Boron-10 effect on the reactivity of the IPR-R1 Triga research reactor Amir Zacarias Mesquita ⇑ , Isabela Carolina Reis, Vitor Fernandes de Almeida, Rogerio Rivail Rodrigues Nuclear Technology Development Center/Brazilian Nuclear Energy Commission (CDTN/Cnen), Campus da UFMG, Pampulha, Belo Horizonte, MG, Brazil article info Article history: Received 17 December 2018 Received in revised form 6 April 2019 Accepted 14 April 2019 Keywords: Chemical shim Triga reactor Boric acid Light water Reactivity abstract The isotope boron-10 (B-10) has a high thermal neutron absorption cross-section and is used to poison the chain reaction in some light-water reactors (chemical shim). The purpose of this work was to show the effectiveness of boron as a neutron absorber in controlling the reactivity of nuclear reactors cooled by light water. In this way, samples were placed in sealed containers with various concentrations of boric acid in the IPR-R1 Triga nuclear research reactor core. Thus, the reactivity variation of this reactor was determined. The sample characterization was performed before and after the experiments by the mea- surement of pH and electrical conductivity. The IPR-R1 reactor is located at the Nuclear Technology Development Center (CDTN) in Belo Horizonte, Brazil. Variations of reactivities were evaluated using the static reactivity null method and the dynamic method. The results obtained made it possible to sim- ulate B-10 consumption during the operation of a reactor and its effect on reactivity with increasing boric acid concentrations. The pH values showed small increases after irradiation and their conductivities showed minor changes. As a result of this experiment, a correlation was drawn between various boric acid concentrations and reactor reactivity. Ó 2019 Elsevier Ltd. All rights reserved. 1. Introduction To maintain a reactor critical, it is necessary to balance the rate at which neutrons are produced and the rate at which they are lost due to leaks and absorptions. For long-term control, chemicals with a high absorption cross sections, such as boric acid, are dis- solved in the primary coolant water of the pressurized water reac- tor. The most common neutron absorber is boron, added as boric acid (H 3 BO 3 ). Natural boron contains 19.9% of B-10, with the remainder as B-11 (80.1%). The B-10 has a cross section for thermal neutrons absorption of approximately 3840 barns. This increases the probability of neutron capture to moderate neutrons. The B- 11 isotope is almost ineffective as a neutron absorber (IAEA, 1996). Acid boric concentration depends on the core’s characteristics and the projection of fuel burnup. In pressurized water reactors (PWR) the boron content starts within the 1000–2000 ppm range and progressively decreases along the fuel cycle while the fuel bur- nup increases. At the end of the fuel cycle, the boron concentration reaches a few ppm or is close to zero and a reactor must be refu- eled. In certain cases, fine power changes can be controlled by the chemical shim. If it is necessary to increase the power, then the boric acid concentration must be diluted, removing 10 B from the reactor core and decreasing its poisoning effect. At the reactor shutdown for refueling, boric acid is injected at high concentra- tions in order to absorb all thermal neutrons, extinguishing the fis- sion reaction. In the exchange of fuel elements, boric acid is used in large concentrations to guarantee core subcriticality (Nordmann, 2004). The thermal neutrons (n thermal ) are captured by isotope 10 of boron, 10 B, contained in the samples as boric acid, following the reaction shown in Eq. (1) (Pastina et al., 1999): 10B þ n thermal ! 7Li þ 4He þ 2:35 MeV ð1Þ The use of boric acid to control reactivity has benefits, including reduction of dependence on control rods, fuel economy, better power distribution in the core, and the reaction products after neu- tron absorption, helium and lithium, are stable isotopes (Giada, 2005; Byrne, 1994). The useful life of a nuclear reactor depends on several factors, such as radiation damage to the pressure vessel, the integrity of the fuel, and especially the number of withdrawals and insertions of the control rods. Chemical shim reduces the mechanical control rod requirement quite considerably. There are several reasons for using chemical shim (chemical control long- term reactivity control). It substantially reduces the number of control rods required in a reactor. Because control rods and their drive mechanisms are expensive, this results in substantial cost savings. The insertion of control rods into the reactor vessel is reduced, increasing the reactor’s life. H 3 BO 3 is more or less uni- formly distributed throughout the reactor core, and changes in https://doi.org/10.1016/j.anucene.2019.04.023 0306-4549/Ó 2019 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: amir@cdtn.br (A.Z. Mesquita). Annals of Nuclear Energy 132 (2019) 64–69 Contents lists available at ScienceDirect Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene