The risk can not be cantitative, but qualitative because, generic, it represents the magnitude (in frequency and consequences) of the impact uncertainty between at least two systems (internal and external). Much more, for these two systems are arbitrarly in nature, also the magnitude is relative, let say scaled somehow. Practicaly, you can use and find more answers using ISO-31000 and -31010 standards, together with any particular PRA or PSA studies related to that phenomenon. Here, the acceptance criteria in scalling the risk has to be carefully chosen based also on engineering judgement and previous experience and statistics, if available.

Well, if you are looking for the methodology of quantitative risk assessment in chemical or process industry, then you may widen your literature search to the following topics because they are the important tools or approached to comprehend the whole risk assessment:

• Consequence Modeling
• Probability Modeling
• Risk Mitigation
• Cost Benefit Analysis

Quantitative risk analysis is a widely accepted technique within the chemical and

process industries. The attached files show some articles that are related to the respective topics. Hope they are of useful to you.

If you are new to this filed. These books are a good starting point. 

Offshore Risk Assessment by Jan-Erik Vinnem

Vol 1.: Principles, Modelling and Applications of Qra Studies (Springer Series in Reliability Engineering) 2013.

Vol 2.: Principles, Modelling and Applications of Qra Studies (Springer Series in Reliability Engineering) 2013

I looked your profile and noticed that two books I mentioned are not for you. They are for offshore engineers. Mr Foo's references are good for chemical engineers.  

This is better for your purpose (I think)

Risk Assessment: Theory, Methods, and Applications  2011     by Marvin Rausand 

 If you explain the context  of risk assessment  you would get better answers. 

  the Dutch Government in the 1990's published a number of books for the safety analysis.  These books are known as Green, Red, Purple and yellow books.  The most widely used "Yellow Book" was revised in 2005. TNO has them as a basis for their software programs EFFECTS and RISKCURVES.

YELLOW BOOK

Methods for the calculation of Physical Effects Due to releases of hazardous materials (liquids and gases) - Third edition Second revised print 2005

GREEN BOOK

Methods for the determination of possible damage to people and objects resulting from releases of hazardous materials - First edition 1992

PURPLE BOOK

Guidelines for quantitative risk assessment – First edition 1999/2005

RED BOOK

Methods for determining and processing probabilities - Second edition 1997/2005

You can download them for free. For example, the yellow book  can be found at

http://content.publicatiereeksgevaarlijkestoffen.nl/documents/PGS2/PGS2-1997-v0.1-physical-effects.pdf

You may refer to our following paper, which implements a quantitative method of risk assessment. It is a generalised method and may be useful for your purpose:

https://www.researchgate.net/publication/265176169_Risk_of_occupational_health_hazard_Assessment_using_CDSwFR_and_minimization_using_EMOwJCS

Article Risk of occupational health hazard: Assessment using CDSwFR ...