Radiation dose and radionuclide inventory characterization following a hypothetical nuclear criticality accident at a uranium processing facility

Abstract

This study focused on a hypothetical nuclear criticality excursion in a Plutonium Uranium Reduction EXtraction (PUREX) processing facility for the recovery of non-irradiated enriched uranium, which are present in scrap material from historical operations. The entire facility, including the fissile process-mass in the workstation where the hypothetical excursion takes place and, the concrete biological shielding walls and roof, was modelled in a series of calculation models using the radiation transport code, MCNP6.2. ENDF/B-8.0 nuclear data was used, and special thermal treatment of scattering kinetics was requested wherever possible, to accurately quantify the impact of the bound state of nuclides, on the thermal neutron energy spectrum. The calculation models were developed to quantify the neutron and photon doses resulting from the criticality excursion at a number of “human detector”² positions placed inside and around the PUREX facility. The design basis criticality accident is an excursion involving 1017 fission events, which is denoted as a CA(10¹⁷f) accident. The calculation model of a “naïve”, baseline facility design was used to quantify the neutron and photon doses. This design was denoted as “modification zero” or MOD0. The conclusion was that the baseline design carries an unacceptable radiological risk — a nuclear criticality excursion inside this baseline facility design was shown to be lethal for every person within a circa 10 m radius from the epicentre of the fissile process mass where the excursion takes place, and also lead to acute radiation syndrome and a substantially elevated cancer risk in all survivors. This baseline design was unable to contain the pulse of ionising radiation, because there was a lack of partitioning shielding walls inside the facility. Radiation therefore streamed freely between processing workstations. Step-by-step facility design modifications, denoted as MOD1, MOD2, until MOD6, were then introduced in order to arrive at a significantly safer facility design, from a personnel exposure perspective. The goal of the stepwise improvements was to compartmentalise the facility into well shielded cubicles, with a shielding wall geometry and thickness able to localise the lethal dose to a single workstation compart-ment or cubicle, whilst maintaining the doses in all other compartments near or below the International Commission on Radiological Protection’s (ICRP’s) recommended annual dose limit of 20 mSv for occupational exposure, which is a factor 50 times below the threshold dose of 1000 mSv for the onset of acute radiation syndrome. It is practically unavoidable that the person(s) in the cubicle where the excursion takes place, will receive a lethal dose. Instead, the aim of the improvements was to ensure that staff in adjacent workstations, the control room, workshop and ablution facilities, as well as persons outside the main processing hall, at the position directly opposite the fissile volume hosting the excursion, would receive effective doses at or below the annual dose limit for normal occupational exposure. The analysis also showed that all the inner partitioning walls must extend all the way to the roof; if this is not done, a significant amount of streaming from the accident cubicle to adjacent cubicles, was observed. This shielding design methodology that proved fruitful and ultimately successful, can be called the radiological de-coupling of process workstations from each other, via an improved placement of inner partitioning concrete walls of adequate thickness and height. The shielded workstations are designed so that the radiation from a criticality excursion is contained and confined to the accident workstation; lethality is localised and the PUREX plant is rescued from being “one large coffin”.