Jan Wallenius

Application of Burnable Absorbers in an Accelerator-Driven System

Abstract The application of burnable absorbers (BAs) to minimize power peaking, reactivity loss, and capture-to-fission probabilities in an accelerator-driven waste transmutation system has been investigated. Boron-10-enriched B4C absorber rods were introduced into a lead-bismuth-cooled core fueled with transuranic (TRU) discharges from light water reactors to achieve the smallest possible power peakings at beginning-of-life (BOL) subcriticality level of 0.97. Detailed Monte Carlo simulations show that a radial power peaking equal to 1.2 at BOL is attainable using a four-zone differentiation in BA content. Using a newly written Monte Carlo burnup code, reactivity losses were calculated to be 640 pcm per percent TRU burnup for unrecycled TRU discharges. Comparing to corresponding values in BA-free cores, BA introduction diminishes reactivity losses in TRU-fueled subcritical cores by ~20%. Radial power peaking after 300 days of operation at 1200-MW thermal power was <1.75 at a subcriticality level of ~0.92, which appears to be acceptable, with respect to limitations in cladding and fuel temperatures. In addition, the use of BAs yields significantly higher fission-to-capture probabilities in even-neutron-number nuclides. Fission-to-absorption probability ratio for 241Am equal to 0.33 was achieved in the configuration studied. Hence, production of the strong alpha-emitter 242Cm is reduced, leading to smaller fuel-swelling rates and pin pressurization. Disadvantages following BA introduction, such as increase of void worth and decrease of Doppler feedback in conjunction with small values of βeff, need to be addressed by detailed studies of subcritical core dynamics.

Muon transfer via meta-stable dtμ∗ molecules and the kinetics of μCF

Abstract We have considered details of the lower stages of the muon cascade following formation of highly excited muonic atoms in D2-T2 mixtures. We show that when the muon reaches the n = 2 states of the muonic atom, an important modification of the hitherto accepted cascade model takes place. Our new theory predicts that fast ( λ dt μ ∗ ⋍ 10 11 s −1 ) formation of metastable dt μ ∗ in tμ(2s)-D2 collisions and their further decay into tμ(1s) + d or dμ(1s) + t together constitute a deexcitation process quickly depleting the n = 2 levels of muonic atoms. The decay into dμ(1s) can be seen as a muon transfer from the tμ(2s) to dμ(1s) via an intermediate resonant state of dtμ. This process provides an efficient mechanism for muon return from the tritium to deuterium atom which enhances the population of dμ(1s), thereby slowing down the cycling rate of the muon and quenching the fusion yield. This new understanding may help to further optimise physical conditions in order to maximise the fusion yield.

Impact of the side-path model on the muon cycling rate density dependence in D-T mixtures

The side-path model previously suggested by us allows for muon transferfrom tritiumto deuterium via intermediate dtµ* resonances formed in tµ(2s)-D2 collisions. Taking this effect into account, the density dependence of the muon cycling rate in D-T mixtures at low temperatures is analysed. Compared with cascade models where only one-sided transfer is present, the predicted density dependence of λc is in better agreement with measurements.

Excited States of Muonic Helium Hydride Ions and Their Possible Impact on Muon Reactivation

Several previously unknown resonances of the μtμ helium hydride ion have been identified using a variational procedure. It is suggested that these resonances may form in αμ(1s)-TD(T2) scattering, for centre of mass collision energies in the range 8–10 keV. If the molecular complex [(αtμ)*dee]* is formed in a dissociative state (with respect to the α tμ-d coordinate), the dissociation energy may in part be transferred to the muonic degrees of freedom, opening the exit channel [(αtμ)*dee]* → tμ + α e + T, effectively amounting to muon transfer from α to t. We present a theoretical formulation of this novel and hypothetical mechanism for muon reactivation together with a numerical calculation of its cross-section for a special case.

Muon transfer in dµ + H2 collisions

The impact of excited state muon transfer in dµ + H2 collisions has been investigated. The formation of metastable pdμ* molecules and their subsequent decay into the pμ (1s) + d channel opens a transfer channel that is otherwise closed at the n=2 level. This mechanism enlarges the fraction of muons arriving to the ground state of the lighter isotope. The resulting ground state population P1spμ as function of deuterium concentration appears to be in qualitative agreement with recent measurements of the Kα X-ray yield in H2/D2 mixtures.