Understanding significant reactor transients

Section 4.1 of the reactor description [https://www.nrc.gov/docs/ML2525/ML25255A207.pdf] describes a regularly occurring event in the regular operation of the reactor:

The most significant reactor transient, apart from startup and shutdown, occurs with the deliberate insertion or removal of high-reactivity irradiation targets while the system is operational. These target movements are planned and executed to ensure that no reactivity limits are exceeded, and that the operating reactor is maintained in a safe state.

A reactor transient is “a change reactor coolant system temperature, pressure, or both, attributed to a change in the reactor’s power output”.

How often does this event take place?

It is likely to be often, because this is how the reactor turns a profit for its operator; materials go in, isotopes come out, they are sold. It’s an isotope factory – the core purpose of its functioning. The frequency is likely a function of the time it takes to irradiate a specific desired isotope to the desired specifications and the manufacturing process efficiency of the site. A commercial manufacturing site is profit-motivated to maximize throughput and yield – unlike a research reactor at a public university.

The frequency of this event is a factor to consider, taken with the probability of an incident during such an event. Each occurrence is another roll of that dice: what are the odds?

What happens if the target material becomes stuck or jammed in the reactor? How is this handled?

This is particularly concerning for two reasons:

  1. The light water pools have to be biologically sealed from operators and personnel in the reactor room. This is necessary because the reactor produces Nitrogen-16, a highly radioactive isotope in gaseous form, which bubbles up from the core. Not great, not terrible. It has a short half-life of about 7 seconds and decays to stable daughter elements – but will damage those breathing it in the room while it lasts.
  2. The core is surrounded by a sealed heavy water tank, some of which consists of tritium.

With both the light water and heavy water tank within sealed, how would such an incident be mitigated?

If target material were to become stuck, could it increase the reactivity beyond acceptable limits?

In other words, could it cause a meltdown – an uncontrolled reaction event.

Could stuck target material prevent an emergency shutdown of the reactor?

Typically when a fission reactor runs too far out of normal operating parameters to control, there is an emergency shutdown mechanism that is able to stop it by retracting control rods, absorbing neutrons, etc.

Could a jammed target capsule prevent this emergency shutdown from working?