Introduction to Earthquake-Resistant Design of Nuclear Power Plants

Abstract

The aim of the earthquake-resistant design of nuclear power plants is to retain three crucial functions, even in the event of a major earthquake and tsunami: to shut down the reactor (shut down), to cool down the reactor under a specified temperature and maintain a stable condition (cool down), and to confine so as to prevent radioactive materials from being released into the surrounding environment (confine). This chapter explains the mechanism of nuclear power generation and the safety assurance of nuclear power plants, and gives an overview of the earthquake-resistant design aiming to retain the three crucial functions, shut down, cool down, and confine. Furthermore, the damage to the Kashiwazaki-Kariwa Nuclear Power Plant caused by the 2007 Niigata-Ken Chuetsu-Oki earthquake and the catastrophic disaster that affected the Fukushima Daiichi Nuclear Power Plant as a result of the 2011 Great East Japan earthquake are described.

Appendices

Appendix 1.1: Boiling Water Reactor and Pressurized Water Reactor

• 1. BWR

  In a BWR, electricity is generated by turbine generators driven by the steam produced by boiling the reactor water (light water). First, primary coolant (light water) is brought into the reactor pressure vessel. The water boils among the fuel rods, evaporates, and produces steam. The steam is then directly sent to the turbine generators and used to rotate the generators. After rotating the generators, the steam is sent to the condenser, where it is cooled and returned to water, which is sent to the reactor pressure vessel again as primary coolant. The water flowing into the reactor pressure vessel is circulated by the recirculation pumps, and the reactor output is controlled by increasing/decreasing the recirculating flow rate and by inserting/withdrawing control rods.

  A characteristic of BWRs is that radiation control is necessary also in the turbine building because the steam that is sent to the turbine generators contains radioactive materials. Figure 1.10 shows the mechanism of a BWR.

  Fig. 1.10

  

  Mechanism of a BWR [6]

  A more recent type of reactor is the advanced boiling water reactor (ABWR). The ABWR offers the advantage of having a lower center of gravity and higher flexibility in the layout of equipment through the use of a reinforced concrete containment vessel instead of the steel containment of conventional BWRs. Additionally, with the use of internal pumps, the reactor building is more compact than the conventional type.

• 2. PWR

  In a PWR, electricity is generated by sending heated and pressurized reactor water (light water) to the steam generators. In the steam generators, the water from a different system is transformed to steam. The steam then rotates the turbine generators to produce electricity. The primary coolant system in which the water circulates in the reactor, and the secondary coolant system in which the steam produced in the steam generators is sent to the turbine generators, are separated by the steam generators.

  In the primary coolant system, light water sent to the reactor vessel by the heat generated by the fission reaction of nuclear fuel becomes hot among the fuel rods and is sent to the steam generators. The water sent to the steam generators is circulated by coolant pumps. The reactor output is controlled by inserting/withdrawing control rods and adjusting the concentration of boric acid dissolved in the coolant.

  In the secondary coolant system, the water on the turbine generator side is transformed to steam by the high-pressure hot water of the primary coolant system that is sent to the steam generators. The steam is then sent to the turbine generators to rotate the turbines. After rotating the generators, the steam is cooled by sea water in the condenser and returns to water, and then is sent back to the steam generators.

  Characteristics of PWRs include that the radiation control area is smaller than that of BWRs because the steam containing radioactive material does not flow into the turbine, and that heat exchange from the primary coolant to the secondary coolant is necessary in the reactor building. Moreover, a high pressure of 15 MPa is applied to the primary coolant system by the pressurizer, and the temperature reaches up to approximately 320 °C. PWRs are characterized by the high pressure and high temperature of the primary coolant; the pressure of the primary coolant in the BWR reactor pressure vessel is approximately 7 MPa, and the temperature is approximately 280 °C. Figure 1.11 shows the mechanism of the PWR.

  Fig. 1.11

  

  Mechanism of a PWR [6]

  PWR technology is being developed to raise the performance of the reactor core system and thus increase the output and duration of cycle operation. There are several plants with advanced PWR systems under construction around the world.

Appendix 1.2: Accident at the Kashiwazaki-Kariwa Nuclear Power Plant by the 2007 Niigata-Ken Chuetsu-Oki Earthquake

At 10:13 on July 16, 2007, a moment magnitude (M _w) 6.8 earthquake centered at a depth of 17 km off the coast of Chuetsu in Niigata Prefecture struck. In the cities of Nagaoka, and Kashiwazaki, and the village of Kariwa in Niigata Prefecture and in the town of Iizuna in Nagano Prefecture, the seismic intensity measured upper 6 on the Japan Meteorological Agency scale (MMI, Modified Mercalli Intensity: IX). In the cities of Joetsu, and Ojiya, and the town of Izumozaki in Niigata Prefecture, seismic intensity of lower 6 (MMI: VIII) was measured. In addition to these cities and towns, shaking was measured in much of the western and northern regions of the main island of Japan, but also extending to the Tohoku, Kinki, and Chugoku regions. A tsunami with a height of 32 cm to 1 m was recorded at Kashiwazaki.

Among the seven reactors of the Kashiwazaki-Kariwa Nuclear Power Plant, the reactors under operation or in the process of startup at the time of the earthquake (Units 2, 3, 4, and 7) all automatically shut down safely, and reached a cold shutdown status after cooling and depressurization. Confinement of radioactive material in fuel was maintained in all seven reactors, including the reactors of other units (i.e., Units 1, 5, and 6). Thus, the most important functions required in a time of emergency to ensure reactor safety—shut down, cool down, and confine—were secured.

The acceleration measured at the plant during the earthquake largely exceeded the maximum acceleration assumed in the design phase. For example, a maximum acceleration of 680 gal was recorded in the fifth basement of the Unit 1 reactor building (on the foundation rock), whereas the maximum acceleration assumed in the design phase was 273 gal. The measurements at Units 1–4 were approximately six times the average ground motion evaluated using the epicentral distance attenuation formula, and the measurements at Units 5–7 were approximately three times the design ground motion. However, no damage due to the shaking of the earthquake was found.

The analysis of the records indicates the following causes of the large ground motion.

• 1. The seismic source caused ground motion 1.5 times that of an earthquake of the same magnitude (owing to the effect of the source properties).

• 2. The ground motion was amplified by the effect of the irregularity of deep geological structures (approximately 4–6 km deep).

• 3. There was a difference in the amplification of the ground motion at the site owing to the presence of an old fold structure in the shallow ground (approximately 2–4 km deep).

Major damage caused by the earthquake included a fire at the Unit 3 onsite transformer (Fig. 1.12). The fire resulted from insulation oil leaking from the transformer and being ignited by an arc induced by a short circuit. Although the transformer itself was supported by foundation piles, the cable duct was supported by a flat foundation. As a result, it was reported that only the foundation of the duct subsided because of earthquake ground motion, and this damaged the bushing (which takes in electric cables from the outside to the equipment such as transformers, and insulates and supports the cables from the equipment and walls), and caused an arc, which ignited the leaked insulation oil.

Fig. 1.12

View of the transformer fire [7]

Additionally, owing to the long-period component of earthquake motion, there was sloshing vibration in Units 1–7, and the spent fuel pool water flooded the units (Fig. 1.13). In Unit 6, the flood water flowed into the discharge water tank (through cable holes, for example), which does not contain radioactive material, and was released into the sea. The amount of radiation release and the radiation dose were approximately 9 × 10⁴ Bq and approximately 2 × 10⁻⁹ mSv, respectively.

Fig. 1.13

View of flooding from the spent fuel pool [7]

Additional reported damage was a trace amount of radioactivity being released from the ventilation duct of Unit 7, the failure of a joint of the reactor building ceiling crane of Unit 6, the opening of a blowout panel of the reactor building of Unit 3, and the misalignment of the main ventilation ducts in Units 1–5 (i.e., air conditioning ducts for sending the air exhausted from reactor buildings to air vents). It was also reported that damage to the administrative building and the resulting deformation of the door to the emergency response center prevented personnel from entering the room, and that failure of the facilities inside the room made it difficult to establish the response organization immediately after the earthquake.

Appendix 1.3: Accidents at the Fukushima Daiichi Nuclear Power Plant by the 2011 Great East Japan Earthquake

When the 2011 Great East Japan earthquake struck, Units 1–3 of the Fukushima Daiichi Nuclear Power Plant were operating at the rated output, while Units 4–6 were undergoing periodic inspection. Units 1–3 automatically tripped in response to the earthquake ground motion. At that time, a total of six lines were connected to offsite power sources from the plant, but they all stopped supplying electricity because of, for example, damage to breakers and the collapse of transmission line towers due to the earthquake ground motion. Therefore, the emergency generators of each unit started operation. However, when the tsunami struck the site several dozen minutes later, the sea water pumps for cooling, emergency diesel generators, and switchboards were flooded, causing all emergency diesel generators except that in Unit 6 to stop, and all units except Unit 6 suffered a blackout. In Unit 6, one emergency diesel generator (air-cooled) and the switchboard were not flooded and continued to operate. Additionally, the equipment of the residual heat removal system that releases residual reactor heat to the sea stopped functioning because the sea water pumps for cooling were flooded by the tsunami.

As the emergency core cooling function powered by AC was lost in Units 1–3, attempts were made to actuate the core cooling function using DC power sources. Specifically, the isolation condenser of Unit 1, the reactor core isolation cooling system of Units 2 and 3, and the high-pressure coolant injection system of Unit 3 were actuated. However, the core cooling functions of these DC power sources later stopped when their batteries ran out. Thus, core cooling was switched to the alternate means of injecting fresh water or sea water from the fire extinguishing system line, using fire extinguishing pumps.

As the pressure inside each reactor was very high and water could not be injected into each reactor pressure vessel for some time, the nuclear fuel in the core of each unit started to be exposed, resulting in core meltdown; some of the molten fuel accumulated in the lower part of the reactor pressure vessel. Meanwhile, a huge amount of hydrogen was generated by the chemical reaction between the zirconium contained in the fuel cladding and the steam. The damaged fuel cladding caused the radioactive materials inside the fuel rods to be released to the outside of the reactor pressure vessels. Moreover, in the process of depressurizing the reactor pressure vessels, the hydrogen and radioactive material were released to the containment.

The water injected into the reactor pressure vessel was turned into steam by the heat generated by the nuclear fuel and the pressure inside the reactor pressure vessel continued to rise without core cooling. Therefore, the steam was released to the containment through safety valves. Although the containment can be ultimately cooled down by the cooling system using sea water as a cooling source under normal conditions, this was not possible owing to the damage and collapse of the sea water system components, and the units lost their ultimate heat sink status. Consequently, the temperature and pressure inside the containment gradually rose. In Units 1–3, several attempts were made to release the gas inside the containment from the vapor phase (i.e., the part above the water surface) of the suppression chamber (a water-cooled facility that restricts the pressure rise in the containment caused by, for example, steam pressure) into the atmosphere through air vents (i.e., containment wet-well vents).

At Units 1 and 3, explosions thought to be caused by the hydrogen that leaked from the reactor containment occurred in the upper part of the reactor building after the containment wet-well vent. As a result, the top floor of each reactor building was blown away, causing significant amounts of radioactive materials to be released into the atmosphere. Subsequent to the explosion in the building of Unit 3, there was also an explosion, considered to be caused by hydrogen too, in the reactor building of Unit 4, where all fuel in the core had been conveyed to the spent fuel pool for periodic inspection, and the top of the reactor building was destroyed. At the same time, it was reported that a failure occurred in a location presumed to be around the suppression chamber of the reactor building of Unit 2 (Fig. 1.14).

Fig. 1.14

Status of the accident at the Fukushima Daiichi Nuclear Power Plant [5]

Attempts were also made to inject water into the spent fuel pools in Units 1–4 in conjunction with the continuous efforts to recover the power sources and to inject water into the reactor pressure vessels. The water level of the spent fuel pool of each unit kept falling owing to the evaporation of water by the heat generated from the spent fuel as the function to cool the pool water had been lost through the loss of power sources. Therefore, water was dropped into the spent fuel pools by helicopters and water cannons by the self-defense forces, fire brigade, and police. After all these attempts, concrete pump trucks were used to inject fresh water from a nearby reservoir to cool the pools.

The maximum acceleration of 550 gal that was observed on the foundation ground (i.e., the bottom of the fourth floor of the reactor building of Unit 2) exceeded the basic earthquake ground motion for the original design S _s, but most other measured accelerations were below the design acceleration.

The damage to facilities caused by the earthquake ground motion could not be clearly confirmed owing to the tsunami that struck approximately 40 min after the earthquake. Tokyo Electric Power Co., Inc. stated that “it is considered that major facilities maintained their safety-related functions during and immediately after the earthquake,” on the basis of instrument measurements showing the plant status and alarm issuance records, dynamic response analysis of the reactor buildings and major facilities using observed acceleration on the basement of the buildings, and onsite visual inspections. In this regard, all the reports on the accident made by various organizations concluded that the direct cause of the accident was the blackout caused by the tsunami, and only the Independent Investigation Commission of the National Diet questioned limiting the accident cause to the tsunami, stating “it cannot be assumed that important safety facilities were not damaged by the earthquake ground motion.”

Although the tsunami height for the original design was O.P. + 6.1 m, the actual tsunami height was O.P. + 13.1 m. The inundation height at the site was reported to be O.P. + 15.5 m around Units 1–4, and O.P. + 14.5 m around Units 5 and 6 (Fig. 1.15).

Fig. 1.15

Status of inundation due to the tsunami at the Fukushima Daiichi Nuclear Power Plant [8]

Damage by the tsunami included the loss of function of the emergency diesel generators and the switchboards due to flooding, and the loss of function of the residual heat removal system in the reactor and the cooling system due to the flooding of the sea water pumps. Thus, the power sources and most of the functions related to cooling were lost (Fig. 1.16). In addition, innumerable drifting objects, including a destroyed and drifting heavy oil tank and vehicles, filled the plant site and impeded emergency responses and activities.

Fig. 1.16

Status of damage due to tsunami at the Fukushima Daiichi Nuclear Power Plant [7]

Appendix 1.4: Status of the Onagawa Nuclear Power Plant by the 2011 Great East Japan Earthquake

When the Great East Japan earthquake struck, Units 1 and 3 were in operation and the reactor of Unit 2 was in the process of starting up at the Onagawa Nuclear Power Plant. The reactors of all units were automatically tripped by the limit signal from measured accelerations. Among the five supply lines from the offsite electric power sources, Matsushima Line No. 2 continued to function. Thus, Units 2 and 3 continued to receive offsite power and onsite electric power was secured.

In Unit 1, although offsite power could not be received as the startup transformer tripped in response to the earthquake, power was secured by the actuation of the emergency diesel generator. Additionally, the reactor core cooling system was successfully actuated using a DC power source and could be used to cool the reactor. Pressure was controlled by the main steam relief valve, and after depressurization, water was injected into the reactor by the control rod drive water pressure system. The suppression chamber and the reactor were cooled by the residual heat removal system, and approximately 12 h after the earthquake, the unit was brought to the cold shutdown status.

In Unit 2, the reactor was subcritical and the reactor water temperature was below 100°C before the earthquake struck. Therefore, the reactor was brought to the cold shutdown status by shifting the reactor mode switch to the shut down position. The cooling water system (B) of the reactor building and the cooling water system of the high-pressure core spray lost their functionality because the sea water flowing from the sea water intake canal lifted the lid of the level gauge box installed in the sea water pump room. The sea water then flowed into the underground trenches through the pipe penetrations after passing through the cable trays and finally poured into the reactor building. However, as another reactor cooling water system (A) remained intact, the reactor cooling function of the residual heat removal system (A) was secured.

In Unit 3, the level gauge of the sea water pump was damaged by the spilling waters of the tsunami, resulting in the tripping of the circulation pump. Furthermore, as the cooling sea water system of the turbine building lost its functionality owing to the influx of sea water, the reactor feed water pump was stopped and the reactor core isolation cooling system was started manually to cool the reactor. In addition, the pressure was controlled by the main steam relief valve, and the water was injected into the core after depressurizing the reactor with the makeup water condensate system. The suppression chamber and the reactor were cooled by the residual heat removal system, and approximately 13 h after the earthquake, the unit was brought to the cold shutdown status.

Additionally, although the cooling system for the spent fuel pool automatically tripped in response to the earthquake motion, it was restarted after confirming that there was no abnormality in the system, and no significant increase in the temperature of the fuel pool was observed.

Despite the confirmation of the fire at the high-pressure power panel in the turbine building of Unit 1, the failure of the glass windows in the fuel handling machine room in Unit 2, the opening of the blowout panel in the turbine building of Unit 3, and the collapse of the ceiling panel of the Unit 1 main control room in response to the earthquake, it was confirmed that important safety-related functions was properly maintained.

The height of the tsunami at the plant was O.P. + approximately 13 m at the maximum and did not exceed the ground height of the site (O.P. + approximately 13.8 m). (The ground level subsided by approximately 1 m owing to tectonic deformation, which is taken into consideration in the calculation of this value.) From the beginning of the design and construction of Unit 1, countermeasures against a tsunami were repeatedly discussed at survey meetings with external experts. As a result, the height of the plant site was set to be 14.8 m, reflecting the survey result that “the height of the plant site shall be the primary countermeasure against tsunami and O.P. + 15 m is considered to be a sufficient height.” This is one of the reasons why damage to the plant due to the tsunami was limited.

Although the tsunami caused some trouble for cooling water systems of the reactor and the turbine buildings, and automatic tripping of the circulation pump due to damage of the sea water pump level detector, the cooling function was successfully maintained. Additionally, although a heavy oil tank collapsed, it did not affect the safety functions (Figs. 1.17, 1.18 and 1.19).

Fig. 1.17

Collapse of the heavy oil tank at Onagawa Unit 1 [9]

Fig. 1.18

Burnt-out of the high-pressure power panel at Onagawa Unit 1 [9]

Fig. 1.19

Influx of sea water into the reactor building at Onagawa Unit 2 [9]