We will be using the Three Mile Island incident to demonstrate how a Technique for Human Error Rate Prediction (THERP) analysis is done. First, take a look at this Summary Sheet on THERP (PDF) (Links to an external site.) hosted by Alion’s System Reliability Center website.

Next do the following activities. Steps 1-3 of the Summary Sheet on THERP are given below as a guide for your THERP analysis:

Step 1: Define the system or process. This involves describing the system goals and functions and the consequences of not achieving them. It also requires identifying mission, personnel, and hardware/software characteristics.

The “Meltdown on Three Mile Island” video ( 

https://www.youtube.com/watch?v=D8W5hq5dsZ4)in Module 1 gave you an understanding of the situation on that fateful day, Wednesday, March 28, 1979. Review the video if needed (use time segment 3:15 – 4:10 to focus on the key aspects needed for this assignment).

Step 2:  Identify and list all the human operations performed and their relationships to the system or process tasks and functions. This requires an analysis of all operator and maintainer tasks.

Take a look at the human operations depicted in the Fault Tree (PDF) of what occurred in segment 3:15 – 4:10 of the video. 

Step 3: Predict error rates for each human operation or group of operations. Errors likely to be made in each task or subtask must be identified. Errors that are not significant in terms of system operation are ignored. This step includes estimating the likelihood of each error occurring and the likelihood of an error not being detected. 

For this step, use the Event Tree (PDF) provided to fill in the error rates.

Determine the basis human error probability (HEP) based on the event tree provided.  You will be using data found in the NRC: Handbook of Human Reliability Analysis with Emphasis on Nuclear Power Plant Applications (PDF) (Links to an external site.) (NUREG/CR-1278).  Go to Chapter 20 to see how to use the THERP tables to locate probability values.

For this case, the data sources in the NUREG/CR-1278 tables are shown for each action in the table below.  Given that the operators omitted a procedural step from a long (>10 items) list of procedures, what should the Basic Human Error Probability (HEP) for each action be?  

ActionsHEP (error factor)

Data Source in

NUREG/CR-1278

Operators fail to restore signal power

20-7 (2)

Operators fail to restore power to controls

20-7 (2)

Operators fail to close the relief valve

20-13 (2)

Operators fail to close the drain valve

20-13 (2)

Upload your event tree with associated Basic HEP for each branch of the tree.