Emergency response | Power plant cooling

‘Extra helpings, please’

1 June 2012

The US industry has responded to post-Fukushima regulatory requirements by devising a generic all-hazard strategy relying on multiple sets of emergency equipment. The programme is an upgrade of the industry’s response to the September 11 (2001) attacks. By Will Dalrymple

The starting point for the US industry’s main post-Fukushima emergency response, called FLEX, is to assume a loss of AC power, which drives standard reactor safety equipment, and a loss of ultimate heat sink, which is the essential removal of decay heat. FLEX aims to provide additional layers of protection in these circumstances. It will form part of the industry’s legally-binding response to the US regulator’s post-Fukushima orders to be able to mitigate beyond-design-basis accidents.

In a press conference, Charles Pardee, Exelon chief operating officer and chairman of the US industry Fukushima response steering group, said that it is trying to come up with a way to provide cooling to the reactor core, spent fuel pond, to remove decay heat, and maintain the integrity of containment to contain fission product releases.

The response to such an emergency would proceed in a three-stage sequence. First, workers use installed equipment on site, be that turbine-driven equipment powered by steam decay heat, the reactor coolant injection system, or auxiliary pumps. Powering these systems first of all are diesel generators, usually the first port of call in a power failure, although their duration is limited by the capacity of site diesel tanks. Nuclear plants’ secondary site power source are batteries. Most US plants are required by law to be able to subsist for four hours on battery power alone, but that period could be significantly extended if some of their loads were reduced, said the Nuclear Energy Institute’s senior vice president Tony Pietrangelo.

Onsite installed equipment would then be supplemented by onsite portable equipment. Post-9/11, utilities supplemented installed equipment with portable equipment stored on site to fight fires and explosions under a February 2002 NRC security order, section B.5.b of which required them to identify strategies to maintain reactor cooling with ‘readily available’ equipment. These requirements, and initiatives, have come to be called by that section number, B.5.b. The FLEX-based concept was originally devised post- 9/11 in response to the effects of a large fire and explosion on a reactor site, explains the Nuclear Energy Institute’s executive director of strategic programs, Adrian Heymer. “The idea was, if you have emergency equipment located on site but outside the plant, workers can connect it into the plant, and get water into the reactor and spent fuel pool for cooling, and also fight the fire, and that would deal with the issue,” he said. When Fukushima occurred, the NEI team discovered that although the cause of the event was different—natural disaster—its effect was the same.

A key part of onsite portable equipment is redundancy, Heymer said, so that a set of emergency equipment is available wherever an incident occurs on site, and also for extra capacity in case of failure or damage. One set might be stored inside the plant, another outside the plant but within the site boundary. Site supplies also include consumables (water, diesel, compressed gas canisters). The post-Fukushima focus on risks posed by the failures of multiple units on a single site has further heightened awareness of the need for redundancy of portable equipment sets, which are likely to be purchased in a 2:1 or n+1 relationship to the number of reactors, he said. Also, FLEX will focus on the means of connection, making sure that there are multiple alternatives for connection of electricity and water pipes, so that they can function in different parts of the plant.

Having connected that equipment, site workers will be able to keep the reactors stable until they can receive phase 3 support from offsite regional equipment storage and support organisations. Equipment held offsite would be similar, if larger, than onsite emergency gear, such as 1 MW-2 MW generators, high-pressure pumps, additional self-contained breathing apparatus, and consumables, such as radiation monitors, additional cabling and hoses.

Heymer said that the offsite equipment repository idea came partly out of an awareness that commercially-available portable equipment would not be as reliable as equipment permanently-installed on site. “We recognised from experiences dealing with natural events that when we use portable equipment, we need a backup. We don’t necessarily want to have everything on site, but in severe natural events such as hurricanes and tornadoes, we can normally get equipment to the station in 48-72 hours. At Fukushima Daiichi there were fire trucks arriving at the station within 24 hours that came from regions to the north and southwest.”

The exact location of such an equipment depot, or depots, is still under review. Heymer said that a location that seems ideal from a transport point of view may not ideal because of risk of exposure to natural events such as tornadoes and earthquakes. He also said that it remained to be decided whether the depot would be run by industry or a third-party service provider.

The NEI has also drafted site-by-site guidance on how to implement the FLEX programme, including how to assess external hazards and protection criteria. And there will be NRC and INPO oversight of periodic equipment testing to make sure that it will work as needed.

Orders for the first tranche of portable equipment, comprising 300 pieces including injection pumps, air compressors, small generators for recharging batteries, and larger generators, were due to be placed by the end of March, for delivery into 2013. A second order will follow later in 2012, after site calculations are completed, and the NRC requirements are clearer, Heymer said. Plant modifications, even if simple, will take more time to be engineered, reviewed, and scheduled into an outage. He said that it is now aiming to complete all of the work by the end of 2016.

Although the cost of implementing FLEX will vary, it will probably be relatively low; Pietrangelo said it was around a ‘couple’ of million dollars per station, although it was unclear whether that sum included equipment stored centrally.

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This article was first published in the May 2012 issue of Nuclear Engineering International

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