The parsing is a little difficult here, but I think that's the idea.

I basically agree with Massimo, in that it refers to a control rod ejection accident, but I wanted to mention just what the "haz" refers to. "Beam" is in fact a misinterpretation of the word "haz" here, as it's not referring to anything we would call a "beam" in English; it's referring to a "bundle," i.e., the fuel "bundle" or "assembly":

"haz
1. m. Atado de mieses, lino, hierba, leña o cosas semejantes."
https://dle.rae.es/haz?m=form

"En los elementos combustibles PWR, las barras de material absorbente que se utilizan para controlar la reacción nuclear ("barras de control") se insertan por la parte superior en sitios especiales dentro del elemento combustible.

Las barras combustibles terminadas se agrupan formando haces, manojos o elementos combustibles, cada haz con entre 100 a 400 barras combustibles dependiendo la cantidad del diseño específico de cada reactor. Un núcleo de un reactor nuclear contiene varios cientos de haces o manojos de combustible."
https://es.wikipedia.org/wiki/Combustible_nuclear


"Pressurized water reactor (PWR) fuel consists of cylindrical rods put into bundles. A uranium oxide ceramic is formed into pellets and inserted into Zircaloy tubes that are bundled together. The Zircaloy tubes are about 1 cm in diameter, and the fuel cladding gap is filled with helium gas to improve the conduction of heat from the fuel to the cladding. There are about 179-264 fuel rods per fuel bundle and about 121 to 193 fuel bundles are loaded into a reactor core. Generally, the fuel bundles consist of fuel rods bundled 14×14 to 17×17. PWR fuel bundles are about 4 meters long. In PWR fuel bundles, control rods are inserted through the top directly into the fuel bundle."

"PWR fuel assembly (also known as a fuel bundle)"
https://en.wikipedia.org/wiki/Nuclear_fuel

So,

barra de control = control rod
haz (o elemento o manojo de combustible) = (fuel) bundle (or assembly)

"Most of the reactivity control devices are vertically oriented and are located between fuel channels in the unpressurized, low-temperature moderator. (Liquid poison injection nozzles in the second shutdown system are located horizontally between fuel channels.) This removes high pressure as a possible driving force in accidently [sic] ejecting control rods from the reactor"
https://www.sciencedirect.com/topics/engineering/reactivity-...

So, as Massimo says, it's effectively referring to a "control rod ejection accident".