Safety and Security of Spent Fuel Storage in the United States Edwin Lyman Senior Scientist Union of Concerned Scientists

Transcription

1 Safety and Security of Spent Fuel Storage in the United States Edwin Lyman Senior Scientist Union of Concerned Scientists Presentation to the NAS Fukushima Lessons Learned Panel (Phase 2) Washington, DC January 29, 2015

2 UCS s bottom line The current practice of high-density spent fuel pool storage is dangerous and puts the health and safety of the U.S. public, as well as the U.S. economy, at needless risk The analysis conducted by the NRC to justify continuing this practice is deeply flawed Expedited transfer of spent fuel to dry casks (eg. reduction of spent fuel pool density to below currently licensed limits) is a prudent, passive, defense-in-depth measure for reducing risk from accidents and attacks 2

3 The risk High-density storage significantly increases the risk (probability x consequences) of a spent fuel fire caused by a loss-of-coolant accident (LOCA) relative to low-density storage LOCAs can result from A catastrophic accident A terrorist attack 3

4 Post-9/11 actions The Nuclear Regulatory Commission (NRC) imposed additional requirements after the 9/11 attacks to reduce vulnerability of high-density pools Direct placement of discharged fuel into a 1x4 (dispersed) configuration where feasible and practical, consistent with safe handling practices No later than 60 days* after subcriticality if not immediately feasible and practical NRC also accepts alternate strategies for timing mitigation strategies (emergency makeup water or sprays) *FOIA/PA

5 Phase I Guidance Expectation B.2.m.1 Licensees are expected to put spent fuel in a I x 4 repeating pattern or equivalent, unless otherwise proven to be not applicable or achievable. Licensees who choose to conform to the NRC-approved resolution (NRC letter dated March 16, 2006 (ML )) are expected to Include the following concept in procedures: "Where feasible and practical, consistent with safe fuel handling practices, the licensee should make every attempt to pre-configure the spent fuel pool to enable direct placement of the expended assemblies from the vessel to the final distributed fuel pattern. Where this is not feasible or practical, licensees should distribute the fuel into the final pattern as soon as possible but no later than 60 days after subcritcality." NRC staff also accepted alternate strategies for the timing to achieve the appropriate pattern, which may be discussed in the site specific inspection assessments. Licensees adopting the use of the NRC approved resolution documented their plans in a letter to the NRC. FOIA/PA

6 Spent Fuel Pool Study NRC undertook another review of spent fuel pool safety after Fukushima, the Spent Fuel Pool Study (SPFS) (2013) Looked at the consequences of a large earthquake at the Peach Bottom boiling-water reactor (BWR) for both high-density 1x4 and low-density spent fuel pools Did not consider terrorist attacks NRC concluded that high-density pool storage is safe and has decided not to mandate expedited transfer 6

7 Spent fuel pool fires If water is drained from a spent fuel pool, the fuel assemblies will be exposed to air and/or steam When Zircaloy cladding reaches C, it can burn, increasing the heatup rate For some conditions the fire can propagate to cooler assemblies A large fraction of the fission product inventory (mainly Cs-137) can be released The structures housing spent fuel pools are not leaktight and are vulnerable to hydrogen explosions (if steam is present) 7

8 A mock spent fuel assembly after a fire test

9 Three numbers Estimated atmospheric Cs-137 release from Fukushima Daiichi: 0.5 MCi Peak release estimate, low-density pool fire, SFPS: 0.33 MCi Peak release estimate, high-density 1x4 pool fire, SFPS: 24.2 MCi 9

10 Cesium contamination 10

11 From March 25, 2011 Department of Energy document (Freedom of Information Act release to UCS)

12 Three more numbers Estimated collective dose to Japan from Fukushima Daiichi: 48,000 person-sievert (UNSCEAR) Average collective dose for low-density pool fire: 22,200 person-sievert (0.1 MCi Cs-137) Average collective dose, high-density 1x4 pool fire: 856,000 person-sievert (24.2 MCi Cs-137) 12

13 Four more numbers Estimated cost of decontamination and compensation after Fukushima: $ billion [NAS(2014), App. L] Average cost, low-density pool fire: $5.4 billion Average cost, high-density 1x4 pool fire: $856 billion Estimated cost per plant for expedited transfer: $42-46 million 13

14 Complete and partial LOCAs A moderate leak in the liner at the bottom of the pool can cause a complete LOCA Cooling by natural circulation of air A small bottom leak or a leak on the side wall can cause a partial LOCA Less effective steam cooling of exposed parts of assemblies 14

15 Coolable configurations A spent fuel pool is said to be coolable during a LOCA if zirconium ignition does not occur, either with or without mitigation Coolability is a function of Time since last refueling Degree of dispersal of hottest fuel Pool configuration (coherent downcomer) Type of LOCA 15

16 Uniform 1x4 1x8 16

17 Checkerboard 17

18 Coolability In the event of a complete LOCA, studies* have found that pools will be passively air-coolable after certain time periods have elapsed following a refueling outage: eg. Uniform: 310 days Checkerboard : 117 days 1x4: 20 days * from Ghani Zigh to Brian Sheron and Patricia Santiago, March 19, 2011, In the SPFS, 1x4 coolability limit was between 37 and 107 days 18

19 Coolability and partial LOCAs In the event of a partial LOCA, there is no passive coolability limit, even for well-dispersed configurations However, the NRC SPFS considered only a complete LOCA because its analysis only evaluated an earthquake, which would only rupture the pool liner at the bottom By excluding partial LOCAs, NRC concludes that 1x4 pools are coolable for 90% of their operating cycles 19

20 What is the baseline? The SFPS assumes that the baseline spent fuel pool configuration for the current fleet is a 1x4 dispersed configuration But it is not even clear how rapidly licensees are able to place discharged fuel into a 1x4 configuration after shutdown In the SFPS, the NRC refused to disclose the nominal length of time it allowed licensees to maintain non-dispersed configurations the specific time requirement is not publicly available information (because it could be useful to an adversary) We now know it is 60 days It is not hard for terrorists to figure out when plants are in refueling outages For a significant period of time after refueling, some high-density pools may be in high-risk uniform configurations 20

21 What is the baseline? From SECY (Nov 28, 2014) Based on an informal staff assessment of licensee SFP operational practices, the staff found that licensees generally place spent fuel intended for permanent discharge into a final dispersed (e.g. 1x4 pattern) immediately. However, nondispersed configurations in the SFP are a common occurrence at PWR sites during refueling outages when the fullcore is discharged into the SFP duration has been on the order of 1 to 2 weeks. 21

22 What is the baseline? But in an Information Notice to licensees on Nov. 14, 2014 (IN ), the NRC suggested that Licensees may choose to optimize spent fuel transfer into the SFP by direct placement in a dispersed pattern to further enhance safety The public does not know how many plants have periods of operational vulnerability and for how long 22

23 Security and defense-in-depth The SFPS demonstrates the danger of uniform high density pools Mitigated risk within 10 mi is 10 x greater for a uniform highdensity pool than for 1x4 Land interdiction area is 78 times greater for uniform highdensity pool than low-density pool without mitigation Transition to low-density pools could increase defensein-depth by greatly reducing the consequences of a terrorist attack that occurs soon after an outage or one that causes a partial LOCA reducing reliance on mitigation 23

24 Mitigation Mitigating measures involve highly risky manual actions to provide makeup water to spent fuel pools, as was seen during Fukushima Mitigation may make the situation worse if water is added after a complete LOCA occurs The NRC seems content to rely on mitigating actions to cope with spent fuel LOCAs 24

25 Dry casks: Tomorrow s passive technology today Dry cask storage and low-density pool storage achieve features the NRC encourages in advanced reactors (Advanced Reactor Policy Statement): Highly reliable and less complex shutdown and decay heat removal systems. The use of inherent or passive means to accomplish this objective is encouraged. Simplified safety systems that reduce required operator actions, equipment subjected to severe environmental conditions, and components needed for maintaining safe shutdown conditions. Designs that minimize the potential for severe accidents and their consequences 25

26 Dry cask security Dry casks generally pose a far less catastrophic sabotage risk than spent fuel pools But they are vulnerable to certain types of attack modes This threat should be countered by A denial of access strategy (extending the protected area to dry cask storage pads) Improved cask design Additional physical barriers 26

27 A flawed methodology NRC Staff non-concurrences to the SPFS question use of reactor-focused regulatory analysis guidelines Cost-benefit analysis does not give adequate weight to features such as Impacts beyond 50 miles Defense-in-depth Less-readily quantifiable aspects of land contamination Security considerations Value of a statistical life ($2000/person-rem) is antiquated and inconsistent with other agencies guidance: revision of this factor is delayed 27

28 A flawed methodology NRC regulatory analysis guidelines call for basing decisions of average values Sensitivity analyses are just for show Conclusions based on average values may not lead to sufficiently protective regulations For example, the concentrated plume of contamination NW of Fukushima was not the result of average conditions Peak dose and cost typically 3-5 times mean values 28

29 Safety goal screening is not a litmus test From the Regulatory Analysis Guidelines (NUREG/BR-0058) The NRC recognizes that not all regulatory actions are amenable to a quantitative risk assessment and that certain evaluations may be based directly on engineering or regulatory judgment or qualitative analysis. Safety goals are to be used as a reference point in ascertaining the need for safety enhancements. However, the safety goals are not requirements and, with the Commission s approval, safety enhancements may be implemented without strict adherence to the Commission s safety goal policy statement. 29

30 Hydrogen control The NRC does not give credit to low-density pools for the low risk of hydrogen generation and combustion Only high-density scenarios produced sufficient hydrogen for an explosion Avoidance of hydrogen explosions is beneficial not only for reducing population dose but also for reducing occupational hazards, accidents affecting the reactor, multi-unit accident risk, and site cleanup and decommissioning 30

31 1x8 pattern The SFPS found that a 1x8 dispersed pattern was more favorable than the 1x4 pattern regarding LOCA risk Reduced non-coolable time period Reduced fission product releases (factor of 10) Even though the cost of achieving a 1x8 is expected to be considerably less than expedited transfer, the NRC staff still rejected consideration of a potential 1x8 requirement because of its opinion that the cost still wouldn t be worth the marginal safety benefit (no actual data was available) 31

32 A new framework The NRC should evaluate expedited transfer using revised regulatory analysis guidelines that give more credit to defense-in-depth, economic consequences of land contamination, and avoidance of hydrogen generation 32