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Edison Engineering Manager Jerry Stephenson explains how the ISFSI Radiation Monitoring System at the San Onofre Nuclear Generating Station works. Photo by Samantha Nelson
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Edison aims for transparency with radiation monitoring

REGION — As spent nuclear fuel continues to be stored away while the San Onofre Nuclear Generating Station begins to be dismantled, Southern California Edison has installed a radiation monitoring system to stay transparent and ease concerns of area residents regarding radiation coming from the plant.

In the last six months, Edison installed four monitors on the plant’s site. Three of those monitors sit on the independent spent fuel storage installation (ISFSI) pad and the fourth sits in the parking lot up on the hill above the plant site to provide baseline radiation readings.

The monitors detect levels of gamma radiation emitted by the spent nuclear fuel in storage as well as the plant’s general background radiation. The monitors measure in microrem — a thousandth of a millirem — per hour.

Typically speaking, the monitors on the pad read around 18 microrem while the baseline monitor in the parking lot reads about 10 microrem. 

“This is tiny,” said Edison Engineering Manager Jerry Stephenson.

According to the National Council on Radiation Protection and Measurements, the average annual radiation dose per person in the United States is 620 millirem or 620,000 microrem. That amount includes a naturally occurring dose of radiation from the sun, air travel and food as well as manmade sources like medical and dental x-rays.

It’s hard to see, but this radiation monitor on the dry spent fuel storage pad at SONGS is measuring 13 microrems per hour of radiation. The average person receives an annual dose of 620,000 microrems. There are three monitors like this around the dry storage pads and a fourth in the parking lot to measure baseline radiation readings. Photo by Samantha Nelson

“We had to choose very sensitive radiation monitors, otherwise they would have just read zero all the time,” Stephenson said.

Edison purchased the radiation monitoring system from Mirion Technologies, a radiation measurement and detection company based in Irvine. Edison got the idea to use the monitoring system from Prairie Island in Minnesota, which is the only other nuclear plant in the country to use such a system.

The monitoring system will measure elevated levels of radiation when a canister of spent nuclear fuel passes by to be downloaded into dry storage. 

According to Edison spokesman John Dobken, the maximum reading on Feb. 28 when a canister was moved onto the storage pad was 0.337 millirem, or 337 microrem per hour. The readings were about 10 times lower the day before and after that.

Radiation is so low on the dry storage pad due to the ISFSI’s thick, concrete shielding that workers are not required to wear dosimeters, which measure the effective dose of radiation received by the human body through exposure to external ionizing radiation.

Edison is collecting data from the radiation monitoring system and sending it to three off-site agencies: The California Department of Public Health’s Radiologic Health Branch, California State Parks and the city of San Juan Capistrano.

Installation of the new radiation monitors came around the same time as Edison received approval to begin demolishing the SONGS plant. 

As The Coast News has previously reported, SONGS has been a fixture in coastal Southern California for more than 40 years. The nuclear plant was retired in 2013, and since then talk of its complete removal has been underway.

As dismantlement begins, moving canisters of spent nuclear fuel into dry storage will continue.

By March 13, Edison had downloaded 54 of a total of 73 spent fuel canisters into its HI-STORM UMAX dry storage system. Edison expects to complete downloading by midsummer. 

Another 50 canisters are already stored in the TN-NUHOMS system, the other ISFSI on site.

Though the demolition process has begun, Edison is making sure to continue moving spent fuel into dry storage without pause.

“We’re being very careful to make sure the decommissioning work does not get in the way of the fuel transfer operations,” said Doug Bauder, the vice president of decommissioning and chief nuclear officer at SONGS. “Completing those safely is our top priority.”

The two dry storage facilities will remain on site even after everything else from demolition is shipped off over a period of nine years until the federal government is ready to store it somewhere else.