THE DESIGN BASIS

The design basis of a plant specifies the necessary capabilities of the plant to cope with a specified range of operational states and design basis accident conditions, in compliance with the defined requirements for radiation protection. The design basis typically includes the specification for normal operation, the conditions created by PIEs, important assumptions and, in some cases, the particular methods of analysis. 

The performance of the plant should also be addressed for certain events for which the plant has not been designed, i.e. beyond design basis (or severe) accident conditions. I C systems important to safety play an important part in such an eventuality, since they may be called upon to provide critical information about the status of the plant or to operate outside the design ranges of the mechanical plant systems. 

The Requirements for Design require that the plant states be identified and grouped into a limited number of categories according to their probability of occurrence (Ref. [1], para. 5.7). The categories typically cover normal operation, anticipated operational occurrences, design basis accidents and severe accidents. 

The Requirements for Design require (Ref. [1], para. 5.25) that the potential for accidents to occur in low power and shutdown states such as startup, refuelling and maintenance, when the availability of certain I C safety systems may be reduced, be 

The safe normal operation of a nuclear power plant, intended to cover all normal modes of operation, should be considered in the design process. The design process should establish a set of requirements and limitations on the normal operation of the l C system as necessary for safe operation of the plant. These requirements should cover (Ref. [1], para. 5.26)  

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Dual-beUows assembhes, ie, universal-type expansion joints, are particularly vulnerable to squirm, and can experience elastic squirm at one-fourth the pressure of an individual bellows. When large amounts of offset are encountered, as is often the design basis, a pinwheel effect occurs because of unbalanced pressure forces. This effect tends to rotate the center-spool pipe which may lead to bellows mpture. Eor this reason the center spool should always be stabilized by hinges or tie-rod lugs to prevent such rotation. 

The following information has been extracted from the design basis for an actual wastewater treatment plant ... 

Conventional Flare System - The majority of pressure relief valve discharges which must be routed to a closed system are manifolded into a conventional blowdown drum and flare system. The blowdown drum serves to separate liquid and vapor so that the vapor portion can be safely flared, and the separated liquid is pumped to appropriate disposal facilities. The blowdown drum may be of the condensible or noncondensible type, according to the characteristics of the streams entering the system. Selection criteria, as well as the design basis for each type of blowdown drum, are detailed later in this volume. The design of flares, including seal drums and other means of flashback protection, is described later. 

The design basis for condensible blowdown drums is as follows ... 

Another example of an unsteady state condensible blowdown system is the design for a phenol condensible blowdown tank. A blowdown tank is used in phenol treating plants to handle streams containing phenol and heavy hydrocarbons (lubricating oil stocks). The blowdown tank is illustrated in Figure 4. The design basis is as rollows ... 

The design basis is the most important eonsideration when determining the size of a biologieal reaetor. Other pertinent faetors are the final produet, the mieroorganism used, its growth rate and oxygen requirement, the produet eoneentration (e.g., expressed in mg/L broth for proteins or 100 mg/L for organie aeids), and the type of produet (intereellular or extraeellular). 

A primary objective of any safety program is to maintain or reduce the level of risk in the process. The design basis, especially inherently safer features that are built into the installation, must be documented. Management of change programs must preserve and keep the base record current and protect against elimination of inherently safer features. For identical substitution, the level of risk in the process is... 

During the facilities design scoping and development prior to completion of the design basis to focus on equipment and configuration (refer to Sections 4.3 and 4.4). 

The analyses of system failures which could challenge the containment or lead to the release of radioactivity form the licensing process. The design basis analyses are deterministic, and degraded core accidents are not considered. PSA determines the probabilities of the numerous sequences that could lead to core degradation and how the core behaves. 

PSAs calculate that accidents more severe than those postulated in the design basis are the principal risk contributors. This indicates that safety designed with a specific goal is successful. 

The DPSE found an acceptably low risk that was dominated by releases during normal operation. The mean risk of the station was estimated to be 9E6 Sv/y to the individual at ihe sue boundary and 7E4 person-Sv/yr to the surrounding population to a distance of 100 km. The consequences of events beyond the design basis were not calculated but their frequency was predic(cd to be 4E-6 /rcactor-y which is acceptably low. The mean economic risk was estimated to be about 10 M /reactor-yr. 

The Pickering A Risk Assessment (PARA) (Ontario Hydro, 1995) is also a level 3 PSA for 1 of the 4 units at Pickering. A difference between PARA and DPSE is that sequences beyond the design basis were modeled using the MAAP-CANDU codes with best estimate assumptions. Other parts of the analysis used licensing-type conservative assumptions. 

Establish the rado of the naximurn anticipated flow rate for system, Qjj, to the design basis rate, Q,d or Q,m/Q.d-When Qv] is not known, nor can it be anticipated, use Qm/Qd of 1.1 for flow control and 1.25 for level pressure and temperature control valves to anticipate the flow rate transients as the control loop recovers from a disturbance [9]. 

The use of properly designed relief panels or free floating vessel covers are usually effective for dust explosions in silos tanks, filter housings, and the like. The design basis has been pre dousIy discussed. 

Provide at least three, and perhaps four feed nozzles in addition to the one theoretically calculated to be the optimum location. Select these feed locations approximately two and four trays above and below the design basis or theoretical location. These extra nozzles must be oriented on the column so they have proper feed entry spargers or distributors (entry can be onto the tray or into the downcomer) and can be valved from a feed manifold to select the alternate desired location for testing purposes. 

The design basis includes the process requirements specification and the safety requirements specification. The process requirements specification is typically developed by process engineering, with input from operations personnel. The process requirements are provided to the instrumentation, electrical, or controls systems personnel to develop the safety requirements specification with input from operations and maintenance personnel. 

Engineering, Installation, Commissioning, and Validation (EICV) This phase involves the physical realization of the design basis, which is developed in response to process risk identified in an H RA study. The bulk of the work in this phase is not a process... 

The design basis of the facility and its safety systems, including operating and maintenance procedures, are established and documented. 

By contrast, the nature of certain accident scenarios could prove to be quite sensitive to some design parameters. It should not be ruled out during the risk assessment phase, especially during detailed design, that discoveries during consequence analysis could lead to the revision of the design basis of the facility or some equipment or components. 

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