aging nuclear reactors, containment, corrosion, neutron activation, neutron bombardment, neutron embrittlement, neutron irradiation, neutrons, nuclear accidents, nuclear bombs, nuclear dangers, nuclear pressure vessels, nuclear reactor containment, nuclear reactor cracked containment, nuclear safety, nuclear war, Radiation protection, shoddy construction
One of the least discussed and bizarre and frightening commonalities between nuclear bombs and nuclear reactors is their ability to make previously non-radioactive materials become radioactive, including within the human body! Neutron radiation, from nuclear reactors or bombs can induce this neutron “activation”.
Neutron bombardment also degrades the materials which make up nuclear reactors at a faster rate than would normally occur. This is the reason that nuclear reactors were licensed for a fixed period of time and one major reason why it is a dangerously bad idea to extend their life, as is currently happening. While on the surface this suggests that new reactors would be better, the reactors recently under construction in the US, Finland and France have been characterized by shoddy construction, including substandard concrete and improper spacing of rebar, as documented by the governments themselves. So, on the one hand there is the problem of materials weakened, and even cracked, through age, corrosion and neutron bombardment, and, on the other hand, materials which may be weak from the start. Combined with the serious dangers of nuclear radiation, the future looks bleak indeed, unless nuclear power is stopped dead in its tracks now. Dead nuclear or dead everything!
Frightening, indeed, that according to the US Oak Ridge National Lab,
“there is currently little or no data on long-term concrete performance” for nuclear power plants, even though they are largely built of concrete!
According to Oak Ridge National Lab: “Nuclear power plant operating environments create material degradation mechanisms that may be unique or environmentally exacerbated. In this figure, Irradiation-Assisted Stress Corrosion Cracking has resulted in cracking at the head of a baffle bolt.”
According to Chopra, 2010, for the US NRC: “Austenitic stainless steels are used extensively as structural alloys in the internal components of light water reactor (LWR) pressure vessels because of their relatively high strength, ductility, and fracture toughness. However, exposure to neutron irradiation for extended periods changes the microstructure and microchemistry of these steels and degrades their fracture properties. This report presents a critical assessment of the susceptibility of LWR core internal materials to irradiation effects such as irradiation-assisted stress corrosion cracking (IASCC), neutron embrittlement, void swelling, and irradiation-induced stress relaxation.” (http://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr7027/: “Degradation of LWR Core Internal Materials Due to Neutron Irradiation (NUREG/CR-7027)“, Prepared by: O.K. Chopra (2010)
“Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce radiation.” http://en.wikipedia.org/wiki/Neutron_radiation
“Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus often decays immediately by emitting gamma rays, or particles such as electrons (beta rays), alpha particles, or fission products and neutrons (in nuclear fission). Thus, the process of neutron capture, even after any intermediate decay, often results in the formation of an unstable activation product. Such radioactive nuclei can exhibit half-lives ranging from small fractions of a second to many years.
Neutron activation is the only common way that a stable material can be induced into becoming intrinsically radioactive. All naturally-occurring materials, including air, water, and soil, can be induced (activated) by neutron capture into some amount of radioactivity in varying degrees, as a result of production of neutron-rich radioisotopes.” Sea salt is easier to activate than water. http://en.wikipedia.org/wiki/Neutron_activation “For physicians and radiation safety officers, activation of sodium in the human body to sodium-24, and phosphorus to phosphorus-31, can give a good immediate estimate of acute accidental neutron exposure.” http://en.wikipedia.org/wiki/Neutron_activation
“Neutron radiation is often called indirectly ionizing radiation. It does not ionize atoms in the same way that charged particles such as protons and electrons do (exciting an electron), because neutrons have no charge. However, neutron interactions are largely ionizing, for example when neutron absorption results in gamma emission and the gamma ray (photon) subsequently removes an electron from an atom, or a nucleus recoiling from a neutron interaction is ionized and causes more traditional subsequent ionization in other atoms. Because neutrons are uncharged, they are more penetrating than alpha radiation or beta radiation. In some cases they are more penetrating than gamma radiation, which is impeded in materials of high atomic number. In materials of low atomic number such as hydrogen, a low energy gamma ray may be more penetrating than a high energy neutron.“…
“Neutron radiation is also used in select facilities to treat cancerous tumors due to its highly penetrating and damaging nature to cellular structure.” http://en.wikipedia.org/wiki/Neutron_radiation
“Health hazards and protection
In health physics neutron radiation is considered a fourth radiation hazard alongside the other types of radiation. Another, sometimes more severe hazard of neutron radiation, is neutron activation, the ability of neutron radiation to induce radioactivity in most substances it encounters, including the body tissues of the workers themselves. This occurs through the capture of neutrons by atomic nuclei, which are transformed to another nuclide, frequently a radionuclide. This process accounts for much of the radioactive material released by the detonation of a nuclear weapon. It is also a problem in nuclear fission and nuclear fusion installations, as it gradually renders the equipment radioactive; eventually the hardware must be replaced and disposed of as low-level radioactive waste.
Neutron radiation protection relies on radiation shielding. Due to the high kinetic energy of neutrons, this radiation is considered to be the most severe and dangerous radiation to the whole body when exposed to external radiation sources. In comparison to conventional ionizing radiation based on photons or charged particles, neutrons are repeatedly bounced and slowed (absorbed) by light nuclei, so hydrogen-rich material is more effective than iron nuclei. The light atoms serve to slow down the neutrons by elastic scattering, so they can then be absorbed by nuclear reactions. However, gamma radiation is often produced in such reactions, so additional shielding has to be provided to absorb it. Care must be taken to avoid using nuclei which undergo fission or neutron capture that results in radioactive decay of nuclei that produce gamma rays.
Neutrons readily pass through most material, but interact enough to cause biological damage. The most effective shielding materials are hydrocarbons, e.g. polyethylene, paraffin wax or water. Concrete (where a considerable amount of water molecules are chemically bound to the cement) and gravel are used as cheap and effective biological shields due to their combined shielding of both gamma rays and neutrons. Boron is an excellent neutron absorber (and also undergoes some neutron scattering) which decays into carbon or helium and produces virtually no gamma radiation, with boron carbide a commonly used shield where concrete would be cost prohibitive. Commercially, tanks of water or fuel oil, concrete, gravel, and B4C are common shields that surround areas of large amounts of neutron flux, e.g. nuclear reactors. Boron-impregnated silica glass, high-boron steel, paraffin, and Plexiglas have niche uses.
Because the neutrons that strike the hydrogen nucleus (proton, or deuteron) impart energy to that nucleus, they in turn will break from their chemical bonds and travel a short distance before stopping. Such hydrogen nuclei are high linear energy transfer particles, and are in turn stopped by ionization of the material through which they travel. Consequently, in living tissue, neutrons have a relatively high relative biological effectiveness, and are roughly ten times more effective at causing biological damage compared to gamma or beta radiation of equivalent radiation exposure. Neutrons are particularly damaging to soft tissues like the cornea of the eye. http://en.wikipedia.org/wiki/Neutron_radiation The NRC gives neutrons a weighting factor of 10. However, the ICRP gives neutrons a weighting factor of 20 or more, to reflect their dangerousness to the body. Alpha particles are given a weighting factor of 20.” http://en.wikipedia.org/wiki/Equivalent_dose (Note that boron, itself, contributes to material degradation in nuclear reactors, as demonstrated in the Davis Besse reactor).
Neutron activation is the only common way that a stable material can be induced into becoming intrinsically radioactive. Neutrons are only free in quantity in the microseconds of a nuclear weapon’s explosion AND IN AN ACTIVE NUCLEAR REACTOR.
Effects on materials…
In any location with high neutron fluxes, such as within the cores of nuclear reactors, neutron activation contributes to material erosion; periodically the lining materials themselves must be disposed of, as low-level radioactive waste…” http://en.wikipedia.org/wiki/Neutron_activation
(All bold and caps added)
“Neutrons also degrade materials; bombardment of materials with neutrons creates collision cascades that can produce point defects and dislocations in the materials. At high neutron fluences this can lead to embrittlement of metals and other materials, and to swelling of some of them. This poses a problem for nuclear reactor vessels, and significantly limits their lifetime (which can be somewhat prolonged by controlled annealing of the vessel, reducing the number of the built-up dislocations). Graphite moderator blocks are especially susceptible to this effect, known as Wigner effect, and have to be annealed periodically; the well-known Windscale fire was caused by a mishap during such an annealing operation.
Neutron radiation and nuclear fission
The neutrons in reactors are generally categorized as slow (thermal) neutrons or fast neutrons depending on their energy. Thermal neutrons are similar to a gas in thermodynamic equilibrium but are easily captured by atomic nuclei and are the primary means by which elements undergo atomic transmutation.
In order to achieve an effective fission chain reaction, the neutrons produced during fission must be captured by fissionable nuclei, which then split, releasing more neutrons. In most fission reactor designs, the nuclear fuel is not sufficiently refined to be able to absorb enough fast neutrons to carry on the fission chain reaction, due to the lower cross section for higher-energy neutrons, so a neutron moderator must be introduced to slow the fast neutrons down to thermal velocities to permit sufficient absorption. Common neutron moderators include graphite, ordinary (light) water and heavy water. A few reactors (fast neutron reactors) and all nuclear weapons rely on fast neutrons. This requires certain changes in the design and in the required nuclear fuel. The element beryllium is particularly useful due to its ability to act as a neutron reflector or lens. This allows smaller quantities of fissile material to be used and is a primary technical development that led to the creation of neutron bombs.” http://en.wikipedia.org/wiki/Neutron_radiation
(Bold added throughout this and the other wikipedia articles).
Oak Ridge National Lab Research into Material Aging and Degradation in Nuclear Reactors
“The major R&D areas under the Materials Aging and Degradation Pathway include the following:
Reactor Metals – Numerous types of metal alloys can be found throughout the primary and secondary systems of NPPs. Plant operating environments create material degradation mechanisms that may be unique or environmentally exacerbated….
Concrete – Large areas of most NPPs have been constructed using concrete. However, there is currently little or no data on long-term concrete performance in these plants. As such, the objective is to assess the long-term performance of concrete in nuclear applications.
Cabling – Cable aging is a concern that currently faces existing NPPs. Degradation of cables is primarily caused by long-term exposure to high temperatures. Additionally, stretches of cables that have been buried underground are frequently exposed to groundwater. Wholesale replacement of cables would likely be a ‘show stopper’ for long-term NPP operation.
Nondestructive Evaluation (NDE) – The understanding of aging-related phenomena and their impacts on SSCs is expected to be a significant issue for any NPP planning for extended service. The management of those phenomena and their impacts can be better enabled by improved methods and techniques for the detection, monitoring and prediction of SSC degradation.
Mitigation Technologies – These technologies include weld repair, post-irradiation annealing, and water chemistry modifications.
In addition, the Materials Aging and Degradation Pathway is actively involved in efforts to harvest selected materials (RPV metals, concrete and low-voltage cabling) from the decommissioned Zion plant for conducting aging effects R&D and a joint DOE/industry project to study and identify improved inspection and aging prediction techniques from NPPs currently in their extended service periods.” http://www.ornl.gov/science-discovery/nuclear-science/research-areas/reactor-technology/light-water-reactor-sustainability
Of additional interest regarding materials, from US National Sandia Lab:
Printed July 2011
Nuclear Containment Steel Liner Corrosion Workshop: Final Summary and Recommendation Report
Jason P. Petti Structural and Thermal Analyses, Org. 06233 Sandia National Laboratories; Dan Naus Oak Ridge National Laboratory; Alberto Sagüés University of South Florida; Richard E. Weyers Virginia Tech University;Bryan A. Erler Erler Engineering Ltd.; Neal S. Berke Tourney Consulting Group, LLC, Kalamazoo, MI 49048
This report documents the proceedings of an expert panel workshop conducted to evaluate the mechanisms of corrosion for the steel liner in nuclear containment buildings. The U.S. Nuclear Regulatory Commission (NRC) sponsored this work which was conducted by Sandia National Laboratories. A workshop was conducted at the NRC Headquarters in Rockville, Maryland on September 2 and 3, 2010. Due to the safety function performed by the liner, the expert panel was assembled in order to address the full range of issues that may contribute to liner corrosion. This report is focused on corrosion that initiates from the outer surface of the liner, the surface that is in contact with the concrete containment building wall. Liner corrosion initiating on the outer diameter (OD) surface has been identified at several nuclear power plants, always associated with foreign material left embedded in the concrete. The potential contributing factors to liner corrosion were broken into five areas for discussion during the workshop. Those include nuclear power plant design and operation, corrosion of steel in contact with concrete, concrete aging and degradation, concrete/steel non-destructive examination (NDE), and concrete repair and corrosion mitigation. This report also includes the expert panel member’s recommendations for future research“. Entire report here: http://pbadupws.nrc.gov/docs/ML1121/ML112150012.pdf (emphasis added) The issue of “foreign material left embedded in the concrete” suggests shoddy construction. This should serve as a reminder of the dangers of letting shoddily constructed nuclear power plants go online, as well as the dangers of letting old ones stay online. Some of the older plants were so poorly constructed that they never went online as nuclear power plants, although sometimes as other types of power plants! So, shoddy construction isn’t totally new.