ACCIDENT TYPE TESTS

The main classical examples of fuel testing under neutron flux conditions are the accident type tests. Fuel behaviour under accident conditions has to be studied in tests representative of the addressed mechanisms. For the present industrial power reactors, the most common accidents studied are the loss of coolant accident (LOCA) and the reactivity insertion accident (RIA).

Furthermore, special investigations are performed in terms of the simulation of severe accident conditions.

4.1. LOCA conditions

Loss of coolant accidents are notably studied for LWRs. The test sequences typically consist of a blowdown, a cladding heat-up under steam cooling conditions provoking clad burst and oxidation, a quench and a

post-quench phase. The power level is normally very moderate, of the order of a few W/g (corresponding to 10–20 W/cm at PWRs).

The current regulatory safety criteria for LOCA, still in use in most countries, are derived from the emergency core cooling system (ECCS) acceptance criteria that were issued by the US Atomic Energy Commission in December 1973 and published in the US Code of Federal Regulations (10.CFR50, part 50.46). Among the relevant criteria, it is emphasized that the calculated total oxidation of the cladding shall nowhere exceed 0.17 times the total cladding thickness before oxidation.

This accident is characterized by the assumption of a certain number of fuel failures, of the non-fragmentation of rods (the rods keep their geometry), and of ensuring long term core cooling.

Technical issues associated with LOCA can be investigated either through separate effect or integral experiments to be performed, depending on constraints, under non-irradiated conditions or under irradiation in dedicated research reactors (see Part 8), or in multipurpose research reactors.

Historically, separate effect programmes have been undertaken on the kinetics of cladding oxidation at a temperature of around 1200°C and on the assessment of the ductile–fragile limit. Integral tests in research reactors have also been performed and are foreseen in several countries. Such experiments involve submitting fuel samples to accident type thermal hydraulic transients, with coolant restriction and/or evacuation according to a given scenario (typically involving sample deflooding and final reflooding). The temperature finally reached and the final sample state depend on the test input.

The very stringent test conditions require that these tests be performed in dedicated research reactors with special safety features. However, using standard multipurpose research reactors can be envisioned (and has been effected) for less stringent test conditions (typically, with automatic power transient shutdown when clad decoupling temperature criteria are reached).

Key aspects of such tests are data acquisition during the transient (sample temperature monitoring, fission product release monitoring, etc.) and management of the test device after test sequence completion. Figure 4 shows the design concept of a device for fuel testing under LOCA conditions, Fig. 5 adds the flow diagram of a real device for LOCA tests.

In this context, specific attention has to be paid to contamination and safety aspects. With regard to contamination, the main issue is the involvement of fission product release (in any form, not only gaseous) into the environment of the sample (notably following clad rupture), with consequential contami-nation of the coolant in the test device and of the sampling lines. Furthermore, the problem of contaminated sampling lines as for gas sampling in the FGR tests is extended to liquid sampling here. Specific technical solutions have to be

provided in order to prevent short term as well as long term contamination in the test facility originating from the performance of such tests, and to ensure radiological protection of the staff.

With regard to safety issues, by definition, such tests involve fast thermal hydraulic power transients on fuel samples. Therefore a key aspect is to prevent the propagation (due to any design or operation failure) from standard test input conditions to more degraded fuel conditions (e.g. with regard to fuel temperature) during the test, which could generate sample coolant interaction phenomena beyond those taken into account in the test input conditions. This aspect requires very specific design and operation considerations before any such test is undertaken. These considerations have to address both the test device as well as the hosting reactor design and operation, with particular regard to their robustness against active or passive failures or their accumu-lation.

FIG. 4. Conceptual design of an in-pile separate effect test in a multipurpose research reactor for studying LOCA type phenomena.

In principle, due to the constraints recalled above, such tests should not be undertaken in multipurpose research reactors (using irradiation devices in those MTRs) without relevant design and safety provisions to handle the risks.

Furthermore, the size of sample tested can be minimized deliberately and, apart from generic design and operating conditions, analyses can result in the implementation of temporary supplementary provisions in those MTRs when such tests are performed.

4.2. Reactivity insertion accident conditions

RIAs are very low probability events characterized by accidental injections of extra reactivity in reactor cores. The time period of the transient may vary from 10 ms up to a few hundreds of milliseconds and the power level during the pulse can be very high, e.g. more than 1000 times greater than the rated power.

RIA tests in research reactors are important for determining a fuel safety margin in view of rupture and for assessing potential failure consequences, such as fuel degradation and dispersal. Such tests are essential also for under-standing the mechanisms acting during the transient sequence, prior to and after the failure, and for writing and testing reliable transient codes.

Two approaches are typically used in safety analyses. The first one is based on energy deposited during the test (at the time of failure). The second

FIG. 5. Flow diagram of a LOCA type experiment: the FLASHMOX programmme.

approach takes into account a criterion based on the correlation between the strain level and the occurrence of a failure. Sometimes, fission gas release can be an important issue. In any case, the objective is to determine a safety domain in which there is no fuel failure or degradation, and/or dispersion and the function of cooling the fuel are preserved.

By design, RIA experiments involve submitting limited quantities of fuel samples to representative power scenarios. For integral tests, that condition can only be met by an adequate reactivity pulse provided by a suitable reactor core, which feeds the excess neutrons into the test fuel samples.

The neutronics characteristics of that neutron source have to be such that they provide enough feedback to achieve the neutronics transient by pure physical feedback while respecting decoupling criteria acceptable for the normal operation of that source. Nonetheless, the final reactor shutdown is achieved by a reactor scram at the end of the test sequence, to ensure a sufficient shutdown margin on the long term.

Therefore, integral tests require dedicated reactor cores (pulsed cores) (see Part 8). The test conditions can also be very demanding on the test device features, especially when considering that representative coolant conditions prevail during the test.

When MTR core features are not designed to perform such neutron pulses, such pulsed experiments have to be excluded for safety reasons.

Nonetheless, from a theoretical point of view, an alternative for multi-purpose MTRs would be to simulate fast power transients by experimental device design with the core remaining at a stable core power level (no need for control rod action; an automatic preprogrammed reactor scram can terminate the test). A theoretical solution could involve fast transit (e.g. electromagnetic motion) of a small experimental sample in a guide tube throughout the core at nominal power.

In any case, achievable performances would be much more limited (separate effect experiments with access to restricted levels of power on samples which could, however, enable the reproduction of mechanisms that are active during a real RIA sequence).

Specific precautions in terms of safety issues should be carried out before envisioning any such tests, and only multipurpose research reactors with relevant design and safety provisions should be considered. In any case, parametric tests in multipurpose research reactors should be performed during a dedicated reactor cycle with no other irradiation test, to prove specific design and operation provisions.

At this moment, it seems that no practical implementation of RIA type separate effects experiments in MTRs, as described above, has been effected.

4.3. Severe accident conditions

Severe accident conditions in LWRs are multiple failure sequences beyond design basis during which the core would undergo a partial or total loss of its integrity and initial geometry. During such sequences, fuel assemblies will suffer a dramatic degradation and release a significant part of their material in airborne or particle form.

In case of severe accident sequences, the main concerns for reactor safety are core coolability and the parameters of the source term as determined by fission product behaviour and containment integrity.

Severe accident studies require integral trial runs in research reactors (e.g. quantitative studies of core melting under irradiation conditions in terms of assessing radiological releases and dissemination).

Corresponding technical requirements for the design of a related test device, insertion of such a device into a research reactor and operation of the reactor plus the device during the different phases of the test cycle (prepa-ration, re-irradiation, accident sequence management, post-irradiation operation) impose peculiar technical and safety constraints.

In accordance with the technical and safety analyses, those constraints lead to dedicated test devices and often even to single experiment dedicated (research) reactors (see also Part 8).

5. NECESSARY PLANT PROVISIONS AND EXPERIMENTAL