Radiation from Accelerating the Transfer of Spent Fuel from Pools to Casks

September 11, 2013 | 10:40 am
David Wright
Former Contributor

Contrary to claims by some in the nuclear industry, accelerating the transfer of spent fuel from cooling pools to dry casks would not pose a significant risk to workers from increased radiation exposure, and does not outweigh the benefits of such transfers.

To see this, consider the results of an August 2012 study by the Energy Power Research Institute (EPRI), “Impacts Associated with Transfer of Spent Nuclear Fuel from Spent Fuel Storage Pools to Dry Storage after Five Years of Cooling, Revision 1,” which estimates the increase in radiation dose to workers loading fuel into casks in scenarios of accelerated transfers.

The EPRI Study

The August 2012 EPRI report considers a “Base case” in which spent fuel is only transferred out of the pool to dry casks to free up enough space for new spent fuel to be moved from the reactor core into the pool. It compares that case with two accelerated-transfer cases in which all of the accumulated fuel that has been in the pool longer than five years is transferred to dry casks over a period of either 10 years or 15 years, with spent fuel after that period transferred to casks on an ongoing basis once it has been in the pool for five years. Here I consider the 10-year case, which uses the most accelerated transfer rate (Case 2 in the EPRI study).

EPRI’s report calculates the total dose for workers at representative reactors over the time period from 2011 to 2099.  As expected, the study finds an increase in total worker exposure in the 10-year case over the Base case. This is because workers will be loading somewhat hotter fuel into dry casks over the next 20 to 30 years of operation of these reactors, since the fuel will not sit in the cooling pools as long as in the base case before being transferred to dry casks.

Total Industry-Wide Worker Dose

I look here at the increased dose to workers loading casks, since that is by far the biggest change due to accelerated transfers.

EPRI estimates that the dose to workers loading the casks will increase by about 40% over the Base case during the roughly 30 years that casks are assumed to be loaded at the reactors. In particular, EPRI estimates an increase of 16 person-rem per reactor for existing pressurized water reactors (PWRs) and 11 person-rem per reactor for existing boiling water reactors (BWRs) (p. 3-18). Assuming 100 reactors in the U.S. fleet roughly two-thirds of which are PWRs, this would mean a collective increased dose summed up over all the workers loading fuel in the fleet of 1,400-1,500 person-rem. And in fact, Table 4.3 (p. 4-14) estimates a total increase for the fleet for loading casks of 1,520 person-rem.

Current models for health risks from low levels of radiation assume a “linear no-threshold (LNT) model,” meaning that there is no threshold below which risks from radiation exposure can be ignored, so that even very low levels of radiation can cause cancers. Applying the LNT model predicts that the additional dose of 1,520 person-rem would result in a 75% chance of having one additional cancer death across all workers loading casks in the nuclear industry over that roughly 30 year time span. (Both the recent ICRP and BEIR VII studies use a risk value at low dose (less than about 10 rem) of about 5×10-4 deaths/rem.)

Minimizing health risks is of course an important goal. But the risks from accelerating the loading of casks must be compared to the risks of radiation exposure to the population around a reactor if there were an accident at the pool. A recent NRC study looked at the potential consequences of a pool accident at the Peach Bottom nuclear plant in Pennsylvania. The study shows that removing spent fuel older than five years from the pool could reduce the amount of radioactive cesium released in a severe accident by factor of 80, the number of cancers by a factor of 10, and the total amount of land that would be uninhabitable by a factor of 50. Debate continues about the probability of such an accident, but a defense-in-depth safety philosophy means taking steps to reduce the consequences of an accident should it occur.

Rather than consider industry-wide limits on radiation, the NRC instead sets radiation limits based on annual dose limits for individual workers. It’s therefore useful to convert the dose increase EPRI calculates to an average annual per-worker dose and compare it to the annual per-worker dose limit set by the NRC.

 Average Annual Worker Dose per Worker

In both the PWR and BWR cases, the increase in annual per-person dose due to loading casks is 0.005 rem (see Table 1). The average background exposure in the U.S. is about 0.3 rem per year, which is roughly 20 times the estimated dose from loading casks and 60 times larger than the increase in dose caused by the accelerated transfer rate.

The NRC sets an annual exposure limit for individual workers of 5 rem. The increase due to loading casks is therefore 1,000 times smaller than the annual exposure limit used by the NRC.

The safety goal is to ensure that the radiation exposure of workers is kept below the NRC annual limit. The tiny dose increase from loading casks would only put a worker over the annual limit if the worker was already at 99.9% of the limit.

If the industry and NRC believe the increased radiation dose to workers from accelerating the transfer of spent fuel to dry casks presents a significant health risk, then they should reassess their current limits on radiation exposure.

The Techical Note at the end of the post explains how the numbers in the white boxes were calculated.

It’s interesting to compare the dose that workers receive from loading dry casks with that from other activities related to waste storage. According to the EPRI study, the largest contribution to worker dose is not from loading casks but from annual maintenance and inspection at dry cask storage sites. These activities result in an average annual per-worker dose of 0.027 rem.

What this shows is that the per-worker dose from these activities, which EPRI assumes are the same in the Base and 10-year case, contributes roughly twice as much per year to the exposure of workers in the Base case as does loading casks.

Industry and NRC Concerns?

Dave Lochbaum has pointed out that recent actions by the nuclear industry and NRC suggest they are not genuinely concerned about the dose increase to workers that would result from accelerated transfers of spent fuel to dry casks:

  • Neither the nuclear industry nor the NRC raised concerns about worker exposure when the industry reduced the duration of refueling outages and began discharging irradiated fuel from reactor cores to spent fuel pools within hours rather than days, as had been the practice previously. Yet the worker dose issue is much more significant when irradiated fuel is moved within hours of reactor shut down rather than many years later.
  • Indian Point has two operating reactors, but only Unit 2 has a crane and the infrastructure necessary to handle a fully loaded standard dry cask weighing nearly 100 tons. Yet Unit 3 recently reached the point that it needed to begin transferring spent fuel to dry casks to free up space in the pool. Rather than spend the money to upgrade Unit 3, Indian Point’s owner requested and the NRC approved in July 2012 a plan in which workers load up to 12 spent fuel assemblies from the Unit 3 spent fuel pool into a smaller cask that weighs 40 tons when loaded. Workers then move this smaller cask into the Unit 2 spent fuel pool, unload it, and then reload the irradiated fuel into a standard, larger cask and move this standard cask to the storage pad. This multistep process triples the number of loadings and unloadings for each fuel assembly compared to the usual practice, significantly increasing the handling of spent fuel and therefore worker exposure. However, this additional exposure was not seen as a reason to disallow this process, which could have been avoided by buying an additional crane.

In accepting Indian Point’s request, the NRC stated: “The evaluation concluded that the radiation dose to workers would be within the dose limits specified in 10 CFR 20.1201. The NRC staff reviewed the dose estimates for the transfer operations in its safety evaluation for the proposed action and concluded that the dose estimates for the operations activities are reasonable. Based on the above, there are no significant occupational dose impacts associated with the proposed action.”

Given its conclusion in the Indian Point case, it is difficult to see how the NRC could believe that the increased radiation exposure from accelerated transfers to dry casks could have any “significant occupational dose impacts.”


Technical Note

The average annual, per-worker dose is given by dividing the total dose at a reactor by the number of workers involved with loading casks at the plant and by the number of years the loading operations take place. The EPRI report estimates that the total worker dose for loading casks at its representative PWR is 38 person-rem for the Base case and 54 person-rem for the 10-year case. Similarly, it estimates that the total worker dose for loading casks at its representative BWR is 31 person-rem for the Base case and 42 person-rem for the 10-year case.

The EPRI study assumes cask loading will take place over a period of 31 years for its representative PWR case and 28 years for its BWR case.(EPRI assumes the PWR will stop operating in 2037 and the BWR in 2034, so that dry cask loading would end five years later in each case.)  According to the 2012 GAO report , “Spent Nuclear Fuel: Accumulating Quantities at Commercial Reactors Present Storage and Other Challenges,” there are some 9,500 workers in the nuclear industry involved in loading operations, or about 90 per reactor (p. 40, footnote 41 states that an increase of 7.5% over the current set of workers who load casks represents 710 people, implying a current workforce of 9500 across the industry, or about 90 at each of the 103 operating reactors at the time of the report.)

The EPRI study assumes an annual dose of 0.12 person-rem per storage site per year for inspection and security surveillance activities and 1.5 person-rem per year per site for operations and maintenance, for a total of 1.62 person-rem per site for year (p. 2-29). According to the 2012 GAO report there are about 60 people per reactor involved in this work. (Footnote 41 states that an increase of 1% over the current set of workers who load casks represents 63 people, implying a current workforce of 6300 across the industry, or about 60 at each of the 103 operation reactors.) That leads to the average annual, per-worker dose of 0.027 rem.