Metal Dissolution Example Assessment cont'd

DOE-HDBK-3010-94

7.0 Application Examples; Dissolving Operations Examples

The questioning nature of an evaluation independent of source term is apparent in other

questions that could be asked in this example. Is it worth considering altering the system so

as to use nitrogen as the purge gas instead of air? Should a branch of the nitrogen line be

attached to the purge line to provide back-up purging capability if process air is lost? The

hydrogen deflagration event discussed above does not seem to warrant a large, complicated

set of redundant safeguards, but it is important to at least consider such issues in an

evaluation. Other obvious questions exist with regard to the heating system. If there is a

heat exchanger leak, and steam flow has been shut off due to high temperature in the slab

tank, pressure on the steam condensate line will be quickly lost and process solution will leak

into the condensate line. The process steam condensate is collected in raschig ring tanks that

are sampled for contamination twice before being returned to the site steam plant. Is that

good enough? Should a change in the operation of the heat exchanger be considered? What

is the proper tradeoff between worker man-rem, potential contamination, or even waste

generation and increased maintenance to check for leaks? There are no "right" answers per

se to these questions, and the original operation is not negligent simply because the questions

can be asked.

These are the kinds of considerations other than source term estimates mentioned at the

beginning of this chapter that are related to developing a simple understanding of why things

are done the way they are. Such considerations are an example of why the Introduction to

this document states that the source term formula alone cannot be a vehicle for determining

the complete facility safety basis. It would be a misuse of the overall process in which ARF

models are developed to simply note that site boundary consequences are below a given value

and do no further evaluation. In this example, such an approach would have continued to

overlook improper use of an air-to-open valve on the purge line that creates an obvious

mechanism for obtaining explosive concentrations of hydrogen in a vessel. Likewise, it

would be extremely inefficient to try to use a restrictive dose criteria that would force

"backing into" the problem. The ARF and RF data of this handbook are not an effective

mechanism for fine parsing of results, or for replacing process and operational knowledge.

The main value of the release estimate obtained by application of ARF values indicates that

the accident consequences are not large, let alone catastrophic, which places it firmly within

the basic spectrum of industrial accidents. As part of a graded approach to safety, it does

not demand any unique or excessive expenditure of resources beyond industrial norms. That

expenditure would rightly include a basic explosion analysis, of the type summarized in this

example, indicating that detonation is not a major issue and vessel venting capability is

adequate. Such a basic analysis needs to be performed if one does not already exist on file

for the metal dissolution process. The analysis would not, however, be expected to require

integration with a large-scale quantitative risk assessment or extensive computer modelling.

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