Spent Fuel Management

Introduction to Spent Nuclear Fuel and Greater Than Class C Waste (GTCC) at DCPP

Creation of Radioactive Spent Nuclear Fuel

The electricity produced at the DCPP is fueled by uranium, a chemical element found all over the world.  The uranium is mined from rock, enriched, and formed into ½ inch sized pellets.  The pellets are placed into zirconium alloy-clad rods, which are then grouped together into fuel assemblies (See Figure 1).  An 1100 MWe PWR core may contain 193 fuel assemblies composed of over 50,000 fuel rods and some 18 million fuel pellets.  The fuel assemblies are then placed into the core of the nuclear reactor. Within the reactor, the nuclear fission (atom splitting) process is initiated.  This process produces heat, which boils water to create steam.  The steam then turns a turbine, creating electrical energy.

After about five years, the nuclear fuel assemblies in the reactor no longer produce sufficient energy and are removed.  At that point it is deemed “spent nuclear fuel” and is replaced with new nuclear fuel assemblies.  At DCPP, about 88 of the 193 fuel assemblies placed in each reactor are replaced during a refueling outage that occurs approximately every 18 months. This refueling process will end before the two DCPP nuclear reactors are shut down by 2025.

The unused uranium that is in original new fuel assemblies have only low levels of radiation and thus have low risk associated with its handling.  However, once the fuel is used in the fission process (and becomes spent nuclear fuel), the radiation levels are dangerously high – and have the potential to kill an exposed human within minutes.  This spent nuclear fuel requires highly specialized and careful handling, not only as it leaves the reactor, but for tens of thousands of years thereafter.

High Burnup Fuel

Before it is made into fuel, uranium is processed to increase the concentration of atoms that can split in a controlled chain reaction in the reactor.  In general, the higher the concentration of those atoms, the longer the fuel can sustain a chain reaction. And the longer the fuel remains in the reactor, the higher the burnup. 

In other words, burnup is a way to measure how much uranium is burned in the reactor. It is the amount of energy produced by the uranium. Burnup is expressed in Gigawatt-Days per Metric Ton of Uranium (GWd/MTU). Average burnup, around 35 GWd/MTU two decades ago, is over 45 GWd/MTU today. Utilities are now able to get more power out of their fuel before replacing it. This means they can operate longer between refueling outages. It also means they use less fuel.  High burnup fuel is used at DCPP.

High burnup fuel is hotter and more radioactive than low burnup fuel because more uranium was “burned” (that is, split during nuclear fission into smaller atomic fragments and the consequent conversion of some atomic mass of uranium into heat energy.)  It is the extra high abundance of these atomic fragments (including isotopes of iodine, cesium, strontium, xenon and barium, plutonium, and many other radioactive isotopes) in high burnup fuel that causes such high levels of radioactivity and accompanying release of more heat energy as further fission processes occur.  Because the fuel is very hot, both thermally and radioactively, it must be cooled for a longer period of time in the spent fuel pool before the spent nuclear fuel assemblies can be moved to dry cask storage.

Spent Fuel Pools

After being removed from the reactors, the spent nuclear fuel assemblies are shielded and moved to one of two DCPP spent fuel pools (See Figure 2).  The assemblies are placed within specialized racks in stainless-steel lined, concrete-walled pools filled with borated water, which is continuously circulated. The pools protect the workers and public from radiation exposure and cool the fuel assemblies.  The zircaloy cladding (.5 mm. thick) on the hot fuel rods will spontaneously combust in the presence of oxygen and if the fuel rods reach a temperature of 900 degrees Celsius; therefore, the fuel assemblies must constantly be kept under water.

When originally constructed, the spent fuel pools were expected to be used for a low-density configuration of 270 assemblies per pool. As of December 2018, however, there are 744 and 768 assemblies in pools 1 and 2, respectively.  By the time DCPP is shut down, there will be 1,261 in spent fuel pool 1 and 1,281 assemblies in spent fuel pool 2.  The assemblies are held in a checkerboard pattern, where hotter assemblies are surrounded by cooler assemblies.  This measure is intended to create additional emergency response time before a catastrophic fire could result in the event the pool water is unexpectedly drained.

Historically, PG&E has removed spent nuclear fuel assemblies from the spent fuel pools after about ten years. As discussed later (Section 2e), this time frame is under analysis and either longer or short storage times for individual assemblies may be used in order to accelerate the total time during which the DCPP spent fuel pools are in service.

Independent Spent Fuel Storage Installation (ISFSI)

After the spent nuclear fuel assemblies are removed from the spent fuel pools, they are placed in sealed, helium-filled canisters and set into an approximately 20-foot tall, concrete-filled storage cask made of steel. The storage casks are placed within the Independent Spent Fuel Storage Installation (ISFSI) area which is located on-site, inland from the reactors.  The casks are bolted to a 7½ foot thick, steel-reinforced concrete pad to ensure seismic stability.  This is known as “dry cask storage” (See Figures 3 and 4).  DCPP employs a cask system called Holtec HI-STORM 100 cask system, each of which holds 32 fuel assemblies.  As of December 2018, a total of 1,856 assemblies are stored at the Diablo ISFSI, within 58 casks. 

SA System

DCPP ISFSI Pad in 2017 with 49 Loaded Casks

Future Spent Nuclear Fuel Storage Options

When DCPP was constructed, there was an expectation that the federal government would create a federal repository for all spent nuclear fuel generated in the United States.  As described in greater detail later in this section, plans for the completion of a federal repository at Yucca Mountain are at a standstill because Congress has not yet appropriated funding for the processing of the license application by the NRC.  The Trump Administration did allocate funding for the project in the 2019 proposed budget. The Nevada Governor, state Attorney General, and congressional delegation, as well as leaders from Clark County and the City of Las Vegas, and the vast majority of Nevada’s citizens oppose the Yucca project.  In addition, there are numerous legal challenges to the site based on alleged unsuitability, including water seepage and seismic activity.

In 2016 and 2017, two separate companies applied to NRC for licenses to build interim consolidated facilities for the centralized storage of spent nuclear fuel until a federal repository such as Yucca Mountain is opened.  In the meantime, spent nuclear fuel will remain at the DCPP ISFSI.

Greater Than Class C Waste (GTCC)

In addition to spent nuclear fuel assemblies, another category of highly radioactive materials will exist at DCPP.  This waste is known as Greater Than Class C Waste (GTCC). GTCC includes all the materials that have been irradiated during the nuclear fission process, such as the reactor itself, which must be dismantled and removed when the plant is decommissioned.  An estimated ten casks will be needed to store the GTCC, which is expected to be ultimately placed at the ISFSI.  The existing ISFSI is not sized nor licensed for GTCC, and so PG&E would have to obtain an amended permit and licensing to either construct new storage pad space or reconfigure the existing dry cask placement. 

DCPP Spent Nuclear Fuel Storage Program

Current DCPP Spent Nuclear Fuel Management Cycle – From Plant to Pools to ISFSI

A simple graphic helps to summarize the Spent Fuel Cycle at DCPP


Each of the two nuclear reactor vessels at DCPP holds 193 nuclear fuel assemblies.  At the end of a cycle lasting approximately 18 months, one-third of the assemblies are replaced with new fuel assemblies.  Assemblies that have been used for three cycles (approximately 54 months) are removed and placed in the spent fuel pools.  Currently, PG&E keeps fuel assemblies in the spent fuel pools for approximately 10+ years during which time the spent fuel assemblies cool sufficiently to be removed from the pool and placed in specially designed casks to be stored in the ISFSI.  In its 2018 Triennial NDCTP filing, PG&E proposes to shorten the time fuel assemblies remain in the spent fuel pools by using a new generation of casks capable of handling higher heat loads.  This could allow removal to the ISFSI in seven years or less.  The casks were expected to be removed from the ISFSI to a federal repository, such as the Yucca Mountain Nuclear Waste Repository. However, in 2010 the Administration attempted to withdraw the Department of Energy (DOE) application for the Yucca Mountain Nuclear Waste Repository.  The U.S. Court of Appeals rejected this attempt, and ordered DOE and NRC to continue processing the Yucca application. This occurred, but the project has now been stymied due to lack of federal funds.

Although a private Consolidated Interim Storage Facility in Texas (discussed later in this section) could be open and available to start accepting DCPP spent nuclear fuel casks as early as 2027, it is likely that some (if not all) spent nuclear fuel casks will remain onsite for many years and perhaps even decades into the future.

Description of Spent Fuel Pools

The spent fuel pools at DCPP are built on solid bedrock and constructed with six-foot thick reinforced concrete walls.  The pools are lined with stainless steel, are 40 feet deep and designed to withstand the most destructive projected earthquake on the nearby Hosgri fault. The pools are filled with very pure water mixed with boric acid (boron being a neutron-absorbing element). Boric acid is added to the water in order to prevent a self-sustaining nuclear chain reaction.  The pools contain a system of racks capable of holding up to 1,324 fuel assemblies that are approximately 14 feet tall and are covered in a minimum of 23 feet of water (sufficiently deep to keep radiation risk to workers at low and acceptable levels).  The fuel assemblies that have been removed from the reactor are very hot and continue to release heat for years as a result of radioactive decay of fission products from the original uranium, including: 90Sr, 137Cs, 99Tc and 129I among dozens of other radioactive isotopes.  So much heat is produced that it is necessary to have very large compressors and pumps to continuously circulate and cool the water.  This is an active cooling process and requires continuous electrical energy to power the compressors, pumps and supplies of water to replace any water lost by evaporation or even a leak caused by some extraordinary event such as earthquake or terrorist attack.  Because of the critical nature of this system, DCPP maintains doubly redundant backup systems for compressors and pumps, plus backup diesel generators in event regular power is lost.  Large reservoirs of water are maintained on-site to rapidly replace any water in the event of a leak from the spent fuel pools.  Even if power is lost altogether, the pools can be filled using simple gravity through a system of pipes with mechanical, hand-operated valves.  The emergency reservoirs could cool the spent fuel pools for several days.

An important aspect of the DCPP spent fuel pools operating license is that “hot” (cooled for less than 120 days) spent nuclear fuel assemblies when placed into the spent fuel pool racks must be surrounded on four sides by “cold” (cooled for greater than 1 year) spent nuclear fuel assemblies. This requirement is in place to provide a heat sink for the hot assemblies in the event of a catastrophic loss of water in the spent fuel pools.  Having such heat sinks adjacent to the hot assemblies significantly lengthens the amount of time for emergency efforts to replace the water lost from the pool or otherwise address the risk of an uncontrollable spent fuel fire from the loss of water in the pool.  This requirement of four adjacent cold assemblies becomes particularly important at the end of power generation when the full load of 193 fuel assemblies from the reactor have to be unloaded into the spent fuel pools all at once.  That means that PG&E must have an inventory of at least 772 cold spent nuclear fuel assemblies still in the pool from previous unloading campaigns.  The result is that an unusually large number of assemblies will be in the pool after the final unloading campaign.  This is what has led to PG&E, in part, to halt the transfer of spent nuclear fuel assemblies to the ISFSI until after end of power generation.  The projected number of spent nuclear fuel assemblies stored in pools 1 and 2 at time of Unit 2 shutdown in 2025 is 1,261 and 1,281 respectively, if there are no additional loading campaigns prior to final shutdown.

Once power generation in Units 1 and 2 ceases in 2024 and 2025 and all spent nuclear fuel assemblies have been transferred into the spent fuel pools, then the licenses for operating the pools and other plant equipment convert to possession-only licenses, which allow continuing operation for non-generation purposes.  No license renewal is required for this transition.

Also relevant to operation of the spent fuel pools post-power generation is this statement from the DCPP 2018 Triennial NDCTP filing (Volume 1, Chapter 3, Section G.2):

Several existing plant systems are used to ensure there is adequate cooling of the spent fuel pools. These existing systems could continue to be used for SFP cooling during decommissioning; however, to facilitate safe and efficient decommissioning, the nuclear industry has implemented the SFP Island (SFPI) concept. A SFPI is an independent cooling system for the SFPs that allows the licensee to abandon the in-place plant systems supporting SFP cooling. PG&E plans to develop and install an SFPI to reduce the risk of decommissioning activities impacting the SFPs.

The NRC deems the spent fuel pools to be a safe storage system for spent nuclear fuel, both in the construction of the pools and in continuing operation.  The operation of the pools is continuously monitored by PG&E staff, and reviewed by full-time on-site NRC representatives as well as by the staff of the Diablo Canyon Independent Safety Committee (DCISC) which operates under the auspices of the CPUC.

DCPP ISFSI System and Dry Cask Design

The DCPP ISFSI, where the spent nuclear fuel is placed after being cooled in the spent fuel pools, is located 310 feet above sea-level, thus assuring protection from the largest tsunami that would be expected along this section of the California coastline. The installation also is constructed on bedrock, consisting of seven reinforced concrete pads, each 7½ feet thick and approximately 105 feet by 68 feet in size. There are 140 cask locations, each marked by an embedment ring which is used to anchor each cask to the pad. This system is compliant with the seismic requirements of the ISFSI license. As of March 2019, there are 58 loaded casks at the ISFSI.  Current projections forecast use of 138 cask locations with two locations being reserved to facilitate aging management activities such as allowing the PG&E transporter access to casks located on the interior of the ISFSI.

Casks for storing spent nuclear fuel assemblies use the concept of “passive” cooling, with ambient air drawn in through openings at the bottom of the casks, circulating upward along the sealed inner unit and discharging out at the top in a chimney effect (which steadily removes the heat that still is being produced as a result of continuing radioactive decay of the fission products from the spent nuclear fuel).  Because this system of removing the continuing heat production from the spent fuel is passive and does not depend on any compressors, pumps and assured electrical supply, it is typically considered safer than keeping the spent nuclear fuel assemblies in the pools.  There is some concern about possibilities of stress cracks developing in the casks over time.  As a result, it is considered critical that PG&E continuously monitor the casks as part of an aging management plan.

The storage cask used by PG&E through 2018 is the Holtec Hi-STORM 100 model that holds 32 fuel assemblies per cask.  There are specific guidelines required by the NRC for loading into these casks that require a knowledge of the heat being generated by each fuel assembly.  Each relatively hot fuel assembly must be accompanied by a relatively cool assembly. The relative heat is basically a function of how long the assembly was cooled in the spent fuel pools and the degree to which the original uranium atoms have undergone fission to produce the array of highly radioactive fission by-products.  A detailed knowledge of each fuel assembly is needed and careful calculations are required to assure that the total amount of heat being emitted does not exceed the capacity of the Holtec cask.

In 2019 PG&E plans to solicit bids from all qualified suppliers for a new generation of casks that have higher heat capacity ratings and could potentially reduce the amount of time required in the spent fuel pools from 10+ years to seven or fewer years.

The ISFSI at DCPP was constructed and is operating under a separate license from the NRC which provides for its use through March 2024. There are spaces for 140 casks to be stored on the ISFSI. PG&E intends to seek a license renewal for an additional 40 years, through March of 2064. If necessary, PG&E will seek a further renewal as 2064 nears.  With the lack of a long-term solution to storage of highly radioactive spent nuclear fuel, the spent nuclear fuel will remain on site into the foreseeable future.

For more information about the spent nuclear fuel cycle and storage, please access PG&E’s video at:  Diablo Canyon Used Fuel Management

Existing Ten-Year Transfer Program of All Spent Nuclear Fuel to ISFSI

PG&E currently retains spent nuclear fuel assemblies in the spent fuel pools for 10+ years.  After this time period, spent nuclear fuel assemblies are loaded into casks for dry storage, transferred to the ISFSI and secured there in a multi-step operation called a loading campaign.  The history of these loading campaigns is as follows:


A total of 58 casks have been loaded and transferred to the ISFSI.  As of January 2019, all loading campaigns have been discontinued until the end of power generation.  This step is being taken as part of a larger plan to empty the spent fuel pools of all spent nuclear fuel assemblies at an earlier end date than would otherwise be possible if the existing loading campaigns were continued.

Proposed Seven-Year Transfer Program of Spent Nuclear Fuel to ISFSI

As a result of the 2015 Triennial NDCTP Filing with the CPUC, PG&E was asked to consider shortening the residence time of the spent nuclear fuel assemblies in the spent fuel pools from 10 to seven years, thus matching standards that have been approved by the NRC and are being adopted more broadly in the industry.  PG&E has proposed to do so in its 2018 Triennial NDCTP filing, however this requires use of a new generation of casks that have the capacity to handle higher heat.  Any such casks would have to meet the demanding seismic requirements unique to DCPP. PG&E is preparing to solicit bids for such new generation casks from all qualified suppliers.  Obtaining qualifying bids may be complicated by the fact that any acceptable supplier would have to meet the additional seismic requirements at DCPP.  The outcome of this bidding process is one that the DCDEP will follow closely. Complicating this process is the need for NRC approvals of any modified cask design.  If such approvals were needed, it could incur delays that may threaten the timing in the plans outlined in PG&E’s 2018 NDCTP.

Alliance for Nuclear Responsibility (A4NR) Objection to the Proposed Seven-Year Transfer Program of Spent Nuclear Fuel to ISFSI

The A4NR has filed an objection to the 2018 NDCTP seven year campaign.  The complaint alleges the following:

  1. PG&E failed to adequately collaborate with the California Energy Commission in the preparation of the 2018 NDCTP;
  2. The significant build-up spent fuel assemblies in the spent fuel pools is not acceptable; and
  3. PG&E should return to the original open (lower density) racking in the spent fuel pools in order to reduce overall number of spent nuclear fuel assemblies in each pool and to improve water circulation, efficiency of cooling and safety.

A4NR’s objection and PG&E’s response will be considered by the CPUC as a part of its regulatory review of the 2018 NDCTP.

High Bridge Associates Finding Regarding Transfer of Spent Fuel

High Bridge Associates (HBA), an independent expert hired by PG&E to help in the preparation of its December 2018 NDCTP, had the following comment regarding PG&E’s spent fuel pool transfer program (Volume 2, Attachment A, Page 9 – Findings):

The most significant finding is the overall fourteen (14) year schedule duration for the decommissioning work from shutdown of Unit 1 to the end of site restoration is longer than the current industry norm.  This duration is primarily due to a longer than expected period for fuel cool down and other activities that could be managed so they are off the schedule critical path.

High Bridge Associates compared DCPP against other similar nuclear reactors and stated the following (Volume 2, Attachment A, Page 12 – Overall Schedule Duration Section):

The first major period examined, DCPP’s Fuel on Pad period is near the high end of all planned and executed decommissioning schedules.  When compared against results from past plants, DCPP is above average.  Because of DCPP’s unique seismic profile and operating history, HBA does not expect it to be as short as other plants in this comparison.  However, decommissioning project in similar stages of planning to DCPP are several years shorter than DCPP.

Comparison of Existing Spent Fuel Storage Programs in California

There are three existing ISFSIs in California, in addition to the DCPP ISFSI.  These are located at Rancho Seco, San Onofre Nuclear Generating Station (SONGS), Humbolt Bay (HBPP).  The spent nuclear fuel management at DCPP has more in common with the SONGS facility than that of Rancho Seco or HBPP.

Rancho Seco ISFSI

Rancho Seco is host to 228.8 metric tons of spent fuel (493 spent fuel assemblies) and 13.6 metric tons of GTCC waste from the reactors.  Altogether, 22 canisters are stored horizontally at the ISFSI.  None of the spent nuclear fuel is classified as high burnup, and so the challenges of spent nuclear fuel storage are less than those posed at the DCPP, where a significant portion of the spent nuclear fuel is classified as high burnup.


There are six dry casks stored at the HBPP. None of them contain high burnup fuel. The ISFSI at HBVPP is an I-shaped, subterranean concrete vault with six cylindrical vault liners poured in place. Each liner with its surrounding concrete is considered a separate cell within the vault.  The spent nuclear fuel is able to be stored in this manner because of the age of the HBPP fuel and the low decay heat associated with it.  The ISFSI storage casks do not require the normal atmospheric cooling, therefore the casks may be stored underground without fear of overheating.

The concrete vault provides structural stability as well as lateral restraint to resist seismic forces. The concrete vault also provides radiation shielding to lower the potential dose to the public in close proximity to the vault (i.e. along the public trail between the ISFSI and Humboldt Bay.)


SONGS Unit 1 commenced operation in 1968, and was shut down in 1992.  SONGS Units 2 and 3 were taken out of service in 2012 after a radioactive leak from a new steam generator whose design had been modified by the manufacturer, Mitsubishi, without obtaining a license amendment from the NRC.  On June 7, 2013, Southern California Edison (SCE) announced its decision to permanently retire SONGS Units 2 and 3. SCE announced in a press release that the decision was driven by regulatory uncertainty concerning the restart of both units and the associated economic impacts. Dismantlement of Unit 1 is essentially complete.

There are two separate ISFSIs at SONGS. The older installation uses horizontally-oriented Areva casks, while the newer ISFSI is employing the Holtec HI-STORM UMAX design, which is vertically oriented but built below grade, just a few feet above the mean tide level.  The loading of spent nuclear fuel into the Holtec UMAX casks was halted in August 2018 because of a near-miss during loading of a canister.  The 5/8 inch thick Holtec canister became lodged on an interior rim of the transfer cask and could have fallen 18 feet into the storage cask. Loading has not yet resumed, though it is anticipated to begin again soon. The NRC issued a violation against SCE and levied a fine of $116,000. Blame for the near-miss was attributed to Holtec, whose employees were operating the machinery under contract with SCE.

The Areva and Holtec ISFSI installations hold 124 casks, and altogether the site hosts 1,773 tons of spent nuclear fuel. Much of the spent nuclear fuel still housed in the pools can be classified as high burnup, and so it must be cooled for a longer period of time.

The following chart outlines the dry cask storage programs for SONGS, HBPP and Rancho Seco:

LocationSan Diego County, CaliforniaHumboldt County, CaliforniaSacramento County, California
Dry Cask Storage SystemAreva NUHOMS canister-based system (consists of a dry shielded canister (DSC) and reinforced concrete horizontal storage module (HSM)

Holtec UMAX (consists of Multi-Purpose Container (MPC) stored in below-grade reinforced concrete vault)
HI-STAR 100 HB system (consists of MPC-HB and HB overpack stored in below-grade reinforced concrete vault)Areva NUHOMS canister-based system (consists of a DSC and reinforced concrete HSM
Canisters StoredUnit 1: 18 casks of spent fuel and GTCC waste

Units 2&3: Projected 124 casks of spent fuel and GTCC waste
6 casks of spent fuel and GTCC waste22 casks of spent fuel and GTCC waste
NRC License-Type / YearGeneral

Year is N/A since it is a general license

Licensed in 2005

Licensed in 2000
Plant Permanent Shutdown YearUnit 1: 1992

Units 2&3: 2013
Reference(s)SONGS Website

NRC SONGS Decommissioning Webpage

Holtec UMAX Overview

SONGS Irradiated Fuel Management Plan
HB ISFSI License Renewal Application and Presentation to NRC

Funding Report to NRC
Rancho Seco ISFSI License Renewal Application and Presentation to NRC
DCPP ISFSI Spent Nuclear Fuel Casks

Current DCPP Licensed Cask Design

The current dry cask storage system at Diablo Canyon uses the Holtec International HI-STORM 100SA overpack, HI-TRAC 125D transfer cask, and Multi-Purpose Canister (MPC) capable of holding 32 fuel assemblies (MPC-32). This system is approved for use by general licensees under NRC Docket Number 72-1014. The canisters are half-inch thick stainless steel nestled within a concrete “overpack” that is 27-1/2 inches thick and lined with a 1 inch thick stainless steel liner around both the inner and outer diameters. No mechanism for inspecting the canisters for cracking or loss of helium currently exists, though research is underway.

When the spent nuclear fuel is transferred to dry storage from the spent fuel pools, it must be shielded against radiation. The fuel assemblies are loaded into the MPC which is inside the transfer cask underwater in the spent fuel pools. The MPC, shielded by the transfer cask, is raised out of the water, dried, filled with helium, sealed, and then carefully moved to the ISFSI, where the transfer cask is removed while the canister is lowered into the concrete overpack. The canister and concrete overpack are moved into place on the ISFSI pad and bolted down. This transfer process is designed to protect workers and the environment from radiation exposure.

Upcoming Request for Proposal (RFP) for New Casks

In July 2019, PG&E plans to issue a Request for Proposal (RFP) to begin the process of selecting a new dry cask storage system at the DCPP ISFSI. Because of the high seismic threat at Diablo Canyon, a site-specific dry storage plan must be used. At the DCDEP’s informational meeting held on February 22, 2019, three dry cask manufacturers (Orano, Holtec, and GNS) presented information regarding their products to the DCDEP. PG&E has noted that all manufacturers with dry cask storage systems are welcome to submit proposals. 

Technical Variables Associated with New Cask Design

Relevant to dry cask storage systems is this statement from the PG&E 2018 Triennial NDCTP filing (Volume 1, Chapter 6, Section B.2):

For a general license, the dry cask vendor performs the licensing to gain the NRC’s approval for the dry cask design to be used. However, DCPP is not authorized as a general licensee, but rather uses the system under a site-specific ISFSI license (NRC Docket Number 72-26). PG&E chose to obtain a site-specific ISFSI license to adequately address DCPP site-specific conditions including seismic design basis requirements and the associated impacts to the system’s thermal capacity.

Because design requirements are exceptional for the Diablo Canyon site, dry cask vendors must modify their designs to meet additional safety parameters. Each canister and cask is manufactured to order, and so the process may take some time to complete. The DCDEP will review all of the proposed cask designs submitted to PG&E and will make its recommendation to PG&E based on design safety and longevity. Although the recommendations will be advisory only, the DCDEP feels there is a responsibility to do so.

Cask Housing Options

During the DCDEP workshops held on spent nuclear fuel storage, three options for dry spent fuel storage were discussed: open air unmonitored storage, Hardened On-Site Storage (HOSS), and Hardened Extended-life Local Monitored Surface Storage (HELMS).

The system currently used at the DCPP ISFSI is an open air unmonitored system.  The dry casks are affixed to 7½ foot thick concrete pads in the open air.  The spent nuclear fuel emits radiation (like light from a light bulb) and continues to cool using a passive system: that is, it relies upon a combination of heat conduction through solid materials and natural convection or thermal radiation through air to move decay heat from the spent fuel into the ambient environment.  There is no real-time radiation monitoring at each cask.  Four radiation monitors (Thermoluminescent Dosimeters or TLD’s) are placed at the outer edges of the inner perimeter of the ISFSI.  Eight additional radiation monitors are placed around the exterior perimeter of the ISFSI.  TLD’s are replaced and the doses read quarterly.  Resultant doses are reported in the Annual Radiological Environmental Operating Report.  The most recent dose results are from 2017 and can be found in the Annual Radioactive Effluent Report (https://www.nrc.gov/docs/ML1813/ML18130A025.pdf.– page 83).  Thus far, radiation levels at the ISFSI are as expected.

The HOSS concept is still under development.  The principles are as follows:

  • Irradiated fuel must be stored as safely as possible as close to the site of generation as possible;
  • The facilities are not regarded as a permanent waste solution and should not be constructed underground rendering the waste irretrievable;
  • The facility must have real-time radiation and heat monitoring for early detection of problems with containers;
  • The amount of releases projected, in even severe attacks, should be low enough so that the storage system would be unattractive as a terrorist target; and
  • Placement of individual dry casks in a manner that detection from outside the site boundary is difficult.
  • Casks must be:
    • Retrievable
    • Capable of being re-containerized
    • Transportable

The HELMS concept is also under development and has been submitted to the NRC by Citizens Oversight, an activist organization based out of San Diego.  The following is from the Citizens Oversight website (citizensoversight.org):

HELMS stands for Hardened, Extended-life, Local, Monitored Surface Storage. Hardened to deal with the reality of the terrorist and other unpredictable events, Extended-Life to embrace a 1,000 year DESIGN LIFE, 300 year PASSIVE LIFE, while still allowing a 40-year license term. Local, to imply that the waste will likely be moved to perhaps a half-dozen Consolidated Interim Storage (CIS) sites which are near the source of the waste but away from the coastal areas and other waterways. Monitored, by defining and included a standard monitoring electronics package that can provide 7/24 monitoring during the initial decades of storage. Surface, to embrace the fact that a) the waste is simply too hot to place in any geologic repository, b) no geologic repository actually exists, and c) if the SNF is emplaced in the repository, it would need to be actively ventilated for up to 200 years.

In Germany, by comparison, dry casks are stored in passively cooled buildings in order to keep them out of sight of terrorists and to protect from potential environmental harm caused by excessive humidity and dust. A monolithic cask body is made of ductile cast iron with machined cooling fins to improve the heat removal. A bolted double lid system – the primary lid and the secondary lid– with metal seals and a permanent pressure monitoring of the interspace allows proof of leak tightness. Each cask has a pressure switch that sounds an alarm when a pressure limit is reached or if the switch doesn’t function. That switch sits in the secondary lid and surveys the helium pressure (higher than inside the cask, so that a leak would go to the inside of the cask and not to the environment) within the space between the primary and secondary lid. The radiation of any individual cask is measured during loading at the power plant and verified upon arrival at the ISFSI and then connected to the pressure switch, which surveys the leak tightness. Radiation is surveyed inside and outside the building, in particular at the fence.  Beginning in 1998, Germany has required onsite storage at nuclear power plants to be located in buildings with reinforcement that are 1.2 to 1.4 meters-thick. Japan also stores its spent nuclear fuel casks inside buildings.

DCPP Greater than Class C Waste (GTCC) Storage Program

When the DCPP is decommissioned, there will be waste generated from dismantling the reactor pressure vessel internals and appurtenances. This waste is classified as Greater-Than-Class-C (GTCC) Waste.  GTCC Waste cannot be shipped off-site like lower class demolition wastes, but must be stored in a long-term repository, similar to spent nuclear fuel.  The current ISFSI is not large enough to accommodate an additional approximately 10 casks of GTCC that will be stored onsite with the spent nuclear fuel. As part of the RFP process, PG&E will be evaluating dry cask storage systems for storage of GTCC waste at the DCPP ISFSI until such time as transfer to an approved, off-site facility can occur. 

The DCDEP is evaluating the storage of GTCC waste in a holistic manner.  In addition to the 58 spent fuel casks already onsite at Diablo Canyon, PG&E plans to add an additional 80 casks after offloading all spent nuclear fuel from the spent fuel pools and adding the GTCC waste. The addition of GTCC waste and the need to contain it in the ISFSI presents an opportunity for a fresh look at spent nuclear fuel storage at Diablo Canyon.

DCPP Spent Nuclear Fuel Security Program

Current Security Measures

Currently, NRC-regulated nuclear facilities, such as DCPP, are considered among the most secure of the nation’s critical infrastructure. This security is achieved through multiple approaches working concurrently.  DCPP is a strong structure, built to withstand adverse weather and earthquakes.  It is also surrounded by open space that is controlled by the utility or its subsidiaries.  DCPP is not visible from public roads. Additional security measures include trained and armed security officers, physical barriers, intrusion detection and surveillance systems. 

The NRC requires that DCPP, as well as all nuclear power plants, be able to defend against a set of adversary characteristics called the Design Basis Threat (DBT). The details of the DBT are not public. But, in general, it outlines threats and adversary characteristics these facilities must demonstrate they can protect against. The DBT is based on realistic assessments of the tactics, techniques and procedures used by terrorist groups and organizations. The NRC is constantly re-evaluating the threat environment and considers changes to the DBT if necessary.  The NRC’s security baseline inspection program is the primary way the agency verifies nuclear power plants are operating according to security regulations. Force-on-force security inspections are part of this program. In these inspections, a specially trained mock adversary force “attacks” the facility.

Security Measures During Decommissioning

TheNRC staff evaluates the overall security and emergency preparedness posture during decommissioning on a site-specific basis. The NRC requires a level of security commensurate with the potential consequences to public health and safety and common defense and security.  Each decommissioning power reactor has unique characteristics, such as the age of the fuel, amount of fuel in the pool, pool construction/location, and spent fuel load pattern.  Although some of the components of the DCPP security program during operation will remain during decommissioning, the NRC allows for changes based on reduced risks that exist after plant shut down.

Proposed Security Measures Beyond Decommissioning

After all the spent fuel has been moved from the pools to dry cask storage, the security program shifts to focus on the ISFSI.  The NRC continues to regulate the required security programs through the license it issues for the ISFSI.  This remains in place until all spent fuel is removed from the site. 

Additional information about how the NRC regulates plant security can be found at https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/security-enhancements.html

DCPP Inspection and Monitoring Program for ISFSI

NRC Mandated ISFSI Monitoring

NRC-mandated radiation monitoring requirements are not specific. The following is excerpted from Inspection Procedure 60855 – Operation of a Spent Fuel Storage Installation:

Review radiological records for the loading of several recent casks to confirm that radiation levels measured on the casks were within limits specified by the TS or CoC and consistent with values specified in the SAR. Contamination incidents since the last inspection should be reviewed to verify the licensee is continuing to maintain effective control of contamination during work activities.

Review the environmental dosimetry records since the last inspection for the areas around the ISFSI pad to verify that accumulation of casks on the ISFSI pad have not caused dose rates in the area to exceed 10 CFR Part 20 limits without posting the area. Verify that workers in nearby buildings are not experiencing elevated dose rates that would be inconsistent with the principles of ALARA (as low as reasonably achievable) and that areas accessible by the public are not exceeding doses to the public specified in 10 CFR Part 20.

NRC Mandated ISFSI Inspection

Below is an excerpt from the NRC document NUREG-1927, Revision 1, Standard Review Plan for Renewal of NRC Specific Licenses and NRC Certificates of Compliance (CoC) for Dry Storage of Spent Nuclear Fuel:

Both the specific-license and the CoC renewal applications must contain requirements and operating conditions (fuel storage, surveillance and maintenance, and other requirements) for the ISFSI or DSS that address aging mechanisms and aging effects that could affect structures, systems, and components relied upon for the safe storage of spent fuel. Renewal applications must include (1) time-limited aging analyses, if applicable, that demonstrate that structures, systems, and components important to safety will continue to perform their intended function for the requested period of extended operation, and (2) aging management programs for management of issues associated with aging that could adversely affect structures, systems, and components important to safety. Licensees and applicants are encouraged to meet with the NRC staff at public pre-application meetings to discuss their proposed plans for the renewal application.

PG&E Monitoring of the DCPP ISFSI

PG&E has chosen to use a “bounding” radiation dose measurement each year as a direct measurement of the amount of radiation exposure at the plant. For the eight Thermoluminescent Dosimeters (TLD) outside the perimeter of the ISFSI, PG&E has chosen to use this method of dose measurement:

Direct Radiation (line-of-sight plus sky-shine)

Direct radiation to a member of the public has been evaluated per 40 CFR 190 to ensure members of the public did not receive more than 25 mrem per year to the whole body.  The 2017 Land Use Census did not identify any members of the public that live in a location that can receive direct radiation from the DCPP site.

Instead of calculating dose to an hypothetical member of the public at the site boundary, direct radiation for 2017 was calculated for the operators of the makeup water treatment plant located near the site boundary and approximately 200 meters from the both the ISFSI and the centerline between the Unit 1 and Unit 2 plant vent exhausts. The makeup water operators have been estimated to spend a maximum of 2920 hours a year at their work location.

The makeup water plant is unique at Diablo Canyon because it is near the northern site boundary and receives direct radiation from multiple plant sources.  The makeup water plant operators work to support plant operation within the owner-controlled area and outside the protected area, but inside the site boundary.  Therefore, they are not evaluated to be members of the public not associated with the nuclear fuel cycle as defined in 40 CFR Part 190.

Because of these factors, dose received by makeup water plant operators is considered bounding – a maximum greater than the dose that could be received by any real member of the public in the unrestricted area.  The 2017 dose calculated for the makeup water operator as a receptor was 4.7 millirem. This is approximately 1/5 of the 25 millirem limit from 40 CFR Part 190 that would apply to members of the public not associated with the nuclear fuel cycle due to activities inside the site boundary.” (Page 23 – 2017 Annual Radioactive Effluent Release Report)

For the area within the ISFSI, the radiation monitoring is as follows:

  • TLDs are placed inside body phantoms (a block of human tissue equivalent material. to represent the human body)
  • Background radiation is subtracted using control TLDs.
  • TLDs are exchanged and read out quarterly.
  • Resultant doses are reported in the Annual Radioactive Effluent Release Report.
  • Radiation to members of the public in 2017 are reported in the 2017 Annual Radioactive Effluent Release Report- DCL 18-028.

The most recent report on radiation releases from Diablo Canyon is available in the 2017 Annual Radioactive Effluent Report. https://www.nrc.gov/docs/ML1813/ML18130A025.pdf

PG&E Inspection of the DCPP ISFSI

Below is a summary of the current ISFSI inspections completed at the DCPP:

Daily Inspection

  • PG&E’s Operations Services conducts daily checks to see that cask inlets and outlets are clear and undamaged
  • Radiation dosimeters are worn by staff during inspections and any changes in dosimetry readings are recorded

Monthly Inspection

Monthly inspection is performed by maintenance staff for:

  • Cask fastener integrity
  • Inlet and outlet screen integrity

Annual Inspection

Annual engineering inspection performed to assure:

  • Painted surfaces are relatively free of corrosion, and chipped, cracked or blistered paint
  • Nameplates are present, legible, and in good general condition
  • Lid surfaces are relatively free of dents, scratches, gouges or other damage
  • Lid lift hole plugs are installed
  • Lid retention studs are installed
  • Lid holes are in good condition
  • Anchor hardware is installed, and visible portions are in good condition.

Voluntary Electric Power Research Institute, Inc. (EPRI) Inspection. 

EPRI is an American independent, nonprofit organization that conducts research and development related to the generation, delivery, and use of electricity to help address challenges in electricity, including reliability, efficiency, affordability, health, safety, and the environment.

DCPP performed a voluntary EPRI inspection as a proof-of-technology verification in January of 2014 to help EPRI validate accessibility and inspection technologies for viewing canister exterior surfaces. The following is from the EPRI Report (3002002822):

  • The inspection provided remote access to the canister surface to collect surface samples, take temperature measurements, and obtain visual evidence of the surface condition.
  • The chemical analysis results confirmed very low chloride concentrations, less than 5 mg/m2, despite being located close to the ocean.
  • Sea salt aerosols were identified in some of the dust samples, indicating that the chlorides from the ocean are being transported inside the overpack to the canister surface, although very slowly as indicated by the low concentration.
  • The measured temperatures, ranging from about 120°F (49°C) near the bottom of the canister to well over 200°F (93°C) on the top, indicate that most of the canister is above the temperature where CISCC is expected to occur, yet the coolest areas near the bottom of the canister may already be below this threshold. One of the 2 year-old canisters tested was surprisingly cooler near the bottom than was expected, indicating vulnerability to deposition of salts from the sea air.
  • Visual inspection found a small amount of dust on the top surface; however, the sides were free of visible dust and debris, and there was no sign of gross degradation.
Permanent Federal Spent Nuclear Fuel Storage Facility Proposal

History of Federal Nuclear Waste Repository

When the nuclear industry was first developing, the National Academy of Sciences released a study in 1957 recommending that the best means of protecting the environment and public health and safety would be to dispose of the nuclear waste in rock deep underground.

In 1982 the federal government enacted the Nuclear Waste Policy Act (NWPA) which mandated the creation of a federal “repository” for spent nuclear fuel disposal.  The NWPA calls for a “permanent deep geologic disposal of high-level radioactive waste and spent nuclear fuel.”  The law specified that the disposal facility should begin accepting spent nuclear fuel in 1998.   The NWPA law specifies that Environmental Protection Agency (EPA) would set the human health protection standards that such a repository must meet, the Department of Energy (DOE) would identify a site that it believes complied with EPA’s standards, and the NRC would decide, after a full adjudicatory proceeding, whether or not the site chosen by DOE actually satisfies EPA’s standards.  If so, the facility would be built.  If not, DOE would select an alternative site, and the process would begin again.  The NWPA process specifies that public interest groups and State and local authorities could challenge and litigate the DOE and NRC decisions.

Yucca Mountain Nuclear Waste Repository

After extensive research, DOE identified a site in Nevada called Yucca Mountain that it believed met the EPA health protection standards.  In 2008, the DOE applied to the NRC for a license to construct the Yucca Mountain Nuclear Waste Repository.  However, the project has been a hotly debated national topic.  The majority of Nevadans, including the Governor and state Attorney General, as well as the state’s congressional delegation, leaders from Clark County, the City of Las Vegas, and the Western Shoshone Nation, continue to oppose the project.  Only the local county in which Yucca Mountain site is located, Nye County, supports the development of the repository. 

In 2010, the Obama Administration directed DOE to withdraw the Yucca Mountain application.  This decision was challenged by states where spent nuclear fuel was accumulating.  In 2013, the U. S. Court of Appeals for the District of Columbia rejected the DOE withdrawal and ordered DOE and NRC to resume processing the Yucca Mountain license application.  DOE and NRC restarted the Yucca process, but soon ran out of money because the Obama Administration would not appropriate funding for the project.  More recently, the budgets that were proposed by the Trump Administration for 2018 and now 2019 included approximately $120 million to restart the Yucca Mountain licensing process.  The 2018 Budget proposal did not pass.  The 2019 Budget proposal is currently pending in Congress.  

If the Yucca MountainNuclear Waste Repository licensing process at the NRC resumes, it will be litigated for several years.  And even if the project were approved by NRC (and the federal courts), it is unlikely that Yucca Mountain would begin accepting spent nuclear fuel before 2050. 

Prospects for Completion

This leaves all nuclear power plants in the US without any designated long-term federal disposal site.  As a result, most nuclear power plants, including DCPP, must store their spent nuclear fuel, indefinitely, on site in dry cask storage systems made of steel and concrete casks. The prospects for completion of Yucca Mountain Nuclear Waste Repository or any other such permanent repository in the near future are low and there is currently no approved funding for further development.   However, there was a Bill in the last Congress (the Nuclear Waste Policy Amendment Act of 2017) that directs the DOE to develop a federal Consolidated Interim Storage Facility (CISF) to be used until the development, construction and operation of a permanent federal nuclear waste repository is developed.  That bill (HR 3053), passed the House of Representatives by 370 – 72, but Senator Heller (R- NV) prevented it from coming to a vote in the Senate.  Senator Heller has since lost his seat.  A similar Bill could be introduced in the current Congress.

PG&E reached a settlement agreement with the DOE in 2012 for yearly reimbursement for the costs of on-site storage of spent nuclear fuel and yearly claims are submitted to the DOE.  This means that, until Yucca Mountain Nuclear Waste Repository or another federal repository opens, the federal government (taxpayers), not PG&E nor its ratepayers, pays the costs of storing spent nuclear fuel on the DCPP site.

Consolidated Interim Private Spent Nuclear Fuel Storage Facility (CISF) Proposals

 Texas and New Mexico CISF Proposals

As an interim measure until the federal government opens a permanent federal spent nuclear fuel repository, two private entities have submitted applications to the NRC for Consolidated Interim Storage Facilities (CISF).  These CISFs would be large ISFSIs, located either above or below grade.  Holtec International submitted an application for a CISF in Lea County, New Mexico in 2017 which may be approved as early as 2021. Holtec stated to the DCDEP that, once the license is issued, the facility could be constructed and open to accept spent nuclear fuel within 2 or 3 years. In 2016 another company, Interim Storage Partners, LLC submitted an application for a CISF in Andrews County, Texas with an estimated approval date of 2022.

Legal challenges to building CISF’s in both New Mexico and Texas are currently on appeal at the NRC. The Governor of New Mexico, both U.S. Senators, and two of three New Mexico Congressional representatives have expressed their opposition to building a CISF in their state, whereas local elected officials are supportive of the project. In Texas, local government officials have expressed their opposition to the proposed CISF, whereas Federal officials support it

Timeframe for Readiness

Both of these pending CISF proposals are seeking a specific license from the NRC under 10 CFR Part 72 and are not co-located with a power reactor.  The NRC is currently performing a technical review of all the safety and environmental protection aspects of the proposed CISFs.   If approved, the license could be valid for up to 40 years.

Once these privately owned and operated CISFs are licensed and constructed, the decision as to which commercial nuclear power plants get to send their spent nuclear fuel to the CISF first has not yet been decided.  The DOE has an informal Acceptance Policy Ranking for a federal repository, which states that the oldest fuel from a particular location should be transported first.  However, it is not known if this approach would apply to the CISFs.

At the federal level, Representative Mike Levin (D- San Juan Capistrano) has introduced HR 2699 – the Nuclear Waste Policy Amendments Act of 2019, which would give priority to waste from: (1) decommissioned plants or those in the process of being decommissioned, (2) sites located near dense population centers and (3) locations where an earthquake hazard is present.  Levin’s legislation would supersede the “oldest first” principle, which is not codified under any law or regulation but has been accepted by some in the industry. If Levin’s bill becomes law, the old standards would be replaced and new criteria would be established to determine which sites would move to the front of the queue for transporting used spent nuclear fuel to a CISF or permanent federal repository.  This new standard may accelerate the transfer of spent nuclear fuel from DCPP due to its location near fault lines.

DCDEP Position on CISF

Although the recommendations put forward in the Vision Statement include support of CISF and the desire to transfer spent nuclear fuel to these facilities if available, DCDEP member Linda Seeley has presented an opposition paper entitled “Opposition to Consolidated Interim Storage”recommending the spent nuclear fuel remain at the DCPP site until such time as a permanent federal repository exists. Her paper can be accessed at this link.

Transportation of GTCC Waste / Spent Nuclear Fuel

Transportation Impacts of Spent Nuclear Fuel

The transportation related impacts of decommissioning which could include moving both radiologically contaminated and non-contaminated demolition materials (and GTCC waste and spent nuclear fuel in the future) over the local highway and rail systems are of critical importance to the county and in particular, the community of Avila Beach.  It is imperative that the movement of demolition materials, GTCC waste and spent nuclear fuel be done safely and with limited impacts to surrounding communities.

Transportation Casks and Canisters

At the DCPP, the existing dry casks overpacks in use for spent nuclear fuel storage do not meet the NRC’s transportation cask specifications.  Transporting spent nuclear fuel from DCPP to either a CISF or a federal repository must be preceded by transferring the multi-purpose spent fuel canisters from existing overpacks to transportation casks

As part of the RFP process being used to select a new dry storage system at the DCPP ISFSI, PG&E is not requiring that future casks that are evaluated be designed and licensed for both storage and transportation, but is not opposed to assessing casks that meet this criteria. As part of the RFP, PG&E is looking at canisters that meet the transportation requirements for eventual transfer to a transportation cask, if they are not already in a licensed transportation cask.

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