Safety Analysis and Project Proposal for UC Berkeley Sub-Critical Assembly

For my senior design project in nuclear engineering at UC Berkeley, I led a project proposal to restore and rethink the university’s sub-critical assembly. A steel basin houses 344 Uranium pins, hidden beyond a locked door inside a room with a leaking roof.

For at least a decade or more, the Sub-Critical Assembly has been left underutilized. When it rained, the assembly itself would fill with rainwater, a neutron moderator, increasing the criticality of the assembly due to the higher rate of thermal neutron flux. By charge of incoming UC Berkeley NE professor Dr. Daniel Siefman, under the advisory of him and Dr. Peter Hosemann, myself and two other undergraduates set out to provide a safety analysis and create a proposal for its revitalization.

The SubCritical Assembly is, as the name suggests, an array of 344 natural Uranium pins seated in a steel basin. The arrangement of them is such that under any normal conditions and the accounted for abnormal conditions, the assembly will not reach criticality. Criticality is the condition under which the amount of neutrons produced equals the amount of neutrons needed for a self-sustaining fission reaction. This is the normal state that nuclear reactors operate when generating power, however, the SubCritical Assembly is not designed to reach a k-effective of 1, or criticality. As such, it does not have any cooling systems, but is nevertheless emitting tens of thousands of becquerel.

The room is separated behind a foot of concrete wall, and 6 inch thick steel doors, locked with a key in another lab space under a Radiation Use Authentication (RUA). The ceiling to this room additionally was impacted by a leak, which filled the assembly with rain water. Due to the excellent neutron moderation properties of water, the activity of the water-logged assembly would increase, but even still, it remains sub critical.

My role was Assembly Lead, and informally I engaged our very own Dr. Alan Bolind, a UC Berkeley Environmental Health and Safety (EHS) Radiation Safety Liaison. I had prior Radiation Use Authorization from my years of research work under Dr. Kai Vetter's RUA, so I could enter the room, take measurements, and create detailed sketches of the space, as well as coordinate further radiation measurements later used in the risk assessment.

I turned my drawings into a 3D CAD model in Inventor which was used to inform the MCNP model, which used Monte Carlo methods to run neutron flux simulations. Both models accounted for the tank dimensions, pin placement, support structures, materials, and potential modifications or proposed experiment designs. I proposed a new water fill system using a square storage tank, a single reversible pump, and external piping, so students could run approach-to-criticality style experiments without being at risk. I also collected pricing information from quotes from several companies regarding materials for the upgrades and potential experiments. I also designed a concept for a motorized linear rail system that could move neutron detectors across the top of the tank, allowing us to measure flux profiles with precision and keep students safely outside the room while data was being collected. I used a geiger counter to collect exposure rates in air, as well as Liquid Scintillation detection to confirm the rainwater had zero leeched uranium in it and was safe to dispose of as non-rad material.

The bottom of the tank grate had corrosion and what looked like mold growing on it, which was of course very pleasant to present to my professor and lab staff that oversaw our project and presentation. Dr. Siefman was particularly excited for our proposal, as it presented hands on learning opportunities for his courses in neutronics prior to the start of his teaching the next semester. The SubCritical assmebly experienced years of neglect, and through our project we put together an informed risk assessment with project deliverables and realistic budget as outlined by Nuclear Engineering administrators.

This was one of the last things I worked on as a student in Berkeley, and it gave me hands on experience with risk assessment and the extensive levels of documentation and cross-department communication needed for a project on its scale of about several hundred thousand to over 2 million in proposed upgrades.