2.1. NUCLEAR PLANT DESIGN AND OPERATION

Nuclear plants are used in the United States and many other countries primarily to meet baseload² demands for electricity. These plants are especially well suited for this purpose because they can be operated for long periods without maintenance outages and can produce electricity at constant rates.

Nuclear plants generate electricity using the Rankine thermal cycle: the plant's nuclear reactors produce heat that is used to convert water to steam. The steam drives a turbine that spins a generator to produce electricity. After passing through the turbine, the steam is cooled, condensed, and recirculated. This “steam engine” cycle is also used to produce electricity in other types of thermal power plants, particularly coal- and gas-fired plants.

The primary fuel for nuclear plants is slightly enriched uranium,³ usually in the form of 1-cm-long cylindrical uranium dioxide pellets. These pellets are encased in metal tubes, referred to as nuclear fuel cladding, each ~10 mm in diameter and about 4 m in length, made of various zirconium alloys (Zircaloy, ZIRLO, M5) containing 98 percent or more zirconium. The cladding provides structural support for the fuel pellets, serves as a barrier to the release of radioactive material from the fuel, and provides an efficient geometry for cooling. The ensemble of pellets and cladding is referred to as a nuclear fuel rod. Fuel rods are grouped into bundles, or fuel assemblies (Figure 2.1 [top]), each containing between about 64 and 300 rods.

FIGURE 2.1

(Top) Side view of a BWR fuel assembly. Some fuel rods have been removed to reveal construction details. (Bottom) Cross-section showing four BWR fuel assemblies and a control rod. The control rod consists of four blades in the shape of a cross. It can (more...)

The entire set of fuel assemblies, along with control rods and associated structural supports (Figure 2.1 [bottom]), constitutes the reactor core. The control rods contain materials that are highly neutron absorbing, such as hafnium, boron, or silver. The control rods can be used to shut down the reactor when fully inserted.

The reactor core is enclosed in a robust steel pressure vessel, the reactor pressure vessel (RPV) (Figure 2.2). The RPV contains numerous penetrations for steam and water lines, instrumentation, and controls. The robust RPV design allows the reactor to operate at high temperature and pressure to increase its thermal efficiency.⁴ It also provides a major barrier to the release of radioactive material from the reactor during an accident. Water circulation through the RPV is used to control reactor pressure and temperature and generate steam for electricity production. The movement of water and/or steam out of the RPV is pressure-driven and is controlled by opening and closing valves.

FIGURE 2.2

Schematic of a BWR/5 or BWR/6 reactor pressure vessel. SOURCE: ANS (2012, Fig. 5).

The RPV is located within the containment of the building that houses the reactor (Figure 2.3). The containment is constructed of reinforced concrete a meter in thickness or a carbon steel shell a few centimeters thick and contains heavy metal bulkheads to allow access for maintenance work. Like the RPV, the containment also has numerous penetrations for steam and water lines, instrumentation, and controls. The containment serves as a barrier to the release of radioactive material to the environment during a severe accident (Sidebar 2.1).

FIGURE 2.3

Simplified schematics for nuclear power reactors. (Top) Boiling water reactor. (Bottom) Pressurized water reactor. SOURCE: ANS (2012, Fig. 2 [Top] and Fig. 3 [Bottom]).

SIDEBAR 2.1

Radioactive Material Releases in Severe Accidents. Nuclear reactors generate radioactive by-products, primarily fission products (e.g., iodine and cesium) and transuranic isotopes (e.g., plutonium), that build up inside the fuel and fuel rods during the (more...)

Reactor power is regulated by manipulating the positions of the control rods in the reactor core.⁵ The reactor can be “started” by partially withdrawing the control rods from the core. This allows a sustained nuclear fission chain reaction to be initiated in the uranium fuel, which generates large quantities of heat. This heat is removed by the constant circulation of cooling water through the core. As noted previously, the reactor can be shut down by fully inserting the control rods into the core. A reactor is said to be scrammed when all of the reactor's control rods are fully inserted and the fission process is halted after an off-normal condition is detected. Shutdown may occur automatically or can be initiated by reactor operators.

The operation of a reactor produces a wide range of radioactive isotopes:

• Fission of the uranium fuel results in the production of dozens of highly radioactive fission products, for example, cesium-137, iodine-131, and strontium-90. Some of these fission products, notably cesium and iodine, are volatile.
• Absorption of neutrons by materials in the reactor core produces transuranic elements such as plutonium-239 as well as neutron activation products such as cobalt-60.

These isotopes continue to decay and generate heat (referred to as decay heat) even after the reactor is shut down. Decay-heat generation immediately following reactor shutdown can be up to about 6 percent of the reactor's licensed power. Heat generation decreases rapidly as short-lived isotopes (primarily fission products) decay (see Figure 2.4). Cooling is crucial in the first few days after the reactor is shut down and continues to be required for years (but at lower levels) to remove the heat generated from the decay of long-lived fission products. Reactor cooling systems are designed to remove this heat so as to prevent excessive temperature rise.

FIGURE 2.4

Estimated thermal power output of reactor cores at the Fukushima Daiichi plant following shutdown. SOURCE: Based on methodology used in Gauntt et al. (2012b) and Phillips et al. (2012).

Reactor cooling is provided by several safety systems. Some safety systems operate during normal conditions to maintain RPV pressures and temperatures and water levels within a set range. Other safety systems are part of the emergency core cooling system (ECCS). These systems are used to cool the core during off-normal conditions. The effectiveness of these systems depends on their ability to remove decay heat through a combination of heating and boiling of water in the reactor while maintaining the water level in the RPV above the top of the reactor core.

In the United States, nuclear fuel in a reactor must be replaced every 4 to 6 years depending on the reactor's design and operation. U.S. reactors are typically shut down every 18-24 months⁶ for replacing a portion of the reactor fuel. The used (or spent) fuel is transferred from the reactor to a spent fuel pool. The pool has its own cooling system (the pool water is circulated through a heat exchanger) to remove decay heat from the fuel.

Nuclear plants can contain one or more reactors and their support systems, including water and electrical supplies, mechanical systems, and spent fuel pools. All nuclear plant sites in Japan and most in the United States have multiple reactors (see Sections 2.3 and 2.4 in this chapter).

More than 400 nuclear power reactors⁷ are currently operating throughout the world and 70 more are currently under construction. The large majority of nuclear power plants in the world and all plants in the United States and Japan are light-water reactors; these reactors are cooled and moderated by regular water.⁸ Two types of light-water reactors have been deployed worldwide for electricity production, including in Japan and the United States: BWRs and pressurized water reactors (PWRs). The design of these reactors is illustrated in Figure 2.3.

The primary difference between BWRs and PWRs is the mechanism for generating steam to produce electricity. BWRs produce steam directly in the core; that steam is separated, dried, and used to drive turbines and the electrical generators connected to them. PWRs produce high-temperature water that is circulated through a heat exchanger (referred to as the steam generator) to produce steam in a secondary water circulation loop. The steam in this secondary loop drives the turbines and their associated electrical generators. In both plant designs, the steam is condensed to water after passing through the turbines and the condensed water is recirculated. The water used to condense the steam is taken from a nearby ocean, river, or other water supply.

The reactors at the Fukushima Daiichi plant are BWRs. Consequently, the discussion in the remainder of this chapter focuses primarily on the design and operation of this reactor type.

2.2. BOILING WATER REACTORS

BWRs were initially developed by General Electric Company during the 1950s and have evolved through “generations,” with each generation representing iterative evolutions in the design of steam systems, water recirculation systems, safety systems, and containments. In the United States, BWR containments are designated Mark I, Mark II, and Mark III (the oldest to most recent designs). These designs are illustrated schematically in Figure 2.5. Additionally, there are six reactor generations denoted BWR/1,⁹ BWR/2, BWR/3, BWR/4, BWR/5, and BWR/6. The design of reactor cooling systems (see Section 2.2.3) has also evolved in these BWR reactor generations.

FIGURE 2.5

BWR containment designs. The location of the spent fuel pool is not shown in the Mark II containment. SOURCE: ANS (2012, Fig. 16).

Newer BWR designs, the advanced boiling water reactor (ABWR) and economic simplified boiling water reactor (ESBWR), have been developed by General Electric. The ABWR design has been approved by the U.S. Nuclear Regulatory Commission (USNRC), and several ABWRs have been constructed in Japan (see Section 2.3). The ESBWR design has been submitted to the USNRC for approval and its review is nearing completion. The discussion in the remainder of this section focuses on first-generation BWRs (BWR/1-BWR/6).

Unit 1 at the Fukushima Daiichi plant is a BWR/3 with a Mark I containment: Units 2-5 are BWR/4 with Mark I containments; and Unit 6 is a BWR/5 with a Mark II containment (Table 2.1). A number of U.S. nuclear plants have reactors and containments that are similar to those in Units 1-4 at the Fukushima Daiichi plant (Table 2.2).

TABLE 2.1

BWR Reactor Designs .

TABLE 2.2

Mark I-BWRs in the United States That Are Similar to Units at the Fukushima Daiichi Plant.

The discussion of containment systems below focuses on the Mark I containment because of its relevance to the Fukushima Daiichi accident.

2.2.3. Core Cooling System

BWRs have various engineered safety features to cool their cores depending on their generation. Three systems played key roles in the Fukushima nuclear accident (see Chapter 4):

• Isolation condenser (IC) system: Used in BWR/2s and BWR/3s, including Unit 1 at the Fukushima Daiichi plant.
• Reactor core isolation cooling (RCIC) system: Used in BWR/4s, including Units 2-4 at the Fukushima Daiichi plant, BWR/5s, BWR/6s, and the Advanced Boiling Water Reactor.
• High-pressure coolant injection (HPCI) system: Used in BWR/3s and BWR/4s, including Units 2-4 at the Fukushima Daiichi plant.

These systems are designed to remove decay heat from the reactor in the absence of AC power. They require DC power for control purposes but in some situations can operate for extended periods without any power. These systems are described in subsequent sections of this chapter. More complete descriptions can be found in technical information documents such as the Reactor Concepts Training Manual (USNRC, 2012b).

AC power is required to operate other safety systems. These include the core spray, residual heat removal (RHR), and containment spray systems. Containment and suppression pool spray systems also can be powered by the diesel-driven fire protection system or emergency water sources. These systems played little or no role in the Fukushima nuclear accident and so they are described only briefly in the next section.

2.2.3.1. Low-Pressure Core Cooling Systems

Low-pressure core cooling systems comprise two separate and independent systems: the core spray system and the low-pressure coolant injection (LPCI) system of the RHR system (Figure 2.6). These systems require AC power to operate pumps, controls, and valves.

FIGURE 2.6

Schematic of the (A) core spray system for Unit 1 and (B) residual heat removal (RHR) system for Units 2 and 3 at the Fukushima Daiichi plant. Motor-operated (MO) valves are indicated by connected triangles. SOURCE: Government of Japan (2011b, Figs. IV-2-1 (more...)

The core spray system pumps water from the suppression pool into the RPV (to remove decay heat) using two separate and independent pumping loops. The core spray system sprays water from above the core onto the tops of the fuel assemblies. Water is supplied by AC-powered, high-volumetric flow pumps. The core spray system and the LPCI mode of the RHR system operate only when the RPV is at low pressure.

The RHR system is a multipurpose system that uses AC-powered, high-volumetric flow pumps in different configurations to supply plant needs. The RHR system is normally aligned in the LPCI configuration to supply water makeup to the RPV for core cooling under loss-of-coolant conditions.¹² During LPCI operation, RHR pumps take water from the suppression pool and discharge it into the RPV directly or after flowing through a heat exchanger that transfers heat to the ultimate heat sink.

2.2.3.2. Isolation Condenser System (Fukushima Daiichi Unit 1 Reactor)

The IC system (Figure 2.7) is used to remove decay heat and conserve reactor water inventory when the RPV becomes isolated from the power conversion system (i.e., the turbine and condenser; see Figure 2.3) at or near operating pressures. It has two trains of equipment (labeled “Train A” and “Train B” in Figure 2.7), each consisting of a large heat exchanger and associated piping. The secondary (shell) side of the heat exchanger, basically a large tank, contains enough water to remove decay heat from the RPV for several hours. The shell-side water can be replenished using the makeup-water or fire protection systems or fire trucks.

FIGURE 2.7

Schematic of the isolation condenser (IC) system for Unit 1 at the Fukushima Daiichi plant. Motor-operated (MO) valves are indicated by connected triangles. Black indicates valve closed during normal operations; white indicates valve open during normal (more...)

The system can operate without electrical power or operator intervention as long as the system valves are open and there is water in the shell side of the heat exchanger. The system operates by gravity flow: Steam enters the heat exchanger via a steam line from the RPV, and condensate is returned to the RPV through a recirculation pump line.

As shown in Figure 2.7, there are four valves for each IC train. The two valves outside of containment are operated by DC power from batteries; the two valves inside containment are operated by AC power. If DC power is lost, a separate DC-powered interlocking logic circuit causes all four valves in each train to close, effectively shutting down the IC system. Once closed, the valves inside containment cannot be reopened unless AC power is available.¹³ This system logic affected the operation of the valves for the IC in Unit 1 of the Fukushima Daiichi plant during the accident (see Chapter 4).

2.2.3.3. Reactor Core Isolation Cooling System (Fukushima Daiichi Unit 2 and 3 Reactors)

The RCIC system (Figure 2.8) is designed to make up water inventory losses from the RPV caused by water boil-off when the RPV is isolated from the turbine and condenser. It is designed to operate independently of auxiliary AC power, service air, or external cooling-water systems and can provide adequate makeup water to the RPV in the following circumstances:

FIGURE 2.8

Schematic of the reactor core isolation cooling (RCIC) system for Units 2 and 3 at the Fukushima Daiichi plant. Valves are indicated by connected triangles. Black indicates valve closed; white indicates valve open. Power sources (AC or DC power) for motor-operated (more...)

• RPV is isolated from the power conversion system (turbine and condenser) and is being maintained at operational pressures and temperatures.
• Reactor is shut down and at high pressure¹⁴ with loss of normal feedwater.
• AC power is lost.

The RCIC system consists of a steam-driven turbine pump and associated piping, valves, and instrumentation necessary to implement several flow paths. The system is driven by steam produced by decay heat in the RPV. Steam exits through isolation valves and is routed through the turbine pump to provide the motive force for pumping makeup water into the reactor. The steam is exhausted to the suppression pool after exiting the turbine. Makeup water can be supplied from either the condensate storage tank (CST) or the suppression pool, with the preferred source being the CST. Makeup water enters the RPV through the feedwater injection line (see Figure 2.8).

As shown in Figure 2.8, the valve outside of containment is operated by DC power supplied by batteries; but the valve inside containment is operated by AC power. If DC power is lost, a separate DC-powered interlocking logic circuit causes both the DC and AC valves to close, effectively shutting down the RCIC system. This logic circuitry was specifically intended as an isolation function to prevent leakage from the containment if a break occurs in the RCIC piping. Once closed, the valve inside containment cannot be reopened unless AC power is available (see Footnote 13).

The RCIC system is designed to operate over a wide range of RPV pressures—from full operating pressures (~7 MPa¹⁵) to ~1 MPa. The suppression pool acts as the heat sink for steam generated by reactor decay heat. Decay heat can be removed from the suppression pool using the heat exchangers in the RHR system when AC power is available.

The continued operation of the RCIC system following a loss of DC and AC power depends on the timing of the power losses in the AC and DC circuits that control the valves and the “failsafe” control logic—similar to the IC system operation that was described in Section 2.2.3.2. In the case of an extended loss of AC power, such as occurred at the Fukushima Daiichi plant following the tsunami, the RCIC system may stop operation for the following reasons:

• DC power for the failsafe logic control has failed, causing the system's valves to close (if motive power for the valves is still available).
• Suppression pool temperature is too high, possibly leading to failure of the turbine and pump bearings.
• Containment pressure is too high, causing the RCIC system turbine to shut down.¹⁶

2.2.3.4. High-Pressure Coolant Injection System (All Fukushima Daiichi Reactors)

The HPCI system (Figure 2.9) is similar to the RCIC system in function except that it has about seven times the flow capacity (680-1,270 m³/h). It is designed to operate when the RPV remains at high pressure; such conditions might occur when a small pipe break causes water levels in the RPV to drop, but the diameter of the broken pipe is not large enough to depressurize the RPV. The HPCI can also act as a backup to the RCIC system. The same types of actuation signals initiate and terminate both the HPCI and RCIC, and DC power is needed to operate the HPCI pump and some HPCI system valves.

FIGURE 2.9

Schematic of the high-pressure core injection (HPCI) system for Units 1-3 at the Fukushima Daiichi plant. Valves are indicated by connected triangles. Black indicates valve closed; white indicates valve open. Power sources (AC or DC power) for motor-operated (more...)

2.3. NUCLEAR PLANTS IN JAPAN

Prior to the Fukushima Daiichi accident, Japan had 54 operating nuclear power reactors at 16 sites (see Figure 2.10 and Table 2.3). These reactors provided about 30 percent of Japan's electricity supply. In early 2011, Japan was the world's third-largest producer of electricity from nuclear power, after the United States and France. Tokyo Electric Power Company, the owner/operator of the Fukushima Daiichi plant, owns 17 nuclear reactors at three sites: Fukushima Daiichi (6 reactors), Fukushima Daini (4 reactors), and Kashiwazaki Kariwa (7 reactors) (see Figure 2.10). Collectively, these reactors supplied about a third of Japan's nuclear power–generated electricity before the accident.

FIGURE 2.10

Locations of nuclear power plants in Japan. SOURCE: Nuclear Energy Agency (available at http://www.oecd-nea.org/press/2011/NEWS-02.html. Accessed on June 3, 2014).

TABLE 2.3

Operating Nuclear Plants in Japan Prior to the Fukushima Daiichi Accident.

The nuclear plant fleet in Japan consists of 24 PWRs and 26 BWRs. All but four of these plants are Generation II designs.¹⁷ Four ABWRs, at Hamaoka, Kashiwazaki-Kariwa, and Shika (Figure 2.10), are Generation III designs.

Figure 2.11 shows the operating electrical generating capacity of nuclear plants in Japan in 2011 and 2012. There was a decrease in capacity following the Fukushima Daiichi accident in March 2011 as reactors were taken offline for scheduled maintenance and were not allowed to restart. All Japanese reactors were shut down by April 2012. Two of the reactors at the Kansai Electric Power Co.'s Ohi plant in western Japan (Figure 2.10) were allowed to restart in July 2012 because of concerns about power shortages in the Kyoto region. These reactors were subsequently shut down for scheduled maintenance in September 2013 and were not allowed to restart.

FIGURE 2.11

Electrical generating capacity from operating Japanese nuclear power plants prior to and following the Fukushima Daiichi accident. SOURCE: Electrical generating capacity from Nuclear Regulation Authority, written communication; reactor shutdown dates (more...)

All nuclear reactors in Japan must undergo a safety review by the new nuclear plant regulator (Nuclear Regulation Authority; see next section) before they can be restarted. These reviews are currently under way, and no completion date has been announced.

2.3.1. Regulation of Nuclear Plants in Japan

Prior to the Fukushima Daiichi accident, the Nuclear and Industrial Safety Agency (NISA) within the Ministry of Economy, Trade and Industry (METI) was responsible for nuclear plant regulation in Japan. NISA was overseen by the Nuclear Safety Commission (NSC), a senior government body responsible for formulating safety policy, and the Atomic Energy Commission (AEC), which was responsible for nuclear power and research policy. Both the NSC and AEC were part of the Cabinet Office¹⁸; however, they were advisory and neither had direct authority over nuclear plant regulation.

Following the Fukushima Daiichi accident, NISA's association with METI was seen to compromise its independence and pose a conflict of interest because METI also promotes nuclear energy. The Japanese government decided to eliminate NISA and establish a new organization in its place. This new organization, the Nuclear Regulation Authority (NRA), was established as an extra-ministerial organization of the Ministry of Environment in September 2012. NRA combines the roles of NISA and NSC and also assumed the nuclear-related functions of the Ministry of Education and Science (see Figure 2.12).

FIGURE 2.12

(Left) Japanese government nuclear organizations prior to the Fukushima Daiichi nuclear accident. (Right) Organization of new Japanese nuclear regulator (NRA) effective September 2012. JAEA = Japan Atomic Energy Agency. JNES = Japan Nuclear Energy Safety Organization. (more...)

The NRA is headed by a five-member commission composed of a chairman and four commissioners who are appointed by the Japanese prime minister and confirmed by the National Diet for 5-year terms. A secretary general directs the activities of the Secretariat of the NRA and carries out the policies and decisions of the commission. Most of the staff of NRA were transferred from METI and the Ministry of Education, Culture, Sports, and Science and Technology (MEXT); they will not be allowed to return to METI or MEXT in the future because they were hired by the NRA under a “no-return” rule.

2.4. NUCLEAR PLANTS IN THE UNITED STATES

There are 100 nuclear power reactors currently licensed to operate at 65 sites in 31 states (Figure 2.13, Table 2.4). Collectively, these reactors provide about 20 percent of U.S. electricity supply. Thirty-five of these reactors are BWRs and 65 are PWRs, all of Generation II design. One Generation II reactor (Watts Bar Unit 2) and four Generation III reactors (Vogtle Units 3 and 4 and V.C. Summer Units 2 and 3) are under construction. These Generation III plants are PWR designs (AP1000).

FIGURE 2.13

Locations and names of currently operating nuclear plants in the United States. SOURCE: USNRC (2013b, Fig. 16).

TABLE 2.4

Operating Nuclear Plants in the United States (June 2014) .

2.5. COMPARISON OF JAPANESE AND U.S. BWR PLANTS

Twenty-one BWRs in the United States have the same reactor and containment designs as the Fukushima Daiichi units (see Table 2.2).²¹ Six are of the same design as the Fukushima Unit 1 (BWR/3 Mark I) and 15 are the same design as Fukushima Units 2, 3, and 4 (BWR/4 Mark I).²² Several safety enhancements have been made to Japanese and U.S. Mark I BWRs since they began operating; some of these enhancements are described in the following sections.

Footnotes

1

The terms “nuclear power plant” and “nuclear plant” are used interchangeably in this report.

2

That is, the continuous demand for electricity from customers in regions served by the plant. Countries such as France have such a high percentage of nuclear power that the output of some of their plants is varied according to demand.

3

Enriched uranium contains uranium-235 in higher-than-natural abundances. Natural uranium contains about 0.7 percent uranium-235. Uranium used in most reactors contains 3-5 percent uranium-235.

4

BWRs operate at pressures and temperatures of about 7 MPa and 285ºC. Pressurized water reactors (PWRs) operate at pressures and temperatures of about 15 MPa and 315ºC. See Footnote 15 for a definition of MPa.

5

Other means are used to regulate reactor power as well. The power in BWRs can be regulated by varying water flow through the core. The power in PWRs can be regulated by varying the concentration of boron, a neutron absorber, in reactor cooling water.

6

Japanese and some European reactors are shut down every 12 months for refueling.

7

Prior to the Fukushima nuclear accident, there were 442 operating power reactors in 30 countries (World Energy Council, 2012).

8

Moderation refers to the slowing down of fission neutrons to thermal energies to increase their nuclear fission cross-section. The CANDU (Canadian Deuterium Uranium) reactor uses heavy water as moderator and accounts for about 10 percent of reactors worldwide.

9

No first-generation (BWR/1) reactors are operating today.

10

Also sometimes referred to as the wetwell and torus.

11

The reactors at the Three Mile Island plant are PWRs.

12

For example, when water levels in the RPV drop below acceptable levels because of a pipe break or other off-normal condition.

13

There are several possible sources of AC power: offsite AC power, onsite emergency diesel generators, and onsite DC sources via inverters.

14

That is, before the reactor is depressurized to a level where the low-pressure cooling systems can be operated. However, it is also possible to lower the reactor pressure to below the shutoff head of the RHR/LPCI or LPCS so that one of these sources of water can be injected to the RPV while still having enough RPV pressure to provide steam for RCIC operation (150- to 200-psi range).

15

Megapascals (10 pascals). Pascal is the SI-derived unit for pressure and is equal to 1 N/m; 1 MPa ≈ 145 pounds per square inch (psi).

16

BWR emergency operating procedures are being rewritten to override the high containment backpressure trip for the RCIC turbine.

17

Reactor generation terminology was developed by the U.S. Department of Energy. Generation II reactors were constructed beginning in the 1960s. They have mechanically and electrically operated safety systems that can be started automatically or by operator control. Most of the world's current reactor fleet consists of Generation II reactors. Generation III reactors were constructed beginning in the 1990s. They incorporate more passive safety systems and have other design improvements. See Goldberg and Rosner (2011) for additional information.

18

The Cabinet Office is Japan's executive branch of government. It consists of the Japanese prime minister and other state ministers.

19

Independent agencies in the U.S. government are run by commissions or boards with oversight from the U.S. Congress. The members of the commissions and boards are appointed by the president; some appointments require U.S. Senate confirmation.

20

This section of Title 10 of the Code of Federal Regulations (CFR) is available at http://www​.nrc.gov/reading-rm​/doc-collections​/cfr/part054/part054-0031.html. Accessed on June 3, 2014.

21

Although the reactors and containments have a standard design, the design, of the remainder of the plant, including reactor buildings, control rooms, and locations of safety systems, are not standardized.

22

In addition, there are two BWR/2 Mark I plants in the United States: Nine Mile Point Unit 1 (New York) and Oyster Creek (New Jersey).

23

Installation of a Hardened Wetwell Vent (Generic Letter 89-16). Available at http://i2​.cdn.turner​.com/cnn/2012/images/02/16/nrc​.gl.89.16.pdf. Accessed on June 3, 2014.

24

The exception is the James A. FitzPatrick plant, which is located in New York.

25

Compliance with Order EA-13-109, Order Modifying Licenses with Regard to Reliable Hardened Containment Vents Capable of Operation Under Severe Accident Conditions (JLD-ISG-2013-02). Available at http://pbadupws​.nrc.gov​/docs/ML1330/ML13304B836.pdf. Accessed on June 3, 2014.

26

Nitrogen is used to displace air within the primary containment vessel when the reactor is operating. By reducing the concentration of oxygen to less than 4 percent, it is possible to prevent explosions or fires within the containment even if hydrogen is generated and released from the RPV into containment. BWR Mark III containments are not inerted. They have hydrogen control systems that are designed to burn hydrogen at low concentrations.

27

The modifications are described in Borchardt (2012a, Enclosure 2).

28

It is difficult to make a direct comparison between Japan and the United States; control rooms in Japan are different in terms of reliance on computer controls and number of staff.

29

10 CFR § 50.63, Loss of all alternating current power. Available at http://www​.nrc.gov/reading-rm​/doc-collections​/cfr/part050/part050-0063.html. Accessed on June 3, 2014.

30

The quoted material is taken from an unofficial translation of the guide.

31

Interim Compensatory Measures Order EA-02-026 (this document is designated as Safeguards Information and is not available to the public).

32

Section B.5.b of the USNRC Order for Interim Safeguards and Security Compensatory Measures was designated by the USNRC as Safeguards Information, and so it was exempt from public release. Consequently, TEPCO would not have had direct access to this information. However, as discussed in , the USNRC shared some B.5.b information with Japanese government authorities. Moreover, the USNRC requirements were made publicly available in 2009 (USNRC, 2009a) and were incorporated into reactor designs that were being developed by Japanese vendors for sale in the United States.