Damage Control - Interim Acceptance Criteria

SAM-N – To their activist peers at universities and elsewhere during the late 1960s and early 1970s, nuclear safety engineers and scientists in places like the National Reactor Testing Station in Idaho and the Oak Ridge National Laboratory in Tennessee were seen as conspirators fulfilling some nefarious government agenda. In hindsight, such feelings are understandable given the pace at which the commercial nuclear power industry was moving and the skepticism of the times. With doubts about nuclear power safety drawn from the Semiscale experiment and the immaturity of analysis tools like RELAP3 and THETA2-1B, the public, given voiced by the Union of Concerned Scientists and Ralph Nader, demanded constraint and introspection from it government.

Unappreciated at the time, the Atomic Energy Commission responded and actually did so fairly quickly. The time between the last Semiscale test demonstrating the bypass of early coolant injection in January of 1971 and the publication of new rules regarding the conduct of safety analysis was less than six months. In those six months, the staff at the NRTS was busy studying the Semiscale results and their computer models to derive meaningful lessons for the new industry.

The “Interim Acceptance Criteria” (IAC) appeared in the U.S. Federal Register on June 29, 1971. Its content addresses more than can be presented here; but, the primary takeaways are the acceptance criteria and the rules for analysis. IAC identified four acceptance criteria. In their words,

“[The] performance of the emergency core cooling system is judged to be acceptable if the calculated course of the loss-of-coolant accident is limited as follows:

1. The calculated maximum fuel element cladding temperature does not exceed 2,300°F …

2. The amount of fuel element cladding that reacts chemically with water or steam does not exceed 1 percent of the total amount of cladding in the reactor.

3. The clad temperature transient is terminated at a time when the core geometry is still amenable to cooling …

4. The core temperature is reduced and decay heat is removed for an extended period of time, as required by the long-lived radioactivity remaining in the core.”

With the IAC, loss-of-coolant accident calculations prepared for demonstrating safety had to abide to a particular set of instructions. First, the IAC specifically identified that RELAP3 and THETA1-B be used, and then it described how these codes should be applied to address uncertainties in the principal phenomenological contributors impacting fuel temperature. For the application of RELAP3, these included:

• applying the conservative Moody discharge flow model (with a nominal break discharge coefficient);
• applying the decay heat curve described in the proposed ANS Standard, with a 20 percent allowance for uncertainty;
• examining two break models: 1) the double-ended severance (guillotine), which assumes that there is break flow from both ends of the broken pipe, but no communication between the broken ends and 2) assume discharge from a single node (split).
• setting the time after the break for the onset of departure from nucleate boiling at the hot spot to 0.1 second.
• assuming that for cold leg breaks, all of the water injected by the accumulators prior to end-of-blowdown bypasses the reactor core region;
• applying a containment back pressure less than the initial pre- break pressure plus 90 percent of the increase in pressure calculated for the accident under consideration;
• using a pump resistance, K, based on the more conservative of two assumptions (locked or running).

Several additional requirements were then listed for dictating the application of THETA1-B. These mostly related to conservatively impacting the rate of vessel reflood, such as maximizing the effect of steam binding.

It was true that regarding policy, it was (and still is) the job of nuclear safety engineers and scientists to serve the decision-makers in Washington, D.C. From experiments and mathematical models developed at the national laboratories, the AEC staff was able to prepare the foundation of a new nuclear safety rule - a practice that continues today. This particular rule would become the focal point of congressional hearings beginning in January 1972. It is useful to note that for all that engineering and science developed at the laboratories in Idaho and elsewhere, the conclusions drawn and the requirements drafted were translated into very simple language. An important lesson from this is that no matter how complete or consistent the “science” may seem to the scientist or engineer or even the regulator, if it can’t be communicated to its intended audience, tension, distrust, and a loss of credibility should be expected.