Lessons Learned from the Fukushima Nuclear Accident for Improving Safety and Security of U.S. Nuclear Plants Phase 2 (2016) / Chapter Skim
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2 Fukushima Daiichi Nuclear Accident: Lessons Learned for Spent Fuel Storage
2 Fukushima Daiichi Nuclear Accident: Lessons Learned for Spent Fuel Storage
Pages 19-74
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From page 19... ...
, • In a common pool, and • In a dry cask storage facility. Table 2.1 provides information about the quantities of fuel being stored at the plant and the decay heat in the spent fuel pools.
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From page 20... ...
Their spent fuel pools are located on the fifth-floor refueling decks in the upper portions of the reactor buildings (Figure 2.2)
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From page 21... ...
. a Actual pool volumes are slightly larger than the volumes reported here because of the displacement of the fuel and racks and the control of water levels below the top of the pool.
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. FIGURE 2.3 Typical BWR spent fuel rack with fuel assemblies.
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Water levels (m) relative to the bottom of the spent fuel pool are shown on the right-hand side of the figure.
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Their refueling decks were configured as shown in Figure 2.2. The gates separating the spent fuel pools from the reactor wells were closed, and their reactor wells and dryer-separator pits were dry.
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From page 25... ...
2.2 IMPACTS OF EARTHQUAKE AND TSUNAMI ON THE UNIT 1-4 SPENT FUEL POOLS NRC (2014) provides a discussion of key events at the Fukushima Daiichi plant following the March 11, 2011, earthquake and tsunami.
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From page 26... ...
The loss of AC and DC power and cooling functions also affected the Unit 1-4 spent fuel pools: The pools' FPC systems, secondary cooling systems, and pool water-level and temperature instrumentation became inoperable. High radiation levels and explosion hazards prevented plant personnel from accessing the Unit 1-4 refueling decks.
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From page 27... ...
TEPCO added the Unit 3 pool to its priority list on the morning of March 16 after steam7 was observed billowing from the top of the Unit 3 reactor building. Details about key events in the Unit 1-4 spent fuel pools and operator responses are described in the following sections.
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From page 28... ...
A participant on the call commented that "Right now, Unit 4 doesn't have a spent-fuel pool anymore. It appears that the walls have crumbled and you've just got fuel there." .
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From page 29... ...
The Japanese government issued a measured public response to the U.S. government announcements about conditions in the Unit 4 spent fuel pool:f "Because we have been unable to go the scene, we cannot confirm whether there is water left or not in the spent fuel pool at Reactor No.
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From page 30... ...
FIGURE S2.1 Photograph of Unit 4 obtained during the March 16, 2011, helicopter overflight of the Fukushima Daiichi site. The photo shows the refueling deck region in the vicinity of the spent fuel pool.
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From page 31... ...
This system used the spent fuel pool heat exchanger and pump and a temporary air-cooled heat exchanger and closed-loop circulation system outside the building. TEPCO estimated that the water level in the Unit 1 pool decreased by 3 m (to about 4 m above the top of the racks)
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From page 32... ...
If TEPCO's sloshing loss estimates are included and no credit is given for water additions by the concrete pump truck, then water levels could have been as low as 2.9 m above the top of the racks.10 2.2.2 Unit 2 Pool There was no explosion in the Unit 2 reactor building, but a blowout panel on the east side of the refueling deck became dislodged after the Unit 1 explosion on March 12, 2011. Steam was occasionally observed to emerge from this opening.
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From page 33... ...
The temperature fluctuations were likely caused by changes in pool water levels: When the pool was full, the temperature sensor was submerged and indicated the water temperature; when the water level dropped, the sensor was exposed and indicated the air-vapor temperature near the top of the pool. 2.2.3 Unit 3 Pool The explosion in the Unit 3 reactor building on March 14, 2011, damaged the northwest side of the fifth floor, collapsing the steel and concrete structure.
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From page 34... ...
to pinpoint the origin of the steam: It was found to originate from near the reactor well cover and the dryer-separator pit, not the spent fuel pool. Thermal imaging on March 20, 2011, showed that the pool water temperature was about 60°C, whereas temperatures adjacent to the reactor cover and dryer-separator pit measured by thermal imaging were over 100°C.
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From page 35... ...
If TEPCO's sloshing losses are included, then water levels could have been as low as 3.0 m above the top of the racks if no
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From page 36... ...
of this hydrogen inside the spent fuel pool building can damage the structure and provide a pathway for radioactive material releases into the environment. Further temperature increases can drive more volatile fis sile products out of the fuel pellets and cause the fuel rods to buckle, resulting in the physical relocation of rod segments and the dispersal of fuel pellets within the pool.
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From page 37... ...
had been removed from the reactor and placed in the pit, and the reactor core comprising 548 fuel assemblies had been removed from the reactor and placed into contiguous racks in the spent fuel pool (Figure 2.10)
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From page 38... ...
Quarterly inspections have been carried out since May 2012 and no significant issues have been found. The Unit 4 pool's inventory of 1,535 fuel assemblies was moved into the common pool (spent fuel)
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From page 39... ...
The damage to the Unit 3 and 4 building structures and steam emissions from both buildings raised grave concerns about the spent fuel pools in those units (see Sidebars 2.1 and 2.3)
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If release from other spent fuel pools occurred, then contamination could extend as far as Tokyo, requiring compulsory evacuations a NAIIC (2012) describes the circumstances of the report (see p.
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From page 41... ...
. This study assumed a source term of the entire core of the Unit 2 reactor over 24 hours as well as 50 percent of the Unit 3 spent fuel pool and 100 percent of the Unit 4 spent fuel pool over 48 hours.
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From page 42... ...
This is almost three times the volume of the Unit 4 building above the refueling deck. TABLE S2.1 Estimated Mass of Hydrogen Production by Oxidation of Spent Fuel Assemblies in the Unit 4 Spent Fuel Pool at the Fukushima Daiichi Plant Zr Oxidationa H2 Produced No.
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From page 43... ...
Consequently, plant operators shifted their attention to the Unit 3 pool from March 16 to March 20. The extensive visible damage to the Unit 4 reactor building and high level of decay heat in the Unit 4 pool continued to drive concerns about pool water levels.
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that as water levels in the pool dropped because of evaporation, forces on the gates from water in the reactor well caused leakage around the gate seals, allowing additional water to enter the pool. This process is depicted in Figure 2.14.
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From page 45... ...
The solid lines are TEPCO-modeled water levels and temperatures. SOURCE: TEPCO (2012a, Attachment 9-5, Figure 2)
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From page 46... ...
Visual inspection of the fuel assemblies removed from the Unit 4 pool did not reveal any damage. 2.3 COMMITTEE ANALYSIS OF UNIT 4 POOL WATER LEVELS TEPCO did not provide enough technical documentation for its waterlevel estimates in the Unit 4 spent fuel pool (Figure 2.13)
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From page 47... ...
Water was assumed to flow in one direction from the reactor well to the spent fuel pool and only if the reactor water level was higher than the pool level; see Appendix 2C for further discussion of this issue. The committee's estimates for the Unit 4 pool and reactor well and dryer-separator pit water levels are indicated by the blue and orange curves, respectively, in Figure 2.15.
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From page 48... ...
. • Pool water levels dropped to less than 2 m above the tops of the racks on April 13 and again on April 20, 2011.23 The water-level drop between April 4 and April 12 was a consequence of insufficient water injection amounts because TEPCO evaluated the need for additional water based on unreliable instrumentation for measur ing water levels in the pool.
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From page 49... ...
The ORNL and com mittee estimates indicate that water levels ranged between 1 and 2 m above the top of active fuel between April 10 and 22; TEPCO estimates that water levels never fell below 2 m above the top of active fuel. These differences are apparently due to TEPCO's assumption that the reactor well and spent fuel pool were "hydraulically connected" before April 22.
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From page 50... ...
As noted previously, the water-level indicator in the skimmer surge tank provided misleading information on pool water levels prior to this date.
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The results are shown by the black curve in Figure 2.15. Without water leakage, pool water levels could have dropped well below the top of active fuel (located 4 m above the bottom of the pool)
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From page 52... ...
and had there been no water leakage from the reactor well and dryer-separator pit. The committee estimates that pool water levels would have reached 50 percent of fuel assembly height (Figure 2.17)
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From page 53... ...
at the Fukushima Daiichi plant maintained their containment func tions during and after the March 11, 2011, earthquake and tsunami. However, explosions in the Unit 1, 3, and 4 reactor buildings dam aged spent fuel handling facilities and equipment, introduced heavy debris into the pools, and provided enhanced pathways for releases of radioactive materials from the damaged reactors into the environment.
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From page 54... ...
The recommended improvements in plant monitoring systems include the following: • Remote surveillance of pools and refueling decks, • Radiation levels on the refueling deck, • Pool temperatures, and • Pool water levels. The lack of reliable information on pool water levels and temperatures at the Fukushima Daiichi plant created unnecessary anxiety about the condition of the stored spent fuel and may have also created false priorities for allocating resources.
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From page 55... ...
The systems must be seismically rugged, must be operable under severe accident conditions,27 and must be installed so that they have reasonable protection in case of damage to the structure over the pool. They must also be designed so that spent fuel pool water levels can be read from the control room, an alternate shutdown panel, or other accessible locations.
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From page 56... ...
The U.S. nuclear industry is already making good progress in improving the ability of plant operators to maintain adequate cooling of stored spent fuel during severe accidents or terrorist attacks: • Under its B.5.b initiative,29 the industry has pre-positioned equip ment and developed procedures to add makeup water to spent fuel pools and cool the stored fuel assemblies with water sprays (see Chapter 3)
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From page 57... ...
Solutions that avoid these problems include the concrete pump trucks employed at the Fukushima Daiichi plant or pre-positioned nozzles (protected from falling debris) that can be remotely operated using water connections external to the spent fuel enclosure or reactor building.
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From page 58... ...
the report goes on to identify the most prevalent reason for loss of pool inventory was leaking fuel pool gates. Given the potential for gate leakage under normal operations it is not surprising that it is also an issue under severe accident conditions.
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From page 59... ...
2A.2 COMMON SPENT FUEL POOL The earthquake and tsunami knocked out power for the FPC system, causing pool water temperatures to rise to about 73°C before the power was restored on March 24 (TEPCO, 2012a, p.
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60 LESSONS LEARNED FROM THE FUKUSHIMA NUCLEAR ACCIDENT: PHASE 2 were damaged. However, the casks were not damaged or displaced, and air flows were not significantly obstructed (TEPCO, 2012a, p.
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From page 61... ...
• Water evaporated from the pool is completely dispersed into the atmosphere and does not condense on the remaining structure and flow back into the pool. As a consequence of these simplifying assumptions, the model results are limited in applicability to pool water levels above the top of the fuel racks, and the quantitative results have an associated uncertainty that the committee has not characterized in detail.
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From page 62... ...
and are modest but not negligible. The committee has used the values1 proposed by TEPCO for the purposes of estimating approximate times for heat-up and evaporation of spent fuel pools.
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From page 63... ...
In certain MELCOR simulations, the net effect is to decrease the rate at which the pool water level decreases by up to a factor of 2 over the situation in Unit 4 where the building structure was demolished (Gauntt et al., 2012, p.
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From page 64... ...
is assumed to be constant at 4.184 kJ/kg K and the heat of vaporization is also assumed to be constant, ΔHfg = 2.3 MJ/kg. At Fukushima, the added water temperature was about 10°C, and the initial pool temperatures were about 30°C before the loss of pool cooling.
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From page 65... ...
Without mitigation, the pool will heat up until a quasi-steady-state equilibrium condition is reached in which the rate of energy loss due. to vaporization balances the thermal power input from the spent fuel, Qsf, correcting .for the heat lost by radiation, conduction, and convection with the term Qloss, which is estimated using engineering correlations2 for heat transfer: .
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From page 66... ...
2B.1 MODEL RESULTS FOR FUKUSHIMA POOLS The committee applied a simplified version of this model to the initial stage of a Fukushima-like scenario of loss of pool cooling without mitigation or leakage. The energy addition to pool water is divided into two 4 The committee's computation used the "stagnation film boundary layer" mass transfer analysis accounting for induced convection and the significant mass fraction of water in the region above the pool surface; see the discussion by Lienhard and Lienhard (2015, Section 11.8)
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From page 67... ...
First, pool water heats up until the surface reaches the equilibrium temperature with negligible loss of mass: dT !
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From page 68... ...
The amount of water loss, ΔMmid, corresponding to a reduction in pool collapsed water level to the midplane of the fuel will depend on the occupancy of each pool. Accounting for the reduction in pool volume within the rack area, we estimate values of about 700 tonnes for Unit 1, 950 tonnes for Units 2-6, and 2,400 tonnes for the common pool.
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From page 69... ...
Unfortunately, the status of the pools was initially unknown so that, while these estimates are straightforward, it was difficult for the Fukushima Daiichi plant operators and TEPCO to have confidence in these results in the absence of any measurements of pool level and temperature. To evaluate alternate outcomes of the Fukushima accident for Unit 4, one must evaluate the thermal power in the pool for times earlier than March 11.
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From page 70... ...
ΔH fg Madd = C ΔT 1+ p ΔH fg As expected, the mass added must be sufficient to compensate for both the leak as well as the evaporation. The term in the denominator accounts for the thermal power used to heat the cooler added water to the pool temperature; evaluating this for a pool temperature of 90oC, we find that the FIGURE 2B.5 Wigner-Way model of decay heat for the Unit 4 reactor core after reactor shutdown at the end of November 2010.
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From page 71... ...
LESSONS LEARNED FOR SPENT FUEL STORAGE 71 denominator is less than or equal to 1.1 and makes a negligible correction to the intuitive result, !
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From page 72... ...
, leakage can occur when the water level in the reactor well is higher than in the spent fuel pool, that is, when ΔP is positive. The force of water created by a sufficiently higher level on the pool side than the well side pushes the gate toward the well side and squeezes an elastomer seal to stop the flow of water out of the pool.2 When the water level on the well side is higher than in the pool, the gate is mounted such that the force due to the difference in water level can displace the gate sufficiently that the seal is not effective and water will flow from the well into the pool; see the discussion in TEPCO (2012a, Attachment 9-1, p.
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From page 73... ...
1000 8 mass (tonne) 800 6 600 4 400 2 200 0 0 3/11/11 0:00 3/21/11 0:00 3/31/11 0:00 4/10/11 0:00 4/20/11 0:00 4/30/11 0:00 5/10/11 0:00 time FIGURE 2C.2 Committee model for Unit 4 pool water level showing the effect of assuming four sizes of the gate passage opening on pool water-level history as well as assuming that prior to April 22 for one case (5 mm)
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From page 74... ...
In Figures 2.15 and 2.16, a value of 1.0 mm for the effective width of the flow-passage area was used and the height of the flow passage was taken to be equal to the difference in levels between the reactor well and spent fuel pool. Once the leakage rate is computed, water levels in the reactor well and dryer-separator pit can be estimated using the mass balance as discussed in Appendix 2B.
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