Shiga studies impact on Lake Biwa from possible Fukui nuke accident

Chunichi Shimbun

The Shiga Prefectural Government plans to develop a computer system able to simulate the amount and dispersal of radioactive materials that would contaminate Lake Biwa in the event of an accident at one of the nuclear power plants in neighboring Fukui Prefecture.

The system, which the prefecture will start developing in the next fiscal year from April, is a modified version of the central government’s System for Prediction of Environmental Emergency Dose Information (SPEEDI), which predicts the volume and spread of radioactive materials in the environment by analyzing the terrain and wind direction.

Shiga Prefecture, the proud home of Lake Biwa, is eager to develop the system and start running simulations as the lake supplies drinking water for 14 million residents in the Keihanshin region, which includes the cities of Osaka, Kobe and Kyoto.

The prefecture hopes to use the system to forecast how contamination would affect the lake’s water and ecosystems in a nuclear accident in Fukui Prefecture. It also plans to revise its disaster-prevention programs based on the data it would gather from computer simulations.

Various organizations have developed systems that predict radioactive contamination in seawater, but this would be the first time a system is developed to forecast how a lake would be affected.

Shiga already has a system that forecasts environmental contamination by radioactive substances. A simulation using the system has shown that radioactive iodine and xenon would spread across Lake Biwa if emitted in springtime or wintertime by Kansai Electric Power Co.’s Mihama nuclear power plant in Fukui Prefecture. The plant is located about 30 km from the lake.

If Lake Biwa is tainted with radioactive materials from a nuclear accident at the Mihama plant, experts believe it would take more than 10 years before all the contaminated water is gone.

Some radioactive materials emitted during the Chernobyl disaster still remain in some of Japan’s lakes, 25 years after the world’s worst nuclear accident.

As many questions remain unanswered, the ability to simulate how much water in Biwa would be contaminated by a nuclear disaster in Fukui, as well as how radioactive materials would be dispersed under various meteorological scenarios, would be indispensable to Shiga’s government.

The prefecture plans to improve the accuracy of its data on how water flows in the lake using about 40 observation points.

Lake Biwa’s ecosystems also would be greatly affected by the spread of radioactive materials.

Radioactive cesium, which sinks faster in freshwater than in seawater, would likely fall to the bottom of the lake and contaminate the mud on its floor. The cesium could then enter the food chain through shrimp that live on the bottom, and as the shrimp are eaten by other fish it could keep moving up the food chain.

A system able to take such factors into account would thus be invaluable to the prefectural government.

“In the Fukushima No. 1 nuclear plant accident we actually saw widespread contamination because of radioactive materials (emitted by the crippled reactors). As we are a local government situated next to many nuclear reactors (in Fukui Prefecture), we need to clarify what the effects would be on drinking water and agricultural products” in the event of a nuclear accident, a prefectural official said.

But Katsumi Hirose, a visiting professor of environmental radiation at Sophia University in Tokyo, sounded a note of caution.

“It’s quite difficult to predict the movement of radioactive materials in water. In addition, cesium spreads in a different way in freshwater than in seawater, which requires alternative simulation calculations than those used to forecast the contamination of oceans,” Hirose said.

“It’s highly likely the contamination will remain for a long time in a closed environment such as a lake. So it is crucial that a simulation that can accurately estimate freshwater safety is developed,” he said.

This section, appearing Saturdays, features topics and issues from the Chubu region covered by the Chunichi Shimbun. The original article was published Nov. 23.