# The Impact of Tritium in the Environment
## 2 Artificial Sources
### Nuclear Power Engineering  
With the cessation (restriction) of nuclear testing, the nuclear industry and nuclear power engineering, in particular, nuclear reactors and reprocessing plants, have become the most important sources of tritium in the environment. In nuclear reactors, tritium is formed inside fission reactions of \({}^{235}\)U and \({}^{239}\)Pu nuclei, as well as in the interaction of neutrons with Li, B, and D. More than 96% of tritium in light-water reactors and gas-cooled reactors is produced by the ternary fission of uranium and plutonium [17; 18]. When irradiating plutonium nuclei in the fuel elements, the tritium yield is three times more efficient than when irradiating uranium [19]. Figure 1: Evolution of tritium inventory in the biosphere. Sources are as follows: 1—nuclear tests, 2—nuclear energy, 3—natural sources, 4—integrated value. Amounts were calculated by taking into account decay using a database of nuclear explosions [11; 12] and the IAEA and WNA databases [13; 14]. Figure 2: Modern distribution of “global” tritium in the biosphere, plotted according to [15]. In water reactors, tritium is also produced by the activation of deuterium present in the coolant/moderator water. In light-water reactors, due to the small amount of deuterium in natural water (D\({}_{2}\)O:H\({}_{2}\)O = 1.5 \(\times\) 10\({}^{-4}\)) and the low probability of neutron capture, this source of tritium is insignificant. In heavy-water reactors where D\({}_{2}\)O is used as a moderator/coolant, deuterium activation is the main source of tritium (Table 1). Tritium emissions to the environment during a reactor campaign vary from 0.36 (BWR) to 8.8% (HWR), depending on the process. In a two-loop PWR, the intensity of the chain reaction (reactivity) is controlled by changing the concentration of boron in the water of the first circuit. The dissolution of boric acid in the primary cooling circuit of a PWR allows for the regulation of the amount of \({}^{10}\)B in the reaction medium and the removal of heat from the reactor core. The concentration of \({}^{10}\)B controls the rate of the nuclear reaction and, consequently, the energy it generates. Unlike PWRs, BWR cooling circuits do not use borates. In BWRs, boron is used in control rods fabricated from boron carbide, fuel pools, and the control system fluid reserve tank. The normalized tritium generation amount due to ternary fission in PWRs and BWRs is of the same value. In addition, about 30% of the tritium in BWRs is generated by neutron capture reactions with \({}^{10}\)B nuclei in the control rods, which is an order of magnitude more than in the primary cooling circuit of PWRs. Most of the tritium released from PWRs appears in the liquid effluent (about 85%), whereas 75% of the tritium released from BWRs is in the form of airborne effluents [20]. When a neutron is captured, the generated tritium atoms can be incorporated into the boron carbide structure, compensating for the 'broken' bonds and forming a stable B\({}_{12}\)C\({}_{3}\)(H, T)\({}_{16}\) structure, which undoubtedly reduces tritium emissions. Due to the difference in boron control technology as well as the construction peculiarities of reactors and despite the fact that tritium generation in PWRs is 30% less than in BWRs, the amount of tritium released to the environment from PWRs is about six times greater than from BWRs. The expected tritium generation in a fusion reactor is four orders of magnitude higher than in PWRs and BWRs, and the environmental release from a fusion reactor is estimated at 1.4-2.2%\({}_{\circ}\), which, in absolute values, is two times higher than the release from a heavy-water reactor (Table 1). The content of tritium generated by the global nuclear power complex in the biosphere was calculated using the WNA and IAEA reactor databases [13] and taking into account the type of reactor, the amount of electricity produced, and radioactive decay (Figure 3). Figure 3: Integrated Tritium Generation by nuclear power engineering.