three mile island hra using therp part 2
Paper details
We have been using the Three Mile Island incident to demonstrate how a THERP analysis is done. Take another look at the SRC: Summary Sheet on THERP (PDF) (Links to an external site.) hosted by Alion’s System Reliability Center website. You have successfully covered steps 1-3 and it’s time for you to put it all together!
Step 4: Determine the effect of human errors on the system or process, including the consequences of the error not being detected. This requires the development of event trees. The left limbs of the event trees are success paths; the right limbs are failure paths. Probabilities are assigned to each path. The tree reflects the effects of task dependence. The relative effects of performance-shaping factors, e.g., stress and experience, are estimated.
For this step, use the event tree from Activity 3.2
Complete the table below by providing the missing information. Acronyms to focus on are Human Error Probability (HEP), Step By Step (SBS), Dynamic (DYN), Zero Dependency (ZD) and Moderate Dependency (MD).
Actions
Basic HEP
(error factor)
*from 3.2*
Data Source in
NUREG/CR-1278
Tables
Stress Modifier
(Experience,
SBS, DYN)
Modifier Source in
NUREG/CR-1278
Tables
Dependency
Median HEP
(error factor)
Operators fail to restore signal power
20-7 (2)
20-16, item 4
Operators fail to restore power to controls
20-7 (2)
20-16, item 4
Operators fail to close the relief valve
20-13 (2)
20-16, item 4
Operators fail to close the drain valve
20-13 (2)
20-16, item 4
Upload your event tree with associated Median HEP for each branch of the tree.
Step 5: Develop and recommend changes that will reduce the system or process failure rate. The recommended changes can be developed using sensitivity analyses, in which factors and values are varied and effects monitored. THERP makes no assumptions about the dependence or independence of personnel behaviors. Data are taken from available sources.
Upload a 1- 2 page of your final assessment and recommendations to the Plant Supervisor.
- Summarize the incident using only what was stated in the 3:15 – 4:10 segment of the Three Mile Island video.
- Explain why you chose the modifier for stress (experience, SBS or DYN), dependencies and how you arrived at the median HEP in your final answer.
- Clearly state your final assessment.
- Provide at least two changes you recommend (you may use anything you learned from the entire video to come up with these recommendations).
