Small Defects, Big Impacts

Before we dig in to the details outlined in the previous section, let's first explore the huge impacts that small defects can have. In this case, we'll motivate the study of these tiny defects by the role they had in one of the most serious nuclear accidents in history. Most of you have heard of Chernobyl, Fukushima, and Three Mile Island. Indeed, these disasters were the some of the worst in history, receiving International Nuclear Event Scale (INES) ratings of 7, 7, and 5, respectively. Note - the INES scales are logarithmic, so Chernobyl and Fukushima are approximated to be 100$\times$ worse than Three Mile Island. These disasters resulted in both direct and indirect cost of human lives, as well as billions of dollars in property damage and environmental remediation. Indeed, the area around Chernobyl has been designated an exclusion zone since 1968, and remediation will continue at Fukushima for the next 30-40 years.

There have been three other nuclear disasters above INES ratings of 4 which most people haven't heard of. One is the Windscale fire on the Northwest coast of England, in 1957. As we'll see, this event is closely related to these tiny defects that can exist in crystals, and we'll use a simulation to explore why. I invite you to watch the video in Video 6.3.1 and read the technical synopsis in Section 6.3.2.

Windscale Technical Synopsis

The Windscale facility was constructed to produce plutonium for use in the UK's nuclear weapons program in the 1950s. The UK targeted the use of plutonium ($^{239}\ce{Pu}$) for use in these weapons is produce, which is fission product of $^{238}\ce{U}$ and neutrons:

$$^{238}\ce{U} + ^{1}\ce{n} \rightarrow ^{239}\ce{U} \stackrel{\beta^-}{\ce{\rightarrow}} \,^{239}\ce{Np} \stackrel{\beta^-}{\rightarrow} \,^{239}\ce{Pu} \tag{6.3.1}$$

The source of the neutron in these reactors is $^{235}\ce{U}$, which decays to produce Ba and K isotopes, as well as a bunch of energetic neutrons:

$$^{1}\ce{n} + ^{235}\ce{U} \rightarrow \,^{141}\ce{Ba} + \,^{92}\ce{Kr} + 3\,^{1}\ce{n} \tag{6.3.2}$$

In order to sustain the production of $^{239}\ce{Pu}$, engineers have to control the energy of the neutrons released during the decay of $^{235}\ce{U}$. For these neutrons to efficiently interact with $^{238}\ce{U}$ to produce $^{239}\ce{Pu}$, they must be "moderated" or slowed down. The substances typically used to do have low-atomic-masses. Water is the world's most popular moderator in nuclear reactors, but graphite is used in 20% of reactors, and was more popular in the 50s and 60s. Graphite - a regular, periodic arrangement of carbon atoms (a crystal) - was used in the Windscale facility.

While neutrons are slowed down by the reactor, sometimes the impact of these neutrons can also lead to a "defect" in the graphite crystal lattice. A neutron with enough energy can actually displace a carbon atom from the graphite lattice, leaving a "vacant" site behind. This, as one might imagine, can have serious consequences. Let's explore on the next page.