A Scientific Critique of the Accident Risks from the Cassini Space Mission

By: Dr. Michio Kaku

Henry Semat Professor of Theoretical Physics
Physics Department
City University of New York
New York NY 10031

Table of Contents:

• I. Introduction
• II. Calculation of Casualties from a Maximum Accident
  • A. Launch Phase
  • B. Fly-by
• III. Calculation of Risk
  • A. Launch Phase
  • B. Fly-by
• IV. Calculation of Solar Alternative
• V. Conclusions and recommendations
• VI. Short biography

I. Introduction

What divides the experts is:

• (a) how much plutonium can be realistically released in a maximum credible accident and
  
• (b) the likelihood of such an event.

All parties are agreed that such an event is unlikely. It may happen that the Cassini mission may be a resounding, flawless success. However, it's only a matter of time before some disaster strikes. Instead of relying on misleading computer programs which tell you what you want to hear, one should carefully examine the actual track record of accidents in the space program, with numerous booster rocket failures and space probes which malfunction.

Unfortunately, the true risks from such an accident and the consequences have been downplayed. In a democracy, the American people can make rational decisions only on the basis of scientific truth, not simplistic, misleading press releases. It is inevitable that there will be spectacular accidents with the space program, some involving casualties, and the American people have a democratic right to know what the true risks are. Thus, it is a matter of scientific interest to go over line-by-line the calculation of the FEIS.

NASA calculates in its FEIS that up to 2,300 people might come down with fatal cancer over a 50 year period from the dispersal of plutonium-238 over a populated area. More recently, it has lowered this figure to 120. However, the calculation of these figures depends on three important steps, each of which has been underestimated by NASA:

• the calculation of the "source term," i.e. the amount of plutonium-238 which actually escapes and is dispersed into the environment
• the calculation of the land contamination area over which the plutonium-238 is spread
• the calculation of the population density and how many may come down with cancer.

• a) the FEIS consistently underestimates the possible risks, avoiding the maximum credible scenarios.
• b) since NASA has never conducted a full-scale test of a realistic accident scenario, the FEIS simply makes up numbers to compensate for its ignorance. However, the FEIS consistently fabricates these numbers in a certain way: to arrive at the lowest casualty figures.
• c) the FEIS disguises this fact by giving the results to three significant figures, which makes the figures seem authoritative and accurate, when in fact they are largely created by fiction. Of course, it is justified to make estimates. But it is then standard procedure within the scientific community to give error bars or estimates of uncertainty. However, one immediately spots a glaring error: no uncertainties are ever given in the FEIS, which is a serious flaw. No uncertainties are given because their numbers are simple educated guesses, not real experimental numbers at all.

II. Calculation of Casualties from a Maximum Accident

A Launch Phase (Phase 1,5 and 6)

• a) Source Term
  The most important component of the calculation is the determination of the "source term." The FEIS admits that plutonium will escape from the RTGs during an accident both in the launch phase as well as the fly-by. However, the FEIS typically concedes that only a tiny fraction of a percent of the plutonium inventory will escape. This severely underestimates the true impact of a maximum credible accident and results in artificially low casualty figures. This is the main weakness of the FEIS.

  The FEIS admits that plutonium in the RTGs will be subject to three extreme conditions during a launch phase accident: high temperatures, shrapnel, and explosive over-pressure. However, the essential problem is that NASA engineers have failed to perform a full-scale, realistic test of an explosion involving the RTGs.

  In other areas of engineering, we have a good understanding of what happens when many different types of catastrophes happen, e.g. plane crashes and train wrecks, because we have a large body of experimental data. However, we have no experimental data by which to estimate the true dispersion of plutonium during a launch phase explosion because no realistic tests have ever been conducted.

  NASA, however, has conducted some partial tests, which already reveal the vulnerability of the RTGs to extreme environments. The FEIS in fact, concedes that plutonium will escape the RTGs during a launch phase explosion, but its analysis is purely hypothetical and results in only a rough estimate.

  In particular, we find:
  

  • i) High temperatures. The iridium casing surrounding the RTGs begins to oxidize and degrade at 1,000 degrees C, and begins to melt at 2,425 degrees C. Graphite eutectic melting points are even lower: 2,269 C. Experiments with the fuel cladding show that they may resist temperatures of about 2,360 C found in propellant fires, which is just 65 degrees below the melting point of the iridium casing, but are expected to fail beyond that.

    Several conclusions can be drawn:
    

    • The laws of thermodynamics show that there is a statistical distribution of molecules at kinetic energies beyond the average one, given by the Maxwell-Boltzmann distribution, indicating that structurally the iridium casing will begin to soften and weaken even as it approaches its melting point. In other words, the structural integrity of the iridium casing will degrade as it approaches its melting point and make it possible for shrapnel and explosive over-pressure to burst open the casing. Thus, the combination of temperature, shrapnel, and over-pressure may be sufficient to burst most of the containers wide open.
    • Temperatures even beyond 3000 degrees C can typically be found locally in chemical explosions and reactions (e.g. an acetylene torch typically burns at 3,315 C). This is well beyond the melting point of the iridium casing. As a rough estimate, we know from the Stefan-Boltzmann and Wien's law that the color of a flame is roughly correlated with temperature, and the color red typically found in combustive reactions (at wavelengths of 7,000 angstroms) will be correlated with temperatures of about 4,000 C. Thus, we can expect some melting of the iridium casing due to local heating within the fireball, although the average temperature may be lower than the melting point.
  • ii) Shrapnel. Tests have shown that aluminum bullets fired at the RTGs at velocities of 1,820 ft/sec and titanium bullets fired at l,387 ft/sec have caused a breach of containment. Edge-on fragments at velocities as low as 312 ft/s can rupture the leading fuel clads. So even at room temperature, we can expect high-velocity fragments to pierce the RTGs. But at high temperatures near the melting point of iridium, where the RTG casings are weakened by high temperatures and pressures, we can expect shrapnel to do even more damage to the RTG casings, bursting many of them open.
  • iii) Over-pressure. Chemical explosions can cause local over-pressures of several thousand pounds per square inch. The RTGs have been tested to 2,210 lb/ft^2 without fuel release. However, under the weakened conditions created by high temperature, shrapnel, etc., it is not known how much can actually escape.

    The point is that a full-scale test involving the simultaneous conditions of high temperature, shrapnel, and over-pressure has never been done. It is likely that the combination of all three will cause severe rupturing of the RTGs.

    In spite of all these factors and uncertainties, the FEIS on p. 4-48 confidently concludes that a maximum of 28.7 curies, or less than .01% of the plutonium, will escape during a launch phase accident.

    Several points can be made:

    • This estimate is sheer speculation. The number is made up. Since no one has ever done a full-scale test of the RTGs in the explosive environment of a booster rocket failure, it pure guesswork as to how much plutonium will escape.
    • However, the estimates are given as a statement of fact, with no error bars or indications of reliability. We have no indication of the confidence level of this number. This is a severe statistical mistake.
    • The figure of 28.7 curies of plutonium is given to three significant figures, which is rather surprising, revealing a lack of grasp of statistical analysis on the part of the engineers. According to the laws of statistics, the propagation of errors determines that a calculation is no more reliable than its largest source of error. The largest source of error in this calculation is the fact that the engineers have made up many of the numbers out of thin air. Thus, calculating the plutonium release to three significant figures reveals a remarkable lack of understanding of even elementary statistics.
    • Given the fact that the simultaneous effect of high temperature, shrapnel, and over-pressure has never been fully tested, and given the fact that in combination they will probably cause a large failure of the iridium casing, a figure of 30% to 40% release is probably more realistic.

• b) Area of Impact.
  In typical radiological computer programs conducted by the military and the commercial nuclear industry, the area of impact of the accident is largely a function of wind conditions. Computer calculations involve solving a simple second-order partial differential equation (the standard Helmholtz equation with source term) by iterations. Because we have the conservation of mass, the source term is the driving term within this second order differential equation, sometimes called the diffusion equation.

  In addition, actual experiments have shown that micron-sized particles of natural uranium, U-238, can be dispersed by the wind over 25 miles. In nuclear power plant accidents, radiation has been dispersed several thousand miles from the original accident. (For example, in the Windscale disaster in England in 1957, which was completely hushed up by British authorities, the radioactive cloud emerging from the carbon-moderate reactor was tracked going over London, sailing over the English channel, and finally dispersing over Cairo, Egypt. More recently, the radiation from Chernobyl was widely tracked over Europe and even the U.S.)

  However, what is rather remarkable is that the FEIS totally ignores wind conditions and merely postulates that the plutonium will be dispersed, in one scenario, within an area of 7.18 x 10^-2 square miles. This is a roughly a square area 1,000 feet on each side. Again, the fact that this is presented without any error bars, and to three significant figures, shows the ignorance of the engineers who calculated this number.

  But what is revealing is that the FEIS assumes that almost all the plutonium will be confined to the launch facility. According to the FEIS, no plutonium is expected to leave the launch pad area. In other words, NASA engineers have discovered a new law of physics: the winds stop blowing during a rocket launch.

  But anyone who lived through the Challenger explosion, the Delta rocket explosion, etc., will realize that debris has been pulverized and spread over a significant area. Eyewitness accounts of the recent Delta rocket explosion indicated debris scattered over several miles.

  In fact, experiments conducted on metal oxides have shown that a significant percent of the inventory can be pulverized into a fine dust of micron-sized particles, which can then be blown miles from the original site by the winds. These micron-sized particles are especially dangerous because they stay lodged deeply in the lungs for decades, where ciliary action is useless in expelling these particulates. Thus, these particles can emit radiation at close range to nearby lung tissue for decades to come, causing cancer.

• c) Population density.
  Yet another reason for attaining low estimates of risk is the FEIS's assumption that the population density is rather low. In this calculation, one problem is determining the number of person-rems which will initiate a cancer. One can reasonably assume that 5,000 person-rems will induce a single cancer. (Although some critics have placed the true figure as low as 300 person-rems/cancer.)

  However, what is in dispute is the fact that the FEIS assumes a rather average density of people per square mile. This is therefore not a maximum credible accident, which would assume that the winds blow the plutonium into a major city.

  For example, the FEIS assumes that, for a Phase 5 accident over Africa, the expected health risk would be 1.5 x 10^-4 over a population of only 1,000 people. This is low even for Africa. Not to mention that the rocket may misfire during the launch phase and tumble in a partial orbit, thereby landing almost anywhere on the earth, rather than in Africa.

  A Phase 1 accident would release plutonium in an area populated by only 100,000 people. But if the winds blow, then the area affected within 5 counties of the launch site could total over a million people.

B. Fly-by Phase (VVEJGA Phase)

• a) Source term.
  The FEIS admits that about 32% to 34% of the plutonium is expected to be released high in the atmosphere. However, the FEIS then dismisses this factor by diluting it over the population of the entire earth. This neglects the fact that the mixing of plutonium in the atmosphere takes a considerable amount of time, and in the meantime it may concentrate or hover over certain regions of the earth. This effect is ignored by the FEIS.

  The FEIS then calculates how much plutonium may actually land on the earth, and again underestimates the real risks.

  The FEIS first divides the source term into three parts: a rock impact, soil impact, and water impact, and then calculates the percent distribution of each on the planet earth. For example, the FEIS estimates that 4% will hit rock, 21% will hit soil, and 75% will hit water.

  This is a rather odd way of calculating maximum risks, because it confuses probability of an accident with the consequences of that accident. The calculation of how much surface area of the earth is divided into rock, soil, and water belongs in a calculation of the probability of mishap, not in the calculation of maximum risk.

  The calculation of maximum credible risk necessarily assumes maximum risk by definition, i.e. that all the plutonium will hit rock, since that is the maximum credible scenario. Rather than 4% of the plutonium hitting rock, one should assume that all of it does.

  Second, the FEIS calculates the percent of plutonium that can be released on impact with rock, soil, and water. Again, these numbers are simply pulled out of a hat, with no justification. For example, in one scenario, it assumes that all of the plutonium will be dispersed if it hits rock, 25% of the plutonium hitting soil will escape, and none hitting water will escape. However, no justification is given for these estimates, because there are none.

  The important point is that no one has ever done an experiment calculating the effect of entering the atmosphere with RTGs at 40,000 miles per hour. Until this experiment is done (using a replacement for plutonium), all these numbers are purely speculative.

• b) Area of impact
  The estimated land contamination for this plutonium accident is on the order of 2,000 sq. km. That is roughly equivalent to a square about 27 miles on a side.

  More recently, on April 1997, the Supplemental Environmental Impact Statement has revised the early estimate of cancer fatalities from 2,300 to 120 (p. 2-19). This may seem strange, until one realizes that their assumptions have become even more conservative. Instead of assuming that land contamination can be 2,000 sq. km, the new estimate puts it at a surprisingly small area of 7.9 sq. km. This is a square about 1.7 miles on each side. In other words, the new EIS assumes that the Cassini probe, coming down in flames from outer space at 40,000 miles per hour, will hit a bull's eye and then remain there, without any winds whatsoever.

  This is a remarkable reduction by a factor of 250, which once again is pulled out of a hat, without any justification. Not surprisingly, the casualty figures have also dropped significantly, from 2,300 to 120, a factor of 20.

• c) Population density
  Again, the FEIS assumes average figures for population density, and totally neglects the fact that there are large population concentrations on the earth where tens of millions live. Within a 50 mile radius of Manhattan, for example, there are about 20 million people, or about 8% of the entire population of the U.S. Similarly, there are other concentrations of people on the earth with even larger densities, such as around Tokyo, Mexico City, and Shanghai.

III. Calculation of Risk

Although these methods are standard for the field, these methods have largely been discredited by the actual operating record of nuclear power accidents. Three Mile Island, for example, was a Class IX accident which was largely unforeseen by MIT's WASH-1400, the standard reference within the industry, which largely ignored small pipe breaks.

The methodology is flawed for several reasons: