclear reactors, has been measured using the total absorption technique. We have coupled for the first time a total absorption spectrometer to a Penning trap in order to obtain sources of very high isobaric purity. Our results solve a significant part of a long-standing discrepancy in the component of the decay heat for 239 Pu in the 4-3000 s range. DOI: 10.1103/PhysRevLett.105.202501 PACS numbers: 23.40.Às, 27.60.+j, 28.41.Fr, 29.30.Kv Nuclear reactors provide a significant fraction of the world's electricity. A burgeoning population and an associated growth in economic activity suggest that world demand will double by 2050. Until now, the bulk of this has come from the burning of fossil fuels. There is general concern that reserves of fossil fuels are limited and their burning damages the environment. In particular, it contributes to the emission of large amounts of CO 2 . In this context, nuclear power, based on the fission process, will be less damaging to the environment. Accordingly there is now a renaissance in the building of nuclear power stations around the world. Modern reactor designs, based on many years of operating experience, are much more efficient, more economical, and safer than earlier designs. Although the basic principles are well established, we still lack certain information, such as a knowledge of the decay properties of specific nuclei that are important contributors to the heating of the reactor during and after operation. The estimation and control of the heat emitted by the decay of fission products plays a key role in the safe operation of reactors. The primary aim of this work is to study the decay properties of specific nuclei that are important contributors to this source of heat.Approximately 8% of the total energy generated during the fission process is related to the energy released in the natural decay of fission products, and is commonly called decay heat [1]. Once the reactor is shut down, the energy released in radioactive decay provides the main source of heating. Hence, coolant needs to be maintained after termination of the neutron-induced fission process in a reactor, and the form and extent of this essential requirement needs to be specified on the basis of decay-heat summation calculations. Decay heat varies as a function of time after shutdown and can be determined theoretically from known nuclear data. Such computations are based on the inventory of nuclei created during the fission process and after reactor shutdown and their radioactive decay characteristics:where fðtÞ is the power function, E i is the mean decay energy of the ith nuclide ( , , and components), i is the decay constant of the ith nuclide, and N i ðtÞ is the number of nuclide i at cooling time t. These calculations require extensive libraries of cross sections, fission yields, and decay data. Obviously, an accurate assessment of the decay heat is highly relevant...

Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications

“…Total Absorption Gamma-ray Spectroscopy (TAGS) has been successfully used to determine the β − strength functions of a significant number of complex decay schemes [37][38][39][40][41] on the basis of calculations involving the data derived from 4π γ-ray spectroscopy. Normally, the main spectrometer consists of a substantial well-type NaI(Tl) detector in which the γ -ray emitting source is contained, along with an auxiliary NaI(Tl) detector that fits into and closes the well to achieve an approximately 97% 4π solid-angle geometry.…”

“…TAGS has proved to be of importance in the development of a more correct and comprehensive understanding of the nucleus, reaching beyond some of the specific limitations of γ singles and coincidence measurements [39,40]. Furthermore, TAGS studies have been used to determine the average β − and γ energies of particular fission produces, aiding considerably in calculations of the decay heat emitted by irradiated reactor fuel [38,41] -see also Sect. 4.2.…”

“…Hardy et al have demonstrated that ∼ 20% of the true γ -ray intensity above 1.7 MeV for a fictional radionuclide (Pandemonium) may remain undetected, impacting significantly on the use of γ -ray singles data to calculate β − transitions by means of gamma population-depopulation balances of the proposed nuclear levels [82]. TAGS (Total Absorption Gamma-ray Spectroscopy) measurements can overcome these difficulties, and provide the necessary mean beta and gamma decay energies for sound decay heat calculations [37,38,41,83]. Fig.…”

“…Decay-heat calculations for irradiated 239 Pu as a function of cooling time in terms of (a) total energy release rate per fission second, (b) light-particle/electron energy release rate per fission second, constituting the β − , β + , Auger and conversion electron components, and (c) electromagnetic energy release rate per fission second, constituting the γ , X-rays, annihilation radiation and internal bremsstrahlung components. Tobias (1989) denotes benchmark data for 239 Pu as determined by a least squares fit to all available measured data [84]; JEFF311 defines the calculated decay heat when the JEFF3.1.1 decay-data sublibrary was adopted [69] in which TAGS data from Greenwood et al had been included [38]; JEFF311 + TAGS are equivalent decay-heat calculations in which TAGS data from Algora et al had also been added [41]. 86 Se, 84 Br, 89 Br, 87 Kr, 91 Kr, 88 Rb, 94 Rb, 92 As determined from the TAGS measurements of Greenwood et al [38], total mean β − and γ energies (Ē β − and E γ ) for twenty-nine fission products of importance in decayheat calculations have been incorporated into the JEFF3.1.1 decay-data sub-library [69] to replace inadequate values that arose from the evaluation of gamma-ray spectroscopic studies.…”

“…Some of the listed radionuclides represent repeat measurements of Greenwood et al as a sensible means of cross checking different sets of TAGS data [38]. A consequence of these particular assessments is that recent dedicated TAGS studies have focussed on the β − decay of 101 Nb, 105 Mo and 102,104,105,106,107 Tc [41]. As depicted in Fig.…”