Production and transport modelling of Po-210 in DEMO reactor

Abstract

One of the generic designs of the nuclear fusion DEMO reactor proposed by the EUROfusion consortium foresees the development of a tritium Breeding Blanket (BB) relying on the use of the liquid-metal PbLi eutectic alloy as both neutron multiplier and tritium breeder, namely the Water-Cooled Lithium Lead (WCLL) BB, whose strengths and weaknesses are well known. This paper focuses the attention on one of the possible disadvantages of such a technology: the production of the highly radiotoxic radionuclide 210Po, which could become a safety issue to be accounted for. The 210Po concentration within the PbLi circuit has been assessed by solving a modified version of Bateman’s equations to consider the alloy circulation, so a one-dimensional convective fluid-dynamic model has been set up. Nuclear quantities have been evaluated by Monte Carlo neutron transport analyses using MCNP code and adopting a fully heterogeneous model of DEMO equipped with the WCLL BB. Moreover, rough sensitivity analyses have been performed to assess the influence on the results of the uncertainties related to the 209Bi radiative-capture cross section and the initial concentration of this nuclide which is present in the PbLi as an impurity. Results obtained have been critically discussed and some safety issues have been addressed to evaluate the possible hazard in case of a leak of PbLi accident.

Water activation products generation and transport in DEMO divertor

“…This circumstance is due to the fact that the phenomenology investigated is characterised by three interacting time constants, the period of the source, the time required for the water to complete the given loop and, indeed, the mean lifetime of the radioisotopes. It is interesting to observe that a similar dynamic was found in a related work concerning the assessment of the polonium inventory in the liquid PbLi breeder loop of one of the blanket concepts envisaged for DEMO [19]. Finally, we conclude by noting that the results obtained highlight a couple of safety-related issues.…”

“…The activation of the coolant and its impurities due to the passage through the reactor and exposition to high neutron fluxes is a topic which designers have dealt with since the early steps of nuclear energy use for heat and electric power applications [32]. When water is used as coolant, 16 N, 17 N and 19 O are the intrinsic activation products that present the highest activities in the loop. These short-lived products present no problem after reactor shutdown, but their high concentration prohibits access to the cooling system during plant operation and influences the shielding design for the equipment compartments and closest cubicles within the reactor building [33].…”

“…So, five activation products of the water constituents have been considered ( 16 N, 17 N, 19 O, 14 C, 41 Ar) some details of which are summarized in table 1 [12]. In a nuclear system cooled by water, the endothermic charged-particle reactions 16 O (n,p) 16 N (with threshold energy of ∼10.2 MeV) and 17 O (n,p) 17 N (with threshold energy of ∼8.4 MeV) are the principal sources of water radioactivity during operation [13,14].…”

“…16 N decays by emission of β particle and emits γ rays with a half-life (T 1/2 ) of 7.13 s while 17 N decays by β particles and emits neutrons with a half-life of 4.173 s. 17 N is also produced by the reaction 18 O (n,d) 17 N (with threshold energy of ∼14.6 MeV) even if to a very lower extent as it will be shown in the following. The activation product 19 O is produced by the reaction 18 O (n,γ) 19 O which already takes place at thermal energies. Such isotope decays by β emission with subsequent γ emission 100% of the time.…”

“…Gammas of 0.11 and 1.44 MeV are produced 6% of the time (3% and 3%, respectively). The half-life of 19 O is 26.47 s. 14 C has been also considered for its potential radiological risk as it is a pure β emitter and its half-life is very great, 5730 years. It is produced both via the reaction 17 O (n,α) 14 C and 14 N (n,p) 14 C although the concentration of 14 N in the coolant is rather low (4.28 • 10 19 atoms per −1 kg of water can be estimated [15]) so that this second production channel is rather negligible.…”