Safety and Risk Assessement Flashcards
The three bodies responsible for nuclear safety and their respective responsibilities
The Government - Establish a regulatory framework
- Establish a regulatory body
Regulatory body - Sets safety standards and ensures
that licence conditions are being
met through inspections
- License new installations
- Ensure corrective measures are
taken
The Operator - Responsible for upholding safety
standards set by the regulator and
creating a safety culture
How are reactors designed to reduce risks?
Ensuring that a single action cannot disable a safety feature (single failure)
Multiple independent systems perform the same activity (redundancy)
Components don’t suffer from the same fault (diversification)
Physically separate safety systems (segregation)
Failure of a component or instrument should send the plant into a safe mode (failsafe)
What is defence in depth?
The existence of multiple layered barriers/levels to protect a system.
For a nuclear reactor, DiD is key for the containment of fission products (pellets -> cladding -> reactor core -> containment)
Generally applied in 5 stages, from the prevention of faults to the mitigation of consequences
What is a potential risk?
Risks that would be present in the absence of protective measures
What is a residual risk?
The risks that remain after protective measures have been installed (will never be zero)
What is an acceptable risk
Residual risks which have been reduced to be low enough to be considered acceptable
What is a Probabilistic Safety Analysis?
PSA considers risks from identified faults and assigns a probability of failure to each fault.
Contains both fault trees and event trees.
What is a cut set?
A route between an initiator (fault) to an event (major safety event)
What is a minimal cut set?
The smallest list of events to cause top-level event.
What is a fault tree?
Fault trees determine the failures of systems. It is quantified using boolean algebra OR/AND gates. Is used to determine the probability of a top-event occuring.
What is an event tree?
Event trees develop a sequence of failures. They use binary logic to questions. Is used to determine the frequency of consequences
What is a safety case?
A set of documents describing the hazards of the site and its facilities, the modes of operation and the measures needed to prevent harm.
It should cover experience from the past, be written in the present and set expectations for the future.
What are the 5 levels of defence in depth?
Level 1: Prevent faults- Prevention of abnormal operation and failures by design
Level 2: Ensure detection if protection fails- Prevention and control of abnormal operation and detection of failures
Level 3: Limit consequences- Control of faults within design basis
Level 4: Prevent escalation- Control of severe conditions, beyond design basis
Level 5: Mitigate consequences- Mitigation of radiological consequences
What are the specific risks posed by a nuclear installation?
Reactors contain a large inventory of radionuclides
Significant energy release occurs for a long time after the fission reaction has been shut down (decay heat)
The heat source cannot be simply turned off by isolating the fuel supply, unlike fossil-fuelled plants
What are the fundamental safety functions of a nuclear installation?
Control reactivity- effective control of the fission chain reaction
Remove heat- fuel cooling must be assured at all times to maintain the integrity of the fuel clad and the structure of the core
Containment of radioactivity- containment of the fission products that were produced in the fuel and activation products in the primary coolant
