Fuel assemblies with the design called “BWR STEP 3” had been loaded in the reactors. Each new fuel assembly contains six kinds of uranium dioxide (UO2) fuel (Fig. 21.1, Table 21.1). The most popular initial 235U/U enrichment in the fuels is

4.4 wt%, whose inventory per assembly is 76.8 kgU. The fuel of 9.6 kgU per assembly has the highest initial enrichment of 4.9 wt%. The initial uranium inventory in total is 170.9 kgU per assembly, including fuels of other enrichments and of the UO2-gadolinium oxide (Gd2O3) composite [4].

The Unit 1 reactor in 1FNPS had 400 assemblies, which consisted of six batches of burn-up. Each of the Unit 2 and 3 reactors had 548 assemblies of five batches. Among these assemblies, 64 in the Unit 1 reactor, and 116 in the Unit 2 reactor, had a low burn-up of only 3-5 GWD/t (Table 21.2). Other assemblies of the same number are older but still have a burn-up as low as 15-16 GWD/t. The oldest assemblies have a burn-up of about 40 GWD/t [5].

Assembly Pitch 15.24cm

U02 23SU 4.9wt% @ U02 235U 3.4wt%

2 U02 23SU 4.4wt% U02 235U 2.1 wt%

© U02 235U 3.9wt% (G) U02-Gd203 3.4wt%

w w 2 2 3 Gd203 5.0wt%

Fig. 21.1 Benchmark model of the BWR STEP3 fuel assembly

The condition of the fuel debris has not yet been identified in any reactor except estimations by severe accident analysis codes. Study of the TMI-2 fuel debris [6], however, suggests that various kinds of fuel debris may also be produced in the 1FNPS reactors, such as hard and loose debris. Especially, loose debris may show a wide variety of composition including structural materials such as Zircaloy and

Table 21.1 Initial uranium inventory in a boiling water reactor (BWR) STEP 3 fuel assembly

steel. Boron originating from the control rods cannot be expected necessarily to coexist with the fuel debris. It is also possible that the fuel debris in CVs has been generated through the molten core-concrete interaction (MCCI). It must be con­sidered that the fuel debris is not uniform and will be found at various locations.

The fuel debris is being cooled with nonborated water although it is highly preferable to add neutron poison and to maintain enough concentration in the water to secure the subcritical condition such as was performed after the TMI-2 accident. Boration is not realistic at present because of the coolant water leakage from CVs and underground water inflow to the coolant water circulation. Boron will be injected only in the event of re-criticality [7].