A PROCESS-INHERENT, ULTIMATE-SAFETY (PIUS), BOILING-WATER REACTOR

Transcription

1 A PROCESS-INHERENT, ULTIMATE-SAFETY (PIUS), BOILING-WATER REACTOR Charles W. Forsberg Oak Ridge National Laboratory* P.O. Box X Oak Ridge, Tennessee (615) Summary fcr 1985 American Nuclear Society Annual Meeting; June 9-14, 1985; Boston, Massachusetts. CONF January 1985 DE DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, <or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors exp/essed herein do not necessarily state or reflect those of the United States Government or any agency thereof. *0perated by Martin Marietta Energy Systems Inc. under contract No. DE-ACO5-84OR214OO with the U.S. Department of Energy. DI3T1II8UTI0N OF THIS DOCUMENT IS UNLIMITED ft

2 A PROCESS-INHERENT, ULTIMATE-SAFETY, BOILING-WATER REACTOR Charles W. Forsberg A new type of boiling-water reactor (BWR) has been conceived the Process-Inherent, Ulv i.^ate-safety, Boiling-Water Reactor (PIUS BWR). The proposed design described in this report extends t'ae PIUS design philosophy from pressurized-water reactors (PWRs) to BWRs. BWR, safety systems are passive, with no active components. In a PIUS The reactor core and nonnuclear portion of the plant are similar to current BWR designs, but a special, prestressed concrete reactor vessel (PCRV) with ^ unique internal design replaces the conventional pressure vessel, emergency core cooling systems, containment shell, spent-fuel storage ponds, diesel generators, and most other components of the nuclear island. The PIUS BWR (Fig. 1) is a natural-circulation BWR inside a large PCRV. The height of the PCRV allows for a 20+-m riser; hence, available pressure drop is sufficient to use modified commercial BWR core designs. Top-entry control rods are used. The reactor pressure, temperature, and thermal efficiencies match current BWR design conditions. The PCRV is divided into two components: a reactor zone (core, riser, downcomer, and steam separators) and a cool, borated-water zone. The two zones are separated by an Insulated wall that is not a pressure boundary. The two zones are In contact near the top of the PCRV cavity through a hot/cold water interface zone, which is stable because hot reactor water is on top of the cool, borated water. 1 2 zone PCRV concept is similar to the PIUS PWR. ' In design, the two- Near the bottom of the

3 -2- ORNL DWG R STEAM FEEDWATER STEAM SEPARATOR WATER LEVEL HOT/COLD WATER INTERFACE ZONE COLD BORATED WATER FIVES WATER PUMP *». ":*'.'*?»'.' :':':'. i- : A. W/////////7/, \ RISER (1 OF 2) DOWNCOMER (ANNULAR RING AROUND RISERS) CONTROL ROD DRIVES SPENT FUEL STORAGE REACTOR CORE FLUIDIC VALVE HOT/COLD WATER INTERFACE ZONE PCRV Fig. 1. Schematic of Process Inherent Ultimate Safety Boiling- Water Reactor.

4 -3- PCRV cavity, the two zones are separated by a fluidic valve. If the valve opens, the borated water enters the reactor core and shuts the reactor down. The volume of borated water in the PCRV is sufficient to remove core-decay heat for a period exceeding one week, by boiling of the borated water. For a 750-MW(e) PIUS BWR, the required PCRV internal volume is smaller than that required for existing British Advanced Gas-Cooled Reactors (AGR) rated at 620 MW(e) (4350 «3 versus 70A3 m 3 ) but the operating pressure is somewhat greater (6.9 MPa versus 4.6 MPa). Reactor safety is assured if (1) the cool, borated water 1B allowed to enter the reactor if there is insufficient reactor water, and (2) reactor power levels are limited to match the available coolant- The first condition is met with a Fluidic In-Vessel Emergency Core-Cooling System (FIVES) which consists of (Figs. 1 and 2): (1) a water pump (<100 kw), (2) a fluidic valve separating the cool, borated water from the reactor coolant, and (3) a hot/cold interface zone below the fluidic valve. The fluidic valve remains closed if it receives a steady flow of high-pressure water from the water pump. Protection against a low water level in the reactor vessel is provided by positioning the water pump above the reactor core in the downcomer (Fig. 1). If there is a loss of feedwater, the pump will go dry before the reactor core is uncovered; thus, no water will be sent to Che fluidic valve, and the valve opens flooding the reactor with cool, borated water. The fluidic valve is a vortex fluidic valve that works like e centrifugal pump with a blocked exit line. The water from the pump in

5 -4 ORNL OWC 84-1O4R2 BOTTOM CORNER OF REACTOR CORE PRESSURE = P ( P 2 >P, DOWNCOMER (MOT REACTOR COOLANT) HIGH PRESSURE WATER FROM WATER PUMP HIGH IN DOWNCOMER l"l in K UJ I Ul o m a* OPENINGS ON OUTSIDE SURFACE OF VORTEX ) CASING AT PRESSURE OF COLD, 8ORATED WATER INSTRUMENT LINES TO CONTROL WATER PUMP OUTPUT SCREEN HOT REACTOR COOLANT _WATER CLEANUP SYSTEM EXIT LINE COLD BORATED WATER ZONE OF HOT/COLD INTERFACE Fig. 2. Vortex fluidic valve assembly.

6 -5- the downcomer Is injected at high velocities into the device tangentially, causing the water to move in a circular path. The centrifugual forces create higher pressures near the outside surface of the device and lower pressures near the inside. The outside surface has short pieces of pipe connected to a zone of clean, higher-pressure, reactor water which, in turn, is in contact with the borated water zone at the hot/cold interface. Ths inside is connected to the downcomer water. By adjusting water pump input, these pressures can be made to match the pressures of the two water zones. Below the fluidic valve is a hot/cold interface zone with hot coolant on top and cool, borated water on the bottom. The >20% difference in density between the two fluids makes this a stable interface. Temperature sensors determine the location of the interface, and control the speed of the water pump. If the interface rises, the pump speeds up, the pressure of the reactor coolant in the vortex valve box increases, and the interface is pushed back to its correct position. The reverse operation occurs if the interface drops. The fluidic valve has no moving parts; hence it cannot fail in. a closed condition, for it is the hydraulic forces (not pieces of metal) that block water flow through the valve. In the PIUS BWR, power is limited to safe levels by use of a lowexcess-reactivity core and the negative void coefficient. would be used for operations but not required for safety. Control rods The reactor core is designed so that all the control rods could be pulled out of the core at any time in the fuel cycle without excess power levels being generated. In practice, this condition exists at the end of nornal BWR

7 -6- core life when there is no excess reactivity in the core. This condition may be achievable throughout core life with the use of burnable Gd2O3 poisons. This was partially demonstrated at the Barsebeck Nuclear Power Station in southern Sweden in Based on current technology, the preferred PIUSBWR power levels would be between 300 and 1200 WJ(e). Below 300 MW(e), the small size of the PCRV and resultant small riser height requires use of recirculation pumps or low-pressure-drop cores. Above 1200 MW(e), there is no PCRV experience. The PCRV adds to plant costs but eliminates the need for a steel pressure vessel, biological shield walls, containment dome, emergencycere cooling systems, emergency cliesel generators, and spent-fuel storage ponds. The PCRV contains about 60 m 3 of concrete per MW(fc) of plant capacity and represents more than half the requirements for plant construction materials. In comparison, current BWRs require ~135 K 3 of concrete per MM(e). This suggests! that the PIUS EWR may have lower capital costs and shorter construction times than existing LMRs. The BWR, as compared to a PWR, lends itself naturally to the PIUS approach because the lower operatitig pressures and the lack of steam generators minimize the strength and size required for the PCRV to store the required borated water. Considerable experimental and analytical work will be needed before this conceptual design can be considered for fuilg scale development.

8 -7- References 1. K. Hannerz, "Towards Intrinsically Safe Light-Water Reactors," ORAU/IEA-83-2(M)-REV, Oak Ridge Associated Universities (July 1983). 2. K, Hannerz, "Applying PIUS to Power Generation: The Secure-P LWR," Nucl. Eng Int., 28(349): 41 (December 1983). 3. National Nuclear Corp., "Heysham 2/Torness, New Advanced Gas-Cooled Reactors Incorporate Design Improvements." Nucl. Eng. Int., ^6(316): 27 (March 1981). 4. D. N. Wormley and H. H. Richardson, "A Design Basis for Vortex-Type Fluid Amplifiers Operating in the Incompressable Flow Regime," J. Basic Eng. (Transactions of the ASME, Series D) 92.(2): 369 (June 1970). 5. S. Lundberg, "Reactor Core Operation Using Fuel with Heavy Gadolinium Loadings," Pamphlet by Southern Sweden Power Company (1984). 6. S, Lundberg, "Reactor Operation Using Heavy Gadolinium Loadings," Trans. Am. Nucl. Soc, 45: 738 (Oct. 30-Nov. 4, 1983). 7. S. Olsson, "Operation with Monosequences in ASEA-ATOMBWR A Way of Reducing the Impact of PCI," IAEA Specialists' Meeting on Pellet-Cladding Interaction in Water Reactors, RISO National Laboratory, Roskilde, Denmark, CONF (Sept , 1980). 8. Patents applied for.