The Chernobyl Nuclear Reactor accident is often given as an example of what can or will happen if reactors are used power sources. It is not usually stated what did happen any why. Recently on the RiskAnal Server, the Chernobyl Reactor has been discussed. The following are three postings by Jim Dukelow. They are the best lay description, I have seen, of this nuclear accident and why it is not a precedent for other possible accidents in Western reactors. It is quite clear from these postings that Chernobyl was not a safely designed reactor, that operations leading to the accident were ill-considered, and that well-designed reactors will not behave in a similar manner.

Disclaimer by Jim:

Jim Dukelow

Pacific Northwest National Laboratory
Richland, WA
js_dukelow@pnl.gov

These comments are mine and have not been reviewed and/or approved by my management or by the U.S. Department of Energy.

1. Posting in response to question on risk assessment of nuclear reactors.

Probabilistic risk assessments (PRAs)have been performed for many/most Western nuclear plants. They show core damage probabilities (CMPs) on the order of 1.0E-3 to 1.0E-5 per year. For U.S. plants with CMPs close to 1.0E-3, the NRC has typically required the owner to make operational and/or hardware changes to bring the risk down closer to 1.0E-4 (which is the NRCs Safety Goal for CMP). With a hundred plants operating in the U.S., we might expect core damage once every 30 to 300 years. We have had TMI, which melted about half the core, and Fermi, which caused minor core damage, in about 45 years of operation of commercial nuclear power plants. Western plants also have robust containments, which are intended to contain the consequences of a core damage accident. The NRCs safety goal for containment is that it will contribute another factor of 100 safety factor., that is, that only 1 in a 100 core damage accidents will result in a significant release of radionuclides to the environment. PRAs and some experimental work suggest that current plants may meet that goal, but there is little actual experience (happily enough), other than the fact that the TMI containment contained most of the radionuclides released from the core. The TMI accident released about a million and a half Curies of noble gases (krypton and xenon), which have very little dose consequence, and about 15 Curies of radio-iodine, for which we can calculate a small exposure to the surrounding population.

Soviet designed reactors (VVERs and RBMKs) are generally considered to be more dangerous, enough so that western countries are spending significant amounts of money to help the nations of the former Soviet Union upgrade the safety of their reactors.

Comparative risk analysis of energy sources was a substantial cottage industry in the 1970s. These studies showed electricity generation by coal and oil to be orders of magnitude more risky than nuclear power, natural gas and hydropower roughly equal in risk or slightly more risky. The underlying causes of higher risk for fossil fuels is the much greater amounts of material that must be mined or pumped, transported, and processed in order to generate a specified amount of power. In addition, all three cause morbidity and mortality due to air pollution. One item that was not considered in most of these studies was the transportation impact for coal and other fossil fuels. About a quarter of the volume of rail freight is coal moving from mine to power plant. About 600-800 people a year are killed in railroad grade crossing accidents. About 15 years ago a liquefied petroleum gas tanker went off the road into a roadside campsite in Spain, killing about 170 campers. About 10 years ago, a train ignited a cloud of natural gas escaping from a pipeline in the Urals in the Soviet Union, killing about 600 people on the train. Coal mine accidents killing a few 10s of employees and oil refinery accidents killing 5 to 10 employees are pretty common.

One of the more interesting of the comparative risk studies was Herbert Inhaber's Risk of Energy Production, AECB-1119, Rev 1, an Atomic Energy Control Board (Canada) report. Inhaber looked at 10 energy sources, including six renewable energy sources, and considered risks associated with construction, fueling, and operation (including accidents). He found total deaths per Megawatt-year of electricity to be lowest for natural gas, next lowest for nuclear, slightly higher for hydro, solar space heating, and ocean thermal energy conversion, and one to two orders of magnitude higher for coal, oil, wind power, methanol, and solar electric technologies. Results for man days lost due to occupational accidents and public health impacts were similar. As far as I can recall, none of the 1970s studies consider global warming issues.

2. Posting describing the cause and nature of the Chernobyl accident.

The Chernobyl accident we all know about (at Chernobyl Unit 4) was very much a nuclear accident. It occurred because of a combination of design defects of the Soviet-designed RBMK reactor with performance of a experiment with the reactor that, ironically, was intended to establish safer ways of bringing the reactor to a safe-shutdown condition after an operational upset. During the course of the experiment, the reactor drifted into a condition of "prompt criticality" . When barely prompt critical, an RBMK reactor (the Chernobyl reactor type) will increase in power by a fraction of 1% every 0.001 second. At Chernobyl, the prompt criticality resulted in a power spike from the 20% of normal full power where the reactor had been operating to approximately 100 times full power, followed by a dip to 10 or 20 times full power, followed by a second spike to approximately 500 times full power, all of this happening within about four seconds.

The two power spikes had several side effects. The energy they deposited in the fuel essentially blew it apart, fragmenting it. The fragmented fuel transferred much of its heat to the surrounding cooling water, which did two things: 1) produced a lot of steam (this is called a steam explosion and can occur in non-nuclear contexts also, particularly in metal foundries), and 2) the steam reacted with the fuel, the fuel cladding, and the graphite to produce a lot of hydrogen, carbon monoxide, and other flammable/explosive gases, which then exploded. The steam explosion and the gas explosion blew apart the reactor compartment, blew a hole in the roof of the reactor building, and completed the job of igniting the graphite stack, whose fire provided a continuing source of energy for dispersing the contents of the core over the next couple of weeks.

A hint of the force of the initial power spikes can be found in the Chernobyl folklore that micron sized pieces of unmelted fuel were found in Finland and Scandinavia a day or two after the accident. The fuel was fragmented BEFORE it had a chance to melt.

What George may have been thinking about was a serious fire at Chernobyl Unit 2, which was caused by the ignition of hydrogen used to cool the electrical generator and which caused serious damage to the turbine hall. That fire/accident did not involve the nuclear portion of the plant, although, if memory serves, the operators did have some difficulty shutting down the reactor and getting the plant to a stable, shutdown condition while the fire was being fought.

A TECHNICAL NOTE: One physical phenomenon that allows nuclear reactors to operate in a safe, controllable fashion is the delayed neutron. A small fraction of the neutrons produced by fission are actually emitted by fission products a few seconds (ranging from 0.2 seconds up to about 55 seconds) after the fission event. This means that when a reactor becomes slightly critical, its power will increase, but slowly, at a rate determined by the delayed neutrons. On the other hand, if the reactor becomes sufficiently supercritical to be prompt supercritical, then it is supercritical using prompt neutrons alone, and its rate of power increase will be controlled by the prompt neutron generation time, which is the average time between the release of a neutron in a fission event and the fission that it causes. The prompt generation time varies from around 0.0001 second up to 0.001 second, depending on the type of reactor. A design feature that permits reactors to operate safely is their intrinsic stability, which depends on their coefficients of reactivity. If a reactor is operating at steady power (i.e., it is critical, not sub- critical or super-critical), what happens if temperature, or void fraction (the fraction of steam bubbles in the cooling water), or power increase for some reason? If the reactor core is designed with negative reactivity coefficients, then these increases will produce negative feedbacks that tend to pull it back to a lower power. Intrinsic stability is a de facto requirement for licensing reactors in the U.S. and most of the rest of the world. RBMKs, at the time of the Chernobyl accident, had a positive void coefficient and a positive power coefficient (which was directly responsible for the first power spike). I believe that RBMKs have since been modified so that their power coefficient of reactivity is negative.

3. Posting clarifying the nature of the Chernobyl Operations leading to the accident

I didn't intend my lack of a ringing denunciation of the experiment to be taken as evidence that I thought it was OK. The goal of the experiment, to establish that the continued production of steam in the core after a shutdown could be used to drive a turbine/generator to produce the electrical power needed for essential safety equipment, was, perhaps, legitimate. However, the design and implementation of the experiment did not take into account some of the trickier aspects of the neutronics of the reactor (i.e., the behavior of neutrons in the core) and the coupling of the neutronics with the conditions of the cooling water (i.e., the thermal hydraulics of the reactor). In the run-up to the accident, the plant operators, without realizing they were doing it, brought the plant onto a neutronic and thermal hydraulic knife-edge, where a slight perturbation could drive it into a prompt criticality. Paradoxically, the perturbation was probably provided by the operator's attempt to scram the reactor. The control rod design was such that the first fraction of a second of rod insertion would add reactivity rather than subtract it.

At their root, almost all errors and failures are "human" errors. In the case of the Chernobyl accident, these included design errors and bureaucratic errors. Some of the safety systems had to be locked out to even perform this experiment, which should have told the reactor designers and the plant engineers that they needed to consider the experiment very carefully before performing it.

Jim Dukelow

Pacific Northwest National Laboratory
Richland, WA
js_dukelow@pnl.gov

These comments are mine and have not been reviewed and/or approved by my management or by the U.S. Department of Energy.