Conservative assumptions

A conservative assumption is that O can be set equal to zero. When the stress O equals the characteristic strength O the faUure probabUity is 63.2%. Under conditions other than tensUe loading, the stress distribution in a body is inhomogeneous. To account for this, a loading factor k is used to calculate the effective volume under stress and kVreplaces V. 

Another way to evaluate risks is to calculate the sensitivity of the total risk estimates to changes in assumptions, frequencies, or consequences. Risk analysts tend to be conservative in their assumptions and calculations, and the cumulative effect of this conservatism may be a substantial overestimation of risk. For example, always assuming that short-term exposure to chemical concentrations above some threshold limit value will cause serious injury may severely skew the calculated risks of health effects. If you do not understand the sensitivity of the risk results to this conservative assumption, you may misallocate your loss prevention resources or misinform your company or the public about the actual risk. 

The above criteria are a guide for detecting potential problems with gas letdown systems and apply for the first 90 m of piping downstream of the pressure reducer under concern. Systems with only liquid flow are not considered potential problems and need not be investigated. For systems with two phase flow, use the conservative assumption of the total mass flow rate as gas. Any system exceeding these criteria should be further evaluated. 

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. 

The process was iterative because of the need to modify conservative assumptions as the av. L mbly process proceeded. The results are summarized in Table 11.2-2,... 

The quantity of mixture present beneath the cars possibly parked in the parking lot can only be estimated, so a conservative assumption should be used. Assume that a maximum number of 100 cars are parked closely together. An average car occupies an area of approximately 6 x 3 = 18 m (including intervening space), and the free space beneath an average car is approximately 0.3 m high. Then, 100 cars create a partially confined space of... 

More attention lias been recently focused on exposure assessment. Tliis is because many of the risk assessments performed in tlie past used too many and overly conservative assumptions. This in turn, caused an overestimation of the actual exposure. 

For VCEs, consequence screening is performed in a number of different ways, depending upon the level of detail desired and the tools available to the analyst. VCE consequence screening is based on an estimate of the blast parameters that could reasonably occur, given conservative assumptions about the quantity and nature of possible releases. 

This equation assumes the pressure head at the wetting front to be equal to zero. The value of the pressure head is, however, a source of disagreement and one of the sources of uncertainty in this test. The assumption that the pressure head is zero is a conservative assumption, tending to give a high hydraulic conductivity. 

The constants in the heat transfer and pressure drop correlations are functions of the fluid physical properties, volumetric flowrate, tube size and pitch. In preliminary design, it is reasonable to assume either 20 mm outside diameter tubes with a 2 mm wall thickness or 25 mm outside diameter tubes with 2.6 mm wall thickness. The tube pitch is normally taken to be pj = l.25do- A square tube pitch configuration can be assumed as a conservative assumption. Baffle cut can be assumed to be 0.25 in preliminary design. 

The worst operating condition in a common design practice consists of overly conservative assumptions on the hot-channel input. These assumptions must be realistically evaluated in a subchannel analysis by the help of in-core instrumentation measurements. In the early subchannel analysis codes, the core inlet flow conditions and the axial power distribution were preselected off-line, and the most conservative values were used as inputs to the code calculations. In more recent, improved codes, the operating margin is calculated on-line, and the hot-channel power distributions are calculated by using ex-core neutron detector signals for core control. Thus the state parameters (e.g., core power, core inlet temper-... 

Response to Blast Waves The effect of blast waves upon equipment and people is difficult to assess because there is no single blast wave parameter which can fully describe the damage potential of the blast. Some targets respond more strongly to the peak incident overpressure and others to the impulse (J p dt) of the blast. The blast parameters are usually based on the conservative assumption that the... 

The blast parameters are usually based on the conservative assumption that the blast strikes the target normal to its surface, so that normal reflection parameters may be used. 

Production of HC1 has certain advantages in operating costs. These amount to approximately 100 000 per year, or to 17 per tonne of chlorine content. Much of the difference is due to the conservative assumption of a membrane life of one year. If membranes lasted two years, the difference would almost disappear. An... 

The FDA and EPA believe that the tolerance process is protective of human health because it is based on extensive testing and on a combination of conservative assumptions and risk assessment practices developed using current scientific knowledge. 

A conservative assumption of a 10% reduction in activation energy moves the peak desorption temperature from equation 1 from 335 C to 278C. The peak is made more narrow, and the maximum temperature for desorption is significantly reduced. This will greatly reduce the tendency for the material to reform and agglomerate, allowing the porous matrix to remain largely intact. 

The number of naturally occurring proteins is few in comparison with those that can be made from the twenty commonly occurring amino acids. There are 10 °° possible 150-residue proteins and there are 10 that have less than 20% sequence homology and that therefore represent unique folds. If the conservative assumption is made that one in a billion of the unique folds will form stable tertiary structures there are still 10 new proteins to be discovered The number of naturally occurring ones are less than 10 , probably because they have... 

The most common methods for dealing with variability and uncertainty in the past have been the use of conservative assumptions, safety factors, and assessment scenarios. Each of these approaches has limitations that may often lead to inappropriate decisions. 

Both large and small amounts of radiation are damaging to health. Current scientific consensus is radiation can also increase the probability of cancer, and a conservative assumption is no threshold level exists below which there is no additional risk of cancer. There is considerable debate about how great the cancer risks are when people are chronically exposed to very low levels of radiation. Since everyone is environmentally exposed to a small amount of radiation, the minimum amount of additional radiation that may constitute a health hazard is not well known. 

Overly conservative assumptions of the basic design data... 

The Boyle method (see A5.13) can be used to estimate the diameter required for a dump system. It will usually be safe to assume liquid-only flow if dumping occurs well before the maximum rate (the original Boyle method). Alternatively, the conservative assumption that a homogeneous two-phase mixture (rather than liquid alone) enters the dump system from the reactor could be made (the modified Boyle method). 

Leung[341 has proposed an alternative sizing method for tempered hybrid systems (see A5.11). This method makes the same assumptions as that above, except that the conservative assumption that the allowable temperature rise is the same as that in a closed vessel does not need to be made. The method is therefore likely to yield smaller relief sizes than the method above. However, the method is more time-consuming to evaluate as it requires a trial and error procedure. ... 

Fauske[19] has "produced an equation that combines the DIERS equation (7.1) with equation (A2.4) from Annex 2, which obtains the value of QGmax from experimental measurements. Fauske s equation neglects friction and assumes that the two-phase relief system flow is. not choked (this is not a conservative assumption, but he justifies it by the conservatism of the assumptions in the DIERS equation). Fauske s equation is ... 

We can next compare these oral equivalent doses with regulatory limits. The EPA OPP chronic dietary reference doses (RfD) for diuron [84] and 2,4-D [84] are 0.003 and 0.005 mgkg-1 per day, respectively. The lowest oral equivalent doses in Table 2.3 are above the RfDs by factors of 87 and 6, respectively. Note that the rTK Css uses a series of conservative assumptions and provides low (and therefore conservative) values for the doses at which bioactivity may be seen. Based on these results, we would not expect bioactivity, at least as tested for in the ToxCast assays, for either diuron or 2.4-D at or below the RfD. 

These types of calculations indicate that, even when adopting very conservative assumptions, the risk to the general population by human penetration of the repository in the distant future is very small. 

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