How safe is Diablo Canyon's nuclear waste?

Japan’s Fukushima disaster has prompted PG&E to accelerate transfer of Diablo Canyon’s spent fuel out of its cooling pools — though not to the lowest capacity for which the pools were originally designed

dsneed@thetribunenews.comJune 18, 2011 

Together, Diablo Canyon’s two spent fuel pools can hold 2,648 fuel assemblies. They currently hold 2,176, and in coming years plans call for that number to be reduced to about 1,200.


Seventy-five percent of the highly radioactive fuel used by the nation’s nuclear plants remains today in cooling pools at those facilities.

At Diablo Canyon nuclear power plant, the amount is higher — 81 percent of spent fuel sits in two pools. In response to the nuclear disaster in Japan, PG&E plans to reduce the amount of used fuel assemblies it keeps in its pools at Diablo Canyon. But it has no plans to reduce them to their original and safest low-density configuration.

The safety of the spent fuel storage pools at the nation’s 104 nuclear reactors has come under renewed scrutiny in the wake of the March 11 nuclear disaster in Japan. In addition to the crippled reactors at the Fukushima Dai-ichi plant, a spent fuel pool also contributed to the crisis. Workers there had to take emergency steps to keep it full of water to avoid a catastrophic fire.

Nuclear watchdog and public safety groups say the first step that should be taken to increase the safety of U.S. nuclear plants is to transfer spent fuel to dry-cask storage, leaving only the newest assemblies that must cool off in the pools for five years for safety reasons.

“In protecting America from nuclear catastrophe, safely securing the spent fuel by eliminating highly radioactive, crowded pools should be a public safety priority of the highest degree,” wrote Robert Alvarez in May in a report for the Institute for Policy Studies, a liberal Washington, D.C., think tank.

Operators at California’s two nuclear plants — Diablo Canyon and the San Onofre Nuclear Generating Station near San Clemente — say they have no plans to take that step. The federal Nuclear Regulatory Commission does not require it.

“The NRC has determined that storage and management of used fuel in either an engineered pool or in sealed canisters as part of an independent fuel storage installation are technologies that protect public health and safety,” said Gil Alexander of Southern California Edison, the utility that operates San Onofre.

According to the Nuclear Energy Institute, an industry group, about 75 percent of the nation’s spent fuel is stored in crowded pools, with the remaining 25 percent in dry casks. California’s two nuclear plants mirror this national trend. Diablo Canyon has 81 percent of its spent fuel still in the two pools, and San Onofre has 68 percent of its spent fuel in its two pools.

Dangerous end product

Spent fuel — a form of high-level nuclear waste — is one of the most hazardous known materials. Direct exposure to its intense radiation would kill a person within minutes, and it stays dangerous for tens of thousands of years.

Studies have shown that spent fuel in densely loaded pools could catch fire if the water drained out. Conversely, loosely stored fuel has enough space between the assemblies to prevent a fire.

Critics charge that spent fuel pools lack some of the safety features that protect a nuclear power plant’s reactors. For example, at Diablo Canyon, the spent fuel pools are robust concrete-and-steel-lined structures, but they are located outside the containment domes that house the reactors.

One of the main purposes of the containment domes is to prevent the spread of radiation in the event of an accident. According to a 2004 study by the National Academy of Sciences, smoke from a fire in a pool could spread “radioactive aerosols hundreds of miles downwind under appropriate atmospheric conditions.”

Used fuel assemblies are stored in pools for five years before they can be transferred to dry casks. These casks are huge, sealed steel-and-concrete containers.

Sixteen loaded casks sit bolted to a thick concrete slab behind the Diablo Canyon plant.

Most nuclear experts say that dry-cask storage is safer than pools because it is passive and does not require pumps and other equipment to keep it cool. Heat that builds up in each cask is slowly released into the air.

The hardened casks are also a less attractive target for terrorists. It would be much easier for terrorists to drain a spent fuel pool than breach a dry cask.

In 2008, Gregory Jaczko, then a board member of the NRC and now its chairman, admitted that “the most clear-cut example of an area where additional safety margins can be gained involves additional efforts to move spent nuclear fuel from pools to dry-cask storage.”

In spite of this, the NRC’s official policy remains that the spent fuel pools are sufficiently safe. The agency maintains that there is no pressing safety or security reason to mandate earlier transfer of fuel from pool to cask.

Stored at Diablo

At Diablo Canyon, the spent fuel pools contain more than five times the number of assemblies than the reactors. The utility plans to reduce the density of the pools by about 45 percent over the next five years, to about 600 assemblies per pool.

“It is important to try to create a buffer,” said Jearl Strickland, Diablo Canyon’s used fuel manager. “We want to show people that we are serious about reducing the risk.”

However, there are no plans to return the pools to their original low-density configuration, called open racking, of 270 assemblies per pool. Doing so would not allow the utility to load the spent fuel casks with the ideal mixture of older and new used fuel, Strickland said.

The dry casks are filled using a storage technique with newer, hotter fuel surrounded by older fuel in the casks. If PG&E technicians reduced the density to open racking, they would not have enough older fuel available to load the spent fuel casks with the proper ratio of older and newer fuel, Strickland said.

Concern about nuclear waste at Diablo Canyon is long-standing. In 2006, the anti-nuclear group San Luis Obispo Mothers for Peace sued the NRC to force the agency to look at the environmental effects of a radiation release caused by a terrorist attack at Diablo Canyon. The agency determined that the probability of such an incident is so low that it need not be taken into account.

“The ongoing catastrophe in Fukushima illustrates the folly of that position,” said Jane Swanson, a Mothers for Peace spokeswoman.

In the aftermath of the Japanese disaster, the NRC is requiring that all utilities review and test their emergency procedures and report the results. A recent NRC inspection found more than 20 problems with Diablo Canyon’s emergency preparedness.

The most significant was a backup cooling pump that failed to start; it has since been repaired. In spite of these problems, the NRC said plant operators would still have been able to deal with a major emergency.

PG&E is correcting those problems. The utility also takes several precautions to keep the spent fuel pools safe, Strickland said.

They center on ensuring that the pools are kept filled with water, which is circulated continually to prevent it from boiling away. The plant’s emergency diesel generators produce enough electricity to power the pumps that keep cooling water circulating around the used assemblies, even in the event of a blackout, as happened in Japan.

If the pumps fail, procedures are in place to use emergency fire pumps and hoses to keep the water circulating, Strickland said. As a last resort, water from two freshwater ponds behind the plant, containing a total of 5 million gallons, could be gravity-fed into the pools.

Additionally, the NRC requires that spent fuel in the pools be arranged in a checkerboard fashion, with the newest and hottest assemblies surrounded by older fuel. This reduces the chances that the fuel would catch fire if the assemblies were ever exposed in an emergency.

No permanent solution

The nation got into this dilemma because its nuclear power plants were built with the assumption that used fuel would be taken by the federal government to be reprocessed into new fuel or stored in a central underground facility at Yucca Mountain in Nevada. Neither of those permanent solutions has come about, forcing each nuclear plant to store its used fuel on-site for the foreseeable future.

Utilities solved this problem on an interim basis by installing new, higher-density racks while they applied to build dry-cask storage facilities. At one time, Diablo Canyon’s pools even had temporary racks installed to accommodate the load.

Since then, Diablo Canyon’s dry-cask storage facility has opened, and 16 casks with 32 spent fuel assemblies each are stored there. The spent fuel pools contain 1,072 and 1,104 assemblies. The pools can hold a total of 2,648 assemblies.

The establishment of a dry-cask facility at Diablo Canyon comes after a nationwide trend, with about half of the nation’s nuclear plants using them. Another 12 storage casks are due to be delivered to Diablo this summer. The utility will start filling them in January. Ten more casks are on order for delivery later in 2012, Strickland said.

One-hundred-thirty-eight casks will be needed to store all the spent fuel the plant will generate in the 40 years of its initial operating license. PG&E has applied to the NRC to extend the life of the plant by 20 years starting in 2024 and 2025.

If approved, license renewal will cause the dry-cask facility to eventually expand to 207 casks.

Graphic: Spent fuel storage

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More nuclear power plant coverage coming this week

Editor’s note:

We’ve long kept readers apace of developments at Diablo Canyon nuclear power plant, and in recent months have taken close looks at seismic and design issues, relicensing and emergency preparedness.

Today’s explanatory story on spent fuel pools at Diablo Canyon and at San Onofre near San Clemente, the state’s other nuclear power plant, addresses a top safety concern raised in light of the March 11 nuclear disaster in Japan: How much spent fuel should be transferred to dry-cask storage and how quickly?

David Sneed, who has covered the environment for The Tribune for the past 14 years, is widely regarded as an authoritative reporter on the nuclear industry. He will report on the future of on-site storage nationwide — and what that means long-term for sites like Diablo — on Sunday, June 26.

Beyond Sneed’s insightful coverage, the Associated Press has produced a four-part investigative series called Aging Nukes. The first story — to be published Monday — examines how regulators and the U.S. industry have worked in concert to keep older plants operating, and the potential problems that poses.

— Sandra Duerr, executive editor

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