A Primer on Nuclear Safety: 2.1 Defense in Depth

A Primer on Nuclear Safety:
2.1.1 Defense in Depth
Light Water Reactors - Physical Barriers

The Defense in Depth philosophy is applied to the release of radioactive materials from inside the core of light water reactors. Helen Caldicott, the ceaseless critic of nuclear power notes

Nuclear power creates massive quantities of radioactive isotopes, which are classified as nuclear waste. Among these materials are strontium 90, . . cesium 137 . . . plutonium, . . . lutonium has a radioactive life of half a million years. It enters the body through the lung, where it is known to cause cancer. It mimics iron in the body. Hence it migrates to the bone, where it can induce bone cancer or leukemia, or to the liver, causing liver cancer; and it crosses the placenta into the embryo, where, like the drug thalidomide, it can cause gross birth deformities. Finally, it has a predilection for the testicles, thus inducing genetic mutations in humans and other animals that are passed from generation to generation for the rest of time. Meanwhile, the plutonium itself lives on to enter testicle after testicle, lung after lung, liver after liver for the rest of time as well. Children are 10 to 20 times more susceptible to the carcinogenic effects of radiation than are adults.

That it is possible for such radioactive materials to escape in massive amounts from some reactors is certain given the Chernobyl accident. Even though massive amounts of radioactive materials that escaped during the Chernobyl incident did not lead to the sort of human disaster Dr. Caldicott imagined large scale releases of bioactive readio active materials from reactors is highly undesirable.   

During the 1950's and 60's nuclear chemists at Oak Ridge National Laboratory did extensive theoretical, field and Laboratory research on routs to radioisotope release from reactors.   This research was of major importance to nuclear safety because by identifying routs for radioisotope escape, the researchers alerted reactor designers to those escape routes and the possible means of mitigating events that could potentially lead to radioisotope escape.  

Nuclear safety researchers were by no means satisfied with their acomplishments. In 1967 my father, C.J. Barton, Sr. wrote

In order to promote confidence in such large reduction factors, continued research into the efficiency of removal for al l the various forms of the released fission products will be required.

During the 1960's researchers at ORNL. Battelle Northwest, and Phillips-Idaho conducted sophisticated containment and reactor accident research with facilities that were designed to simulate nuclear accidents. Again the findings of this research were fundamental to reactor safety design. As I have pointed out elsewhere in this blog, the continuation of nuclear safety research at AEC facilities became during the late 1960'sane early 1970's became a major matter of political controversy.

Even though the politically inspired attack on nuclear safety research was never completely rectified by the American political establishment, enough progress had been made to allow for great improvements in Light Water Reactor safety.

Physical Barriers increase Light Water Reactor safety

Defense in Depth against the release of radioisotopes required a series of physical barriers that inhibited the movement of radioisotopes from the nuclear fuel pellets, into the environment. In order to illustrate the defense in depth of civilian light water reactors, a brief comparison to the Soviet RBMK reactor is in order. The failure of the safety features of one of the RBMK at Chernobyl lead to the release of large amounts of radioisotopes from the reactor core. The RBMK reactor like Western Light Water Reactors featured ceramic uranium fuel elements made of uranium dioxide baked at high heat. In Western reactors the fuel pellets are clad with Zirconium a sturdy metal that resists the reactors heat and radiation.
The Uranium Oxide fuel is itself the first barrier in the defense in depth, and it is a one of the strongest barriers in the whole defense. Fission products are basically locked in to the rock like fuel pellet. As George Parker and my father were to observe that the release of fission products from Light Water Reactor fuel was cause by a variety of mechanisms that were all triggered by overheating. Thus the first barrier could be breached by reactor over heating.

The Zirconium cladding adds protection against fission product escape. Zirconium has a high melting temperature, although not as high as uranium oxide. Like uranium oxide, zirconium and zirconium alloys are dependent on reactor cooling to prevent to maintain integrity as a barrier to fission product escape. A further consequence of the failure of Zirconium cladding would be that it would subject uranium oxide fuel to mechanisms that promote fission product loss.

A Zirconium tubes in which the fuel pellets rest in the reactor core constitute a third barrier to fission product release, however in practice if reactor core heat is sufficiently high to cause the failure of Zirconium cladding, it will also cause the failure of zirconium tubes. The outer structure of the reactor provides a further barrier to fission product release. In LWRs the pressure vessel is a major barrier to solid fission product release, although radioactive gases can work their way around the barrier in major reactor accidents. The RBMK does not have a pressure vessel, which is perhaps the most significant reason for the massive release of radioisotopes in the Chernobyl accident. The Chernobyl RBMK appears to have included an outer structure designed to maintain the RBMK core in a helium environment in order to prevent graphite burning. This containment structure failed during the Chernobyl incident, and the resulting graphite fire contributed greatly to the fission product release during the Chernobyl incident.

The next barrier to fission product release is the reactor outer radiation shield. Although this shield is seldom mentioned in discussions of defenses in depth, it does provide a barrier to the release of solid and molten fission particles whose movement is limited by the forces of gravity. Thus in the event of a core melt down which penetrated the pressure vessel, the radiation shield would offer considerable containment of the molten fission particles. Because of its massive nature, the radiation barrier would also mitigate a steam explosion powerful enough to rupture the wall of the pressure vessel. The sideways and downward pressure of the steam explosion would be baffled by the massive radiation shield while gravity would contribute to containing the movement of non-gaseous fission products within the outer containment structure. The Chernobyl reactor was surrounded by a radiation containment structure which failed because the of a powerful steam explosion. The cause of the blast was a combination of design flaws that caused a dramatic rise power levels in the reactor when an operator attempted to shut the reactor down.

The destruction of the radiation shield of the Chernobyl reactor removed the last level of containment for that reactor, while another level of containment, represented by the outer containment dome, would have still survived a Chernobyl like explosion. The failure of the Chernobyl radiation shield and the subsequent graphite fire lead the the massive release of radioisotopes from the burning Chernobyl reactor. Thus the critical features that lead to the radioisotope release from the Chernobyl radiation release were not present and two outer barriers to the release of solid radioactive materials, the massive 8" thick steel pressure vessel, and the even more massive outer dome of the reactor were not features of the Chernobyl reactor design. Other unique features of the RBMK reactor design including the use of a graphite moderator, and numerous design flaws that created safety problems contributed to the accident.

Anti-nuclear critics of nuclear safety often point to the Chernobyl accident as evidence of the fundamental safety flaws of all reactors, without noting the significant differences in safety features between RBMK reactors and LWRs. In fact during the Three Mile Island accident the outer safety barriers, the pressure vessel, the radiation shield, and the containment dome all remained in tact. There were no verified cases of radiation related health problems as a result of the Three Mile Island accident, and subsequent research failed to identify any increase in the number of cancer cases that could be associated with the accident. Thus the defense in depth deployed at the Three Mile Island Reactor was successful.

A Note on Radioactive Gases

The radioactive material released as a consequence of the Three Mile Accident were primarily nobel gases. The nobel gases and other radioactive gases are fission bi-products that are present in the uranium oxide fuel pellets, Normally they would remained trapped in the uranium oxide pellets, but if the reactor core heats enough to melt down, the zirconium cladding will rupture or melt, and the melting of the uranium oxide pellets will release the nobel gases. The gases escape from the reactor core through the cooling system. The gases are quickly dispersed by the atmosphere. While nuclear critics rase the issue of radioactive gases as an issue in justifying their opposition to nuclear power, nuclear critics often display a strange inconsistency. Radon, a radioactive gas is also released by coal burning coal fired power plants. In addition natural gas contains radon. More radioactive gas is released into the environment by the use of fossil than by nuclear power plants, yet nuclear critics rarely raise their voices in concern about radioactive gasses released by the use of fossil fuels. In fact, many supposedly pro-environmental, anti-nuclear organizations, accept funds from foundations with ties to fossil fuel produces, sometimes with stipulations that the funds will be used to promote fossil fuel use. Needless to say, these organizations never raise talk about the association of radon gas with fossil fuel use.

I will in a later post discuss a methods of preventing or at least limiting the release of radioactive gases associated with the development of the LFTR.