Additional radiation dose due to atmospheric dispersion of tritium evaporated from a hypothetical reservoir

Highlights

• A new radiation exposure pathway associated to liquid tritium discharge was detailed.

• Tritium evaporation rate was estimated under various discharge conditions.

• Additional radiation dose was assessed due to tritium evaporation factor.

• A deep discharge instead of surface discharge was suggested from ALARA criterion.

Abstract

With regard to an inland nuclear power plant bordered by a reservoir, a major concern was that fresh water might be polluted and the human body might be radiation exposed due to the discharge of liquid radioactive effluents. In contrast to other radionuclides in the effluents, tritium has specific dispersion behavior in the aquatic environment such as emission into the air along with water evaporation. Further, the evaporated tritium in the air could go toward the territorial system where the wind blows. As a result, the person staying in the vicinity of the plant discharge point would be exposed with an additional radiation dose. In light of this characteristic, this study first introduced this new exposure pathway and investigated the additional radiation dose on the basis of a hypothetical reservoir. The results indicated that annual tritium evaporation fraction is approximately 2.5%, which is a comparable level with the radioactive decay factor. This would produce an additional radiation dose of 0.63 μSv/a to a person staying 50 m away from the plant discharge point for the case of 1 g/a tritium discharge. Tritium evaporation effects could be decreased through controlling the discharge depth. Thus, a preliminary suggestion to adopt a deep discharge instead of surface discharge was proposed from the ALARA (as low as reasonably achievable) criterion of radiation protection.

Introduction

Nuclear power has played an important role in the energy generation since it was discovered by the scientific community in the last century. Nuclear contributed 10–18% to the total electricity generation around the world based on the past 34a’ (1985–2019) data provided by the International Atomic Energy Agency (IAEA, 2019). The nuclear fraction varies greatly in different nations, for instance, this fraction was more than 70% in France while only 4.9% in China in 2019. A positive policy has been made to develop nuclear power in China because of various advantages of nuclear energy. Inland nuclear power plant (NPP) is regarded as a significant option as available coastal sites would become increasingly scarce along with the implementation of the ambitious development plan in China (Zeng et al., 2016). In reality, site selection study for inland NPP has been performed for over a decade and several sites have been identified, such as the Xianning site in Hubei province, Pengze site in Jiangxi province and Taohuajiang site in Hunan province. However, they were suspended affected by the Fukushima nuclear accident in 2011. It is still under debate whether the inland NPP projects should be restarted.

A site bordered by a reservoir was regarded to be one of the suitable choice for the inland NPP construction (e.g. Xianning site bordered by the Fushui reservoir) as a large quantity of cooling water are needed. While some express warry that fresh water would be polluted due to the discharge of liquid radioactive effluents, particularly tritium. Taking liquid radioactive effluents from Korean NPPs from example, tritium contributed 45–57% to the total released radioactivity during the years 2011–2015 (Kong et al., 2017). This is the main argument against inland NPP (Ke et al., 2017). Thus, identification of potential water pollution and individual radiation dose is one of the significant concerns in relation to liquid tritiated effluent discharge.

Previous scholars have performed some numerical studies on general radionuclides dispersion in the aquatic system. Among these work, several models have been developed and some typical models are as follows. The Japan Atomic Energy Agency (JAEA) developed an Oceanic Radionuclide Dispersion Model (SEA-GEARN) to simulate radionuclide dispersion in the ocean on the basis of Ocean General Circulation Model (KOGCM) (Kobayashi et al., 2007). The Korea Atomic Energy Research Institute (KAERI) developed a Lagrangian Oceanic Radiological Assessment System (LORAS) which can output dissolved, suspended and bottom sediment concentrations (Min et al., 2013). In addition, Federal University of Rio de Janeiro (UFRJ) in Brazil developed a software named SisBaHiA including both hydrodynamic and transport model (Filho et al., 2013).

With regard to identification of tritium dispersion characteristics, few publications were reported except some experimental researches on measurement for tritium concentration in the water and aquatic biota (Harms et al., 2016; Jefanova et al., 2018; Kim et al., 2019; Matsumoto et al., 2013). In reality, some features should be identified when simulating tritium dispersion in the aquatic environment, such as tritium evaporation behavior which was also proposed by Marang et al. (2011). It is worth noting that the evaporated tritium might go toward the territorial system where the wind blows. As a result, the person staying in the vicinity of plant discharge point would be exposed with an additional radiation dose. This is a new exposure pathway and the early understanding that ingestion of aquatic products was regarded as the only exposure pathway associated to liquid radioactive discharge might be incomplete.

Given the research gap, we attempted to investigate the additional radiation dose due to tritium evaporation factor based on a hypothetical reservoir case. Both tritium evaporate fraction and additional radiation dose would be estimated for the first time. It is attempted to propose some preliminary strategic suggestions on tritium discharge from the nuclear facility, particularly the inland NPPs.