GE Hitachi – ESBWR
Last week, GE Hitachi Nuclear Energy announced an agreement with Detroit Edison to submit a joint application for a nuclear power plant to be located 35 miles south of Detroit.
Detroit Edison, Michigan’s largest electric utility, will apply for an operating license using the GE Hitachi’s reactor plant design known as the Economic Simplified Boiling Water Reactor (ESBWR). Under the agreement, GE Hitachi will work with Detroit Edison to plan the initial development stages of the reactor plant if the project receives approval from various state and federal authorities.
If the NRC approves the ESBWR design, GE Htiachi will proceed with its plans to introduce the ESBWR to a global market. Last week, the Senior Vice President Danny Roderick informed the Dow Jones Newswire that he expects to resubmit the ESBWR design for approval in the United Kingdom. The design was previously pending approval there, but was withdrawn in September of 2008 so that GE Hitachi can focus on obtaining approval in the United States.
The renewed interest in the ESBWR comes on the heels of several setbacks for the ESBWR project. In November of 2008, for example, Exelon pulled the plug on plans to build the ESBWR in Victoria, Texas, citing delays in the certification process with the NRC. Then, in January of 2009, Entergy and Dominion Resources announced that they were not able to reach an agreement with GE Hitachi with licensing the ESBWR.
GE Hitachi has been struggling to satisfy questions posed by the U.S. Nuclear Regulatory Commission (NRC) relating to the certification of the ESBWR design. In August of 2009, GE Hitachi submitted revision 6 of its design documents in an attempt to provide the details that the NRC claims is lacking. Although the number of outstanding issues raised by the NRC has been greatly reduced as a result of its August submission, publicly available letters on NRC’s website indicate that questions about certain aspects of the design were not sufficiently answered.
For example, so far in December the NRC and GE Hitachi have exchanged communications relating to: (1) operations during high-wind conditions; (2) hydrogen combustion concerns in drywell areas; (3) radiation shielding details; (4) air circulation inhabited areas of the reactor plant; (5) seismology data; (6) spent fuel racks ; (7) data regarding reactor vessel integrity testing; (8) water flow calculations in the reactor pressure vessel; (9) emergency cooling system piping angles; (10) and operating temperatures of various equipment.
Natural Circulation
Instead of using reactor coolant pumps to circulate water over the fuel rods in the reactor core as in conventional reactor plants, the ESBWR relies solely on natural circulation to provide enough coolant flow to remove heat.
As can be seen in the diagram that was included in documents filed with the NRC, relatively cool feedwater enters the reactor vessel and then to travels down an outer annulus to the bottom of the reactor vessel. The feedwater then travels up through the center of the vessel in order to cool the nuclear fuel. As the water absorbs the heat from the nuclear fuel it turns to steam and continues to rise to the top of the vessel before finally exiting the reactor. This steam can then be directed to turbines in order to generate electricity.
The fact that there is no need for expensive pumps and its associated piping allows the reactor plant to be housed in a much smaller containment system than most other reactor plant designs. The ESBWR design reduces the total construction cost of the reactor plant, as well as maintenance costs since there are no reactor coolant pumps that require periodic maintenance.
Not only does the ESBWR rely on natural circulation for its reactor coolant system, but its emergency cooling system also relies on natural circulation. U.S. Patent No. 7,558,360, issued on July 7, 2009, describes a system which contains the molten reactor core in the event of a severe nuclear accident.
This figure taken from the patent shows the emergency containment and circulation system regarding the ESBWR. The wetwell (26) encloses the suppression pool (30) and is separated from the drywell (24) by the drywell wall (28) and a valve (32). The valve (32) remains shut at normal operating temperatures, but opens once the temperature rises to a predetermined level.
In the event of a nuclear fuel accident, the nuclear fuel contained in the reactor core (14) may overheat and become a molten radioactive mass that may melt through the lower reactor vessel (20). Once the temperature rises to a predetermined level, the valve (32) opens and allows the water in the suppression pool (30) to enter the molten mass contained in the drywell (24). In addition, water is pumped into the reactor through pipe (34) and exists as steam through pipe (36).
The emergency cooling system also includes two sets of condensers (44, 42) physically located above the reactor vessel. In the event of a severe reactor casualty, steam would flow into piping (54, 52) and would be directed to the condensers (44, 42) which are submersed in a pool of water. As the steam condenses to water, it gravity drains down through piping (58, 60) to the suppression pool (30).
A diagram from the documents recently submitted to the NRC shows the general containment system layout in the ESBWR. As previously discussed, the water from the suppression pool will be released into the drywell area in the event of a severe nuclear accident. Although not visible in this diagram, the ESBWR design also includes the condensers (44, 42) located above the reactor.
Fuel Assembly
A recently published patent application discusses the nuclear fuel assembly design in the ESBWR.
U.S. Patent Application No. 20090135989, published on May 28, 2009, describes the use of segmented fuel rods. Nuclear fuel is placed inside the hollow fuel rods (102) which are then sealed on both ends.
As can be seen in the diagram, the ends of the fuel rods (102) can be connected to the ends of more fuel rods (102) by way of the spacer plate (150).
In one version of the spacer plate (160), the fuel rods (102) are connected to one another through the joint rings (155).
The joint rings (155) are all connected forming a spacer grid. This design includes flow holes (158) through which coolant can flow over the fuel rods (102) removing the heat created by the fission process created by the nuclear fuel. The spacer plate (160) also has mixing tabs (161) that are curved in order to promote flow twisting of the reactor coolant. The mixing tabs (161) are designed to encourage the turbulent flow of water in order to promote boiling.
The fuel assembly GE Hitachi describes in documents submitted to the NRC for the EBSWR certification has a similar segmented design for the fuel rods that are connected through holes in the spacer plates. The fuel assembly in the EBSWR contains two holes in the center for water to flow – similar to the two holes in the spacer grid described in the patent
Conclusion
Although no ESBWR reactor plant has yet been built, GE Hitachi plans on receiving certification for the ESBWR in late 2011. Before GE Hitachi filed revision 6 of its design document in August of 2009, the NRC needed clarification regarding over 1000 items. Now, based on publicly available letters, only about 100 outstanding issues remain. Although the NRC is not yet satisfied with the level of detail the GE Hitachi has provided regarding the design of the ESBWR, the frequency of responses filed with the NRC indicate that the ESBWR project has moved to the top of GE Hitachi’s priority list.
Originally published on 12/23/2009 at www.nuclearstreet.com.