NIKE

For the past few decades, Louisiana Emergency Response Plans for a potential accident at a Nuclear Power Plant have been based on the 1992 Environmental Protection Agency (EPA) Protective Action Guides (PAG). In 2017 the EPA issued a new PAG manual which updated the dose assessment methodology and made other recommended changes to policy, including a supplemental guideline for administering Potassium Iodide (KI) based on the child thyroid dose, rather than evacuation on adult thyroid dose, as previously.

Louisiana Department of Environmental Quality (LDEQ) has elected to implement a modified form of these recommendations, using the 2017 PAG Manual dose assessment methodology with an additional evacuation threshold based on the child thyroid dose, in lieu of KI administration, to become effective by the end of 2020. It will be some time before the Nuclear Power Plants can adopt the 2017 PAGs, if ever, and during this transitional period there may be accident scenarios in which LDEQ recommendations to government officials would differ from those of the Nuclear Power Plant. Either the difference in dose assessment methodology, or the additional Louisiana Child thyroid dose evacuation threshold, can lead to Louisiana and the Nuclear Power Plants being bound by their planning and licensing basis to give evacuation recommendations that differ from each other.

This study will perform a comprehensive ensemble of simulations of potential accidents designed to determine which scenarios would lead to these contradictory recommendations. The result will be a tool which tabulates this data for a wide variety of accident and weather conditions, and will allow utility and LDEQ personnel to know in advance which conditions will likely lead to a contradictory set of recommendations so that they can prepare how to communicate and explain this difference beforehand. The results of these simulations will also be useful to other states as they consider implementing the 2017 PAGs.

Figure: In the event of an accident at a nuclear power plant, the EPA Protective Action Guidelines (PAG) establish dose thresholds at which an evacuation for a given geographical area should be ordered if the ‘most exposed individual’ in that area exceeds that threshold dose. Under the 1992 EPA Guidelines, there were two such thresholds at which an evacuation would be ordered if either one of the two were exceeded: A 1 rem adult whole body dose (TEDE) and a 5 rem adult thyroid dose (CDE), both of which were calculated using the ICRP 26/30 dose assessment methodology.  The 2017 EPA PAG Manual replaced the two separate evacuation thresholds with a single adult whole body dose (TED) threshold of 1 rem, which will now be calculated using the ICRP 60/72 dose assessment methodology. Because of the increased weighting given to the thyroid gland in the ICRP 60/72 methodology, this single threshold will normally be as conservative as the older combined thresholds, even in cases in which substantial radioactive iodine is released.

Due to the increased sensitivity of the child thyroid gland to radioactive iodine, Louisiana has added an additional threshold of 5 rem to the Child Thyroid. An example of an accident in which this additional evacuation threshold would lead to a different evacuation order for the same accident is shown above. A Loss of Coolant Accident (LOCA) is simulated for the Waterford 3 Nuclear Power Plant near New Orleans, which involves a substantial release of radioactive iodine. Yellow indicates areas where the evacuation thresholds are exceeded. These results indicates that while the new Whole Body Dose does not exceed evacuation thresholds past 5 miles, the new Child Thyroid Dose threshold added by the state of Louisiana is exceeded past 10 miles, leading to a very different evacuation order.