Nuclear Reactor Fuel Safety: The Waiting Gain

September 27, 2018 | 9:21 am
Dave Lochbaum
Former Contributor

Nuclear power reactors spilt atoms to release energy used to generate electricity. Many of the byproducts formed when atoms split are unstable (radioactive) and release particles or gamma rays in search of stability. These radioactive emissions produce energy. Whether in the core of an operating reactor, in the core of a shutdown reactor, in the spent fuel pool after discharge from a reactor core, or in dry storage after offloading from a spent fuel pool, the energy released from nuclear reactor fuel must be removed before it damages the fuel from overheating. This commentary describes the energy levels associated with nuclear fuel in various locations at various times to illustrate the factors that affect the associated hazard levels.

Nuclear Fuel Locations

The San Onofre nuclear plant near San Clemente, California is used to describe the nuclear reactor fuel locations and energy levels for this commentary. San Onofre has been permanently shut down, but data from when its reactors operated and for the spent fuel remaining onsite represent conditions at nuclear plants across the country.

Figure 1 is an aerial view of San Onofre Units 2 and 3 during their construction in 1980. The reactor cores resided within the reactor containment domes—robust structures made from thick reinforced concrete. Each unit had a spent fuel pool housed within its own Fuel Handling Building, an industrial-grade structure designed not to fall down when the wind blows or ground shakes.

Figure 1 (Source: Source: Nuclear Regulatory Commission)

Figure 2 is an aerial view of the dry storage locations at San Onofre. Concrete vaults stored metal canisters of spent fuel assemblies from Unit 1 in horizontal vaults. The owner opted to place metal canisters of spent fuel assemblies from Units 2 and 3 in vertical vaults within an unground concrete slab. The dry storage area is on the plant site. The Unit 2 and 3 buildings are off the picture to the right.

Figure 2 (Source: Southern California Edison presentation, November 2, 2017)

Nuclear Fuel Energy Levels

Table 1 provides information on the energy levels of nuclear reactor fuel under various conditions for San Onofre Unit 3. Its reactor core contained 217 fuel assemblies. The reactor was licensed to operate at power levels up to 3,438 Megawatts thermal (Mwt). When operating at full power, the average fuel assembly generated 15.8 Mwt. When operators flipped switches to rapidly insert control rods that interrupted the nuclear chain reaction, the reactor might be shut down but the decay of unstable fission byproducts continued to produce about six percent of the core’s output at full power. The average fuel assembly released just under 1 Mwt minutes after a reactor shut down from full power. As fission byproducts decayed, the radioactive emissions continued to release energy at steadily decreasing amounts. Fifteen days after a reactor shut down from full power, the power level of an average fuel assembly dropped to 0.41 Mwt.

The Unit 3 spent fuel pool was licensed to hold up to 1,542 fuel assemblies. After the plant was permanently shut down and all fuel assemblies were offloaded from the reactor core, the Unit 3 spent fuel pool contained 1,350 fuel assemblies. The Unit 3 spent fuel pool had two limits on decay heat from the spent fuel it held. The maximum limit of 15.035 Mwt assumed the entire reactor core was offloaded into the spent fuel pool as quickly as allowed by the license in addition to the decay heat from the rest of the spent fuel in the pool. The normal limit of 7.239 Mwt applied to times when only a portion of the reactor core was discharged to the spent fuel pool during refueling and replaced with new fuel assemblies.

The owner calculated the actual decay heat load in the Unit 3 spent fuel pool at various times since permanent shut down of the plant. The actual decay heat load was 0.953 Mwt at the end of 2013 and has steadily declined since then.

The owner is transferring fuel assemblies from the Unit 3 spent fuel pool into the underground dry storage vaults using Multi-Purpose Canisters holding up to 37 assemblies (MPC-37). Each MPC-37 canister is certified for storing up to 37 spent fuel assemblies with a maximum total decay heat load of 0.037 Mwt. The canisters being loaded at San Onofre have actual decay heat loads of about 0.028 Mwt.

The fifth column of Table 1 compares the relative power levels of fuel in various locations to the power level in an MPC-37 loaded to the maximum limit. The power level of the reactor core at full power is nearly 93,000 times higher than that in the MPC-37.

The sixth column of Table 1 shows the power level of the average fuel assembly in the spent fuel pool to roughly equal the power level of a fuel assembly in the MPC-37. While the fifth column shows that the individual fuel assembly power levels are about the same, the larger inventory of fuel assemblies in the spent fuel pool yields a higher overall power (energy) level.

Table 1. (Source: Union of Concerned Scientists)

Nuclear Fuel Amounts

The third column of Table 1 provides the inventories of fuel assemblies in the reactor core and spent fuel pool in terms of number of equivalent MPC-37 canisters. It would take about six MPC-37 canisters to hold the fuel assemblies from one reactor core. It would take more than 36 MPC-37 canisters to store the fuel assemblies from the Unit 3 spent fuel pool before the current loading campaign began. Thus, the spent fuel pool contained about six reactor cores’ worth of fuel assemblies while the reactor core contained about six MPC-37’s worth of fuel assemblies.

Nuclear Fuel Populations

Table 2 provides the information on energy levels of nuclear reactor fuel for San Onofre Unit 2. The results are identical to Unit 3’s information for the reactor core and MPC-37 cases, and very similar for the spent fuel pool case.

Table 2 contains information on a few additional conditions for Unit 2 than presented for Unit 3. I estimated the inventory and heat load in the Unit 2 spent fuel pool after 5, 10, 15, 20, 25, and 30 MPC-37s had been loaded. This analysis shows that while the average fuel assembly energy level (column 6) remains the same, the overall energy level (column 4) in the spent fuel pools decreases as fuel assemblies are transferred into dry storage.

Table 2 (Source: Union of Concerned Scientists)

To help put the fuel assembly relative power data in context, three additional columns are provided in Table 2. These columns linked the populations of three nearby cities to the power levels relative to the MPC-37 maximum power level (i.e., the column 5 data). As the power levels decreased when the reactor was shut down, fuel offloaded to the spent fuel pool, and fuel transferred into dry storage, the population levels were reduced by the same percentage.

All but one person (I think it’s Amy although it might be Earl) must depart San Clemente to match the reduction in power level from full power to MPC-37 storage.

 UCS Perspective

Tables 1 and 2 illustrate the relative hazards of nuclear fuel in reactor cores, spent fuel pools, and dry storage. Nuclear fuel in the reactor core, even in the core of a shutdown reactor, has a significantly higher energy level than when in the spent fuel pool or dry storage. The higher energy level has two associated hazard implications. First, it translates into less time to successfully intervene to prevent fuel damage when cooling is lost or impaired. Second, it provides a larger catalyst or engine to expel radioactive materials from damaged fuel. Risk is defined as the product of the probability of an accident times its consequences. The first factor affects the probability of an accident while the second factor affects its consequences. Combined, these factors can cause risk to increase.

Nuclear fuel in spent fuel pools has lower energy levels than when in reactor cores. The average fuel assembly energy levels are lower than the maximum energy level permitted in a MPC-37 canister. But the associated inventories indicate why spent fuel pools have higher risks than dry storage. The collective higher energy levels in spent fuel pools once again translate into less time to respond should cooling be lost or impaired. And the larger inventory of fuel assemblies emits a larger radioactive cloud should intervention fail.

Nuclear fuel in dry storage represents the least amount of fuel at the lowest energy level. If cooling is lost or impaired, more time is available to successfully intervene and less nasty spread gets out when efforts fail. But fuel in dry storage is far from absolutely safe. If it were even close to being so safe, the US would not be spending billions of dollars looking for, but not yet finding, a geological repository that can isolate this hazardous material from people and the environment for at least 10,000 years into the future.

Dry storage is the safest and securest way to manage nuclear fuel risks today. However, the more of the 10,000-year period we waste looking for a geological repository, the less competent and responsible we reveal ourselves to be.

We can do better. And not just because it would be hard for us to mess this mess up any worse than we’ve mismanaged so far.