Kirk H. McDaniel and Norbert T. Rempe
Westinghouse Electric Corporation
Waste Isolation Division
U.S. Department of Energy
Carlsbad Area Office
PO Box 2078
Carlsbad, New Mexico 88221

ABSTRACT 

The Underground Ventilation System at the Waste Isolation Pilot Plant is similar to that of other underground mine environments in many respects, and yet it possesses unique features which sets it apart. It provides three primary functions.

• A safe, healthy, and comfortable work environment for underground personnel and equipment during normal operations
• The confinement, channeling, and filtration of potential airborne radioactive material in the event of an accidental release
• The confinement and reversal of the underground airflow in the event of a fire

The system consists of three intake shafts, drifts interconnected with crosscuts, and one exhaust shaft. At the facility horizon, ventilation is divided into four separate splits: construction (mining), maintenance, waste handling, and waste disposal areas. Underground openings are separated by bulkheads, airlocks, and salt pillars. Underground air flows converge into a common exhaust shaft that, in turn, is tied to the main surface fans and the High Efficiency Particulate Air filtration system. 

A total of five surface ventilation fans supply air flow to the underground. Two of these are the unfiltered main fans, and three can run either unfiltered or in filtration mode. They are operated in various configurations to provide the necessary airflow to the underground. In the unlikely event of an underground or shaft fire, three underground booster fans can be used to reverse the airflow in select parts of the facility. To enhance safety and performance, the WIPP is currently installing a third main mine fan with the same capacity as the existing main fans. 

A high level of monitoring has been incorporated into the underground ventilation system at WIPP. Significant parameters such as airflow, differential pressure, and psychrometrics are monitored and trended in real-time. State-of-the-art modeling software is used to analyze the data, and the results are used to assist in maintaining proper system configuration. Future enhancements include the development of an "logic based" mine ventilation simulator.  

The design of the airflow network, coupled with the extensive monitoring and control features give the WIPP an unparalleled ability to fine tune operations and to ensure the efficiency, versatility and safety of the underground ventilation system.  

 The ventilation system at the U.S. Department of Energy (DOE) Carlsbad Area Office's Waste Isolation Pilot Plant (WIPP) is designed to perform three distinct functions. First, it fulfills the normal mine ventilation requirements of complying with all state and federal mine regulations. Second, it prevents the uncontrolled release of radioactive contaminants from the facility. Third, it provides for the confinement and reversal of select portions of the underground airflow circuits in the event of a fire. Although a radioactive material release in the facility is considered extremely unlikely, the ventilation system incorporates many special features to prevent the possible spread of contamination. 

The underground facility at the WIPP contains four main ventilation pathways. These air splits service the north maintenance, mining, waste disposal, and the waste shaft station areas. The facility was designed and constructed to keep occupational exposures to radiation and radioactive materials "as low as reasonably achievable" (ALARA). This concept led to a design that separates the nuclear waste handling and disposal areas from the mining and non-radioactive experimental areas. The ventilation system is designed so that air leakage is from the mining and experimental areas to the waste disposal areas. Furthermore, radiation detectors are strategically located underground, and an exhaust filtration building is installed on the surface to mitigate the accidental release of radiation to the environment. Additional elements of the underground ventilation system are monitored (such as airflow, differential pressure, regulator settings, psychrometrics, and bulkhead position) on a regular basis. This data are used both as an engineering tool, and to assist in maintaining the system within desired performance parameters.  

 The DOE has determined that the visco plastic deformation of salt under a significant thickness of overburden will provide the best feasible permanent solution to isolate transuranic (TRU) waste from the biosphere. TRU waste is waste that contains radioactive isotopes with a higher atomic number than that of uranium (92). The TRU waste that is to be disposed of at the WIPP site is at present retrievably stored in a variety of locations around the country. Initial evaluations of the WIPP began in 1974. In 1979, the U.S. Congress enacted Public Law 96-164 (1) authorizing construction and development of the WIPP Project. The mission of the WIPP is to provide the safe, long-term disposal of TRU waste generated by the national defense programs of the U.S.

 The WIPP site is located approximately 47 km (29 mi) east of Carlsbad, New Mexico in the Chihuahuan Desert. The repository is located in the 630 m (2000 ft) thick Salado Formation. This Permian Basin salt deposit is about 250 million years old and its continued presence after such a long time is convincing testimony to the long-term stability of the region and the site, ensuring permanent isolation of the emplaced waste. The underground facility is 660 m (2,150 ft) below the surface, approximately halfway through the Salado Formation. Since 1984 underground non-radioactive experiments have been performed at the WIPP for site characterization and to determine it's performance as a nuclear waste repository.

 The repository is connected to the surface by three intake (fresh air) shafts and one exhaust (dirty air) shaft. The three intake shafts at the WIPP are the Salt Handling Shaft (SHS), Waste Handling Shaft (WHS), and Air Intake Shaft (AIS). The Exhaust Shaft (ES) is the only exhaust airway for the facility. During normal operation most of the intake air enters the underground through the Air Intake Shaft. The Salt Handling Shaft, which also provides personnel and material access, is used for the removal of the mined salt, and is a secondary intake shaft. The Waste Handling Shaft is equipped with an enclosed head-frame, and will be used for lowering nuclear waste. This shaft also provides access for personnel and materials to the repository horizon. Air enters this shaft and is used to ventilate the Waste Shaft Station Area only before being routed to the Exhaust Shaft. Figure 1 shows the underground ventilation system configuration. 

A total of five ventilation fans can supply air flow to the underground. They are located on the surface of the WIPP facility near the Exhaust Shaft. Two of these are the unfiltered main fans, and three are smaller fans that can be used with or without filtration. These fans are operated in various configurations to provide the necessary airflow to the underground. There are also three booster fans located in the underground repository. These can be used to reverse the airflow in various parts of the facility in the event of an underground or shaft fire. 

Once the facility becomes operational (expected May 1998), normal ventilation through the facility is achieved by running one or both of the 450 kw (600 hp) centrifugal main fans. During concurrent mining and waste handling operations, both fans operate in parallel to provide 230 m³/s (490,000 cfm). This airflow quantity is required for proper operation of diesel equipment throughout the facility. When either only mining or waste emplacement is active, the ventilation demand can be satisfied with one main fan in operation. This results in an airflow of 140 m³/s (300,000 cfm). In the unlikely event of an underground radioactive material release, the ventilation system is either automatically or manually shifted to a filtration mode. The airflow during filtration mode is reduced to 28 m³/s (60,000 cfm). This shift consists of turning off the main fans, and starting one of three 175 kw (235 hp) centrifugal stand-by fans. A series of isolation dampers then diverts the air through the filtration system. The air is routed through a series of High Efficiency Particulate Air (HEPA) filters. 

The air flow in the underground is driven by the negative pressure induced by the exhaust fans. A second pressure component resulting from the difference in density of the air in the various air shafts can effect the performance of the system. This phenomena is called the natural ventilation pressure (NVP).  

During hot weather, the air going underground is warmer and less dense (lighter) than the air returning from the underground. This air resists being drawn underground by the mechanical fans. The NVP itself acts as a fan pointing up the intake shafts. Hence in hot weather there is an NVP that opposes the mechanical fan pressure. This reduces the flow and reduces the differential pressures between the Waste Shaft Station, Waste Disposal Area, and the other areas. The air in the WHS will be cooler than that in the AIS and SHS which further reduces the differential pressure between the WHS Station and the mining area. 

During cold weather, the air going underground is colder and denser (heavier) than the air returning from the underground. This air prefers to go underground, even without the aid of a mechanical fan. The NVP acts like a booster fan inside the intake shafts. Hence in cold weather there is an NVP which augments the fan pressure. The positive NVP reduces the fan (constant flow control) suction pressure, increases the downcast air flow in one or more shafts and increases the differential pressure between the Waste Shaft Station, Waste Disposal Area and the other underground areas. The air feeding the WHS consists of leakage from the Auxiliary Air Intake Tunnel, leakage from the Waste Hoist Tower and leakage from the Waste Handling Building. The result is that the air feeding the WS is warmer than the surface air feeding the AIS. The consequences of this are a reduction in flow down the WHS and possible reverse flow in the WHS. Administrative actions are taken to adjust the underground regulators to avoid reverse flow in the WS. Under these same conditions of cold weather NVP, lower NVP in the SHS compared to the AIS can potentially cause the SHS flow to reverse.

A high degree of monitoring has been incorporated into the design of the underground ventilation system. Some fundamental capabilities include fan status, bulkhead status, and key differential pressures.  

In addition, two specially designed remote monitoring systems are in place at the WIPP to collect data on the status of the underground ventilation system. In its efforts to provide real time monitoring and modeling capability of the underground ventilation system, the WIPP has designed systems emphasizing interactive capabilities and multiple uses for the data. The first is the Underground Ventilation Remote Monitoring and Control System (UVRMCS) (2). The second is the Mine Weather Stations (MWS) (3). 

The UVRMCS consists of 15 air velocity sensors, eight differential pressure sensors (strategically placed throughout the repository), and provides for local or remote control of the position of the air regulators controlling the four main ventilation splits. The sensors send a 4-20 ma signal to one of four Central Monitoring System (CMS) Local Processing Units (LPU) in the repository. The LPUs digitize and process this signal into predetermined engineering units (air velocity in ft/min, differential pressure in inches water gauge, and regulator position in percent open), depending on the nature of the signal being received. Each signal or "point" has a specific identifier that the LPU software recognizes. The software performs the calculations necessary to convert the signal to its appropriate engineering unit. The LPUs then transmit the signal onto the data highway which extends throughout the underground and surface facilities. On the surface is a Central Monitoring Room (CMR). The CMR is equipped with Operator-Machine Interface (OMI) stations. A CMR operator can selectively retrieve the signals from the data highway and display them on the OMI terminal in graphical or tabular formats as programmed into the OMI software. The CMR operator has the capability to control the system by remotely opening or closing the main regulators. 

The ventilation system also includes eight psychrometric Mine Weather Stations (MWSs) to collect data for use in calculating natural ventilation pressures. These monitoring stations measure dry-bulb temperature (in Co ), pressure (in kPa) and relative humidity (in %) every five minutes. The MWSs are currently interfaced with the CMS through the LPUs in a manner similar to the operation of the UVRMCS.  

Information on the underground ventilation system main fans is also available from the CMR. These data show which fans are operating, the static pressure, and an indication of the airflow quantity.  

The ventilation engineer can retrieve real time data by interfacing with the CMS through a computer program called PI Process Book, commercially available from Oil Systems, Inc. This Virtual Address Extension (VAX) interface software program transfers data from the CMS VAX Interface Unit onto the VAX computer network. PI then statistically analyzes the data, screens them, and eventually archives them to the VAX for final storage. The PI system connects to the site CMS through computer drivers developed by Oil Systems, Inc. The VAX network can be accessed by the ventilation engineer, giving him access to real-time data on the status and performance of the system.  

WIPPVENT is an interactive mine ventilation simulation software program developed by Westinghouse Electric Corporation for exclusive use at the Waste Isolation Pilot Plant (4). It is based on the commercially available VNETPC for Windows Version 1.0 (Copyright of Mine Ventilation Services, Inc., 1996). The characteristic which makes WIPPVENT unique is that it is designed to interact with the WIPP underground ventilation monitoring systems (the UVRMCS and the MWS) through the site-wide CMS. Data from the remote monitoring systems are accessed directly from the CMS through PI and used by WIPPVENT to simulate the current condition of the underground ventilation system. WIPPVENT also incorporates characteristic resistance curves specific to the site's four main underground regulators . These features give WIPPVENT the ability to retrieve real time data and to use these data to create and continuously update a real time ventilation model. In addition to being used for WIPPVENT modeling and NVP calculations, the archived psychrometric data from the remote monitoring systems available on PI/Process Book in a Windows environment for trending in either historical or real time. Figure 2 shows the relationship of all system components.  

The underground ventilation system at WIPP represents an unparalleled effort to provide a safe and controlled environment. It uses both standard industrial components coupled with state-of-the-art technology and specialized software. At the present time, the WIPP facility is completing the installation of a third main mine fan similar to the two existing fans. Once all these main fans are completed and operational, any two fans will operate in parallel to provide full ventilation. Future enhancements include making fully automatic the shift to filtration functions of the surface fan array, and transforming WIPPVENT into a "logic based" mine ventilation system. 

1. Public Law 96-164, 1979, "Department of Energy National Security and Military Applications of Nuclear Energy Authorizations Act of 1980," United States Congressional Record. 
2. M.T. Strever, K.G. Wallace, and K.M. McDaniel, "Underground Ventilation Remote Monitoring and Control System," Proceedings of the 7th U.S. Mine Ventilation Symposium, June, 1995, Chapter 11, pp. 69-74, Society of Mining, Metallurgy and Exploration Inc. 
3. K.M. McDaniel, and K.G. Wallace, "The Development of WIPPVENT: An Interactive Mine Ventilation Simulation Computer Program at the Waste Isolation Pilot Plant". The 1996 Annual Meeting and Exhibit of the Society of Mining, Metallurgy and Exploration Inc., Phoenix, Arizona, March 1996. 
4. K.M. McDaniel, "Real-time Psychrometric Data Collection". The 1997 Annual Meeting and Exhibit of the Society of Mining, Metallurgy and Exploration Inc., Denver, Colorado, March 1997. 

Processing and final preparation of this paper were performed by the Westinghouse Electric Corporation's Waste Isolation Division, the management and operating contract for the Waste Isolation Pilot Plant, under U.S. Department of Energy contract DE-AC04-86AL31950.

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