A review and analysis of the state of the art on beryllium poisoning in research reactors
Introduction
Beryllium content in Earth’s igneous rocks is estimated to occur to the extent of 0.0002% wt, in about 30 recognized minerals. It is mined mainly in the USA, China, Kazakhstan, Mozambique, Madagascar and Portugal (Hanusa, 2016). It has a wide range of non-nuclear and nuclear applications. It is used in the electronics and telecommunication industry. The near transparency to X-radiation makes it usable in medical imaging and diagnostics. Due to its light atomic weight (Z = 4) combined with high fusion temperature (1287 °C), it is used as a component for rockets, satellites and aircraft (Use of beryllium, 2007).
Beryllium is used in numerous nuclear reactors, either as a reflector or as a moderator, thanks to a few of its distinctive properties. It is commonly used in material testing reactors (MTR). Some examples of facilities using beryllium reflector are ATR (USA) (Marshall, 2005), JMTR (Japan) (Hori et al., 2012), HFIR (USA) (Cook et al., 1990), MURR (USA) (Woolstenhulme et al., 2013), JHR (France) (Estrade et al., 2017), ORR (USA) (Cole and Gill, 1957), SAFARI (South Africa) (Tillwick et al.). Some of the beryllium-moderated units - where beryllium is used within the core – are BR-2 (Belgium) (Joppen et al., 2012), MARIA (Poland) (Krzysztoszek, 2007), EBOR (USA) (Moore, 1961), IVG.1 M (Kazakhstan) (Irkimbekov et al., 2019), IVV.2 M (Russian Federation) (Russkikh, 2017), MIR.M1 (Russian Federation) (Arkhangelsky et al., 2004).
In fact, most of the MTRs use beryllium and they all face some similar issues with its exploitation: buildup of neutron absorbers/poisons, irradiation-induced structural damage, gas production, swelling, distortion, formation of cracks, deterioration, spalling and loose part dissemination, corrosion, activation, transmutation, tritium release, reprocessing and waste disposal.
In all facilities having beryllium in the reactor’s core, some methods were developed to examine the beryllium state and to predict its maximum exploitation duration. Most of them use numerical calculation tools to estimate fast neutron fluence, or neutron poison build-up, or the evolution of mechanical properties in irradiated beryllium. Detailed measurements were held on irradiated beryllium samples to collect experimental data, and much effort was devoted to improve modelling of the evolution of mechanical properties, to monitor swelling or cracks formation. However, despite these efforts, the assessment of beryllium state always relies on visual inspections of structural damage, and swelling is still checked by simple geometric measurement or by water displacement methods.
An experimental alternative exists, which has never been proposed: it relies on beryllium's poisons measurement by neutron transmission technique. This novel approach is currently investigated in the MARIA reactor and it could bring a relevant and universal solution for the characterization of irradiated beryllium.
After briefly reviewing some of the characteristics of nuclear grade beryllium for use in reactors (Section 2), main nuclear reactions and neutronics properties of beryllium are recalled (Section 3), with details on specific features related to beryllium poisoning. Irradiation damage mechanisms are explained in Section 4, with emphasis on the experimental data used to establish swelling correlations. The experience gained from years of operation in several facilities using beryllium is compiled in Section 5 to compare the various strategies and criteria used for deciding on beryllium replacement. Finally, a basic principle of the neutron transmission experiment is presented and an application of this original method is proposed for the characterization of poisoned beryllium blocks in the MARIA reactor (Section 6).
Section snippets
Fabrication, storage and recycling
Nuclear grade beryllium is manufactured from beryllium powder in a form of a vacuum hot-pressed block. Although this operation results in a uniform, fully dense, fine-grained beryllium, the material usage requires caution, especially in handling ((Materion Brush Beryllium, 2020)), as inhalation of airborne beryllium may cause serious lung disorders in susceptible individuals.
A chemically pure beryllium powder with low oxide content (<2% of BeO) is preferred for structural and nuclear grade
Principal nuclear reactions
Light water cooled nuclear test reactors operate with the use of thermal neutrons for sustaining the fission process in the fuel. Neutrons emitted by fission are initially at high energy (“fast”), ~2 MeV on average. Beryllium, when present in the core, interacts as a moderator for these neutrons, to reduce their energy through scattering reactions so they become thermal. Beryllium undergoes several nuclear reactions presented below. These reactions depend on the particle type and its energy.
Irradiation damage mechanisms
Aside from impacting the neutronic properties of beryllium, irradiation induces mechanical damage. These mechanisms are shortly summarized in this section. Two different approaches used to characterize the problem of beryllium swelling are presented in parts 4.1 and 4.2.
An illustration of all major issues related to the use of beryllium in MTRs is presented in Fig. 13. As beryllium is exposed to neutron and gamma radiation, structural defects are developed and several gaseous products, such as 3
Experience gained in various facilities
A brief literature review is carried out to compare the experience gained from years of operation in several facilities using beryllium. Of particular interest for the present study is the information regarding the neutronic poisoning effects (loss of reactivity and degradation of neutron flux performance), the observed data on irradiation-induced swelling and embrittlement as well as various strategies and criteria for decision making on beryllium replacement. Difficulties related to the open
On using neutron transmission technique for irradiated beryllium characterization
As presented in the previous section, safe management of beryllium in material testing reactors relies on numerical simulations and visual inspections. These methods only serve for approximated evaluation of beryllium usability, to anticipate failure before replacement, but none of them allows for detailed analysis of neutron poison distributions when beryllium is employed either as a reflector or as moderator of reactor’s matrix element. The unknown fraction of gas release during irradiation
Conclusion
Beryllium use in research reactors allows to moderate neutrons and to reflect them to the core. These favourable neutronic properties make it one of the most frequently used material in MTR, where it is exposed to significant radiation dose and damages. Neutron-induced atomic displacements and defects clustering cause embrittlement and material hardening, meanwhile transmutations of 9Be targets yield helium and tritium that also contribute to beryllium swelling and embrittlement. Besides,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was done in collaboration between National Centre for Nuclear Research in Poland and Commissariat à l'énergie atomique et aux énergies alternatives in France. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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